Initiation of swimming activity in the medicinal leech by glutamate, quisqualate and kainate.

The excitatory amino acid, glutamate, and several of its agonists are known to produce locomotory activity in the lamprey (Brodin et al. 1985), rat (Kudo and Yamada, 1987) and the embryos and larvae of amphibians (Dale and Roberts, 1984; McClellan and Farel, 1985). It is hypothesized that glutamate acts as a neurotransmitter in the neuronal pathway that generates locomotion in these systems. In both the central and peripheral nervous systems of invertebrates, glutamate also acts as a neurotransmitter. For example, at many crayfish neuromuscular junctions, exogenously applied glutamate mimics the effects of the endogenous excitatory transmitter (Takeuchi and Takeuchi, 1964; Kawagoe et al. 1981). In other invertebrates, application of glutamate to central neurons produces a variety of responses depending on the neuron stimulated and the preparation (Walker et al. 1980; Shinozaki, 1988). However, in no invertebrates has glutamate been implicated in eliciting any centrally generated motor patterns. In the medicinal leech, Hirudo medicinalis, local application of L-glutamate (L-glu) onto a segmental ganglion produces prolonged excitation of the Retzius cell (James and Walker, 1979). This L-glu-induced excitatory response is provocative, given that Trl stimulation, which can initiate swimming activity in the ventral nerve cord (Brodfuehrer and Friesen, 1986), elicits a similar type of sustained excitation in the Retzius cells prior to the onset of swimming. These similarities suggest that L-glu could act as a neurotransmitter in the initiation of leech swimming. In this paper we tested whether pressure ejection of L-glu can elicit the swim motor pattern in the isolated leech nerve cord and determined which of the three glutamate receptor subtypes (quisqualic acid, kainic acid or N-methylD-aspartic acid receptors) mediate this effect. To determine if L-glu and its agonists can elicit swimming in an isolated nerve cord (head ganglion or segmental ganglion 2-19), a micropipet containing L-glu or an agonist dissolved in leech physiological saline (pH7.40±0.03) was positioned