Functional organization of the parvocellular red nucleus in the cat

Distribution of the Extracellular Matrix in the Pararubral Area of the Rat

Previously we described similarities and differences in the organization and molecular composition of an aggrecan based extracellular matrix (ECM) in three precerebellar nuclei, the inferior olive, the prepositus hypoglossi nucleus and the red nucleus of the rat associated with their specific cytoarchitecture, connection and function in the vestibular system. The aim of present study is to map the ECM pattern in a mesencephalic precerebellar nucleus, the pararubral area, which has a unique function among the precerebellar nuclei with its retinal connection and involvement in the circadian rhythm regulation. Using histochemistry and immunohistochemistry we have described for the first time the presence of major ECM components, the hyaluronan, aggrecan, versican, neurocan, brevican, tenascin-R (TN-R), and the HAPLN1 link protein in the pararubral area. The most common form of the aggrecan based ECM was the diffuse network in the neuropil, but each type of the condensed forms was also recognizable. Characteristic perineuronal nets (PNNs) were only recognizable with Wisteria floribunda agglutinin (WFA) and aggrecan staining around some of the medium-sized neurons, whereas the small cells were rarely surrounded by a weakly stained PNNs. The moderate expression of key molecules of PNN, the hyaluronan (HA) and HAPLN1 suggests that the lesser stability of ECM assembly around the pararubral neurons may allow quicker response to the modified neuronal activity and contributes to the high level of plasticity in the vestibular system.

The functional anatomy of the cerebrocerebellar circuit: A review and new concepts

The cerebrocerebellar circuit is a feedback circuit that bidirectionally connects the neocortex and the cerebellum. According to the classic view, the cerebrocerebellar circuit is specifically involved in the functional regulation of the motor areas of the neocortex. In recent years, studies carried out in experimental animals by morphological and physiological methods, and in humans by magnetic resonance imaging, have indicated that the cerebrocerebellar circuit is also involved in the functional regulation of the nonmotor areas of the neocortex, including the prefrontal, associative, sensory and limbic areas. Moreover, a second type of cerebrocerebellar circuit, bidirectionally connecting the hypothalamus and the cerebellum, has been detected, being specifically involved in the regulation of the hypothalamic functions. This review analyzes the morphological features of the centers and pathways of the cerebrocerebellar circuits, paying particular attention to their organization in different channels, which separately connect the cerebellum with the motor areas and nonmotor areas of the neocortex, and with the hypothalamus. Actually, a considerable amount of new data have led, and are leading, to profound changes on the views on the anatomy, physiology, and pathophysiology of the cerebrocerebellar circuits, so much they may be now considered to be essential for the functional regulation of many neocortex areas, perhaps all, as well as of the hypothalamus and of the limbic system. Accordingly, clinical studies have pointed out an involvement of the cerebrocerebellar circuits in the pathophysiology of an increasing number of neuropsychiatric disorders.

Effects of gonadotrophin-releasing hormone outside the hypothalamic-pituitary-reproductive axis

Gonadotrophin-releasing hormone (GnRH) is a hypothalamic decapeptide with an undisputed role as a primary regulator of gonadal function. It exerts this regulation by controlling the release of gonadotrophins. However, it is becoming apparent that GnRH may have a variety of other vital roles in normal physiology. A reconsideration of the potential widespread action that this traditional reproductive hormone exerts may lead to the generation of novel therapies and provide insight into seemingly incongruent outcomes from current treatments using GnRH analogues to combat diseases such as prostate cancer.

Immunoreactive GnRH type I receptors in the mouse and sheep brain

Gonadotropin Releasing Hormone-I (GnRH) has been implicated in an array of functions outside the neuroendocrine reproductive axis. Previous investigations have reported extensive GnRH binding in numerous sites and this has been supported by in situ hybridization studies reporting GnRH receptor mRNA distribution. The present study on mice and sheep supports and extends these earlier investigations by revealing the distribution of cells immunoreactive for the GnRH receptor. In addition to sites previously shown to express GnRH receptors such as the hippocampus, amygdala and the arcuate nucleus, the improved resolution afforded by immunocytochemistry detected cells in the mitral cell lay of the olfactory bulb as well as the central grey of the mesencephalon. In addition, GnRH receptor immunoreactive neurons in the hippocampus and mesencephalon of the sheep were shown to colocalize with estrogen receptor beta. Although GnRH may act at some of these sites to regulate reproductive processes, evidence is accumulating to support an extra-reproductive role for this hypothalamic decapeptide.

