Caffeine treatment prevents rapid eye movement sleep deprivation‐induced impairment of late‐phase long‐term potentiation in the dentate gyrus

The CA1 and dentate gyrus (DG) are physically and functionally closely related areas of the hippocampus, but they differ in various respects, including their reactions to different insults. The purpose of this study was to determine the protective effects of chronic caffeine treatment on late‐phase long‐term potentiation (L‐LTP) and its signalling cascade in the DG area of the hippocampus of rapid eye movement sleep‐deprived rats. Rats were chronically treated with caffeine (300 mg/L drinking water) for 4 weeks, after which they were sleep‐deprived for 24 h. L‐LTP was induced in in anaesthetized rats, and extracellular field potentials from the DG area were recorded in vivo. The levels of L‐LTP‐related signalling proteins were assessed by western blot analysis. Sleep deprivation markedly reduced L‐LTP magnitude, and basal levels of total cAMP response element‐binding protein (CREB), phosphorylated CREB (P‐CREB), and calcium/calmodulin kinase IV (CaMKIV). Chronic caffeine treatment prevented the reductions in the basal levels of P‐CREB, total CREB and CaMKIV in sleep‐deprived rats. Furthermore, caffeine prevented post‐L‐LTP sleep deprivation‐induced downregulation of P‐CREB and brain‐derived neurotrophic factor in the DG. The current findings show that caffeine treatment prevents acute sleep deprivation‐induced deficits in brain function.     

Sex differences in depolarizing actions of GABAA receptor activation in rat embryonic hypothalamic neurons

GABA_A receptor activation exerts trophic actions in immature neurons through depolarization of resting membrane potential. The switch to its classical hyperpolarizing role is developmentally regulated. Previous results suggest that a hormonally biased sex difference exists at the onset of the switch in hypothalamic neurons. The aim of this work was to evaluate sex differences in GABA_A receptor function of hypothalamic neurons before brain masculinization by gonadal hormones. Hypothalamic cells were obtained from embryonic day 16 male and female rat foetuses, 2 days before the peak of testosterone production by the foetal testis, and grown in vitro for 9 days. Whole‐cell and perforated patch‐clamp recordings were carried out in order to measure several electrophysiological parameters. Our results show that there are more male than female neurons responding with depolarization to muscimol. Additionally, among cells with depolarizing responses, males have higher and longer lasting responses than females. These results highlight the relevance of differences in neural cell sex irrespective of exposure to sex hormones.     

Effect of Melanotan‐II on Brain Fos Immunoreactivity and Oxytocin Neuronal Activity and Secretion in Rats

Melanocortins stimulate the central oxytocin systems that are involved in regulating social behaviours. Alterations in central oxytocin have been linked to neurological disorders such as autism, and melanocortins have been proposed for therapeutic treatment. In the present study, we investigated how systemic administration of melanotan‐II (MT‐II), a melanocortin agonist, affects oxytocin neuronal activity and secretion in rats. The results obtained show that i.v. , but not intranasal, administration of MT‐II markedly induced Fos expression in magnocellular neurones of the supraoptic (SON) and paraventricular nuclei (PVN) of the hypothalamus, and this response was attenuated by prior i.c.v. administration of the melanocortin antagonist, SHU‐9119. Electrophysiological recordings from identified magnocellular neurones of the SON showed that i.v. administration of MT‐II increased the firing rate in oxytocin neurones but did not trigger somatodendritic oxytocin release within the SON as measured by microdialysis. Our data suggest that, after i.v. , but not intranasal, administration of MT‐II, the activity of magnocellular neurones of the SON is increased. Because previous studies showed that SON oxytocin neurones are inhibited in response to direct application of melanocortin agonists, the actions of i.v. MT‐II are likely to be mediated at least partly indirectly, possibly by activation of inputs from the caudal brainstem, where MT‐II also increased Fos expression.     

