Research reportConvulsant actions of the neurosteroid pregnenolone sulfate in mice
Introduction
Several endogenous steroids have been identified that can rapidly alter the excitability of neurons by direct (nongenomic) actions on membrane ion channels (for reviews, see Refs. 9, 20, 29). Modulatory effects of these neurosteroids are best characterized at the GABAA receptor chloride channel complex. For example, the progesterone metabolites pregnanolone (5β-pregnan-3α-ol-20-one) and allopregnanolone (5α-pregnan-3α-ol-20-one), produce a potent enhancement of GABAA receptor responses in vitro 11, 16, 22, 30 and, like other positive GABA modulators, show potent anticonvulsant, anxiolytic and sedative activities when administered in vivo 2, 3, 16, 36. In contrast to the GABA potentiating activity of pregnanolone and allopregnanolone, the neurosteroid pregnenolone sulfate (PS) has been shown, in in vitro experiments, to inhibit GABAA receptor responses 23, 24, 27 and also to potentiate NMDA receptor-mediated excitatory responses 4, 13, 14, 37, 38. The net effect of these actions on GABAA and NMDA receptors is expected to be an enhancement of brain excitability. PS is present in the mammalian brain and, in fact, can be synthesized de novo from cholesterol in situ 1, 7, 12. Thus, PS could conceivably serve as an endogenous proconvulsant substance. To examine the ability of PS to act as a modulator of seizure susceptibility, we administered the steroid to naive mice and also to mice treated with the convulsants pentylenetetrazol (PTZ) and NMDA. Our results indicate that PS has powerful convulsant properties.
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Animals
Male NIH Swiss mice (25–30 g) were obtained from the National Institutes of Health (NIH) animal program. Animals were allowed to acclimatize with free access to food and water for a 24-h period before surgery. All procedures were carried out under strict compliance with the NIH guide for the Care and Use of Laboratory Animals under a protocol approved by the NIH Animal Use Committee.
Surgery
Intracerebroventricular (i.c.v.) injections were made via an indwelling 23-gauge stainless steel cannula 10 mm in
Seizure induction by PS
Systemic (i.p.) injection of PS at doses of 30, 50 and 100 mg/kg (eight mice tested at each dose) did not produce any obvious behavioral effects during the 60-min observation period following administration. However, when PS was injected i.c.v. (30–300 nmol), the steroid produced severe limbic and clonic–tonic seizures. Table 1 indicates the typical sequence of behavioral effects occurring after i.c.v. administration of PS. In general, the behavioral signs occurred at earlier times with higher
Discussion
The results presented here show for the first time that the neurosteroid PS induces an acute convulsant effect when administered directly into the central nervous system. A low subconvulsant i.c.v. dose of the steroid dramatically potentiated seizures induced by the convulsants PTZ and NMDA. Higher doses of PS (>50 nmol) produced seizure activity in the absence of other convulsants. The intense seizures induced by PS had limbic, clonic and tonic components, and progressed to status epilepticus
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