Hypothalamic-pituitary-adrenocortical axis: neuropsychiatric aspects

Activation of the hypothalamic-pituitary-adrenal axis in lithium-induced conditioned taste aversion learning

Intraperitoneal injections (ip) of lithium chloride at large doses induce c-Fos expression in the brain regions implicated in conditioned taste aversion (CTA) learning, and also activate the hypothalamic-pituitary-adrenal (HPA) axis and increase the plasma corticosterone levels in rats. A pharmacologic treatment blunting the lithium-induced c-Fos expression in the brain regions, but not the HPA axis activation, induced CTA formation. Synthetic glucocorticoids at conditioning, but not glucocorticoid antagonist, attenuated the lithium-induced CTA acquisition. The CTA acquisition by ip lithium was not affected by adrenalectomy regardless of basal corticosterone supplement, but the extinction was delayed in the absence of basal corticosterone. Glucocorticoids overloading delayed the extinction memory formation of lithium-induced CTA. ip lithium consistently induced the brain c-Fos expression, the HPA activation and CTA formation regardless of the circadian activation of the HPA axis. Intracerebroventricular (icv) injections of lithium at day time also increased the brain c-Fos expression, activated the HPA axis and induced CTA acquisition. However, icv lithium at night, when the HPA axis shows its circadian activation, did not induce CTA acquisition nor activate the HPA axis, although it increased the brain c-Fos expression. These results suggest that the circadian activation of the HPA axis may affect central, but not peripheral, effect of lithium in CTA learning in rats, and the HPA axis activation may be necessary for the central effect of lithium in CTA formation. Also, glucocorticoids may be required for a better extinction; however, increased glucocorticoids hinder both the acquisition and the extinction of lithium-induced CTA.

USP7 Acts as a Molecular Rheostat to Promote WASH-Dependent Endosomal Protein Recycling and Is Mutated in a Human Neurodevelopmental Disorder

Endosomal protein recycling is a fundamental cellular process important for cellular homeostasis, signaling, and fate determination that is implicated in several diseases. WASH is an actin-nucleating protein essential for this process, and its activity is controlled through K63-linked ubiquitination by the MAGE-L2-TRIM27 ubiquitin ligase. Here, we show that the USP7 deubiquitinating enzyme is an integral component of the MAGE-L2-TRIM27 ligase and is essential for WASH-mediated endosomal actin assembly and protein recycling. Mechanistically, USP7 acts as a molecular rheostat to precisely fine-tune endosomal F-actin levels by counteracting TRIM27 auto-ubiquitination/degradation and preventing overactivation of WASH through directly deubiquitinating it. Importantly, we identify de novo heterozygous loss-of-function mutations of USP7 in individuals with a neurodevelopmental disorder, featuring intellectual disability and autism spectrum disorder. These results provide unanticipated insights into endosomal trafficking, illuminate the cooperativity between an ubiquitin ligase and a deubiquitinating enzyme, and establish a role for USP7 in human neurodevelopmental disease.

Circadian activation of the hypothalamic-pituitary-adrenal axis may affect central, but not peripheral, effect of lithium in conditioned taste aversion learning in rats

Activation of the hypothalamic-pituitary-adrenal (HPA) axis has been implicated in conditioned taste aversion (CTA) learning induced by lithium chloride. This study investigated if circadian activation of the HPA axis affects the lithium-induced CTA formation. The pairing of conditioned stimulus (sucrose) and unconditioned stimulus (lithium chloride) was performed at night (shortly after light-off) when the HPA activity shows its circadian increase. Intraperitoenal injection of lithium chloride (0.15M, 3ml/kg or 12ml/kg) at night induced CTA formation and the HPA axis activation and increased c-Fos expression in both the parabrachial nucleus (PBN) and the nucleus tractus of solitarius (NTS) in a dose dependent manner. However, intracerebroventricular lithium (0.6M, 5µl) at night failed to induce CTA or the HPA axis activation, although it increased c-Fos expression in the PBN and NTS. Results suggest that circadian activation of the HPA axis may affect central, but not peripheral, effect of lithium in CTA formation, and the lithium-induced c-Fos expression in brain regions may not be effective to induce CTA unless it is coupled with the HPA axis activation. It is concluded that the HPA axis activation may play an important role mediating not only peripheral but also central effect of lithium in CTA formation.

Hydration and beyond: neuropeptides as mediators of hydromineral balance, anxiety and stress-responsiveness

Challenges to body fluid homeostasis can have a profound impact on hypothalamic regulation of stress responsiveness. Deficiencies in blood volume or sodium concentration leads to the generation of neural and humoral signals relayed through the hindbrain and circumventricular organs that apprise the paraventricular nucleus of the hypothalamus (PVH) of hydromineral imbalance. Collectively, these neural and humoral signals converge onto PVH neurons, including those that express corticotrophin-releasing factor (CRF), oxytocin (OT), and vasopressin, to influence their activity and initiate compensatory responses that alleviate hydromineral imbalance. Interestingly, following exposure to perceived threats to homeostasis, select limbic brain regions mediate behavioral and physiological responses to psychogenic stressors, in part, by influencing activation of the same PVH neurons that are known to maintain body fluid homeostasis. Here, we review past and present research examining interactions between hypothalamic circuits regulating body fluid homeostasis and those mediating behavioral and physiological responses to psychogenic stress.

Increased antidepressant sensitivity after prefrontal cortex glucocorticoid receptor gene deletion in mice

Our laboratory has previously shown that antidepressants regulate glucocorticoid receptor (GR) expression in the prefrontal cortex (PFC). To determine if PFC GR are involved in antidepressant effects on behavior or hypothalamic-pituitary-adrenocortical (HPA) axis activity, we treated floxed GR male mice with saline or 15 or 30 mg/kg/d imipramine after PFC injection of adeno-associated virus 2/9 vectors transducing expression of Cre recombinase, to knock-down GR (PFC-GRKD), or green fluorescent protein (PFC-GFP), to serve as a control. The pattern of virally transduced GR deletion, common to all imipramine treatment groups, included the infralimbic, prelimbic, and medial anterior cingulate cortex at its largest extent, but was confined to the prelimbic and anterior cingulate cortex at its smallest extent. PFC GR knock-down increased behavioral sensitivity to imipramine, with imipramine-treated PFC-GRKD but not PFC-GFP mice exhibiting significant decreases in depression-like immobility during forced swim. PFC GR deletion did not alter general locomotor activity. The 30 mg/kg dose of imipramine increased plasma corticosterone levels immediately after a 5-min forced swim, but PFC GR knock-down had no significant effect on plasma corticosterone under these experimental conditions. We conclude that PFC GR knock-down, likely limited to the medial prelimbic and anterior cingulate cortices, can increase behavioral sensitivity to antidepressants. These findings indicate a novel role for PFC GR in influencing antidepressant response.