Adolescent Intermittent Alcohol Exposure: Deficits in Object Recognition Memory and Forebrain Cholinergic Markers

Abstract

The long-term effects of intermittent ethanol exposure during adolescence (AIE) are of intensive interest and investigation. The effects of AIE on learning and memory and the neural functions that drive them are of particular interest as clinical findings suggest enduring deficits in those cognitive domains in humans after ethanol abuse during adolescence. Although studies of such deficits after AIE hold much promise for identifying mechanisms and therapeutic interventions, the findings are sparse and inconclusive. The present results identify a specific deficit in memory function after AIE and establish a possible neural mechanism of that deficit that may be of translational significance. Male rats (starting at PND-30) received exposure to AIE (5g/kg, i.g.) or vehicle and were allowed to mature into adulthood. At PND-71, one group of animals was assessed using the spatial-temporal object recognition (stOR) test to evaluate memory function. A separate group of animals was used to assess the density of cholinergic neurons in forebrain areas Ch1-4 using immunohistochemistry. AIE exposed animals manifested deficits in the temporal component of the stOR task relative to controls, and a significant decrease in the number of ChAT labeled neurons in forebrain areas Ch1-4. These findings add to the growing literature indicating long-lasting neural and behavioral effects of AIE that persist into adulthood and indicate that memory-related deficits after AIE depend upon the tasks employed, and possibly their degree of complexity. Finally, the parallel finding of diminished cholinergic neuron density suggests a possible mechanism underlying the effects of AIE on memory and hippocampal function as well as possible therapeutic or preventive strategies for AIE.

Fig 1. Depiction of the three test trials used in the stOR task.

Animals were placed in the chamber with two objects (trial 1). After a 1-hour delay, animals were returned to the chamber with two new objects; after another 1-hour delay animals were returned to the chamber containing the four previously encountered objects. One object from trial 1 and one object from trial 2 were placed in new positions while the other objects remained in their original positions.

Fig 2. AIE produces deficits in object recognition memory in adulthood.

Partial regression plots demonstrating the relationship between pre-treatment condition (saline v. AIE) and recency preference (panel A) and position preference (panel B); and between age (adolescent v. adult) and recency preference (panel C) and position preference (panel D). These plots represent the relationship between the independent variable (pre-treatment or age) and the dependent variable (recency preference or position preference) after controlling for time spent inspecting objects in Trials 1 and 2. Pretreatment was significantly predictive of the recency preference (panel A), but was not significantly predictive of position preference (panel B). Age was not predictive of recency or position preference (panels C and D). Panel A inset depicts the group mean (+/- SEM) predicted preference indices (derived from the regression model, denoted by “P”) and the observed preference indices based on uncorrected data (denoted by “O”).

Fig 3. AIE decreases ChAT+IR in the basal forebrain in adulthood.

AIE decreases ChAT+IR in the Ch1 and Ch2 nuclei of the basal forebrain of adult rats (a). Left Panel—the cell density of ChAT+IR is significantly decreased in the Ch1 and Ch2 nuclei at 0.70 ~ 0.20 mm from bregma 25 days after AIE, **p<0.01. Right Panel—representative photomicrography ChAT+IR neurons in the Ch1 and Ch2 nuclei from a control animal (Control), and an AIE-exposed animal (ETOH). AIE decreases ChAT+IR in the Ch3 and Ch4 nuclei of the basal forebrain of adult rats (b). Left Panel—the cell density of ChAT+IR is significantly decreased in the Ch3 and Ch4 nuclei at 0.48 ~ 0.40 mm from bregma 25 days after AIE, *p = 0.046. Right Panel—representative photomicrography ChAT+IR neurons in the Ch3 and Ch4 nuclei from a control animal (Control), and an ethanol-exposed animal (ETOH). Scale bar = 50 μm.