Muscimol infusions in the brain stem reticular formation reversibly block ingestion in the awake rat

Abstract

Previous studies have localized a central pattern generator for mastication to the midline pontomedullary reticular formation (RF) based on cortically induced ororhythmic movements. The present study determined whether this same substrate mediated licking responses evoked by more natural stimuli. Licking in the awake rat was initiated either through an appetitive response to sucrose presented in a bottle or by intraoral (IO) infusions. Oral rejection responses also were obtained by IO infusions of quinine hydrochloride. Small volumes of the GABA(A) agonist muscimol bilaterally infused into the lateral medullary RF significantly reduced licking and oral rejection responses measured electromyographically from the anterior digastric and geniohyoid muscles. Other than the decrement or absence of ororhythmic activity, rats appeared normal and actively approached and probed the water bottle. The suppression was reversible and returned to baseline within 3 h. In contrast, midline infusions of muscimol did not affect licking or rejection responses. We postulate that the lateral medullary RF is an essential final common path for ingestive consummatory responses.

Functional circuitry involved in the regulation of whisker movements

“…Many of these labeled pre-FMNs were closely apposed to anterogradely Drew et al, 1986;Nozaki et al, 1993;Chen et al, 2001; for review see Buttner-Ennever and Holstege, 1986). Similarly, the reticular formation in birds contains the CPG for rhythmic acts such as pecking and jaw movements (Berkhoudt et al, 1982;Wild et al, 1985).…”

“…Several lines of anatomical and physiological evidence support the conclusion that pre-FMNs in the brainstem reticular formation are part of a CPG that controls rhythmic whisking. Neurons in this region are involved in other types of rhythmic motor acts in mammals, such as licking, mastication, and locomotion (Drew et al, 1986;Nozaki et al, 1993;Chen et al, 2001; for review see Buttner-Ennever and Holstege, 1986). Similarly, the reticular formation in birds contains the CPG for rhythmic acts such as pecking and jaw movements (Berkhoudt et al, 1982;Wild et al, 1985).…”

Activation of NPY receptors suppresses excitatory synaptic transmission in a taste-feeding network in the lower brain stem

“…One region, immediately subjacent to the nucleus of the solitary tract (NST), which includes both the intermediate subdivision of the medullary RF (IRt) and the more lateral parvocellular RF (PCRt), contains a dense constellation of preoromotor neurons (29, 37, 58) that provide a potent source of excitatory drive to the oromotor nuclei (59, 60). Functional inactivation of the IRt/PCRt with either the GABA agonist muscimol or glutamatergic antagonists suppresses consummatory behavior in the awake freely moving rat (10,11,57). …”

μ-Opioid modulation in the rostral solitary nucleus and reticular formation alters taste reactivity: evidence for a suppressive effect on consummatory behavior

“…These relatively low volumes and doses were chosen to optimize our ability to observe anatomically discrete but reliable effects. To limit drug diffusion, we used the smallest volume practical for making consistent infusions in awake, behaving animals, one slightly lower than that used in similar experiments by J. Travers and colleagues (5,6,67). The dose was similar to the lower range employed in earlier investigations of behavioral effects of intracranial Damgo infusions made in a variety of brain regions [50 pmol unilaterally in raphe magnus, pain-elicited behavior (51); 60 pmol unilaterally in rostral ventromedial medulla, pain-elicited behavior (63); 97 pmol, nucleus accumbens, sucrose-licking (64); 97 pmol, nucleus accumbens, taste reactivity (61)].…”

“…MOR modulation of ingestive behavior also seems likely at the level of the medulla; in particular, in the first gustatory relay, the rostral nucleus of the solitary tract (rNST), and the adjacent parvocellular and intermediate zones of the reticular formation (RF). The rNST and RF are anatomically connected and together exert a major influence on taste processing and oromotor integration (5,6). Both the rNST and RF contain MORs (40,47), and their endogenous ligands, the enkephalins and endomorphins (9,28,44,48,56).…”