Dopamine agonists at repeated "autoreceptor-selective" doses: effects upon the sensitivity of A10 dopamine autoreceptors.
Previous reports have established the ability of dopamine (DA) agonists to stimulate inhibitory DA autoreceptors at doses which minimally stimulate postsynaptic DA receptors, suggesting that hyperactive DA transmission may be controlled clinically by treatment with DA agonists. Little is known, however, about the possible loss of autoreceptor sensitivity that may occur after repeated treatment with low doses of DA agonists. Extracellular single cell recording and microiontophoretic techniques were used to measure the sensitivity of impulse-regulating DA autoreceptors on A10 DA cells in the ventral tegmental area (VTA) of chloral hydrate-anesthetized rats pretreated for seven days with repeated subcutaneous (s.c.) doses of the DA agonist apomorphine (APO). The ability of intravenous (i.v.) administration of the potent D2 DA agonist quinpirole (QUIN) to inhibit the firing of A10 cells was not attenuated in rats pretreated with repeated low doses (2 x 50 micrograms/kg/day, s.c.) of APO for 7 days, although higher doses (2 x 250 or 500 micrograms/kg/day) did cause subsensitive responses to QUIN. In rats pretreated with repeated low doses of APO, microiontophoretic application of DA on A10 cells revealed somewhat subsensitive responses. However, ibotenic acid lesions of postsynaptic cells in the nucleus accumbens (NAc) prior to initiation of APO treatment (2 x 50 micrograms/kg/day) did not alter the response of A10 cells to systemic QUIN, contradicting the possibility that the feedback projection from the NAc to the VTA was compensating for autoreceptor down-regulation during systemic challenge with QUIN. In contrast, administration of the irreversible DA antagonist EEDQ (2 mg/kg, i.p.) to control and APO-treated rats (2 x 50 micrograms/kg/day) 24 hr prior to recording did reveal a difference in A10 cell sensitivity to systemic QUIN and to microiontophoretic DA between the two groups, suggesting that "spare" DA autoreceptors may have concealed the down-regulation of autoreceptors induced by repeated low doses of APO. Challenge of A10 DA cells with the partial DA autoreceptor agonist (-)-3-(3-hydroxyphenyl)-N-n-propylpiperidine [(-)3-PPP], for which an autoreceptor reserve should not exist, produced slightly attenuated responses in APO-treated rats (2 x 50 micrograms/kg/day). These findings provide evidence for the existence of spare somatodendritic DA autoreceptors on A10 DA cells with respect to potent DA agonists, suggesting that repeated administration of "autoreceptor-selective" doses of DA agonists may not result in a diminished inhibition of DA neuronal activity.