Diabetic Schwann cells suffer from nerve growth factor and neurotrophin‐3 underproduction and poor associability with axons

Schwann cells (SCs) are integral to peripheral nerve biology, contributing to saltatory conduction along axons, nerve and axon development, and axonal regeneration. SCs also provide a microenvironment favoring neural regeneration partially due to production of several neurotrophic factors. Dysfunction of SCs may also play an important role in the pathogenesis of peripheral nerve diseases such as diabetic peripheral neuropathy where hyperglycemia is often considered pathogenic. In order to study the impact of diabetes mellitus (DM) upon the regenerative capacity of adult SCs, we investigated the differential production of the neurotrophic factors nerve growth factor (NGF) and neurotrophin‐3 (NT3) by SCs harvested from the sciatic nerves of murine models of type 1 DM (streptozotocin treated C57BL/6J mice) and type 2 DM (LepR^?/? or db/db mice) or non‐diabetic cohorts. In vitro, SCs from diabetic and control mice were maintained under similar hyperglycemic and euglycemic conditions respectively. Mature SCs from diabetic mice produced lower levels of NGF and NT3 under hyperglycemic conditions when compared to SCs in euglycemia. In addition, SCs from both DM and non‐DM mice appear to be incapable of insulin production, but responded to exogenous insulin with greater proliferation and heightened myelination potentiation. Moreover, SCs from diabetic animals showed poorer association with co‐cultured axons. Hyperglycemia had significant impact upon SCs, potentially contributing to the pathogenesis of diabetic peripheral neuropathy. GLIA 2013;61:1990–1999     

ERK磷酸化对糖尿病周围神经病雪旺氏细胞Netrin-1表达水平及凋亡的影响

目的 探讨ERK通路对糖尿病周围神经病雪旺氏细胞凋亡以及Netrin-1表达水平的影响。方法 建立动物和细胞模型分析ERK通路对雪旺氏细胞以及Netrin-1表达水平的影响。结果 观察糖尿病模型小鼠坐骨神经组织中p-ERK的表达增加,ERK抑制剂PD98059抑制高糖诱导雪旺氏细胞中ERK的磷酸化,即ERK通路促进糖尿病周围神经疾病的发生; 糖尿病模型小鼠坐骨神经组织中Netrin-1表达减少,ERK抑制剂PD98059增加高糖刺激中雪旺氏细胞中Netrin-1的表达,即ERK抑制剂通过增加Netrin-1的表达来发挥神经保护作用; PD98059可逆转高糖诱导雪旺氏细胞中Caspase-3的表达和细胞凋亡,即ERK抑制剂降低雪旺氏细胞凋亡,从而防止糖尿病周围神经疾病的发展。结论 ERK抑制剂可抑制糖尿病周围神经疾病雪旺氏细胞的凋亡以及增加Nertin-1的表达,从而发挥保护作用     

Cocaine self‐administration disrupts mesolimbic dopamine circuit function and attenuates dopaminergic responsiveness to cocaine

Dopaminergic projections from the ventral midbrain to the nucleus accumbens (NAc) have long been implicated in encoding associations between reward availability and environmental stimuli. As such, this circuit is instrumental in guiding behaviors towards obtaining maximal rewards based on previous experience. Cocaine acts on the dopamine system to exert its reinforcing effects and it is thought that cocaine‐induced dysregulation of dopamine neurotransmission contributes to the difficulty that cocaine addicts exhibit in selecting environmentally appropriate behaviors. Here we used cocaine self‐administration combined with in vivo fast scan cyclic voltammetry in anesthetised rats to examine the function of the ventral tegmental area to NAc projection neurons. Over 5 days of cocaine self‐administration (fixed‐ratio 1; 1.5 mg/kg/injection; 40 injections/day), animals increased their rate of intake. Following cocaine self‐administration, there was a marked reduction in ventral tegmental area‐stimulated NAc dopamine release. Additionally, there was a decreased augmentation of stimulated dopamine overflow in response to a cocaine challenge. These findings demonstrate that cocaine induces a hypodopaminergic state, which may contribute to the inflexible drug‐taking and drug‐seeking behaviors observed in cocaine abusers. Additionally, tolerance to the ability of cocaine to elevate dopamine may lead to increased cocaine intake in order to overcome decreased effects, another hallmark of cocaine abuse.     

