Hydrocortisone acts as a negative allosteric modulator of 5-HT3 receptors

NATALIA ANDERSEN; JEREMÍAS CORRADI; CECILIA BOUZAT

Huerta Grande, Córdoba

Congreso; First Joint Meeting of the Argentine Society for Neurosciences; 2009

Sociedad Argentina de Neurociencias

5-HT3A receptor is a member of the Cys-loop family of ligand-gated ion channels. Due to its low conductance, kinetic analysis of this receptor has been restricted to the macroscopic level. We introduced mutations in the 5-HT3A subunit to obtain a high-conductance form so that single-channel currents can be detected. We used electrophysiological techniques to study the effect of the neuroactive steroid, hydrocortisone (HC), in the high-conductance form of the 5-HT3A receptor. At the single-channel level, currents triggered by 1 mM 5-HT appear as a series of long opening events of 4.7±0.4 pA (-70 mV) grouped in long clusters. A low-conductance population of opening events occurs at a very low frequency (relative area <0.1, amplitude ~2.7 pA). Open-time histograms show three components. In the presence of HC, a reduction in the duration of the slowest open component (~100 ms) is observed, suggesting an open channel block that can be interpreted on the basis of a simple linear blocking scheme. In addition, an increase in the frequency of the low-conductance events occurs in a dose-dependent manner. The relative area of the low-amplitude population is >4-fold higher at 400 mM HC than in the absence of the steroid. Macroscopic currents evoked by 100 mM 5-HT decay slowly and decays are well fitted by two exponential components (tfast = 60 ± 30 ms and tslow = 400 ± 190 ms). In the presence of HC, the peak current is reduced (~50% at 400 µM HC), and decay rates are increased. The results reveal that the neuroactive steroid hydrocortisone negatively modulates 5-HT3A receptors and shows a novel mechanism which involves the stabilization of a sub-conductance form.