The nictitating membrane response: an electrophysiological study of the abducens nerve and nucleus and the accessory abducens nucleus in rabbit

Synaptic Mechanisms of Delay Eyeblink Classical Conditioning: AMPAR Trafficking and Gene Regulation in an In Vitro Model

An in vitro model of delay eyeblink classical conditioning was developed to investigate synaptic plasticity mechanisms underlying acquisition of associative learning. This was achieved by replacing real stimuli, such as an airpuff and tone, with patterned stimulation of the cranial nerves using an isolated brainstem preparation from turtle. Here, our primary findings regarding cellular and molecular mechanisms for learning acquisition using this unique approach are reviewed. The neural correlate of the in vitro eyeblink response is a replica of the actual behavior, and features of conditioned responses (CRs) resemble those observed in behavioral studies. Importantly, it was shown that acquisition of CRs did not require the intact cerebellum, but the appropriate timing did. Studies of synaptic mechanisms indicate that conditioning involves two stages of AMPA receptor (AMPAR) trafficking. Initially, GluA1-containing AMPARs are targeted to synapses followed later by replacement by GluA4 subunits that support CR expression. This two-stage process is regulated by specific signal transduction cascades involving PKA and PKC and is guided by distinct protein chaperones. The expression of the brain-derived neurotrophic factor (BDNF) protein is central to AMPAR trafficking and conditioning. BDNF gene expression is regulated by coordinated epigenetic mechanisms involving DNA methylation/demethylation and chromatin modifications that control access of promoters to transcription factors. Finally, a hypothesis is proposed that learning genes like BDNF are poised by dual chromatin features that allow rapid activation or repression in response to environmental stimuli. These in vitro studies have advanced our understanding of the cellular and molecular mechanisms that underlie associative learning.

Presumed canine trigemino-abducens synkinesis in a dog

A ten-year-old male neutered Rhodesian ridgeback cross dog was presented for the investigation of abnormal bilateral protrusion of the third eyelid when chewing. Physical, ophthalmological, and neurological examinations were unremarkable. Thoracic radiographs, abdominal ultrasound, and magnetic resonance of the brain and orbits failed to reveal any abnormalities. Cerebrospinal fluid analysis revealed elevated protein, but the nucleated cell count was normal. trigemino-abducens synkinesis was presumptively diagnosed. Aetiopathogenesis of this condition is discussed. To the authors' knowledge, this is the first report of presumed trigemino-abducens synkinesis in a dog.

Physiological and anatomical evidence for an inhibitory trigemino-oculomotor pathway in the cat

During blink down-phase, the levator palpebrae superioris (levator) muscle is inactivated, allowing the orbicularis oculi muscle to act. For trigeminal reflex blinks, the excitatory connections from trigeminal sensory nuclei to the facial nucleus have been described, but the pathway whereby the levator is turned off have not. We examined this question by use of both physiological and anatomical approaches in the cat. Intracellular records from antidromically activated levator motoneurons revealed that periorbital electrical stimulation produced bilateral, long latency inhibitory postsynaptic potentials (IPSPs). Central electrical stimulation of the principal trigeminal nucleus produced shorter latency IPSPs. Intracellular staining revealed that these motoneurons reside in the caudal central subdivision and have 10 or more poorly branched dendrites, which extend bilaterally into the surrounding supraoculomotor area. Axons penetrated in this region could be activated from periorbital and central electrodes. Neurons labeled from tracer injections into the caudal oculomotor complex were distributed in a crescent-shaped band that lined the ventral and rostral aspects of the pontine trigeminal sensory nucleus. Double-label immunohistochemical procedures demonstrated that these cells were not tyrosine hydroxylase-positive cells in the Kölliker-Fuse area. Instead, supraorbital nerve afferents displayed a similar crescent-shaped distribution, suggesting they drive these trigemino-oculomotor neurons. Anterograde labeling of the trigemino-oculomotor projection indicates that it terminates bilaterally, in and above the caudal central subdivision. These results characterize a trigemino-oculomotor pathway that inhibits levator palpebrae motoneurons in response to blink-producing periorbital stimuli. The bilateral distributions of trigemino-oculomotor afferents, levator motoneurons, and their dendrites supply a morphological basis for conjugate lid movements.

Evidence from retractor bulbi EMG for linearized motor control of conditioned nictitating membrane responses

Classical conditioning of nictitating membrane (NM) responses in rabbits is a robust model learning system, and experimental evidence indicates that conditioned responses (CRs) are controlled by the cerebellum. It is unknown whether cerebellar control signals deal directly with the complex nonlinearities of the plant (blink-related muscles and peripheral tissues) or whether the plant is linearized to ensure a simple relation between cerebellar neuronal firing and CR profile. To study this question, the retractor bulbi muscle EMG was recorded with implanted electrodes during NM conditioning. Pooled activity in accessory abducens motoneurons was estimated from spike trains extracted from the EMG traces, and its temporal profile was found to have an approximately Gaussian shape with peak amplitude linearly related to CR amplitude. The relation between motoneuron activity and CR profiles was accurately fitted by a first-order linear filter, with each spike input producing an exponentially decaying impulse response with time constant of order 0.1 s. Application of this first-order plant model to CR data from other laboratories suggested that, in these cases also, motoneuron activity had a Gaussian profile, with time-of-peak close to unconditioned stimulus (US) onset and SD proportional to the interval between conditioned stimulus and US onsets. These results suggest that for conditioned NM responses the cerebellum is presented with a simplified "virtual" plant that is a linearized version of the underlying nonlinear biological system. Analysis of a detailed plant model suggests that one method for linearising the plant would be appropriate recruitment of motor units.

