Differential effects of chronic ethanol consumptions or thiamine deficiency on spatial working memory in Balb/c mice: a behavioral and neuroanatomical study

Re-evaluating the role of the mammillary bodies in memory

Although the mammillary bodies were among the first brain regions to be implicated in amnesia, the functional importance of this structure for memory has been questioned over the intervening years. Recent patient studies have, however, re-established the mammillary bodies, and their projections to the anterior thalamus via the mammillothalamic tract, as being crucial for recollective memory. Complementary animal research has also made substantial advances in recent years by determining the electrophysiological, neurochemical, anatomical and functional properties of the mammillary bodies. Mammillary body and mammillothalamic tract lesions in rats impair performance on a number of spatial memory tasks and these deficits are consistent with impoverished spatial encoding. The mammillary bodies have traditionally been considered a hippocampal relay which is consistent with the equivalent deficits seen following lesions of the mammillary bodies or their major efferents, the mammillothalamic tract. However, recent findings suggest that the mammillary bodies may have a role in memory that is independent of their hippocampal formation afferents; instead, the ventral tegmental nucleus of Gudden could be providing critical mammillary body inputs needed to support mnemonic processes. Finally, it is now apparent that the medial and lateral mammillary nuclei should be considered separately and initial research indicates that the medial mammillary nucleus is predominantly responsible for the spatial memory deficits following mammillary body lesions in rats.

Chronic Ethanol Consumption Impairs Learning and Memory After Cessation of Ethanol

Acute consumption of ethanol results in reversible changes in learning and memory whereas chronic ethanol consumption of six or more months produces permanent deficits and neural damage in rodents. The goal of the current paper was determine whether shorter durations of chronic ethanol ingestion in mice would produce long-term deficits in learning and memory after the cessation of ethanol. We first examined the effects of four and eight weeks of 20% ethanol followed by a three week withdrawal period on learning and memory in mice. We determined that three weeks after eight, but not four, weeks of 20% ethanol consumption resulted in deficits in learning and long-term memory (seven days) in T-maze footshock avoidance and Greek Cross brightness discrimination, step-down passive avoidance and shuttlebox active avoidance. Short-term memory (1 hr) was not affected. The deficit was not related to changes in thiamine status, caloric intake, or nonmnemonic factors, such as, activity or footshock sensitivity. Lastly, we examined if the mice recovered after longer durations of withdrawal. After eight weeks of ethanol, we compared mice after three and 12 weeks of withdrawal. Mice that had been off ethanol for both three and 12 weeks were impaired in T-maze footshock avoidance compared to the controls. The current results indicate that a duration of ethanol consumption as short as eight weeks produces deficits in learning and memory that are present 12 weeks after withdrawal.

Proton MR spectroscopic studies of chronic alcohol exposure on the rat brain

PURPOSE

To better understand the long-term pathophysiologic mechanisms of alcoholism-related organic brain damage by serially assessing brain metabolites in chronically exposed rats using both in vivo magnetic resonance spectroscopy (MRS) and high-resolution nuclear magnetic resonance (NMR) from brain extracts.

MATERIALS AND METHODS

The alcoholic regimen was continued up to 60 weeks. In vivo proton MRS studies were performed at 200 MHz using a small animal imaging/spectrometer. In vitro rat brain extracts were also examined using a 500 MHz vertical bore magnet. Comparison measurements were also obtained in an age-matched control group.

RESULTS

In vivo results showed that there is a significant increase in the Cho/NAA ratio in the chronic alcohol-exposed group that reached a maximum around 16 weeks. After 44 weeks of alcohol exposure, Cho/NAA in the alcohol group decreased significantly from its maximum value to a value that was significantly lower than those from the control groups. Brain extract studies demonstrated that PC and GPC were the main components responsible for the observed in vivo spectral changes after 16 and 60 weeks of alcohol consumption, respectively.

CONCLUSION

The fluctuation of choline-containing metabolites during alcohol intoxication could explain sometimes seemingly conflicting and confusing results from MRS studies in human and animal studies in which the duration of alcohol consumption and amount are varied widely.

Evidence for a role for GABA(A) and NMDA receptors in ethanol inhibition of long-term potentiation

We have investigated the mechanisms by which acute ethanol inhibits the induction of long-term potentiation (LTP) in area CA1 of the rat hippocampal slice. In a previous report [Alcohol. Clin. Exp. Res. 21 (1997) 404] we demonstrated that ethanol produces only a modest inhibition of pharmacologically isolated N-methyl-D-aspartate receptors (NMDAR) in the CA1 region of the hippocampus. Moreover, this level of inhibition was not sufficient to account for ethanol's complete inhibition of LTP induction in this brain region. One possible explanation of these results is that we may have underestimated ethanol's ultimate effect on the NMDAR by focusing on pharmacologically isolated NMDAR responses. Ethanol might indirectly inhibit the NMDAR by, for example, potentiating the GABA(A)R. To explore this possibility, we first examined the effects of the GABA(A)R antagonist picrotoxin (PTX) and the allosteric GABA(A)R modulator flunitrazepam on NMDAR responses. We demonstrate that these modulators of GABA(A)R activity significantly affect the magnitude of synaptically evoked NMDAR responses. We next examined the effects of ethanol on NMDAR responses in the presence and absence of PTX. We see a significantly greater ethanol inhibition of the NMDAR when GABA(A)Rs are functional, i.e. in the absence of PTX. These data suggest that ethanol produces an inhibition of the NMDAR indirectly by affecting the GABA(A)R neurotransmission. Moreover, we found that ethanol inhibition of NMDAR activity, both directly through actions on the NMDAR, and indirectly, possibly through potentiation of GABA(A)R activity, is sufficient to account for ethanol's complete blockade of LTP induction.

