EphA4 is Involved in Sleep Regulation but Not in the Electrophysiological Response to Sleep Deprivation.

STUDY OBJECTIVES: Optimal sleep is ensured by the interaction of circadian and homeostatic processes. Although synaptic plasticity seems to contribute to both processes, the specific players involved are not well understood. The EphA4 tyrosine kinase receptor is a cell adhesion protein regulating synaptic plasticity. We investigated the role of EphA4 in sleep regulation using electrocorticography in mice lacking EphA4 and gene expression measurements. METHODS: EphA4 knockout (KO) mice, Clock(Δ19/Δ19) mutant mice and littermates, C57BL/6J and CD-1 mice, and Sprague-Dawley rats were studied under a 12 h light: 12 h dark cycle, under undisturbed conditions or 6 h sleep deprivation (SLD), and submitted to a 48 h electrophysiological recording and/or brain sampling at different time of day. RESULTS: EphA4 KO mice showed less rapid eye movement sleep (REMS), enhanced duration of individual bouts of wakefulness and nonrapid eye movement sleep (NREMS) during the light period, and a blunted daily rhythm of NREMS sigma activity. The NREMS delta activity response to SLD was unchanged in EphA4 KO mice. However, SLD increased EphA4 expression in the thalamic/hypothalamic region in C57BL/6J mice. We further show the presence of E-boxes in the promoter region of EphA4, a lower expression of EphA4 in Clock mutant mice, a rhythmic expression of EphA4 ligands in several brain areas, expression of EphA4 in the suprachiasmatic nuclei of the hypothalamus (SCN), and finally an unchanged number of cells expressing Vip, Grp and Avp in the SCN of EphA4 KO mice. CONCLUSIONS: Our results suggest that EphA4 is involved in circadian sleep regulation.

Neonatal stress augments the hypoxic chemoreflex of adult male rats by increasing AMPA receptor-mediated modulation.

Neonatal stress disrupts the developmental trajectory of homeostatic systems. Adult (8- to 10-week-old) male rats exposed to maternal separation (a form of neonatal stress) display several traits reported in patients suffering from sleep-disordered breathing, including an augmented hypoxic chemoreflex. To understand the mechanisms behind this effect, we tested the hypothesis that neonatal stress augments glutamatergic neurotransmission in three regions involved in respiratory regulation, namely the nucleus of the solitary tract, the paraventricular nucleus of the hypothalamus and the phrenic motor nucleus. Maternal separation was performed for 3 h day(-1) from postnatal day 3 to 12. Control pups were undisturbed. Adult rats were instrumented for intracerebroventricular injection of the AMPA/kainate receptor antagonist CNQX (0-4.3 µm). Using plethysmography, ventilatory activity was measured at rest in awake animals during normoxia (fractional inspired O2 = 0.21) and during acute hypoxia (fractional inspired O2 = 0.12; 20 min). Following vehicle injection, the hypoxic ventilatory response of stressed rats was 35% greater than that of controls. Microinjection of CNQX attenuated the hypoxic ventilatory response, but the effect observed in stressed rats was greater than that in control animals. Autoradiography experiments showed that neonatal stress augments expression of AMPA receptors within the paraventricular nucleus of the hypothalamus and the phrenic motor nucleus. Quantification of brain-derived neurotrophic factor showed that neonatal stress augments brain-derived neurotrophic factor expression only within the paraventricular nucleus. We conclude that neonatal stress augments the hypoxic chemoreflex by increasing the efficacy of glutamatergic synaptic inputs projecting onto key respiratory structures, especially the paraventricular nucleus of the hypothalamus. These data provide new insight into the aetiology of sleep-disordered breathing.

Challenged sodium balance and expression of angiotensin type 1A receptor mRNA in the hypothalamus of Wistar and Dahl rat strains.

The present study investigates the influence of a chronic high Na+ diet (8% Na+) on the expression of the angiotensin type 1A (AT1A) receptor gene in the lamina terminalis and paraventricular nucleus of the hypothalamus (PVH) in normotensive Wistar (W) rats, as well as in Dahl salt-resistant (DR) and Dahl salt-sensitive (DS) rats. Three weeks of 8% Na+ diet led to a higher blood pressure in DS rats compared to DR and W rats. Moreover, the high Na+ diet was correlated with a decreased expression of AT1A receptor mRNA in the median preoptic nucleus (MnPO) and in the PVH of DS rats, compared to DR and W rats. Contrastingly, the AT1A receptor mRNA expression was not altered by the high Na+ diet in the forebrain circumventricular organs of all the rat strains. Interestingly, a furosemide-induced Na+ depletion was correlated with an increased expression of AT1A receptor mRNA in the PVH, MnPO and SFO of both the DS and DR rats. It is concluded that chronic high Na+ diet did differently regulate the expression of AT1A receptor mRNA in two hypothalamic integrative centers for hydromineral and cardiovascular balance (the PVH and MnPO) in DS rats, compared to DR and W rats. However, the AT1A receptor mRNA expression was similarly regulated in DS and DR rats in response to an acute Na+ depletion, suggesting a distinct high Na+ -induced regulation of the AT1A receptor gene in the PVH and MnPO of DS rats.

