Effects of adrenergic and serotonergic agonists in the amygdala on the hypothalamo-pituitary-adrenocortical axis

Prior stress and vasopressin promote corticotropin-releasing factor inhibition of serotonin release in the central nucleus of the amygdala

Corticotropin-releasing factor (CRF) is essential for coordinating endocrine and neural responses to stress, frequently facilitated by vasopressin (AVP). Previous work has linked CRF hypersecretion, binding site changes, and dysfunctional serotonergic transmission with anxiety and affective disorders, including clinical depression. Crucially, CRF can alter serotonergic activity. In the dorsal raphé nucleus and serotonin (5-HT) terminal regions, CRF effects can be stimulatory or inhibitory, depending on the dose, site, and receptor type activated. Prior stress alters CRF neurotransmission and CRF-mediated behaviors. Lateral, medial, and ventral subdivisions of the central nucleus of the amygdala (CeA) produce CRF and coordinate stress responsiveness. The purpose of these experiments was to determine the effect of intracerebroventricular (icv) administration of CRF and AVP on extracellular 5-HT as an index of 5-HT release in the CeA, using in vivo microdialysis in freely moving rats and high performance liquid chromatography (HPLC) analysis. We also examined the effect of prior stress (1 h restraint, 24 h prior) on CRF- and AVP-mediated release of 5-HT within the CeA. Our results show that icv CRF infusion in unstressed animals had no effect on 5-HT release in the CeA. Conversely, in rats with prior stress, CRF caused a profound dose-dependent decrease in 5-HT release within the CeA. This effect was long-lasting (240 min) and was mimicked by CRF plus AVP infusion without stress. Thus, prior stress and AVP functionally alter CRF-mediated neurotransmission and sensitize CRF-induced inhibition of 5-HT release, suggesting that this is a potential mechanism underlying stress-induced affective reactivity in humans.

Heterosynaptic modulation in the paraventricular nucleus of the hypothalamus

The stress response-originally described by Hans Selye as "the nonspecific response of the body to any demand made upon it"-is chiefly mediated by the hypothalamic-pituitary-adrenal (HPA) axis and is activated by diverse sensory stimuli that inform threats to homeostasis. The diversity of signals regulating the HPA axis is partly achieved by the complexity of afferent inputs that converge at the apex of the HPA axis: this apex is formed by a group of neurosecretory neurons that synthesize corticotropin-releasing hormone (CRH) in the paraventricular nucleus of the hypothalamus (PVN). The afferent synaptic inputs onto these PVN-CRH neurons originate from a number of brain areas, and PVN-CRH neurons respond to a long list of neurotransmitters/neuropeptides. Considering this complexity, an important question is how these diverse afferent signals independently and/or in concert influence the excitability of PVN-CRH neurons. While many of these inputs directly act on the postsynaptic PVN-CRH neurons for the summation of signals, accumulating data indicates that they also modulate each other's transmission in the PVN. This mode of transmission, termed heterosynaptic modulation, points to mechanisms through which the activity of a specific modulatory input (conveying a specific sensory signal) can up- or down-regulate the efficacy of other afferent synapses (mediating other stress modalities) depending on receptor expression for and spatial proximity to the heterosynaptic signals. Here, we review examples of heterosynaptic modulation in the PVN and discuss its potential role in the regulation of PVN-CRH neurons' excitability and resulting HPA axis activity. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.

Blunted 5-HT1A receptor-mediated responses and antidepressant-like behavior in mice lacking the GABAB1a but not GABAB1b subunit isoforms

RATIONALE

There is accumulating evidence for a role of GABAB receptors in depression. GABAB receptors are heterodimers of GABAB1 and GABAB2 receptor subunits. The predominant GABAB1 subunit isoforms are GABAB1a and GABAB1b. GABAB1 isoforms in mice differentially influence cognition, conditioned fear, and susceptibility to stress, yet their influence in tests of antidepressant-like activity has not been fully investigated.

OBJECTIVES

Given the interactions between GABAB receptors and the serotonergic system and the involvement of 5-HT1A receptors (5-HT1AR) in antidepressant action, we sought to evaluate 5-HT1AR function in GABAB1a-/- and GABAB1b-/- mice.

