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 [³H]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 [³H]GBR-12935 in a biphasic manner (IC₅₀ values of 0.4 and 2.0 μM [high affinity] and 34.8 and 52.7 μM [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 [³H]GBR-12935 binding, while the selective 5-HT₃ receptor antagonist ICS 205–930 exhibited an IC₅₀ > 100 μM. The maximum density of [³H]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 [³H]dopamine uptake into synaptosomes of both brain regions, however it was significantly more potent in the caudate putamen (IC₅₀: 5.1 vs. 6.5 μM). 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.