Continued morphine modulation of calcium channel currents in acutely isolated locus coeruleus neurons from morphine‐dependent rats

The actions of the opioid agonists morphine and methionine‐enkephalin (met‐enkephalin) on the calcium channel currents (IBa) of acutely isolated locus coeruleus (LC) neurons from morphine‐dependent and vehicle‐treated rats were examined using whole cell patch clamp techniques. In LC neurons maintained in 5 μM morphine, co‐superfusion of naloxone (1 μM) or the μ‐opioid receptor antagonist CTAP (D‐Phe‐Cys‐Tyr‐D‐Trp‐Arg‐Thr‐Pen‐Thr‐NH2 1 μM) with morphine resulted in a significant increase in the amplitude of IBa. The increases in IBa were not different in neurons from morphine‐dependent or vehicle rats. The increase in IBa was mimicked by washing off morphine, but not by co‐superfusion of the κ‐receptor antagonist norbinaltorphimine (300 nM) or the δ‐receptor antagonist ICI‐174864 (1 μM). In spontaneously withdrawn LC neurons from morphine‐dependent rats, met‐enkephalin (pD2 7.1, maximum inhibition 49%) and morphine (pD2 6.5, maximum inhibition 33%), inhibited IBa in all cells. In cells from vehicle rats the pD2 for met‐enkephalin was 7.3, maximum inhibition 52%, while the pD2 for morphine was 6.6 and the maximum inhibition 43% (P