IS PREGNENOLONE SULFATE AN ENDOGENOUS MODULATOR OF NICOTINIC ACH RECEPTOR?

Pregnenolone sulfate (PregS) is an important endogenous modulator of brain activity. PregS reduces the inhibitory activity of GABAA receptors, increases excitatory neurotransmission potentiating NMDA receptors function, trafficking and glutamate release. These latter actions are mediated by PregS at the level of sigma I receptor, calcium and TRP channels [1]. The peleiotropic mechanism of action of PregS could explain some of the pharmacological effects of the steroid: in preclinical studies PregS is neuroprotective, it potentiates synaptic LTP and improve spatial cognitive performance in rats eventhough the pro-mnestic effect of PregS likely depends on the mode of administration and behavioral model being tested [2]. Because cholinergic transmission is fundamental for cognitive functions such as learning and memory we studied the effect of PregS on the activity of nicotinic Ach receptors (nAchR). Using the patch-clamp technique in the whole-cell configuration we tested PregS on the currents mediated by nAChRs expressed in SH-SY5Y cells and in neurons grown in primary cultures. In SH-SY5Y application of PregS reduced NIC (10 μM) - evoked currents with an IC50 in the low micromolar range (14+9 μM) and a maximal effect of 58+10 %. To investigate the mechanism of PregS antagonism we applied the same concentration (10 μM) of the neurosteroid together with different concentrations of NIC (from 5 to 100 μM). PregS effect was not dependent on agonist concentration suggesting a non competitive mechanism of action. The analysis of the dose-response curves of PregS effect at 10 and 100 μM NIC evidentiates that potency and efficacy of the steroid is unchanged. PregS reduces nAchR-mediated currents by increasing receptor desensitization and this effect is more pronounced at high agonist concentrations. Experiments in primary cultures of cortical and cerebellar neurons have confirmed the antagonistic effect of PregS on NIC-evoked current. The inhibitory effect of PregS of nAchR- mediated current cannot support its activity as cognitivie enhancer. To find a rationale it has to be taken into account an additional complexity resulting from the activity of PregS on a neuronal network: it is possible that the reduction of cholinergic activity be limited to specific inhibitory circuits and that the overall effect will be an improved excitatory neurotransmission. It is also conceivable that PregS effect could be served as a compensatory mechanism