Red nuclear and interposate nuclear excitation of pontine nuclear cells

A novel inhibitory nucleo-cortical circuit controls cerebellar Golgi cell activity

The cerebellum, a crucial center for motor coordination, is composed of a cortex and several nuclei. The main mode of interaction between these two parts is considered to be formed by the inhibitory control of the nuclei by cortical Purkinje neurons. We now amend this view by showing that inhibitory GABA-glycinergic neurons of the cerebellar nuclei (CN) project profusely into the cerebellar cortex, where they make synaptic contacts on a GABAergic subpopulation of cerebellar Golgi cells. These spontaneously firing Golgi cells are inhibited by optogenetic activation of the inhibitory nucleo-cortical fibers both in vitro and in vivo. Our data suggest that the CN may contribute to the functional recruitment of the cerebellar cortex by decreasing Golgi cell inhibition onto granule cells.

A hypothetical universal model of cerebellar function: reconsideration of the current dogma

The cerebellum is commonly studied in the context of the classical eyeblink conditioning model, which attributes an adaptive motor function to cerebellar learning processes. This model of cerebellar function has quite a few shortcomings and may in fact be somewhat deficient in explaining the myriad functions attributed to the cerebellum, functions ranging from motor sequencing to emotion and cognition. The involvement of the cerebellum in these motor and non-motor functions has been demonstrated in both animals and humans in electrophysiological, behavioral, tracing, functional neuroimaging, and PET studies, as well as in clinical human case studies. A closer look at the cerebellum's evolutionary origin provides a clue to its underlying purpose as a tool which evolved to aid predation rather than as a tool for protection. Based upon this evidence, an alternative model of cerebellar function is proposed, one which might more comprehensively account both for the cerebellum's involvement in a myriad of motor, affective, and cognitive functions and for the relative simplicity and ubiquitous repetitiveness of its circuitry. This alternative model suggests that the cerebellum has the ability to detect coincidences of events, be they sensory, motor, affective, or cognitive in nature, and, after having learned to associate these, it can then trigger (or "mirror") these events after having temporally adjusted their onset based on positive/negative reinforcement. The model also provides for the cerebellum's direction of the proper and uninterrupted sequence of events resulting from this learning through the inhibition of efferent structures (as demonstrated in our lab).

Regional (14C) 2-deoxyglucose uptake during forelimb movements evoked by rat motor cortex stimulation: cortex, diencephalon, midbrain

The caudal forelimb region of right "motor" cortex was repetitively stimulated in normal, conscious rats. Left forelimb movements were produced and (14C) 2-deoxyglucose (2DG) was injected. After sacrifice, regions of increased brain (14C) 2DG uptake were mapped autoradiographically. Uptake of 2DG increased about the stimulating electrode in motor (MI) cortex. Columnar activation of primary (SI) and second (SII) somatosensory neocortex occurred. The rostral or second forelimb (MII) region of motor cortex was activated. Many ipsilateral subcortical structures were also activated during forelimb MI stimulation (FLMIS). Rostral dorsolateral caudate-putamen (CP), central globus pallidus (GP), posterior entopeduncular nucleus (EPN), subthalamic nucleus (STN), zona incerta (ZI), and caudal, ventrolateral substantia nigra pars reticulata (SNr) were activated. Thalamic nuclei that increased (14C) 2DG uptake included anterior dorsolateral reticular (R), ventral and central ventrolateral (VL), lateral ventromedial (VM), ventral ventrobasal (VB), dorsolateral posteromedial (POm), and the parafascicular-centre median (Pf-CM) complex. Activated midbrain regions included ventromedial magnocellular red nucleus (RNm), posterior deep layers of the superior colliculus (SCsgp), lateral deep mesencephalic nucleus (DMN), nucleus tegmenti pedunculopontinus (NTPP), and anterior pretectal nucleus (NCU). Monosynaptic connections from MI or SI to SII, MII, CP, STN, ZI, R, VL, VM, VB, POm, Pf-CM, RNm, SCsgp, SNr, and DMN can account for ipsilateral activation of these structures. GP and EPN must be activated polysynaptically, either from MI stimulation or sensory feedback, since there are no known monosynaptic connections from MI and SI to these structures. Most rat brain motor-sensory structures are somatotopically organized. However, the same regions of R, EPN, CM-Pf, DMN, and ZI are activated during FLMIS compared to VMIS (vibrissae MI stimulation). Since these structures are not somatopically organized, this suggests they are involved in motor-sensory processing independent of which body part is moving. VB, SII, and MII are activated during FLMIS but not during VMIS.

