Direct glutamate-mediated presynaptic inhibition of sensory afferents by the postsynaptic motor neurons.

An in vitro preparation of the crayfish central nervous system was used to study a negative feedback control exerted by the glutamatergic motor neurons (MNs) on to their presynaptic cholinergic sensory afferents. This negative control consists in small amplitude, slowly developing depolarizations of the primary afferents (sdPADs) strictly timed with MN bursts. They were not blocked by picrotoxin, but were sensitive to glutamate non-N-methyl-D-aspartate (NMDA) antagonists. Intracellular recordings were performed within thin branches of sensory terminals while electrical antidromic stimulation were applied to the motor nerves, or while glutamate (the MN neurotransmitter) was pressure-applied close to the recording site. Electrical motor nerve stimulations and glutamate pressure application had similar effects on to sensory terminals issued from the coxo-basipodite chordotonal organ (CBTs): like sdPADs, both stimulation-induced depolarizations were picrotoxin-resistant and were dramatically reduced by non-NMDA antagonist bath application. These results indicate that sdPADs are likely directly produced by MNs during locomotor activity. A functional scheme is proposed.