Glucocorticoid modulates the sensitivity of the GABAA receptor on primary afferent neurons of bullfrogs

Defective cerebral gamma-aminobutyric acid-A receptor density in patients with systemic lupus erythematosus and central nervous system involvement. An observational study

Gamma-aminobutyric acid-A (GABA-A) receptors play a crucial role in regulating neuronal excitability and cognitive functions. Single-photon emission computerized tomography (SPECT) analysis of GABA-A receptors binding by 123I-labelled Iomazenil (123I-IMZ) has been applied in some neuropsychiatric disorders to investigate conditions where GABA-A receptor density can be detected in several pathophysiological conditions. In this study we investigate cerebral GABA-A receptor density in a small series of patients with systemic lupus erythematosus (SLE) and cognitive impairment characterized by recurrent, episodic memory loss. Nine female patients with SLE and cognitive alterations underwent to a clinical neuropsychiatric evaluation including digital video-EEG, brain MRI, 99mTc-ECD brain SPECT and 123I-IMZ brain SPECT. All patients tested showed diffuse or focal GABA-A receptor density reduction. This is, to our knowledge, the first report on GABA-A receptor density abnormalities associated with cognitive defects in SLE patients. We hypothesize that in our series a decrease in GABA-A receptor density might be related to the neurological manifestations. Further studies are needed to clarify this aspect and the possible mechanisms. GABA-A receptor density impairment might be due to the SLE-related cerebral vasculopathy, or to neuronal-reacting auto-antibodies or drugs which could interfere with GABA-A receptors expression/binding. This study may support the concept that cognitive impairment in systemic lupus erythematosus could be the outcome of fine-tuned neurotransmission alterations.

Synergistic effects of corticosterone and kainic acid on neurite outgrowth in axotomized dorsal root ganglion

Corticosterone is the main adrenal glucocorticoids induced by stress in rats. Therapeutic use of high concentration of synthetic glucocorticoids in clinical treatment of spinal cord injury suggests that pharmacological action of glucocorticoids might be beneficial for nerve repair. In this article we cultured axotomized rat dorsal root ganglion neurons to investigate the effects of corticosterone and a glutamate receptor agonist kainic acid on neurite outgrowth. Our results revealed a synergistic effect of corticosterone and kainic acid in promoting neurite outgrowth when applied as early as one and two days in vitro, but not effective at three and four days in vitro. In addition, applied corticosterone and kainic acid were neurotoxic at three and four days in vitro but not at one and two days in vitro. The minimal concentrations of corticosterone and kainic acid to be effective were 10 microM and 1 mM, respectively. The neurotrophic effect of corticosterone and kainic acid was attenuated by the receptor tyrosine kinase A (TrkA) inhibitor AG-879. Western blot analysis and immunocytochemical studies revealed an increase of expressions of both TrkA and growth-associated protein GAP-43 in dorsal root ganglion neurons with combined treatment of corticosterone and kainic acid. Immunocytochemistry showed that corticosterone+kainic acid increase nerve growth factor immunoreactivity in dorsal root ganglion neurites and enhance GAP-43 immunointensity in dorsal root ganglion neurons. These results suggest that the neurotrophic effect of glucocorticoids on axonal regeneration might require facilitation of excitatory stimulation at an early stage of nerve injury, and nerve growth factor may mediate a growth signaling to accomplish the effect.

Rapid neuromodulation by cortisol in the rat paraventricular nucleus: an in vitro study

1. We have used a range of in vitro electrophysiological techniques to investigate the mechanism of rapid cortisol neuromodulation of parvocellular neurones in the rat paraventricular nucleus. 2. In our study, we found that cortisol (10 microM) increased spontaneous action-current firing frequency to 193%. This effect was insensitive to the glucocorticoid intracellular-receptor antagonist mifepristone. 3. Cortisol (0.1-10 microM) had no detectable effects on whole-cell GABA current amplitudes, or GABA(A) single-channel kinetics. 4. Cortisol (10 microM) inhibited whole-cell potassium currents in parvocellular neurones by shifting the steady-state activation curve by 14 mV to the right. 5. Additionally, in a cell line expressing both the glucocorticoid intracellular receptor and recombinant, fast inactivating potassium channels (hKv1.3), cortisol (1 and 10 microM) inhibited potassium currents by shifting their steady-state activation curves to the right by 12 mV (10 microM cortisol). This effect was also insensitive to the cortisol antagonist, mifepristone. 6. These data suggest that inhibition of voltage-gated potassium channels may contribute to the rapid neuromodulatory effects of cortisol, possibly by direct interaction with the ion channel itself.

