The effects of type I and type II corticosteroid receptor agonists on exploratory behavior and spatial memory in the Y-maze

We investigated the effects of two adrenal steroid agonists on adrenalectomized (ADX) rats' performance on the Y-maze. The Y-maze was chosen because memory can be assessed quickly and because it is sensitive to various parameters of exploratory behavior and spatial memory performance. Four days after surgery, ADX rats were injected with aldosterone (ALDO, a selective Type I receptor agonist), RU362 (a selective Type II receptor agonist) or sesame vehicle at three different time points (120 min prior to Trial 1, immediately after Trial 1 or 120 min after Trial 1). SHAM-operated rats injected with vehicle were also tested. The results indicate that vehicle-treated ADX rats were impaired on spatial recognition memory compared to SHAM rats. Treatment with ALDO restored spatial recognition memory performance of ADX rats to a level comparable to SHAM-treated rats by acting on acquisition and consolidation, whereas treatment with RU362 did not change the poor spatial recognition memory performance of ADX rats. Discrimination memory was improved only when either agonist was injected prior to the first trial, strongly suggesting a non-selective effect of corticosteroids on discrimination memory such as increasing arousal. A detailed analysis of exploratory behavior showed that both the ALDO- and RU362-treated rats explored the Y-maze more than the ADX and SHAM groups at all periods of the experiment. These results show that the non-specific increase in exploratory behavior induced by replacing corticosteroids targeted at Type I and Type II receptors was used differentially with the ALDO-treated rats learning and consolidating spatial information better than the RU362-treated rats. These data are discussed along with other evidence to suggest that Type II receptors may require the simultaneous occupancy of Type I receptors to affect learning and memory processes.

Dexamethasone suppression test: corticosteroid receptors regulation in mononuclear leukocytes of young and aged subjects

The dexamethasone suppression test (DST) is considered an indicator of the function of the adrenal pituitary axis. The effect of the steroid is mediated by its binding to corticosteroid receptors. We previously suggested that the measurement of corticosteroid receptors in lymphocytes is an index of an analogous pattern in brain. In the present study, corticosteroid Type I and Type II receptors in mononuclear leukocytes were measured in 10 elderly subjects and in 9 young adults, before and after overnight DST (1 mg). Receptors were measured by radioreceptor assay. In all the subjects, dexamethasone was able to suppress plasma cortisol. The number of Type I and Type II receptors before the test was lower in elderly subjects than in adults. In the control group, dexamethasone produced a significant depression of Type I receptors (from 267 +/- 72 to 169 +/- 71 receptors per cell), which can be interpreted as a primary involvement of Type I receptors in the response to dexamethasone; Type II receptors decreased in half the subjects (from 2849 +/- 703 to 2345 +/- 569 receptors per cell). In elderly healthy subjects, Type II receptors were also significantly decreased (from 1796 +/- 671 to 720 +/- 345). We suggest that in young subjects Type II receptors are initially up-regulated by dexamethasone, and then down-regulated, while in aged subjects an up-regulation cannot be achieved, as suggested by the higher values of plasma cortisol usually found in aging subjects.

5-HT1A receptor expression is modulated by corticosteroid receptor agonists in primary rat hippocampal culture

The effects of corticosteroid receptor agonists on the expression of 5-HT1A receptor mRNA were measured in rat hippocampal cultures using in situ hybridization histochemistry. In our normal culture system, grown in serum and steroid-free media, moderate to heavy signal for 5-HT1A mRNA transcripts were detected in hippocampal neurons and glial cells. Aldosterone, a type I corticosteroid receptor agonist (10(-9) M), significantly reduced the expression of 5-HT1A mRNA both in neurons and glial fibrillary acidic protein (GFAP)-immunoreactive (IR) cells. The type II corticosteroid receptor agonist, Ru28362 (10(-8) M), also significantly decreased neuronal 5-HT1A mRNA expression. However, it was not as effective as aldosterone in reducing the label over GFAP-IR cells. These data indicate that corticosteroids may directly regulate the expression of hippocampal 5-HT1A receptors at the mRNA level in cultured hippocampal cells.

Enhanced synaptophysin immunoreactivity in rat hippocampal culture by 5-HT 1A agonist, S100b, and corticosteroid receptor agonists

Serotonin (5-HT) has been shown to modulate brain maturation during development and adult plasticity. This effect in the whole animal may be due to activation of 5-HT1A receptors and a corresponding increases in S100b and corticosterone. Synaptophysin, an integral protein of the synaptic vesicle membrane that correlates with synaptic density and neurotransmitter release, is reduced by depletion of 5-HT in the cortex and hippocampus of the adult rat. Injections of a 5-HT1A agonist or dexamethasone can reverse the loss of synaptophysin immunoreactivity (IR). In this study we used morphometric analysis of synaptophysin-IR to study the effects of the 5-HT1A agonist, ipsapirone, and the neuronal extension factor, S100b on hippocampal neurons grown in a serum and steroid free media. Both compounds increased the synaptophysin-IR at doses previously established to be highly specific. Ipsapirone (10(-9)M) was more effective on neuronal cell bodies staining and S100b (10 ng/ml) was more effective in increasing the number of synaptophysin-IR varicosities on neuronal processes. In addition both types of corticosteroid receptor agonists, at previously established specific doses, Ru28362 (10(-8) M) and aldosterone (10(-9) M) produced smaller increases compared to control groups in both the cell body staining and the number of varicosities. The effect of these differentiating factors on the expression of synaptophysin-IR suggests multiple regulation sites for producing and maintaining pre-synaptic elements in the brain.

