Abstract
When using rats in pain research, strain-related differences in outcomes of tests for pain and nociception are acknowledged. However, very little is known about the specific characteristics of these strain differences. In this study four phylogenetically distant inbred rat strains, i.e. Wistar Kyoto (WKY), Fawn Hooded (FH), Brown Norway (BN) and Lewis (LE), were investigated in different tests related to pain and nociception. During Pavlovian fear conditioning, the LE and WKY showed a significantly longer duration of freezing behaviour than the FH and BN. Additionally, differences in c-Fos expression in subregions of the prefrontal cortex and amygdala between rat strains during retrieval and expression of conditioned fear were found. For example, the BN did not show recruitment of the basolateral amygdala, whereas the WKY, FH and LE did. During the hot plate test, the WKY and LE showed a lower thermal threshold compared to the BN and FH. In a follow-up experiment, the two most contrasting strains regarding behaviour during the hot plate test and Pavlovian fear conditioning (i.e. FH and WKY) were selected and the hot plate test, Von Frey test and somatosensory-evoked potential (SEP) were investigated. During the Von Frey test, the WKY showed a lower mechanical threshold compared to the FH. When measuring the SEP, the FH appeared to be less reactive to increasing stimulus intensities when considering both peak amplitudes and latencies. Altogether, the combined results indicate various differences between rat strains in Pavlovian fear conditioning, nociception related behaviours and nociceptive processing. These findings demonstrate the necessity of using multiple rat strains when using tests including noxious stimuli and suggest that the choice of rat strains should be considered. When selecting a strain for a particular study it should be considered how this strain behaves during the tests used in that study.
Highlights
Inbred rat strains are becoming more and more valuable in pain research because of the availability of extensive genomic information and transgenic technologies in these species [1], enabling the study of genetic background and the role of individual proteins in pain
Post hoc test (Sidak) revealed that the latency time of the Fawn Hooded (FH) was significantly longer compared to Wistar Kyoto (WKY) and LE, the Brown Norway (BN) showed a significantly longer latency time compared to the WKY
A significant difference in mechanical thresholds was found between strains (t15.3 = 2.26, p,0.05), with the WKY showing a lower threshold compared to FH
Summary
Inbred rat strains are becoming more and more valuable in pain research because of the availability of extensive genomic information and transgenic technologies in these species [1], enabling the study of genetic background and the role of individual proteins in pain. Thermal and mechanical (anti)nociception in rats and mice are commonly studied using the hot plate and Von Frey test, respectively [4]. In the hot plate test, the animal is placed on a plate heated at a fixed temperature until a predetermined behavioural endpoint is observed, typically a hind-paw lick or jump response [5,6]. In the automated Von Frey test, a probe is placed under the hind paw which gradually builds up force, until the animal withdraws the paw. Differences in response latencies (i.e. hot plate test) and withdrawal threshold (i.e. Von Frey test) between animals are considered to reflect differences in thermal and mechanical nociceptive threshold, respectively [4]. The literature documents the use of many different rat strains, very little is known about differences in behaviour expressed during the hot plate and Von Frey test
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