Bidirectional effects of the neuroactive steroid tetrahydrodeoxycorticosterone on GABA‐activated Cl⁻ currents in cultured rat hypothalamic neurons

Abstract

1 The non-genomic e ects of tetrahydrodeoxycorticosterone (THDOC; were studied in cultured hypothalamic neurons of the rat.2 The e ects of THDOC (10 nM ± 1 mM) on responses to di erent concentrations of exogenously applied GABA and on spontaneous inhibitory postsynaptic currents (IPSCs) were measured with whole-cell voltage clamp recordings. 3 Application of GABA induced inward currents with dose-dependently increasing amplitudes (up to 3.9 nA at a holding potential of 720 mV). High doses of THDOC (100 nM ± 1 mM) induced small inward currents on its own (14+3 and 24+3 pA, respectively). 4 Simultaneous application of 10 mM GABA with 100 nM or 1 mM THDOC increased current amplitudes by 125 and 128%, respectively. At 10 nM THDOC exerted no consistent e ects on GABA currents. 5 Responses to 1 mM of GABA were modulated in a bidirectional manner by di erent doses of THDOC: 10 nM THDOC reduced the amplitude of GABA responses to 80% (P=0.018, n=15), whereas 100 nM and 1 mM THDOC enhanced the GABA response to 115 and 180% (P=0.0007, n=15), respectively. 6 The time constant of decay of spontaneous inhibitory postsynaptic currents (IPSCs) was reversibly increased from 91+10 to 314+34 ms (n=3) by the application of THDOC (1 mM). The amplitudes of the IPSCs were not a ected by THDOC. 7 These data indicate that THDOC modulates GABA responses of hypothalamic neurons in a bidirectional manner, resulting in a complex tuning of neuronal excitability in the hypothalamus.

Differential modulation of phasic and tonic inhibition underlies serotonergic suppression of long-term potentiation in the rat visual cortex

“…In the present study, the effect of THDOC on sIPSC was different from other neuronal types. High concentration of THDOC increased decay time of sIPSC but had no effect on the amplitude in many cell types (Cooper et al, 1999;Wetzel et al, 1999). However, both amplitude and decay time of sIPSC were increased in the present study.…”

Neurosteroids

“…Recent studies have indicated the existence of at least three neurosteroid binding sites on the GABA-A receptor: one for allosteric enhancement of GABA-evoked currents by allopregnanolone, one for direct activation by allopregnanolone, and one for antagonist action of sulfated neurosteroids such as PS (Lambert et al, 2003; Hosie et al, 2007). Electrophysiological studies are extensively used to confirm that neurosteroids, at low (nM) concentrations, act as positive allosteric modulators of GABA-A receptor function (Harrison et al, 1987; Kokate et al, 1994; Wetzel et al, 1999). Hence, neurosteroids enhance the specific receptor binding of [ 3 H]flunitrazepam, a benzodiazepine receptor agonist, and [ 3 H]muscimol, a specific GABA-site agonist, and inhibit the binding of [ 35 S] t -butylbicycloorthobenzoate (TBPS), a cage convulsant and noncompetitive GABA-A receptor antagonist.…”

Inhibition by α‐Tetrahydrodeoxycorticosterone (THDOC) of Pre‐Sympathetic Parvocellular Neurones in the Paraventricular Nucleus of Rat Hypothalamus

“…When THDOC was added to the bath flow solution at a concentration of 1 μ M , the amplitude of these GABA A currents was increased in a reversible manner by about 50% (Figure 2b, c). Wetzel et al (1999) observed that at low concentrations, THDOC had an inhibitory action. We therefore repeated these experiments at concentrations as low as 10 n M (Figure 2b, d), but failed to detect any inhibition by THDOC at these concentrations.…”

“…In either case, it is now well established that neurosteroid actions are often dependent on the subunit composition of the GABA A receptor (Maitra and Reynolds, 1999; Bianchi and Macdonald, 2001). The neuroactive steroid THDOC itself is remarkable, because it has been observed, that in some preparations, THDOC turns from an inhibitor of GABA currents to an activator as concentration increases (Wetzel et al , 1999). Thus, THDOC could potentially have no effect, an inhibitory or an excitatory activity on any specific population of neurones, depending on nature of the particular GABA A receptors and the local concentration of steroid.…”

“…Many of these neuroactive steroids are positive allosteric modulators of GABA A receptor channels (Majewska et al , 1985), although one such neuroactive steroid, pregnenolone, has been shown to be an inhibitor of GABA A function (Majewska et al , 1988). Another neuroactive steroid, α ‐tetrahydrodeoxycorticosterone (THDOC), has been reported to enhance, inhibit or be inactive on GABA A receptor currents, depending on the dose (Wetzel et al , 1999), the receptor subunit composition (Zhu et al , 1996) or the type of neurone under study (Cooper et al , 1999). Without further experimental evidence, it is therefore impossible to predict whether stress‐induced elevations of THDOC would be excitatory, inhibitory or inactive on parvocellular neurones.…”