Effects of corticosteroids on synaptic transmission in rat dorsolateral septal nucleus

The effects of corticosteroids on synaptic transmission in the rat dorsolateral septal nucleus (DLSN) were examined, in vitro, by using intracellular and voltage-clamp recording methods. Prednisolone (100 microM) increased the amplitude of excitatory postsynaptic potential (EPSP) and depressed both fast and slow inhibitory postsynaptic potentials (IPSP). Under voltage-clamp conditions, prednisolone (100 microM) increased the amplitude of excitatory postsynaptic current (EPSC) and depressed the fast and slow inhibitory postsynaptic currents (IPSCs). Corticosterone (100 microM) mimicked the effects of prednisolone on the postsynaptic currents (PSCs). To examine the direct effects of prednisolone on the EPSC and slow IPSC, the fast IPSC was blocked by bicuculline (20 microM). Under these experimental conditions, prednisolone (100 microM) did not alter the isolated EPSC but depressed slow IPSC by 22 +/- 3% (n = 10). The fast IPSC was isolated by pretreatment with kynurenic acid and CGP55845A, where the EPSC and slow IPSC were blocked. Prednisolone (100 microM) depressed the isolated fast IPSC in DLSN neurons. Prednisolone (100 microM) did not change either the inward current produced by glutamate or the outward current produced by gamma-aminobutyric acid (GABA). The results suggest that corticosteroids facilitate excitatory synaptic transmission in the DLSN by reducing the release of GABA from the presynaptic nerve terminals of interneurons.

Glucocorticoid receptor-mediated inhibition by corticosterone of 5-HT1A autoreceptor functioning in the rat dorsal raphe nucleus

In the rat brain, the dorsal raphe nucleus contains a large proportion of serotoninergic neurons, which are mostly regulated by somato-dendritic 5-HT1A autoreceptors. This nucleus also possesses intracellular glucocorticoid receptors (GR), which may be involved in the well established modulation of serotonin (5-hydroxytryptamine, 5-HT) metabolism by glucocorticoids. Control by corticosteroids of 5-HT1A receptor-mediated inhibitory control of the firing of serotoninergic neurons in the dorsal raphe nucleus was investigated using an in vitro electrophysiological approach. The spontaneous firing rate of serotoninergic neurons recorded in brain stem slices and its inhibition due to 5-HT1A autoreceptor stimulation by 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) were similar in adrenalectomized rats and sham-operated animals. In vitro pretreatment with corticosterone (30-100 nM) significantly reduced 8-OH-DPAT-induced inhibition of the 5-HT cell discharge in slices from adrenalectomized rats. This effect could be prevented by the GR antagonist, 11 beta-(4-dimethyl-amino-phenyl)- 17 beta-hydroxy-17 alpha-(prop-1-ynyl)estra-4,9-dien-3-one (RU) 38486, 30 nM), and mimicked by the GR agonist, 11 beta, 17 beta-dihydroxy-6-methyl-17 alpha (prop-1-ynyl) androsta-1,4,6-trien-3-one (RU 28362, 500 nM). In contrast, the mineralocorticoid receptor (MR) agonist, aldosterone (10 nM), did not alter 8-OH-DPAT-induced inhibition in tissues from adrenalectomized animals. Complementary autoradiographic experiments showed that [3H]8-OH-DPAT specific binding to 5-HT1A sites in the dorsal raphe nucleus (and the hippocampus) was not significantly altered following adrenalectomy and exposure of brain stem slices to corticosterone. These data suggest that GR are involved in the suppressive effects of high levels of corticosterone on the 5-HT1A receptor-dependent regulation of 5-HT neuronal activity in the rat dorsal raphe nucleus.

Mechanism of modulation of GABA-activated current by internal calcium in rat central neurons

GABA-activated currents in Purkinje cells isolated from rat cerebellum were investigated. Increase of intracellular Ca2+ in the physiological range of concentrations caused a decrease in GABA-activated chloride currents. This effect resulted from a decrease of both the maximal values of GABA-activated currents and possibly from the affinity of GABAA receptors. Therefore, the mechanism of Ca2+ effect on GABAA receptors in central rat neurons differs from that in bullfrog sensory neurons.

