Elsevier

Steroids

Volume 67, Issue 6, May 2002, Pages 519-528
Steroids

Withdrawal properties of a neuroactive steroid: implications for GABAA receptor gene regulation in the brain and anxiety behavior

https://doi.org/10.1016/S0039-128X(01)00170-2Get rights and content

Abstract

Early work in the field established that the 5α-reduced metabolite of progesterone 3α-OH-5α-pregnan-20-one (allopregnanolone or 3α,5α-THP) is a potent positive modulator of the GABAA receptor (GABAR), the receptor mediating the effects of the primary inhibitory transmitter in the brain. This steroid acts in a manner similar to sedative drugs, such as the barbiturates, both in terms of potentiating GABA-induced inhibition in vitro and in behavioral assays, by reducing anxiety and seizure susceptibility. Because sedative compounds exhibit withdrawal properties that result in behavioral hyperexcitability, our laboratory has more recently investigated the effect of prolonged application and rapid removal (i.e. ‘withdrawal’) of this steroid, administered in vivo to female rats. Withdrawal from 3α,5α-THP produces a state of increased anxiety and lowered seizure threshold, similar to withdrawal from other GABA-modulatory drugs such as the benzodiazepines and alcohol. Hormone withdrawal also produced increases in the α4-containing GABAR, an effect correlated with insensitivity of the GABAR to modulation by the benzodiazepine class of tranquilizers, as would normally occur under control conditions. In addition, changes in intrinsic channel properties, including a marked acceleration in the decay rate was also observed as a result of declining levels of 3α,5α-THP. Such a change would result in less inhibitory total current, and the resulting increase in neuronal excitability could then underlie the observed behavioral excitability following hormone withdrawal. These results suggest that actions of this steroid on a traditional transmitter receptor in the brain lead to alterations in GABAR subunit composition that result in changes in the intrinsic channel properties of the receptor and behavioral excitability. These results may have implications for endogenous fluctuations in this hormone which may accompany premenstrual dysphoric disorder.

Introduction

Classic actions of steroid hormones involve binding of the steroid molecule to a cytosolic receptor protein, with ensuing genomic effects which typically require a latency of hours to days [1]. However, more recent data has suggested the existence of neuroactive steroids which are defined as hormones which produce rapid actions on neuronal substrates within the central nervous system (CNS) [2]. They include classes of hormones which act as modulators of traditional neurotransmitter receptor molecules to either enhance or depress neuronal activity [3]. In many cases, such hormones are produced by endocrine sources across naturally occurring hormonal rhythms, i.e. the menstrual or circadian cycles [4], or in response to stress or olfactory cues [5], [6]. As steroids are naturally lipophilic molecules which are bound to carrier proteins in the circulation, in most cases, they easily pass the blood-brain barrier where they gain access to neuronal and glial populations. In other cases, neurosteroids are metabolized in the brain from precursor ‘parent’ compounds originating from endocrine sources, such as progesterone (P) [7]. Finally, more recently has come the discovery of ‘neurosteroids,’ which are formed de novo within the CNS from cholesterol via side chain cleavage enzyme [2], [8].

Section snippets

3α,5α-THP

One such neurosteroid is 3α,5α-THP (3α-OH-5α-pregnan-20-one, 3α-OH-DHP or ‘allopregnanolone,’ Fig. 1). Produced from circulating progesterone via two enzymatic conversions (5α reductase and 3α-hydroxy oxidoreductase, Fig. 2), this steroid can be converted both in the periphery as well as within the CNS [7] where levels parallel those of progesterone. 5α- and 5β-reductase enzyme activities are localized to both neurons as well as glia in the CNS [7]. 3α- and 3β-hydroxy additions also occur

GABAR pharmacology

The GABAR is a pentameric receptor protein associated with a chloride ion channel (Fig. 3), and mediates the action of GABA, the primary inhibitory neurotransmitter in the brain [20]. The subunits comprising the receptor come from a pool of as many as 6α, 4β, 4γ, δ, ϵ, π, 3ρ and θ subunits, which yield a multitude of receptor isoforms [20]. The most common isoform is 2α, 2β and 1γ, although other stoichiometries exist [21]. The particular combination of subunits would yield receptors with

Synaptic currents

GABAR in areas of the brain such as the hippocampus, part of the limbic circuitry important for emotional expression, and the cerebellum, an important center for control of motor coordination, have been well-characterized in terms of their specific subcellular localization with regard to synaptic input. Many GABAR subunit subtypes, including the α2 and γ2, are believed to be localized exclusively to the synapse [27]. Others, such as the δ and α5, are believed to reside exclusively in the

Behavioral properties

These positive GABA-modulatory effects of 3α,5α-THP, as described above, are similar to those reported for benzodiazepine tranquilizers and sedatives. Correspondingly, the behavioral properties of this steroid are also similar to those exhibited by benzodiazepine tranquilizers and sedatives, in that administration of 3α,5α-THP can reduce anxiety [7], [39], reduce seizure activity [40], [41], [42], [43], produce sedation [44], and at high doses can act as a general anesthetic [45]. One

Long-term exposure to steroids and withdrawal effects

Progesterone and its metabolites are elevated for 11–12 days during the luteal phase of the menstrual cycle [48] and for nine months during pregnancy before declining precipitously during the premenstrual [50] and postpartum periods, further justifying investigation of longer term hormone exposure and withdrawal effects on GABAR function. Initial studies from the present laboratory established that withdrawal from neuroactive steroids (Fig. 4), such as 3α,5α-THP or 3α,5β-THP, following chronic

Chronic neurosteroid exposure

A number of studies have suggested that chronic exposure to neuroactive steroids, such as 3α,5α-THP result in altered GABAR pharmacology, which in some cases was associated with subunit changes. These changes are comparable to what has been reported for other GABA-modulatory drugs such as the BDZs and ethanol [20]. In early studies, micromolar concentrations of 3α,5α-THP were shown to decrease GABA current recorded from neocortical neurons in culture after 5 days exposure [67]. In addition,

Conclusion

Global activation of circuits in the brain by GABA-modulatory steroids such as 3α,5α-THP would be expected to decrease neuronal activity leading to reduced anxiety, suppression of seizure activity, anticonvulsant, sedative and, at high doses, anesthetic effects. However, chronic exposure to and withdrawal from these steroids could then produce ensuing changes in GABAR isoform through upregulation of gene transcription of the α4 subunit. These changes would yield altered sensitivity to

Acknowledgments

The authors are grateful for the helpful assistance provided by Dr. Maria Gulinello, Dr. Fu Chun Hsu, Qi Hua Gong, Xinshe Li, and Ronald Markowitz. This work was supported by NIH grants R01 AA 12958 and R01 DA 09618, as well as a pharmaceutical contract from Merck.

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