Analysis of co-isogenic prion protein deficient mice reveals behavioral deficits, learning impairment, and enhanced hippocampal excitability

Background

Cellular prion protein (PrP^C) is a cell surface GPI-anchored protein, usually known for its role in the pathogenesis of human and animal prionopathies. However, increasing knowledge about the participation of PrP^C in prion pathogenesis contrasts with puzzling data regarding its natural physiological role. PrP^C is expressed in a number of tissues, including at high levels in the nervous system, especially in neurons and glial cells, and while previous studies have established a neuroprotective role, conflicting evidence for a synaptic function has revealed both reduced and enhanced long-term potentiation, and variable observations on memory, learning, and behavior. Such evidence has been confounded by the absence of an appropriate knock-out mouse model to dissect the biological relevance of PrP^C, with some functions recently shown to be misattributed to PrP^C due to the presence of genetic artifacts in mouse models. Here we elucidate the role of PrP^C in the hippocampal circuitry and its related functions, such as learning and memory, using a recently available strictly co-isogenic Prnp^0/0 mouse model (Prnp^ZH3/ZH3).

Results

We performed behavioral and operant conditioning tests to evaluate memory and learning capabilities, with results showing decreased motility, impaired operant conditioning learning, and anxiety-related behavior in Prnp^ZH3/ZH3 animals. We also carried in vivo electrophysiological recordings on CA3-CA1 synapses in living behaving mice and monitored spontaneous neuronal firing and network formation in primary neuronal cultures of Prnp^ZH3/ZH3 vs wildtype mice. PrP^C absence enhanced susceptibility to high-intensity stimulations and kainate-induced seizures. However, long-term potentiation (LTP) was not enhanced in the Prnp^ZH3/ZH3 hippocampus. In addition, we observed a delay in neuronal maturation and network formation in Prnp^ZH3/ZH3 cultures.

Conclusion

Our results demonstrate that PrP^C promotes neuronal network formation and connectivity. PrP^C mediates synaptic function and protects the synapse from excitotoxic insults. Its deletion may underlie an epileptogenic-susceptible brain that fails to perform highly cognitive-demanding tasks such as associative learning and anxiety-like behaviors.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01203-0.

Sialic Acid and Sialylated Oligosaccharide Supplementation during Lactation Improves Learning and Memory in Rats

Sialic acids (Sia) are postulated to improve cognitive abilities. This study evaluated Sia effects on rat behavior when administered in a free form as N-acetylneuraminic acid (Neu5Ac) or conjugated as 6'-sialyllactose (6'-SL). Rat milk contains Sia, which peaks at Postnatal Day 9 and drops to a minimum by Day 15. To bypass this Sia peak, a cohort of foster mothers was used to raise the experimental pups. A group of pups received a daily oral supplementation of Neu5Ac to mimic the amount naturally present in rat milk, and another group received the same molar amount of Sia as 6'-SL. The control group received water. After weaning, rats were submitted to behavioral evaluation. One year later, behavior was re-evaluated, and in vivo long-term potentiation (LTP) was performed. Brain samples were collected and analyzed at both ages. Adult rats who received Sia performed significantly better in the behavioral assessment and showed an enhanced LTP compared to controls. Within Sia groups, 6'-SL rats showed better scores in some cognitive outcomes compared to Neu5Ac rats. At weaning, an effect on polysialylated-neural cell adhesion molecule (PSA-NCAM) levels in the frontal cortex was only observed in 6'-SL fed rats. Providing Sia during lactation, especially as 6'-SL, improves memory and LTP in adult rats.

Altitude acclimatization improves submaximal cognitive performance in mice and involves an imbalance of the cholinergic system

The aim of this work was to reveal a hypothetical improvement of cognitive abilities in animals acclimatized to altitude and performing under ground level conditions, when looking at submaximal performance, once seen that it was not possible when looking at maximal scores. We modified contrasted cognitive tasks (object recognition, operant conditioning, eight-arm radial maze, and classical conditioning of the eyeblink reflex), increasing their complexity in an attempt to find performance differences in acclimatized animals vs. untrained controls. In addition, we studied, through immunohistochemical quantification, the expression of choline acetyltransferase and acetyl cholinesterase, enzymes involved in the synthesis and degradation of acetylcholine, in the septal area, piriform and visual cortexes, and the hippocampal CA1 area of animals submitted to acute hypobaric hypoxia, or acclimatized to this simulated altitude, to find a relationship between the cholinergic system and a cognitive improvement due to altitude acclimatization. Results showed subtle improvements of the cognitive capabilities of acclimatized animals in all of the tasks when performed under ground-level conditions (although not before 24 h), in the three tasks used to test explicit memory (object recognition, operant conditioning in the Skinner box, and eight-arm radial maze) and (from the first conditioning session) in the classical conditioning task used to evaluate implicit memory. An imbalance of choline acetyltransferase/acetyl cholinesterase expression was found in acclimatized animals, mainly 24 h after the acclimatization period. In conclusion, altitude acclimatization improves cognitive capabilities, in a process parallel to an imbalance of the cholinergic system.

[Hemoperitoenum as presentation of hepatocellular carcinoma: experience in three cases with spontaneous tumoral rupture and review of the literature]

Hemoperitoneum due to spontaneous rupture of hepatocellular carcinoma (HCC) constitutes a life-threatening situation if no appropriate therapy is provided. This complication is a well-known form of HCC presentation in countries with high incidence of liver tumours, but is an unusual event in Western countries, where it has been described in 5% or less of cases with HCC. We report three patients admitted to our centre with acute hemoperitoneum secondary to non-traumatic rupture as a first manifestation of not previously diagnosed HCC. A review of the related literature is also performed.

Aged wild-type and APP, PS1, and APP + PS1 mice present similar deficits in associative learning and synaptic plasticity independent of amyloid load

Wild-type and single-transgenic (APP, PS1) and double-transgenic (APP+PS1) mice were studied at three different (3-, 12-, and 18-month-old) age periods. Transgenic mice had reflex eyelid responses like those of controls, but only 3-month-old mice were able to fully acquire conditioned eyeblinks, using a trace paradigm, whilst 12-month-old wild-type and transgenic mice presented intermediate values, and 18-month-old wild-type and transgenic mice were unable to acquire this type of associative learning. 18-month-old wild-type and transgenic mice presented a normal synaptic activation of CA1 pyramidal cells by the stimulation of Schaffer collaterals, but they did not show any activity-dependent potentiation of the CA3-CA1 synapse across conditioning sessions, as was shown by 3-month-old wild-type mice. Moreover, 18-month-old wild-type and transgenic mice presented a noticeable deficit in long-term potentiation evoked in vivo at the hippocampal CA3-CA1 synapse. The 18-month-old wild-type and transgenic mice also presented a significant deficit in prepulse inhibition as compared with 3-month-old controls. Except for results collected by prepulse inhibition, the above-mentioned deficits were not related with the presence of amyloid beta deposits. Thus, learning and memory deficits observed in aged wild-type and transgenic mice are not directly related to the genetic manipulations or to the presence of amyloid plaques.

Activity-dependent changes of the hippocampal CA3-CA1 synapse during the acquisition of associative learning in conscious mice

Contemporary neuroscientists are paying increasing attention to subcellular, molecular and electrophysiological mechanisms underlying learning and memory processes. Recent efforts have addressed the development of transgenic mice affected at different stages of the learning process, or emulating pathological conditions involving cognition and motor-learning capabilities. However, a parallel effort is needed to develop stimulating and recording techniques suitable for use in behaving mice, in order to grasp activity-dependent neural changes taking place during the very moment of the process. These in vivo models should integrate the fragmentary information collected by different molecular and in vitro approaches. In this regard, long-term potentiation (LTP) has been proposed as the neural mechanism underlying synaptic plasticity. Moreover, N-methyl-d-aspartate (NMDA) receptors are accepted as the molecular substrate of LTP. It now seems necessary to study the relationship of both LTP and NMDA receptors with the plastic changes taking place, in selected neural structures, during actual learning. Here, we review data on the involvement of the hippocampal CA3-CA1 synapse in the acquisition of classically conditioned eyelid conditioned responses (CRs) in behaving mice. Available data show that LTP, evoked by high-frequency stimulation of Schaffer collaterals, disturbs both the acquisition of CRs and the physiological changes that occur at the CA3-CA1 synapse during learning. Moreover, the administration of NMDA-receptor antagonists is able not only to prevent LTP induction in vivo, but also to hinder the formation of both CRs and functional changes in strength of the CA3-CA1 synapse. Thus, there is experimental evidence relating activity-dependent synaptic changes taking place during actual learning with LTP mechanisms and with the role of NMDA receptors in both processes.

