Modulation of the Spontaneous Synaptic Activity of Tortoise Spinal Motoneurons by Group-II Metabotropic Glutamate Receptors

We examined the effects of antagonists and an agonist of the metabotropic glutamate receptors (mGluR) on the frequency and amplitude of spontaneous postsynaptic potentials (PSP) and of a miniature fraction of these potentials in the lumbar segments of the spinal cord of the steppe tortoise (in 2- to 3-mm-thick superfused slices). We demonstrated that a common antagonist of the group-I and group-II mGluR, (+)MCPG (400 μM), as well as selective antagonists, MCCG (200 μM) and EGLU (100-200 μM), and a selective agonist of the group-II, DCG IV (1 μM), change the frequency of spontaneous PSP, including miniature PSP, but practically do not influence their amplitude. This feature shows that mGluR are presynaptically localized both in premotoneuronal links and immediately in synaptic contacts on the motoneurons. Comparison of the effects of antagonists of the mGluR on the normal synaptic activity and on that under conditions of the GABA receptor blockade shows that mGluR are involved in modulation of both glutamatergic and GABA-ergic transmission. We surmise that the NMDA reception plays a special role in the realization of mGluR-mediated modulating effects. The directions of the effects of the above antagonists and an agonist of the mGluR (an increase and a decrease in the frequency of synaptic potentials, respectively) allow us to postulate that the presynaptically localized group-II mGluR causes a decrease in the probability of release of excitatory and inhibitory transmitters in spinal synaptic structures of the tortoise.