Responses to tactile stimulation in deep cerebellar nucleus neurons result from recurrent activation in multiple pathways

In a previous study, we found that neurons in the deep cerebellar nuclei (DCN) respond to 5-ms brief facial tactile stimulation in rats anesthetized with ketamine-xylazine with multiphasic response patterns lasting over 200 ms. It remained unclear, however, to what extent these responses were shaped not only by ascending sensory input from the trigeminal nuclei but also by interactions with other major cerebellar afferent systems, in particular the inferior olive (IO) and cerebral cortex. In the present study, we recorded from the IO, cerebral cortex, cerebellar granule cell layer (GCL), and DCN during the presentation of 5-ms facial tactile stimuli to elucidate potential mechanisms of how extended DCN response patterns are generated. We found that tactile stimulation resulted in robust multiphasic local field potentials responses in the IO as well as in the activation of a wide region of the somatosensory cortex (SI) and the primary motor cortex (MI). DCN neurons responded to electrical stimulation of any of these structures (IO, SI, and MI) with complex temporal patterns strikingly similar to air-puff lip stimulation responses. Simultaneous recordings from multiple structures revealed that long-lasting activation patterns elicited in DCN neurons were based on recurrent network activation in particular between the IO and the DCN with a potential contribution of DCN rebound properties. These results are consistent with the hypothesis that sensory stimulation triggers a feedback network activation of cerebellum, IO, and cerebral cortex to generate temporal patterns of activity that may control the timing of behavior.

Diencephalic damage decreases hippocampal acetylcholine release during spontaneous alternation testing

A rodent model of diencephalic amnesia, pyrithiamine-induced thiamine deficiency (PTD), was used to investigate diencephalic-hippocampal interactions. Acetylcholine (ACh) release, a marker of memory-related activation, was measured in the hippocampus of PTD-treated and control rats prior to, during, and after spontaneous alternation test. During behavioral testing, all animals displayed increases in ACh release. However, both the percent increase of ACh release during spontaneous alternation testing and the alternation scores were higher in control rats relative to PTD-treated rats. Thus, when rats are tested on a task with demands dependent on the hippocampus, it appears that the hippocampus is not fully activated after diencephalic damage.

Cerebellar long-term depression: characterization, signal transduction, and functional roles

Cerebellar Purkinje cells exhibit a unique type of synaptic plasticity, namely, long-term depression (LTD). When two inputs to a Purkinje cell, one from a climbing fiber and the other from a set of granule cell axons, are repeatedly associated, the input efficacy of the granule cell axons in exciting the Purkinje cell is persistently depressed. Section I of this review briefly describes the history of research around LTD, and section II specifies physiological characteristics of LTD. Sections III and IV then review the massive data accumulated during the past two decades, which have revealed complex networks of signal transduction underlying LTD. Section III deals with a variety of first messengers, receptors, ion channels, transporters, G proteins, and phospholipases. Section IV covers second messengers, protein kinases, phosphatases and other elements, eventually leading to inactivation of DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolone-propionate-selective glutamate receptors that mediate granule cell-to-Purkinje cell transmission. Section V defines roles of LTD in the light of the microcomplex concept of the cerebellum as functionally eliminating those synaptic connections associated with errors during repeated exercises, while preserving other connections leading to the successful execution of movements. Section VI examines the validity of this microcomplex concept based on the data collected from recent numerous studies of various forms of motor learning in ocular reflexes, eye-blink conditioning, posture, locomotion, and hand/arm movements. Section VII emphasizes the importance of integrating studies on LTD and learning and raises future possibilities of extending cerebellar research to reveal memory mechanisms of implicit learning in general.

Projection from the cerebellar lateral nucleus to precerebellar nuclei in the mossy fiber pathway is glutamatergic: a study combining anterograde tracing with immunogold labeling in the rat