Classical eyeblink conditioning during acute hypobaric hypoxia is improved in acclimatized mice and involves Fos expression in selected brain areas

This work attempts to evaluate the cognitive aspects of the acclimatization ability of mice submitted to simulated altitude. Critical altitudes were detected by evaluating open field activity, combined or not with object recognition tasks, at different acute simulated altitudes. Results showed impaired cognitive abilities at approximately 3,733 m and above. To evaluate acclimatization capabilities, mice submitted to hypobaric hypoxia at approximately 5,000 m for 1 wk were tested for learning and memory performances with classical eyeblink conditioning at the same altitude or at land altitude. Results showed total acclimatization in mice conditioned at approximately 5,000 m but no improved performance in those conditioned at land altitudes compared with controls. Selected brain sites of conditioned animals were analyzed by immunohistochemistry to detect expression of the protein product of the protooncogene c-fos (Fos) in relation to both motor learning processes and hypobaric conditions. In the nucleus of the solitary tract, a higher expression of Fos was found in the acute hypobaric conditioned animals than in control conditioned and nonconditioned animals. Similar patterns between groups were found in the other brain areas, mainly in the piriform cortex and area 1 of the cingulate cortex and in the hippocampus. Differences between hemispheres were detected only in acute hypobaric animals. The present results show that acclimatization to high altitude prevents the impairment of classical eyeblink conditioning evoked by hypobaric hypoxic conditions but does not improve this task when acquired under land conditions, although it could diminish the activation requirements for its performance.

Kainate receptor-mediated presynaptic inhibition converges with presynaptic inhibition mediated by Group II mGluRs and long-term depression at the hippocampal mossy fiber-CA3 synapse

Kainate receptors (KARs) effect depression of glutamate release at hippocampal mossy fiber-CA3 (MF-CA3) synapses by a metabotropic action involving adenylyl cyclase (AC) inhibition, cAMP reduction, and diminished protein kinase A (PKA) activation. Using hippocampal slices, we show here that KAR activation interferes with the depression of glutamate release produced by Group II metabotropic glutamate receptor stimulation and low frequency stimulation (LFS)-induced long-term depression (LTD), also expressed through presynaptic AC/cAMP/PKA at MF-CA3 synapses. The mutual occlusion of depression mediated by presynaptic KARs, Group II mGluR and LFS-induced LTD suggests their mechanistic convergence at the MF-CA3 synapse and thus invokes KARs in synaptic plasticity manifest in LTD.

Synaptic structural modification following changes in activity induced by tetanus neurotoxin in cat abducens neurons

A low or a high dose of tetanus neurotoxin (TeNT) injected in the lateral rectus muscle of the cat causes respectively, functional block of inhibitory synapses only or of both inhibitory and excitatory synapses simultaneously in abducens neurons (González-Forero et al. [2003] J. Neurophysiol. 89:1878-1890). As a consequence, neuronal firing activity increases (at low dose) or decreases (at high dose). We investigated possible structural modifications of inhibitory synapses in response to these activity alterations induced by TeNT. We used immunofluorescence against postsynaptic (gephyrin) and presynaptic (vesicular gamma-aminobutyric acid [GABA] transporter [VGAT]) markers of inhibitory synapses in combination with cell type markers for abducens motoneurons (calcitonin gene-related peptide or choline acetyltransferase) or internuclear neurons (calretinin). Seven days after high-dose treatment, the number of gephyrin-immunoreactive (IR) clusters per 100 microm of membrane perimeter was reduced on the soma of abducens motoneurons by 55.3% and by 60.1% on internuclear neurons. Proximal dendritic gephyrin-IR clusters were also significantly altered but to a lesser degree. Partial synaptic re-establishment was observed 15 days post injection, and complete recovery occurred after 42 days. Coverage by VGAT-IR terminals was reduced in parallel. In contrast, a low dose of TeNT caused no structural alterations. With electron microscopy we estimated that overall synaptic coverage was reduced by 40% in both types of neurons after a high dose of TeNT. However, F-type terminals with postsynaptic gephyrin were preferentially lost. Thus, the ratio between F and S terminals diminished from 1.28 to 0.39 on motoneurons and from 1.26 to 0.47 on internuclear neurons. These results suggest that the maintenance of proximal inhibitory synaptic organization on central neurons is best related to neuronal activity and not to the level of inhibitory synaptic function, which was equally blocked by the high or low dose of TeNT.

Motoneuron adaptability to new motor tasks following two types of facial-facial anastomosis in cats

The ability of the facial motor system to adapt to a new motor function was studied in alert cats after unilateral transection, 180 degrees rotation and suture of the zygomatic nerve, or transection and cross-anastomosis of the proximal stump of the buccal nerve to the distal stump of the zygomatic nerve. These procedures induced reinnervation of the orbicularis oculi (OO) muscle by different OO- or mouth-related facial motoneurons. Eyelid movements and the electromyographic activity of the OO muscle were recorded up to 1 year following the two types of anastomosis. Animals with a zygomatic nerve rotation recovered spontaneous and reflex responses, but with evident deficits in eyelid kinematics, i.e. the proper regional distribution of OO motor units was disorganized by zygomatic nerve rotation and resuture, producing a permanent defect in eyelid motor performance. Following buccal-zygomatic anastomosis, the electrical activity of the OO muscle was recovered after 6-7 weeks, but air puff-, flash- and tone-evoked reflex blinks never reached the control values on the operated side. Electromyographic OO activities and lid movements corresponding to licking and deglutition activities were observed on the operated side in buccal-zygomatic anastomosed animals up to 1 year following surgery. Mouth-related facial motoneurons did not readapt their discharges to the kinetic, timing and oscillatory properties of OO muscle fibres. A significant hyper-reflexia was observed following both types of nerve repair in response to air puffs, but not to light flashes or tones. In conclusion, adult mammal facial premotor circuits maintain their motor programmes when motoneurons are induced to reinnervate a foreign muscle, or even a new set of muscle fibres.

The role of interpositus nucleus in eyelid conditioned responses

One of the most widely used experimental models for the study of learning processes in mammals has been the classical conditioning of nictitating membrane/eyelid responses, using both trace and delay paradigms. Mainly on the basis of permanent or transitory lesions of putatively-involved structures, and using other stimulation and recording techniques, it has been proposed that cerebellar cortex and/or nuclei could be the place/s where this elemental form of associative learning is acquired and stored. We have used here an output-to-input approach to review recent evidence regarding the involvement of the cerebellar interpositus nucleus in the acquisition of these conditioned responses (CRs). Eyelid CRs appear to be different in profile, duration, and peak velocity from reflexively-evoked blinks. In addition, CRs are generated in a quantum manner across conditioning sessions, suggesting a gradual neural process for their proper acquisition. Accessory abducens and orbicularis oculi motoneurons have different membrane properties and contribute differently to the generation of CRs, with significant species differences. In particular, facial motoneurons seem to encode eyelid velocity during reflexively-evoked blinks and eyelid position during CRs, two facts suggestive of a differential somatic versus dendritic arrival of specific motor commands for each type of movement. Identified interpositus neurons recorded in alert cats during classical conditioning of eyelid responses show firing properties suggestive of an enhancing role for CR performance. However, as their firing started after CR onset, and because they do not seem to encode eyelid position during the CR, the interpositus nucleus cannot be conclusively considered as the place where this acquired motor response is generated. More information is needed regarding neural signal transformations taking place in each involved neural center, and it its proposed that more attention should be paid to functional states (as opposed to neural sites) able to generate motor learning in mammals. The contribution of feedforward mechanisms normally involved in the processing activities of related centers and circuits, and the possible functional interactions within neural systems subserving the associative strength between the conditioned and unconditioned stimuli, are also considered.

Harmaline induces different motor effects on facial vs. skeletal-motor systems in alert cats

Harmaline's effects on reflex and classically conditioned eyelid responses and on tremor picked up by a coil attached to the back were measured in alert cats. Harmaline at a dose of 10 mg/kg produced skeletal muscle tremogenic effects that lasted 4h. Back movements presented a tremor-like displacement with a frequency peak at 10 Hz, but lid responses oscillated as in controls, at 20 Hz during both reflex and conditioned eyelid movements, with no increase in oscillation amplitude or frequency. The learning curves of harmaline-injected animals remained as in controls, but eyelid conditioned responses showed longer latencies, and smaller amplitude and peak velocity. Reflex and already-learned eyelid responses were not modified by harmaline. These results imply that neuronal control systems for skeletal-motor and facial responses are differentially affected by harmaline.

Hippocampal pyramidal cell activity encodes conditioned stimulus predictive value during classical conditioning in alert cats

We have recorded the firing activities of hippocampal pyramidal cells throughout the classical conditioning of eyelid responses in alert cats. Pyramidal cells (n = 220) were identified by their antidromic activation from the ipsilateral fornix and according to their spike properties. Upper eyelid movements were recorded with the search coil in a magnetic field technique. Latencies and firing profiles of recorded pyramidal cells following the paired presentation of conditioned (CS) and unconditioned (US) stimuli were similar, regardless of the different sensory modalities used as CS (tones, air puffs), the different conditioning paradigms (trace, delay), or the different latency and topography of the evoked eyelid conditioned responses. However, for the three paradigms used here, evoked neuronal firing to CS presentation increased across conditioning, but remained unchanged for US presentation. Contrarily, pyramidal cell firing was not modified when the same stimuli used here as CS and US were presented unpaired, during pseudoconditioning sessions. Pyramidal cell firing did not seem to encode eyelid position, velocity, or acceleration for either reflex or conditioned eyelid responses. Evoked pyramidal cell responses were always in coincidence with a beta oscillatory activity in hippocampal extracellular field potentials. In this regard, the beta rhythm represents a facilitation, or permissive time window, for timed pyramidal cell firing. It is concluded that pyramidal cells encode CS-US associative strength or CS predictive value.

[Structure and function of the cerebellum]

INTRODUCTION

The cerebellum is a neural structure, of a crystalline like organization, present in all vertebrates. Its progressive growth from fishes to mammals, and particularly in primates, takes place following the repetition of a primitive cellular plan and connectivity.

