Immunocytochemical localization of tyrosine hydroxylase and gamma-aminobutyric acid in the mesencephalic trigeminal nucleus of the cat: a light and electron microscopic study

Extrasynaptic homomeric glycine receptors in neurons of the rat trigeminal mesencephalic nucleus

The neurons in the trigeminal mesencephalic nucleus (Vmes) innervate jaw-closing muscle spindles and periodontal ligaments, and play a crucial role in the regulation of jaw movements. Recently, it was shown that many boutons that form synapses on them are immunopositive for glycine (Gly+), suggesting that these neurons receive glycinergic input. Information about the glycine receptors that mediate this input is needed to help understand the role of glycine in controlling Vmes neuron excitability. For this, we investigated the expression of glycine receptor subunit alpha 3 (GlyRα3) and gephyrin in neurons in Vmes and the trigeminal motor nucleus (Vmo), and the Gly+ boutons that contact them by light- and electron-microscopic immunocytochemistry and quantitative ultrastructural analysis. The somata of the Vmes neurons were immunostained for GlyRα3, but not gephyrin, indicating expression of homomeric GlyR. The immunostaining for GlyRα3 was localized away from the synapses in the Vmes neuron somata, in contrast to the Vmo neurons, where the staining for GlyRα3 and gephyrin were localized at the subsynaptic zones in somata and dendrites. Additionally, the ultrastructural determinants of synaptic strength, bouton volume, mitochondrial volume, and active zone area, were significantly smaller in Gly+ boutons on the Vmes neurons than in those on the Vmo neurons. These findings support the notion that the Vmes neurons receive glycinergic input via putative extrasynaptic homomeric glycine receptors, likely mediating a slow, tonic modulation of the Vmes neuron excitability.

Direct projection from the lateral habenula to the trigeminal mesencephalic nucleus in rats

Trigeminal mesencephalic nucleus (Vmes) neurons are primary afferents conveying deep sensation from the masticatory muscle spindles or the periodontal mechanoreceptors, and are crucial for controlling jaw movements. Their cell bodies exist in the brain and receive descending commands from a variety of cortical and subcortical structures involved in limbic (emotional) systems. However, it remains unclear how the lateral habenula (LHb), a center of negative emotions (e.g., pain, stress and anxiety), can influence the control of jaw movements. To address this issue, we examined whether and how the LHb directly projects to the Vmes by means of neuronal tract tracing techniques in rats. After injections of a retrograde tracer Fluorogold in the rostral and caudal Vmes, a number of neurons were labeled in the lateral division of LHb (LHbl) bilaterally, whereas a few neurons were labeled in the medial division of LHb (LHbm) bilaterally. After injections of an anterograde tracer, biotinylated dextranamine (BDA) in the LHbl, a small number of labeled axons were distributed bilaterally in the rostral and caudal levels of Vmes, where some labeled axonal boutons contacted the cell body of rostral and caudal levels of Vmes neurons bilaterally. After the BDA injection into the LHbm, however, no axons were labeled bilaterally in the rostral and caudal levels of Vmes. Therefore, the present study for the first time demonstrated the direct projection from the LHbl to the Vmes and the detailed projection patterns, suggesting that jaw movements are modulated by negative emotions that are signaled by LHbl neurons.

Monosynaptic premotor circuit tracing reveals neural substrates for oro-motor coordination

Feeding behaviors require intricately coordinated activation among the muscles of the jaw, tongue, and face, but the neural anatomical substrates underlying such coordination remain unclear. In this study, we investigate whether the premotor circuitry of jaw and tongue motoneurons contain elements for coordination. Using a modified monosynaptic rabies virus-based transsynaptic tracing strategy, we systematically mapped premotor neurons for the jaw-closing masseter muscle and the tongue-protruding genioglossus muscle. The maps revealed that the two groups of premotor neurons are distributed in regions implicated in rhythmogenesis, descending motor control, and sensory feedback. Importantly, we discovered several premotor connection configurations that are ideally suited for coordinating bilaterally symmetric jaw movements, and for enabling co-activation of specific jaw, tongue, and facial muscles. Our findings suggest that shared premotor neurons that form specific multi-target connections with selected motoneurons are a simple and general solution to the problem of orofacial coordination.DOI: http://dx.doi.org/10.7554/eLife.02511.001.