Response linearity determined by recruitment strategy in detailed model of nictitating membrane control

Many models of eyeblink conditioning assume that there is a simple linear relationship between the firing patterns of neurons in the interpositus nucleus and the time course of the conditioned response (CR). However, the complexities of muscle behaviour and plant dynamics call this assumption into question. We investigated the issue by implementing the most detailed model available of the rabbit nictitating membrane response (Bartha and Thompson in Biol Cybern 68:135-143, 1992a and in Biol Cybern 68:145-154, 1992b), in which each motor unit of the retractor bulbi muscle is represented by a Hill-type model, driven by a non-linear activation mechanism designed to reproduce the isometric force measurements of Lennerstrand (J Physiol 236:43-55, 1974). Globe retraction and NM extension are modelled as linked second order systems. We derived versions of the model that used a consistent set of SI units, were based on a physically realisable version of calcium kinetics, and used simulated muscle cross-bridges to produce force. All versions showed similar non-linear responses to two basic control strategies. (1) Rate-coding with no recruitment gave a sigmoidal relation between control signal and amplitude of CR, reflecting the measured relation between isometric muscle force and stimulation frequency. (2) Recruitment of similar strength motor units with no rate coding gave a sublinear relation between control signal and amplitude of CR, reflecting the increase in muscle stiffness produced by recruitment. However, the system response could be linearised by either a suitable combination of rate-coding and recruitment, or by simple recruitment of motor units in order of (exponentially) increasing strength. These plausible control strategies, either alone or in combination, would in effect present the cerebellum with the simplified virtual plant that is assumed in many models of eyeblink conditioning. Future work is therefore needed to determine the extent to which motor neuron firing is in fact linearly related to the nictitating membrane response.

Pathways controlling trigeminal and auditory nerve-evoked abducens eyeblink reflexes in pond turtles

An in vitro brain stem preparation from turtles exhibits a neural correlate of eyeblink classical conditioning during pairing of auditory (CS) and trigeminal (US) nerve stimulation while recording from the abducens nerve. The premotor neuronal circuits controlling abducens nerve-mediated eyeblinks in turtles have not been previously described, which is a necessary step for understanding cellular mechanisms of conditioning in this preparation. The purpose of the present study was to neuroanatomically define the premotor pathways that underlie the trigeminal and auditory nerve-evoked abducens eyeblink responses. The results show that the principal sensory trigeminal nucleus forms a disynaptic pathway from both the trigeminal and auditory nerves to the principal and accessory abducens motor nuclei. Additionally, the principal abducens nucleus receives vestibular inputs, whereas the accessory nucleus receives input from the cochlear nucleus. The late R2-like component of abducens nerve responses is mediated by the spinal trigeminal nucleus in the medulla. Both the principal sensory trigeminal nucleus and the abducens motor nuclei receive CS-US convergence and therefore both, or either, might be considered potential sites of synapse modification during in vitro abducens conditioning. Further data are required to determine the role of the principal sensory trigeminal nucleus in in vitro conditioning.

Projection of the magnocellular red nucleus to the region of the accessory abducens nucleus in the rabbit

The projection of the magnocellular red nucleus (RNm) to the region of the accessory abducens nucleus (AABD) was traced in rabbit using the bidirectional tracer wheat germ agglutinin-horseradish peroxidase (WGA-HRP). In one set of animals, recordings of antidromic responses from RNm neurons elicited by electrical stimulation of the rubrospinal tract were used to localize injections of WGA-HRP for orthograde labeling of RNm terminals. In a different set of animals, horseradish peroxidase was injected into the retractor bulbi muscle to retrogradely label motoneurons of the AABD. The positions of RNm fibers and terminals were examined and compared to the locations and distribution of AABD cell bodies and labeled dendrites. Analyses revealed that along the entire rostrocaudal extent of the AABD, RNm efferents terminate primarily lateral to, or in the lateral aspects of, labeled motoneurons. For the rostral AABD, RNm efferents terminate only lateral to the nucleus. Although the terminals are not positioned to contact cell bodies of the AABD, they could overlap with dendrites that extend in the lateral direction. RNm efferents terminate more extensively within the posterior AABD, overlapping within both dendritic and cell body regions of the nucleus. Even in this posterior region, however, RNm efferents were distributed primarily over the lateral half of the nucleus. These data show that RNm can monosynaptically influence the AABD, through primarily its lateral and posterior aspects. Our findings also show that a major target of RNm efferents is the reticular cell population located lateral to the AABD, suggesting that the RNm also may affect AABD motoneuronal output indirectly through its projection to reticular cells.

