Distinct sites of functional interaction between dopamine, acetylcholine and γ-aminobutyrate within the neostriatum: An electromyographic study in rats

Abstract

In order to study the functional interaction between dopamine, acetylcholine and γ-aminobutyrate within the rat neostriatum, we investigated the effect of intrastriatal injection of different drugs acting on these transmitter systems on muscle tone measured as tonic activity in the electromyogram of the gastrocnemius muscle. Bilateral injection of haloperidol (500 ng) into the rostral neostriatum (rostral injection; A8920-9650 46) induced tonic activity m the electromyogram, whereas injection into the intermediate part (intermediate injection; A7020-7890 46) was ineffective. Muscimol (25 ng) induced tonic activity in the electromyogram, when injected into the intermediate part and not into the rostral part, while bethanechol (1μg) was effective when injected into either site. Haloperidol-induced tonic activity in the electromyogram was prevented by coadministration of apomorphine (500 ng) or scopolamine (1μg), but not of bicuculline (300 ng). Haloperidol-induced tonic activity in the electromyogram was also reduced by subsequent intermediate injection of scopolamine or bicuculline, while apomorphine was ineffective. Tonic activity in the electromyogram induced by rostral injection of bethanechol was prevented by coadministration of scopolamine, but not of apomorphine. Intermediate injection of scopolamine or bicuculline reduced the tonic activity in the electromyogram after rostral or intermediate injection of bethanechol. Tonic activity in the electromyogram induced by intermediate injection of muscimol was prevented by coadministration of bicuculline, but not of scopolamine. Rostral injection of apomorphine or scopolamine failed to alter the tonic activity in the electromyogram induced by intermediate injection of bethanechol or muscimol. These results point to the existence of: (1) a functional interaction between dopamine and acetylcholine in the rostral neostriatum; (2) a functional interaction between acetylcholine and γ-aminobutyrate in the intermediate neostriatum, and (3) a functional flow of information from the rostral to the intermediate neostriatum.