The projection from nucleus reticularis tegmenti pontis onto the cerebellum in the cat

Summary Following stereotactically performed lesions in nucleus reticularis tegmenti pontis (N.r.t.) degenerating fibers are traced to the contralateral N.r.t., to the pontine nuclei, through brachium pontis to restricted areas of the cerebellar nuclei and to most parts of the cerebellar cortex where they terminate in the granular layer. Most degenerating fragments are found in the contralateral half of the cerebellum with the greatest density in the vermal lobules VI and VIIA and in the flocculus.Following injections of HRP in the various cerebellar lobules labeled cells are mainly present within limited groups in the N.r.t.. Injections in vermal lobules VI-VIII B give rise to labeled cells within circumscribed areas in the dorsal and ventral parts throughout the rostrocaudal extent of the N.r.t.. In cases with injections in lobule IX or the ventral paraflocculus labeled cells are found ventrally in the rostral half of the N.r.t., while following injections in the vermal lobules I-V labeled cells are mainly found in the ventral and caudal part of the N.r.t.. Following injections in the intermediate and lateral parts of the anterior lobe, Crus I and II, the paramedian lobule and the dorsal paraflocculus labeled cells occur within groups in medial and lateral parts throughout the rostrocaudal extent of the N.r.t.. Following injections in the flocculus labeled cells are found in a very distinct group in the dorsal and rostral part of the N.r.t., While an injection in the nodulus (lobule X) gave rise to a smaller group of labeled neurons in the dorsolateral corner in the caudal part of the N.r.t.. Labeled cells within processus tegmentosus lateralis (p.t.l.) are only found following injections in lobules VI-VIIIA, Crus I and II and the dorsal paraflocculus.From what is known about afferents to the N.r.t., it is concluded that no cerebellar lobule gets information from one only of these sources via the N.r.t.. Visual information can probably be mediated from the superior colliculus via the N.r.t. to the flocculus and to a minor extent to the vermal lobules VI-VIII B, and from the pretectum via the N.r.t. to both vermal and lateral parts of the cerebellum.     

Vestibular nuclear efferents to the nucleus raphe pontis, the nucleus reticularis tegmenti pontis and the nuclei pontis in the cat

The projection of the vestibular nuclei to the pontine tegmentum was investigated by means of anterograde transport of tritiated leucine. Dense patches of terminal labeling were observed in the contralateral nucleus raphe pontis and the nucleus reticularis tegmenti pontis in cases where the injection involved the medial and descending vestibular nuclei. Following injections in the superior vestibular nucleus and group Y, weaker termination, also patchlike, was observed in the same tegmental nuclei, and in addition in the dorsomedial pontine nuclei proper. The results are discussed in relation to the position of the nucleus raphe pontis and the nucleus reticularis tegmenti pontis in the oculomotor pathways.     

The projection from the nucleus reticularis tegmenti pontis to the cerebellum in the rhesus monkey

Summary Following injections of horseradish peroxidase in various parts of the monkey cerebellum, the distribution of labelled cells in the nucleus reticularis tegmenti pontis (NRT) has been studied. As a rule, labelled cells are found at all rostrocaudal levels of the NRT regardless of the injection size and site. The densest projection from NRT seems to reach the vermal visual area (lobulus VII), a less dense projection supplies the anterior lobe while the paramedian lobule receives a more sparse projection than the anterior lobe. Very few labelled cells were found in the NRT after injections of crus I and II. The projection is topographically organized so that the dorsomedial part of the NRT supplies lobulus VII, a large central region sends fibres to the anterior lobe (and the cerebellar hemispheres), while the lateral extension of NRT (processus tegmentosus lateralis) is connected with the paramedian lobule.The results of the present study are compared with those of a preceding one of cortical afferents to NRT. It is concluded that the cortical areas projecting to the NRT seem likely to exert their influences on largely different parts of the cerebellum via the NRT. The present results are also discussed inrelation to termination of other afferent contingents in the NRT, and it is concluded that the NRT is not homogenous anatomically, and consequently different parts of the nucleus would be expected to play somewhat different functional roles.     

