Modulation of 5HT1A responsiveness in CA1 pyramidal neurons by in vivo activation of corticosteroid receptors.

In this study we describe modulatory effects exerted by in vivo activation of corticosteroid receptors on 5HT responsiveness of rat CA1 pyramidal neurons. In the first series of experiments, adrenalectomized (ADX) rats were injected with corticosterone one hour prior to decapitation (1-1000 micrograms/100 g body weight) after which 5HT1A induced hyperpolarizations were determined in vitro by means of intracellular recordings. It appeared that 5HT responsiveness was dose-dependently affected by corticosterone injections: 5HT responses were relatively large when no corticosteroid receptors were activated (ADX); similar 5HT responses were observed or when both mineralocorticoid receptors (MR) and glucocorticoid receptors (GR) were occupied by injection of high doses of corticosterone (100-1000 micrograms/100 g body weight). However, compared to the latter group, 5HT hyperpolarizations were significantly suppressed in slices from rats that received moderate amounts of corticosterone (10-30 micrograms/100 g). Next, we investigated whether physiological variations of plasma corticosterone levels as occurring in intact rats correlated with the transmitter responsiveness. It was found that high plasma levels of corticosterone due to either stress or exogenous application of high doses of corticosterone correlated with large 5HT-responses in vitro. Interestingly, the large 5HT responses recorded after stress were clearly suppressed by pretreatment with RU38486, a GR antagonist. Altogether, this study presents further evidence that 5HT transmission in hippocampal CA1 area is modulated by differential steroid receptor activation as may occur under physiological circumstances due to different plasma concentrations of corticosterone.