Mineralocorticoid Receptor (MR) is not involved in the noradrenergic hyperactivation that occurs during morphine withdrawal

J Navarro-Zaragoza1,2, ML Laorden1,2, MV Milanés1,2. 1Faculty of Medicine, University of Murcia, Pharmacology, Spain, 2Instituto Carlos III, Red de Trastornos Adictivos, Spain, 3Instituto Murciano de Investigación Biosanitaria, IMIB, Spain

Introduction

Noradrenergic system and the hypothalamus-pituitary-adrenal (HPA) axis represent two of the main systems implicated in stress adaptations. Morphine withdrawal produces an activation of HPA axis in rats due to an activation of NTS-A2 noradrenergic cell group. There are evidences supporting that CRF neurons in the PVN innervate noradrenergic brain stem nuclei and also the existence of a NA-CRF loop in which CRF would activate brainstem noradrenergic activity, which in turn activate PVN CRF activity, effectively closing the loop. HPA axis exerts its function by two family receptors: glucocorticoids (GR) and mineralocorticoids (MR). Several studies have shown the role of GR on aversive symptoms of withdrawal from exposure to drugs of abuse but there is controversial data about MR.Thereby, the aim of present work was to investigate the effects of the selective MR antagonist spironolactone on morphine withdrawal-induced increased NA turnover in the PVN and on tyrosine hydroxylase (TH) phosphorylation in the NTS. Finally, global score of somatic signs of morphine withdrawal was measured.

Methods

Male SD rats were made dependent on morphine by sc implantation of two morphine pellets (75 mg). On day 6, rats were pre-treated with vehicle or spironolactone i.p. (25 mg/kg and 50 mg/kg) 30 min before naloxone injection and the opiate abstinence signs were assessed during a period of 30 min. Sixty minutes later, rats were sacrificed and NA turnover (HPLC) in the PVN was measured. Total TH, TH phosphorylated at Ser31 (TH pSer31) and at Ser40 (TH pSer40) expression were quantified by Western Blot in the NTS. n for different groups of treatment was 6-7 animals each group. Data were analyzed using two‐way analysis of variance (ANOVA) with chronic pretreatment and acute treatment as independent variables.

Results and conclusion

Our results showed that the blockade of MR did not significantly attenuate the increases of NA turnover in the PVN and TH levels in the NTS-A2 in morphine-withdrawn rats. On the other hand, spironolactone significantly attenuated the incidence of morphine withdrawal signs.

These data suggest that MR might not be involved in the activation of brainstem noradrenergic neurons innervating the PVN during morphine withdrawal. Present findings, confirm previous data that reserves to GR a main role on NTS hyperactivation during morphine withdrawal. In contrast, MR could be used as therapeutic target due to its role in the reduction of the somatic signs of morphine withdrawal.