Electrophysiological properties of nucleus reticularis tegmenti pontis cells: Antidromic and synaptic activation

Summary Antidromically and synaptically activated spike and synaptic potentials in the nucleus reticularis tegmenti pontis (NRTP) of the cat were recorded intracellularly. The antidromic firing of the NRTP neurone is composed of IS-SD spikes with short duration and short spike-after-hyperpolarization. Membrane resistance and firing patterns were studied by applying depolarizing and hyperpolarizing current through the recording electrode. Findings indicate that the NRTP neurone has a relatively high membrane resistance and is capable of firing at a high frequency.Single shock stimulation of the interpositus (IP) and lateral nucleus (LN) of the cerebellum, brachium conjunctivum (BC), red nucleus (RN) and cerebral peduncle (CP) induced monosynaptic EPSPs in the NRTP neurones. Superior vestibular nucleus (SVN) stimulation induced monosynaptic IPSPs. Collision tests showed that (1) NRTP neurones are activated by the axons of IP and LN neurones which travel through BC, (2) these axons also send collaterals to RN, and (3) there is convergence of cerebellar, cerebral and brain stem inputs to a single NRTP neurone.     

The pattern in the projection of the intracerebellar nuclei onto the nucleus reticularis tegmenti pontis in the cat. An experimental anatomical study

Summary Intensity and distribution of degeneration in the nucleus reticularis tegmenti pontis (N.r.t.) were mapped in silver impregnated sections following stereotactic lesions of the intracerebellar nuclei. Confirming previous results (Brodal and Szikla, 1972) fibres from the interpositus-lateralis-complex, reaching the N.r.t. by way of the crossed descending limb of the brachium conjunctivum, supply the main central part of the N.r.t. from rostral to caudal. The interpositus posterior does not appear to take part in this projection, nor do the ventralmost parts of the lateral cerebellar nucleus (NL). The caudal part of the latter appears to give off more fibres than its rostral part. Rostral and caudal parts of the interpositus anterior (NIA) appear to contribute approximately equally.Fibres from both nuclei terminate in a largely overlapping fashion in the central main regions of the N.r.t. On the whole the NIA projection is situated a little more medially than the NL projection (Fig. 8). Within both projections caudal parts of the nuclei tend to project somewhat more medially than lateral parts, especially caudally. Certain findings suggest that there may be more detailed topical relations.The organization in the cerebellar-nuclear projection onto the N.r.t. corresponds in principle to the pattern found for its other afferent contingents (especially from the cerebral cortex, Brodal and Brodal, 1971). In spite of wide overlapping there is some degree of topical order.Working in the Anatomical Institute in the University of Oslo with leave of absence from the Laboratory of Normal Anatomy, University of Coimbra, Portugal, with a grant from the Portugese Institute for Higher Culture.     

Responses of nucleus reticularis tegmenti pontis neurons to vestibular stimulation in the rat

Summary Forty-nine neurons were recorded in the nucleus reticularis tegmenti pontis (NRTP) during horizontal vestibular and/or optokinetic stimulation in immobilized pigmented rats. During optokinetic stimulation, the response of NRTP neurons was either unidirectional (51%) or bidirectional (49%). Histological reconstruction showed that unidirectional neurons were located in the dorsal-medial part of NRTP, and bidirectional neurons in the lateral part. All neurons exhibited a response during pure vestibular sinusoidal stimulation in the frequency range 0.025 Hz-0.2 Hz. NRTP neurons were divided into two groups according to their threshold to vestibular stimulation. Group A neurons had a low threshold, a low spontaneous activity and their firing frequency slowly increased with acceleration. Group B neurons showed opposite characteristics. Phase and gain analysis suggested that NRTP neurons carry a head velocity signal. After hemiflocculectomy, the gain of the vestibular response of contralateral NRTP neurons increased. From these data, the role of NRTP in the horizontal vestibulo-oculomotor is discussed.Supported by DGRST 79.7.1012     