METHODS

GABAB1a-/- and GABAB1b-/- mice were assessed in the forced swim test (FST), and body temperature and hypothalamic-pituitary-adrenal (HPA) responses to the 5-HT1AR agonist 8-OH-DPAT were determined. Brain 5-HT1AR expression was assessed by [3H]-MPPF and [3H]-8-OH-DPAT autoradiography and 5-HT1AR G-protein coupling by [35S]GTP-γ-S autoradiography.

RESULTS

As previously described, GABAB1a-/- mice showed an antidepressant-like profile in the FST. GABAB1a-/- mice also demonstrated profoundly blunted hypothermic and motoric responses to 8-OH-DPAT. Furthermore, 8-OH-DPAT-induced corticosterone and adrenocorticotropic hormone (ACTH) release were both attenuated in GABAB1a-/- mice. Interestingly, [3H]-MPPF and [3H]-8-OH-DPAT binding was largely unaffected by genotype. [35S]GTP-γ-S autoradiography suggested that altered 5-HT1AR G-protein coupling only partially contributes to the functional presynaptic 5-HT1AR desensitization, and not at all to the blunted postsynaptic 5-HT1AR-mediated responses, seen in GABAB1a-/- mice.

CONCLUSION

These data demonstrate distinct functional links between 5-HT1ARs and the GABAB1a subunit isoform and suggest that the GABAB1a isoform may be implicated in the antidepressant-like effects of GABAB receptor antagonists and in neurobiological mechanisms underlying depression.

Type-7 metabotropic glutamate receptors negatively regulate α1-adrenergic receptor signalling

We studied the interaction between mGlu7 and α₁-adrenergic receptors in heterologous expression systems, brain slices, and living animals. L-2-Amino-4-phosphonobutanoate (L-AP4), and l-serine-O-phosphate (L-SOP), which activate group III mGlu receptors, restrained the stimulation of polyphosphoinositide (PI) hydrolysis induced by the α₁-adrenergic receptor agonist, phenylephrine, in HEK 293 cells co-expressing α₁-adrenergic and mGlu7 receptors. The inibitory action of L-AP4 was abrogated by (i) the mGlu7 receptor antagonist, XAP044; (ii) the C-terminal portion of type-2 G protein coupled receptor kinase; and (iii) the MAP kinase inhibitors, UO126 and PD98059. This suggests that the functional interaction between mGlu7 and α₁-adrenergic receptors was mediated by the βγ-subunits of the G_i protein and required the activation of the MAP kinase pathway. Remarkably, activation of neither mGlu2 nor mGlu4 receptors reduced α₁-adrenergic receptor-mediated PI hydrolysis. In mouse cortical slices, both L-AP4 and L-SOP were able to attenuate norepinephrine- and phenylephrine-stimulated PI hydrolysis at concentrations consistent with the activation of mGlu7 receptors. L-AP4 failed to affect norepinephrine-stimulated PI hydrolysis in cortical slices from mGlu7^-/- mice, but retained its inhibitory activity in slices from mGlu4^-/- mice. At behavioural level, i.c.v. injection of phenylephrine produced antidepressant-like effects in the forced swim test. The action of phenylephrine was attenuated by L-SOP, which was inactive per se. Finally, both phenylephrine and L-SOP increased corticosterone levels in mice, but the increase was halved when the two drugs were administered in combination. Our data demonstrate that α₁-adrenergic and mGlu7 receptors functionally interact and suggest that this interaction might be targeted in the treatment of stress-related disorders.

Chronic amphetamine treatment enhances corticotropin-releasing factor-induced serotonin release in the amygdala