DEVELOPMENT

The cerebellum is organized in folia located one behind the other in the rostrocaudal axis, and placed transversally on the brain stem. The cerebellar cortex has five types of neuron: Purkinje, stellate, basket, Golgi and granule cells. Apart from granule cells, the other cell types are inhibitory in nature. Afferent fibers to the cerebellar cortex are of two types (mossy and climbing) and carry information from somatosensory, vestibular, acoustic and visual origins, as well as from the cerebral cortex and other brain stem and spinal motor centers. The only neural output from the cerebellar cortex is represented by Purkinje axons that synapse on the underlying deep nuclei. Cerebellar nuclei send their axons towards many brain stem centers and, by thalamic relay nuclei, act on different cortical areas. Functionally, the cerebellum seems to be organized in small modules, similar in structure, but different in the origin and end of their afferent and efferent fibers. The cerebellum is involved in the coordination or integration of motor and cognitive processes.

CONCLUSION

Although cerebellar lesion does not produce severe motor paralysis, loss of sensory inputs or definite deficits in cognitive functions, its certainly affects motor performance and specific perceptive and cognitive phenomena.

Hypoglossal and reticular interneurons involved in oro-facial coordination in the rat

Chewing, swallowing, breathing, and vocalization in mammals require precise coordination of tongue movements with concomitant activities of the mimetic muscles. The neuroanatomic basis for this oro-facial coordination is not yet fully understood. After the stereotaxic microinjection of retrograde and anterograde neuronal tracers (biotin-dextran, Fluoro-Ruby, Fluoro-Emerald, and Fluoro-Gold) into the facial and hypoglossal nuclei of the rat, we report here a direct bilateral projection of hypoglossal internuclear interneurons onto facial motoneurons. We also confirm the existence of a small pool of neurons in the dorsal part of the brainstem reticular formation that project ipsilaterally to both facial and hypoglossal nuclei. For precise tracer injections, both motor nuclei were located and identified by the electrical antidromic activation of their constituent motoneurons. Injections of retrograde tracers into the facial nucleus consistently labeled neurons in the hypoglossal nucleus. These neurons prevalently lay in the ipsilateral side, were small in size, and, like classic intrinsic hypoglossal local-circuit interneurons, had several thin dendrites. Reverse experiments - injections of anterograde tracers into the hypoglossal nucleus - labeled fine varicose nerve fiber terminals in the facial nucleus. These fiber terminals were concentrated in the intermediate subdivision of the facial nucleus, with a strong ipsilateral prevalence. Double injections of different tracers into the facial and the hypoglossal nuclei revealed a small, but constant, number of double-labeled neurons located predominantly ipsilateral in the caudal brainstem reticular formation. Hypoglossal internuclear interneurons projecting to the facial nucleus, as well as those neurons of the parvocellular reticular formation that project to both facial and hypoglossal nuclei, could be involved in oro-facial coordination.

Reversible deafferentation of abducens motoneurons and internuclear neurons with tetanus neurotoxin

Tetanus neurotoxin (TeNT) is a blocker of synaptic vesicle exocytosis in central synapses with preferential affinity for inhibitory neurotransmission. Following its intramuscular injection, TeNT is retrogradely and trans-synaptically transported towards the premotor terminals. Therefore, we have used TeNT as a tool to study the consequences of functional deafferentation on motoneurons following its peripheral administration. For this, we injected the toxin into the lateral rectus muscle at doses of 5 or 0.5 ng/kg and recorded the discharge activity of abducens motoneurons and internuclear neurons in the alert cat. Our results showed that: (i) TeNT blocked selectively the afferent inhibitory signals on abducens neurons only when used at a low dose, whereas both excitatory and inhibitory synaptic drive was lost after the high dose treatment; (ii) all effects were reversible within one month; and (iii) strikingly, the internuclear neurons of the abducens nucleus showed similar discharge alterations to the motoneurons, suggesting a TeNT action on shared common afferences.

Involvement of cerebral cortical structures in the classical conditioning of eyelid responses in rabbits

The classical conditioning of the eyelid motor system in alert behaving rabbits has been used to study the expression of Fos in the hippocampus, and in the occipital, parietal, piriform and temporal cortices. Animals were classically conditioned with both delay and trace conditioning paradigms. As conditioned stimulus, both short and long (20 and 100 ms) tones (600 Hz, 90 dB) or short, weak (20 ms, 1kg/cm(2)) air puffs were used. The unconditioned stimulus was always a long, strong (100 ms, 3 kg/cm(2)) air puff that started 250-270 ms after the onset of the conditioned stimulus. The expression of Fos was significantly increased after both delayed and trace conditioning in the hippocampus, and in the parietal and piriform cortices contralateral to the unconditioned stimulus presentation side, compared with equivalent ipsilateral structures in conditioned animals, or with Fos production in the same contralateral structures in pseudo-conditioned and control animals. Fos expression in some cortical sites was specific to tone versus air puff stimuli when used as conditioned stimulus. Thus, Fos expression was significantly increased in the contralateral temporal lobe when tones were used as conditioned stimulus, for both delayed and trace conditioning paradigms, but not when animals were conditioned to short, weak air puffs. The present results indicate a specific Fos activation in several cerebral cortical structures during associative eyelid conditioning.

Response of abducens internuclear neurons to axotomy in the adult cat

The highly specific projection of abducens internuclear neurons on the medial rectus motoneurons of the oculomotor nucleus constitutes an optimal model for investigating the effects of axotomy in the central nervous system. We have analyzed the morphological changes induced by this lesion on both the cell bodies and the transected axons of abducens internuclear neurons in the adult cat. Axotomy was performed by the transection of the medial longitudinal fascicle. Cell counts of Nissl-stained material and calretinin-immunostained abducens internuclear neurons revealed no cell death by 3 months postaxotomy. Ultrastructural examination of these cells at 6, 14, 24, and 90 days postaxotomy showed normal cytological features. However, the surface membrane of axotomized neurons appeared contacted by very few synaptic boutons compared to controls. This change was quantified by measuring the percentage of synaptic coverage of the cell bodies and the linear density of boutons. Both parameters decreased significantly after axotomy, with the lowest values at 90 days postlesion ( approximately 70% reduction). We also explored axonal regrowth and the possibility of reinnervation of a new target by means of anterograde labeling with biocytin. At all time intervals analyzed, labeled axons were observed to be interrupted at the caudal limit of the lesion; in no case did they cross the scar tissue to reach the distal part of the tract. Nonetheless, a conspicuous axonal sprouting was present at the caudal aspect of the lesion site. Structures suggestive of axonal growth were found, such as large terminal clubs, from which short filopodium-like branches frequently emerged. Similar findings were obtained after parvalbumin and calretinin immunostaining. At the electron microscopy level, biocytin-labeled boutons originating from the sprouts appeared surrounded by either extracellular space, which was extremely dilated at the lesion site, or by glial processes. The great majority of labeled boutons examined were, thus, devoid of neuronal contact, indicating absence of reinnervation of a new target. Altogether, these data indicate that abducens internuclear neurons survive axotomy in the adult cat and show some form of axonal regrowth, even in the absence of target connection.

Discharge characteristics of axotomized abducens internuclear neurons in the adult cat

The aim of the present work was to characterize the axotomy-induced changes in the discharge properties of central nervous system neurons recorded in the alert behaving animal. The abducens internuclear neurons of the adult cat were the chosen model. The axons of these neurons course through the contralateral medial longitudinal fascicle and contact the medial rectus motoneurons of the oculomotor nucleus. Axotomy was carried out by the unilateral transection of this fascicle (right side) and produced immediate oculomotor deficits, mainly the incapacity of the right eye to adduct across the midline. Extracellular single-unit recording of abducens neurons was carried out simultaneously with eye movements. The main alteration observed in the firing of these axotomized neurons was the overall decrease in firing rate. During eye fixations, the tonic signal was reduced, and, on occasion, a progressive decay in firing rate was observed. On-directed saccades were not accompanied by the high-frequency spike burst typical of controls; instead, there was a moderate increase in firing. Similarly, during the vestibular nystagmus, neurons hardly modulated during both the slow and the fast phases. Linear regression analysis between firing rate and eye movement parameters showed a significant reduction in eye position and velocity sensitivities with respect to controls, during both spontaneous and vestibularly induced eye movements. These firing alterations were observed during the 3 month period of study after lesion, with no sign of recovery. Conversely, abducens motoneurons showed no significant alteration in their firing pattern. Therefore, axotomy produced long-lasting changes in the discharge characteristics of abducens internuclear neurons that presumably reflected the loss of afferent oculomotor signals. These alterations might be due to the absence of trophic influences derived from the target.

Cerebellar posterior interpositus nucleus as an enhancer of classically conditioned eyelid responses in alert cats

Cerebellar posterior interpositus neurons were recorded in cats during delayed and trace conditioning of eyeblinks. Type A neurons increased their firing in the time interval between conditioned and unconditioned stimulus presentations for both paradigms, while type B neurons decreased it. The discharge of different type A neurons recorded across successive conditioning sessions increased, with slopes of 0.061-0.078 spikes/s/trial. Both types of neurons modified their firing several trials in advance of the appearance of eyelid conditioned responses, but for each conditioned stimulus presentation their response started after conditioned response onset. Interpositus microstimulation evoked eyelid responses similar in amplitude and profiles to conditioned responses, and microinjection of muscimol decreased conditioned response amplitude. It is proposed that the interpositus nucleus is an enhancer, but not the initiator, of eyelid conditioned responses.