Involvement of PKA and ERK pathways in ghrelin‐induced long‐lasting potentiation of excitatory synaptic transmission in the CA1 area of rat hippocampus

Acute effects of ghrelin on excitatory synaptic transmission were evaluated on hippocampal CA1 synapses. Ghrelin triggered an enduring enhancement of synaptic transmission independently of NMDA receptor activation and probably via postsynaptic modifications. This ghrelin‐mediated potentiation resulted from the activation of GHS‐R1a receptors as it was mimicked by the selective agonist JMV1843 and blocked by the selective antagonist JMV2959. This potentiation also required the activation of PKA and ERK pathways to occur as it was inhibited by KT5720 and U0126, respectively. Moreover it most probably involved Ca²⁺ influxes as both ghrelin and JMV1843 elicited intracellular Ca²⁺ increases, which were dependent on the presence of extracellular Ca²⁺ and mediated by L‐type Ca²⁺ channels opening. In addition, ghrelin potentiated AMPA receptor‐mediated [Ca²⁺]i increases while decreasing NMDA receptor‐mediated ones. Thus the potentiation of synaptic transmission by GHS‐R1a at hippocampal CA1 excitatory synapses probably results from postsynaptic mechanisms involving PKA and ERK activation, which are producing long‐lasting enhancement of AMPA receptor‐mediated responses.     

Crucial Role for the Myelin-associated Glycoprotein in the Maintenance of Axon-Myelin Integrity

It has recently been shown that mice deficient in the gene for myelin-associated glycoprotein develop normal myelin sheaths in the peripheral nervous system. Here we report that in mutant mice older than 8 months the maintenance of axon-myelin units is disturbed, resulting in both axon and myelin degeneration. Morphological features include those typically seen in human peripheral neuropathies, where demyelination-induced Schwann cell proliferation and remyelination lead to the formation of so-called onion bulbs. Expression of tenascin-C, a molecule indicative of peripheral nerve degeneration, was up-regulated by axon-deprived Schwann cells and regenerating axons were occasionally seen. Myelin-associated glycoprotein thus appears to play a crucial role in the long-term maintenance of the integrity of both myelin and axons.     

Hypoxia‐inducible factor 1α may be a marker for vasculitic neuropathy

Neuromuscular biopsy is still an essential method for diagnosing vasculitic neuropathy, although its diagnostic sensitivity is at most 60%. Our objective was to examine the expression of hypoxia‐inducible factor 1α (HIF‐1α) in peripheral nerves and to evaluate its usefulness in diagnosing vasculitic neuropathy, especially for discrimination from other axonal neuropathies. Forty‐one patients with vasculitic neuropathy consisting of 20 definite, 14 probable and seven possible diagnoses, 15 patients with metabolic neuropathy, five with motor neuron disease and six with chronic inflammatory demyelinating polyneuropathy were included. Nerve biopsy specimens were immunohistochemically examined for HIF‐1α and various cell markers. Distinct immunoreactivity (IR) was observed in nuclei of endoneurial cells in 54% (22/41) of vasculitic patients, while specimens from metabolic neuropathies showed less nuclear IR and the difference of mean density of HIF‐1α‐positive nuclei was significant. Two patients with possible vasculitis who showed HIF‐1α‐positive nuclei in endoneurium, were later confirmed to have vasculitis by skin biopsies. Most of the cells expressing HIF were demonstrated to be Schwann cells. There was a trend in the vasculitic patients with early phase nerve damage to display higher endoneurial HIF‐1α‐IR. HIF‐1α may be an immunohistochemical marker for vasculitic neuropathy, especially when the observed section contains no vasculitic lesions.     

