The role of glutamate in swim initiation in the medicinal leech

Antagonists were used to investigate the role of the excitatory amino acid, L-glutamate, in the swim motor program of Hirudo medicinalis. In previous experiments, focal application of L-glutamate or its non-NMDA agonists onto either the segmental swim-gating interneuron (cell 204) or the serotonergic Retzius cell resulted in prolonged excitation of the two cells and often in fictive swimming. Since brief stimulation of the subesophageal trigger interneuron (cell Trl) evoked a similar response, we investigated the role of glutamate at these synapses. Kynurenic acid and two non-NMDA antagonists, 6,7-dinitroquinoxaline-2,3-dione (DNQX) and Joro spider toxin, effectively suppressed (1) the sustained activation of cell 204 and the Retzius cell following cell Trl stimulation and (2) the mono synaptic connection from cell Trl to cell 204 and the Retzius cell, but did not block spontaneous or DP nerve-activated swimming. Other glutamate blockers, including gamma-D-glutamylaminomethyl sulfonic acid, L(+)-2-amino-3-phosphonoproprionic acid and 2-amino-5-phosphonopentanoic acid, were ineffective. DNQX also blocked both indirect excitation of cell 204 and direct depolarization of cell Trl in response to mechanosensory P cell stimulation. Our findings show the involvement of non-NMDA receptors in activating the swim motor program at two levels: (1) P cell input to cell Trl and (2) cell Trl input to cell 204, and reveal an essential role for glutamate in swim initiation via the cell Trl pathway.