Effects of chronic alcohol consumption on spatial reference and working memory tasks

The aim of this work was to determine the spatial memory impairments induced by chronic alcohol consumption in rats. The alcoholization process began on the 21st postnatal day and alcohol concentrations were gradually increased to reach a concentration of 20% that was maintained for 4 mon. Behavioral tests were performed in the Morris Water Maze (MWM). The first study assessed the effects of chronic alcohol intake on two reference memory tasks (a place learning with multiple trials and a new place learning carried out in the same experimental context). Alcohol-treated animals presented no overall impairment in their ability to process spatial information. Deficits were restricted to reduced behavioral flexibility in spatial strategies. The second study assessed working memory in two tasks in which information about platform location was only valid for one trial. In the first working memory task, the animals had to perform one trial per day and in the second task they were submitted to four trials per day. At the end of the second experiment, all animals were trained in a visual-cued task. In the second experiment, the most important deficits in alcohol-treated animals occur in spatial working memory tasks, and this impairment was independent of the intertrial interval used. In the second spatial working memory task, performance of the alcohol-treated animals in the earlier trials affected their performance in subsequent trials, suggesting that a process of proactive interference had taken place. The visual-cued task demonstrated that these behavioral impairments were produced without visuoperceptive impairments.

Effects of prolonged ethanol exposure on neurophysiological measures during an associative learning paradigm

Long-term ethanol exposure has been reported to produce electrophysiological and cognitive impairments in some alcoholics. This study assessed the effects of chronic ethanol exposure on neurophysiological indices of associative learning in rats. Male Wistar rats (46) were exposed to ethanol vapor (EtOH group) or air (control group) for 6 consecutive weeks. After the animals were withdrawn from ethanol, electrodes were implanted in the frontal and parietal cortices and in the amygdala. Following a prolonged abstinence from ethanol (10-15 weeks), rats were exposed to a classical conditioning paradigm in which a food pellet was paired with the presentation of an auditory stimulus. During the first five sessions (conditioning phase), food pellet presentation was paired with the presentation of an infrequently presented tone. During the second five sessions (extinction phase), the association between food pellet presentation and the infrequently presented tone was weakened by no longer presenting food pellets following the infrequent tone. During selected test sessions, event-related brain potentials (ERPs) elicited by each tone (i.e. food-paired tone, non-paired tone) were recorded and analyzed. These analyses revealed differences in ERP responses between the groups. The latency of the N1 and P2 ERP components in the cortex of the control group, but not the EtOH group, increased during sessions when the association between food pellet delivery and tone presentation was being established or extinguished. These data support the hypothesis that chronic ethanol treatment results in a loss of responsivity in ERP components sensitive to changes in food-tone associations, even following a prolonged period of withdrawal from ethanol.

Ethanol's inhibition of LTP may not be mediated solely via direct effects on the NMDA receptor

Acute abuse of alcohol is well known to have deleterious effects on memory. However, the molecular and cellular bases of this effect are not well understood. Ethanol is known to inhibit long-term potentiation (LTP), a putative cellular substrate of memory. However, there is controversy concerning the doses of ethanol required for inhibition of LTP. We examined the doses of ethanol required to inhibit LTP in the CA1 region of the hippocampus. We used two different LTP-inducing paradigms in these studies and found that only doses of ethanol associated with profound intoxication (50-100 mM) can produce significant inhibition of LTP. We also investigated the molecular mechanisms of ethanol's effect on LTP. Activation of the N-methyl-D-aspartate receptor plays a critical role in LTP, and ethanol has been shown to partially inhibit N-methyl-D-aspartate receptor function. We tested directly whether the level of N-methyl-D-aspartate inhibition produced by 100 mM ethanol is sufficient to account for the complete inhibition of LTP produced by 100 mM ethanol. Our data suggest that ethanol's effects on the N-methyl-D-aspartate receptor can account for most, but not all of ethanol's inhibition of LTP.

Localized injections of various compounds effecting neurotransmitter activity in the mammillary complex enhance (T-maze) avoidance retention

The mammillary complex is implicated in the amnesic syndrome associated clinically with Korsakoff's syndrome, Alzheimer's disease and experimentally with lesions in animals. There is however no direct evidence that the mammillary bodies are involved in long term memory processing. Mice were partially trained on a footshock avoidance task. Immediately after training drugs were injected into the mammillary complex. Retention was tested 1 week later by continuing training until each mouse made five avoidance responses in six trials. The results indicated that muscarine, nicotine, dopamine, glutamine and adrenoceptor agonists as well as GABA and 5-HT receptor antagonists and neuropeptide Y improved retention test performance relative to the control. Injection of the same drugs 1 mm above the injection site for the mammillary complex failed to significantly improve retention test performance. It is concluded that the mammillary complex, with its important connections to other areas of the limbic system, is involved in memory processing events that occur shortly after training.