Chronic morphine decreases calbindin D28k immunoreactivity in a subset of cerebellar Purkinje neurons of rat brain.

Calbindin D28k is an intracellular calcium binding protein that is expressed in the cell bodies, nuclei, dendrites, and axons of nearly all Purkinje neurons of the rat cerebellum. Acute morphine administration has been reported to decrease the level of calbindin mRNA in extracts of whole rat cerebellum [75]. Using immunocytochemistry, we studied the effects of chronic morphine administration and morphine abstinence on levels of calbindin in cerebellar Purkinje neurons. Treatment of male rats for 5 days with either morphine injections (10 mg/kg s.c., twice daily) or subcutaneously implanted morphine pellets (75 mg/pellet, once daily) markedly decreased levels of calbindin immunoreactivity in long stretches of Purkinje cell bodies in various folia of cerebellum. After 7 days of abstinence from morphine, the number of calbindin-positive neurons was still significantly decreased, and at 14 days of abstinence, the number of labeled neurons continued to be below that in control rat brain. The effects of morphine in cerebellum were not antagonized by co-administration of the N-methyl-D-aspartate receptor antagonist, MK-801 (0.2 mg/kg, twice daily for 5 days). MK-801 alone also decreased the number of calbindin-positive cells, but in a different pattern from that of morphine. Our findings of decreased calbindin immunoreactivity in Purkinje neurons following chronic morphine administration and abstinence suggest that persistent alterations in intracellular calcium buffering may be associated with opiate tolerance and dependence in cerebellum.