Behavioral effects of chronically elevated corticosterone in subregions of the medial prefrontal cortex.

The medial prefrontal cortex is a key mediator of behavioral aspects of the defense response. Since chronic exposure to elevated glucocorticoids alters the dendritic structure of neurons in the medial prefrontal cortex, such exposure may alter behavioral responses to danger as well. We examined the effects of chronically elevated corticosterone in discrete regions of the medial prefrontal cortex on exploration of the elevated plus-maze. Chronically elevated corticosterone in the prelimbic or infralimbic cortices reduced open arm exploration. This effect was specific to the ventral regions of the medial prefrontal cortex as corticosterone had no effect on plus-maze exploration when administered into the anterior cingulate cortex. Taken together, these findings demonstrate clear regional differences for the effects of corticosterone in the medial prefrontal cortex.

Chronically elevated corticosterone in the dorsolateral bed nuclei of stria terminalis increases anxiety-like behavior.

We previously discovered that corticosterone administration into the amygdala increases anxiety-like behaviors in rats tested on an elevated plus maze. However the behavioral effects of elevated corticosterone in a functionally related structure, the bed nuclei of stria terminalis (BNST) are unknown. The current study examined the effects of corticosterone administration into the dorsolateral BNST on exploratory behavior on an elevated plus maze. Corticosterone reduced open arm exploration on the plus maze indicating an increase in anxiety-like behavior.

Strain differences in anxiety-like behavior: association with corticotropin-releasing factor.

The purpose of the current study was to relate basal corticotropin-releasing factor (CRF) mRNA level in the central nucleus of the amygdala (CeA) with anxiety-like behavior using three strains of rat reported to exhibit a range of behavioral and neuroendocrine responses to stress. Anxiety-like behavior was determined for Fischer (F344), Wistar, and Wistar-Kyoto (WKY) rats with an elevated plus-maze and CRF mRNA level was measured using in situ hybridization. WKY rats exhibited more anxiety-like behavior on the elevated plus-maze than both F344 (p's < 0.01) and Wistar rats (p's < 0.05). WKY rats had higher basal levels of CRF mRNA in the CeA than F344 rats (p < 0.05) with a trend toward higher levels than Wistar rats (p = 0.06). Wistar rats had similar indices of anxiety with F344 rats and comparable levels of CRF mRNA in the CeA. Basal plasma corticosterone was similar for all three strains and repeated experience with the plus-maze had no effect on basal corticosterone levels or CRF mRNA levels in the paraventricular nucleus of the hypothalamus (PVN) for Wistar or WKY rats. Consistent with reported hyperactivity of the hypothalamopituitary adrenal axis of F344 rats with repeated stress, we observed elevated corticosterone following repeated exposure to the elevated plus-maze in F344 rats (p < 0.01) with a trend toward increased CRF mRNA levels in the PVN (p = 0.09). Heightened expression of CRF in the CeA of WKY rats may be involved in anxiety-like behaviors observed in this strain.

Chronically elevated corticosterone in the amygdala increases corticotropin releasing factor mRNA in the dorsolateral bed nucleus of stria terminalis following duress.

Elevated corticosterone in the amygdala leads to cautious avoidance behaviors on the elevated plus maze. The current study examined the effect of elevated corticosterone in the amygdala on corticotropin releasing factor (CRF) mRNA levels in the bed nuclei of stria terminalis (BNST). Exposure to the elevated plus maze increased CRF mRNA in both the dorsolateral and ventrolateral BNST. Corticosterone implants in the amygdala potentiated this effect in the dorsolateral, but not ventrolateral BNST. Corticosterone also resulted in an increased number of neurons expressing CRF in the dorsolateral BNST in response to the behavioral stimuli.

Effect of methamphetamine self-administration on tyrosine hydroxylase and dopamine transporter levels in mesolimbic and nigrostriatal dopamine pathways of the rat.