Corticofugal direct projections to primary afferent neurons in the trigeminal mesencephalic nucleus of rats

Little is known about projections from the cerebral cortex to the trigeminal mesencephalic nucleus (Vmes) which contains the cell bodies of primary sensory afferents innervating masticatory muscle spindles and periodontal ligaments of the teeth. To address this issue, we employed retrograde (Fluorogold, FG) and anterograde (biotinylated dextranamine, BDA) tracing techniques in the rat. After injections of FG into the Vmes, a large number of neurons were retrogradely labeled in the prefrontal cortex including the medial agranular cortex, anterior cingulate cortex, prelimbic cortex, infralimbic cortex, deep peduncular cortex and insular cortex; the labeling was bilateral, but with an ipsilateral predominance to the injection site. Almost no FG-labeled neurons were found in the somatic sensorimotor cortex. After BDA injections into the prefrontal cortex, anterogradely labeled axon fibers and boutons were distributed bilaterally in a topographic pattern within the Vmes, but with an ipsilateral predominance to the injection site. The rostral Vmes received more preferential projections from the medial agranular cortex, while the deep peduncular cortex and insular cortex projected more preferentially to the caudal Vmes. Several BDA-labeled axonal boutons made close associations (possible synaptic contacts) with the cell bodies of Vmes neurons. The present results have revealed the direct projections from the prefrontal cortex to the primary sensory neurons in the Vmes and their unique features, suggesting that deep sensory inputs conveyed by the Vmes neurons from masticatory muscle spindles and periodontal ligaments are regulated with specific biological significance in terms of the descending control by the cerebral cortex.

Neurobiology of orofacial proprioception

Primary sensory fibers innervating the head region derive from neurons of both the trigeminal ganglion (TG) and mesencephalic trigeminal nucleus (MTN). The trigeminal primary proprioceptors have their cell bodies in the MTN. Unlike the TG cells, MTN neuronal somata are centrally located within the brainstem and receive synaptic inputs that potentially modify their output. They are a crucial component of the neural circuitry responsible for the generation and control of oromotor activities. Gaining an insight into the chemical neuroanatomy of the MTN is, therefore, of fundamental importance for the understanding of neurobiology of the head proprioceptive system. This paper summarizes the recent advances in our knowledge of pre- and postsynaptic mechanisms related to orofacial proprioceptive signaling in mammals. It first briefly describes the neuroanatomy of the MTN, which is involved in the processing of proprioceptive information from the face and oral cavity, and then focuses on its neurochemistry. In order to solve the puzzle of the chemical coding of the mammalian MTN, we review the expression of classical neurotransmitters and their receptors in mesencephalic trigeminal neurons. Furthermore, we discuss the relationship of neuropeptides and their corresponding receptors in relaying of masticatory proprioception and also refer to the interactions with other atypical neuromessengers and neurotrophic factors. In extension of previous inferences, we provide conclusive evidence that the levels of transmitters vary according to the environmental conditions thus implying the neuroplasticity of mesencephalic trigeminal neurons. Finally, we have also tried to give an integrated functional account of the MTN neurochemical profiles.

Gamma-aminobutiric acid immunostaining in trigeminal, nodose and spinal ganglia of the cat

Gamma-aminobutyric acid (GABA) is a principal inhibitory neurotransmitter in the vertebrate nervous system. It is found mainly in local circuit neurons, but it has also been described in sensory organs and dorsal root ganglia (DRG). The present study describes the presence of GABA in primary afferent neurons of feline sensory ganglia: trigeminal ganglia (TrG), nodose ganglia (NG), and DRG. Quantitative analysis revealed that approximately 20% of the cells in the TrG, NG and DRG are GABAergic. GABA-expressing neurons varied in size. GABA-containing neuronal fibres were also observed in the neuropil. Some of these were in close apposition to both GABA-positive and GABA-negative ganglionic neuronal perikarya. The localization of GABA in small primary afferent neurons, which are considered to be nociceptors, suggests that the amino acid may function as a pain transmitter or modulator, whereas processing of other sensory modalities, such as somatosensory and proprioceptive, may also be affected by GABA.

