Prolonged pertussis toxin treatment affects morphine's action on tuberoinfundibular dopaminergic neuron activity and on prolactin secretion

Abstract

The effects of pertussis toxin (PTX) pretreatment on basal and morphine-affected changes of tuberoinfundibular dopaminergic (TIDA) neuron activity and serum prolactin level were tested in this study. Adult female Sprague-Dawley rats, ovariectomized and treated with a long-acting estrogen (polyestradiol phosphate, 0.1 mg/rat, s.c.), were used. The activity of TIDA neurons was determined by measuring the turnover rate of dopamine (DA), and the concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) or 3,4-dihydroxyphenylalanine (DOPA) in the median eminence. Acute (30–90 min) treatment of PTX had no significant effect on any of the parameters measured. Prolonged (24 h) treatment of PTX significantly reduced morphine's inhibitory effect on TIDA neuron activity (using DOPA, but not DOPAC as the index), and stimulatory effect on PRL release. Basal TIDA neuron activity as determined by median eminence DOPAC concentration, DOPA accumulation, or DA rate constant was not significantly altered by PTX. Median eminence DA level, however, was significantly reduced. These results suggest that a pertussis toxin-sensitive GTP-binding protein may be responsible for the maintenance of TIDA neurons, and for mediation of the inhibitory effect of morphine on TIDA neuron activity, and in turn, the stimulation of prolactin secretion.