CHARACTERIZATION OF ACUTE HOMOLOGOUS DESENSITIZATION OF MU-OPIOID RECEPTOR-INDUCED CURRENTS IN LOCUS-COERULEUS NEURONS

1 Acute homologous desensitization of mu-opioid receptor-induced currents was pharmacologically characterized in locus coeruleus (LC) neurones by use of intracellular and whole cell recording in superfused brain slices. 2 Following desensitization of opioid receptors by perfusion with a high concentration of [Met(5)] enkephalin (ME) for 5 min, there was a reduction in the maximum response and a rightward shift of the concentration- response curves for ME, [D-Ala(2), N-MePhe(4), Gly-ol]enkephalin (DAMGO) and normorphine. 3 By simultaneously fitting the operational model to the paired pre- and post-desensitization concentration-response data for each agonist, estimates of the level of desensitization were obtained. The values obtained for the three agonists (between 88% and 96%) were similar and did not vary according to the efficacy of the agonist used. 4 Use of whole cell patch recording techniques caused a slow rundown in the amplitude of ME currents (approx. 40% reduction over 60 min) but did not greatly affect the expression of acute desensitization of opioid currents. 5 When included in the patch recording solution, the phosphatase inhibitors, microcystin (50 nM-4 mu M) and okadaic acid (1 mu M) had no effect on the induction of desensitization or the normal ability of opioid or alpha(2)-adrenoceptors to produce currents. Microcystin decreased the rate of recovery of the ME (3.00 nM) currents following desensitization; however, okadaic acid had little effect on the rate of recovery from desensitization. 6 Strong calcium buffering with BAPTA (10-20 mM) had no effect on desensitization or the recovery from desensitization. 7 These results suggest that acute homologous desensitization of mu-opioid receptors in LC neurones entails a rapid loss of responsiveness that involves a majority of the receptor population. The mechanism by which desensitization is reversed may involve a non-calcium-dependent protein phosphatase but the processess that cause desensitization remain unclear.