Central trigeminal and posterior eighth nerve projections in the turtle Chrysemys picta studied in vitro

Recent electrophysiological studies in the turtle Chrysemys picta have suggested that a neural correlate of the eye-blink reflex can be evoked in an in vitro brain-stem-cerebellum preparation by electrical rather than natural stimulation of the cranial nerves. Discharge recorded in the abducens nerve, which is similar to EMG recordings from extraocular muscles during eye retraction, is triggered by a brief electrical stimulus applied to the ipsilateral trigeminal nerve. Evidence also suggests that pairing a one-second electrical stimulus applied to the posterior eighth nerve immediately prior to a single shock to the trigeminal nerve results in classically conditioned abducens nerve discharge in response to the previously neutral eighth nerve stimulus. In view of these physiological findings, the aim of the present study was to examine the central projections of trigeminal and posterior eighth nerve inputs to elucidate the anatomical substrates that may underlie the in vitro eye-blink reflex and possible pathways involved in reflex conditioning. Neurobiotin (NB) or fluorescein dextran (FD) was pressure injected into the cut end of either the trigeminal or posterior eighth nerve of the in vitro brainstem-cerebellum. Following trigeminal nerve injections, both tracers showed label in the ipsilateral trigeminal nuclear complex. Direct projections to the ipsilateral principal and accessory abducens motor nuclei were observed, suggesting that the eye-blink reflex is monosynaptic. Trigeminal nerve axons were also observed to terminate in the ipsilateral cerebellar cortex. The results of the posterior eighth nerve injections showed axonal projections and terminals in the cochlear, vestibular and principal sensory trigeminal nuclei. Terminal label was also observed in the ipsilateral cerebellar cortex, deep cerebellar nuclei, and in the principal and accessory abducens motor nuclei. Results from the NB cases suggested transneuronal transport of this tracer substance, whereas the FD cases did not. The present findings suggest that convergence of trigeminal and posterior eighth nerve inputs occurs in the ipsilateral cerebellar cortex, the principal sensory trigeminal nucleus, and the principal and accessory abducens motor nuclei. These regions of convergence may therefore be considered as potential sites of synaptic modification during in vitro studies of the conditioned abducens nerve reflex.

Properties of conditioned abducens nerve responses in a highly reduced in vitro brain stem preparation from the turtle

Previous work suggested that the cerebellum and red nucleus are not necessary for the acquisition, extinction, and reacquistion of the in vitro classically conditioned abducens nerve response in the turtle. These findings are extended in the present study by obtaining conditioned responses (CRs) in preparations that received a partial ablation of the brain stem circuitry. In addition to removing all tissue rostral to and including the midbrain and cerebellum, a transection was made just caudal to the emergence of the IXth nerve. Such ablations result in a 4-mm-thick section of brain stem tissue that functionally eliminates the sustained component of the unconditioned response (UR) while leaving only a phasic component. We refer to this region of brain stem tissue caudal to the IXth nerve as the "caudal premotor blink region." Neural discharge was recorded from the abducens nerve following a single shock unconditioned stimulus (US) applied to the ipsilateral trigeminal nerve. When the US was paired with a conditioned stimulus (CS) applied to the posterior eighth, or auditory, nerve using a delay conditioning paradigm, a positive slope of CR acquisition was recorded in the abducens nerve, and CR extinction was recorded when the stimuli were alternated. Resumption of paired stimuli resulted in reacquisition. Quantitative analysis of the CRs in preparations in which the caudal premotor blink region had been removed and those with cerebellar/red nucleus lesions showed that both types of preparations had abnormally short latency CR onsets compared with preparations in which these regions were intact. Preparations with brain stem transections had significantly earlier CR offsets as more CRs terminated as short bursts when compared with intact or cerebellar lesioned preparations. These data suggest that a highly reduced in vitro brain stem preparation from the turtle can be classically conditioned. Furthermore, the caudal brain stem is not a site of acquisition in this reduced preparation, but it contributes to the sustained activity of both the UR and CR. Finally, the unusually short CR onset latencies following lesions to the cerebellum are not further exacerbated by removal of the caudal brain stem. These studies suggest that convergence of CS and US synaptic inputs onto the abducens nerve reflex circuitry may underlie acquisition in this reduced preparation, but that mechanisms that control learned CR timing arise from the cerebellorubral system.

Neurons located in the trigeminal sensory complex and the lateral pontine tegmentum project to the oculomotor nucleus in the rabbit

Neurons located in the trigeminal sensory complex (TSC) and the lateral pontine tegmentum (LPT) have been reported to project to both the accessory abducens and the facial nuclei, which innervate the retractor bulbi and orbicularis oculi muscles respectively, in order to control the nictitating membrane (NM) and eyelid defensive reflex. Since muscles innervated by the oculomotor nucleus (OCM) also appear to be involved in this reflex, retrograde and anterograde tracers were used in this study to determine whether there are projections from the TSC and LPT to the OCM in the rabbit. Injections of horseradish peroxidase (HRP) in the OCM nucleus labeled neurons in the LPT surrounding the trigeminal motor nucleus dorsally, laterally and ventrally. Only a few scattered neurons were found in the principal and spinal trigeminal nuclei. Injection of biocytin in the LPT area containing most of the HRP-labeled neurons caused anterograde labeling of fibers that crossed the midline and ascended just dorsal to the contralateral medial lemniscus. A proportion of these fibers coursed in a dorsal direction to enter and terminate within the OCM contralateral to the injection site. The location of the motoneuronal groups innervating the different extraocular muscles was studied by retrograde transport of HRP, and compared with the distribution of biocytin-labeled terminals. It was found that the terminals were located in the superior rectus and the levator palpebrae zone of the nucleus. We discuss the functional significance of this projection for the eyelid and NM response.

