Chronic fluoxetine treatment selectively uncouples raphe 5-HT_1A receptors as measured by [³⁵S]-GTPγS autoradiography

1. Selective Serotonin Reuptake Inhibitors (SSRIs) are thought to have a delay in therapeutic efficacy because of the need to overcome the inhibitory influence of raphe 5-HT_1A autoreceptors. Prolonged SSRI administration has been reported to desensitize these autoreceptors. We have used [³⁵S]-GTPγS autoradiography to determine whether this desensitization occurs at the level of receptor/G protein coupling: 2. Male mice were injected intraperitoneally once a day with saline or 20 mg kg^-1 fluoxetine for either 2 days or 14 days, 5-HT_1A receptor binding and coupling to G proteins were assessed using [³H]-8-OH-DPAT and [³⁵S]-GTPγS autoradiography, respectively. 3. The 5-HT receptor agonist 5-carboxamidotryptamine (5-CT) stimulated [³⁵S]-GTPγS binding in the substantia nigra, as well as in hippocampus and dorsal raphe nucleus. The 5-HT_1A receptor antagonist p-MPPF (4-fluoro-N-(2-[4-(2-methoxyphenyl)l-piperazinyl]ethyl)-N-(2-pyridinyl) benzamide) blocked this effect in the latter regions, whereas the 5-HT_1B/D antagonist GR-127,935 (2′-methyl-4′-(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4- carboxylic acid [4-methoxy-3-(4-methyl-piperazin-1-yl)-phenyl]-amide) only decreased labelling in substantia nigra. 4. Fourteen-day flouxetine treatment decreased 5-CT-stimulated [³⁵S]-GTPγS binding in dorsal raphe (saline: 112 ± 12% stimulation; fluoxetine: 66 ± 13%), bur not in substantia nigra (99 ± 14% vs 103±7%) or hippocampus (157±3% vs 148±18%). Two-day fluoxetine treatment did not alter 5CT-stimulated [³⁵S]-GTPγS binding in any of the brain areas investigated. 5. Decreased [³⁵S]-GTPγS binding was not due to receptor down-regulation, since the density of raphe [³H]-8-OH-DPAT binding sites was unaffected by fluoxetine treatment. 6. These results suggest that the desensitization of presynaptic 5-HT_1A receptor function occurs at the level of receptor-G protein interaction on dorsal raphe neurons, and may underlie the therapeutic efficacy of long-term SSRI treatment.