Ethanol affects hypothalamic neurons projecting to the hippocampus and inhibits dentate granule cell LTP

Conventional cardiovascular risk factors in Transient Global Amnesia: Systematic review and proposition of a novel hypothesis

Transient Global Amnesia (TGA) is an enigmatic amnestic syndrome. We conducted a systematic review to investigate the relationship between the conventional cardiovascular risk factors and TGA. MEDLINE, CENTRAL, EMBASE and PsycINFO were comprehensively searched and 23 controlled observational studies were retrieved. The prevalence of hypertension, diabetes mellitus, dyslipidemia and smoking was lower among patients with TGA compared to Transient Ischemic Attack. Regarding the comparison of TGA with healthy individuals, there was strong evidence suggesting a protective effect of diabetes mellitus on TGA and weaker evidence for a protective effect of smoking. Hypertension was associated with TGA only in more severe stages, while dyslipidemia was not related. In view of these findings, a novel pathophysiological hypothesis is proposed, in which the functional interactions of Angiotensin-II type-1 and N-methyl-D-aspartate receptors are of pivotal importance. The whole body of clinical evidence (nature of precipitating events, associations with migraine, gender-based association patterns) was integrated.

The Interplay between the Hippocampus and Amygdala in Regulating Aberrant Hippocampal Neurogenesis during Protracted Abstinence from Alcohol Dependence

The development of alcohol dependence involves elevated anxiety, low mood, and increased sensitivity to stress, collectively labeled negative affect. Particularly interesting is the recent accumulating evidence that sensitized extrahypothalamic stress systems [e.g., hyperglutamatergic activity, blunted hypothalamic-pituitary-adrenal (HPA) hormonal levels, altered corticotropin-releasing factor signaling, and altered glucocorticoid receptor signaling in the extended amygdala] are evident in withdrawn dependent rats, supporting the hypothesis that pathological neuroadaptations in the extended amygdala contribute to the negative affective state. Notably, hippocampal neurotoxicity observed as aberrant dentate gyrus (DG) neurogenesis (neurogenesis is a process where neural stem cells in the adult hippocampal subgranular zone generate DG granule cell neurons) and DG neurodegeneration are observed in withdrawn dependent rats. These correlations between withdrawal and aberrant neurogenesis in dependent rats suggest that alterations in the DG could be hypothesized to be due to compromised HPA axis activity and associated hyperglutamatergic activity originating from the basolateral amygdala in withdrawn dependent rats. This review discusses a possible link between the neuroadaptations in the extended amygdala stress systems and the resulting pathological plasticity that could facilitate recruitment of new emotional memory circuits in the hippocampus as a function of aberrant DG neurogenesis.

The CNS renin-angiotensin system

The renin-angiotensin system (RAS) is one of the best-studied enzyme-neuropeptide systems in the brain and can serve as a model for the action of peptides on neuronal function in general. It is now well established that the brain has its own intrinsic RAS with all its components present in the central nervous system. The RAS generates a family of bioactive angiotensin peptides with variable biological and neurobiological activities. These include angiotensin-(1-8) [Ang II], angiotensin-(3-8) [Ang IV], and angiotensin-(1-7) [Ang-(1-7)]. These neuroactive forms of angiotensin act through specific receptors. Only Ang II acts through two different high-specific receptors, termed AT1 and AT2. Neuronal AT1 receptors mediate the stimulatory actions of Ang II on blood pressure, water and salt intake, and the secretion of vasopressin. In contrast, neuronal AT2 receptors have been implicated in the stimulation of apoptosis and as being antagonistic to AT1 receptors. Among the many potential effects mediated by stimulation of AT2 are neuronal regeneration after injury and the inhibition of pathological growth. Ang-(1-7) mediates its antihypertensive effects by stimulating the synthesis and release of vasodilator prostaglandins and nitric oxide and by potentiating the hypotensive effects of bradykinin. New data concerning the roles of Ang IV and Ang-(1-7) in cognition also support the existence of complex site-specific interactions between multiple angiotensins and multiple receptors in the mediation of important central functions of the RAS. Thus, the RAS of the brain is involved not only in the regulation of blood pressure, but also in the modulation of multiple additional functions in the brain, including processes of sensory information, learning, and memory, and the regulation of emotional responses.