Muscle activity during unconditioned and conditioned eye blinks in the rabbit

EMGs were recorded from the orbicularis oculi, retractor bulbi and superior rectus muscles in rabbits to investigate the time course of muscle activation during unconditioned and conditioned eye blinks. EMGs from the three muscles showed two responses, with the responses of the orbicularis oculi and retractor bulbi showing the same latency, and the responses of the superior rectus lagging. The latency of responses to periorbital electrostimulation was about 5 ms, and to air puff stimulation about 10 ms. Results showed a tight coupling of activity between muscles, with cross-correlograms peaking at 0.65 to 0.85 and showing little time shift. Stimulus-response curves showed clear non-linearities in the response of the muscle to changes in stimulus strength. Local anesthesia of the cornea had little effect on unconditionally evoked responses. The form of unconditionally evoked responses was similar with periorbital electrostimulation and air puff stimuli but differed in latency. These results show the form of the eye blink reflex response and will be of importance in interpreting electrophysiological studies of the classically conditioned eye blink of rabbits.

Changes in the motor pattern of learned and unlearned responses following cerebellar lesions: a kinematic analysis of the nictitating membrane reflex

Kinematic and dynamic analyses were employed to study the effects of cerebellar lesions on conditioned and unconditioned nictitating membrane responses in the rabbit. It was found that conditioned responses acquired to an auditory stimulus accelerated in two bursts as indicated by two distinct peaks of acceleration. The second peak of acceleration was very weak during the early portions of conditioning but became a prominent feature of the conditioned response over 16 sessions of conditioning. The second peak of acceleration in the conditioned response was more sensitive to cerebellar damage than was the first peak. When lesions of the cerebellum permanently reduced the amplitude of conditioned responses, but did not affect their frequency, the second peak of acceleration was nearly abolished while the first peak was unaffected. When cerebellar lesions profoundly impaired both the amplitude and frequency of conditioned responses, large and permanent impairments occurred in both peaks of acceleration. Lesions of the anterior interpositus nucleus most severely impaired both peaks of acceleration in the conditioned response and significantly reduced the acceleration of unconditioned responses across a wide range of intensities of corneal air puff. The deficit in the acceleration of unconditioned responses became manifest only after membrane extension exceeded 0.12 mm. The impairment in the amplitude of the unconditioned response after cerebellar lesions more closely approximated the impairment in the amplitude of the conditioned response when the force-generating properties of the conditioned and unconditioned stimuli were equated. It was hypothesized, therefore, that one reason why conditioned responses are so easily disrupted by cerebellar lesions is because they are of low force and not simply because they are learned. It was proposed that the two peaks of acceleration that characterize the conditioned response represent the function of two distinct anatomical systems. The first, a short-latency system, initiates the response and is most likely mediated by circuits that traverse the pontomedullary reticular formation. The second, a longer-latency system, amplifies response amplitude and its neural basis remains to be elucidated. The two components of the conditioned response may reflect two sequential bursts of activity in the accessory abducens nucleus, the principal site of the motoneurons for the retractor bulbi muscle, or may reflect the synergistic activity of the accessory abducens nucleus and the motor nuclei of the other extraocular muscles. It was concluded that the vulnerability of the second component of the conditioned response to cerebellar damage reflects an important role for the cerebellum in modulating the degree to which long-latency neural systems contribute to the ongoing performance of learned and unlearned behaviors.

Effects of MDA upon differential serial compound conditioning and reflex modification of the rabbit's nictitating membrane response

The present investigations sought to determine the effects of 3,4-methylenedioxyamphetamine (MDA) on: 1) differential conditioning of the rabbit's nictitating membrane response to the serial compounds A-X-US (tone-light-reinforced compound) and B-X (white noise-light-unreinforced compound) by examining differential responding to A and B and their conditional control over responding to X within the compounds (Experiment 1); and 2) the ability of the compound stimuli and their components to modify the amplitude of the unconditioned nictitating membrane response (Experiment 2). Experiment 1 revealed that MDA decremented differential responding to the serial compounds and their A and B components, while enhancing conditioned responding to the X component. In addition, Experiment 2 indicated that MDA attenuated reflex modification to the compounds and their A and B components, but facilitated reflex modification to X alone. The results of these experiments indicated that MDA operated to alter the intensity, distinctiveness, and persistence (short-term memory) of stimulus representations.

Electrical activity of the lateral rectus muscle and abducens nerve during unconditioned eye retraction induced by corneal stimulation in the cat

Horizontal eye movements, the electromyogram of the lateral rectus muscle and the evoked electroneurogram of the VIth nerve were recorded in the alert cat during air puff stimulation of the cornea. A burst of activity was observed in the lateral rectus muscle following air puff stimulation of the ipsilateral or contralateral eye. This activity produced rotational eye movement. The VIth nerve produced two volleys of activity that were interpreted as the R1 and R2 parts of the reflex. It can be concluded that the lateral rectus muscle participates in the ocular retraction following trigeminal stimulation. It is further suggested that motoneurons in the main abducens nucleus participate in the corneal reflex, permitting several final pathways for conditioned and unconditioned nictitating membrane extension.

Activity of spinal trigeminal pars oralis and adjacent reticular formation units during differential conditioning of the rabbit nictitating membrane response

Spinal trigeminal nucleus pars oralis (SpoV) is anatomically linked to brain circuitry thought to subserve unconditioned and conditioned nictitating membrane responses in rabbit. Single-unit recording from SpoV and adjacent reticular formation obtained during conditioning from awake, behaving animals revealed modulation of unit firing related to CS, US, and CR occurrence. SpoV participates directly in the unconditioned response and probably relays US information to other brain areas subserving conditioning. The presence of CR-related activity suggests that SpoV may participate in the CR motor output pathway, and may also provide CR-related information to cerebellum. Sensory convergence and CR-related activity in reticular formation mark this structure as a candidate locus of primary neuronal plasticity in this example of conditioning.

