5-HT(2A/2C) receptor-mediated hypopnea in the newborn rat: relationship to Fos immunoreactivity.

Previous data derived from anesthetized, decerebrate, or in vitro preparations suggested that 5-HT(2) receptor activation might be responsible for respiratory dysfunction. Such a mechanism has not yet been documented in the intact animal, but might be of clinical relevance to the apneic spells of the premature infant. In the present investigation on conscious newborn rats we analyzed the respiratory response to the activation of 5-HT(2A/2C) receptors by the agonist 1-(2.5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), and we delineated central structures possibly involved in this response, using Fos expression as a marker of neuronal activation. We demonstrated that intraperitoneal injection of 5 mg/kg DOI produced a long-lasting decrease in respiratory frequency and tidal volume, which could be blocked by the antagonist ritanserin. Fos immunohistochemistry suggested that the rostral ventrolateral medulla and the lateral paragigantocellular nucleus might have a key role in the respiratory response to 5-HT(2) receptor activation. In addition, double immunostaining for Fos and tyrosine hydroxylase suggested that the contribution of catecholaminergic neurons to this response might be modest and indirect.