Pain thresholds in alcohol preferring and non-preferring rats: diurnal and repeated trial line differences.

Abstract

Studies in humans and animals suggest that those with a genetic predisposition toward high levels of alcohol consumption have decreased pain thresholds. The present study explores this association in rat lines selectively bred for alcohol preference or nonpreference. 13 HAD1, 11 LAD1, 16 iP5C, and 16 iNP1 rats were tested in the Ugo Basile Plantar Analgesiometer and the IITC Tail Flick device. Five trials were conducted in each test session with one session per device conducted in the light and one in the dark. Trial one latency represents baseline latency. Decline in Latency over Repeated Trials (DLRT) was analyzed using linear regression. Testing in the Plantar Analgesiometer revealed no significant line differences in baseline latency (iP5C versus iNP1; HAD1 vs. LAD1). The alcohol preferring lines (iP5C and HAD1), however, both demonstrated an increase in baseline latency in the dark phase compared to the light phase (p < 0.05). The iNP1 line demonstrated highly significant DLRT in both the light phase (p < 0.001) and dark phase (p < 0.01) while the iP5C line demonstrated a significant DLRT only at night (p < 0.01). In the tail flick apparatus, the HAD1 line demonstrated a significantly increased baseline latency compared to the LAD1 line in both the light (p < 0.05) and dark (p < 0.01) phases. The HAD1 line also demonstrated a significant increase in baseline latency in the dark compared to the light phase (p < 0.05) and a significant DLRT in the light (p < 0.01) and the dark phase (p < 0.001). The alcohol preferring HAD1 line demonstrates a significantly increased baseline tail latency compared to the nonpreferring LAD1 line in both light and dark phases. There is a significant diurnal rhythm of paw and tail latency in both of the alcohol preferring lines (iP5C and HAD1) but not in the nonpreferring lines (iNP1 and LAD1). A novel finding of Decline in Latency with Repeated Trials (DLRT) was found in both alcohol preferring and nonpreferring lines; degree of DLRT differed as a function of line and circadian period.