Mauthner cell-initiated electromotor behavior is mediated via NMDA and metabotropic glutamatergic receptors on medullary pacemaker neurons in a gymnotid fish.
Weakly electric fish generate meaningful electromotor behaviors by specific modulations of the discharge of their medullary pacemaker nucleus from which the rhythmic command for each electric organ discharge (EOD) arises. Certain electromotor behaviors seem to involve the activation of specific neurotransmitter receptors on particular target cells within the nucleus, i.e., on pacemaker or on relay cells. This paper deals with the neural basis of the electromotor behavior elicited by activation of Mauthner cells in Gymnotus carapo. This behavior consists of an abrupt and prolonged increase in the rate of the EOD. The effects of specific glutamate agonists and antagonists on basal EOD frequency and on EOD accelerations induced by Mauthner cell activation were assessed. Injections of both ionotropic (AMPA, kainate, and NMDA) and metabotropic (trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid) glutamate agonists induced increases in EOD rate that were maximal when performed close to the soma of pacemaker cells. In contrast, injections in the proximity of relay cells were ineffective. Therefore, pacemaker neurons are probably endowed with diverse glutamate receptor subtypes, whereas relay cells are probably not. The Mauthner cell-evoked electromotor behavior was suppressed by injections of AP-5 and (+/-)-amino-4-carboxy-methyl-phenylacetic acid, NMDA receptor and metabotropic glutamate receptor antagonists, respectively. Thus, this electromotor behavior relies on the activation of the NMDA and metabotropic glutamate receptor subtypes of pacemaker cells. Our study gives evidence for the synergistic effects of NMDA and metabotropic receptor activation and shows how a simple circuit can produce specific electromotor outputs.