The organization of the nucleus reticularis tegmenti pontis in the cat in the light of experimental anatomical studies of its cerebral cortical afferents

Summary The distribution of degenerating fibres in the nucleus reticularis tegmenti pontis (N.r.t.) has been examined in Nauta (1957) impregnated sections from cats with discrete lesions of various cortical regions. The following cortical regions send fibres to the N.r.t.: Ms I, Sm I, Sm II, the orbital gyrus, the proreate gyrus, the parietal cortex and parts of the medial surface of the frontal lobe. The projection is bilateral, but mainly ipsilateral. The main terminal area of fibres from all cortical regions mentioned is the ventral part of the N.r.t. at middle rostrocaudal levels. Within this territory most cortical regions have their particular terminal sites in the N.r.t., but there is considerable overlapping.The anatomical organization and the role of the N.r.t. as a cerebrocerebellar relay station are discussed. The regions of the N.r.t. not receiving cortical fibres are supplied by fibres from other sources. These fibre groups have their preferential, although overlapping, areas of termination. In its organization the N.r.t. differs markedly from the pontine nuclei proper. Like the two other precerebellar reticular nuclei the N.r.t. appears to provide possibilities for an integration of impulses from the cerebral cortex with those from many other sources before they influence the cerebellum.     

Convergence of cortico- and cuneopontine projections onto components of the pontocerebellar system in the rat: an anatomical and electrophysiological study

Summary Previous studies in the rat have demonstrated that corresponding peripheral tactile and somatosensory cortical inputs converge within the granule cell layer of various cerebellar lobules and further that descending corticopontine projections from the forelimb sensory cortex (FLSCx) partially overlap with the projection zones of ascending basilar pontine afferents from nucleus cuneatus (NC). The present study employed anatomical and electrophysiological procedures to determine whether cortical and dorsal column nuclear afferent projections converge on pontine neurons that, in turn, provide mossy fiber input to the granule cell layer of the paramedian lobule (PML), i. e., that portion of the rodent cerebellum shown to receive forelimb peripheral inputs. The combination of the orthograde and retrograde axonal transport of horseradish peroxidase (HRP) conjugated to wheat germ agglutinin (WGA) was used light microscopically to demonstrate that orthogradely labeled projections from injections of the FLSCx and NC converged with ponto-paramedian projection neurons that were retrogradely labeled from injections of the PML. These studies were also repeated in conjunction with ablations of either the FLSCx or NC which resulted in the ultrastructural identification of degenerating, as well as WGA-HRP labeled axonal boutons of these pontine afferent projections thus confirming that such projections actually formed synaptic contacts with the retrogradely labeled pontoparamedian projection neurons. Single unit recording analyses of neurons in the ventromedial region of the basilar pons following combined electrical stimulation of various regions of the sensorimotor cortex and the contralateral body surface indicated that approximately 40% of all cells recorded responded to electrical stimulation of corresponding regions of the cortex and periphery, particularly the FLSCx and the forepaw. Natural cutaneous stimuli applied to the forepaw that also elicited responses in these same groups of basilar pontine neurons and were associated with relatively small receptive fields. Taken together, these observations indicate that the previously observed convergence of peripheral and somatosensory cortical inputs within the granule cell layer of the cerebellar cortex may be at least partially organized at the level of the basilar pons.     

Neuronal activity in nuclei pontis and reticularis tegmenti pontis related to forelimb movements of the monkey

The activity of the pontine nucleus (PN) neuron was recorded in 3 monkeys moving a handle alternately from start to target zones in a simple extension-flexion movement at the wrist. Of 73 PN neurons related to the task, 44 were related to movement, 19 to handle holding and 5 to both movement and holding of the handle. Of the 44 movement-related neurons, 16 were related to flexion, 22 to extension, and 6 to both. In 37 of 54 analyzed movements of the PN neurons which were related to movements, or to both movements and handle holding, the change of the activity occurred before the movement. However, in most of these cases (24/37), discharge occurred less than 100 ms earlier than the start of the movement. In the remaining one-third of movements (17/54), neurons discharged after the onset of the movement. Locations of the 73 neurons were histologically verified in the pontine nucleus. Somewhat similar observations were made of 14 cells located in the nucleus reticularis tegmenti pontis (NRTP). Considering that the majority of movement-related PN and NRTP neurons discharged immediately before or even after the onset of movement, these neurons may play a role in the execution of movement, at least of a simple movement, rather than in the initiation or planning of movement.     

