Dopamine release and presynaptic dopaminergic regulation in guinea pig spinal cord

Using pharmacological approaches, we obtained evidence for the release of dopamine (DA) and its dopaminergic regulation in the guinea pig spinal cord. Electrical stimulation of the cord pieces increased the endogenous DA release and the efflux of [3H]dopamine [( 3H]DA) from tissues preloaded with [3H]DA. The evoked release of [3H]DA was current- and frequency-dependent and was prevented by tetrodotoxin (10(-6) M) or Ca2+-free medium containing EGTA (10(-4) M), while benztropine allowed a recovery of a more extensive amount of [3H]DA in the superfusing medium by inhibiting the reuptake process. The stimulated [3H]DA release was reduced by LY-171555, but not by SKF-38393 and 8-Br-cAMP. (-)Sulpiride enhanced the stimulated endogenous DA and [3H]DA release. (-)Sulpiride reversed the inhibition of evoked [3H]DA release induced by LY-171555, while SKF-38393 and 8-Br-cAMP did not affect LY-171555-induced inhibition of evoked [3H]DA release. These findings provide additional evidence for the neurotransmitter role of DA in the spinal cord of the guinea pig, and they strongly suggest the presence of presynaptic regulation of DA release via the dopamine receptor which is the D2 type.