THE ACTIONS OF 5-HYDROXYTRYPTAMINE ON THE MEMBRANE OF PUTATIVE SYMPATHOEXCITATORY NEURONS IN THE ROSTRAL VENTROLATERAL MEDULLA OF THE ADULT-RAT INVITRO

Intracellular recordings were obtained in vitro from neurones lying within the rostral ventrolateral medulla of the adult rat. Neurones could be classified into three groups: silent neurones, irregularly firing neurones and neurones which had a regular pattern of action-potential generation. Membrane hyperpolarization of regularly firing neurones failed to reveal underlying EPSPs or disrupt the regular pattern of action-potential generation. Superfusion of a high Mg2+, low Ca2+ aCSF did not abolish action-potential generation but the regular pattern of firing of these neurones was lost. 5-Hydroxytryptamine evoked a slow concentration-dependent hyperpolarization in both spontaneously active and silent neurones, accompanied by a decrease in cell-input resistance. This study has provided further evidence for pacemaker-like neurones within the RVLM and for the modulation of these neurones by 5-hydroxytryptamine.