Amphetamine use is associated with dysphoric states, including heightened anxiety, that emerge within 24h of withdrawal from the drug. Corticotropin-releasing factor increases serotonin release in the central nucleus of the amygdala, and this neurochemical circuitry may play a role in mediating fear and anxiety states. We have previously shown that chronic amphetamine treatment increases corticotropin-releasing factor receptor type-2 levels in the serotonergic dorsal raphe nucleus of the rat. Therefore, we hypothesized that chronic amphetamine treatment would enhance the amygdalar serotonergic response to corticotropin-releasing factor infused into the dorsal raphe nucleus. Male rats were injected once-daily with d-amphetamine (2.5mg/kg i.p., or saline) for two weeks. Serotonin release within the central nucleus of the amygdala in response to intra-raphe infusion of corticotropin-releasing factor (100 ng) was measured 24h after the last treatment in urethane-anesthetized (1.8 mg/kg, i.p.) rats using in vivo microdialysis. Rats pretreated with amphetamine showed significantly enhanced serotonin release in the central nucleus of the amygdala in response to corticotropin-releasing factor infusion when compared to saline pretreated rats. Furthermore, this enhanced response was blocked by the corticotropin-releasing factor type-2 receptor antagonist antisauvagine-30 (2 microg) infused into the dorsal raphe nucleus. These results suggest increased sensitivity to corticotropin-releasing factor as mediated by type-2 receptors following chronic amphetamine treatment, which may underlie dysphoric states observed during amphetamine withdrawal.

Restraint stress increases serotonin release in the central nucleus of the amygdala via activation of corticotropin-releasing factor receptors

Decreases in serotonergic activity in the central nucleus of the amygdala reduce responses to stressors, suggesting an important role for serotonin in this region of the amygdala in stress reactivity. However, it is not known whether exposure to stressors actually increases serotonin release in the central nucleus of the amygdala. The current experiment tested the hypothesis that restraint stress increases extracellular serotonin within the central nucleus of the amygdala and adjacent medial amygdala using in vivo microdialysis in awake male rats during the dark phase of the light-dark cycle. Serotonin release in the central nucleus increased immediately in response to restraint stress. In contrast, there was no change in serotonin release within the adjacent medial amygdala during or following restraint. Since corticotropin-releasing factor is an important mediator of both responses to stressors and serotonergic activity, subsequent experiments tested the hypothesis that central nucleus serotonergic response to restraint stress is mediated by central corticotropin-releasing factor receptors. Administration of the corticotropin-releasing factor type 1 and 2 receptor antagonist d-Phe-CRF (icv, 10 microg/5 microl) prior to restraint stress suppressed restraint-induced serotonin release in the central nucleus. The results suggest that restraint stress rapidly and selectively increases serotonin release in the central nucleus of the amygdala by the activation of central corticotropin-releasing factor receptors. Furthermore, the results imply that corticotropin-releasing factor mediated serotonergic activity in central nucleus of the amygdala may be an important component of a stress response.

Stress-induced and alcohol cue-induced craving in recently abstinent alcohol-dependent individuals

BACKGROUND

Research has shown that exposure to stress/negative affect and to alcohol cues can each increase alcohol craving and relapse susceptibility in alcohol-dependent individuals. However, whether the emotional and physiological states associated with stress-induced and alcohol cue-induced craving are comparable has not been well studied. Therefore, this study examined the craving, emotional, and physiological responses to stress and to alcohol cues in treatment-engaged, 4-week abstinent, alcohol-dependent individuals using analogous stress and alcohol cue imagery methods.

METHOD

Twenty treatment-seeking, alcohol-dependent participants (18 males/2 females) were exposed to a brief 5-minute guided imagery procedure that involved imagining a recent personal stressful situation, a personal alcohol cue-related situation, and a neutral-relaxing situation, 1 imagery per session presented in random order. Alcohol craving, anxiety and emotion rating scales, cardiovascular measures, and salivary cortisol were compared across the 3 conditions.

RESULTS

Exposure to stress and to alcohol cues each produced significant increases in alcohol craving, anxiety, and negative emotions and decreases in positive emotions. Stress-induced alcohol craving was significantly correlated with increases in sadness, anger, and anxiety ratings, but alcohol cue-induced craving was associated with decreases in positive affect (joy and neutral relaxed state) and increases in anxiety and fear ratings. Furthermore, stress increased systolic and diastolic blood pressure responses, but significant increases in salivary cortisol were only observed in the alcohol cue condition.

CONCLUSIONS

Although both stress and alcohol cues produce increases in anxiety associated with alcohol craving, each produced a dissociable psychobiological state involving subjective emotional, cardiovascular, and cortisol responses. These data could have significant implications for understanding the specific psychobiology associated with stress or alcohol cue exposure and their potential effects on alcohol relapse susceptibility.