Excitatory GABAergic synaptic potentials in the mesencephalic trigeminal nucleus of adult rat in vitro

The mesencephalic trigeminal nucleus (MesV) contains the somata of primary afferent neurons innervating masticatory muscle spindles and the periodontal membrane. MesV afferent somata are unique in receiving synaptic inputs. Intracellular recordings in coronal pontine slices from adult rats were made from MesV neurons identified by having Cs-sensitive inward rectification and pseudounipolar morphology. Stimuli near the MesV evoked either a cluster of action potentials superimposed on a postsynaptic potential (PSP) or an antidromic spike at resting membrane potential (RMP). Membrane hyperpolarization revealed that each cluster of action potentials consisted of an antidromic spike and a subsequent PSP. Evoked PSPs in slices and miniature postsynaptic currents (mPSCs) recorded using whole-cell patch in dissociated MesV neurons were resistant to glutamate antagonists and strychnine but were reversibly abolished by 40 microM bicuculline. Superfusion of 1-10 mM GABA decreased input resistance and depolarized the membrane. Reversal potentials for evoked PSPs and GABA-induced depolarizations were similar and close to that for mPSCs which matched the Cl- equilibrium potential. Thus activation of synapses on MesV somata evokes GABAergic PSPs that generate action potentials at RMP in the adult. These data also indicate that primary afferent MesV neurons can act as interneurons in the central control of mastication.

Synaptic Inputs to Trigeminal Primary Afferent Neurons Cause Firing and Modulate Intrinsic Oscillatory Activity

In this paper, we investigated the influence of synapses on the cell bodies of trigeminal muscle spindle afferents that lie in the trigeminal mesencephalic nucleus (NVmes), using intracellular recordings in brain stem slices of young rats. Three types of synaptic responses could be evoked by electrical stimulation of the adjacent supratrigeminal, motor, and main sensory nuclei and the intertrigeminal area: monophasic depolarizing postsynaptic potentials (PSPs), biphasic PSPs, and all or none action potentials without underlying excitatory PSPs (EPSPs). Many PSPs and spikes were abolished by bath-application of 6,7-dinitroquinoxaline (DNQX) alone or combined with d,l-2-amino-5-phosphonovaleric acid (APV), suggesting that they are mediated by non– N-methyl-d-aspartate (NMDA) and NMDA glutamatergic receptors, while some action potentials were sensitive to bicuculline, indicating involvement of GABAA receptors. A number of cells showed spontaneous membrane potential oscillations, and stimulation of synaptic inputs increased the amplitude of the oscillations for several cycles, which often triggered repetitive firing. Furthermore, the oscillatory rhythm was reset by the stimulation. Our results show that synaptic inputs to muscle primary afferent neurons in NVmes from neighboring areas are mainly excitatory and that they cause firing. In addition, the inputs synchronize intrinsic oscillations, which may lead to sustained, synchronous firing in a subpopulation of afferents. This may be of importance during rapid biting and during the mastication of very hard or tough foods.

Comparative analysis of the chemical neuroanatomy of the mammalian trigeminal ganglion and mesencephalic trigeminal nucleus

A characteristic peculiarity of the trigeminal sensory system is the presence of two distinct populations of primary afferent neurons. Most of their cell bodies are located in the trigeminal ganglion (TG) but part of them lie in the mesencephalic trigeminal nucleus (MTN). This review compares the neurochemical content of central versus peripheral trigeminal primary afferent neurons. In the TG, two subpopulations of primary sensory neurons, containing immunoreactive (IR) material, are identified: a number of glutamate (Glu)-, substance P (SP)-, neurokinin A (NKA)-, calcitonin gene-related peptide (CGRP)-, cholecystokinin (CCK)-, somatostatin (SOM)-, vasoactive intestinal polypeptide (VIP)- and galanin (GAL)-IR ganglion cells with small and medium-sized somata, and relatively less numerous larger-sized neuropeptide Y (NPY)- and peptide 19 (PEP 19)-IR trigeminal neurons. In addition, many nitric oxide synthase (NOS)- and parvalbumin (PV)-IR cells of all sizes as well as fewer, mostly large, calbindin D-28k (CB)-containing neurons are seen. The majority of the large ganglion cells are surrounded by SP-, CGRP-, SOM-, CCK-, VIP-, NOS- and serotonin (SER)-IR perisomatic networks. In the MTN, the main subpopulation of large-sized neurons display Glu-immunoreactivity. Additionally, numerous large MTN neurons exhibit PV- and CB-immunostaining. On the other hand, certain small MTN neurons, most likely interneurons, are found to be GABAergic. Furthermore, NOS-containing neurons can be detected in the caudal and the mesencephalic-pontine junction portions of the nucleus. Conversely, no immunoreactivity to any of the examined neuropeptides is observed in the cell bodies of MTN neurons but these are encircled by peptidergic, catecholaminergic, serotonergic and nitrergic perineuronal arborizations in a basket-like manner. Such a discrepancy in the neurochemical features suggests that the differently fated embryonic migration, synaptogenesis, and peripheral and central target field innervation can possibly affect the individual neurochemical phenotypes of trigeminal primary afferent neurons.