Effects of glucocorticoids and norepinephrine on the excitability in the hippocampus

The CA1 pyramidal neurons in the hippocampus contain a high density of adrenal corticosteroid receptors. By intracellular recording, CA1 neurons in slices from adrenalectomized rats have been found to display a markedly reduced afterhyperpolarization (that is, the hyperpolarizing phase after a brief depolarizing current pulse) when compared with their sham controls. No differences were found for other tested membrane properties. Brief exposure of hippocampal slices from adrenalectomized rats to glucocorticoid agonists, 30 to 90 minutes before recording, greatly enhanced the afterhyperpolarization. In addition, glucocorticoids attenuated the norepinephrine-induced blockade of action potential accommodation in CA1 neurons. The findings indicate that glucocorticoids can reduce transmitter-evoked excitability in the hippocampus, presumably via a receptor-mediated genomic action.

Steroid modulation of the GABA/benzodiazepine receptor-linked chloride ionophore

Recent findings suggest that steroids with sedative-hypnotic properties interact specifically with the gamma-aminobutyric acidA/benzodiazepine receptor-chloride ionophore complex (GBRC). They show positive heterotropic cooperativity by allosterically enhancing the binding of GABA agonists and the clinically useful benzodiazepines (BZs) to their respective recognition sites. These steroids have stringent structural requirements for activity at the GBRC, with the essential requirements for high potency being a 3 alpha-hydroxyl group and a 5 alpha-reduced A-ring. Some of these steroids are naturally occurring metabolites of progesterone and deoxycorticosterone and have nanomolar potencies as potentiators of chloride channel conductance. These 3 alpha-hydroxylated, 5 alpha-reduced steroids do not act through any known sites on the GBRC. Thus, the exact site and mechanism of action remain to be determined. Together with the observation that physiological levels of these metabolites are sufficient to influence the function of the GBRC, the evidence clearly suggests a role for these steroids in the normal regulation of brain excitability by potentiating the postsynaptic effects of gamma-aminobutyric acid (GABA). Pharmacological studies of the GBRC-active steroids show that they possess anxiolytic and anticonvulsant activities. The potential therapeutic application of these steroids in the treatment of mood disorders and catamenial exacerbation of seizures associated with the menstrual cycle is discussed. Collectively, the evidence from the studies of these steroids imply that another mechanism by which the endocrine system influences brain function has been identified. Its characterization will provide important insight into how steroids modulate brain excitability under normal and pathophysiological states.

Voltage-clamp studies of the inhibition of gamma-aminobutyric acid response by glucocorticoids in bullfrog primary afferent neurons

Acute effects of glucocorticoids on the response to gamma-aminobutyric acid (GABA) were examined in primary afferent neurons in bullfrog spinal ganglia, using intracellular and voltage-clamp recording techniques. Prednisolone and hydrocortisone (5 microM to 1 mM) caused a dose-dependent decrease in the amplitude of GABA-induced depolarization, while having no effect on the membrane potential and resistance of the neuron. Prednisolone depressed the muscimol-induced depolarization. Nipecotic acid, a blocker of GABA uptake, did not influence the inhibitory action of prednisolone. Voltage-clamp analyses showed that the inward current induced by an iontophoretic application of GABA (GABA current) was suppressed by prednisolone and hydrocortisone. The depression of the GABA current is neither due to a blockage of open channels nor a facilitation of the desensitization of GABA receptors. Prednisolone shifted the dose-response curve of the GABA current downward. The double-reciprocal (Lineweaver-Burk) plot showed that the maximum GABA current was reduced by prednisolone, suggesting a non-competitive antagonism. These results suggest that glucocorticoids suppress the GABA-induced chloride current, decreasing the number of functional channels associated with GABAA receptor.

Modulation of the GABAA receptor by progesterone metabolites

The naturally occurring progesterone metabolites 5 beta-pregnan-3 alpha-ol-20-one and 5 beta-pregnane-3,20-dione reversibly enhance membrane currents elicited by locally applied GABA in bovine adrenomedullary chromaffin cells. Such potentiation was not influenced by the benzodiazepine antagonist Ro 15-1788. At concentrations in excess of those necessary to evoke potentiation of GABA currents, 5 beta-pregnan-3 alpha-ol-20-one and 5 beta-pregane-3,20-dione directly activated a membrane conductance. The resulting currents were potentiated by phenobarbitone and diazepam, and abolished by the GABAA-receptor antagonist, bicuculline. On outside-out membrane patches, 5 beta-pregnan-3 alpha-ol-20-one and 5 beta-pregnane-3,20-dione activated single channel currents of similar amplitude to those evoked by GABA. The results suggest that certain naturally occurring steroids potentiate the actions of GABA and, additionally, directly activate the GABAA receptor.