The possibility that the terminal serotonin (5-HT) autoreceptor in the rat hippocampus is coupled to G(i), G(o) or G(s) regulatory proteins was investigated using the electrically evoked overflow of [H-3]5-HT from preloaded slices. Pertussis toxin, which inactivates G(i/o) or cholera toxin, which stimulates G(s), was injected directly in the hippocampus 3 to 11 days prior to the experiments. Hippocampus slices were prepared, loaded with [H-3]5-HT, superfused continuously, and stimulated electrically 72 min (S1) and 116 min (S2) after the beginning of superfusion. In the absence of any drug, the evoked overflow of [H-3]5-HT in S1 was not altered by either toxin. The enhancing effect of the 5-HT reuptake blocker paroxetine (1-mu-mol/l) on the evoked [H-3]5-HT overflow was also unaltered by these toxins. 5-Carboxyamidotryptamine, a 5-HT autoreceptor agonist, inhibited in a concentration-dependent manner the stimulation-evoked release of [H-3]5-HT. The concentration-effect curve (0.001-0.1-mu-mol/1) for this drug was not altered by pretreatment with either pertussis or cholera toxin. Similarly, the effect of another 5-HT autorecetor agonist, 5-methoxytryptamine (0.1 and 1-mu-mol/l), was not altered in the pretreated rats. In addition, the reduction of [H-3]5-HT overflow obtained by increasing the stimulation frequency from 1 Hz to 5 Hz, which is due to an increase in terminal 5-HT autoreceptor activation at the higher frequency, was not altered by either toxin. The enhancing effect of the 5-HT autoreceptor antagonist methiothepin (1-mu-mol/l) on stimulation-evoked [H-3]5-HT overflow was not changed by either pretreatment. N-Ethylmaleimide inactivates G(i/o) proteins by alkylation. Preincubation with 30-mu-mol/l N-ethylmaleimide for 30 min did not alter the efficacy of the 5-HT reuptake blocker paroxetine to enhance [H-3]5-HT overflow nor did it alter the attenuating effect of 5-methoxytryptamine and the differential effectiveness of 1 and 5 Hz stimulations on the overflow of [H-3]5-HT. In conclusion, the results suggest that the terminal 5-HT autoreceptor in the rat hippocampus may not be coupled to G(i), G(o) or G(s) proteins.