Scopolamine impairs information processing in the hippocampus and performance of a learned eyeblink response in alert cats

The object of this work was to determine whether the changes in field activity and neuronal excitability recorded in the hippocampus during eyeblink classical conditioning are induced by the cholinergic input, and their relationships with the conditioned response performance. The pyramidal layer field activity, its response to fornix stimulation and eyelid responses were recorded during paired tone-air puff presentations, under scopolamine (25, 50 and 100 microg/kg) or saline administration, following well-established eyeblink conditioning in a trace paradigm in cats. Scopolamine impaired behavioral performance, and, in the hippocampus, disrupted conditioned stimulus-evoked field potential, high frequency shift in field activity, and paired presentation-induced hyperexcitability. These findings indicate that the cholinergic input participates in hippocampal information processing in a way that allows precise conditioned response performance and memory trace formation.

Why move the eyes if we can move the head?

To see while moving is a very basic and integrative sensorimotor function in vertebrates. To maintain visual acuity, the oculomotor system provides efficient compensatory eye movements for head and visual field displacements. Other types of eye movement allow the selection of new visual targets and binocular vision and stereopsis. Motor and premotor neuronal circuits involved in the genesis and control of eye movements are briefly described. The peculiar properties and robust biomechanics of the oculomotor system have allowed it to survive almost unchanged through vertebrate evolution.

Limits to the capacity of transplants of olfactory glia to promote axonal regrowth in the CNS

Olfactory bulb ensheathing cell (OBEC) transplants promoted axonal regeneration in the spinal cord dorsal root entry zone and in the corticospinal tract. However, OBECs failed to promote abducens internuclear neuron axon regeneration when transplanted at the site of nerve fibre transection. In experiments performed in both cats and rats, OBECs survived for up to 2 months, lining themselves up along the portion of the regrowing axons proximal to the interneuron cell body. However, OBECs migrated preferentially towards abducens somata, in the direction opposite to the oculomotor nucleus target. OBECs seem to promote nerve fibre regeneration only where preferred direction of glial migration coincides with the direction of axonal growth towards its target.

Kinetic and frequency-domain properties of reflex and conditioned eyelid responses in the rabbit

Eyelid position and the electromyographic activity of the orbicularis oculi muscle were recorded unilaterally in rabbits during reflex and conditioned blinks. Air-puff-evoked blinks consisted of a fast downward phase followed sometimes by successive downward sags. The reopening phase had a much longer duration and slower peak velocity. Onset latency, maximum amplitude, peak velocity, and rise time of reflex blinks depended on the intensity and duration of the air puff-evoking stimulus. A flashlight focused on the eye also evoked reflex blinks, but not flashes of light, or tones. Both delayed and trace classical conditioning paradigms were used. For delayed conditioning, animals were presented with a 350-ms, 90-dB, 600-Hz tone, as conditioned stimulus (CS). For trace conditioning, animals were presented with a 10-ms, 1-k/cm(2) air puff, as CS. The unconditioned stimulus (US) consisted of a 100-ms, 3-k/cm(2) air puff. The stimulus interval between CS and US onsets was 250 ms. Conditioned responses (CRs) to tones were composed of downward sags that increased in number through the successive conditioning sessions. The onset latency of the CR decreased across conditioning at the same time as its maximum amplitude and its peak velocity increased, but the time-to-peak of the CR remained unaltered. The topography of CRs evoked by short, weak air puffs as the CS showed three different components: the alpha response to the CS, the CR, and the reflex response to the US. Through conditioning, CRs showed a decrease in onset latency, and an increase in maximum amplitude and peak velocity. The time-to-peak of the CR remained unchanged. A power spectrum analysis of reflex and conditioned blink acceleration profiles showed a significant approximately 8-Hz oscillation within a broadband of frequencies between 4 and 15 Hz. Nose and mandible movements presented power spectrum profiles different from those characterizing reflex and conditioned blinks. It is concluded that eyelid reflex responses in the rabbit present significant differences from CRs in their profiles and metric properties, suggesting different neural origins, but that a common approximately 8-Hz neural oscillator underlies lid motor performance. According to available data, the frequency of this putative oscillator seems to be related to the species size.

Different discharge properties of rat facial nucleus motoneurons

In this paper, we describe two types of putative facial motoneuron based on their electrophysiological properties and on their firing frequency adaptation as recorded in rat brainstem slices. Type I motoneurons (n = 33, 61%) were characterized by a sustained spike firing during depolarizing current injections and a marked depolarizing sag (inward rectification) during hyperpolarizing pulses. The time-course and voltage-dependence of the inward rectification together with the finding that it was blocked by Cs+ are consistent with the involvement of a Na+ -and K+ -mediated Q current. Type II motoneurons (n = 21, 39%) were identified by a fast spike firing adaptation. Type II cells showed a less pronounced inward rectification with hyperpolarizing current pulses and a higher discharge rate than type I cells during depolarizing current pulses. These distinct discharge properties imply the activation of a Ca2+ -dependent K+ current, because when carbachol was added to the bath, or the slice was exposed to a Ca2+ -free solution, a decrease was noticed in the firing frequency adaptation. The two types of motoneuron were further differentiated by the initial delay of the first spike, observed only in type I cells, which was blocked by bath application of 4-aminopyridine, indicating the presence of a K+ -mediated A current. The addition of 4-aminopyridine to the bath also increased the firing rate due to a decrease of the post-spike afterhyperpolarization. However, the two types of motoneuron were not morphologically differentiated. Facial motoneurons exhibited rhythmic membrane potential oscillations (8-20 Hz) at depolarized membrane potentials or during the silence following spike frequency adaptation. It is suggested that the intrinsic properties of these two types of facial motoneuron may be relevant in the government of distinct facial muscle activities. The fact that their discharge rate and the level of spike frequency adaptation were modified by altering some K+ currents suggests a potential plasticity in the modulation of motoneuron firing activities depending upon functional motor needs.

Discharge profiles of abducens, accessory abducens, and orbicularis oculi motoneurons during reflex and conditioned blinks in alert cats

The discharge profiles of identified abducens, accessory abducens, and orbicularis oculi motoneurons have been recorded extra- and intracellularly in alert behaving cats during spontaneous, reflexively evoked, and classically conditioned eyelid responses. The movement of the upper lid and the electromyographic activity of the orbicularis oculi muscle also were recorded. Animals were conditioned by short, weak air puffs or 350-ms tones as conditioned stimuli (CS) and long, strong air puffs as unconditioned stimulus (US) using both trace and delayed conditioning paradigms. Motoneurons were identified by antidromic activation from their respective cranial nerves. Orbicularis oculi and accessory abducens motoneurons fired an early, double burst of action potentials (at 4-6 and 10-16 ms) in response to air puffs or to the electrical stimulation of the supraorbital nerve. Orbicularis oculi, but not accessory abducens, motoneurons fired in response to flash and tone presentations. Only 10-15% of recorded abducens motoneurons fired a late, weak burst after air puff, supraorbital nerve, and flash stimulations. Spontaneous fasciculations of the orbicularis oculi muscle and the activity of single orbicularis oculi motoneurons that generated them also were recorded. The activation of orbicularis oculi motoneurons during the acquisition of classically conditioned eyelid responses happened in a gradual, sequential manner. Initially, some putative excitatory synaptic potentials were observed in the time window corresponding to the CS-US interval; by the second to the fourth conditioning session, some isolated action potentials appeared that increased in number until some small movements were noticed in eyelid position traces. No accessory abducens motoneuron fired and no abducens motoneuron modified their discharge rate for conditioned eyelid responses. The firing of orbicularis oculi motoneurons was related linearly to lid velocity during reflex blinks but to lid position during conditioned responses, a fact indicating the different neural origin and coding of both types of motor commands. The power spectra of both reflex and conditioned lid responses showed a dominant peak at approximately 20 Hz. The wavy appearance of both reflex and conditioned eyelid responses was clearly the result of the high phasic activity of orbicularis oculi motor units. Orbicularis oculi motoneuron membrane potentials oscillated at approximately 20 Hz after supraorbital nerve stimulation and during other reflex and conditioned eyelid movements. The oscillation seemed to be the result of both intrinsic (spike afterhyperpolarization lasting approximately 50 ms, and late depolarizations) and extrinsic properties of the motoneuronal pool and of the circuits involved in eye blinks.

