Effects of iontophoretically applied (+)- and (-)-naloxone on rat hypothalamic and septal neurons

The purpose of this study was to investigate the possible independent actions of both (+)-and (-)-naloxone on individual neurons in the preoptic/anterior hypothalamus (POAH) and septal area (SA) of the rat brain. Morphine and (-)-naloxone were applied to 31 neurons in the SA (n = 11) and the POAH (n = 20). Morphine depressed the spontaneous activity in 19 of 31 neurons. (-)-Naloxone at currents less than 10 nA did not influence these neurons. However, (-)-naloxone applied in excess of 10 nA reduced spontaneous activity in 28 of 29 neurons. This effect of (-)-naloxone was stereospecific; (+)-naloxone did not alter the spontaneous rate in 12 of 14 cells when alternately applied with (-)-naloxone at the same current intensity. Application of (+)- and (-)-naloxone at supramaximal currents produced a diminution of spike amplitude and an increase in the duration of the action potential. The results of this study indicate that naloxone reduces spontaneous activity via two mechanisms. One involves a direct stereospecific action, and a second produces a non-specific reduction in spike amplitude and a prolongation of spike duration.