Effects of d-amphetamine and dopamine synthesis inhibitors on dopamine and acetylcholine neurotransmission in the striatum. II. Release in the presence of vesicular transmitter stores

The release of endogenous dopamine (DA) elicited by electrical stimulation and by d-amphetamine (AMPH) from superfused striatal slices of untreated rabbits was examined. AMPH (0.3-10 μM) produced a concentration-dependent increase in basal DA efflux (30-fold increase at 10 μM) and stimulation-evoked (SE) DA overflow (11-fold increase at 10 μM). Although AMPH had little effect on the basal efflux of dihydroxyphenylacetic acid (DOPAC), the drug was an effective inhibitor of the SE overflow of the DA metabolite (66% inhibition at 0.3 μM). AMPH increased significantly the total basal efflux of endogenous compounds (DA + DOPAC) only at high concentrations (3-10 μM) whereas the total SE overflow of total endogenous compounds was decreased at all concentrations of AMPH tested. AMPH inhibited SE [3H]acetylcholine (ACh) release in a concentration-dependent manner (71% inhibition at 10 μM). Inhibition of DA synthesis with α-methyl-p-tyrosine (100 μM) or 3-iodotyrosine (100 μM) reduced both the basal efflux and SE overflow of endogenous DA and DOPAC; synthesis inhibition had greater effects on the SE overflow. Neither synthesis inhibitor altered SE [3H]ACh release. α-Methyl-p-tyrosine and 3-iodotyrosine reduced the absolute values of the basal efflux and SE overflow of DA elicited by AMPH by approximately 60%; however, the inhibition of SE [3H]ACh release produced by AMPH was attenuated only slightly (~20%). Synthesis inhibitors also reduced tissue DA levels (~30%). These results suggest that: 1) basal efflux of endogenous DA from superfused rabbit striatal slices may be derived both from DA newly synthesized in the cytoplasm and from spontaneous leakage of DA from storage vesicles. In addition, synthesis may provide a continuous supply of DA to vesicles that are used for exocytotic DA release during electrical stimulation. However, the depletion of tissue DA produced by synthesis inhibitors as well as other extraneous pharmacological actions of these drugs makes firm conclusions difficult. 2) AMPH increases the synaptic concentration of DA by accelerating the basal efflux as well as the SE overflow of unchanged DA. At concentrations less than 1 μM AMPH has no effect on basal efflux of DA or DOPAC but reduces SE overflow of DOPAC via an unknown mechanism. At higher concentrations (≥1 μM) acceleration of carrier-mediated DA efflux coupled with displacement of DA from vesicular stores, as well as interference with the uptake of exocytotically released DA produces a marked increase in synaptic DA which in turn inhibits SE ACh release; 3) AMPH does not distinguish 'newly synthesized' and 'older storage' pools of DA but rather releases DA from a single (axoplasmic) pool. This pool is probably maintained by both vesicular and recently synthesized transmitter. 4) A hyperbolic function describes the relationship between the amount of endogenous DA in the effluent superfusate and SE inhibition of [3H]ACh release. Consequently, modulation of [3H]ACh release by DA occurs within a narrow range of DA concentrations and thus [3H]ACh release may be a potentially misleading index of synaptic DA concentration under certain conditions.