Proceedings of The Physiological Society
King's College London (2011) Proc Physiol Soc 22, PC11
Short term, low dose fluoxetine treatment increases brain allopregnanolone concentrations in female rats and abolishes estrous cycle-related stress-induced hyperalgesia
A. J. Devall1, J. P. Fry2, J. W. Honour3, T. A. Lovick1
1. School of Clinical and Experimental Medicine, University of Birmingham, Birmingham, United Kingdom. 2. Department of Neuroscience, University College London, London, United Kingdom. 3. Department of Clinical Biochemistry, University College London Hospital, London, United Kingdom.
Fluctuations in brain concentrations of female gonadal hormones during the ovarian cycle can impact significantly on behaviour. During late diestrus (LD) in rats, a sharp fall in the concentration of allopregnanolone (ALLO), a neuroactive metabolite of progesterone (PROG) triggers a withdrawal-like response, leading to increased responsiveness to anxiogenic stress (1). We investigated whether an acute replacement of ALLO with the synthetic analogue ganaxolone or an enhancement of ALLO production by fluoxetine (FLX) treatment can prevent the stress-induced hyperalgesia of LD. Female Wistar rats were confined in a Perspex tube for measurement of tail flick latencies (TFLs) in response to noxious radiant heat applied to the tail. Nociceptive thresholds were similar at all stages of the estrous cycle. Exposure to anxiogenic vibration stress (4 Hz for 5 min) during LD, but not at other stages, evoked hyperalgesia (21±3.4% average decrease in TFL n=16, mean±SEM, P<0.01, one-way ANOVA). Pre-treatment with FLX (1.75mg Kg-1 i.p. at 16:30h on the day of early diestrus and again on the morning of LD 1h prior to behavioural testing) or the synthetic analogue of ALLO, ganaxolone (GNX, 7mg Kg-1, same dosing protocol as FLX) prevented development of stress-induced hyperalgesia (-4±5.8%, n=10 and +5.4±3.6%, n=7 change in TFL, respectively following stress). FLX or GNX had no effect when administered at other stages of the cycle. Since the dose of FLX used is reported to be subthreshold for effects on 5-HT systems (2) we investigated whether its effect was due to central steroidogenic activity (2). Rats were dosed with FLX or vehicle (n=5-6) as described above but instead of undergoing behavioural testing, they were killed by decapitation and the brain, minus olfactory bulbs, snap frozen at -80oC. Free steroids were extracted and fractionated from individual whole brain samples and derivatised for identification and assay by gas capillary chromatography-electron impact mass spectrometry (3). FLX induced a significant (P<0.05, Student’s t-test) increase in whole brain concentrations of ALLO (1.4±0.2 v. 0.7±0.2ng/g) and its reduced metabolites allopregnanediol (2.7±0.4 v.1.2±0.4ng/g) and 5α-pregnan-3α,17-diol-20-one (1.0±0.2 v. 0.4±0.1ng/g). Concentrations of pregnenolone, the metabolic precursor of PROG decreased (3.0±0.5 v. 1.5±0.2ng/g) but there was no change in PROG or 5α-dihydroprogesterone. These measurements suggest an enhancement by FLX of the reducing activity of the 3α-hydroxysteroid dehydrogenase enzyme which catalyses ALLO production. Our results show that short term replacement of ALLO by ganaxolone, or the enhancement of the production of this steroid by FLX treatment prevents development of the stress-induced hyperalgesia normally associated with the fall in brain ALLO at LD.
Where applicable, experiments conform with Society ethical requirements