Optokinetic response of cells in the nucleus reticularis tegmenti pontis of the pigmented rabbit

Summary In immobilized pigmented rabbits anesthetized with N₂O (70%) and halothane (2–4%), extracellular spikes were recorded from neurons in the nucleus reticularis tegmenti pontis (NRTP) and their responses to optokinetic stimulation (OKS) were examined. OKS was delivered using constant-velocity (0.1–4.0°/s) movements of a random dot pattern (60° × 60°) at 0°, 45°, 90° or 135° to the horizon. With OKS delivered to the contralateral eye (n=43), the preferred directions of NRTP cells were forward (F, n = 10), backward (B, n = 7), downward (D, n = 5), and the remaining cells showed no response (N, n = 21). With OKS delivered to the ipsilateral eye (n = 43), the preferred directions were F (n = 8), B (n = 8), upward (U, n = 2), D (n= 1) and N (n = 24). The majority of cells which responded to OKS (17/22 for contralateral, and 16/19 for ipsilateral OKS) preferred the horizontal orientation. The optimum velocity ranged from 0.2 to l°/s. The results suggest that the NRTP cells mainly transfer horizontal optikinetic signals to the flocculus and control horizontal optokinetic eye movements.     

Immunohistochemical studies of the structural bases of inhibition in the central cerebellar nuclei in mice

The distribution of glutamate decarboxylase-immunoreactive structures in the central nuclei of the cerebellum, its first afferent component, was studied at the light and electron microscope levels. Axosomatic, axodendritic, and axospinous synapses were detected, in which the presynaptic parts contained glutamate decarboxylase (GDC); this enzyme is involved in GABA synthesis. Additionally, these investigations revealed axoaxonal synapses in which both poles were GDC-reactive. The central nuclei of the cerebellum were found to have an intrinsic GABAergic system. Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 84, No. 12, pp. 1325–1332, December, 1998.     

GABAergic inhibition in the rat pontine nuclei is exclusively extrinsic: evidence from an in situ hybridization study for GAD67 mRNA

 As clearly indicated by our electrophysiological work, GABAergic inhibition plays a powerful role in the pontine nuclei (PN), the major link between cerebral cortex and the cerebellum. Using the technique of in situ hybridization for the mRNA encoding for the γ-aminobutyric acid (GABA)-synthesizing isoenzyme glutamic acid decarboxylase₆₇ (GAD₆₇), we demonstrate here the total absence of potentially GABAergic neurons from the rat PN. This negative finding supports the notion that GABAergic inhibition in the PN of rats, unlike that of higher mammals, is exclusively based on extrapontine GABAergic afferents. Received: 20 August 1998 / Accepted: 29 October 1998     

The mossy fiber projection of the nucleus reticularis tegmenti pontis to the flocculus and adjacent ventral paraflocculus in the cat

The pontine projection of the flocculus and adjacent ventral paraflocculus was investigated with antegrade and retrograde axonal tracer techniques. Injections of horseradish peroxidase into the floccular complex revealed subsets of labeled neurons in the nucleus reticularis tegmenti pontis, the nucleus raphe pontis and the medial lemniscus. Following injections of tritiated leucine in these subsets, the topographical distribution of labeled mossy fibers in the floccular complex was studied. Cells clustered in the central part of the nucleus reticularis tegmenti pontis project to the rostral flocculus and the rostral part of the caudal flocculus. The terminal field of cells in the nucleus raphe pontis and of cells associated with the lateral aspect of the medial lemniscus covered the same area. The number of mossy fiber terminals arising from these cells is small and concentrated in a medial position. The medial extension of the ventral paraflocculus and its most caudal sublobule do receive a very dense mossy fiber projection from cells associated with the medial edge of the medial lemniscus next to the rostral nucleus reticularis tegmenti pontis and beyond. Concomitantly, a collateral projection terminates in a restricted part of the uvula. Labeled mossy fiber terminals were never observed in the nodulus. The nucleus reticularis tegmenti pontis does not project to any part of the lower brain stem. The connections described in this paper are discussed in relation to the possible role of the nucleus reticularis tegmenti pontis as a relay nucleus in brain stem pathways transmitting visual information. It is concluded that in the cat this nucleus is an exclusively pre-cerebellar relay, not involved as a final link in the non-cerebellar pathway transmitting visual information to the vestibular nuclei.     