Effect of adrenal steroids on preproneurokinin-A gene expression in discrete regions of the rat brain

An in situ hybridization histochemical procedure was developed to monitor the cellular distributions of the three major alternatively spliced alpha, beta and gamma species of mRNA encoding neurokinin molecules of the CNS. Two oligodeoxyribonucleotide probes were synthesized corresponding to common sequences of the alpha, beta, and gamma NK-A mRNA. The first experiment used rats that were sham-operated (Sham), adrenalectomized (ADX), and ADX rats treated with corticosterone (ADX+CORT). Intense labelling was observed within the habenula (Hb), while strong labelling was detected within the olfactory tubercle (OT), the lateral olfactory tubercle (LOT), the horizontal diagonal band of Broca (HDB), the bed nucleus of the stria terminalis (BNST), and the dorsal and ventral part of the caudate putamen (d-CP, v-CP). Moderate labelling of a number of cells was observed within the medial preoptic area (mPOA), the postero-dorsal part of the medial amygdala (MePD), and the dorsal and ventral part of the premamillary nucleus of the hypothalamus (PM-D; PM-V). ADX decreased NK-A mRNA in OT, LOT, HDB, BNST, CP compared to sham-operated rats, whereas CORT replacement elevated NK-A mRNA to above sham levels in OT, LOT, HDB, BNST and CP. There was no effect of ADX or CORT in Hb, while smaller, and often non-significant, effects of ADX and CORT replacement were found in other areas. Since there are two types of adrenal steroid receptors in brain, we next investigated the effects of agonists for type I and type II adrenal steroid receptors. ADX rats were given either aldosterone (ALDO, 10 micrograms/ml/h, Alzet minipumps) or RU 28362 (10 micrograms/ml/h, Alzet minipumps) for 8 days. Type I receptor activation by ALDO partially reversed the ADX-induced decrease in NK-A mRNA, whereas type II steroid receptor activation by RU 28362 restored the decrease caused by ADX and caused an elevation of NK-A mRNA above sham levels in OT. These findings show that adrenal steroids regulate NK-A gene expression through both type I and type II receptors in a number of brain areas. The results are consistent with a role for adrenal steroids and neurokinins in the regulation of body fluid homeostasis.

Glucocorticoid receptor activation potentiates the morphine-induced adaptive increase in dopamine D-1 receptor efficacy in gamma-aminobutyric acid neurons of rat striatum/nucleus accumbens

The present study was designed to evaluate the hypothesis that enhanced corticosterone levels may facilitate the enduring neuroadaptive effects in the brain caused by drugs of abuse. Treatment of primary neuronal cultures of the rat striatal complex (striatum/nucleus accumbens, consisting for more than 90% of gamma-aminobutyric acid neurons) with 10 microM morphine for 2 hr to 3 days, enhanced the maximal stimulatory effect of the dopamine D-1 receptor agonist SKF38393 on adenylyl cyclase activity. This adaptive increase in D-1 receptor efficacy upon long-term mu-opioid receptor activation was about doubled after simultaneous or previous exposure of the neurons to the glucocorticoid receptor agonist dexamethasone (EC50 about 2 nM). A similar facilitation of the effect of morphine was observed upon exposure of the neurons to relatively high (nanomolar) concentrations of corticosterone, whereas the mineralocorticoid receptor agonist aldosterone appeared to be ineffective in this respect, indicating the involvement of glucocorticoid receptors. Interestingly, whereas morphine exposure also enhanced isoprenaline-stimulated adenylyl cyclase activity, this increase of beta adrenoceptor efficacy was not at all affected by dexamethasone. In both morphine-treated and untreated neurons, low concentrations (< .3 nM) of corticosterone or aldosterone, but not dexamethasone, caused a slight (about 20%) reduction of dopamine D-1 receptor-stimulated adenylyl cyclase activity, indicating the involvement of mineralocorticoid receptors. These data show that the morphine-induced adaptive increase of postsynaptic dopamine D-1 receptor efficacy (also observed in striatal slices of rats weeks after repeated treatment with morphine or cocaine) is strongly enhanced after previous or simultaneous glucocorticoid receptor activation.(ABSTRACT TRUNCATED AT 250 WORDS)

Hippocampal homosynaptic long-term depression/depotentiation induced by adrenal steroids

The effects of adrenal steroids on synaptic plasticity were investigated in the dentate gyrus of the hippocampus. Experiments were performed in either adrenalectomized or intact (non-adrenalectomized), anesthetized rats. High-frequency stimulation was applied to the medial perforant pathway at three different frequencies; 100, 200 or 400 Hz, either post- or pre- and post-administration of the specific Type-II adrenal steroid receptor agonist RU 28362. High-frequency stimulation prior to RU 28362 administration produced a frequency-dependent long-term potentiation of the population spike, with 100 Hz showing no long-term potentiation and 400 Hz the highest degree of potentiation. In contrast, following administration of RU 28362, high-frequency stimulation produced a long-term depression (in comparison to baseline). In the experiments in which high-frequency stimulation was applied both pre- and post-RU 28362 administration, the size of the population spike was initially potentiated and then depotentiated after the RU 28362 injection. This effect was also frequency dependent, although opposite to the long-term potentiation effect. That is, 400 Hz was the least effective frequency for inducing long-term depression/depotentiation, while 100 Hz was the most effective. Long-term depression/depotentiation was immediate following high-frequency stimulation and lasted for the extent of the recording session, in some cases longer than 1 h. Similar to the finding reported in the accompanying paper, induction of long-term potentiation was substantially suppressed by RU 28362. However, in a number of experiments long-term potentiation could still be induced after RU 28362 administration, even after long-term depression/depotentiation had been established. In these cases, stimulation at the higher frequencies was necessary.(ABSTRACT TRUNCATED AT 250 WORDS)