Regional (14C) 2-deoxyglucose uptake during forelimb movements evoked by rat motor cortex stimulation: pons, cerebellum, medulla, spinal cord, muscle

Electrical stimulation of the right forelimb motor (MI) sensory (SI) cortex in normal, adult rats produced repetitive left forelimb movements. Regions of increased (14C) 2-deoxyglucose (2DG) uptake were mapped auto-radiographically during these movements. MI stimulation activated the ipsilateral reticular tegmental pontine nucleus (RTP) and the middle (rostral-caudal) third of the pontine nuclei including pyramidal (P), medial (POM), ventral (POV), and lateral (POL) pontine nuclei. The ipsilateral inferior olivary complex was activated including dorsal accessory olive (DAO), principal olive (PO), and medial accessory olive (MAO). The contralateral lateral reticular (LR) nucleus and nucleus cuneatus (CU) were activated. Lateral vermal, paravermal, and hemispheric portions of the contralateral cerebellum were also activated. Parts of vermian lobules IV, V, VI, VII, and VIII, and lobulus simplex, crus I, crus II, paramedian lobule, and copula pyramidis were activated. Granule cell layers were activated much more than molecular layers. Discrete microzones of high granule cell 2DG uptake alternated with zones of low uptake in left paramedian lobule and copula pyramidis and may correlate with the fractured cerebellar somatotopy described physiologically by Welker and his associates. Portions of the left lateral and interpositus nuclei were metabolically activated. Medial portions of laminae I-VI were activated in the dorsal horn of cervical spinal cord. The 2DG uptake was either unchanged or decreased in the ventral horn. Thoracic and lumbar spinal cord were not activated. Monsynaptic MI and SI connections to P, POM, POV, POL, RTP, DAO, PO, MAO, LR, CU, and spinal cord could account for activation of those structures. However, there are no direct MI or SI connections to the deep cerebellar nuclei, the cerebellar hemisphere, or the muscles. Activation of these structures must be due to activation of polysynaptic pathways, sensory feedback from the moving forelimb, or both. The present experiments cannot distinguish these possibilities. Comparison of the regions activated during forelimb MI stimulation (FLMIS) to those activated during vibrissae MI stimulation (VMIS) suggests that the pontine nuclei, cerebellar hemisphere, and possibly the deep cerebellar nuclei are somatotopically organized. RTP, LR, CU, and spinal cord were activated during FLMIS but were not activated during VMIS. The failure to activate the ventral horn of cervical spinal cord may be due to known inhibition of alpha-motor neurons during motor cortex stimulation.

Learning- and cerebellum-dependent neuronal activity in the lateral pontine nucleus

The effects of inactivation of cerebellar deep nuclei and the lateral pontine nucleus on classical eyeblink conditioning with tone or lateral reticular nucleus (LRN) stimulation as conditioned stimuli (CSs) were examined. Inactivation of cerebellar deep nuclei abolished eyeblink conditioned responses (CRs) when the CS was either a tone or LRN stimulation. Inactivation of the lateral pontine nucleus prevented only the acquisition and retention of tone-evoked eyeblink CRs. Multiple-unit recording demonstrated that when LRN stimulation was used as the CS, inactivation of the interpositus nucleus abolished learning-related neuronal activity in the lateral pontine nucleus, whereas inactivation of pontine nucleus had little effect on similar activity in the interpositus nucleus. Thus, the learning-induced neuronal activity in the lateral pontine nucleus was most likely driven by the cerebellar interpositus nucleus.