Inhibitory effect of the nucleus reticularis pontis oralis on the pontine micturition center and pontine urine storage center in decerebrate cats

The influence of the nucleus reticularis pontis oralis (PoO) on the pontine micturition center (PMC) and pontine urine storage center (PUSC) was examined in decerebrate cats by electrical and chemical stimulations of the PMC, PUSC or PoO. Microinjection of carbachol into the rostral and dorsolateral part of the PoO rapidly inhibited reflex micturition and external urethral sphincter (EUS) activity. After confirming the inhibition of reflex micturition and EUS activity by microinjection of carbachol into the PoO, intravenous injection of atropine sulfate or its microinjection into the PoO recovered both reflex micturition and EUS activity. Microinjection of carbachol into the PMC evoked micturition and then inhibited reflex micturition, but intravenous injection of atropine or its microinjection into the PoO recovered reflex micturition. After confi rming the inhibition of reflex micturition and EUS activity by microinjection of carbachol into the PoO, electrical stimulation of the PUSC enhanced EUS activity, but electrical stimulation of the PMC failed to evoke micturition. However, electrical stimulation of the PMC evoked micturition after microinjection of atropine into the PoO. These results suggest that the PoO strongly inhibits the PMC and less strongly inhibits the PUSC. Therefore, the PoO seems to be the pontine micturition inhibitory area.     

Effects of kainic acid lesions of the nucleus reticularis tegmenti pontis on fast and slow phases of vestibulo-ocular and optokinetic reflexes in the pigmented rat

Summary The nucleus reticularis tegmenti pontis (NRTP) and adjacent pontine reticular formation were lesioned chemically using the neurotoxic agent kainic acid, and the effects of these lesions on horizontal ocular optokinetic and vestibular nystagmus were examined. Eye position was measured in the alert, NRTP-lesioned animals with the electromagnetic search coil technique. Optokinetic and vestibular stimuli consisted of steps of rotations or sinusoidal oscillations of a fullfield visual pattern surrounding the animal or of the animal in total darkness, respectively. In a first group of animals, small unilateral NRTP lesions were produced by placing a single kainic acid injection in the area of the left NRTP. In one third of the animals, ipsilateral quick phases of optokinetic and vestibular nystagmus were abolished. In the remaining animals, quick phases were deficient to various degrees or not affected at all. There were no changes in the characteristics of optokinetic step responses to ipsilateral pattern rotations which activate predominantly optokinetic pathways on the side of the brainstem lesion. In animals with ipsiversive quick phase deficits, contralateral pattern rotations elicited tonic eye deviations. In a second group of animals, large uni- or bilateral lesions were produced by injecting kainic acid into three separate rostral, middle and caudal levels of the right NRTP. These animals had uni- or bilateral quick phase deficits during optokinetic and vestibular nystagmus. Optokinetic nystagmus in response to velocity steps of pattern rotation towards the lesion side was strongly reduced in gain even in those animals that had no apparent deficits in the fast contraversive reset phases. In four out of six animals, responses to sinusoidal optokinetic pattern oscillations were reduced in gain and showed increased phase lags compared to controls. Vestibulo-ocular responses to velocity steps of head rotations were of normal gain but reduced in duration (measured from onset of stimulation to reversal of nystagmus). Sinusoidal vestibulo-ocular responses evoked by head oscillations exhibited reduced gain values and strongly increased phase leads in the frequency range below 0.5 Hz. The vestibular time constant was found to be around 4.5 s in animals with NRTP lesions compared to about 7.5 s in control animals. The present results show that large kainic acid lesions of the NRTP (and adjacent area) do not abolish optokinetic eye movements in the rat, in contrast to what has been reported after electrolytic lesions. The data suggest, however, that there is a failure of slow build-up of OKN slow phase velocity as well as a shortening of the vestibular time constant which correlates with the kainic acid lesions extending into rostromedial and caudal parts of the NRTP. The implications of these findings with respect to an involvement of these structures in velocity storage are discussed.Abbreviations CN cochlear nucleus - DpSC decussation, peduncle, superior, cerebellar - ip interpeduncular nucleus - MLF medial longitudinal fasciculus - NOT nucleus of optic tract - NRTPc nucleus reticularis tegmenti pontis, central subdivision - NRTPp nucleus reticularis tegmenti pontis, pericentral subdivision - p pontine nuclei - ph praepositus hypoglossi nucleus - pMC peduncle, middle cerebellar - pSC peduncle, superior cerebellar - Pyr pyramidal tract - Rcs raphe central superior - Rm raphe magnus - rpc reticular nucleus, pontine, caudal - rpo reticular nucleus, pontine, oral - TB trapezoid body - tM trapezoid nucleus, medial - tGd tegmental nucleus of von Gudden, dorsal - tGv tegmental nucleus of von Gudden, ventral - 5 trigeminal tract or trigeminal nerve - 5m mesencephalic trigeminal nucleus - 5mt motor trigeminal nucleus - 6n abducens nucleus - 7 facial nerve Prof. W. Precht died on March 12, 1985     

