THE 5-HT3, RECEPTOR AGONIST 1-(M-CHLOROPHENYL)-BIGUANIDE INTERACTS WITH THE DOPAMINE TRANSPORTER IN RAT-BRAIN SYNAPTOSOMES

The ability of the 5-HT3 receptor agonist 1-(m-chlorophenyl)-biguanide to bind to the dopamine transporter and inhibit [H-3]dopamine uptake was investigated in rat brain synaptosomes from the nucleus accumbens and caudate putamen. Competitive displacement experiments showed that 1-(m-chlorophenyl)-biguanide inhibited the binding of [H-3]GBR-12935 in a biphasic manner (IC50 values of 0.4 and 2.0 mu M [high affinity] and 34.8 and 52.7 mu M [low affinity] for caudate putamen and nucleus accumbens, respectively), and the high affinity binding site differed between brain regions. Serotonin was ineffective at competing for [H-3]GBR-12935 binding, while the selective 5-HT3 receptor antagonist ICS 205-930 exhibited an IC50 > 100 mu M The maximum density of [H-3]GBR-12935 binding sites was more than two-fold greater in the caudate putamen than in the nucleus accumbens (6.9 vs. 2.7 pmol/mg protein), and K-D values were similar (4.7 and 4.2 nM). 1-(m-chlorophenyl)-biguanide was able to inhibit [H-3]dopamine uptake into synaptosomes of both brain regions, however it was significantly more potent in the caudate putamen (IC50:5.1 vs. 6.5 mu M) The results demonstrate that some of the reported dopamine releasing effects of 1-(m-chlorophenyl)-biguanide may be due in part to activity at the dopamine transporter, and further suggest a possible difference in dopamine uptake parameters between the caudate putamen and nucleus accumbens.