Effects of neurosteroid 3alpha-hydroxy-5alpha-pregnan-20-one on ethanol-mediated paired-pulse depression of population spikes in the CA1 region of rat hippocampal slices

Ethanol inhibits long-term potentiation in hippocampal CA1 neurons, irrespective of lamina and stimulus strength, through neurosteroidogenesis

Ethanol inhibits memory encoding and the induction of long-term potentiation (LTP) in CA1 neurons of the hippocampus. Hippocampal LTP at Schaffer collateral synapses onto CA1 pyramidal neurons has been widely studied as a cellular model of learning and memory, but there is striking heterogeneity in the underlying molecular mechanisms in distinct regions and in response to distinct stimuli. Basal and apical dendrites differ in terms of innervation, input specificity, and molecular mechanisms of LTP induction and maintenance, and different stimuli determine distinct molecular pathways of potentiation. However, lamina or stimulus-dependent effects of ethanol on LTP have not been investigated. Here, we tested the effect of acute application of 60 mM ethanol on LTP induction in distinct dendritic compartments (apical versus basal) of CA1 neurons, and in response to distinct stimulation paradigms (single versus repeated, spaced high frequency stimulation). We found that ethanol completely blocks LTP in apical dendrites, whereas it reduces the magnitude of LTP in basal dendrites. Acute ethanol treatment for just 15 min altered pre- and post-synaptic protein expression. Interestingly, ethanol increases the neurosteroid allopregnanolone, which causes ethanol-dependent inhibition of LTP, more prominently in apical dendrites, where ethanol has greater effects on LTP. This suggests that ethanol has general effects on fundamental properties of synaptic plasticity, but the magnitude of its effect on LTP differs depending on hippocampal sub-region and stimulus strength.

Acute and chronic effects of ethanol on learning-related synaptic plasticity

Alcoholism is associated with acute and long-term cognitive dysfunction including memory impairment, resulting in substantial disability and cost to society. Thus, understanding how ethanol impairs cognition is essential for developing treatment strategies to dampen its adverse impact. Memory processing is thought to involve persistent, use-dependent changes in synaptic transmission, and ethanol alters the activity of multiple signaling molecules involved in synaptic processing, including modulation of the glutamate and gamma-aminobutyric acid (GABA) transmitter systems that mediate most fast excitatory and inhibitory transmission in the brain. Effects on glutamate and GABA receptors contribute to ethanol-induced changes in long-term potentiation (LTP) and long-term depression (LTD), forms of synaptic plasticity thought to underlie memory acquisition. In this paper, we review the effects of ethanol on learning-related forms of synaptic plasticity with emphasis on changes observed in the hippocampus, a brain region that is critical for encoding contextual and episodic memories. We also include studies in other brain regions as they pertain to altered cognitive and mental function. Comparison of effects in the hippocampus to other brain regions is instructive for understanding the complexities of ethanol's acute and long-term pharmacological consequences.

Ethanol enhances neurosteroidogenesis in hippocampal pyramidal neurons by paradoxical NMDA receptor activation

Using an antibody against 5α-reduced neurosteroids, predominantly allopregnanolone, we found that immunostaining in the CA1 region of rat hippocampal slices was confined to pyramidal neurons. This neurosteroid staining was increased following 15 min administration of 60 mm but not 20 mm ethanol, and the enhancement was blocked by finasteride and dutasteride, selective inhibitors of 5α-reductase, a key enzyme required for allopregnanolone synthesis. Consistent with a prior report indicating that N-methyl-D-aspartate (NMDA) receptor (NMDAR) activation can promote steroid production, we observed that D-2-amino-5-phosphonovalerate (APV), a competitive NMDAR antagonist, blocked the effects of 60 mm ethanol on staining. We previously reported that 60 mm ethanol inhibits the induction of long-term potentiation (LTP), a cellular model for memory formation, in the CA1 region. In the present study, LTP inhibition by 60 mm ethanol was also overcome by both the 5α-reductase inhibitors and by APV. Furthermore, the effects of ethanol on neurosteroid production and LTP were mimicked by a low concentration of NMDA (1 μm), and the ability of NMDA to inhibit LTP and to enhance neurosteroid staining was reversed by finasteride and dutasteride, as well as by APV. These results indicate that ethanol paradoxically enhances GABAergic neurosteroid production by activation of unblocked NMDARs and that acute LTP inhibition by ethanol represents a form of NMDAR-mediated metaplasticity.

Selected line difference in the effects of ethanol dependence and withdrawal on allopregnanolone levels and 5alpha-reductase enzyme activity and expression

BACKGROUND

Allopregnanolone (ALLO) is a progesterone derivative that rapidly potentiates gamma-aminobutyric acid(A) (GABA(A)) receptor-mediated inhibition and modulates symptoms of ethanol withdrawal. Because clinical and preclinical data indicate that ALLO levels are inversely related to symptoms of withdrawal, the present studies determined whether ethanol dependence and withdrawal differentially altered plasma and cortical ALLO levels in mice selectively bred for differences in ethanol withdrawal severity and determined whether the alterations in ALLO levels corresponded to a concomitant change in activity and expression of the biosynthetic enzyme 5alpha-reductase.

