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.