Electrophysiological properties of rat pontine nuclei neurons In vitro II. Postsynaptic potentials

We investigated the postsynaptic responses of neurons of the rat pontine nuclei (PN) by performing intracellular recordings in parasagittal slices of the pontine brain stem. Postsynaptic potentials (PSPs) were evoked by brief (0.1 ms) negative current pulses (10-250 microA) applied to either the cerebral peduncle or the pontine tegmentum. First, excitatory postsynaptic potentials (EPSPs) could be evoked readily from peduncular stimulation sites. These EPSPs exhibited short latencies, a nonlinear increment in response to increased stimulation currents, and an unconventional dependency on the somatic membrane potential. Pharmacological blockade of the synaptic transmission using 6,7-dinitroquinoxaline-2, 3-dione and ,-2-amino-5-phosphonovaleric acid, selective antagonists of the alpha-amino-3-hydroxy-5-methyl-4-isoxazilepropionate- (AMPA) and the N-methyl--aspartate (NMDA)-type glutamate receptors, showed that these EPSPs were mediated exclusively by excitatory amino acids via both AMPA and NMDA receptors. Moreover, the pharmacological experiments indicated the existence of voltage-sensitive but NMDA receptor-independent amplification of EPSPs. Second, stimulations at peduncular and tegmental sites also elicited inhibitory postsynaptic potentials (IPSPs) in a substantial proportion of pontine neurons. The short latencies of all IPSPs argued against the participation of inhibitory interneurons. Their sensitivity to bicuculline and reversal potentials around -70 mV suggested that they were mediated by gamma-aminobutyric acid-A (GABAA) receptors. In addition to single PSPs, sequences consisting of two to four distinct EPSPs could be recorded after stimulation of the cerebral peduncle. Most remarkably, the onset latencies of the following EPSPs were multiples of the first one indicating the involvement of intercalated synapses. Finally, we used the classic paired-pulse paradigm to study whether the temporal structure of inputs influences the synaptic transmission onto pontine neurons. Pairs of electrical stimuli applied to the cerebral peduncle resulted in a marked enhancement of the amplitude of the second EPSP for interstimulus intervals of 10-100 ms. Delays >200 ms left the EPSP amplitude unaltered. These data provide evidence for a complex synaptic integration and an intrinsic connectivity within the PN too elaborate to support the previous notion that the PN are simply a relay station.

Electrophysiological properties of rat pontine nuclei neurons In vitro. I. Membrane potentials and firing patterns

We used a new slice preparation of rat brain stem to establish the basic membrane properties of neurons in the pontine nuclei (PN). Using standard intracellular recordings, we found that pontine cells displayed a resting membrane potential of -63 +/- 6 mV (mean +/- SD), an input resistance of 53 +/- 21 MOmega, a membrane time constant of 5.3 +/- 2.4 ms and were not spontaneously active. The current-voltage relationship of most of the PN neurons showed the characteristics of inward rectification in both depolarizing and hyperpolarizing directions. A prominent feature of the firing of pontine neurons was a marked firing rate adaptation, which eventually caused the cells to cease firing. Several types of membrane conductances possibly contribute to this feature. For one, a medium and a slow type of afterhyperpolarization (AHP) control the pattern of firing. The medium AHP was partly susceptible to blockade of calcium influx, whereas it was abolished completely by blockade of potassium channels with tetraethylammonium, indicating that it is based on at least two conductances: a calcium-dependent and a calcium-independent one. The slow AHP was carried by potassium ions and could be blocked effectively by preventing calcium influx into the cell. It was present after single spikes but was strongest after a high-frequency spike train. Calcium entry into the cell was mediated by high-threshold calcium channels that were detected by the generation of calcium spikes under blockade of potassium channels. Furthermore, the early phase of the firing rate adaptation was shown to be related to the time course of a slow, tetrodotoxin (TTX)-sensitive, persistent sodium potential, which was activated already in the subthreshold range of membrane potentials. This potential was time dependent and imposed as a depolarizing "hump" with a maximum occurring in most cases between 50 and 100 ms after stimulus onset. In the suprathreshold range, it generated plateau potentials following fast spikes, if potassium channels were blocked. After the complete adaptation of the firing rate, PN neurons were observed to display irregular fluctuations of the membrane potential, which sometimes reached firing threshold thereby eliciting an irregular low-frequency spike train. As these fluctuations could be blocked with TTX, they probably are based on the persistent sodium currents. The opposing drive in hyperpolarizing direction may be provided by strong outward currents that generated a marked outward rectification in the current-voltage relationship under TTX. In conclusion, PN neurons show complex membrane properties that are reminiscent in many ways to cerebrocortical "regular firing" neurons.

