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Abstract
The purpose of the present study was to characterize the metabolism of dopamine (DA)
in tuberoinfundibular (TI) neurons terminating in the median eminence and to examine
the effects of procedures that alter the synthesis and turnover of DA in these neurons
on the concentrations of dihydroxyphenylacetic acid (DOPAC) in the median eminence.
The DA uptake inhibitor nomifensine (25 mg/kg, i.p.; 30 min) failed to alter median
eminence DOPAC concentrations indicating that very little released DA is recaptured
and metabolized by TIDA neurons. Within 5 min following the administration of the
monoamine oxidase inhibitor pargyline (50 mg/kg, i.v.) median eminence DOPAC concentrations
declined to 15% of control demonstrating that this metabolite has a high turnover
rate and is rapidly removed from the median eminence. Median eminence DOPAC concentrations
in diestrous female rats, whose TIDA neuronal activity is higher than in the male,
were two-fold greater than in male rats. Prolactin (10 micrograms/rat, i.c.v.; 12
h), which increases TIDA neuronal activity, produced a corresponding increase in median
eminence DOPAC concentrations in male rats. Restraint stress (30 min), which decreases
TIDA neuronal activity, produced a corresponding decrease in median eminence DOPAC
concentrations in diestrous female rats. The results from the present study suggest
that DOPAC concentrations in the median eminence can be used as an index of TIDA neuronal
activity.