Regulation of central noradrenergic activity by 5-HT3 receptors located in the locus coeruleus of the rat

A functional interaction between serotonergic and noradrenergic systems has been shown in the locus coeruleus (LC). Noradrenaline (NA) levels in the prefrontal cortex (PFC) are dependent on the firing rate of LC neurons, which is controlled by alpha(2) adrenoceptors (alpha 2ADR). The aim of the present study was to investigate the role of 5-HT3 receptors (5HT3R) in the modulation of central noradrenergic activity. We measured extracellular NA concentrations in the LC and PFC by dual-probe microdialysis in awake rats and the firing rate of LC neurons by electrophysiological techniques in vitro. Administration of the 5HT3R agonists SR57227 (1-100 mu M) and m-chlorophenylbiguanide (mCPBG, 1-100 mu M) into the LC increased NA in this nucleus (E-max = 675 +/- 121% and E-max = 5575 +/- 1371%, respectively) and decreased NA in the PFC (E-max = -49 +/- 6% and E-max = -25 +/- 11%, respectively). Administration of the 5HT3R antagonist Y25130 (50 mu M) into LC attenuated SR57227 effect in the LC (E-max = 323 +/- 28%) and PFC (E-max = -37 +/- 7%). The alpha 2ADR antagonist RS79948 (1 mu M) blocked the SR57227 effect in the PFC but it did not change the effect in the LC (E-max = 677 +/- 202%). In electrophysiological assays, both mCPBG (1-10 mu M) and SR57227 (1-10 mu M) reduced the firing rate of about 50% of tested LC neurons (maximal effect = -37 +/- 2% and -31 +/- 4%, respectively); this effect was partially blocked by Y25130 (50 mu M). Administration of RS79948 (1 mu M) reversed the inhibition induced by mCPBG. Competition radioligand assays against [H-3]UK14304 and [H-3]RX821002 (alpha 2ADR selective drugs) in the rat brain cortex showed a very weak affinity of SR57227 for alpha 2ADR, whereas the affinity of mCPBG for alpha 2ADR was 17-fold higher than that of SR57227 for alpha 2ADR. The present results suggest that 5HT3R stimulate NA release in the LC, which promotes simultaneously a decrease in the firing rate of LC neurons through alpha 2ADR and then a decrease of NA release in terminal areas such as the PFC. (C) 2012 Elsevier Ltd. All rights reserved.