Glutamic acid decarboxylase-like immunoreactive axon terminals in synaptic contact with mesencephalic trigeminal nucleus neurons in the rat

The purpose of the present study was to obtain reliable evidence for the presence of gamma-aminobutyric acid-ergic (GABAergic) synapses upon mesencephalic trigeminal nucleus (MTN) neurons in the rat. For confocal laser-scanning microscopy, phosphate-activated glutaminase-like immunoreactivity (-IR) of MTN neurons was visualized with red fluorescence of Texas Red, while glutamic acid decarboxylase (GAD)-IR of GABA axons was visualized with green fluorescence of dichlorotriazinyl aminofluorescein. Many GAD-axon terminals were in close apposition to the cell bodies of MTN neurons. For electron microscopy, MTN neurons were labeled with wheat germ agglutinin-horseradish peroxidase injected into the masseter nerve, while axon terminals were labeled with GAD-IR. GAD-axon terminals were in symmetric synaptic contact with the cell bodies of MTN neurons. Primary proprioceptive neurons in the orofacial regions might be regulated post-synaptically by GABA neurons.

Serotonergic innervation of mesencephalic trigeminal nucleus neurons: a light and electron microscopic study in the rat

Neurons of the mesencephalic trigeminal nucleus (MTN) are considered to be homologous to mechanosensitive neurons in the sensory ganglia. The sites of origin of serotonin (5HT)-immunoreactive axons on neuronal cell bodies in the MTN were studied in the rat by combining immunofluorescence histochemical techniques with retrograde tracing of Fluoro-Gold (FG) and anterograde tracing of biotin-conjugated dextran amine (BDA). The tracing studies, which were combined with multiple-labeling immunohistochemistry and confocal microscopy, indicated that 5HT-immunoreactive axon terminals on the cell bodies of MTN neurons originated from the medullary raphe nuclei, such as the nucleus raphes magmus (RMg), alpha part of the nucleus reticularis gigantocellularis (GiA) and nucleus raphes obscurus (ROb), as well as from the mesopontine raphe nuclei, such as the nucleus raphes dorsalis (DR), nucleus raphes pontis (PnR) and nucleus raphes medianus (MnR); mainly from the RMg, GiA and DR, and additionally from the ROb, PnR and MnR. The cell bodies in close apposition to 5HT-immunoreactive axon terminals were found through the whole rostrocaudal extent of the MTN. Electron microscopically a number of axon terminals that were labeled with BDA injected into the raphe nuclei were confirmed to be in asymmetric synaptic contact with the cell bodies of MTN neurons. It was also indicated that substance P existed in some of the 5HT-containing axosomatic terminals arising from the ROb, RMg and GiA. The present results indicated that proprioceptive sensory signals from the muscle spindles or periodontal ligament might be modulated at the level of the primary afferent cell bodies in the MTN by 5HT-containing axons from the mesopontine and medullary raphe nuclei including the GiA.

