Abstract
The method of trans-striatal dialysis has been applied here to the study of the release and metabolism of dopamine (DA) in the awake rat. DA and its acidic metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), present in the dialysates were separated by high performance liquid chromatography on reverse phase columns and estimated by electrochemical detection. In the awake rat, DA, DOPAC, and HVA could be recovered and quantitated in the dialysates for at least 4 days from the time of implantation of the dialysis tube. At a constant 2-microliters/min flow of Ringer in the dialysis tube, the output of the substances recovered in 20-min samples 24 hr after the implantation was as follows: DA, 0.318 +/- 0.035; DOPAC, 41.3 +/- 4.84; HVA, 32.98 +/- 3.79 (mean picomoles +/- SEM of six 40-microliters samples). The output of DA, DOPAC, and HVA decreased slowly so that 4 days after the implantation the output of DA was reduced by about 35% in respect to the 24-hr values. After a 24-hr recovery, drugs were administered and their effect on DA release and metabolism was investigated. Drugs of different chemical structure and spectrum, but having in common the property of blocking DA receptors and being effective neuroleptics such as haloperidol, sulpiride, and flupentixol, stimulated DA release and DOPAC and HVA output. Threshold doses for this effect were very low, being 0.012 mg/k, s.c., for haloperidol, 2.5 mg/kg, s.c., for (-)-sulpiride, and 0.025 mg/kg, s.c., for cis- flupentixol. This effect was stereospecific as the (+) form of sulpiride and the trans- form of flupentixol were at least 10 to 100 times less potent than their enantiomer. The stimulation of DA release was shorter-lasting than the stimulation of DA metabolism and sedation or catalepsy. Moreover, whereas DA release did not increase by more than 100% over basal values, DOPAC and HVA increased by more than 3 times after maximally effective doses of neuroleptics. gamma- Butyrolactone (200 mg/kg, i.p.) reversed haloperidol (0.1 mg/kg, s.c.), and sulpiride (20 mg/kg, s.c.) induced stimulation of DA release while it potentiated the stimulation of DOPAC and HVA output. These data indicate that stimulation of DA release by neuroleptics is strictly dependent upon stimulation of DA firing and that different mechanisms underline their effects on DA release and on DA metabolism.