Ultrastructural study of the GABAergic and cerebellar input to the nucleus reticularis tegmenti pontis

The nucleus reticularis tegmenti pontis is an intermediate of the cerebrocerebellar pathway and serves as a relay centre for sensorimotor and visual information. The central nuclei of the cerebellum provide a dense projection to the nucleus reticularis tegmenti pontis, but it is not known to what extent this projection is excitatory or inhibitory, and whether the terminals of this projection contact the neurons in the nucleus reticularis tegmenti pontis that give rise to the mossy fibre collaterals innervating the cerebellar nuclei. In the present study the nucleus reticularis tegmenti pontis of the cat was investigated at the ultrastructural level following anterograde and retrograde transport of wheat germ agglutinin coupled to horseradish peroxidase (WGA-HRP) from the cerebellar nuclei combined with postembedding GABA immunocytochemistry. The neuropil of this nucleus was found to contain many WGA-HRP labeled terminals, cell bodies and dendrites, but none of these pre- or postsynaptic structures was double labeled with GABA. The vast majority of the WGA-HRP labeled terminals contained clear spherical vesicles, showed asymmetric synapses, and contacted intermediate or distal dendrites. Many of the postsynaptic elements of the cerebellar afferents in the nucleus reticularis tegmenti pontis were retrogradely labeled with WGA-HRP, while relatively few were GABAergic. We conclude that all cerebellar terminals in the nucleus reticularis tegmenti pontis of the cat are nonGABAergic and excitatory, and that they contact predominantly neurons that project back to the cerebellum. Thus, the reciprocal circuit between the cerebellar nuclei and the nucleus reticularis tegmenti pontis appears to be well designed to function as an excitatory reverberating loop.

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.

The learning-related activity that develops in the pontine nuclei during classical eye-blink conditioning is dependent on the interpositus nucleus

A growing body of research now implicates the cerebellum in the formation and storage of the critical neural plasticity that subserves the classically conditioned eye-blink response. Previous anatomical, physiological, and behavioral research suggests that auditory-conditioned stimulus information is routed to the cerebellum by the pontine nuclei. However, it has also been observed from multiple unit recordings that some populations of pontine cells, in addition to showing auditory-evoked responses, also show changes in activity that is learning-related. It is unknown whether this learning-related activity is generated by the pontine cells or whether it is generated by some other structure and projected to the pontine nuclei. Because the cerebellum has been implicated in the formation of the essential plasticity that subserves this learned behavior, we examined how multiple unit recordings of learning-related activity within the pontine nuclei are affected by reversible inactivation of the interpositus nucleus of the cerebellum. The results indicated clearly that when the interpositus nucleus was inactivated, the learning-related activity in the pontine nuclei was abolished completely and the auditory stimulus-evoked activity was unaffected. In contract, when the facial nucleus was inactivated, both the auditory stimulus and the learning-related activity were still present. These results indicate that the learning-related activity exhibited by some populations of pontine nuclei cells is dependent on the interpositus nucleus and may represent feedback from the cerebellum.

The cerebellopontine system: an electrophysiological study in the rat

We examined the effects of electric stimulation of the cerebellar lateral nucleus (LN) in the rat on the activity of single pontocerebellar neurons in the basilar pontine nuclei (BPN) and the reticulotegmental nucleus (RtTg). We found that about half of the cells of these nuclei that were influenced by LN stimulation were inhibited. A significant fraction of both excitatory and inhibitory responses had latencies of less than 4 ms and were able to follow high frequency stimulation, compatible with a monosynaptic linkage. Extracellular field potential recordings within the BPN and RtTg were interpreted as arising from impulses propagating along inhibitory axons projecting in a bundle from the cerebellum to these pontine structures. Microiontophoretic administration of GABA antagonists bicuculline or picrotoxin abolished or attenuated most inhibitory effects. Therefore, we conclude that LN-induced inhibition is most likely mediated by cerebellopontine GABAergic fibers. The functional significance of this cerebellopontine inhibitory circuit is discussed.