Selective involvement of the spinal trigeminal nucleus in the conditioned nictitating membrane reflex of the rabbit

These experiments were performed to test the hypothesis that a region associated with the trigeminal nuclear complex is selectively involved in mediating the classically conditioned nictitating membrane reflex in the rabbit. Microinjections of Lidocaine were used to produce a temporary, localized block of neural activity following the conditioning of the reflex using a standard tone/air puff-paired stimulus paradigm. Data indicate that the injection of Lidocaine at the medial pars oralis/reticular formation junction results in a selective suppression of the conditioned reflex.

Single-unit activity in red nucleus during the classically conditioned rabbit nictitating membrane response

Previous investigations have suggested that the cerebellum and associated brainstem structures, including the red nucleus, are essential for the expression of the classically conditioned nictitating membrane (NM) response. The present study examined the firing patterns of extracellularly-recorded single units in the red nucleus of the awake rabbit during differential conditioning. Tones were used as conditioned stimulus (CS+ and CS-) and periocular electrostimulation was used as the unconditioned stimulus (US). Most units exhibited one or more changes in firing rate during the presentation of the CS, and increases in firing were much more common than decreases. The onset of some of these changes appeared to be time-locked to the onset of the CS ('CS-locked' responses), while other changes were time-locked to the onset of the CR ('CR-locked' responses). About one-third of all CS-locked changes were CR-dependent, meaning that the neuronal response was reduced when the CR did not occur. About two-thirds of all CR-locked responses preceded the onset of the CR, and lead times varied considerably across units. Many CR-locked units were located in what has been described as a dorsal face region of the red nucleus. Most units responded to the US, and some of the US responses were CR-dependent: i.e., a smaller US response was evoked when a CR preceded the US than when the CR was absent. Our results support the notion that cerebellum-brainstem circuits are involved in generating NM CRs.

Development of the abducens nuclei in the Xenopus laevis

The development of the main (nVI) and the accessory abducens (nVIa) nuclei was studied with the horseradish peroxidase and cobaltic-lysine labeling techniques in Xenopus laevis tadpoles. In earliest labeling was obtained at stage 39, and neuroblasts of both nuclei formed two separate groups according to their definitive positions in relation to other rhombencephalic structures in this young age of development. Conspicuous morphological differences were observed between the two nuclei: the accessory abducens neuroblasts were twice as big as the abducens neuroblasts and the characteristic nVIa 'knee' was present from this time of the first successful labeling. The two different dendritic arborization patterns, which clearly distinguished the abducens neurons from the accessory abducens neurons, gradually developed in tadpoles. It is suggested that the form and position of abducens and accessory abducens neurons are determined at a prefunctional stage, probably before the beginning of axonal outgrowth, and neurobiotaxis may not play the role attributed previously in the differentiation of these two nuclei.

Pharmacological analysis of the magnocellular red nucleus during classical conditioning of the rabbit nictitating membrane response

Previous experiments have suggested that the red nucleus is an essential structure in the neural pathways subserving the conditioned responses (CRs) elicited in several simple associative learning paradigms. The present investigation confirms the involvement of the magnocellular red nucleus in production of the classically conditioned nictitating membrane response in the rabbit and suggests that gamma-aminobutyric acid (GABA) processes within this structure are involved in expression of the CR. Specifically, these studies demonstrate that microinfusion of a GABA antagonist (either picrotoxin or bicuculline methiodide) into the magnocellular red nucleus can selectively and reversibly reduce or abolish retention of the CR, without altering the unconditioned reflex response. Furthermore, these pharmacological manipulations that disrupt the CR are both anatomically and pharmacologically specific, and demonstrate a predictable dose-dependent function. These findings suggest that GABAergic processes within the magnocellular red nucleus are part of the critical circuitry subserving the CR.

Asymmetric uptake of 2-deoxy-D-[14C]glucose in the dorsal cochlear nucleus during Pavlovian conditioning in the rabbit

Uptake of 2-deoxy-D-[14C]glucose was measured during Pavlovian conditioning of the rabbit's nictitating membrane response by both qualitative autoradiography and by quantitative measurement of radioactivity in samples of brain tissue. Conditioning was accomplished by pairing a tone stimulus delivered to both ears with an air-puff stimulus delivered to the right eye. Infusion of 2-deoxy-D-[14C]glucose during the first day of conditioning when there was no evidence of acquisition or during the 7th day of conditioning when animals demonstrated 68% conditioned responses resulted in a significantly greater uptake of radioactivity by the caudal portions of the left as compared with the right dorsal cochlear nucleus. Similar changes were not observed in other auditory and non-auditory nuclei. Rabbits that had acquired conditioned responses across 6 days of training and were exposed only to the tone-conditioned stimulus on the 7th day of testing exhibited 69% conditioned responses but no asymmetry in the uptake of 2-deoxy-D-[14C]glucose. Control animals receiving unpaired presentations of tone and air puff or no stimulation did not acquire conditioned responses and did not demonstrate asymmetric uptake of radioactivity in the dorsal cochlear nucleus. These results indicate that the asymmetric uptake of radioactivity by the dorsal cochlear nucleus did not result from the effects of stimulation per se or the prior occurrence of learning but was due to the explicit pairing of the tone stimulus with the asymmetric delivery of the air puff. It would appear that the caudal dorsal cochlear nucleus not only serves as a signal transducer for auditory stimuli but also receives inputs from other sensory systems thus allowing it to both recognize when an auditory stimulus is followed by a biologically significant event and to transmit such information to other brain regions that are, in turn, responsible for learning.