Sex differences in the basolateral amygdala: the extracellular levels of serotonin and dopamine, and their responses to restraint stress in rats

The sex difference in the emotional response to stress suggests a sex-specific stress response in the amygdala. To examine the sex difference in extracellular levels of serotonin (5HT) and dopamine (DA) in the basolateral amygdala (BLA) and their responses to restraint stress, in vivo microdialysis studies were performed in male and female rats. In experiment I, dialysates were collected from the BLA at 15-min intervals under the freely moving condition. Mean extracellular levels of 5HT or DA in the BLA were higher in male rats than in female rats. In experiment II, rats were subjected to restraint stress for 60 min to examine the stress response of 5HT or DA levels. Although restraint stress significantly increased extracellular 5HT levels in both sexes of rats, female rats showed a greater response than male rats. Moreover, restraint stress significantly increased extracellular DA levels in female rats, but not in male rats. In experiment III, rats were subjected to restraint stress for 30 min to examine behavioral responses to restraint stress. Although no sex difference was observed in the number of audible vocalizations, male rats defecated a larger number of fecal pellets than female rats. In experiment IV, rats were tested for freezing behavior to examine contextual fear responses. Conditioned male rats showed a longer freezing time than conditioned female rats. We found sex differences in the extracellular levels of 5HT and DA in the BLA and their responses to restraint stress, which may be involved in the sex-specific emotional response to stress in rats.

Corticotropin-releasing factor in the dorsal raphe elicits temporally distinct serotonergic responses in the limbic system in relation to fear behavior

The neurotransmitters serotonin and corticotrophin-releasing factor are thought to play an important role in fear and anxiety behaviors. This study aimed to determine the relationship between corticotrophin-releasing factor-evoked changes in serotonin levels within discrete regions of the limbic system and the expression of fear behavior in rats. The effects of corticotrophin-releasing factor administration to the serotonin cell body regions of the dorsal raphe nucleus on fear behavior, behavioral activity, and extracellular serotonin levels were assessed in freely moving rats with microdialysis probes implanted into the central nucleus of the amygdala and the medial prefrontal cortex. Infusion of corticotrophin-releasing factor (0.5 microg) into the dorsal raphe rapidly induced freezing behavior, which was positively correlated with an immediate increase in serotonin release in the central nucleus of the amygdala. In contrast, cessation of freezing behavior correlated with a delayed and prolonged increase in serotonin release within the medial prefrontal cortex. Our findings suggest that corticotrophin-releasing factor-induced freezing behavior is associated with regionally and temporally distinct serotonergic responses in the limbic system that may reflect differing roles for these regions in the expression of fear/anxiety behavior.

Effects of chronic paroxetine pretreatment on (+/-)-8-hydroxy-2-(di-n-propyl-amino)tetralin induced c-fos expression following sexual behavior

Chronic treatment with the selective serotonin reuptake inhibitor paroxetine impairs the functioning of 5-HT(1A) receptors involved in ejaculation. This could underlie the development of delayed ejaculation often reported by men treated with paroxetine. The neurobiological substrate linking the effects of selective serotonin reuptake inhibitor-treatment and 5-HT(1A) receptor activation with ejaculation was investigated. Male Wistar rats that were pretreated with paroxetine (20 mg/kg/day p.o.) or vehicle for 22 days and had received an additional injection with the 5-HT(1A) receptor agonist 8-OH-DPAT ((+/-)-8-hydroxy-2-(di-n-propyl-amino)tetralin; 0.4 mg/kg s.c.) or saline on day 22, 30 min prior to a sexual behavior test, were perfused 1 h after the sexual behavior test. Brains were processed for Fos-, and oxytocin immunohistochemistry. The drug treatments markedly changed both sexual behavior and the pattern and number of Fos-immunoreactive cells in the brain. Chronic pretreatment with paroxetine caused delayed ejaculation. Acute injection with 8-OH-DPAT facilitated ejaculation in vehicle-pretreated rats, notably evident in a strongly reduced intromission frequency, whereas 8-OH-DPAT had no effects in paroxetine-pretreated rats. Chronic treatment with paroxetine reduced Fos-immunoreactivity in the locus coeruleus, and prevented the increase in Fos-immunoreactive neurons induced by 8-OH-DPAT in the oxytocinergic magnocellular part of the paraventricular nucleus as well as in the locus coeruleus. Since oxytocin and noradrenalin facilitate ejaculation, the alterations in Fos-IR in these areas could connect selective serotonin reuptake inhibitor treatment and 5-HT(1A) receptor activation to ejaculation. Chronic paroxetine treatment and 8-OH-DPAT changed c-fos expression in a number of other brain areas, indicating that Fos-immunohistochemistry is a useful tool to find locations where selective serotonin reuptake inhibitors and 8-OH-DPAT exert their effects.

