Blockade of Striatal Neurone Responses to Morphine by Aminophylline: Evidence for Adenosine Mediation of Opiate Action

1 The responses of cortical and striatal neurones to morphine and adenosine applied iontophoretically have been studied in the male rat. 2 The majority of cells (57%) within the corpus striatum were profoundly inhibited, and a smaller proportion (18%) excited by morphine. Adenosine depressed the firing rate of 30/44 cells in the striatum, excitation never being observed. In contrast, the responses of cortical cells to morphine were typically weak and required longer ejection pulses to effect comparable changes in firing rate. 3 Aminophylline applied iontophoretically, as an anion, proved able to antagonize reversibly both morphine and adenosine in parallel. 4 On a number of cells, gamma-aminobutyric acid (GABA) was used as a control depressant but aminophylline did not appear to reduce these responses. 5 The responses to morphine (both inhibitory and excitatory) were not easily antagonized by naloxone. Typically, excitatory reponses were easier to antagonize than the inhibitory ones. 6 It is concluded that a consequence of the interaction of morphine with its receptors may be the release of adenosine which subsequently produces the inhibition observed with morphine.