Topography and organization of cranial nerve nuclei in the sand lizard, Lacerta agilis

Cobaltic-lysine complex compound was used to label cranial nerves of the ventrolateral (branchiomotor) and dorsomedial (somatomotor) nuclear columns in the sand lizard, Lacerta agilis. The dendritic arborizations and axonal trajectories of neurons of the respective nuclei were reconstructed from serial sections. A fairly uniform neuronal morphology was found in the nuclei of the ventrolateral column: a spindle-shaped perikaryon gave rise to dorsomedial and ventrolateral dendritic trees, the latter arborizing in a characteristic broomlike manner within a narrow region in the lateral white matter. Axons of all neurons converged upon the medial longitudinal fasciculus and after making a hairpin turn formed the corresponding motor roots. A group of small neurons constituted a separate subnucleus within the V motor nucleus. The VII and IX nuclei were fused into a single nuclear complex. The nucleus ambiguus was found dorsal to the XII nucleus and lateral to the dorsal vagal nucleus. The latter nucleus extended rostrally to the caudal pole of the VI nucleus, and its neurons sent axons to the VII, IX, and X nerves. The term "dorsal visceromotor column" designates the extended dorsal vagal nucleus. A number of small polygonal neurons lying scattered in the lateral part of the medulla were labeled via the VII, IX, and X nerves. This loose aggregate of labeled neurons was termed the "lateral visceromotor area." On the basis of nuclear topography and cellular morphology, the existence of a bulbar XI nucleus was excluded. Three different types of neurons could be distinguished in the dorsomedial nuclear column. Neurons with oval or spherical perikarya and radially oriented dendrites constituted the nuclei innervating external eye muscles. Except for the IV nucleus, axons followed a ventral trajectory. The accessory VI nucleus was composed of a second type of neuron with elongated soma and dorsoventral dendrite orientation; the dorsally directed axon turned ventrally at the VI nucleus. The XII nucleus contains a third type of neuron with strongly decussating dendrites. The distinct differences in the neuronal morphology did not support the classical assumption that all of the nuclei of the dorsomedial motor column supply muscles derived from somitic mesoderm. Sensory fibers of the trigeminal nerve formed the familiar spinal tract, which partially decussated in the medullospinal transition zone and could be followed as far as the lumbar segments on the ipsilateral side of the spinal cord. Neurons of the mesencephalic root of the trigeminal nerve were localized in the optic tectum; their descending fibers joined the medial aspect of the spinal tract.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of abducens motoneurons, accessory abducens motoneurons, and abducens internuclear neurons in the chick by retrograde transport of horseradish peroxidase

The location of the motoneurons innervating the lateral rectus, pyramidalis, and quadratus muscles of the chick has been determined by application of horseradish peroxidase (HRP) to these muscles and their nerve branches, and internuclear neurons in the abducens nucleus have been identified by injection of HRP into the oculomotor nucleus. Quantitative results were obtained by means of a semiautomatic image analyzer. Lateral rectus motoneurons were observed only in the ipsilateral principal abducens nucleus, where they numbered 500-550, and quadratus and pyramidalis motoneurons only in the ipsilateral accessory abducens nucleus. The 325-375 internuclear neurons that appeared in the principal abducens nucleus contralateral to the oculomotor nucleus injected with HRP were practically confined to the rostral two thirds of the nucleus, where they tended to surround the lateral rectus motoneurons in dorsal or lateral positions, though a minority of interneurons also mingled with the motoneurons in the center or at the medial face of the nucleus. Most interneurons were small and elongated, but a minority of larger interneurons morphologically similar to the lateral rectus motoneurons were also distinguishable. The 100-110 quadratus motoneurons and the 45-55 pyramidalis motoneurons mingled in the accessory abducens nucleus were larger than the lateral rectus motoneurons and sent their axons into the ipsilateral abducens nerve.

Innervation of extraocular muscles in the rabbit

The innervation of extraocular muscles in the rabbit was studied by using the methods of horseradish peroxidase (HRP) histochemistry, gross dissection, and quantitative morphology. Subdivisions of the oculomotor complex that innervate the superior rectus, inferior rectus, medial rectus, and inferior oblique and levator palpebrae are described, and our results are in agreement with previous accounts of the projections of this nucleus. Our analysis of the innervation of the lateral rectus and retractor bulbi muscles, however, differs from previous descriptions. The axons of approximately 80% of neurons in the abducens nucleus are in the VIth nerve and innervate the lateral rectus muscle, and approximately 15-20% are internuclear neurons both surrounding and intermingling with the motor neurons of the abducens nucleus. The interneurons project to the medial rectus subdivision of the contralateral oculomotor complex via the medial longitudinal fasciculus (MLF). Neurons in both the abducens and the accessory abducens nucleus innervate the retractor bulbi muscles via the VIth nerve. All neurons in the accessory abducens nucleus innervate the retractor bulbi muscles, but gross dissection revealed that the retractor bulbi is also innervated by the IIIrd nerve. The bases for differences between our data and previously published descriptions are discussed. The trochlear nucleus of the rabbit has not been previously studied by methods of axonal transport. The body of the nucleus, its caudal tail, the trajectories of axons entering the trochlear nerve, and soma size distributions are described. The trochlear nucleus contains approximately 900 neurons; most are motoneurons the axons of which travel in the trochlear nerve and decussate in the anterior medullary velum. Approximately 3% of trochlear motor neurons innervate the ipsilateral superior oblique muscle. Their soma size is significantly smaller than that of contralaterally projecting neurons. For comparative purposes, the innervation of extraocular muscles by the trochlear nerve was also investigated in several rodents and carnivores. In all animals studied, the percentage of trochlear neurons innervating the ipsilateral superior oblique muscle was strikingly uniform (2-4%). Gross dissection of the extraocular muscles revealed in the rabbit a muscle, innervated by the trochlear nerve, for which we propose the name "tensor trochleae." In the rabbit, this muscle is innervated by approximately one-third of the trochlear motor neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization of motoneurons innervating the extraocular muscles in Salamandra salamandra L. (Amphibia, Urodela)

