Abstract
We studied the effects of GABA B receptor activation on either glycine or GABA A receptor-mediated synaptic transmission to hypoglossal motoneurons (HMs, P8-13) using a rat brainstem slice preparation. Activation of GABA B receptors with baclofen, a GABA B receptor agonist, inhibited the amplitude of evoked glycine and GABA A receptor-mediated inhibitory postsynaptic currents. Additionally, with blockade of postsynaptic GABA B receptors baclofen decreased the frequency of both glycine and GABA A receptor-mediated spontaneous miniature inhibitory postsynaptic currents (mIPSCs), indicating a presynaptic site of action. Conversely, the GABA B receptor antagonist CGP 35348 increased the frequency of glycine receptor-mediated mIPSCs. Application of the GABA transport blocker SKF 89976A decreased the frequency of glycinergic mIPSCs. Lastly, we compared the effects of baclofen on the frequency of glycine and GABA A receptor-mediated mIPSC during HM development. At increased postnatal ages (P8-13 versus P1-3) mIPSC frequency was more strongly reduced by baclofen. These results show that presynaptic GABA B receptors inhibits glycinergic and GABAergic synaptic transmission to HMs, and the presynaptic sensitivity to baclofen is increased in P8-13 versus P1-3 HMs. Further, endogenous GABA is capable of modulating inhibitory synaptic transmission to HMs.
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