Function and modulation of GABAergic and glycinergic transmission in neonatal rat hypoglossal motoneurons

In the present study, I investigated the function and modulation of glycinergic and GABAergic transmission in neonatal rat hypoglossal motoneurons. Whole cell recording were made from brain stem slice preparation, and kynurenic acid (2 mM), strychnine (0.4 μM) or bicuculline (10 μM) were used as pharmacological tools to block glutamatergic, glycinergic or GABAergic transmissions, respectively. Current clamp experiments showed that cell firing, elicited by current pulse injection, is inhibited by evoked glycinergic or GABAergic postsynaptic potentials. The mechanism proposed is a shunt inhibition of the membrane excitability, due to the fall in input resistance induced by chloride channel opening. In voltage clamp experiments, I tested the effect of muscarine application on spontaneous postsynaptic currents (sPSCs), miniature postsynaptic currents (mPSCs), electrically evoked postsynaptic currents ( ePSCs) and pressure-pulse evoked postsynaptic currents. Muscarinic receptor activation strongly reduced amplitude and frequency of spontaneous glycinergic and GABAergic transmission (with no major differences between the two transmitters). Miniature GABAergic currents resulted completely unaffected by muscarine, while glycinergic ones were slightly reduced in amplitude. Evoked glycinergic and GABAergic currents were also reduced in amplitude by muscarine application. Postsynaptically, muscarine depressed the maximal amplitude of both glycinergic and GABAergic dose-response curves. To identify the muscarinic receptor subtypes involved in the modulatory actions of muscarine, I used pirenzepine, AFDX-116, DAMP or tropicamide as selective antagonists against Mi, M2, M3, or M4 receptor subtypes, respectively. Muscarininc actions on evoked glycinergic or GABAergic transmission were apparently mediated by M 2 or Mi and M3 receptors, respectively, while, as far as spontaneous transmission is concerned, the pharmacological pattern of antagonist block was more complex. However, experiments indicated a predominant role for putative, presynapticallylocated M2 receptors. Finally, postsynaptic M2 and M3 receptors accounted for muscarine action on postsynaptic glycinergic or GABAergic currents, respectively. These results suggest that various muscarinic receptors down-regulated action-potential dependent inhibitory transmission via postsynaptic and network-based action. Moreover, muscarinic modulation of glycine or GABA transmission was due to different receptor subtypes, indicating that these two transmitters are released from different interneurons, and that their co-release from the same synaptic terminal, if present, represents a limited phenomenon only.