Effects of botulinum neurotoxin type A on the expression of gephyrin in cat abducens motoneurons

In this study, we investigated the effects of long-term synaptic blockade on postsynaptic receptor clustering at central inhibitory glycinergic synapses. High doses of botulinum neurotoxin type A injected in the lateral rectus muscle completely abolishes inhibitory postsynaptic potentials onto abducens motoneurons within 2 days postinjection, and transmission remains blocked for at least 2 months. Using this model, we analyzed the expression of gephyrin, a glycine receptor clustering protein, on the membrane of motoneuron somata after botulinum neurotoxin type A injection in their target muscle. Immunofluorescence or electron microscopy immunohistochemistry revealed gephyrin-immunoreactive clusters (most < 0.5 microm in diameter) densely covering the surface of control abducens motoneurons. Ultrastructurally, presynaptic terminals containing flattened synaptic vesicles (F terminals) were found associated with multiple gephyrin-immunoreactive postsynaptic densities (average 1.24 gephyrin clusters/F+ profile). No significant changes in gephyrin-immunoreactive clusters were observed at 5 days postinjection, but we found significant reductions (25-40%) in the density of gephyrin clusters 19 and 35 days postinjection. Hence, the physiological alterations reported in this model precede structural changes on postsynaptic receptor cluster density. The decrease in gephyrin-immunoreactive clusters was paralleled by reductions in synaptic covering (F+ terminals per 100 microm of membrane). Presumed inactive F+ terminals that remained attached to the motoneuron surface displayed normal gephyrin-immunoreactive clusters; however, the pre- and postsynaptic membranes in between synaptic active zones frequently appeared separated by enlarged extracellular spaces. We concluded that postsynaptic receptor cluster dissolution seemed more directly related to terminal retraction than to inactivity alone.

Output-to-input approach to neural plasticity in vestibular pathways

Some thoughts on current interpretations of available data regarding vestibular compensation at functional, network, and neural levels are presented. Basic concepts related to neural plasticity (or elasticity) underlying motor learning and regeneration also are discussed briefly. Modifiability in vestibular pathways, at both the functional and structural levels, after peripheral and central axotomy, and subsequent to transient or permanent chemical target removal, is presented as an experimental ground to explain similarities and differences between regenerative, compensatory, and adaptive mechanisms in the mammal central nervous system.

Localization of parvalbumin, calretinin, and calbindin D-28k in identified extraocular motoneurons and internuclear neurons of the cat

Calcium-binding proteins have been shown to be excellent markers of specific neuronal populations. We aimed to characterize the expression of calcium-binding proteins in identified populations of the cat extraocular motor nuclei by means of immunohistochemistry against parvalbumin, calretinin, and calbindin D-28k. Abducens, medial rectus, and trochlear motoneurons were retrogradely labeled with horseradish peroxidase from their corresponding muscles. Oculomotor and abducens internuclear neurons were retrogradely labeled after horseradish peroxidase injection into either the abducens or the oculomotor nucleus, respectively. Parvalbumin staining produced the highest density of immunoreactive terminals in all extraocular motor nuclei and was distributed uniformly. Around 15-20% of the motoneurons were moderately stained with antibody against parvalbumin, but their axons were heavily stained, indicating an intracellular segregation of parvalbumin. Colchicine administration increased the number of parvalbumin-immunoreactive motoneurons to approximately 85%. Except for a few calbindin-immunoreactive trochlear motoneurons (1%), parvalbumin was the only marker of extraocular motoneurons. Oculomotor internuclear neurons identified from the abducens nucleus constituted a nonuniform population, because low percentages of the three types of immunostaining were observed, calbindin being the most abundant (28.5%). Other interneurons located within the boundaries of the oculomotor nucleus were mainly calbindin-immunoreactive. The medial longitudinal fascicle contained numerous parvalbumin- and calretinin-immunoreactive but few calbindin-immunoreactive axons. The majority of abducens internuclear neurons projecting to the oculomotor nucleus (80.7%) contained calretinin. Moreover, the distribution of calretinin-immunoreactive terminals in the oculomotor nucleus overlapped that of the medial rectus motoneurons and matched the anterogradely labeled terminal field of the abducens internuclear neurons. Parvalbumin immunostained 42% of the abducens internuclear neurons. Colocalization of parvalbumin and calretinin was demonstrated in adjacent semithin sections, although single-labeled neurons were also observed. Therefore, calretinin is proven to be a good marker of abducens internuclear neurons. From all of these data, it is concluded that parvalbumin, calretinin, and calbindin D-28k selectively delineate certain neuronal populations in the oculomotor system and constitute valuable tools for further analysis of oculomotor function under normal and experimental conditions.

Localization of parvalbumin, calretinin, and calbindin D-28k in identified extraocular motoneurons and internuclear neurons of the cat

Calcium-binding proteins have been shown to be excellent markers of specific neuronal populations. We aimed to characterize the expression of calcium-binding proteins in identified populations of the cat extraocular motor nuclei by means of immunohistochemistry against parvalbumin, calretinin, and calbindin D-28k. Abducens, medial rectus, and trochlear motoneurons were retrogradely labeled with horseradish peroxidase from their corresponding muscles. Oculomotor and abducens internuclear neurons were retrogradely labeled after horseradish peroxidase injection into either the abducens or the oculomotor nucleus, respectively. Parvalbumin staining produced the highest density of immunoreactive terminals in all extraocular motor nuclei and was distributed uniformly. Around 15-20% of the motoneurons were moderately stained with antibody against parvalbumin, but their axons were heavily stained, indicating an intracellular segregation of parvalbumin. Colchicine administration increased the number of parvalbumin-immunoreactive motoneurons to approximately 85%. Except for a few calbindin-immunoreactive trochlear motoneurons (1%), parvalbumin was the only marker of extraocular motoneurons. Oculomotor internuclear neurons identified from the abducens nucleus constituted a nonuniform population, because low percentages of the three types of immunostaining were observed, calbindin being the most abundant (28.5%). Other interneurons located within the boundaries of the oculomotor nucleus were mainly calbindin-immunoreactive. The medial longitudinal fascicle contained numerous parvalbumin- and calretinin-immunoreactive but few calbindin-immunoreactive axons. The majority of abducens internuclear neurons projecting to the oculomotor nucleus (80.7%) contained calretinin. Moreover, the distribution of calretinin-immunoreactive terminals in the oculomotor nucleus overlapped that of the medial rectus motoneurons and matched the anterogradely labeled terminal field of the abducens internuclear neurons. Parvalbumin immunostained 42% of the abducens internuclear neurons. Colocalization of parvalbumin and calretinin was demonstrated in adjacent semithin sections, although single-labeled neurons were also observed. Therefore, calretinin is proven to be a good marker of abducens internuclear neurons. From all of these data, it is concluded that parvalbumin, calretinin, and calbindin D-28k selectively delineate certain neuronal populations in the oculomotor system and constitute valuable tools for further analysis of oculomotor function under normal and experimental conditions.

Effects of botulinum neurotoxin type A on abducens motoneurons in the cat: alterations of the discharge pattern

The discharge characteristics that abducens motoneurons exhibit after paralysis of the lateral rectus muscle with botulinum neurotoxin type A were studied in the alert cat. Antidromically identified motoneurons were recorded during both spontaneous and vestibularly induced eye movements. A single injection of 0.3 ng/kg produced a complete paralysis of the lateral rectus muscle lasting for about 12-15 days, whereas after 3 ng/kg the paralysis was still complete at the longest time checked, three months. Motoneurons recorded under the effect of the low dose showed differences in their sensitivities to both eye position and velocity according to the direction of the previous and ongoing movements, respectively. These directional differences could be explained by post-saccadic adaptation of the non-injected eye in the appropriate direction for reducing ocular misalignment. Thus, backward and forward post-saccadic drifts accompanied on- and off-directed saccades, respectively. The magnitude of the drift was similar to the magnitude of changes in eye position sensitivity. The discharge of the high-dose-treated motoneurons could be described in a three-stage sequence. During the initial 10-12 days, motoneuronal discharge resembled the effects of axotomy, particularly in the loss of tonic signals and the presence of exponential-like decay of firing after saccades. In this stage, the conduction velocity of abducens motoneurons was reduced by 21.4%. The second stage was characterized by an overall reduction in firing rate towards a tonic firing at 15-70 spikes/s. Motoneurons remained almost unmodulated for all types of eye movement and thus eye position and velocity sensitivities were significantly reduced. Tonic firing ceased only when the animal became drowsy, but was restored by alerting stimuli. In addition, the inhibition of firing for off-directed saccades was more affected than the burst excitation during on-directed saccades, since in many cells pauses were almost negligible. These alterations could not be explained by adaptational changes in the movement of the non-injected eye. Finally, after 60 days the initial stages of recovery were observed. The present results indicate that the high dose of botulinum neurotoxin produces effects on the motoneuron not attributable to the functional disconnection alone, but to a direct effect of the neurotoxin in the motoneuron and/or its synaptic inputs.

Quantal organization of reflex and conditioned eyelid responses

Quantal organization of reflex and conditioned eyelid responses. J. Neurophysiol. 78: 2518-2530, 1997. Upper lid movements and the electromyographic activity of the orbicularis oculi muscle were recorded in behaving cats during spontaneous and experimentally evoked reflex blinks, and conditioned eyelid responses. Reflex blinks evoked by the presentation of air puffs, flashes, or tones consisted of a fast downward lid movement followed by late, small downward waves, recurring at approximately 50-ms intervals. The latency, maximum amplitude, peak velocity, and number of late waves depended on the modality, intensity, and duration of the evoking stimulus. The power spectra of acceleration records indicated a dominant frequency of approximately 20 Hz for air puff-evoked blinks. Flashes and tones usually evoked small and easily fatigable reflex responses of lower dominant frequencies (14-17 and 9-11 Hz, respectively). A basic approximately 20-Hz oscillation was also noticed during lid fixation, and ramplike lid displacements evoked by optokinetic stimuli. Five classical conditioning paradigms were used to analyze the frequency-domain properties of conditioned eyelid responses. These learned lid movements differed in latency, maximum amplitude, and profile smoothness depending on the modality (air puff, tone), intensity (weak, strong), and presentation site (ipsi-, contralateral to the unconditioned stimulus) of the conditioned stimulus. It was found that the characteristic ramplike profile of a conditioned response was not smooth, but appeared to be formed by a succession of small waves at a dominant frequency of approximately 20 Hz. The amplitude (and number) of the constituting waves depended on the characteristics of the conditioned stimulus and on the time interval until unconditioned stimulus presentation. Thus conditioned responses seemed to be formed from lid displacements of 2-6 degrees in amplitude and approximately 50 ms in duration, which increased in number throughout conditioning sessions, until a complete (i.e., lid closing) conditioned response was reached. It is suggested that a approximately 20-Hz oscillator underlies the generation of reflex and conditioned eyelid responses. The oscillator is susceptible to being neurally modulated to modify the velocity of a given quantum of movement, and the total duration of the lid response. Learned eyelid movements are probably the result of a successively longer release of the oscillator as a function of the temporal-spatial needs of the motor response.