Orexin-A (Hypocretin-1) and leptin enhance LTP in the dentate gyrus of rats in vivo

Orexin-A (Hypocretin-1) has been localized in the posterior and lateral hypothalamic perifornical region. Orexin containing axon terminals have been found in hypothalamic nuclei and many other parts of the brain; for example, the hippocampus. Two types of orexin receptors have been discovered. Orexin 1 type of receptors have been described and been shown to be widely distributed in the rat brain including the hippocampus. Subsequently Orexin-A was found to impair both water maze performance and hippocampal long term potentiation (LTP). Leptin is expressed in adipose tissue and released into the blood where it affects food intake and can also produce widespread physiological changes mediated via autonomic preganglionic neurons, pituitary gland, and cerebral cortex. Immunoreactivity for leptin receptors has been found in various hypothalamic nuclei including the lateral hypothalamic area as well as the hippocampus especially in the dentate gyrus and CA1. Leptin receptor deficient rats and mice also show impaired LTP in CA1 and poor performance in the water maze. The present study was conducted to determine the effects of 0.0, 30, 60, 90, and 100 nM, orexin-A, and leptin, 0.0, 1.0, 100 nM, 1, and 10 microM, in 1.0 microl of ACSF, applied directly into the dentate gyrus, on LTP in medial perforant path dentate granule cell synapses in urethane anesthetized rats. Orexin-A specifically enhanced LTP at the 90 nM dose; and it was possible to block the enhancement by pretreating the animals with SB-334867, a specific orexin 1 receptor antagonist. Leptin enhanced normal LTP at 1.0 microM but inhibited LTP at lower and higher doses. These results and previous data indicate that the same peptide could possibly have different modulatory post synaptic effects in different hippocampal synapses dependent upon different types of post synaptic receptors.

Ethanol effects on dentate granule cell LTP

In previous studies we identified a lateral hypothalamic area (LHA) sensitive to ethanol, < 5.0 mM, when the perifornical region of the area is perfused with different concentrations of ethanol. Some of these perifornical neurons contain angiotensin (Ang) and project directly to the dentate gyrus where angiotensin is released and inhibits LTP in medial perforant path-dentate granule cell synapses. The AT1 subtype receptor is involved because pretreatment with losartan, an AT1 antagonist, prevents Ang II, diazepam, and ethanol impairment of LTP as well as their effects on behavior. There is a possibility that these effects were not specific to the LHA; but might be attributable to direct effects of ethanol on postsynaptic granule cells due to diffusion of the ethanol in the extracellular space or by the circulatory system. The purpose of the present study was to determine a dose effect of ethanol on LTP in these same synapses when the dentate gyrus was perfused with several different concentrations of ethanol under the same conditions in urethane anesthetized rats. Ethanol was administered directly into the dentate gyrus by means of a fine stainless steel cannula attached approximately 1.0 mm from the tip of the glass capillary recording electrode. Results show that the threshold for ethanol in the dentate is higher by a factor of ten, > 30 mM and < 50 mM; and that at higher doses ethanol can have a direct effect on the LHA; and possibly toxic due to increasing ethanol in the blood circulatory system.

Orexin-A (hypocretin-1) impairs Morris water maze performance and CA1-Schaffer collateral long-term potentiation in rats

Glucose-sensitive neurons in the lateral hypothalamic area produce orexin-A (hypocretin-1) and orexin-B (hypocretin-2) and send their axons to the hippocampus, which predominantly expresses orexin receptor 1 showing a higher sensitivity to orexin-A. The purpose of the present study was to assess the effects of orexin-A on the performance of Wistar rats during the Morris water maze test and then to determine the effects of orexin-A on both the long-term potentiation and long-term depression in Schaffer collateral/commissural-CA1 synapses in hippocampal slices. The results of the Morris water maze test show that 1.0 and 10 nmol of orexin-A, when administered intracerebroventricularly, retarded spatial learning. A probe test examined after training of water maze task also showed an impairment in spatial memory. The results of an electrophysiological study using hippocampal slices demonstrated that 1.0 to 30 nM of orexin-A applied to the perfusate produces a dose-dependent and time dependent suppression of the long-term potentiation. In addition, the long-term depression was not affected by orexin-A. The results of a paired-pulse facilitation experiment indicated that the effects of orexin-A were post-synaptic and not due to presynaptic transmitter release. These results show that orexin-A impairs spatial performance and these impairments can be attributed to a suppression of long-term potentiation in the Schaffer collateral-CA1 hippocampal synapses.

