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 neuron s (MNs) on to their presynaptic cholinergic sensory afferents. This negativ e control consists in small amplitude, slowly developing depolarizations of the primary afferents (sdPADs) strictly timed with MN bursts. They were no t blocked by picrotoxin, but were sensitive to glutamate non-N-methyl-D-asp artate (NMDA) antagonists. Intracellular recordings were performed within t hin branches of sensory terminals while electrical antidromic stimulation w ere applied to the motor nerves, or while glutamate (the MN neurotransmitte r) 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-resi stant and were dramatically reduced by non-NMDA antagonist bath application . These results indicate that sdPADs are likely directly produced by MNs du ring locomotor activity. A functional scheme is proposed.