Cytochemical characteristics of neurons in the trigeminal mesencephalic nucleus of hatchling chicks

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.

Immunocytochemical localization of aromatase in sensory and integrating nuclei of the hindbrain in Japanese quail (Coturnix japonica)

The distribution of the estrogen synthesizing enzyme (aromatase) in the hindbrain (rhombencephalon and mesencephalon) of male adult quail was investigated by immunocytochemistry. Aromatase-immunoreactive neuronal structures (perikarya and fibers bearing punctate structures) were observed in sensory (trigeminal, solitary tract, vestibular, optic tectum) and integrating (parabrachial, periaqueductal, cerulean, raphe) nuclei. Besides the expression of aromatase in these well-delineated nuclei, dense to scattered networks of immunoreactive fibers were found dispersed throughout the hindbrain and, in particular, in its rostral and dorsal parts. To a lesser extent, they were also present throughout the premotor nuclei of the reticular formation and in various fiber tracts. In contrast, no immunoreactive signal was found in motor nuclei, and in most of the statoacoustic (cerebellum, cochlear, olive, pontine, part of vestibular) nuclei. The expression of aromatase in perikarya and fibers in areas of the adult hindbrain where estrogen receptors have been identified previously suggests a role for estrogens locally produced in the regulation of sensory and integrating functions, contrary to the widespread assumption that these functions are regulated exclusively by steroids produced in the gonads.

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.

Central connections of the nucleus mesencephalicus nervi trigemini in the mallard (Anas platyrhynchos L.)

BACKGROUND

In the mallard duck, functionally distinct groups of jaw muscles are each innervated by a different subnucleus of the main trigeminal (mV) or facial (mVII) motor nucleus. The other subnuclei of mV and mVII innervate several head muscles, including lingual muscles. The reticular premotor cells of the trigeminal and facial jaw motor subnuclei occupy different areas in the parvocellular reticular formation (RPc). The cell bodies of jaw muscle spindle afferents are situated in the mesencephalic nucleus (MesV). In the present study, the central connections of MesV with jaw motor subnuclei and their premotor areas are investigated.

METHODS

In a first series of experiments, horseradish peroxidase (HRP) injections were made in electrophysiologically identified trigeminal and facial subnuclei. In a second series of experiments, HRP was delivered iontophoretically at different parts of RPc. Anterograde tracing with tritiated leucine was used to confirm the central connections of MesV. Double labeling with fluorescent tracers was used to investigate whether MesV collaterals reach both the rostral and caudal parts of RPc.

RESULTS

MesV projects to only two of the five different subnuclei of the trigeminal motor nucleus. The subnuclei that receive spindle afferents innervate jaw adductor muscles (mV2) or pro- and retractors of the mandible (pterygoid muscles; mV1). The three other subnuclei innervate jaw-opener muscles or other head muscles. MesV fibers also project to the rostral part of the dorsolateral RPc (RPcdl), which serves as a premotor area for the motor subnuclei of adductor and pterygoid muscles. The intermediate part of RPcdl does not contain premotor cells of mV or mVII, and a clear projection of MesV to this area is absent. The caudal part of RPcdl projects to the mV and mVII subnuclei that innervate jaw-opener muscles. This part of RPc receives a projection from the same MesV cells as the rostral RPcdl. The MesV projection to RPc does not include premotor cells of mV and mVII in the ventromedial part of RPc (RPcvm).

CONCLUSIONS

Spindle afferents from jaw-closer muscles project only to mV subnuclei innervating jaw-closer muscles (mV1, mV2) and to a population of premotor cells in the rostral RPcdl that innervates these subnuclei. The mixed population of premotor cells in RPcvm, which innervates both jaw-opener and jaw-closer subnuclei, does not receive a MesV projection. However, a premotor area for jaw-opener subnuclei in the caudal part of RPcdl does receive MesV input and may serve as a relay through which proprioceptive information from jaw closer spindles can reach jaw opener muscles.

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.