Ethanol-induced suppression of LTP can be attenuated with an angiotensin IV analog

Hippocampal slices taken from animals chronically or acutely treated with ethanol exhibit significant inhibition of long-term potentiation (LTP). This inhibition appears to be associated with impaired activity of N-methyl-D-aspartate (NMDA) receptors, perhaps via ethanol-induced increases in GABAergic synaptic transmission. Recently, a role for the octapeptide angiotensin II (AngII) in ethanol's inhibition of LTP has been reported. Complementary to these findings our laboratory has shown that the application of the hexapeptide metabolite of AngII, angiotensin IV (AngIV), significantly facilitated normal tetanic-induced LTP in the hippocampal slice. This facilitation is presumably by activation of the angiotensin receptor subtype, AT(4). The present study tested whether an AT(4) receptor agonist could overcome ethanol-induced suppression of LTP. The results indicate that Nle(1)-AngIV could offset ethanol-induced suppression of LTP in the CA(1) region of the hippocampus. Pretreatment with the specific AT(4) receptor antagonist Nle(1), Leual(3)-AngIV blocked this facilitation implicating the involvement of the AT(4) receptor subtype. These results suggest that an AT(4) receptor agonist is effective in overcoming ethanol's suppressing influence on LTP, and encourage further investigation of the cognitive enhancing properties of such compounds.

Craving for alcohol in the rat: adjunctive behavior and the lateral hypothalamus

A review of previous results and the new data in this report show clearly that the Falk model of adjunctive behavior is an adequate analogue of human alcoholism and can be applied to induce excessive ethanol consumption. New data on the consumption of sweet flavored ethanol solutions and, especially, sweet alone solutions during brief periods of ethanol withdrawal provide some significant insights concerning the possible physiological basis for cravings in humans. Because voluntary consumption of ethanol is the normal process by which alcoholism develops, a general set of environmental and other experimental conditions that produce behavioral excess; adjunctive behavior, electrical stimulation of the brain, and salt arousal of drinking are discussed in some detail. Neuronal circuits of the lateral hypothalamus are important because some of the cells are chemosensitive and monitor osmolality of the blood and initiate drinking in the normal regulation of body fluids. Alcohol in very small amounts has a direct effect on these cells that also project to lower spinal motor neurons and modulate the level of excitability in spinal reflexes and thereby reactivity to environmental stimulation. Taste and other sensory information from the mouth arrives in presynaptic endings on these same cells by a multitude of indirect multisynaptic pathways. A theoretical model is developed to explain how tactile and taste sensory information and what is initially a nonspecific general state of motor arousal interact together to produce an excessive consumption or craving for ethanol.

The consequences of imbibing alcohol in the absence of adequate nutrition: the salt and water hypothesis

Alcohol can have an effect on almost every cell in the human body and it is becoming increasingly clear that when alcohol is consumed the prior nutritional status of the individual may be an important factor for long-term health. The salt and water hypothesis integrates the biochemical findings from the current alcohol literature and proposes a mechanism by which alcohol consumption prior to food intake may cause a transient alteration in the functioning of the hypothalamic-pituitary-adrenal axis due to an alcohol-induced impairment in electrolyte regulation.

Septohippocampal pathway as a site for the memory-impairing effects of ethanol

Ethanol affects behavior by interacting with synaptic sites at many levels of the nervous system. However, it targets most readily and at the lowest concentrations those sites mediating higher cognitive functions such as attention and memory. The memory-impairing effects of ethanol are thought to involve the hippocampus, a structure particularly vulnerable to the effects ethanol at low concentrations and early in the rising phase of the blood ethanol concentration curve. One of the early, low-dose effects of ethanol is an interruption of the normal physiological regulation of the hippocampus by the ascending septohippocampal pathway originating in the medial septal area (MSA). Ethanol enhances GABAergic transmission in the MSA, thereby reducing the regularity and vigor with which rhythmically bursting neurons of the MSA drive the hippocampal theta rhythm. Disruption of septohippocampal activity also has consequences on the response of the hippocampus to cortical inputs. Ethanol produces a loss of hippocampal responsivity that reduces the ability of the hippocampus to encode and retrieve relevant stimulus information necessary for accurate memory. This paper examines the behavioral and neural evidence for hippocampal vulnerability to ethanol and explores the hypothesis that these effects are due to ethanol disrupting septohippocampal modulation of the hippocampus, resulting in impairments of memory.