The cerebellopontine system in the rat. II. Electron microscopic studies

Cerebellopontine axonal boutons in the neuropil of the basilar pontine nuclei (BPN) were marked for ultrastructural identification by producing unilateral electrolytic lesions of the superior cerebellar peduncle (SCP) as it exited from the cerebellum and before its decussation in the caudal midbrain. Three varieties of degenerating boutons were distinguished on the basis of size, type of degeneration, and postsynaptic locus. A relatively large variety of bouton (2.5-6.0 microns) that exhibited filamentous degeneration throughout the range of survival times employed (1-14 days) was the most frequently observed type of degenerating cerebellopontine bouton. Such boutons formed synaptic contacts with several small, dendritic, spinelike profiles as well as the shafts of intermediate or proximal dendrites. A second, far less numerous and somewhat smaller type of bouton (1.5-4.5 microns) was distinguished by the fact that it exhibited advanced dark degenerative changes after a 2-day survival period, formed multiple spine contacts (but not shafts), and was no longer apparent in the neuropil after a postlesion survival time of 6 days. The third variety of degenerating bouton was small (0.8-2.0 microns), exhibited dark degeneration with a 2-6 day survival period, contacted primarily shafts of small-diameter dendrites, and was observed more frequently than the larger dark boutons but less often than the large filamentous boutons. All three types of degenerating boutons contained round, clear, synaptic vesicles and formed only asymmetric synaptic active sites. It is suggested that the three types of degenerating axon terminals arise from at least three varieties of neurons in the deep cerebellar nuclei. Further it is suggested that such boutons originate from cerebellar efferent axons which distribute in collateral fashion to the thalamus, red nucleus, and/or the inferior olive.

The cerebellopontine system in the rat. I. Autoradiographic studies

This study utilized light microscopic autoradiographic procedures to describe the projections from the three major subdivisions of the deep cerebellar nuclei (DCN) to the basilar pontine nuclei (BPN). Although the vast majority of cerebellopontine axons reached the BPN via the descending limb of the brachium conjunctivum (BC) after crossing the midline within the midbrain, a relatively small number of ipsilaterally projecting fibers was also observed. Fascicles of cerebellopontine axons left the main bundle of descending limb fibers throughout much of the rostrocaudal length of the BPN and passed around and through the medial lemniscus and cerebral peduncle to enter the pontine gray. The lateral cerebellar nucleus gave rise to the largest number of cerebellopontine fibers, whose terminal fields exhibited both diffuse and patchlike labeling patterns within each of the major subdivisions of the BPN including medial, ventral, lateral, and dorsal areas. Projections from the interpositus complex exclusive of its posterior division were fewer and less widely distributed than those from the lateral nucleus. Interpositopontine fibers terminated primarily in the caudal one-half of the BPN in medial, ventral, and lateral regions and overlapped somewhat with projections from the lateral cerebellar nucleus. Pontine projections emanating from the medial cerebellar nucleus were the fewest and most restricted in distribution relative to the other two cerebellar efferent systems. Such fibers formed a patchlike network of terminal fields which extended throughout much of the rostrocaudal length of the BPN in medial and dorsomedial regions. A relatively small but considerable number of ipsilateral cerebellopontine fibers terminated in pontine regions, which often mirrored the typical contralateral projection fields. Although it proved difficult to determine the precise origin of the ipsilateral fiber systems, it appeared that each of the three major DCN subdivisions made some contribution. Also it was apparent that considerable overlap existed between cerebellopontine projection zones and those of other pontine afferents including sensorimotor, visual, and auditory cortices, the superior colliculus, and the mammillary nuclei of the hypothalamus. Moreover, cerebellopontine terminal fields were congruent in some instances with discrete clusters of BPN neurons which serve as the source of pontocerebellar fiber systems, reaching portions of the lateral cerebellar hemispheres, posterior vermis, and the paraflocculus.