Differential startle reactivity following central CCK-8S and systemic Boc CCK-4 administration in mice: antecedent stressor history and testing condition.

The influence of intraventricular cholecystokinin-8S (CCK-8S) and systemic N-t-Boc-Trp-Met-Asp-Phe-amide (Boc CCK-4) was evaluated in the acoustic and fear-potentiated startle paradigms in CD-1 mice. In the light + tone startle condition. CCK-8S increased startle 168 hr after administration, compared with saline. In the tone startle condition, CCK-8S decreased startle immediately and 24 hr after administration, compared with saline. Among nonshocked mice, CCK-8S increased startle at 48 and 168 hr, compared with saline. In the light + tone condition, 5 microg Boc-CCK-4 did not influence startle, whereas 15 microg Boc CCK-4 decreased startle immediately, 24 hr, and 48 hr following administration. Results demonstrate that antecedent environmental experiences interact with subsequent pharmacological challenges in provoking the temporal expression of alterations in startle magnitude.

Exposure of mice to a predator odor increases acoustic startle but does not disrupt the rewarding properties of VTA intracranial self-stimulation.

The present investigation assessed the propensity of an acute psychogenic stressor exposure to induce behavioral change in paradigms assessing fear/anxiety (acoustic startle) and motivation/anhedonia (intracranial self-stimulation) in CD-1 mice. In the acoustic startle paradigm, a 10-min exposure of 2-4 month old mice (young adult mice) to fox odor (2,5-dihydro-2,4,5-trimethylthiazoline; TMT) was associated with decreased acoustic startle relative to mice exposed to the control odor, butyric acid (BA), immediately and relative to both saline and BA exposure 24 h following odor exposure in the home cage. In contrast, a 2-min exposure of young adult mice to TMT was associated with an increase in startle relative to saline and BA during the immediate post-odor test session only. In young adult mice a 2-min and a 10-min exposure to BA resulted in a startle profile of mice reminiscent of saline-treated mice. In comparison to young adult mice, a 2-min exposure of mature adult mice (5-7 months old) to TMT enhanced startle for up to 48 h relative to both saline and BA, while a 10-min exposure of mature adult mice to TMT enhanced startle for 168 h post-odor exposure relative to saline-exposed mice only. However, the greatest increase in startle amplitude (i.e. 48 h) was acquired following the 2-min exposure of mature mice to TMT. Among mature adult mice, a 10-min exposure to BA in the home cage eventuated in enhanced startle relative to saline-exposed animals 168 h following odor exposure. In comparison, exposure of mice to 10 min of TMT depressed responding for VTA brain stimulation at the initial 80 Hz frequency, but was ineffective in elevating reward thresholds relative to mice merely exposed to saline. Mice assessed in the ICSS paradigm were approximately 2-4 months old at the time of surgery and 5-7 months old at the completion of testing. These data suggest that acute odor exposure may induce a fear gradient dependent upon the perceived stressor severity and that the resultant anxiety-like effects are dependent on the duration of odor exposure, age of the animals and the temporal interval between odor presentation and behavioral testing. Moreover, the anxiogenic properties of psychogenic stressors can be separated from their anhedonic effects. The implications of these data for clinical psychopathology are discussed.

Acute sodium deficit triggers plasticity of the brain angiotensin type 1 receptors.

The brain renin-angiotensin system (bRAS) is involved in the control of hydromineral balance. However, little information is available on the functional regulation of the bRAS as a consequence of sodium deficit in the extracellular fluid compartments. We used a pharmacological model of acute Na+ depletion (furosemide injections) to investigate changes of a major component of the bRAS, the hypothalamic angiotensin type 1A (AT(1A)) receptors. Furosemide induced a rapid and long-lasting expression of the AT(1A) mRNA in the subfornical organ, the median preoptic nucleus (MnPO), and the parvocellular division of the paraventricular nucleus (pPVN). Na+ depletion increased the number of cells expressing AT(1A) mRNA in the pPVN, but not in the MnPO. The enhancement of AT(1A) mRNA expression was associated with an increase in AT(1) binding sites in all the regions studied. It is of interest that in the paraventricular nucleus, the majority of the neurons expressing AT(1A) mRNA also showed an increase in metabolic activity (Fos-related antigen immunoreactivity [FRA-ir]). By contrast, in the MnPO, we observe two distinct cell populations. Our data demonstrated that an acute Na+ deficit induced a functional regulation of the hypothalamic AT(1A) receptors, indicating that these receptors are subject to plasticity in response to hydromineral perturbations.