Vagal pulmonary afferents and central respiratory effects of 5-HT in newborn rats.

In decerebrate newborn rats, serotonin (5-HT) is a respiratory depressant via activation of 5-HT2 receptors, whereas it evokes respiratory stimulant effects when applied to the isolated brainstem obtained from the newborn rat. This discrepancy could be due to deafferentation in the in vitro preparation. The aim of our study was to analyse the role of vagal afferents in the modulation of central respiratory effects of 5-HT.

Central effects of 5-HT on respiratory rhythm in newborn rats in vivo.

The role of 5-HT in inducing apnoeas (a major element in sudden infant death syndrome) is controversial because while 5-HT is a respiratory depressant in vivo, it evokes respiratory analeptic effects when applied to the isolated brainstem of the newborn rat. In decerebrate newborn rats, the electrical activity of the diaphragm and that of a hypoglossally innervated tongue muscle, as well as the cardiac frequency (Fc), were recorded before and after the application of 5-HT and related agents to the floor of the IVth ventricle. To assess the spontaneous variability in inspiratory frequency (Fi) and Fc, a sham group was studied.

Central effects of 5-HT on activity of respiratory and hypoglossally innervated muscles in newborn kittens.

1. In decerebrate kittens (n = 29), electrical activity was studied in the 3rd intercartilaginous (inspiratory), the 9th internal intercostal (expiratory) and the hypoglossally innervated muscles (geniohyoid m. and sternohyoid m.

Central effects of 5-HT on respiratory and hypoglossal activities in the adult cat.

The activities of the diaphragmatic, internal intercostal and hypoglossal-innervated muscles were studied in adult decerebrate cats in response to 5-HT and related agents (8-OH-DPAT and DOI). The drugs were placed on the floor of the IVth ventricle. The mean respiratory frequency (Fi) increased (124-193% of the control value) within 3 min of the 5-HT application, and decreased thereafter (30-90%).