Unit responsivity in various hippocampal layers and the depressive effects of ethanol

Effects of ethanol on the excitability of hippocampal granule neurons

The effects of low dose ethanol treatment on neuronal firing threshold were studied in dentate granule neurons using intracellular recording. A higher threshold for orthodromic activation and lower threshold for somatic activation were observed in 50 mM of ethanol, based on an examination of the excitatory postsynaptic potentials, strength duration and repetitive firing properties. Experiments involving blockade of glutamate-receptor subtypes suggest that an N-methyl-D-aspartate (NMDA) receptor mediated mechanism is implicated in this action of ethanol.

Changes in Ca2+ ion activity within unrestrained rat's hippocampus perfused with alcohol or acetaldehyde

In the freely moving rat, the kinetics of Ca2+ ion activity were determined at circumscribed sites in the hippocampus, which was perfused with ethanol, tertiary-butyl alcohol or acetaldehyde. Initially, a region in CA1 or other cell field of the dorsal hippocampus was prelabelled by microinjection of 45Ca2+ through a permanently implanted guide tube. Then the tip of a concentric push-pull cannula assembly was lowered through the guide tube to the labelled site, and an isotonic artificial cerebrospinal fluid was repeatedly perfused at a rate of 25 microliter/min. Each perfusion was timed for 5.0 min with a 5.0 min interval between each. Once the washout curve of 45Ca2+ activity had begun to approach its asymptote, ordinarily in the midpoint of a series of perfusions, an isotonic solution of ethanol (188-942 mM), tertiary-butyl alcohol (12-580 mM) or acetaldehyde (10-98 mM) was added to the fourth perfusate. Thereafter, the hippocampal site was again perfused with the normal cerebrospinal fluid for the remainder of the experiment. Although the lowest concentration of ethanol exerted no effect on 45Ca2+ ion activity, an intermediate concentration caused mixed effects in either enhancing or suppressing the efflux into the perfusate of this cation. The highest concentration of ethanol produced in most experiments an initial suppression in Ca2+ ion efflux which was followed frequently by an elevation in the release of 45Ca2+. Similar changes in Ca2+ ion activity were produced by tertiary-butyl alcohol, but the magnitude of its effect was generally less than that of ethanol, suggesting that its effect on brain tissue differs from that of ethanol. Acetaldehyde evoked an intense and concentration-dependent enhancement of Ca2+ ion efflux from the perfused tissue at all of the sites in the hippocampus examined. These results suggest that in the unrestrained rat ethanol could unbind Ca2+ ions from hippocampal membranes or retard their uptake into cells of the hippocampus. The dual excitatory and inhibitory effect of ethanol on Ca2+ ion activity corresponds to the electrophysiological effects of this alcohol and could alter neurotransmitter release from neurons in this subcortical structure. The mechanism of action of acetaldehyde is envisaged to be due to its affinity to membrane sulfhydryl groups which alters protein conformation and thus interferes with both Ca2+ channels and Ca2+ binding properties.

Biphasic action of ethanol on single units of the dorsal hippocampus and the relationship to the cortical EEG

The effects of four doses of ethanol (100, 200, 400, 800 mg/kg) administered IV, on the spontaneous firing rate of single units in the dorsal hippocampal regin of the rat were studied. At the lower doses, a mixture of excitatory and inhibitory effects occurring in that order was seen, reflecting a biphasic action of ethanol at the level of the single neuron. As the dose increased, the excitation disappeared and successively greater degrees of response inhibition prevailed. The pattern of the fronto-cortical EEG changed from predominantly low amplitude fast activity with a few episodes of high amplitude slow activity at low doses, to more sustained episodes of slow activity at high doses. The time-response curve showed that the peak of maximum inhibition seen at the highest dose occurred with a longer latency than the peak of maximum excitation seen at the lower doses. Finally, the changes in unit firing appeared to follow four general patterns and to be correlated with the mode of fronto-cortical EEG activity.

Changes in the rhesus monkey's EEG responses to ethanol during chronic exposure

This study was designated to document the changes in the CNS response to ethanol during chronic exposure in 5 male, 3.5--5.0 kg rhesus monkeys. Electrodes were implanted bilaterally into the amygdala, hippocampus and the calvarium over the frontal and temporal cortex. Ethanol or control solutions were administered intragastrically through indwelling cannulae. The 1.25 g/kg ethanol challenge dose was administered during EEG recordings. After one challenge dose, the animals received 60 days of chronic alcohol exposure (3.0 g/kg/day increasing to 8.0 g/kg/day). EEG was recorded every 10 days and analyzed by period analysis. Changes in the effect of the challenge dose were assessed by determining the percentage change of the EEG from pre-dose levels to selected times post-dose throughout the chronic alcohol exposure. The EEG response changed significantly during chronic alcohol treatment. Although each structure exhibited a slightly different pattern of change, the overall change was a shift from an excitatory response in the non-tolerant animal to an EEG slowing during chronic exposure. We suggest that such a change may be useful as a diagnostic marker for alcohol tolerance. In addition, the differential nature of the in vivo expression of alcohol tolerance in each brain area suggests that such analysis may provide a valuable tool for understanding the mechanism and expression of alcohol tolerance in the CNS.