The paraventricular nucleus of the hypothalamus (PVN) is central to integration of the stress response. It consists of neurones which release CRF, thus controlling cortisol secretion from the adrenal cortex, and neurones which project to sympathetic centres in the spinal cord and modulate cardiovascular function (Jansen et al. 1995). During acute stress, the characteristic increase in heart rate and blood pressure are accompanied by elevation in hypothalamic concentrations of neurosteroids such as tetrahydroxycorticosterone (THDOC, Paul & Purdy, 1992). Neurosteroids, in general, decrease sympathetic outflow, apparently by acting to enhance GABA inhibitory tone in the medulla (Laiprasert et al. 1998). However, THDOC has been reported to both enhance and inhibit GABA (Wetzel et al. 1999), and furthermore, the activity of neurosteroids on the GABA_A channel is likely to be subunit dependent. We therefore chose to investigate whether THDOC modulates the activity of spinally projecting (pre-sympathetic) neurones (SPNs) of the PVN.

The methods used were similar to those described previously (Barrett-Jolley et al. 2000), but briefly: PVN SPNs were labelled by injection of a red (DiI) retrograde tracer into the spinal cord IML (T2-T4) of 3- to 4-week-old rats under general anaesthesia (I.P. injection of metatomidine-ketamine 0.3ml (100 g)^-1, Home Office Procedure 40/2211.6) Approximately 1 week later, rats were humanely killed by an overdose of anaesthesia (I.P. sagittal) and 200 µm hypothalamic slices prepared. Results are given as means ± S.E.M.

Action currents were measured from identified SPNs, before and during the bath application of THDOC. Spontaneous action current frequency was reduced with a pIC₅₀ of 6.9 ± 0.2 n = 5.

Under these recording conditions, THDOC could be acting directly on GABA_A receptors of the recorded neurone, or indirectly via GABA_A receptors of neurones projecting to the recorded neurone. To investigate whether SPNs actually express GABA_A receptors, we combined an immunohistochemical study with retrograde labelling. We found all retrogradely labelled SPNs to express the predominant PVN GABA_A α-subunit, α₂.

In whole-cell patch-clamp experiments, we found SPN GABA_A currents (elicited by pressure injection of 300 µM GABA) to be enhanced by 1 µM THDOC to 148 ± 15 % n = 5 (P < 0.05, one-sample t test).

These results suggest that during stress, when concentrations of hypothalamic neurosteroids rise, pre-sympathetic neurones projecting from the PVN to the spinal cord will be inhibited by THDOC.

This work was funded by the BHF.