Effects of botulinum neurotoxin type A on abducens motoneurons in the cat: ultrastructural and synaptic alterations

The synaptic alterations induced in abducens motoneurons by the injection of 3 ng/kg of botulinum neurotoxin type A into the lateral rectus muscle were studied using ultrastructural and electrophysiological techniques. Motoneurons identified by the retrograde transport of horseradish peroxidase showed a progressive synaptic stripping already noticeable by four days post-injection which increased over the study period. By 35 days post-injection, the normal coverage of motoneurons by synaptic boutons (66.4 +/- 4.0%) significantly decreased to 27.2 +/- 4.0%. Synaptic boutons detached by a widening of the subsynaptic space but remained apposed by synaptic contacts and desmosomes to the motoneuron. Detachment did not affect equally flat and round vesicle-containing boutons. The control motoneuron had almost equal numbers of both types of boutons, but after 35 days post-injection the ratio of round to flat vesicle-containing boutons was 1.20 +/- 0.01. Synaptic boutons impinging on motoneurons showed signs of alterations in membrane turnover, as indicated by an increase in the number of synaptic vesicles and a decrease in the number of coated vesicles and synaptic vesicles near the active zone. Abducens motoneurons had a transient increase in soma size by 15 days that returned to normal at 35 days, but no signs of chromatolysis or organelle degeneration were seen. Accompanying the swelling of motoneurons, a 15-fold increase in the number of spines, very infrequent in controls, was observed. Spines located in the soma and proximal dendritic trunk received synaptic contacts from both flat and round vesicle-containing boutons that could be either partly detached or completely attached to the motoneuron. An increased turnover of the plasmatic membrane of the motoneuron was observed, as indicated by a four-fold increase in the number of somatic coated vesicles. Animals were implanted with bipolar electrodes in the ampulla of both horizontal semicircular canals for evoking contralateral excitatory and ipsilateral inhibitory postsynaptic potentials. Motoneurons were antidromically identified from the lateral rectus muscle. Synaptic potentials of vestibular origin were recorded in abducens motoneurons. In the period between two and six days post-injection, a complete abolition of inhibitory synaptic potentials was observed. By contrast, excitatory synaptic potentials remained, but were reduced by 82%. The imbalance between excitatory and inhibitory inputs to motoneurons induced a progressive increase of firing frequency within a few stimuli applied to the contralateral canal. Between 7 and 15 days post-injection, both excitatory and inhibitory postsynaptic potentials were virtually abolished and remained so up to the longest time checked (105 days). Some motoneurons recorded beyond 60 days post-injection showed signs of recovery of excitatory postsynaptic potentials. During the whole time-span studied, presynaptic wavelets were present, indicating no affecting of the conduction of afferent volleys to the abducens nucleus. Taken together, these data indicate that botulinum neurotoxin at high doses causes profound synaptic alterations in motoneurons responsible for the effects seen in the behavior of motoneurons recorded in alert animals.

Dose-dependent, central effects of botulinum neurotoxin type A: a pilot study in the alert behaving cat

We investigated, in alert behaving cats, the long-term effects of botulinum neurotoxin (BoNT) type A injected into the lateral rectus muscle of the eye. We studied orthodromic field potentials recorded in the injected muscle, eye movements, and the discharge characteristics of the innervating abducens motoneurons. Single BoNT injections at doses from 0.01 to 0.3 ng/kg reduced, or even completely eliminated, eye movements in the abducting direction for up to 2 months without affecting the motoneuron discharge profile that remained related to actual eye movements of the contralateral unparalyzed eye. This result indicates that abducens motoneurons were still under the influence of the ocular motor central control system regardless of their ineffective action on lateral rectus muscle fibers. We also conclude that paralysis per se is not enough to initiate axotomy-like neural responses in ocular motoneurons. The injection of BoNT at a dose of 3 ng/kg produced significant changes in the discharge pattern of abducens motoneurons lasting up to 3 months-the maximum time checked. This finding was probably due to retrograde and, perhaps, transneuronal effects of BoNT when injected in a high dose. The results give some indications of the maximum allowable dose that can be used without the induction of unwanted side effects in the motoneuronal pool innervating the injected muscle.

Involvement of cerebellar cortex and nuclei in the genesis and control of unconditioned and conditioned eyelid motor responses

The eyelid motor system of the cat was used here for the study of the kinetic properties of reflex and conditioned lid movements, and of the role played by the cerebellum in the acquisition and/or performance of both types of motor responses. Spontaneous blinks, eyelid reflex responses, eye-guided lid movements and conditioned lid responses were recorded in alert cats in simultaneity with unitary and field electrical activity of cerebellar cortex and nuclear zones related to the eyelid motor system. Results indicate that nuclear unitary activity does not precede unconditioned or conditioned lid responses, but that cerebellar nuclei are directly involved in the performance of the late components of reflex lid movements and in the acquisition of conditioned lid responses.

Influence of the postsynaptic target on the functional properties of neurons in the adult mammalian central nervous system

In this review we have attempted to summarize present knowledge concerning the regulatory role of target cells on the expression and maintenance of the neuronal phenotype during adulthood. It is well known that in early developmental stages the survival of neurons is maintained by specific neurotrophic factors derived from their target tissues. Neuronal survival is not the only phenotype that is regulated by target-derived neurotrophic factors since the expression of electrophysiological and cytochemical properties of neurons is also affected. However, a good deal of evidence indicates that the survival of neurons becomes less dependent on their targets in the adult stage. The question is to what extent are target cells still required for the maintenance of the pre-existing or programmed state of the neuron; i.e., what is the functional significance of target-derived factors during maturity? Studies addressing this question comprise a variety of neuronal systems and technical approaches and they indicate that trophic interactions, although less apparent, persist in maturity and are most easily revealed by experimental manipulation. In this respect, research has been directed to analyzing the consequences of disconnecting a group of neurons from their target-by either axotomy or selective target removal using different neurotoxins-and followed (or not) by the implant of a novel target, usually a piece of embryonic tissue. Numerous alterations have been described as taking place in neurons following axotomy, affecting their morphology, physiology and metabolism. All these neuronal properties return to normal values when regeneration is successful and reinnervation of the target is achieved. Nevertheless, most of the changes persist if reinnervation is prevented by any procedure. Although axotomy may represent, besides target disconnection, a cellular lesion, alternative approaches (e.g., blockade of either the axoplasmic transport or the conduction of action potentials) have been used yielding similar results. Moreover, in the adult mammalian central nervous system, neurotoxins have been used to eliminate a particular target selectively and to study the consequences on the intact but target-deprived presynaptic neurons. Target depletion performed by excitotoxic lesions is not followed by retrograde cell death, but targetless neurons exhibit several modifications such as reduction in soma size and in the staining intensity for neurotransmitter-synthesizing enzymes. Recently, the oculomotor system has been used as an experimental model for evaluating the functional effects of target removal on the premotor abducens internuclear neurons whose motoneuronal target is destroyed following the injection of toxic ricin into the extraocular medial rectus muscle. The functional characteristics of these abducens neurons recorded under alert conditions simultaneously with eye movements show noticeable changes after target loss, such as a general reduction in firing frequency and a loss of the discharge signals related to eye position and velocity. Nevertheless, the firing pattern of these targetless abducens internuclear neurons recovers in parallel with the establishment of synaptic contacts on a presumptive new target: the small oculomotor internuclear neurons located in proximity to the disappeared target motoneurons. The possibility that a new target may restore neuronal properties towards a normal state has been observed in other systems after axotomy and is also evident from experiments of transplantation of immature neurons into the lesioned central nervous system of adult mammals. It can be concluded that although target-derived factors may not control neuronal survival in the adult nervous system, they are required for the maintenance of the functional state of neurons, regulating numerous aspects of neuronal structure, chemistry and electro-physiology.(ABSTRUCT TRUNCATED)

Kinematics of spontaneous, reflex, and conditioned eyelid movements in the alert cat

