Inactivation of Amino Acid Receptors in the Medullary Reticular Formation Modulate and Suppress Ingestion and Rejection Responses in the Awake Rat

Inactivation of amino acid receptors 2 ABSTRACT The lateral medullary reticular formation (RF) is the source of many pre-oromotor neurons and is essential for the generation of ingestive consummatory responses. Although the neurochemistry mediating these responses is poorly understood, studies of fictive mastication suggest that both excitatory and inhibitory amino acid receptors play important roles in the generation of these ororhythmic behaviors. The present study tested the hypothesis that amino acid receptors modulate the expression of ingestion and rejection responses elicited by natural stimuli in awake rats. Licking responses were elicited by either intraoral (IO) gustatory stimuli or sucrose presented in a bottle. Oral rejection responses (gaping) were elicited by IO delivery of quinine hydrochloride. Bilateral microinjection of the NMDA receptor antagonist D-CPP suppressed licking and gape responses recorded electromyographically from a subset of orolingual muscles. Likewise, infusion of the non-NMDA receptor antagonist CNQX significantly reduced licking and gape responses, but was accompanied by spontaneous gasping responses. Rats still actively probed the bottle, indicating an intact appetitive response. Neither D-CPP nor CNQX differentially affected ingestion or rejection, suggesting that the switch from one behavior to the other does not simply rely on one glutamate receptor subtype. Nevertheless, a glutamate-receptor mediated switch from consummatory behavior to gasps following CNQX infusions suggests a multifunctional substrate for coordinating the jaw and tongue in different behaviors. Bilateral infusions of the GABA A receptor antagonist bicuculline, or the glycine receptor antagonist strychnine, enhanced the amplitude of IO stimulation-induced oral responses. These data suggest that the neural Inactivation of amino acid receptors 3 substrate underlying ingestive consummatory responses is under tonic inhibition. Release of this inhibition may be one mechanism by which aversive oral stimuli produce large amplitude mouth openings associated with the rejection response. Inactivation of amino acid receptors 4 Studies in decerebrate preparations demonstrate that the caudal brainstem is sufficient for generating the consummatory components of ingestion and rejection in response to natural gustatory stimuli (29). Deafferentation studies indicate that these rhythmic movements are organized by a central pattern generator (18, 73). Although there is some question as to the precise anatomical location of a central pattern generator (CPG) for rhythmic consummatory behavior (52, 55, 78), reversible lesion studies in awake rat preparations suggest a necessary role for the rostral, lateral medullary reticular formation (14). Moreover, this RF substrate appears to be multifunctional. Ingestion (licking) and the oral component of taste rejection (the gape) …