The pontine nuclei (PN) and the nucleus reticularis tegmenti pontis (NRTP) are sources of an excitatory projection to the cerebellar cortex via mossy fibers and a direct excitatory projection to the cerebellar nuclei. These precerebellar nuclei, in turn, receive a feedback projection from the cerebellar nuclei, which mostly originate in the lateral nucleus (LN). It has been suggested that the feedback projection from the LN partially uses gamma-aminobutyric acid (GABA) as a transmitter. We tested this hypothesis by using a combination of anterograde tracing (biotinylated dextran amine injection into the LN) and postembedding GABA and glutamate immunogold histochemistry. The pattern of labeling in the PN and the NRTP was compared with that of cerebellonuclear terminals in two other target structures, the parvocellular part of the nucleus ruber (RNp) and the ventromedial and ventrolateral thalamus (VM/VL). The projection to the inferior olive (IO), which is known to be predominantly GABAergic, served as a control. A quantitative analysis of the synaptic terminals labeled by the tracer within the PN, the NRTP, and the VL/VM revealed no GABA immunoreactivity. Only one clearly labeled terminal was found in the RNp. In contrast, 72% of the terminals in the IO were clearly GABA immunoreactive, confirming the reliability of our staining protocol. Correspondingly, glutamate immunohistochemistry labeled the majority of the cerebellonuclear terminals in the PN (88%), the NRTP (90%), the RNp (93%), and the VM/VL (63%) but labeled only 5% in the IO. These data do not support a role for GABAergic inhibition either in the feedback systems from the LN to the PN and the NRTP or within the projections to the RNp and the VM/VL.

Cerebellar nuclei: the olivary connection

This chapter gives an overview of the relation between the inferior olive and the cerebellar nuclei based on tracing and electrophysiological experiments in rats and cats. The olivary and cerebellar nuclear masses appear to maintain a precise topographical relationship. The olivary projection to the cerebellar nuclei is strictly contralateral, originates as climbing fiber collaterals, and is excitatory. The cerebellar projection to the inferior olive is exactly reciprocal to the olivonuclear projection, but, in addition, also has a variable but definite ipsilateral component, and has been demonstrated to be GABAergic. Stimulation and lesion experiments carried out in cat imply that the cerebellar nucleo-olivary pathway may be involved in regulating the oscillatory capabilities of olivary cells and as such may also have an effect on the synchrony of olivary firing. Subsequent stimulation experiments performed in rats suggests that synchronous firing of olivary neurons may result in profound effects in cerebellar nuclear neurons.

Cerebellar influence on olivary excitability in the cat

This study examines the influence of the cerebellum on the excitability of inferior olivary neurons in the cat. Two major pathways from the cerebellar nuclei to the inferior olive have been investigated by electrophysiological and anatomical techniques. The first, excitatory pathway connects the cerebellar nuclei through nuclei at the mesodiencephalic junction with the inferior olive. The second is the direct, GABAergic, nucleo-olivary pathway. Intra- as well as extracellular recordings obtained in the rostral part of the medial accessory and principal olives revealed that electrical stimulation with a short burst of three pulses delivered at the mesodiencephalic junction results in short-latency activation (4-8 ms) of most olivary neurons. More than half of the units showed, in addition to the short-latency activation, a consistent response with a much longer latency (approximately 180 ms). Many units (66%) that responded to mesodiencephalic stimulation could also be activated by superior cerebellar peduncle stimulation with a similar stimulation paradigm (latency 9-15 ms). However, in such cases consistent long-latency responses were only rarely recorded (7%). To distinguish between the effect of the two pathways, both of which are activated by superior cerebellar peduncle stimulation, an electrolytic lesion of the nucleo-olivary fibres was made in the brainstem in six experiments. The effect of this lesion was verified in three cases by retrograde horseradish peroxidase tracing from the rostral inferior olive at the end of the experiment. This time only extracellular recordings were made. Stimulation of the mesodiencephalic junction still resulted in easily activated olivary units which showed an increased probability of firing a long-latency action potential. Stimulation of the superior cerebellar peduncle now resulted in a 50% decrease in probability of activating olivary units in the short-latency range. However, a five-fold increase in the chance of triggering action potentials in the long-latency interval was noted, implying that many units reacted only with a long-latency action potential. The results obtained with our experimental paradigm appear enigmatic since it is well established that the nucleo-olivary pathway is GABAergic and thus, by convention, should be inhibitory to the olivary neurons. However, it is possible to explain these results in terms of dynamic coupling of olivary neurons. This concept ascribes an important role to the nucleo-olivary pathway in regulating the degree of electrotonic coupling between olivary neurons (probably by a shunting mechanism) and as such may be an important instrument in the regulation of synchronous and rhythmic olivary discharges.(ABSTRACT TRUNCATED AT 400 WORDS)

Corticorubral synaptic organization in Macaca fascicularis: a study utilizing degeneration, anterograde transport of WGA-HRP, and combined immuno-GABA-gold technique and computer-assisted reconstruction

