Dopamine (DA) inhibits the evoked release of the neurotransmitters DA, acetylcholine (ACh) and gamma-aminobutyric acid (GABA) in rat striatal. The unifying hypothesis of this proposal is that these striatal release-modulating receptors are all of the D-2 subtype but that their second messengers and regulation may differ. Differences in the pharmacological profiles of these three receptors have been reported. These may reflect differences in (1) the neuronal location of the receptors, (2) the amounts of synaptic DA to which the receptors are exposed and (3) the contributions from D-1 DA receptor activation. The pharmacological profiles will be determined from dose-response curves for DA receptor agonist and antagonist modulation of electrically-evoked 3H-DA, 14C-ACH and 3H-GABA release from rat striatal slices. Tetrodotoxin will be used to determine whether axonal transmission is necessary for the action of these receptors. Synaptic DA levels will be depleted with alpha-methyl-p-tyrosine. The influence of concomitant D-1 DA receptor activation will be evaluated using Schild analysis. Next, the involvement of multiple mechanisms - including coupling to GTP binding proteins, inhibition of adenylate cyclase activity, decreased activation of voltage-sensitive calcium channels and activation of a potassium conductance - in striatal D-2 receptor modulation of neurotransmitter release will be investigated. Specifically, the effects of (1) treatment with pertussis toxin, (2) agents that elevate cAMP and calcium levels and (3) calcium and potassium channels blockers on the actions of these striatal D-2 release modulatory receptors will be investigated. Lastly, it is likely that the D-2 receptors that modulate evoked 3H-DA release, because of their inhibitory presynaptic nature, will be regulated differently from those that modulate 14C-ACh and 3H-GABA release. Changes in receptor sensitivity will be induced by chronic administration of two DA receptor antagonists or by nigrostriatal denervation. In order to determine whether changes in the sensitivity of these release-modulating receptors are those measured by radioligand binding assays, the changes in the release- modulating receptors will be compared with those in the DA receptors measured with quantitative autoradiographic receptor analysis. Striatal D-2 DA receptors appear to be important both in the therapeutic actions of antiparkinsonian agents and in the untoward side effects of antipsychotic agents. It is our hope that the results of the proposed experiments will help to distinguish both pharmacologically, as well as physiologically, the specific subpopulation of release-modulating D-2 DA receptors in the striatum.
