A comparison of the effects of methylphenidate and amphetamine on the simultaneous release of radiolabelled dopamine and p- or m-tyramine from rat striatal slices.

The release of [14C]dopamine (DA) from slices of rat caudate nucleus was studied simultaneously with the release of either [3H]para-tyramine (pTA) or [3H]meta-tyramine (mTA). Amphetamine (10(-5) M) caused a large concurrent release of [14C]DA and [3H]pTA; similar results were obtained when [14C]DA and [3H]mTA release were studied. The release of all three amines by amphetamine was quantitatively similar. In contrast, methylphenidate caused a release of [3H]pTA similar to that seen with amphetamine, but only a very small simultaneous release of [14C]DA. [3H]mTA was also strongly released by methylphenidate concurrent with a minimal release of [14C]DA. The inclusion of reserpine in the incubation medium had no detectable effect on the release of any of the three amines by amphetamine. Methylphenidate-induced release of tritiated mTA and pTA was also unaffected by reserpine. However, the release of [14C]DA by methylphenidate was potentiated in the presence of reserpine. The uptake of radiolabelled pTA, mTA and DA was inhibited by both amphetamine and methylphenidate, although amphetamine was a stronger inhibitor of the uptake of all three amines. It is suggested that release of endogenous tyramines may be involved in mediating some actions of psychomotor stimulant drugs.