Dual modulation of excitatory synaptic transmission by agonists at group I metabotropic glutamate receptors in the rat spinal dorsal horn.

The effects of group I metabotropic glutamate (mGlu) receptors on excitatory transmission in the rat dorsal horn, but mostly substantia gelatinosa, neurons were investigated using conventional intracellular recording in slices. The broad spectrum mGlu receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S, 3R-ACPD), the group I mGlu receptor selective agonist (S)-3, 5-dihydroxyphenylglycine (DHPG), and the selective mGlu subtype 5 agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), all induce long-lasting depression of A primary afferent fibers-mediated monosynaptic excitatory postsynaptic potential (EPSP), and long-lasting potentiation of polysynaptic EPSP, and EPSP in cells receiving C-afferent fiber input. The DHPG potentiation of polysynaptic EPSP was partially or fully reversed by (S)-4-carboxyphenylglycine (S-4CPG), the mGlu subtype 1 preferring antagonist. 2-Methyl-6-(phenylethynyl)-pyridine, the potent and selective mGlu subtype 5 antagonist, partially reversed the CHPG potentiation of polysynaptic EPSP. The effects of DHPG on monosynaptic and polysynaptic EPSPs were reduced, or abolished, by the N-methyl-D-aspartate (NMDA) receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (AP5). A clear and pronounced facilitation of the expression of DHPG- and CHPG-induced enhancement of polysynaptic EPSP, and EPSP evoked at C-fiber strength, was seen in the absence of gamma-aminobutyric acid subtype A receptor- and glycine-mediated synaptic inhibition. Besides dual modulation of excitatory synaptic transmission, DHPG induces depression of inhibitory postsynaptic potentials evoked by primary afferent stimulation in dorsal horn neurons. In addition, group I mGlu receptor agonists produced a direct persistent excitatory postsynaptic effect consisting of a slow membrane depolarization, an increase in input resistance, and an intense neuronal discharge. Cyclothiazide and (S)-4-CPG, the mGlu receptor subtype 1 preferring antagonists, significantly attenuated the DHPG-induced depolarization. These results demonstrate that the pharmacological activation of group I metabotropic glutamate receptors induces long-term depression (LTD) and long-term potentiation (LTP) of synaptic transmission in the spinal dorsal horn. These types of long-term synaptic plasticity may play a functional role in the generation of post-injury hypersensitivity (LTP) or antinociception (LTD).