Research reportNitrergic innervation of trigeminal and hypoglossal motoneurons in the cat
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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