Swallowing responses induced by microinjection of glutamate and glutamate agonists into the nucleus tractus solitarius of ketamine-anesthetized rats

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Abstract
Swallowing is a patterned motor activity generated by neurons located within the nucleus tractus solitarius (NTS). An excitatory amino acid (EAA) neurotransmitter, such as glutamate (GLU), is suspected of being involved in the initiation of swallowing by NTS neuronal components. However, swallowing can still be elicited in animals anesthetized with ketamine, an antagonist of the N-methyl-D-aspartate (NMDA) subclass of EAA receptors. The present experiments were therefore designed to investigate the influence of EAA administration within the NTS on the swallowing motor acitivity of rats anesthetized with ketamine. Pressure microinjections of GLU in doses ranging from 25 to 500 pmol elicited swallowing. This effect was dose-dependent and was not reproduced when control injections of the vehicle solution were performed. Microinjections of the GLU agonists, quisqualate (QUIS) and NMDA, in doses ranging between 2.5 and 50 pmol, also induced swallowing motor activities. QUIS, like GLU, elicited a short series of swallows at a brief latency while NMDA generated long-lasting rhythmic swallowing with a longer latency. Swallowing induced by GLU microinjections (100 pmol) was suppressed almost completely by local pretreatment with either the broad spectrum EAA receptor antagonist, gamma-D-glutamylglycine (250 pmol), or the more selective non-NMDA antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (50–100 pmol), but not by pretreatment with the selective NMDA antagonist, DL-2-amino-5-phosponovalerate (250 pmol). On the other hand, pretreatment with DL-2-amino-5-phosphonovalerate (50 pmol) suppressed the deglutitions induced by NMDA microinjections (10 pmol) but not those elicited by QUIS microinjections (10 pmol). These results provide evidence that swallowing can be induced by activation of EAA receptors of both the NMDA and the non-NMDA subclasses located within the NTS. Furthermore they indicate that both subclasses may still be active in ketamine-anesthetized animals.