Immunocytochemical evidence for in vitro release of glutamate and GABA from separate nerve terminal populations in the rat pontine nuclei

Summary A quantitative electron microscopic immunocytochemical method was used to study the synaptic handling of glutamate and GABA in slice preparations from the rat pontine nuclei. Slices were subjected to a depolarizing stimulus (55 mM K⁺, 20 min) in the presence of a physiological or low Ca²⁺concentration. Depolarization at physiological [Ca²⁺] evoked a depletion of glutamate-like immunoreactivity from nerve terminals that contain round vesicles and establish asymmetric synaptic contacts. When depolarization was induced in the presence of only 0.1 mM Ca²⁺ (10 mM Mg²⁺ added), the loss of glutamate was significantly reduced or abolished, indicative of a Ca²⁺dependent component of glutamate release. By means of a double labeling immunocytochemical method we could identify a population of nerve terminals that displayed strong GABA-like immunoreactivity, and a level of glutamate like immunoreactivity that was low but yet clearly above background level. This type of terminal contains elongated or pleomorphic vesicles and establishes symmetric synaptic contacts. In these terminals, depolarization evoked a Ca²⁺-dependent depletion of GABA like immunoreactivity, but failed to change the level of glutamate like immunoreactivity. The present study demonstrates that two different types of nerve terminal in the rat pontine nuclei contain releasable pools of glutamate and GABA, respectively, and that the GABA releasing terminals also contain a non releasable pool of glutamate. The glutamate of the latter pool could act as precursor of GABA.     

Profiles enriched in GABA-like immunoreactivity participate in serial synapses in the pontine nuclei of baboon (Papio anubis)

Summary Using a postembedding immunogold procedure with an antiserum against glutaraldehyde-fixed GABA, we demonstrate GABA-like immunoreactivity in two classes of synaptic profiles in the pontine nuclei of baboon. One is an axon terminal in symmetrical synaptic contact with small or medium-sized GABA-immunonegative dendrites, the other is a pale, vesicle-containing profile resembling a dendrite or dendritic process which participates in serial synaptic arrangements. These synaptic arrangements, or triads, consist of a GABA-like immunoreactive, pale vesicle-containing profile being postsynaptic to a GABA-immunonegative axon terminal, and presynaptic to a small or medium-sized GABA-immunonegative dendrite. In at least some of these triads, the GABA-immunonegative axon terminals also contact the GABA-immunonegative dendrite directly.     

