GLUTAMATE AS A PUTATIVE NEUROTRANSMITTER IN THE MOLLUSK, LYMNAEA-STAGNALIS

Bath-applied glutamate (10-1000 mu M) produced excitatory and inhibito ry responses on numerous identified neurons of the mollusc Lymnaea sta gnalis. Using both in situ and in vitro preparations, glutamate or glu tamate agonists produced a depolarization in identified neurons right pedal dorsal 1 and right pedal dorsal 2 and 3. However, attempts to bl ock glutamate-evoked responses with glutamate antagonists were unsucce ssful. We examined a potential glutamatergic neuron, visceral dorsal 4 . Exogenous application of the peptides (GDPFLRFamide and SDPFLRFamide ) could mimic the inhibitory, but not the excitatory effects of viscer al dorsal 4 on its postsynaptic cells, implying the presence of a seco nd transmitter. We tested the possibility that glutamate is this secon d neurotransmitter by using excitatory synapses between visceral dorsa l 4 and postsynaptic cells right pedal dorsal 2 and 3, right pedal dor sal 1, visceral F group and right parietal B group neurons. Of all the putative neurotransmitters tested, only glutamate had consistent exci tatory effects on these postsynaptic cells. Also, the amplitude of the right pedal dorsal 2 and 3 excitatory postsynaptic potentials was red uced in the presence of N-methyl-D-aspartate and other glutamate agoni sts, suggesting desensitization of the endogenous transmitter receptor . In conclusion, some identified Lymnaea neurons respond to glutamate via a receptor with novel pharmacological properties. Furthermore, a L ymnaea interneuron may employ glutamate as a transmitter at excitatory synapses. Copyright (C) 1996 IBRO. Published by Elsevier Science Ltd.