Evidence for functional compartmentalization of trigeminal muscle spindle afferents during fictive mastication in the rabbit

Primary afferent neurons innervating muscle spindles in jaw-closing muscles have cell bodies in the trigeminal mesencephalic nucleus (NVmes) that are electrically coupled and receive synapses. Each stem axon gives rise to a peripheral branch and a descending central branch. It was previously shown that some spikes generated by constant muscle stretch fail to enter the soma during fictive mastication. The present study examines whether the central axon is similarly controlled. These axons were functionally identified in anaesthetized and paralysed rabbits, and tonic afferent firing was elicited by muscle stretch. For the purpose of comparison, responses were recorded extracellularly both from the somatic region and from the central axon in the lateral brainstem. Two types of fictive masticatory movement patterns were induced by repetitive stimulation of the masticatory cortex and monitored from the trigeminal motor nucleus. Field potentials generated by spike-triggered averaging of action potentials from the spindle afferents were employed to determine their postsynaptic effects on jaw-closing motoneurons. Tonic firing of 32% NVmes units was inhibited during the jaw-opening phase, but spike frequency during closing was almost equal to the control rate during both types of fictive mastication. A similar inhibition occurred during opening in 83% of the units recorded along the central branch. However, firing frequency in these was significantly increased during closing in 94%, probably because of the addition of antidromic action potentials generated by presynaptic depolarization of terminals of the central branch. These additional spikes do not reach the soma, but do appear to excite motoneurons. The data also show that the duration and/or frequency of firing during the bursts varied from one pattern of fictive mastication to another. We conclude that the central axons of trigeminal muscle spindle afferents are functionally decoupled from their stem axons during the jaw-closing phase of mastication. During this phase, it appears that antidromic impulses in the central axons provide one of the inputs from the masticatory central pattern generator (CPG) to trigeminal motoneurons.

Neurotrophin-induced rapid enhancement of membrane potential oscillations in mesencephalic trigeminal neurons

We have proposed that neurotrophins, in addition to their trophic actions, act as neuromodulators in the adult central nervous system. As a first step to test this hypothesis, we examined in the adult rat slice preparation whether nerve growth factor and neurotrophin-3 are capable of altering the excitability of neurons of the mesencencephalic trigeminal nucleus. In contrast to vehicle pressure microapplication, which did not evoke changes in the electrophysiological properties of these neurons, neurotrophin application produced a significant increase in amplitude of the membrane potential oscillatory activity that is observed in these cells and a significant decrease in their threshold current. The latency of these effects ranged from 2 to 80 s and the duration ranged from 2 to 11 min. Neurotrophin-3 induced a decrease in input resistance and resting membrane potential in 58% of the cells; nerve growth factor induced a decrease in input resistance and resting membrane potential in 35% of the neurons. The spike configuration and action potential afterhyperpolarization potential remained unchanged following neurotrophin application. Tetrodotoxin blocked the membrane potential oscillatory activity of trigeminal mesencephalic neurons. Neurotrophin-induced effects were not blocked by the tyrosine kinase inhibitor K-252a, whereas IgG-192, an antibody directed to the neurotrophin low-affinity receptor, enhanced excitability, as did neurotrophins. These results demonstrate that neurotrophins are capable of producing a rapid increase in the excitability of trigeminal mesencephalic neurons and suggest that their effects may be mediated by low-affinity neurotrophin receptors.

Oscillatory membrane potential activity in the soma of a primary afferent neuron

In the present report, we provide evidence that mesencephalic trigeminal (Mes-V) sensory neurons, a peculiar type of primary afferent cell with its cell body located within the CNS, may operate in different functional modes depending on the degree of their membrane polarization. Using intracellular recording techniques in the slice preparation of the adult rat brain stem, we demonstrate that when these neurons are depolarized, they exhibit sustained, high-frequency, amplitude-modulated membrane potential oscillations. Under these conditions, the cells discharge high-frequency trains of spikes. Oscillations occur at membrane potential levels more depolarized than -53 +/- 2.3 mV (mean +/- SD). The amplitude of these oscillations increases with increasing levels of membrane depolarization. The peak-to-peak amplitude of these waves is approximately 3 mV when the cells are depolarized to levels near threshold for repetitive firing. The frequency of oscillations is similar in different neurons (108.9 +/- 15.5 Hz) and was not modified, in any individual neuron, by changes in the membrane potential level. These oscillations are abolished by hyperpolarization and by TTX, whereas blockers of voltage-dependent K+ currents slow the frequency of oscillations but do not abolish the activity. These data indicate that the oscillations are generated by the activation of inward Na+ current/s and shaped by voltage-dependent K+ outward currents. The oscillatory activity is not modified by perfusion with low-calcium, high-magnesium, or cobalt-containing solutions nor is it modified in the presence of cadmium or Apamin. These results indicate that a calcium-dependent K+ current does not play a significant role in this activity. We postulate that the membrane oscillatory activity in Mes-V neurons is synchronized in adjoining electrotonically coupled cells and that this activity may be modulated in the behaving animal by synaptic influences.

