THYROTROPIN-RELEASING-HORMONE INPUTS ARE PREFERENTIALLY DIRECTED TOWARDS RESPIRATORY MOTONEURONS IN RAT NUCLEUS AMBIGUUS

In the present study, we assessed the extent of the thyrotropin-releas ing hormone (TRH) input to motoneurons in the ambigual, facial, and hy poglossal nuclei of the rat using a combination of intracellular recor ding, dye filling, and immunohistochemistry. Twelve motoneurons in the rostral nucleus ambiguus were labelled by intracellular injection in vivo of Neurobiotin (Vector). Seven out of 12 ambigual motoneurons dis played rhythmic fluctuations of their membrane potential in phase with phrenic nerve discharge, whereas the other five had no modulations of any kind. Seven facial motoneurons and seven hypoglossal motoneurons were also filled with Neurobiotin. All three motor nuclei contained TR H-immunoreactive varicosities, with the largest numbers found in the n ucleus ambiguus. Close appositions were seen between TRH-immunoreactiv e boutons and every labelled motoneuron. Respiratory-related motoneuro ns in the nucleus ambiguus received the largest number of TRH appositi ons with 74 +/- 38 appositions/neuron (mean +/- S.D.; n = 7). In contr ast, nonrespiratory ambigual motoneurons received significantly fewer TRH appositions (11 +/- 5; n = 5; P < 0.05; Mann-Whitney U test). Faci al motoneurons received about the same number of TRH appositions as no nrespiratory ambigual motoneurons, with 13 +/- 4 (n = 7). Hypoglossal motoneurons received the fewest appositions from TRH-containing bouton s, with 8 +/- 2 (n = 7). There were no differences in the TRH inputs t o respiratory and nonrespiratory motoneurons in the facial and hypoglo ssal nuclei. These results demonstrate that, among motoneurons in the medulla, respiratory motoneurons in the rostral nucleus ambiguus are p referentially innervated by the TRH-immunoreactive boutons. (C) 1995 W iley-Liss, Inc.