THYROTROPIN-RELEASING-HORMONE IMMUNOREACTIVE BOUTONS FORM CLOSE APPOSITIONS WITH MEDULLARY EXPIRATORY NEURONS IN THE RAT

The aim of the present study was to assess the size of the input from TRH immunoreactive varicosities to medullary respiratory neurons in th e Botzinger complex and caudal ventral respiratory group. Neurobiotin was intracellularly injected into seven neurons in the Botzinger compl ex, between 0.4 and 0.9 mm caudal to the facial nucleus. Five of the s even Botzinger neurons had extensive local axonal projections, with bo uton-like varicosities clustered predominantly between their somata an d the nucleus ambiguus. Seven neurons in the caudal ventral respirator y group, located between 1.6 and 2.4 mm caudal to the facial nucleus, were also labelled. All but one caudal respiratory neurons had no, or very few, medullary collaterals. TRH immunoreactive fibres were seen i n many medullary nuclei, including the ventral reticular formation. Bo tzinger neurons were closely apposed by an average of 29 +/- 8 TRH imm unoreactive boutons/neuron (mean +/- S.D., n = 7). In contrast, caudal ventral respiratory group neurons were apposed by only 5 +/- 3 TRH im munoreactive boutons/neuron (n = 7). Botzinger neurons form many intra medullary and bulbospinal inhibitory connections with premotoneurons a nd motoneurons that are important in the timing, amplitude and shape, of respiratory activity. Our findings suggest a role for endogenous TR H-containing neurons in modulating the activity of inhibitory Botzinge r neurons and neurons in the caudal ventral respiratory group. The sig nificance of the apparent difference in size of this input remains to be determined.