Induction of homosynaptic long-term depression at spinal synapses of sensory A delta-fibers requires activation of metabotropic glutamate receptors

The synaptic strength between primary afferent A delta-fibers, many of whic h convey pain-related information, and second order neurons in the spinal d orsal horn can be depressed for prolonged periods of time in a use- and N-m ethyl-D-aspartate receptor-dependent fashion. Here, we have used a transver se spinal cord slice-dorsal root preparation of young rat to characterize t he nature of this form of long-term depression and the role of metabotropic glutamate receptors. Dorsal roots were bisected and intracellular recordin gs were made from lamina II neurons with independent excitatory synaptic in puts from both dorsal root halves. Conditioning stimulation of one dorsal r oot half (1 Hz, 900 pulses) induced long-term depression that was specific for the stimulated pathway, i.e. homosynaptic in nature. The induction of l ong-term depression was prevented by non-selective group I and group II mGl uR antagonist (S)-alpha-methyl-4-carboxyphenylglycine, by selective group I receptor antagonist (S)-4-carboxyphenylglycine and by selective group II m GluR antagonist (RS)-alpha-methylserine-O-phosphate monophenyl ester. Group III mGluR antagonist (RS)-alpha- methylserine-O-phosphate was ineffective. Short-term depression was not affected by any of these antagonists. Thus, a homosynaptic form of long-term depression exists at putative nocice ptive synapses in the spinal dorsal horn and its induction requires the act ivation of both group I and II metabotropic glutamate receptors. (C) 2000 E lsevier Science Ltd. published by Elsevier Science Ltd. All rights reserved .