GABA- and glutamate-immunoreactivity in sensory ganglia of cat: a quantitative analysis

Several amino acids may function as neurotransmitters in the nervous system. The potential role of glutamate (Glu) and aspartate in excitatory responses was demonstrated and it was established that GABA and glycine act as inhibitory agents. The present study aimed at investigating the availability of Glu and GABA in certain feline sensory ganglia, i.e. the trigeminal (TrG), nodose and dorsal root ganglia (DRG). A significant part of the neurons were GABA-positive (19.5% to 23.5%). These were large-sized neurons as well as small- to medium-sized ones. The intensity of immunostaining varied from weak to strong. GABA-containing neuronal fibres were seen in the neuropil and some of them surrounded unstained ganglionic cells. The Glu-immunoreactive (IR) neuronal perikarya in all the investigated ganglia were 63.6% to 66.4%. The majority of positive cells were small- to medium-sized, but large primary sensory neurons were also seen. There was no difference between the intensity of the reaction in the primary sensory and small neurons. Glu-IR neuronal fibres were seen in close apposition to immunopositive as well as immunonegative neurons. In conclusion, in the TrG, nodose and DRG, GABA and glutamate are involved in neurotransmission. There is a significant number of GABAergic neurons in the investigated sensory ganglia of the cat. The difference in the expression of these amino acids suggests that they can act not only as neurotransmitters but also as modulators of sensory information.

Evidence for a possible glycinergic inhibitory neurotransmission in the midbrain and rostral pons of the rat studied by gephyrin

The data on the glycinergic transmission in the rostral brainstem are both few and controversial. The present report provides evidence for a possible glycinergic transmission in Sprague-Dawley rats, based on observations of immunocytochemical labeling for gephyrin, a 93 kDa protein and a component of the functional glycine receptor. A monoclonal antibody against gephyrin was used, and the reaction product was visualized by means of avidin-biotin-peroxidase procedure. The reaction product in midbrain and rostral pons was found in neuronal perikarya and in proximal dendrites but in some cases the most distal dendritic branches were also labeled. The neuropil usually displayed a moderate staining with finely granulated reaction product. The most significant immunocytochemical signal was mainly encountered in large and medium-sized neuronal populations of the motor cranial nerve nuclei (III, IV, V), in the reticular formation (laterodorsal tegmental nucleus, pedunculopontine tegmental nucleus, deep mesencephalic nucleus), in the red nucleus, in the intermediate and deep gray strata of the superior colliculus. Only in the substantia nigra and the inferior colliculus the parvocellular cell populations were mainly labeled. The present data suggest a significant inhibitory glycinergic neurotransmission in the rostral brainstem, probably mediated by interneurons.

Immunocytochemical localization of the AMPA receptor subunits in the mesencephalic trigeminal nucleus of the rat

The mesencephalic trigeminal nucleus is composed of large (35-50 microns) pseudo-unipolar neurons. Closely associated with them are small (< 20 microns) multipolar neurons. An unique peculiarity of the pseudo-unipolar perikarya is that they receive synaptic input from various sources, which sets them apart from the dorsal root and cranial nerves sensory ganglia neurons. Whereas glutamate is the best neurotransmitter candidate in pseudo-unipolar neurons, glutamatergic input into them has not yet been reported. AMPA glutamate receptors are implicated in fast excitatory glutamatergic synaptic transmission. They have been localized ultrastructurally at postsynaptic sites. This study demonstrates that the pseudo-unipolar neurons of the mesencephalic trigeminal nucleus express AMPA glutamate receptor subunits, which indicates that these neurons receive glutamatergic input. Serial sections from the rostral pons and midbrain of Sprague-Dawley rats were immunostained with antibodies against C-terminus of AMPA receptor subunits: GluR1, GluR2/3, and GluR4. The immunoreaction was visualized with avidin-biotin-peroxidase/DAB for light and electron microscopy. With GluR1 antibody only the smallest multipolar neurons were recognized as immunopositive within the mesencephalic trigeminal nucleus. GluR2/3 stained the pseudo-unipolar neurons intensely within the entire rostro-caudal extent of the nucleus. In addition the former antibody stained small multipolar neurons within the mesencephalic trigeminal nucleus, though with somewhat larger dimensions than those immunoreactive for GluR1. Whereas the overall staining with GluR4 antibody was scant, those pseudo-unipolar neurons that were stained, were strongly stained. Furthermore, a considerable number of microglial cells within and surrounding the mesencephalic trigeminal nucleus displayed very intense immunoreactivity for GluR4. These results are discussed in the light of the glutamate receptor subunit composition.

