Neurotransmitter phenotypes of intermediate zone reticular formation projections to the motor trigeminal and hypoglossal nuclei in the rat

Abstract

Numerous studies suggest an essential role for the intermediate (IRt) and parvocellular (PCRt) reticular formation (RF) in consummatory ingestive responses. Although the IRt and PCRt contain a large proportion of neurons with projections to the oromotor nuclei, these areas of the RF are heterogeneous with respect to neurotransmitter phenotypes. Glutamatergic, GABAergic, cholinergic, and nitrergic neurons are all found in the PCRt and IRt, but the projections of neurons with these phenotypes to the motor trigeminal (mV) and hypoglossal nucleus (mXII) has not been fully evaluated. In the present study, after small injections of Fluorogold (FG) into mV and mXII, sections were processed immunohistochemically to detect retrogradely labeled FG neurons in combination with the synthetic enzymes for nitric oxide (nitric oxide synthase) or acetylcholine (choline acetyltransferase) or in situ hybridization for the synthetic enzyme for GABA (GAD65/67) or the brainstem vesicular transporter for glutamate (VGLUT2). In three additional cases, FG injections were made into one motor nucleus and cholera toxin (subunit b) injected in the other to determine the presence of dual projection neurons. Premotor neurons to mXII (pre-mXII) were highly concentrated in the IRt. In contrast, there were nearly equal proportions of premotor-trigeminal neurons (pre-mV) in the IRt and PCRt. A high proportion of pre-oromotor neurons were positive for VGLUT2 (pre-mXII: 68%; pre-mV: 53%) but GABAergic projections were differentially distributed with a greater projection to mV (25%) compared to mXII (8%). Significant populations of cholinergic and nitrergic neurons overlapped pre-oromotor neurons, but there was sparse double-labeling (<10%). The IRt also contained a high proportion of neurons that projected to both mV and MXII. These different classes of premotor neurons in the IRt and PCRt provide a substrate for the rhythmic activation of lingual and masticatory muscles.

Hypoglossal premotor neurons of the intermediate medullary reticular region express cholinergic markers

“…This drive is mainly glutamatergic (11,58), consistent with the evidence that many IRt XII premotor neurons use glutamate as their main transmitter (61). Other than that, little is known about the neurochemical features of IRt XII premotor neurons.…”

“…The medullary reticular formation has been shown previously to contain cholinergic neurons with axonal projections to the facial, trigeminal, and XII motor nuclei (13,14,61), but the incidence of such projections appeared to be relatively small. The cells retrogradely labeled from these nuclei were scattered widely, making it difficult to appreciate the potential importance of these projections.…”

“…In particular, cells that express markers of cholinergic neurons [choline acetyltransferase (ChAT) or nitric oxide synthase (NOS)] are distributed within the IRt region in a pattern reminiscent of that of XII premotor neurons (1,18,64). Indeed, some ChAT-or NOS-immunoreactive neurons of the medullary reticular formation, including the IRt region, were previously shown to project to the trigeminal, facial, or XII motor nuclei (13,14,61). However, their localization relative to other premotor neurons and their other neurochemical markers have been little investigated.…”

Nitric oxide activates hypoglossal motoneurons by cGMP-dependent inhibition of TASK channels and cGMP-independent activation of HCN channels

“…The HMN controls GG muscle activity and is important for a variety of physiological functions including maintenance of airway patency, mastication, and swallowing. It has been known for more than a decade that the HMN receives nitrergic innervation (Pose et al 2005;Travers et al 2005), yet potential involvement of NO in HMN function remains obscure because there is limited evidence describing effects of NO on HMN activity in vivo. A study by Montero et al (2008) showed that microiontophoretic application of an NOS inhibitor or ODQ decreased inspiratory-related HMN activity, whereas injection of DEA or 8-BrcGMP did the opposite.…”

“…The hypoglossal motor nucleus (HMN) is an ideal model system to study NO modulation of neuronal activity because 1) properties of hypoglossal motoneurons are well characterized (Rekling et al 2000); 2) the HMN receives nitrergic innervations (Pose et al 2005;Travers et al 2005); 3) hypoglossal motoneurons express sGC (Gonzalez-Forero et al 2007); and 4) short-term manipulation of NO/cGMP signaling in the HMN can modulate rhythmic output of the region (Montero et al 2008). These results provide compelling evidence that NO is an important modulator of hypoglossal motoneuron activity; however, our understanding of the role of NO in the HMN is far from complete.…”