Repeated administration of milnacipran induces rapid desensitization of somatodendritic 5-HT1A autoreceptors but not postsynaptic 5-HT1A receptors.

The effects of the repeated administration of milnacipran, a serotonin (5-HT)-noradrenaline reuptake inhibitor (SNRI), on the functional status of somatodendritic 5-HT1A receptors, and postsynaptic 5-HT1A receptors were explored using electrophysiological approaches in rats. In-vitro electrophysiological recordings in the dorsal raphe nucleus showed that 5-HT inhibited the firing of serotonergic neurones, and the selective 5-HT1A receptor antagonist, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane carboxamide (WAY 100635), reversed the inhibitory effect of 5-HT. The potency of 5-HT to inhibit the firing of serotonergic neurones was slightly attenuated after 3 days of treatment with milnacipran (30 mg/kg, p.o., twice daily), and significantly attenuated after 7 or 14 days treatment at the same dose. The tricyclic antidepressant, imipramine, did not significantly modify the inhibitory effect of 5-HT. After 7 days treatment at 30 mg/kg, p.o., once daily, milnacipran reduced the potency of 5-HT to inhibit the firing of serotonergic neurones, whereas the selective serotonin reuptake inhibitors, fluvoxamine and fluoxetine (60 and 30 mg/kg, p.o., once daily, respectively), did not modify it under these conditions. Treatment with milnacipran (30 mg/kg, p.o., twice daily) for 14 days did not change the inhibition of the CA1 field potential in rat hippocampal slices by 5-HT. These data suggest that somatodendritic 5-HT1A receptors, but not postsynaptic 5-HT1A receptors, rapidly desensitize in response to the repeated administration of milnacipran.