The macaque red nucleus receives afferents from two major sources, the cerebral cortex and the deep cerebellar nuclei. Approximately 90% of the corticorubral afferent axons project to pars parvicellularis of the red nucleus, the neurons of which transmit information to the cerebellum by way of the inferior olivary nucleus. The remaining 10% project to pars magnocellularis of the red nucleus, the major projection of which is to the spinal cord. In this study, corticorubral terminations labeled following lesions or injections of wheatgerm agglutinin conjugated to horseradish-peroxidase into the topographically defined hand area of the primary motor cortex were quantitatively studied via electron microscopy. Cortical afferent terminals within pars parvicellularis and pars magnocellularis synapse upon all regions of the dendritic arbors of rubral projection neurons. However, the majority of these labeled afferents synapse upon thin-diameter shafts or presumed spinous processes of rubral distal dendrites as well as upon vesicle-containing profiles of presynaptic dendrites of local circuit interneurons that are gamma-aminobutyric acid-immunoreactive, as identified by postembedding immunohistochemistry. Synaptic contacts formed by the labeled cortical terminal were large in width and extended through several serial sections. Synaptic contacts formed by the presynaptic dendritic profiles, on the other hand, were more punctate and could be seen in only one or two serial sections. These latter synaptic interactions probably provide a modification of the effects of cortical input to rubral projection neurons as suggested by previous physiological studies that indicated the dominance of cortical input onto distal dendrites as well as involvement with inhibitory circuits. An example of the complexities of these synaptic interactions is further demonstrated by a three-dimensional computer reconstruction. This quantitative study of corticorubral afferents in the macaque monkey provides insight into the interactions of cerebral cortical afferents with rubral projection neurons and their relationship with local circuit inhibitory interneurons to elucidate the role played by the cortex in the activation of rubral neurons.

Olivary projecting neurons in the nucleus of Darkschewitsch in the cat receive excitatory monosynaptic input from the cerebellar nuclei

The direct projection from the cerebellar nuclei to the inferior olive is GABAergic. In the present study, we investigated the projection from the cerebellar nuclei to the mesodiencephalic junction which is known to provide an excitatory projection to the inferior olive. The mesodiencephalic junction was studied in cat following anterograde transport of wheatgerm agglutinated horseradish peroxidase from the cerebellar nuclei in combination with: (1) retrograde transport of gold-lectin conjugate from the inferior olive; and (2) postembedding GABA-immunocytochemistry. Light microscopic analysis revealed that overlap of the anterograde and retrograde labeling was especially prominent in the nucleus of Darkschewitsch. Electron microscopic examination of this area showed: (1) that many cerebellar terminals made synaptic contacts with neurons that project to the inferior olive; (2) that virtually all cerebellar terminals were non-GABAergic and displayed an excitatory morphology; and (3) olivary projecting neurons were non-GABAergic. It is concluded that the indirect cerebellar projection to the inferior olive via the nucleus of Darkschewitsch is disynaptic and excitatory.

Changes in motor performance and rubral single unit activity in cats after microinjections of serotonin into the red nucleus area

The serotonergic control exerted on the red nucleus (RN) was studied in unrestrained cats during the performance of a simple reaction time task which consisted of releasing a lever in response to an auditory go-signal. The effects of microinjections of serotonin-oxalate salt into the rubral area on the motor activity and on the firing of neurons recorded concomitantly in the red nucleus were investigated. Injections of serotonin (5-HT) (200-400 ng) into the red nucleus or its dorsal border induced subtle alterations in the conditioned motor performances but had no major effects on the spontaneous motor behavior. The changes in the conditioned motor output (an increase in the static force exerted on the lever and a speeding up of the lever release) are reminiscent of the facilitatory influence of serotonin on various motor reflexes previously reported. Changes in the neuronal activity were observed concomitantly with the effects on the motor output: 5-HT either enhanced or reduced the firing rate of the rubral neurons. These effects were apparently dependent on the discharge pattern of the neurons during the static motor activity. The results suggest that the serotonergic input to the red nucleus may participate in motor control by exerting a dual modulatory action on the activity of rubral neurons.

Distribution of motor cortical neuron synaptic terminals on monkey parvocellular red nucleus neurons

We determined the location of 54 horseradish peroxidase (HRP)-labeled motor cortical neuron synaptic terminals on 17 parvocellular neurons in the monkey red nucleus. Synaptic terminals and their postsynaptic elements were identified and reconstructed, using light- and electron-microscopic techniques, from serial thick and thin sections. Terminals were found on proximal and distal dendrites of small and medium-sized parvocellular neurons, where they formed excitatory synapses. Some were 180 microns from cell somata. Approximately half of the labeled terminals, aside from those located at dendritic origins, were situated strategically at or near dendritic branch points. Since monkey parvocellular neurons show little activity during movement, the obvious next question is this: How and in what way does motor cortex influence these cells?