Regulation of hypoxia-induced release of corticotropin-releasing factor in the rat hypothalamus by norepinephrine

Corticotropin-releasing factor (CRF) peptide release was activated by hypoxia in the rat hypothalamus. The mechanisms, however, of the hypoxia-induced CRF release remains unclear. In this study, we demonstrated that the norepinephrine (NE) and its receptors in the paraventricular nucleus (PVN) mediated the CRF release in a simulated altitude hypoxia. When rats were exposed to 5 or 7 km altitude of hypoxia for a short or long term: (1) NE levels in the PVN and the CeA, using the HPLC analysis, were intensity and time course dependently increased, but the increase in the PVN were potential than in the CeA. Restraint-induced NE increase was much higher in both the PVN and the CeA, compared with hypoxia-induced response. (2) Hypoxia and restraint significantly enhanced CRF release in the ME and the PVN but not in the CeA, through RIA assay, which result in stimulating corticosterone secretion. (3) Hypoxia-induced CRF release was reversed by an injection of prazosin (i.c.v.), an alpha-1 adrenoceptor antagonist, while administration of yohimbine (i.c.v.), an alpha-2 receptor antagonist, facilitated further CRF release. These data suggested that hypoxia induced NE activation centrally, via alpha-1 and -2 receptors, leading to improving hypothalamic CRF release, which in turn stimulated pituitary and adrenal cortex. Restraint presented much potential action on NE activation than hypoxia.

Delineation of responsive AVP-containing neurons to running stress in the hypothalamus

Running becomes a stress, termed running stress, if it persists above the lactate threshold (LT) and results in enhanced plasma ACTH level in humans. Although the exact underlying regulation mechanism is still uncertain, hypothalamic AVP has been shown to play a dominant role in running-induced ACTH release. It is still not known, however, whether running stress activates the hypothalamic AVP-containing neurons that are involved in the activation of the ACTH response. For this reason, we applied our rat running stress model, in which both plasma ACTH and osmolality levels increase just above LT running (supra-LT running), to delineate which hypothalamic AVP neurons were responsive to running stress. Rats were previously habituated to running and then subjected to a 30-min run either just below or above the LT. Plasma samples were collected from these animals to determine ACTH and osmolality levels. Brains were prepared for immunocytochemistry for both AVP/Fos in the hypothalamus and enzyme immunoassay for the stalk median eminence (SME) AVP content. Only supra-LT running resulted in an increase in the number of Fos/AVP-immunoreactive neurons in both the parvocellular paraventricular nucleus (pPVN) and the magnocellular supraoptic nucleus (SON) accompanied by increased ACTH and plasma osmolality levels. Similarly, running reduced the SME content of the AVP. We thus found that AVP-containing neurons located in both the pPVN and SON are responsive to running stress just above the LT.

Exposure to a retrieval cue in rats induces changes in regional brain glucose metabolism in the amygdala and other related brain structures

