Opposite effects of antidepressants and corticosterone on the sensitivity of hippocampal CA1 neurons to 5-HT1A and 5-HT4 receptor activation

Using extracellular and intracellular ex vivo recording techniques we studied changes in the reactivity of hippocampal pyramidal CA1 neurons to serotonin (5-HT) and to the 5-HT1A- and 5-HT4 receptor agonists (+/-)-2-dipropylamino-8-hydroxy- 1,2,3 ,4-tetrahydronaphthalene hydrobromide (8-OH-DPAT) and zacopride, respectively, evoked by repeated electroconvulsive shock (ECS), imipramine and corticosterone treatments. Rats were subjected to ECS for 1 or 10 days, treated with imipramine for 1, 7, 14 or 21 days (10 mg/kg p.o., twice daily) and with corticosterone for 7 days (10 mg/kg s.c., twice daily). Hippocampal slices were prepared 2 days after the last treatment. Activation of 5-HT1A receptors decreased the amplitude of population spikes evoked by stimulation of the Schaffer/collateral-commissural pathway and hyperpolarized CA1 cells. Activation of 5-HT4 receptors increased the population spike amplitude and decreased the amplitude of slow afterhyperpolarization. Both repeated ECS and imipramine enhanced the effects related to 5-HT1A receptor activation and attenuated the effects of 5-HT4 receptor activation. The action of imipramine was significant after a 7-day treatment and reached a maximum after 14 daily applications, remaining at the same level in a group of animals treated for 21 days. Repeated corticosterone attenuated the inhibitory effect of 5-HT and 8-OH-DPAT on the population spike amplitude and enhanced the increase in population spike amplitude induced by zacopride. These findings indicate that antidepressant treatments and repeated corticosterone have opposite effects on hippocampal responsiveness to 5-HT1A and 5-HT4 receptor activation. In consequence, antidepressants enhance, whereas corticosterone reduces the 5-HT-mediated inhibition of hippocampal CA1 cells, which may be relevant to the antidepressant and pro-depressant effects of either treatment, respectively.