Differential activation of the periaqueductal gray by acute exposure to mild stress in female rats at different stages of the oestrous cycle

Adam Devall, Thelma Lovick. University of Birmingham, Birmingham, United Kingdom.

In female rats cutaneous hyperalgesia develops after brief exposure to mild non-noxious anxiogenic stress when the animals are in late dioestrus (LD), but not at other stages of the oestrous cycle (Devall et al, 2009). We have now investigated whether acute stress induces activation of the periaqueductal gray (PAG), a brain region which is a source of descending projections to the spinal cord that can both facilitate and inhibit spinal processing of nociceptive transmission (Gebhart, 2004). We hypothesised that acute stress activates the PAG circuitry differentially according to the stage of the oestrous cycle.

Female Wistar rats were habituated to 3 daily 30 min sessions of restraint in a Plexiglas tube in preparation for nociceptive testing, which was carried out on days 4 and 5. Tail flicks in response to noxious radiant heat were evoked at 5 min intervals over a 20 min control period. To induce anxiogenic stress the tube (with the rat inside) was then vibrated for 5 min at 4 Hz before continuing tail flick testing for a further 20 min. Control rats were subjected to the same procedure but the tube was not shaken. A similar experiment was carried out on groups of rats either 24 h after withdrawal from dosing with progesterone (5mg/Kg i.p. for 6 days, PWD) or maintained on progesterone for 7 days (HP). 2 h after exposure to vibration stress in the final experiment the rats were anaesthetised and the brain fixed by perfusion. Immunoreactivity for c-fos in the PAG visualised using anti-fos antibody SC-52 (Santa Cruz Biotech, 1: 5000) with Ni-intensified DAB as the chromogen.

In control i.e. non-stressed rats tail flick latencies (TFLs) and the numbers of fos-labelled nuclei in the PAG were similar regardless of oestrous cycle stage. Following exposure to vibration stress rats in LD (n=5), but not in other cycle stages (5-6 per group) displayed hyperalgesia (16.8±1.8% decrease in TFL, P<0.01, ANOVA and Dunnett’s) and a 302±29%, P<0.01 increase in the density of fos-labelled nuclei in the lateral and ventrolateral columns of the caudal half of the PAG. Vibration stress had no effect on TFLs in HP rats (n=6). In contrast, stress-induced hyperalgesia (16.9±1.9% decrease in TFL, P<0.01) developed in rats undergoing PWD. The density of fos-labelled nuclei in the PAG also increased, by 436±22% (P<0.01, n=6) compared to control (non-stressed) animals.

We suggest that in LD stress-induced hyperalgesia is triggered in response to falling levels of progesterone, which may initiate a shift in the balance between activity in pro- and antinociceptive descending control pathways from the PAG in favour of pro-nociception.

Devall AJ et al., (2009) Psychoneuroendocrinology 34 587-596.
Gebhart GP (2004) Neurosci Biobehav Rev 27 729-737.

The work was supported by the MRC.