The central innervation patterns of the extraocular muscles were investigated in the European fire salamander Salamandra salamandra L. by means of the horseradish peroxidase method. The ipsilateral portion of the nucleus nervi oculomotorii, which is located in the rostral ventral tegmentum mesencephali, supplies the musculi recti inferior and medialis and the musculus obliquus inferior without a clear somatotopic representation of the motoneurons. The musculus rectus superior is innervated mainly by a contralateral portion of this nucleus. A definite nucleus Edinger-Westphal could not be recognized. The nucleus nervi trochlearis, which rostrally joins the nucleus nervi oculomotorii with a gap of only about 40 micron between the nuclei, is situated completely contralateral to the musculus obliquus superior supplied by it. The nucleus nervi abducentis, innervating the musculus rectus lateralis, and the nucleus accessorius nervi abducentis, supplying the musculus retractor bulbi, are found in the ipsilateral medulla oblongata and exhibit a large rostrocaudal extension from the eighth cranial nerve to the first root of the vagus nerve. Dendrites of the nucleus nervi oculumotorii and of the nucleus accessorius nervi abducentis extend into neuropil areas receiving primary sensory afferents.

Anatomical observations on the afferent projections to the retractor bulbi motoneuronal cell group and other pathways possibly related to the blink reflex in the cat

In the cat retractor bulbi (RB) muscle reflexively retracts the eye ball into the orbit. This reflex action is called the nictitating membrane response which, together with the reflex contraction of the orbicularis oculi muscle, constitutes the blink reflex. The retractor bulbi (RB) motoneuronal nucleus is a small cell group located in the lateral tegmentum of the caudal pons, just dorsal to the superior olivary complex. The nucleus is identical to the accessory abducens nucleus and sends its fibers through the abducens nerve. Autoradiographical tracing results indicate that the RB nucleus receives some fibers from the principal and rostral spinal trigeminal nuclei and from the dorsal red nucleus and dorsally adjoining tegmentum. The same areas project to the intermediate facial subnucleus, containing motoneurons innervating the orbicularis oculi muscle. It is suggested that the trigeminal projections take part in the anatomical framework for the R1 component of the blink reflex. Two other brainstem areas i.e.: a portion of the caudal pontine ventrolateral tegmental field and the medullary medial tegmentum at the level of the hypoglossal nucleus were also found to project to the RB motoneuronal cell group and to the intermediate facial subnucleus. These projections were much stronger than those derived from the trigeminal nuclei and red nucleus. Moreover, the medullary premotor area projects not only to the blink motoneuronal cell groups but also to the pontine premotor area. It is suggested that both areas are involved in the R2 blink reflex component. The medullary blink premotor area receives afferents especially from oculomotor control structures in the reticular formation of the brainstem while the pontine blink premotor area receives afferents from the olivary pretectal nucleus and/or the nucleus of the optic tract and from the dorsal red nucleus and its dorsally adjoining area. Because the oculomotor control structures in the reticular formation (by way of the superior colliculus) and the red nucleus receive afferents from trigeminal nuclei, they may play an important role in tactually induced reflex blinking, while the pretectum could take part in the neuronal framework of the visually induced blink reflex.

Classical conditioning of the nictitating membrane response of the rabbit. I. Lesions of the cerebellar nuclei

The classically conditioned nictitating membrane response (NMR) of the rabbit, a simple form of associative motor learning, is crucially dependent upon the cerebellum. Discrete unilateral lesions of the cerebellar nuclei were made in 20 rabbits. Lesions of the anterior interpositus nucleus (IA) abolished NMR conditioning to light and white noise stimuli on the side of the lesion without affecting unconditional responses. Lesions of the posterior interpositus nucleus, fastigial and dentate nuclei were without effect upon NMR conditioning.

The role of the extraocular muscles in the rabbit nictitating membrane response: a re-examination

Berthier and Moore showed that the rabbit nictitating membrane (NM) response principally results from contracting the retractor bulbi muscle which pulls the globe into the socket thereby passively effecting NM extension. They concluded that the remaining extraocular muscles can effect NM extension if the retractor bulbi is denervated. A re-examination of the role of the recti and oblique extraocular muscles in nictitating membrane extension was undertaken in the light of recent results of Marek et al., suggesting that the facial nerve, and not the extraocular muscles, participates in extension of the NM. In contrast to Marek et al., the present results indicated that section of the extraocular muscles was necessary to abolish eyeshock or tactilly elicited NM extension when the abducens and facial nerves were severed. It is therefore likely that extraocular (recti and oblique) muscles participate in globe retraction and NM extension, as originally noted by Lorente de No.

