Effect of α1-adrenergic receptor antagonist on the noradrenaline-induced facilitation in respiratory rhythm in newborn rat pons-medulla-spinal cord preparations

We hypothesized that facilitation of respiratory rhythm by noradrenaline (NA) in rat pons-medulla-spinal cord preparations is mediated through α1-adrenergic receptors. In 0- to 4-day-old rats, the respiratory frequency (fR) was monitored at the C4 ventral root and trigeminal motor (VMO) outputs. fR at temperature (Te) = 23 °C was lower than that at a higher Te (27 °C) and was increased by NA. At 23 °C, lower concentrations of NA were needed to produce the same increases in fR seen at 27 °C. With highest NA concentration we tested (50 μM), activity at C4 was maintained in all preparations at both Te, whereas that at VMO was maintained in 50% (27 °C) or 88% (23 °C) of the preparations. Particularly, tonic activity at C4 appeared in all preparations at both Te, but that at the VMO occurred in 0% (27 °C) or 18% (23 °C) of the preparations. Based on these results, we used the lower Te (23 °C) and applied a low concentration of NA (3 μM) to the preparations. We found that: (1) with the addition of NA, fR was increased without the occurrence of tonic activity and (2) NA-related fR facilitation was inhibited by pre-treatment with the α1-adrenergic receptor antagonist prazosin (2 μM). fR was increased by application of the α1-adrenergic receptor agonist phenylephrine (4 μM), and this response was inhibited by prazosin (4 μM). At Te = 23 °C, fR facilitation by NA in newborn rat pons-medulla-spinal cord preparations was obtained by activation of α1-adrenergic receptors.