Neural elements containing glutamic acid decarboxylase (GAD) in the dorsal lateral geniculate nucleus of the rat; immunohistochemical studies by light and electron microscopy

Immunoreactive constituents of the dorsal lateral geniculate nucleus of adult albino rats were examined by light- and electron-microscopy, using the unlabelled antibody enzyme method, following treatment of brain slices with a purified antibody to glutamic acid decarboxylase. The neuropil of the dorsal lateral geniculate nucleus displayed a conspicuous granular immunoreactivity. In addition, the antibody was bound to a class of small neurons of characteristic morphology. These cells possessed few (commonly 2-4) sparsely branched, long dendrites from some of which immunoreactive appendages were traced. Many cells were bipolar in form, and the dendrites of some appeared to be preferentially orientated. The immunoreactive cells closely resembled intrinsic interneurons characterized in previous Golgi studies of this nucleus. By electron-microscopy, immunoreactive presynaptic elements were present both in the extraglomerular neuropil and in the synaptic glomeruli. The former were axon terminals containing flattened synaptic vesicles and making Gray type II axo-dendritic synaptic contact; they appeared to correspond to axon terminals whose origin in the thalamic reticular nucleus has been established in previous studies, but it is possible that some were axon terminals of intrinsic interneurons. The immunoreactive glomerular components also contained flattened vesicles, were presynaptic to presumptive projection cell dendrites, postsynaptic to retinal axon terminals, and participated in triplet (triadic) and other complex synaptic arrangements. They corresponded in all respects to the synaptic portions of the complex dendritic appendages of intrinsic interneurons, identified and characterized in previous studies. The finding that there are high levels of glutamic acid decarboxylase in the cell bodies, dendritic shafts and dendritic appendages of intrinsic interneurons in the dorsal lateral geniculate nucleus of the rat, and in the axon terminals of fibres projecting to this site from the thalamic reticular nucleus, allows us to conclude that the inhibitory inputs to the geniculo-cortical projection cells from both of these sources are probably mediated by gamma-aminobutyric acid.     

Experimental electron microscopic studies on excitatory and inhibitory synapses in the cerebellar cortex of the rat

Summary Excitatory synapses in the rat cerebellar cortex contain large, spheroid vesicles and inhibitory synapses contain small, ovoid vesicles. Presynaptic terminals can be characterized by the ratio of spheroid: ovoid vesicles (Q-value). Neither the inhibition of acetylcholine synthesis by hemicholinium, nor the inhibition of GABA synthesis by thiosemicarbazide, does induce any significant alterations in the Q values. It is suggested that synaptic vesicles bind mediator substances on their surface membranes.     

Biochemical evidence for γ-aminobutyrate containing fibres from the nucleus accumbens to the substantia nigra and ventral tegmental area in the rat

Glutamate decarboxylase activity, a specific marker for γ-aminobutyrate-containing neurons, has been analysed in microdissected samples from rat mesencephalon following unilateral electrocoagulations of the nucleus accumbens. This lesion resulted in a consistent decrease of 50% in the enzyme activity in the rostromedial substantia nigra, and a slight, but insignificant decrease (?15%) in the medial parts of the caudal pars compacta of the substantia nigra. No change was found in the lateral pars compacta or the central pars reticulata. In the ventral tegmental area, the highest activity was found in the rostromedial part, adjacent to the mammillary body. At this level, a significant decrease of 20% was found in the ventral tegmental area on the lesioned side. In contrast, the activities in the medial accessory optic nucleus and the caudal ventral tegmental area adjacent to the interpenduncular nucleus were unchanged.The results indicate that the nucleus accumbens sends γ-aminobutyrate-containing fibres to the rostromedial substantia nigra and to the rostral ventral tegmental area. The caudal ventral tegmental area, the lateral pars compacta and the central pars reticulata do not receive measurable amounts of such fibres.