Identification of subpopulations of prairie voles differentially susceptible to peer influence to decrease high alcohol intake

Abstract

Peer influences are critical in the decrease of alcohol (ethanol) abuse and maintenance of abstinence. We previously developed an animal model of inhibitory peer influences on ethanol drinking using prairie voles and here sought to understand whether this influential behavior was due to specific changes in drinking patterns and to variation in a microsatellite sequence in the regulatory region of the vasopressin receptor 1a gene (avpr1a). Adult prairie voles' drinking patterns were monitored in a lickometer apparatus that recorded each lick a subject exhibited during continuous access to water and 10% ethanol during periods of isolation, pair housing of high and low drinkers, and subsequent isolation. Analysis of fluid consumption confirmed previous results that high drinkers typically decrease ethanol intake when paired with low drinkers, but that a subset of voles do not decrease. Analysis of bout structure revealed differences in the number of ethanol drinking bouts in the subpopulations of high drinkers when paired with low drinkers. Lickometer drinking patterns analyzed by visual and by cross-correlation analyses demonstrated that pair housing did not increase the rate of subjects drinking in bouts occurring at the same time. The length of the avpr1a microsatellite did not predict susceptibility to peer influence or any other drinking behaviors. In summary, subpopulations of high drinkers were identified, by fluid intake and number of drinking bouts, which did or did not lower their ethanol intake when paired with a low drinking peer, and these subpopulations should be explored for testing the efficacy of treatments to decrease ethanol use in groups that are likely to be responsive to different types of therapy.

Keywords: alcohol; ethanol; genetics; peer pressure; prairie vole; regulatory microsatellite; social behavior; vasopressin.

FIGURE 1

Schematic diagram of lickometer cages and timeline.Custom-designed cages were made to house voles individually (A) or in pairs separated by a mesh divider (B). In both cases, plastic cages with air holes in the top surrounded wire metal racks that covered most of the cage floor. Voles had to step on the wire rack to reach the metal sipper tubes to obtain fluid, completing an electrical circuit to register each lick on the drinking tube. Subjects habituated to drinking water from the tubes for 5 days prior to the start of the experiment. They then had access to 10% ethanol and water for 4 days, after which time they were categorized as high or low drinkers. High and low drinkers were paired and given access to ethanol and water for another 4 days, followed by a final 4 days of isolation with continued access to ethanol and water.

FIGURE 2

Ethanol preference and intake in different housing conditions. Ethanol preference (A) and intake (B) by high (black) and low (white) drinkers in each housing condition. *Significant difference between isolation 1 and subsequent housing conditions for high drinkers; p < 0.05.

FIGURE 3

Ethanol preference and intake across days. Ethanol preference (A) and intake (B) by high drinkers that did (black square) or did not (black triangle) change ethanol intake when paired with low drinkers (white square) is shown across days of isolation and pair housing. In isolation, both high groups are significantly higher than the low drinkers. During pair housing, the high drinkers that change intake and the low drinkers are both significantly lower than the constant high drinkers.

FIGURE 4

Correlation of recorded licks with fluid volume consumed. The relationship between the number of recorded licks from each drinking tube on the X-axis with the volume of water (O) or ethanol (X) consumed on the Y-axis, for each of 4 days in isolation is graphed for one cohort of animals (n = 10) representative of the entire experiment. There was a strong positive correlation for both water (r = 0.815, n = 40, p < 0.0001) and ethanol (r = 0.694, n = 40, p < 0.0001).

FIGURE 5

Ethanol drinking bout features. Bout features of high drinkers that did (black triangle) or did not (black square) change ethanol intake when paired with low drinkers (white square) are shown throughout isolation and pair housing. The number of bouts of ethanol drinking (A), the average length of bouts as measured by number of licks (B) and time (D), the average time between bouts (C), the rate of licks within a bout (E), and the average time until the first lick was recorded (F) are shown for each 22 h period. *Post hoc significant difference between high-no change group and low group; #Post hoc significant difference between high-no change group and high-change group.

FIGURE 6

Cumulative number of licks of ethanol over 22 h for an example pair. (A) The drinking patterns for subjects in a pair on the last day of isolation. (B) The drinking patterns for subjects in the same pair on the last day of pair housing. The high drinker is shown in blue and the low drinker is shown in black. Each “step up” in the graph indicates a bout of drinking while each horizontal line indicates a time when no drinking occurred. The red circle indicates bouts that occurred close together in time, within the applied threshold.

FIGURE 7

Visual assessment of close ethanol drinking bouts between partners in isolation and pair housing.(A) The number of close bouts does not significantly differ between housing conditions or group changes, and there is no significant interaction of effects. (B) The proportion of close bouts relative to the lowest number of bouts one subject exhibited does not significantly differ between housing conditions or group changes, and there is no significant interaction of effects.

FIGURE 8

Cross-correlations between ethanol drinking patterns of peers. (A) Correlations between high and low drinkers in isolation, before they have been housed together. (B) Correlations between high and low drinkers in pairs on the fourth day of pair housing. The Y-axis represents the strength of the correlation (autocorrelation function; ACF), while horizontal dashed lines represent the threshold for significance. The X-axis represents “lag” time, in seconds, between drinking events. A significant correlation at a lag time “h” indicates that h seconds after the high drinker licks, the low drinker is likely to lick; a positive h value indicates that the high drinker leads the low drinker, and a negative value indicates that the low drinker leads the high drinker. Pairs shown are examples of each type of pair observed: those where both subjects changed drinking levels when paired (top panels), those where the high drinker changed to match the low drinker (middle panels show two of many variations of outcomes), and those where neither subject changes (bottom panels).