[Development, cytoarchitecture and ultrastructure of the mesencephalic trigeminal nucleus in domestic ruminants]

The ontogenetic development and cell differentiation of the mesencephalic trigeminal nucleus (Ntm) is lightmicroscopically examined in 58 bovine embryos and fetuses ranging from 2.4 to 80 cm Crown-Rump-Length (CRL). The cytoarchitecture and fine structure in adult cattle, sheep, and goats are investigated with the aid of light- and electronmicroscopy. At 2.4 cm CRL, the proneurons of the Ntm are detectable for the first time within the ventricular zone of the alar plate, possessing one drop-like cytoplasmic protrusion, whereas at 5 cm CRL, two cell types with differing sizes appear. Up to a CRL of 11.5 cm, the nucleus shows advanced maturation processes and has reached his final position at the border of the mesencephalic central grey. From 26 cm CRL onward, three cell types, and at 34 cm CRL four cell types, are discernible based on their nissl-granule arrangement. The cytomorphological differentiation and the maturation of the cells proceeds until 56 cm CRL, at which point the topographical and cytological characteristics of the Ntm are comparable with those of adult animals. In adult cattle, sheep and goats the Ntm consists of large (40-60 microns) and scarce medium-sized (30-40 microns) neurons with round and oval shapes. Scarcer small (20-25 microns) round and medium-sized multipolar neurons occur. The Nissl bodies are scattered throughout the pericaryon of the large neurons in a dust-like pattern and in the medium-sized neurons in a grained form. Within the cytoplasmic streets, which are situated between the membranes of the rough ER, numerous neurofilaments and mitochondria are detectable. Large Golgi complexes are placed in a perinuclear position. The neurons are also characterized by some somatic spines, and by a moderate distribution of axosomatic synapses, in which axon-endings with flattened synaptic vesicles predominate.

Peptidergic innervation of the mesencephalic trigeminal nucleus in the cat

BACKGROUND

The trigeminal processing of proprioceptive information is unique and very little is known about the neurochemical organization of trigeminal primary afferent neurons which mediate the sensory aspects of proprioception. In studies using immunocytochemicalretrograde tracing techniques, some classical neurotramsitters mediating the afferent modulation of the mesencephalic trigeminal nucleus (MTN) have been investigated. This paper summarizes our current understanding of the peptidergic innervation of the cat MTN.

METHODS

The distribution of immunoreactive substances was studied using specific antisera against 11 major neuropeptides. Light and electron microscopic peroxidase-antiperoxidase immunocytochemical staining techniques in colchicine-treated animals were used to clarify the distribution of peptide-identified fibers related to the MTN.

RESULTS

Immunoreactivity to any of the tested neuropeptides could not be detected in the MTN cell bodies. Numerous fibers containing various peptides such as substance P, bombesin, enkephalins, cholecystokinin, vasoactive intestinal polypeptide, vasopressin, and neuropeptide Y were present in the nucleus, however. These thin positive fibers covered the neuronal surface of the MTN cell bodies and some of the immunoreactive varicosities appeared to be in close proximity to profiles of MTN neurons. Electron microscopic observations revealed that perisomatic fibers were in direct apposition to perikarya of unstained large cells and some of them made synaptic contacts with their cell bodies and dendrites.

CONCLUSIONS

The present results demonstrate that the MTN neurons receive dense basket-like innervation from peptidergic neurons on somata and processes and have supported earlier evidence that the MTN of the cat is under influence of peptidergic input. Results of this study provide further evidence that the neuropeptides examined may play an important role in the integration and transmission of trigeminal proprioceptive information. Most likely they may co-exist with a classical but hitherto unknown neurotransmitter(s), that is unique for this region and whose release can be modulated by peptides.