Bicuculline sensitive depressor response to ethanol infusion into the lateral hypothalamus

Decreased GABA function in the hypothalamus increases mean arterial pressure (MAP) and heart rate (HR). Since ethanol acts on GABA-A receptors, blocking GABA-A receptors can prevent a decrease of MAP and HR by ethanol in the lateral hypothalamus (LH). Ethanol at 5-30 mM, with or without 25 ng/microl bicuculline, was infused into the LH, and the activity of the site was validated with 100 nmoles of serotonin. Male rats were anesthetized with pentobarbital, and the femoral artery was catheterized to measure MAP and HR. Microinfusion was performed with a 28-gauge cannula placed into the LH. Serotonin increased MAP and HR within 15 sec. Ethanol decreased the MAP by -21.15 +/- 3.92 mmHg and HR by -53.61 +/- 14.95 BPM, at 15 min, which recovered by 15 min after the infusion was terminated. These maximum decreases were produced by 20 mM ethanol giving a U-shaped dose response. The aCSF vehicle had no effect. Bicuculline prevented ethanol-induced changes and had no effect when administered alone. Both serotonin and ethanol have direct effects on LH neurons with cardiovascular function. Ethanol produces this effect through GABA-A receptors.

Selective effects of alcohol drinking on restraint-induced expression of immediate early genes in mouse brain

BACKGROUND

Analysis of immediate early gene expression in brain is a common contemporary method for mapping changes in neuronal activation with cellular resolution. This method has been applied previously in models of involuntary alcohol exposure. In this study, immunohistochemical expression analysis of immediate early genes c-fos, fosB, and zif268 was performed in brain of C57BL/6J mice after voluntary alcohol consumption.

METHODS

Mice were trained to consume 10% ethanol/10% sucrose, using a 30-min limited-access paradigm. Animals consumed approximately 1.5 g/kg of ethanol per session. Control animals consumed 10% sucrose solution. Gene expression was determined in half of the animals 1.5 hr after the drinking session. Gene expression in the remaining animals was determined after 0.5 hr of restraint stress, which is known to elevate expression of immediate early genes in many brain regions. Analysis of the stressed animals was also performed 1.5 hr after the drinking session.

RESULTS

Blood alcohol concentrations were significantly reduced in animals exposed to restraint stress. However, stressed mice showed the greatest alcohol-induced changes in gene expression. Specifically, animals consuming ethanol/sucrose with subsequent exposure to restraint stress had lower c-Fos expression in the CA3 region of hippocampus, and higher c-Fos expression in nucleus accumbens than mice exposed to restraint stress after drinking the sucrose solution. Consumption of the ethanol/sucrose solution also significantly reduced FosB expression in the basolateral amygdala and lateral hypothalamus, and Zif268 expression in the CA1 region of the hippocampus of stressed animals.

CONCLUSIONS

These data confirm previous observations showing selective effects of alcohol administration on immediate early gene expression. Furthermore, the effects of voluntary alcohol self-administration on immediate early gene expression differ from involuntary alcohol exposure and suggest several brain regions as substrates for alcohol consumption.

Nicotine blocks ethanol and diazepam impairment of air righting and ethanol impairment of maze performance

Results of our previous research in rats demonstrate the following: (a) Angiotensin II (Ang II) inhibits long term potentiation (LTP) in dentate granule cell-perforant path synapses and that this inhibition can be blocked by losartan, an Ang II AT1 receptor antagonist; (b) both ethanol and diazepam inhibit LTP induction and this inhibition can be blocked by losartan; (c) impairment of air righting by ethanol and diazepam (DZ) and eight-arm radial maze performance by ethanol can be blocked by pretreatment with losartan: (d) inhibition of dentate granule cell LTP by Ang II can also be prevented by pretreatment with nicotine. Therefore, it seemed reasonable to hypothesize that ethanol and diazepam impairment of air righting and maze performance might also be blocked by pretreatment with nicotine. The purpose of the present study was to determine the effects of nicotine 0.1, 0.2, 0.3, and 0.4 mg/kg subcutaneously (SC) on 2.0 g/kg ethanol per os (PO) and 1.0 and 2.0 mg/kg DZ intraperitoneally (i.p.) induced impairment of air righting; and to determine if the impaired maze performance due to 2.0 g/kg ethanol PO could be prevented by pretreatment with 0.4 mg/kg of nicotine, SC. Results confirm the hypothesis that moderate doses of ethanol, 2.0 g/kg PO, and DZ, 1.0 mg/kg i.p. impair air righting and that the impairment can be prevented by pretreatment with nicotine SC. Nicotine was not effective in blocking the 2.0 mg/kg DZ impairment of air righting. Nicotine, 0.4 mg/kg SC, prevented the impaired maze performance induced by 2.0 g/kg ethanol PO.

