Four types of GABA receptors in crayfish leg muscles characterized by desensitization and specific antagonist.

The effects of application of GABA were studied in the closer and stretcher muscle of crayfish walking legs and compared to those on the opener muscle. EPSPs were measured intracellularly and extracellularly at single synaptic spots, and the input resistance of the muscle fiber was determined. In contrast to the opener muscle, in the closer and stretcher GABA receptors desensitized nearly completely within 5-10 min in the presence of GABA. The presynaptic receptors desensitized more slowly than the postsynaptic ones. While in the opener muscle betaguanidino propionic acid (betaGPA) activates only the presynaptic GABA receptors, in the closer and stretcher muscles both the pre- and the postsynaptic receptors are activated by betaGPA. The postsynaptic GABA receptor on the closer muscle desensitizes in the presence of betaGPA. The results show that with respect to desensitization and the effect of betaGPA four types of GABA receptors can be distinguished. As far as is known from the literature, the homologous synapses in lobster and crab can be assumed to have the same receptor types as those found in crayfish.