RATIONALE AND OBJECTIVES: Many studies have examined the effect of experimenter-delivered methamphetamine on the mesolimbic and nigrostriatal dopamine pathways. In contrast, little is known about the effect of methamphetamine self-administration on these neuronal pathways. We studied the effect of methamphetamine self-administration on two key regulators of dopamine transmission, tyrosine hydroxylase (TH), and dopamine transporter (DAT), in components of the mesolimbic and nigrostriatal dopamine pathways. METHODS: Rats self-administered methamphetamine (0.1 mg/kg per infusion, fixed-ratio-1 reinforcement schedule) or saline (control condition) for 9 h/day over 10 days. The brains of these rats were collected after 1 or 30 days of forced abstinence and the expression levels of TH and DAT were assayed by in situ, hybridization and western blot. RESULTS: TH mRNA and protein levels were increased in the ventral tegmental area (VTA, the cell body region of the mesolimbic dopamine system) and the substantia nigra pars compacta (SNC, the cell body region of the nigrostriatal dopamine system) after 1 day, but not 30 days, of forced abstinence from methamphetamine. In contrast, methamphetamine self-administration had no effect on TH protein levels in dopaminergic terminals located in the nucleus accumbens and caudate-putamen. In addition, methamphetamine self-administration had no effect on DAT mRNA levels in the VTA. CONCLUSIONS: Results suggest that extended daily access to self-administered methamphetamine results in a transient, short-lasting effect on mesolimbic and nigrostriatal dopamine neurons of the rat brain.

Role of glucocorticoids and cAMP-mediated repression in limiting corticotropin-releasing hormone transcription during stress.

The role of glucocorticoids and the repressor isoform of cAMP response element (CRE) modulator (CREM), inducible cAMP early repressor (ICER), in limiting corticotropin-releasing hormone (CRH) transcription during restraint stress were examined in both intact and adrenalectomized rats receiving glucocorticoid replacement. CRH primary transcript, measured by intronic in situ hybridization, increased after 30 min of restraint and returned to basal levels by 90 min, despite the persistent stressor. The decline was independent of circulating glucocorticoids, because adrenalectomized rats displayed an identical pattern. ICER mRNA in the hypothalamic paraventricular nucleus (PVN) increased after 30 min and remained elevated for up to 4 h in a glucocorticoid-independent manner. Western blot and electrophoretic mobility shift assay analyses showed increases in endogenous ICER in the PVN of rats subjected to restraint stress for 3 h. Chromatin immunoprecipitation assays showed the recruitment of CREM by the CRH CRE in conjunction with decreases in RNA polymerase II (Pol II) binding in the PVN region of rats restrained for 3 h. These data show that stress-induced glucocorticoids do not mediate the limitation of CRH transcription. Furthermore, the ability of CREM to bind the CRH CRE and the time relationship between elevated CREM and reduced Pol II recruitment by the CRH promoter suggest that inhibitory isoforms of CREM induced during stress contribute to the decline in CRH gene transcription during persistent stimulation.

The anxiogenic drug yohimbine reinstates methamphetamine seeking in a rat model of drug relapse.

BACKGROUND: Brain noradrenaline is involved in footshock stress-induced reinstatement of drug seeking in a rat relapse model. We studied whether yohimbine, an alpha-2 adrenoceptor antagonist that increases noradrenaline release and induces anxiety-like responses in human and nonhuman subjects, would reinstate methamphetamine seeking in rats. METHODS: In experiment 1, the effect of yohimbine (1.25-2.5 mg/kg) on reinstatement was compared with that of intermittent footshock (5 min;.2-.6 mA) in rats that were trained to lever press for intravenous methamphetamine (9-11 days) and subsequently underwent 7 days of extinction training. In experiment 2, the effect of yohimbine on reinstatement of drug seeking was determined during early (1 day) and late (21 or 51 days) withdrawal periods. On the test days, rats were first given 3-hour extinction sessions and were then tested for reinstatement induced by yohimbine. RESULTS: In experiment 1, both yohimbine and footshock stress reinstated methamphetamine seeking after extinction. In experiment 2, extinction responding was higher after 21 or 51 withdrawal days than after 1 withdrawal day. In contrast, no significant time-dependent changes in yohimbine-induced reinstatement were observed. CONCLUSIONS: Results indicate that yohimbine is a potent stimulus for reinstatement of methamphetamine seeking in a rat relapse model.