Losartan improves the performance of ethanol-intoxicated rats in an eight-arm radial maze

Results of previous research demonstrate that angiotensin II (Ang II) inhibits long-term potentiation (LTP) in medial perforant path-dentate gyrus granule cells and that the inhibition is mediated by the AT1 receptor because it can be blocked by losartan, a specific AT1 receptor antagonist. Ang II impairment of retention and ethanol inhibition of LTP can both be blocked by pretreatment with losartan. Because losartan pretreatment also prevents ethanol intoxication measured in terms of the aerial righting reflex, the purpose of the present study was to assess the effects of 2.0 g/kg ethanol administered by gavage on performance in an eight-arm radial maze, and then to determine the effectiveness of losartan in reducing the impairment of the learning and memory process. Results confirmed the general hypothesis that ethanol-induced cognitive deficits are mediated by Ang II and the AT1 receptor and that the impairment can be reduced by pretreatment with losartan.

Lateral hypothalamic stimulation inhibits dentate granule cell LTP: direct connections

We discovered that angiotensin II (Ang II) applied directly to the dentate gyrus inhibited LTP induction in medial perforant path-dentate granule cell synapses and that the inhibition can be blocked by losartan, an Ang II AT1 receptor specific antagonist. In the first part of this study we found that electrical stimulation of the lateral hypothalamus (LH) inhibits LTP in these synapses and the inhibition can be blocked by pretreating the animals with losartan, indicating that LH angiotensin-containing neurons project to the dentate gyrus. Results of the second part of the study demonstrate clearly that some angiotensin-containing LH neurons project directly to dentate granule cells. LH neurons were identified by retrograde tracers applied to the granule cell layer. Double-labeled neurons containing angiotensin and HRP were sparsely distributed and both fusiform and multipolar LH neurons appeared in a small cluster lateral and ventral to the fornix at the level of the paraventricular nucleus. Large numbers of angiotensin staining neurons were observed in the hypothalamus. Results support our hypothesis that some angiotensin containing LH neurons project directly to the dentate gyrus.

Nicotine blocks angiotensin II inhibition of LTP in the dentate gyrus

Angiotensin (ANG)-containing axons, terminals, and receptors have been found in the hippocampus. When angiotensin II (ANG II) is administered to the dentate gyrus, long-term potentiation (LTP) induction, in response to medial perforant path stimulation, is inhibited and it can be blocked by losartan, an ANG II AT1 receptor antagonist. ANG II has been shown to mediate impairment of the retention of an inhibitory shock avoidance response and to be involved in ethanol and diazepam inhibition of dentate gyrus LTP, all of which can be blocked by losartan. Nicotine acetylcholine receptors are found in the hippocampus and nicotine is involved in the enhancement of complex and important psychological functions that are mediated by the hippocampus; therefore, the possibility that nicotine prevents the ANG II inhibition of dentate granule cell LTP was examined. Nicotine pretreatment reduced ANG II inhibition of LTP induction in a dose-dependent manner. Mecamylamine blocked the nicotine antagonism of ANG II-induced LTP inhibition and normal LTP occurred, whereas hexamethonium was ineffective in blocking these central effects of nicotine. Nicotine by itself did not affect normal LTP under these conditions. Nicotinic blocking of the ANG II inhibition of a frequency dependent type of synaptic plasticity provides a function for central nicotinic receptors and a possible mechanism of action a) to explain the enhancement of learning and memory by nicotine, b) an explanation for tobacco smoking while drinking alcohol, and c) a possible basis for the excessive use of tobacco in depression and schizophrenia that supports a possible therapeutic use of nicotine in some mental disorders.