1. Upper eyelid position and velocity, and the electromyographic (EMG) activity of the orbicularis oculi muscle, were recorded bilaterally in alert cats during spontaneous, reflexively evoked, and conditioned eyelid movements. 2. Spontaneous blinks appeared randomly (0.2-0.5 per min) and consisted of a fast, large downward lid movement followed by a slower up phase. Blinks of smaller amplitude and slower velocity were also observed mainly accompanying behavioral movements, such as during peering and grimacing. 3. Eyelid response to air puffs applied to the cornea and tarsal lid skin consisted of a short-latency (9-16 ms), fast (up to 2,000 degrees/s) downward movement that lasted for 25-30 ms, followed by late, small downward sags that were sometimes still evident after stimulus offset. Blinks outlasted the duration of the stimulus by approximately 150 ms. Blinks elicited by flashes of light or tones showed longer latency (47.3 +/- 6.3 and 53.7 +/- 8.0 ms, mean +/- SD; respectively), smaller amplitude, and a quicker habituation than air-puff-evoked lid responses. 4. For the down phase of the blink, the peak velocity, but not its duration, increased linearly with blink amplitude. Because the rise time of the down phase remained constant, changes in blink amplitude seemed to be the result of increased blink velocity. The down phase of a typical 10 degrees blink was 10 times faster than the up phase of the same blink or than upward and downward lid saccades of the same amplitude. The peak velocity and duration of the up phases of reflex blinks and upward and downward lid saccades increased linearly with lid movement amplitude. 5. The initial down phase of air-puff-evoked blinks decreased in latency, increased in amplitude and peak velocity, and maintained the same rise time for increasing puff pressure. None of these parameters was dependent on puff duration. The duration of the blink also increased linearly with air puff duration. 6. The amplitude of air-puff-evoked blinks was inversely related to lid position, decreasing with further lid positions in the closing direction. In contrast, neither peak nor integrated EMG activity of the orbicularis oculi muscle was affected by lid position, being only a function of stimulus parameters and of the animal's level of alertness. 7. Air puffs > 20 ms and > 1 kg/cm2 evoked two successive bursts (R(ap) 1 and R(ap) 2) in the EMG activity of the orbicularis oculi muscle. Shorter and/or weaker stimuli evoked only the R(ap) 1 response.(ABSTRACT TRUNCATED AT 400 WORDS)

Botulinum neurotoxin alters the discharge characteristics of abducens motoneurons in the alert cat

1. The effects of botulinum neurotoxin (BoTx) injected into the lateral rectus muscle were examined in alert cats by recording the extracellular activity of abducens motoneurons during spontaneous eye movements. 2. A single high dose (3 ng/kg) of BoTx produced a complete paralysis of abduction that lasted for more than 2 mo. In addition, changes were found in the discharge pattern of abducens motoneurons. Motoneurons discharged steadily at a low firing rate (15-50 spikes/s), which in some instances showed a complete independence of eye position. Their increases in activity during ON-directed saccades were markedly reduced with respect to controls. The loss of inhibitory signals for OFF-directed saccades was even more evident. 3. A low dose (0.3 ng/kg) of BoTx also produced a paralysis of the lateral rectus muscle that lasted for approximately 1 mo. In this case, only minor modifications in the firing characteristics of abducens motoneurons were observed. 4. The present findings indicate that the effects of BoTx observed in the discharge pattern of abducens motoneurons might be due not only to target disconnection, but also to a central action of the neurotoxin on the motoneuron.

Discharge of identified deep cerebellar nuclei neurons related to eye blinks in the alert cat

The activity of identified cerebellar nuclear neurons was recorded in the alert cat during blinks induced by corneal air puffs, light flashes and tones. Eyelid response to air puffs consisted of an early (16.5 +/- 2.7 ms) downward movement followed by two to three late downward steps. Blinks induced by flashes or tones presented longer latencies (52.6 +/- 4.8 and 50.1 +/- 8.0 ms). Type A neurons (n = 86) increased their spike activity in coincidence with the beginning of the blink, regardless of the stimulus modality. The late eyelid downward responses were accompanied by corresponding increases in the firing rate of the neuron. Type A neurons were activated mostly from the red nucleus (48/86) or the restiform body (24/86). Type B neurons (n = 30) fired a brief burst of spikes slightly preceding the blink, followed by a noticeable decrease in their firing rate. As for type A, the discharge response of type B neurons was always the same regardless of the sensory modality. These neurons were activated from the red nucleus (18/30), oculomotor complex (6/30) and restiform body (6/30). Although no precise temporal coupling was found between the beginning of the neuronal response and the start of either the stimulus or the motor response, linear regression analysis demonstrated significant relationships between mean firing rate of type A and B neurons and eyelid position, velocity and/or acceleration. Deep cerebellar nuclei neurons presented here seem to be directly involved in the execution of reflexively induced blinks following the smaller details of eyelid motor performance. The opposite behavior of type A and B cells suggests an interplay of reciprocal actions to determine the ongoing displacements of the lid. Finally, the cerebellum seems to influence blinks through a spread action on many brainstem sites and not exclusively on the red nucleus.

Kinematic analyses of classically-conditioned eyelid movements in the cat suggest a brain stem site for motor learning

Upper eyelid movements were conditioned in the alert cat to the presentation of either tones or short, weak air puffs applied to the ipsi- or contralateral cornea followed by an unconditioned stimulus consisting of a long, strong air puff applied to the ipsilateral cornea. Eyelid movements were measured with the search-coil technique. Electromyographs of the orbicularis oculi muscle were also recorded. Quantitative analysis of the latencies and topographic profiles of eyelid conditioned responses suggests that the primary site for their initiation is the brain stem reflex circuit involved, depending on the sensory modality of and on the side where the conditioning stimulus was applied. However, the kinematic of the conditioned response indicates that other neural structures are involved in its acquisition and consolidation.

Signalling properties of identified deep cerebellar nuclear neurons related to eye and head movements in the alert cat

1. The spike activity of deep cerebellar nuclear neurons was recorded in the alert cat during spontaneous and during vestibularly and visually induced eye movements. 2. Neurons were classified according to their location in the nuclei, their antidromic activation from projection sites, their sensitivity to eye position and velocity during spontaneous eye movements, and their responses to vestibular and optokinetic stimuli. 3. Type I EPV (eye position and velocity) neurons were located mainly in the posterior part of the fastigial nucleus and activated antidromically almost exclusively from the medial longitudinal fasciculus close to the oculomotor complex. These neurons, reported here for the first time, increased their firing rate during saccades and eye fixations towards the contralateral hemifield. Their position sensitivity to eye fixations in the horizontal plane was 5.3 +/- 2.6 spikes s-1 deg-1 (mean +/- S.D.). Eye velocity sensitivity during horizontal saccades was 0.71 +/- 0.52 spikes s-1 deg-1 s-1. Variability of their firing rate during a given eye fixation was higher than that shown by abducens motoneurons. 4. Type I EPV neurons increased their firing rate during ipsilateral head rotations at 0.5 Hz with a mean phase lead over eye position of 95.3 +/- 9.5 deg. They were also activated by contralateral optokinetic stimulation at 30 deg s-1. Their sensitivity to eye position and velocity in the horizontal plane during vestibular and optokinetic stimuli yielded values similar to those obtained for spontaneous eye movements. 5. Type II neurons were located in both fastigial and dentate nuclei and were activated antidromically from the restiform body, the medial longitudinal fasciculus close to the oculomotor complex, the red nucleus and the pontine nuclei. Type II neurons were not related to spontaneous eye movements. These neurons increased their firing rate in response to contralateral head rotation and during ipsilateral optokinetic stimulation, and decreased it with the oppositely directed movements. 6. Saccade-related neurons were located mostly in the fastigial and dentate nuclei. Fastigial neurons were activated antidromically from the medial longitudinal fasciculus, while dentate neurons were activated from the red nucleus. These neurons fired a burst of spikes whose duration was significantly related to saccade duration. Dentate neurons responded during the omni-directional saccades, while some fastigial neurons fired more actively during contralateral saccades. 7. These three types of neuron represent the output channel for oculomotor signals of the posterior vermis and paravermis. It is proposed that type I EPV neurons correspond to a group of premotor neurons directly involved in oculomotor control.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of target depletion on adult mammalian central neurons: functional correlates

The physiological signals and patterns of synaptic connectivity that CNS neurons display after the loss of their target cells were evaluated in adult cats for one year. Abducens internuclear neurons were chosen as the experimental model because of their highly specific projection onto the medial rectus motoneurons of the oculomotor nucleus. Selective death of medial rectus motoneurons was induced by the injection into the medial rectus muscle of ricin, a potent cytotoxic lectin that leaves the presynaptic axons intact. The electrical activity of antidromically identified abducens internuclear neurons was recorded in chronic alert animals, during both spontaneous and vestibularly induced eye movements, before and after target removal. During the three weeks that followed ricin injection, abducens internuclear neurons exhibited several firing-related abnormal properties. There was an overall reduction in firing rate with a corresponding increase in the eye position threshold for recruitment. In addition, neuronal sensitivities to eye position and velocity were significantly decreased with respect to control data. Bursting activity was also altered since low-frequency delayed burst accompanied the saccades in the on-direction and, occasionally, internuclear neurons exhibited low-frequency discharges associated with off-directed saccades. Intracellular recordings carried out seven and 15 days after ricin injection demonstrated no significant changes in their electrical properties, although a marked depression of synaptic transmission was evident. The amplitude of both excitatory and inhibitory postsynaptic potentials of vestibular origin was reduced by 60-85% with respect to controls. However, postsynaptic potentials recorded one month after ricin injection showed normal amplitude values which persisted unaltered one year after target loss. Recovery of synaptic transmission occurred at the same time as the re-establishment of normal eye-related signals in the discharge pattern of abducens internuclear neurons recorded in alert cats from days 25-30 post lesion. The functional restoration of firing properties was maintained in the long term (one year). Conversely, abducens motoneurons showed normal firing and synaptic patterns at all time intervals analysed. These results demonstrate that, after an initial period of altered physiological properties, abducens internuclear neurons survive the loss of their target motoneurons and regain a normal discharge pattern and afferent synaptic connections.