Pre-test exposure to training-related cues is known to improve subsequent retention performance. To identify brain regions engaged in processes promoted by retrieval cues, a brain imaging approach using the [6-14C]glucose autoradiographic technique was used. Sprague-Dawley rats trained in a brightness discrimination avoidance task were submitted to different cueing conditions after a 1- or a 21-day training-to-test interval (TTI). Animals were either non-cued, cued with a box, or cued with a box and the light that served as a discriminative stimulus. Effects of the different cueing conditions on retention performance or on metabolic activity in 58 different brain regions were investigated. Rats cued with the light exhibited a subsequent improvement of their retention performance relative to controls, when tested at the 1- but not 21-days TTI, confirming our previous results. At the 1-day retention interval, a comparison between rats cued with the box and rats cued with the box and the light showed that the light cue significantly increased glucose uptake in a neuronal network composed of the lateral, basal, and central nuclei of the amygdala, the anterior and suprachiasmatic hypothalamic nuclei, the nucleus accumbens, the medial septum, and the insular cortex. In contrast, at the 21-day retention interval, both groups demonstrated similar cerebral metabolic activity. The present results indicate that exposure to a light cue increased metabolic activity in the previously mentioned brain structures only when the light acted as an effective retrieval cue, suggesting an involvement of this network in the processes triggered by a retrieval cue. Arguments are provided supporting the notion that the amygdala may play a key role in these processes. Whether the amygdala is a part of a neural network involved in retrieval processes or in neuromodulating systems that favour the efficacy of retrieval processes is also discussed.

Serotonergic stimulation of corticotropin-releasing hormone and pro-opiomelanocortin gene expression

The neurotransmitter serotonin (5-HT) stimulates adrenocorticotropic hormone (ACTH) secretion from the anterior pituitary gland via activation of central 5-HT1 and 5-HT2 receptors. The effect of 5-HT is predominantly indirect and may be mediated via release of hypothalamic corticotropin-releasing hormone (CRH). We therefore investigated the possible involvement of CRH in the serotonergic stimulation of ACTH secretion in male rats. Increased neuronal 5-HT content induced by systemic administration of the precursor 5-hydroxytryptophan (5-HTP) in combination with the 5-HT reuptake inhibitor fluoxetine raised CRH mRNA expression in the paraventricular nucleus (PVN) by 64%, increased pro-opiomelanocortin (POMC) mRNA in the anterior pituitary lobe by 17% and stimulated ACTH secretion five-fold. Central administration of 5-HT agonists specific to 5-HT1A, 5-HT1B, 5-HT2A or 5-HT2C receptors increased CRH mRNA in the PVN by 15-50%, POMC mRNA in the anterior pituitary by 15-27% and ACTH secretion three- to five-fold, whereas a specific 5-HT3 agonist had no effect. Systemic administration of a specific anti-CRH antiserum inhibited the ACTH response to 5-HTP and fluoxetine and prevented the 5-HTP and fluoxetine-induced POMC mRNA response in the anterior pituitary lobe. Central or systemic infusion of 5-HT increased ACTH secretion seven- and eight-fold, respectively. Systemic pretreatment with the anti-CRH antiserum reduced the ACTH responses to 5-HT by 80% and 64%, respectively. It is concluded that 5-HT via activation of 5-HT1A, 5-HT2A, 5-HT2C and possibly also 5-HT1B receptors increases the synthesis of CRH in the PVN and POMC in the anterior pituitary lobe, which results in increased ACTH secretion. Furthermore, the results indicate that CRH is an important mediator of the ACTH response to 5-HT.

Hypothalamic-pituitary-adrenal responses to weight loss in mice following diet restriction, activity or separation stress: effects of tyrosine

We have studied three different types of weight-loss stress caused by Diet restriction, Activity or Separation, for their effects on the hypothalamic-pituitary axis in young female mice and their responses to tyrosine 100 mg/kg/day. Plasma was assayed for ACTH and glucocorticoid determinations, and brain catecholamine concentrations were measured by HPLC/ECD. A similar weight loss of 24-28% was observed in the models despite significant differences in food intake. Diet restriction to 60% and Separation models produced a significant increase in hypothalamic noradrenaline (p < 0.01), while there was a significant decrease (p < 0.05) in the Diet restriction to 40% that was restored after tyrosine. After Activity, noradrenaline levels did not change. ACTH concentrations decreased following Diet restriction (p < 0.05) but were unaffected by Separation or Activity. The peripheral glucocorticoid response increased significantly after Activity and Diet restriction (p < 0.001), but decreased significantly after Separation (p < 0.001). Tyrosine increased glucocorticoid concentrations in the Activity and Separation models (p < 0.05), but not after Diet restriction. Despite similar weight loss in the three models there were no predictable associations between hypothalamic noradrenaline metabolism and plasma ACTH or glucocorticoid concentrations. Tyrosine might alleviate some of the different pathophysiological problems associated with the stress of weight loss.