CRAYFISH SENSORY TERMINALS AND MOTOR-NEURONS EXHIBIT 2 DISTINCT TYPESOF GABA RECEPTORS

Motor neurones of the crayfish walking system display inhibitory respo nses evoked either by gamma-amino butyric acid (GABA) or glutamate, po ssibly involving the same receptor (Pearlstein et al. 1994). In order to test if this sensibility to both GABA and glutamate was a specific property of crayfrsh GABA receptors, pharmacological characteristics o f GABA-evoked responses in both sensory terminals from CB chordotonal organ and motor neurones of the walking system have been compared. Bot h receptors are GABA-gated Cl- channels activated by specific GABA(A) (muscimol, isoguvacine), GABA(B) (3-aminopropyl phosphinic acid), and GABA(C) (cis-4-amino crotonic acid) agonists, and blocked by competiti ve (beta-guanidino propionic acid) and non-competitive (picrotoxin) an tagonists. They were insensitive to specific GABA(A) (bicuculline, SR- 95531) and GABA(B) (phaclofen) antagonists. Furthermore, in both cases , nipecotic acid and the modulatory drug diazepam had no effect. Howev er, our results demonstrate that GABA receptors of sensory terminals a re different from those of motor neurones. GABA-induced desensitisatio n only occurred in sensory terminals. Moreover, glutamate was shown to activate GABA-gated Cl- channels in motor neurones, but not in sensor y terminals. Therefore, GABA is likely to be the endogenous neurotrans mitter of presynaptic inhibition in sensory terminals, whereas inhibit ion between antagonistic motor neurones would be achieved by glutamate .