A two compartment model of the stepping generator: analysis of the roles of a stage-setter and a rhythm generator

Recent studies on locomotion of the mesencephalic cat demonstrated that activation of the spinal stepping generator and the postural control system are dependent phenomena (Mori et al., 1978, 1980). This has motivated the construction of a new model of the stepping generator to account for interactions with the postural control system. The present model consists of two main compartments, the rhythm generator and the stage-setter. The rhythm generator generates rhythmic bursting discharges of extensor and flexor alpha motoneurons. The function of the stage-setter is to set and reset the excitability of extensor alpha motoneuron to a number of desired levels. This study analyzes interactions in this model between rhythm generating and postural control system. By adding a concept of "stage-setting" to the rhythm generator model, we succeed in simulating a variety of locomotor patterns observed in the mesencephalic cat, including "stepping automatism" (Mori et al., 1979).

Reciprocal connections between the nucleus interpositus of the cerebellum and precerebellar nuclei

Reciprocal excitatory connections which give rise to reverberatory activity were investigated by intracellular recording and retrograde horseradish peroxidase (HRP) transport. Neurons in the interpositus nucleus (IP) were activated antidromically from the nucleus reticularis tegmenti pontis (NRTP) and paramedian reticular nucleus (PMRN). Stimulation of these nuclei and lateral reticular nucleus (LRN) elicited monsynaptic EPSPs in IP neurons. PMRN neurons could be activated antidromically as well as orthodromically from IP. HRP-positive neurons were found in NRTP, PMRN and LRN following injection of HRP into IP. Neuronal connections of IP-NRTP-IP, IP-PMRN-IP and IP-RN-LRN-IP were suggested as possible components of reverberating circuits.

Electron microscopic identification of cerebellopontine axon terminals in the opossum

Following unilateral cerebellar nuclear ablation or transection of the brachium conjunctivum, degenerating axon terminals were identified within the pontine nuclei of adult opossums. Most frequently observed was a category of large boutons (1.5-7.5 microns) exhibiting an early filamentous reaction (1-5 days survival) and later becoming electron dense and shrunken (9-12 days survival) while being engulfed by phagocytic elements. Such boutons characteristically were found nestled within a cluster of spine-like projections taking origin from somata as well as proximal and intermediate dendrites. A smaller variety of dark degenerating boutons (0.5-2.0 micron) was observed after survival periods of intermediate length (6-10 days) and although there was some overlap in size with the smallest filamentous boutons, the majority (71%) were clearly less than 1.5 micron in their greatest dimension. The small dark boutons formed synaptic contacts only with the shafts of intermediate and distal dendrites rather than the claw-like dendritic complex apposed to the large filamentous degenerating boutons. Because of this difference in postsynaptic locus and their small size it was suggested that such boutons might represent the terminals of a second population of cerebello-pontine axons. Such observations lead to the hypothesis that the large filamentous endings contacting the distinctive claw-like somal or dendritic projections from axons of relatively large cerebellar nuclear neurons which also project rostrally to the red nucleus and thalamus where they form similar boutons and synaptic complexes. On the other hand, the small dark boutons may have arisen from small projection-type cerebellar nuclear neurons, the majority of whose axons project caudally to the inferior olive after contributing a relativley small number of collateral branches to the pontine nuclei.