Effect of environmental stressors on opiate and psychostimulant reinforcement, reinstatement and discrimination in rats: a review.

Studies in humans suggest that exposure to life stressors is correlated with compulsive drug abuse and relapse to drugs during periods of abstinence. The behavioral and neurobiological mechanisms involved in the effect of stress on drug abuse, however, are not known. Here, we review data from studies using preclinical models in rats on the effect of environmental stressors on opiate and psychostimulant reinforcement, as measured by the intravenous drug self-administration and conditioned place preference procedures, on relapse to these drugs, as measured by the reinstatement procedure, and on the subjective effects of these drugs, as measured by the drug discrimination procedure. The results of the studies reviewed here suggest that while stressors are important modulators of the behavioral effects of opiate and psychostimulant drugs, the effect of stress on behavior in these animal models is stressor-specific, and to some degree, procedure- and drug-class-specific. The review of studies on the neurobiological mechanisms underlying stress-drug interactions in these animal models indicate that central noradrenaline and extrahypothalamic corticotropin-releasing factor mediate the effect of one form of stress (intermittent footshock) on reinstatement of opiate and psychostimulant seeking after prolonged drug-free periods. At present, however, little is known about the neuronal events that mediate the effect of environmental stressors on opiate and psychostimulant reinforcement or discrimination. The broader implications of the data reviewed here for future research and for the treatment of opiate and psychostimulant addiction are briefly discussed.

Stereotaxic localization of corticosterone to the amygdala enhances hypothalamo-pituitary-adrenal responses to behavioral stress.

The amygdala is involved in behavioral, autonomic, and neuroendocrine responses to stressful stimuli. The goal of the current study was to determine the effect of directly elevating glucocorticoids in the amygdala on hypothalamo-pituitary-adrenocortical (HPA) responses to the elevated plus maze, a behavioral stressor known to activate the amygdala. Micropellets (30 microg) of crystalline corticosterone or cholesterol (control) were implanted bilaterally at the dorsal margin of the CeA in male Wistar rats; vascular catheters were also placed at this time. Five days post-surgery, blood samples were drawn at 07:00 and 19:00 h to assess diurnal rhythm of plasma corticosterone. At 7 days post-implantation, rats were subjected to behavioral stress using an elevated plus maze and blood was collected 15 min prior to stress, and at 15, 45, and 90 min after the initiation of the stressor. Corticotropin releasing factor (CRF) and arginine vasopressin (AVP) mRNA levels were analyzed by in situ hybridization in the medial parvocellular division of the hypothalamic paraventricular nucleus (mpPVN) in corticosterone- and cholesterol-implanted rats either not exposed to the elevated plus maze (control) or 4 h post-behavioral stress. Localization of corticosterone to the amygdala had no effect on diurnal rhythm of corticosterone secretion. Behavioral stress significantly increased peak plasma corticosterone levels in both groups to a similar level. However, in the corticosterone implanted rats, plasma corticosterone concentrations at 45 and 90 min post-stress were significantly greater compared to control rats indicating a prolonged corticosterone response to behavioral stress. In non-stressed rats, corticosterone delivery to the amygdala elevated basal CRF mRNA in the mpPVN to levels similar to those observed post-stress in control animals; no further increase was observed in CRF mRNA following stress. Behavioral stress resulted in a significant elevation in CRF mRNA in cholesterol controls. Basal AVP mRNA levels were unaffected by corticosterone implants. AVP mRNA did not increase in cholesterol implanted rats in response to behavioral stress. However, AVP mRNA levels were higher in corticosterone implanted rats post stress compared to cholesterol treated controls. In conclusion, direct administration of corticosterone to the amygdala increases plasma corticosterone in response to a behavioral stressor without altering the diurnal rhythm in plasma corticosterone. Elevated basal levels of mpPVN CRF mRNA, and the induction of a mpPVN AVP mRNA response to the behavioral stressor implicate enhanced ACTH secretagogue expression in the increased HPA response to corticosterone modulation of amygdala function.