METHODS

Male Withdrawal Seizure-Prone (WSP) and -Resistant (WSR) mice were exposed to 72 hours ethanol vapor or air and euthanized at select times following removal from the inhalation chambers. Blood was collected for analysis of ALLO and corticosterone levels by radioimmunoassay. Dissected amygdala, hippocampus, midbrain, and cortex as well as adrenals were examined for 5alpha-reductase enzyme activity and expression levels.

RESULTS

Plasma ALLO was decreased significantly only in WSP mice, and this corresponded to a decrease in adrenal 5alpha-reductase expression. Cortical ALLO was decreased up to 54% in WSP mice and up to 46% in WSR mice, with a similar decrease in cortical 5alpha-reductase activity during withdrawal in the lines. While cortical gene expression was significantly decreased during withdrawal in WSP mice, there was a 4-fold increase in expression in the WSR line during withdrawal. Hippocampal 5alpha-reductase activity and gene expression was decreased only in dependent WSP mice.

CONCLUSIONS

These results suggest that there are line and brain regional differences in the regulation of the neurosteroid biosynthetic enzyme 5alpha-reductase during ethanol dependence and withdrawal. In conjunction with the finding that WSP mice exhibit reduced sensitivity to ALLO during withdrawal, the present results are consistent with the hypothesis that genetic differences in ethanol withdrawal severity are due, in part, to modulatory effects of GABAergic neurosteroids such as ALLO.

GABAergic neurosteroids mediate the effects of ethanol on long-term potentiation in rat hippocampal slices

We previously found that ethanol has complex effects on hippocampal synaptic plasticity, inhibiting long-term potentiation (LTP) and long-term depression by different mechanisms. The block of long-term depression appears to be mediated by effects on N-methyl-d-aspartate receptors, whereas the block of LTP involves augmented inhibition via gamma-aminobutyric acid-A receptors (GABA(A)Rs). To pursue factors contributing to effects on LTP, we examined the ability of various concentrations of ethanol to block LTP in the CA1 region of rat hippocampal slices. Complete LTP block required 60 mm ethanol. LTP block was enhanced at lower ethanol concentrations in the presence of (3alpha5alpha)-3-hydroxypregnan-20-one, a GABA(A)R-potentiating neurosteroid, suggesting that neurosteroids may be important contributors to the effects of ethanol on LTP. Consistent with this, we found that block of LTP by 60 mm ethanol was overcome by coadministration of a cyclodextrin that binds and removes lipophilic neurosteroids. More specifically, treatment of slices with finasteride, an agent that inhibits the synthesis of 5alpha-reduced neurosteroids, or with an agent that inhibits the effects of 5alpha-reduced neurosteroids on GABA(A)Rs overcame the effects of 60 mm ethanol on LTP. Taken together, these results indicate that acute production of GABA(A)R-enhancing neurosteroids plays a key role in mediating the effects of ethanol on LTP.

Neuroexcitatory actions of Tamiflu and its carboxylate metabolite

Oseltamivir (Tamiflu) is now being stockpiled by several governments as a first line treatment for an anticipated outbreak of avian influenza caused by H5N1. However, abnormal behaviors and death associated with the use of Tamiflu have developed into a major issue in Japan where Tamiflu is often prescribed for seasonal influenza. Thus, it is critical to determine neuropsychiatric effects of oseltamivir and to establish methods for safe administration. Using juvenile rats and rat hippocampal slices, we investigated whether oseltamivir has adverse effects on the central nervous system. Systemic injection of oseltamivir (50mg/kg i.p.) produced no change in behavior within 2h. However, prior injection of oseltamivir significantly altered the duration of loss of lightning reflex following ethanol injection (3.3g/kg, i.p.). Ethanol injection in the presence of oseltamivir also resulted in enhanced hypothermia. In the CA1 region of hippocampal slices, oseltamivir (100 microM) induced paired-pulse facilitation in population spikes without changes in excitatory postsynaptic potentials. Similarly, 3 microM oseltamivir carboxylate, the active metabolite of oseltamivir, facilitated neuronal firing, though the facilitation did not involve GABAergic disinhibition. Moreover, oseltamivir carboxylate produced further facilitation following administration of 60mM ethanol. These findings indicate that oseltamivir has effects on the central nervous system, especially when combined with other agents.

Mechanisms of neurosteroid interactions with GABA(A) receptors

Neuroactive steroids have some of their most potent actions by augmenting the function of GABA(A) receptors. Endogenous steroid actions on GABA(A) receptors may underlie important effects on mood and behavior. Exogenous neuroactive steroids have potential as anesthetics, anticonvulsants, and neuroprotectants. We have taken multiple approaches to understand more completely the interaction of neuroactive steroids with GABA(A) receptors. We have developed many novel steroid analogues in this effort. Recent work has resulted in synthesis of new enantiomer analogue pairs, novel ligands that probe various properties of the steroid pharmacophore, fluorescent neuroactive steroid analogues, and photoaffinity labels. Using these tools, combined with receptor binding and electrophysiological assays, we have begun to untangle the complexity of steroid actions at this important class of ligand-gated ion channel.