The nonuniform distribution of the GABA(A) receptor alpha 1 subunit influences inhibitory synaptic transmission to motoneurons within a motor nucleus

Using immunohistochemistry we studied the distribution of GABA(A) and glyci ne receptor alpha1 subunits in the rat hypoglossal nucleus during postnatal development. In the neonate [postnatal day (P) 1-3] and adult nucleus (P28 -30), GABA(A) receptor alpha1 subunit labeling was relatively modest. Howev er, in the juvenile nucleus (P9-13), labeling was strong in the ventrolater al region and moderate in the dorsal region. Glycine receptor alpha1 subuni t labeling was strong and uniform in the juvenile and adult nucleus and abs ent in the neonate nucleus. GABA and glycine neurotransmitter labeling was uniform throughout the neonatal and juvenile nucleus. To study the function al consequences of this regional differential GABA(A) receptor alpha1 subun it distribution, we voltage clamped juvenile hypoglossal motoneurons (HMs) from the ventrolateral and dorsal regions and recorded spontaneous miniatur e IPSCs (mIPSCs). Pure GABAergic events had slower decay times than glycine rgic events. Although pure GABAergic and glycinergic decay times did not di ffer depending on HM location, the decays of mixed mIPSCs from ventrolatera l HMs, recorded without GABA(A) and glycine receptor antagonists, had signi ficantly slower decays than mIPSCs from dorsal HMs. Focally applied GABA an d glycine onto outside-out patches revealed that the GABAergic to glycinerg ic peak current amplitude ratio was larger for patches from ventrolateral H Ms compared with dorsal HMs. Dual component mIPSCs, presumably caused by co -release of GABA and glycine, were recorded more frequently in the ventrola teral nucleus. These data suggest that the number of synapses using GABA(A) receptor-mediated transmission is greater on ventrolateral HMs than dorsal HMs, demonstrating a nonuniformity of synaptic function within a defined m otor nucleus.