Very short-term potentiation of climbing fiber effects on deep cerebellar nuclei neurons by conditioning stimulation of mossy fiber afferents

Field potentials induced in deep cerebellar nuclei by the electrical stimulation of contralateral red, pontine and reticularis tegmenti pontis oralis nuclei, the restiform body and the inferior olive were recorded in the alert cat. Recording sites in interpositus and fastigial nuclei were selected with the aid of antidromic field potentials induced by red nucleus and restiform body stimulation, respectively. Pontine and reticularis tegmenti pontis oralis nuclei stimulation induced small, but constant amplitude, field potentials consisting of one or two negative waves. Control field potentials induced by inferior olive stimulation consisted of a negative wave at 2-3 ms followed by a late (4-6 ms) positivity. Conditioning stimuli applied to the pontine and reticularis tegmenti pontis oralis nuclei greatly enhanced the amplitude of the inferior olive-evoked synaptic field potential for a time window of about 40 ms. In contrast, inferior olive conditioning stimulation failed to modify the field potentials induced by pontine nuclei stimulation. These facilitatory effects on field potential amplitude showed no long-lasting modifications. The transient modification of inferior olive-induced field potentials could be related to attentional movements made by the animal to natural or electrically-induced sensory cues.

Effects of target depletion on adult mammalian central neurons: morphological correlates

The morphological sequelae induced by target removal were studied on adult cat abducens internuclear neurons at both the somata and terminal axon arborization levels. The neuronal target--the medial rectus motoneurons of the oculomotor nucleus--was selectively destroyed by the injection of toxic ricin into the medial rectus muscle. Retrograde labeling with horseradish peroxidase demonstrated the survival of the entire population of abducens internuclear neurons up to one year after target removal. However, soma size was reduced by about 20% three months postlesion and maintained for one year. At the ultrastructural level, a considerable deafferentation of abducens internuclear neurons was observed at short intervals (i.e. 10 days after lesion). Large regions of the plasmalemma appeared devoid of presynaptic boutons but were covered instead by glial processes. The detachment of synaptic endings was selective on abducens internuclear neurons since nearby motoneurons always showed a normal synaptic coverage. By one month, abducens internuclear neurons recovered a normal density of receiving axosomatic synapses. Anterogradely biocytin-labeled axon terminals of abducens internuclear neurons remained in place after the lesion of medial rectus motoneurons, although with a progressive decrease in density. Ultrastructural examination of the oculomotor nucleus 10 days after the lesion revealed numerous empty spaces left by the dead motoneurons. Targetless boutons were observed surrounded by large extracellular gaps, still apposed to remnants of the postsynaptic membrane or, finally, ensheathed by glial processes. At longer intervals (> one month), the ultrastructure of the oculomotor nucleus was re-established and labeled boutons were observed contacting either unidentified dendrites within the neuropil or the soma and proximal dendrites of the oculomotor internuclear neurons, that project to the abducens nucleus. Labeled boutons were never found contacting with the oculomotor internuclear neurons either in control tissue or at short periods after ricin injection. These results indicate that the availability of undamaged neurons close to the lost target motoneurons might support the long-term survival of abducens internuclear neurons. Specifically, the oculomotor internuclear neurons, which likely suffer a partial deafferentation after medial rectus motoneuron loss, constitute a potential new target for the abducens internuclear neurons. The reinnervation of a new target might explain the recovery of synaptic and firing properties of abducens internuclear neurons after medial rectus motoneuron lesion, which occurred with a similar time course, as described in the accompanying paper [de la Cruz R. R. et al. (1994) Neuroscience 58, 81-97.].

Response diversity of pontine and deep cerebellar nuclear neurons to air puff stimulation of the eye in the alert cat

The discharge of antidromically identified brainstem and cerebellar nuclear neurons involved in the corneal reflex was recorded in the alert cat during corneal air puffs. Eye movements were measured with the search coil technique. Recorded sensory, motor, reticular formation and cerebellar nuclear neurons showed a wide diversity in latencies and patterns of response to air puff stimulation. This diversity suggests that each part of the circuit may contribute different properties to information processing for the corneal reflex, for sustained eyelid closure and, possibly, for the classical conditioning of the nictitating membrane response.

Long-term effects of selective target removal on brainstem premotor neurons in the adult cat

The electrical activity of antidromically identified abducens internuclear neurons selectively deprived of their target motoneurons was recorded in chronic alert cats. Target motoneurons were killed by the injection of the cytotoxic lectin of Ricinus communis into the medial rectus muscle. Following target removal, the discharge pattern of abducens internuclear neurons showed an overall decrease in firing rate, a significant reduction in their sensitivity to eye position and velocity, and the presence of anomalous responses such as bursts of spikes associated with off-directed saccades. The decreased excitability of abducens internuclear neurons correlated well with a marked reduction in the synaptic efficacy of their inputs. Thus, both excitatory and inhibitory synaptic potentials of vestibular origin showed a noticeable decrease in amplitude. The alterations in firing properties and synaptic transmission were only observed during an initial period of 3 weeks following ricin injection. Within 1 month the electrophysiological parameters returned to control values and remained unaltered for 1 year. Retrograde labelling of abducens internuclear neurons revealed that no cell death occurred after target loss. The anterograde axonal labelling of these neurons showed a progressive decrease in the density of their axonal terminals, and no sign of redistribution to other areas was found. These findings indicate that abducens internuclear neurons are not dependent on the presence of their natural target cells, either for the survival or for the maintenance of appropriate physiological signals.

A morphological study of the principal and accessory abducens nuclei in the caspian terrapin (Mauremys caspica)

The location of principal and accessory motoneurons and principal interneurons of the nucleus abducens was determined in the caspian terrapin (Mauremys caspica) by means of horseradish peroxidase histochemical tracing. Enzyme injections were made into the ipsilateral lateral rectus and retractor bulbi muscles and into the contralateral oculomotor nucleus. Labeled principal abducens motoneurons formed a cluster of cells in the rhombencephalon, under the IVth ventricle and adjacent to the medial longitudinal fascicle. The accessory abducens motoneurons were located more deeply in the rhombencephalon and more ventrolaterally than the principal motoneurons forming a compact aggregation of neurons. The principal interneurons of abducens nucleus were arranged as a cluster of cells under the floor of the IVth ventricle and more lateral than the principal motoneurons, with no intermingling.

A physiological study of vestibular and prepositus hypoglossi neurones projecting to the abducens nucleus in the alert cat

1. Vestibular and prepositus hypoglossi (PH) neurones projecting to the abducens (ABD) nucleus were recorded in the alert cat. Their discharge characteristics were analysed to ascertain the origin of the horizontal eye position signal present in ABD neurones. 2. Neurones were classified according to: their location with respect to the ABD nucleus; their antidromic activation from the ABD nucleus; the synaptic field potential they induced in the ABD nucleus with the spike-triggered averaging technique; and their activity during spontaneous and vestibularly induced eye movements. 3. Vestibular neurones projecting to the ABD nucleus were located in the rostral medial vestibular nucleus. They were excitatory on the contralateral and inhibitory on the ipsilateral ABD neurones. Both types of premotor vestibular neurone showed a firing rate weakly related to eye position, increasing for eye fixations in the contralateral on-direction, and decreasing with ipsilateral fixation. Position sensitivity during eye fixations was (means +/- S.D.) 1.8 +/- 0.9 spikes s-1 deg-1 for excitatory neurones and 2.2 +/- 1.3 spikes s-1 deg-1 for inhibitory neurones. Firing rate exhibited a high variability during eye fixations. Their responses during saccades in the off-direction were characterized by a pause that, although less defined, was occasionally present during saccades in the on-direction. Eye velocity sensitivity during spontaneous saccades in the on-direction was 0.17 +/- 0.15 spikes s-1 deg-1 s-1 for excitatory neurones and 0.15 +/- 0.07 spikes s-1 deg-1 s-1 for inhibitory vestibular neurones. During sinusoidal head stimulation at 0.2 Hz, vestibular neurones showed a type I discharge rate with a phase lead over eye position of 86.0 +/- 14.1 deg for excitatory and 80.2 +/- 12.5 deg for inhibitory neurones. Position sensitivity during vestibular stimulation did not differ significantly from values obtained for spontaneous eye movements. However, the velocity sensitivity of premotor vestibular neurones during head rotation was significantly higher (1.6 +/- 0.2 spikes s-1 deg-1 s-1 for excitatory and 1.5 +/- 0.3 spikes s-1 deg-1 s-1 for inhibitory neurones) than during spontaneous eye movements. 4. PH neurones projecting to the ABD nucleus were located in the rostral one-third of the nucleus. These neurones were excitatory on the ipsilateral and inhibitory on the contralateral ABD nucleus. Their firing rates were correlated mainly with eye position, increasing for abducting eye positions of the ipsilateral eye and decreasing with adduction movements.(ABSTRACT TRUNCATED AT 400 WORDS)