Morphological and electrophysiological characteristics of projection neurons in the nucleus interpositus of the cat cerebellum

The populations of neurons in the nucleus interpositus (IP) of the cat cerebellum which project to the ventral lateral nucleus of the thalamus (VL), the red nucleus (RN), the nucleus reticularis tegmenti pontis (NRTP), the pontine nuclei (PN), the inferior olive (IO), and the cerebellar cortex were identified by intracellular and extracellular injections of HRP and studied electrophysiologically. When HRP was simultaneously injected into the VL, RN, and IO, over 95% of the neurons in the IP nuclei were labeled; indicating that there are few, if any, local circuit neurons. The vast majority (86%) of the larger IP neurons (soma length greater than or equal to 20 micrometer) project rostrally to the RN and thalamus. These neurons typically have long, relatively spine free dendrites and axons which in a few cases gave rise to recurrent collaterals. Two intracellularly stained projection neurons which had exceptionally long spiny dendrites had axons which gave rise to nucleocortical collaterals in addition to several local collaterals. IP neurons projecting to the NRTP and PN were located primarily in the lateral aspect of the nucleus interpositus anterior. Electrophysiological experiments established that neurons projecting to the NRTP also project to the VL. The IP neurons projecting to the IO have small fusiform or multipolar somata, long thin dendrites, and receive excitatory inputs from the IO. At least 73% of the small neurons in the IP project to the IO, and some of these, in addition, project to the VL. There are at least three morphologically distinguishable populations of projection neurons, small IO projections neurons, and neurons with nucleocortical collaterals. The projection of the IP to diverse regions of the brain is accomplished mainly by axon collateralization, but regional and morphological specialization also play a role in the organization of the output of the IP.

Electrophysiological and horseradish peroxidase studies of precerebellar afferents to the nucleus interpositus anterior. I. Climbing fiber system

The input to the nucleus interpositus anterior (NIA) of the cat from the inferior olive (IO) was studied by stimulating the IO while recording intracellularly from the NIA, and by the retrograde transport of horseradish peroxidase (HRP). Stimulation of the IO evoked monosynaptic EPSPs in NIA neurons. The cells labeled in the IO following electrophoretic and pressure injection of HRP into NIA were located in the rostral parts of the dorsal and medial accessory olive. Stimulation of the IO also polysynaptically evoked an IPSP and a late disinhibitory depolarization. Data were presented which indicated that these potentials were mediated by the Purkinje cells of the cerebellar cortex.

Electrophysiological and horseradish peroxidase studies of precerebellar afferents to the nucleus interpositus anterior. II. Mossy fiber system

Inputs to the nucleus interpositus anterior (NIA) of the cat from precerebellar nuclei which are thought to give rise to mossy fibers were studied using electrophysiological and anatomical techniques. Stimulation of one of these precerebellar nuclei, the lateral reticular nucleus (LRN) evoked monosynaptic EPSPs in NIA neurons. These EPSPs were followed by polysynaptic IPSPs and late depolarization mediated by the response of the cerebellar cortex. Similar responses were occasionally seen following stimulation of the brachium pontis (BP). When horseradish peroxidase was injected into the NIA, labeled cells were found in the magnocellular and parvicellular LRN, the external cuneate nucleus (ECN), the pontine nuclei and the perihypoglossal nuclei. There was no evidence for a direct projection of the nucleus reticularis pontis to the NIA. It was suggested that most of the tonic excitation of NIA neurons is provided by the LRN and ECN.

Brachium conjunctivum-red nucleus synaptic system in the baboon

Electrophysiological properties of the brachium conjunctivum-red nucleus (BC-RN) synaptic system were studied in barbiturate-anesthetized baboon (Papio). Topographic recordings from the mesencephalon demonstrated that most of the BC-evoked activity was restricted to an area histologically identified as red nucleus; however, some brachium-evoked activity was recorded from the surrounding mesencephalic reticular formation. Short latency BC-evoked activity was also recorded from the pons in the region of nucleus reticularis tegmenti pontis. The majority of the BC fibers were found to conduct at a rate of 44 m/sec; a second group of BC fibers with a slower conduction velocity of 23 m/sec was also observed. Brachium-evoked responses recorded from magnocellular and pravicellular RN neurons were short latency responses consistent with monosynaptic activation of these RN neurons by the BC fibers. The BC-RN synaptic system was found to be a very secure synaptic system and could transmit activity at high rates of stimulation with little or no failure. The responsiveness of the BC fibers was found to be similar to that of optic nerve fibers and pyramidal tract fibers, both of which have been characterized as being similar to peripheral A fibers. The responsiveness of the BC-RN synaptic system began to decrease 5 msec after a single or repetitive transmission and was reduced to about 50% of normal responsiveness at 34 msec. This period of reduced postsynaptic responsiveness was associated with a reduction in presynaptic input to RN and suggest that a disfacilitation at the level of the deep cerebellar nuclei may be in part responsible for the subnormal responsiveness of the BC-RN synaptic system.