The innervation of the monkey accessory lateral rectus muscle

The source and pattern of innervation of the accessory lateral rectus muscle has been re-investigated in 3 Macaca fascicularis monkeys by means of a tracing method employing [125I] wheat germ agglutinin and a morphological analysis of the myo-neuronal junctions. The present findings suggest that this muscle is composed of exclusively singly innervated fibers and that its motoneurons are situated in the accessory abducens nucleus. This is in contrast to a previous study, where the monkey accessory lateral rectus was found to be composed of singly and multiply innervated fibers and to be innervated by a group of motoneurons lying within principle and accessory abducens nucleus. It is concluded that the monkey accessory lateral rectus reflects in principle the organization of the retractor bulbi of other vertebrates, although this muscle is gradually vanishing in primate evolution and remains vestigial in the macaque monkey. The absence of comparable motor units in man is more likely to mean an actual loss of structure and function than their integration into the lateral rectus system.

Eyeball retraction latency in the conscious rabbit measured with a new photodiode technique

A new technique is described for accurate, reliable measurement of eyeball retraction in the rabbit. A narrow film strip, on which a linear light intensity grating has been exposed, is attached to a contact lens which is placed on the animal's cornea. The other end of the intensity grating slides between a photodiode and an LED. The contact lens-film grating assembly moves freely with eyeball retraction and relaxation, causing changes in photodiode output. This device appears to be well tolerated by the animal. Large amplitude eyeball retractions occur in response to air puff stimulation directed at the upper or lower eyelid and to periorbital shock. Average eye retraction latency to stimulation of the abducens (VI) nerve with a chronically implanted stimulating electrode was 5.3 ms (S.D. = 0.75 ms) in the conscious rabbit as measured with our device. Latency to periorbital electrical shock was 9.3 ms (S.D. = 2.1 ms). Eye retraction latency decreased with increasing shock amplitudes. Rabbits readily acquired classically conditioned eyeball retractions, monitored with this device, when a white noise auditory stimulus was paired with an air puff directed at the eyelid.

Red nucleus lesions disrupt the classically conditioned nictitating membrane response in rabbits

Sixteen rabbits received classical conditioning of the right nictitating membrane response using a tone CS and electrostimulation of the right eye as the US. Single electrocoagulating RF lesions of the medial portion of the left magnocellular red nucleus eliminated or severely reduced the previously acquired conditioned response. This finding is consistent with the idea that an essential anatomical substrate of the conditioned response includes a circuit from the cerebellum to the contralateral red nucleus which projects contralaterally in turn to pontine motoneurons mediating the defensive nictitating membrane response.

An HRP study of the brainstem afferents to the accessory abducens region and dorsolateral pons in rabbit: implications for the conditioned nictitating membrane response

Brain projections to the accessory abducens region and dorsolateral pons were investigated in rabbit using implants of crystalline horseradish peroxidase (HRP). Following implantation of HRP in the accessory abducens region (N = 3), labeled cells were observed in the sensory trigeminal nuclei and other regions implicated in the reflex pathway of the defensive nictitating membrane (NM) response. Neurons in the supratrigeminal zone were also labeled, as were portions of the contralateral red nucleus. Implantation of HRP into the dorsolateral pons (N = 5) revealed ipsilateral projections from deep-cerebellar nuclei in some cases. In addition, the parvocellular reticular formation displayed bilateral labeling of cells and an ipsilateral network of fibers and apparent terminations. Many cells of the contralateral supratrigeminal zone were labeled in these cases. Results were discussed in relation to lesioning and electrophysiological studies implicating the supratrigeminal region and other structures in the control of the classically conditioned NM response. Specifically, the possibility that supratrigeminal neurons are premotor elements responsible for the conditioned response is considered. Alternative hypotheses are discussed, including pathways by which cerebellar nuclei could control conditioned responding.

A supratrigeminal region implicated in the classically conditioned nictitating membrane response

The dorsolateral pontine brain stem was investigated as a possible locus of neural elements mediating the classically conditioned nictitating membrane (NM) response in rabbit. Recording and brain stimulation were employed for this purpose. Low-impedance tungsten monopolar microelectrodes were chronically implanted into the pontine brain stem. Multiple-unit recording during classical conditioning revealed a conditioned increase in multiple-unit activity (MUA) which developed and extinguished concurrently with the acquisition and extinction of the behavioral conditioned response. Pseudoconditioning and conditioned inhibition controls indicated that the increase in MUA was an associative learning phenomenon. Histology indicated that electrode tips recording the CR-associated electrical activity were located mostly adjacent or dorsal to the motor trigeminal nucleus. Periocular shock pulses elicited short latency evoked responses throughout most of the dorsolateral pons, suggesting that information concerning the unconditioned stimulus is relayed to this region. Furthermore, electrical stimulation of this region produced a robust ipsilateral nictitating membrane response in a number of cases, suggesting that neural elements of the dorsolateral pons project to the motoneurons that produce membrane extension. A consideration of several criteria based on these experiments implicates a supratrigeminal zone [22] as containing the neural elements of dorsolateral pons most intimately associated with the conditioned NM response. Other interpretations, concerning fibers of passage through this region and