Strain-specific response to anaesthetics and analgesics in rat and rabbit : A pharmacogenetic approach

Abstract

The response of (laboratory) animals to anaesthetics and analgesics is known to show intraspecies variability. Apart from environmental influences, this may also be caused by genetic factors. In this thesis, rabbit and rat inbred strains were used to identify differences in response to anaesthetics and analgesics in order to provide parameters for a genetic analysis to identify candidate genes involved in this variability. The response to single dose iv bolus injections of propofol, ketamine, medetomidine and xylazine was assessed in the AX/JU and IIIVO/JU inbred rabbit strains using the time between injection and loss of righting reflex and the interval between loss and regain of righting reflex (sleep time). IIIVO/JU had a significantly longer sleep time than AX/JU when propofol and ketamine were administered. Xylazine induced severe excitation in AX/JU. Medetomidine did not induce loss of righting in all male and in six out of eight female AX/JU. The biotransformation rate of medetomidine was determined in both strains as well as the activities of the major hepatic cytochrome P450 (CYP) isoenzymes. The formation rate of hydroxymedetomidine and medetomidine carboxylic acid was significantly higher in AX/JU. Significant correlations were found between the formation rates of both metabolites and CYP2D and CYP2E like activity, as well as between these activities and the sleep time. Using amplified fragment length polymorphism markers a linkage map was constructed that was used for testing co-segregation of markers with the medetomidine metabolite formation rates in F2 intercross progeny. Significant quantitative trait loci (QTLs) were thus located on rabbit chromosomes 1 (near the albino and chloride channel locus) and 18 (near the CYP2E1 locus). When MQM (multiple-QTL model or marker-QTL-marker) mapping was used, an additional QTL (sleep time) was found on an unknown linkage group. The response of eight rat inbred strains ACI, BN, COP, F344, LEW, SHR, WAG and WKY to single doses of two analgesics (buprenorphine, nalbuphine) and three anaesthetics (propofol, medetomidine and ketamine) was determined. The analgesic response was measured using a warm water tail withdrawal procedure. The anaesthetic response was assessed in the same way as with the rabbits. ACI was hyperresponsive to buprenorphine whereas no effect was seen in F344 and WKY. None of the strains were responsive to nalbuphine. For all three anaesthetics significant strain differences in sleep time were found. F344 and BN appeared relatively resistant to medetomidine. ACI and BN appeared hypersensitive to ketamine. In all cases the sleep time of albino strains was significantly longer than pigmented strains. The behavioural response to buprenorphine was assessed in ACI and BN by the Laboratory Animal Behaviour Registration and Analysis System (LABORAS). Apart from significant strain differences in unchallenged behaviour, ACI responded to buprenorphine with a decrease in relative duration of locomotion, drinking and eating. In contrast, both strains showed increases in grooming. In conclusion this thesis describes significant rabbit and rat strain differences in response to anaesthetics and analgesics. The differences in response to medetomidine have been genetically analyzed in the rabbit and candidate genes have been identified