Demyelination can proceed independently of axonal degradation during Wallerian degeneration in wlds mice

Peripheral nerve injury induces axonal degeneration and demyelination, which are collectively referred to as Wallerian degeneration. It is generally assumed that axonal degeneration is a trigger for the subsequent demyelination processes such as myelin destruction and de-differentiation of Schwann cells, but the detailed sequence of events that occurs during this initial phase of demyelination following axonal degeneration remains unclear. Here we performed a morphological analysis of injured sciatic nerves of wlds mice, a naturally occurring mutant mouse in which Wallerian degeneration shows a significant delay. The slow Wallerian degerenation phenotype of the wlds mutant mice would enable us to dissect the events that take place during the initial phase of demyelination. Ultrastrucural analysis using electron microscopy showed that the initial process of myelin destruction was activated in injured nerves of wlds mice even though they exhibit morphologically complete protection of axons against nerve injury. We also found that some intact axons were completely demyelinated in degenerating nerves of wlds mice. Furthermore, we observed that de-differentiation of myelinating Schwann cells gradually proceeded even though the axons remained morphologically intact. These data suggest that initiation and progression of demyelination in injured peripheral nerves is, at least in part, independent of axonal degeneration.     

Psychological stress effects on myelin degradation in the cuprizone‐induced model of demyelination

Multiple sclerosis (MS) is known as the most common demyelinating disease worldwide in which previous studies have shown that stress is a risk factor for the disease's onset and progression. Nevertheless, further studies are needed to investigate the consequences of stress in MS pathology. In this study, after 5 days of exposure to psychological and physical stress as a repetitive distress modality, rats were treated with cuprizone. The demyelination degree was compared in animal groups using Luxol fast blue staining, immunohistochemical staining for myelin basic protein and transmission electron microscopy. Outcomes revealed that animals exposed to stress prior to cuprizone ingestion, elicit more intense demyelination. Continuous psychological distress has more severe effects on myelin sheath destruction in the preclinical stage.     

Involvement of Endogenous Brain‐Derived Neurotrophic Factor in Hypothalamic‐Pituitary‐Adrenal Axis Activity

Brain‐derived neurotrophic factor (BDNF) appears to be highly involved in hypothalamic‐pituitary‐adrenal (HPA) axis regulation during adulthood, playing an important role in homeostasis maintenance. The present study aimed to determine the involvement of BDNF in HPA axis activity under basal and stress conditions via partial inhibition of this endogenous neurotrophin. Experiments were conducted in rats and mice with two complementary approaches: (i) BDNF knockdown with stereotaxic delivery of BDNF‐specific small interfering RNA (siRNA) into the lateral ventricle of adult male rats and (ii) genetically induced knockdown (KD) of BDNF expression specifically in the central nervous system during the first ontogenesis in mice (KD mice). Delivery of siRNA in the rat brain decreased BDNF levels in the hippocampus (?31%) and hypothalamus (?35%) but not in the amygdala, frontal cortex and pituitary. In addition, siRNA induced no change of the basal HPA axis activity. BDNF siRNA rats exhibited decreased BDNF levels and concomitant altered adrenocortoctrophic hormone (ACTH) and corticosterone responses to restraint stress, suggesting the involvement of BDNF in the HPA axis adaptive response to stress. In KD mice, BDNF levels in the hippocampus and hypothalamus were decreased by 20% in heterozygous and by 60% in homozygous animals compared to wild‐type littermates. Although, in heterozygous KD mice, no significant change was observed in the basal levels of plasma ACTH and corticosterone, both hormones were significantly increased in homozygous KD mice, demonstrating that robust cerebral BDNF inhibition (60%) is necessary to affect basal HPA axis activity. All of these results in both rats and mice demonstrate the involvement and importance of a robust endogenous pool of BDNF in basal HPA axis regulation and the pivotal function of de novo BDNF synthesis in the establishment of an adapted response to stress.