5-HT2 RECEPTOR-CONTROLLED MODULATION OF MEDULLARY RESPIRATORY NEURONSIN THE CAT

The effects of the 5-HT2 receptor agonist alpha-methyl-5-HT were studi ed on the membrane of expiratory (E2) and post-inspiratory (PI) neuron es, by intracellular recordings in the caudal medulla of anaesthetized cats. Ionophoresis of alpha-Me-5-HT depolarized membrane potential an d increased action potential frequency in a majority of neurones teste d. Depolarization of neurones by alpha-Me-5-HT was accompanied by incr eased input resistance throughout all phases of the respiratory cycle. These effects were antagonized by ionophoresis of cinanserin, a recep tor-blocking agent with high affinity for 5-HT2 receptors. E2 neurones were voltage clamped to measure membrane current changes induced by a lpha-Me-5-HT ionophoresis. alpha-Me-5-HT induced a net inward current by reducing inspiratory-phase outward currents and increasing expirato ry-phase inward currents. These changes were equivalent with steady me mbrane depolarization, decreased inspiratory phase membrane hyperpolar ization and increased expiratory drive potential recorded from the sam e neurones in current clamp. The effects of alpha-Me-5-HT are consiste nt with activation of 5-HT2 receptors on E2 and PI neurones leading to blockade of synaptically activated and persistent conductances to pot assium ions.