The mode of synaptic linkage in the cerebro-ponto-cerebellar pathway of the cat. I. Responses in the brachium pontis

Cerebrally-induced responses of pontine nuclei cells (PN cells) were studied in cats anesthetized with pentobarbitone sodium and with the midbrain transected bilaterally sparing only the cerebral penuncles. After stimulating the subcortical white matter, the internal capsule or the cerebral peduncle, mass potentials were recorded from the cut end of fibres in the brachium pontis (BP) and in the pyramid at the level of the trapezoid body. These potentials were regarded as indicating, respectively, the size of an output volley of PN cells and the size of its causative input volley through the pyramidal tract. BP responses consisted of short- and long-latency potentials which were caused by fast and slow conducting pyramidal tract volleys, respectively. The input-output relations for fast component responses took a characteristic S-shaped form resembling those obtained from the monosynaptic spinal reflex. The input-output relations for slow component responses were almost linear. Both fast and slow BP responses were remarkably potentiated after single or relatively brief repetitive peduncular stimulation, but were depressed after long-lasting high-frequency activation. During repetitive stimulation with varied frequencies, fast and slow BP responses showed different patterns of frequency-dependence of their amplitudes. These results suggest the existence of two separate transmission lines with different properties in cerebro-ponto-cerebellar pathways.

The basilar pontine gray of the opossum: a correlated light and electron microscopic analysis

Neurons within the basilar pontine gray (BPG) of the American opossum can be subdivided into four major nuclei which are named medial, lateral, ventral and peripeduncular in accordance with previous studies. In addition, several smaller subnuclei, such as the median and dorsolateral cell groups, are present, as well as two longitudinal columns of neurons within the ventral nucleus. Neurons in the BPG range in size from 9 to 35 mu and appear randomly distributed so that none of the subdivisions contains exclusively nerve cells of the same perikaryal dimension. Projection neurons as shown in Golgi impregnations have a variable dendritic pattern; those in peripeduncular zones exhibit dendrites closely applied to the surface of the cerebral peduncle, whereas those in other regions generally have a radial type of arrangement. Certain projection neurons can be distinguished on the basis of their dendritic surface, which bears either claw-like protrusions or stalked appendages. Smaller nerve cells measuring less than 18 mu may be intrinsic neurons, since axon-like processes arise from their dendrites and course for some distance near the parent cell before becoming thin and beaded. Ultrastructural observations show profiles of neurons comparable in size to those seen in Golgi impregnations and suggest at least four classes of presynaptic profiles. One category ranges in size from 2 to 8 mu, contains round vesicles (average diameter 450 A) and characteristically forms multiple asymmetric synaptic contacts with several small postsynaptic profiles, some of which appear to be the dendritic claws mentioned above. The other three types of axon terminals measure less than 2 mu in their greatest dimension and are distinguished by their synaptic vesicles; one group containing round vesicles with an average diameter of 380 A, a second group exhibiting larger round vesicles with an average diameter of 500 A and a third group containing flatened or eliptical vesicles. Transection of the superior cerebellar peduncle produces early filamentous and later electron dense degenerative changes in some, but not all, of the larger types of presynaptic profiles. Subsequent to large motor-sensory cortex ablations both filamentous and dark degenerating profiles are simultaneously observed at all survival times. In one case with a cortical lesion restricted to the motor-sensory cortex, mainly dark degenerating terminals are apparent in the ipsilateral pontine gray, whereas in a lesion confined to the visual cortex only filamentous degeneration was observed. It is suggested, therefore, that some of the dark degenerating profiles represent the terminals of collaterals of corticospinal axons and the filamentous boutons are terminal expansions of direct corticopontine fibers.