Elsevier

Brain Research

Volume 1041, Issue 1, 11 April 2005, Pages 29-37
Brain Research

Research report
Nitrergic innervation of trigeminal and hypoglossal motoneurons in the cat

https://doi.org/10.1016/j.brainres.2005.01.092Get rights and content

Abstract

The present study was undertaken to determine the location of trigeminal and hypoglossal premotor neurons that express neuronal nitric oxide synthase (nNOS) in the cat. Cholera toxin subunit b (CTb) was injected into the trigeminal (mV) or the hypoglossal (mXII) motor nuclei in order to label the corresponding premotor neurons. CTb immunocytochemistry was combined with NADPH-d histochemistry or nNOS immunocytochemistry to identify premotor nitrergic (NADPH-d+/CTb+ or nNOS+/ CTb+ double-labeled) neurons. Premotor trigeminal as well as premotor hypoglossal neurons were located in the ventro-medial medullary reticular formation in a region corresponding to the nucleus magnocellularis (Mc) and the ventral aspect of the nucleus reticularis gigantocellularis (NRGc). Following the injection of CTb into the mV, this region was found to contain a total of 60 ± 15 double-labeled neurons on the ipsilateral side and 33 ± 14 on the contralateral side. CTb injections into the mXII resulted in 40 ± 17 double-labeled neurons in this region on the ipsilateral side and 16 ± 5 on the contralateral side. Thus, we conclude that premotor trigeminal and premotor hypoglossal nitrergic cells coexist in the same medullary region. They are colocalized with a larger population of nitrergic cells (7200 ± 23). Premotor neurons in other locations did not express nNOS. The present data demonstrate that a population of neurons within the Mc and the NRGc are the source of the nitrergic innervation of trigeminal and hypoglossal motoneurons. Based on the characteristics of nitric oxide actions and its diffusibility, we postulate that these neurons may serve to synchronize the activity of mV and mXII motoneurons.

Introduction

Somatic motoneurons are perhaps the most completely studied cells in the central nervous system. There is a wealth of data dealing with their structure, function, electrophysiological properties, as well as their synaptic processes, including the neurotransmitters and neuromodulators that control their activity. In a comprehensive review, numerous substances including amino acids, biogenic amines, and peptides that act as neurotransmitters and neuromodulators are described in the synaptic processes that impinge on these cells [40].

In recent years, nitric oxide (NO) has joined the list of neuromodulatory substances that act on neurons in the central nervous system [6], [19]. We have previously reported that synaptic processes, containing the neuronal isoform of nitric oxide synthase (nNOS), are present in close apposition to the dendrites and cell bodies of trigeminal motoneurons in the guinea pig and that nitric oxide exerts an excitatory effect on these cells [1].

Trigeminal motoneurons participate in jaw movements associated with numerous behaviors such as mastication, deglutition, and vocalization [27], [28]. During these and related motor acts, a precise coordination takes place between the jaw musculature innervated by trigeminal motoneurons and the tongue musculature that is innervated by hypoglossal motoneurons. It has been suggested that this coordination depends on circuits that involve overlapping or shared structures controlling both motor pools [2], [13], [14], [24], [25], [47].

The present study was designed to determine the location of nitrergic premotor neurons that project to the trigeminal and hypoglossal motoneurons in the cat. An examination of the origins of the innervation of trigeminal motoneurons, prompted by our previous work [1], was combined with a study of the innervation of hypoglossal motoneurons. By means of retrograde and nNOS labeling techniques, we traced the origin of the nitrergic innervation of these brainstem motor nuclei to a population of cells within the ventro-medial medullary reticular formation. Accordingly, a novel population of premotor neurons was discovered in the present work. A portion of these data has been previously reported [32].

Section snippets

Methods

Thirteen adult cats of both sexes, weighing between 3.0 and 3.5 kg, were utilized. Eight cats were employed for studies of the trigeminal nucleus and five for experiments on the hypoglossal nucleus.

Results

Examples of nitrergic fibers within the trigeminal and hypoglossal motor pools are presented in Figs. 1A, A′ and B, C, respectively. Motoneurons were labeled with the ChAT antibody; their cytoplasm is stained in brown. These sections were also processed to detect NADPH-d activity (stained fibers in blue).

To determine the location of the cell bodies of the neurons that are the source of the nitrergic fibers shown above, CTb was injected into the trigeminal (mV) or into the hypoglossal (mXII)

Discussion

In the present study, we describe a population of premotor neurons within the brainstem that are the source of the nitrergic innervation of trigeminal and hypoglossal motoneurons. The existence of neurons displaying NADPH-d activity and/or nNOS immunoreactivity in the brain stem reticular formation has been demonstrated in earlier publications [4], [12], [41], [43], [48]. In the medullary reticular formation, such neurons were found laterally in the parvocellularis region and medially in the

Acknowledgments

We are grateful to J.K. Engelhardt for his critical review of the manuscript. This work was supported by the following grants from the US. Public Health Service: NS23426, NS09999, MH 43362, and AGO4307.

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