Are the differential effects of chloral hydrate on hooded rats vs. albino rats due to pigmentation or strain differences?

Object recognition testing: rodent species, strains, housing conditions, and estrous cycle

The object recognition task (ORT) allows assessing learning and memory processes in rodents. In this study, two areas in which knowledge about the ORT could be extended were addressed; i.e. generality to species and strains, and intervening variables including housing and estrous cycle. Regarding generality to species and strains, the ORT performance of golden hamsters was assessed. The hamsters showed sufficient exploration times, object recognition performance, and a retention-interval dependent decline similar to rats and mice. Subsequently, we tested three mouse strains which have not been described before in the ORT; i.e. OF1, NMRI, and SJL mice. OF1 and NMRI strains performed equally well, whereas the SJL strain showed low exploration times and no memory retention. Therefore, the SJL strain is unsuited for ORT experiments using a 1h retention interval and a fixed (3 min) trial duration. Furthermore, the sensitivity to a pharmacological memory deficit model (scopolamine) was tested in three rat strains. Each strain showed a dose dependent relationship, but the least effective dose of scopolamine differed among the three strains, the effect being greater in the order of Wistar, Long-Evans, Hooded Lister rats. Finally, to investigate potential intervening variables in the ORT, the effects of housing conditions and estrous cycle were investigated with rats. Single housing resulted in absolute higher performance than social housing. Furthermore, females in pro-estrus/estrus showed better performance compared to females in met-estrus/di-estrus. Taken together, object recognition appears to be a common ability of rodent species, but different strains have different memory capacities and sensitivities to scopolamine, individual housing leads to higher performance, and performance of females is dependent on the estrous cycle phase. Thus, rodent species, strain, housing, and estrous cycle should be taken into consideration in ORT studies.

Strain differences in response to traumatic brain injury in Long-Evans compared to Sprague-Dawley rats

The selected strain of rodent used in experimental models of traumatic brain injury is typically dependent upon the experimental questions asked and the familiarity of the investigator with a specific rodent strain. This archival study compares the injury responsiveness and recovery profiles of two popular outbred strains, the Long-Evans (LE) and the Sprague-Dawley (SD), after brain injury induced by lateral fluid percussion injury (LFPI). General findings include a significantly longer duration of unconsciousness in LE rats, but similar durations of apnea. Both strains displayed the same level of initial FPI-induced behavioral deficits, followed by a more rapid rate of functional recovery in SD rats. Cortical volume loss was not significantly different, but close inspection of the data suggests the possibility that LE rats may be more susceptible to damage in the hemisphere contralateral to the injury site than are SD rats. It is hoped that the information provided here encourages greater attention to the subtle differences and similarities between strains in future pre-clinical efficacy studies of traumatic brain injury.

The locus of origin of augmenting and reducing of visual evoked potentials in rat brain

Humans who are high sensation seekers and cats who demonstrate comparable behavioral traits show increasing amplitudes of the early components of the cortical visual evoked potential (VEP) to increasing intensities of light flash; low sensation seekers show VEP reducing. Roman high-avoidance (RHA) and Roman low-avoidance (RLA) rats have behavioral traits comparable to human and cat high and low sensation seekers, respectively. Previously, we showed that RHA and RLA rats are cortical VEP augmenters and reducers, respectively. The goal of this study was to determine if augmenting-reducing is in fact a property of the visual cortex or if it originates at the lateral geniculate nucleus and is merely reflected in recordings from the cortex. EPs to five flash intensities were recorded from the visual cortex and dorsal lateral geniculate of RHA and RLA rats. As in the previous study, the slope of the first cortical component as a function of flash intensity was greater in the RHA than in the RLA rats. The amplitude of the geniculate component that has a latency shorter than the first cortical component was no different in the two lines of rats. The finding from the cortex confirms the earlier finding of augmenting and reducing in RHA and RLA rats, respectively. The major new finding is that the augmenting-reducing difference recorded at the cortex does not occur at the thalamus, indicating that it is truly a cortical phenomenon.

Possible confounding influence of strain, age and gender on cognitive performance in rats

There are substantial differences in the performance of various rat strains in tasks of learning, memory and attention. Strain, age and sex differences are not consistent over procedures: poor performance in one paradigm does not predict poor performance in a different paradigm. Some strain differences are not readily apparent until a direct comparison is made between one or more strains. Moreover, large differences in nominally the same strain but obtained from different suppliers have been observed in behavioural, pharmacological and physiological parameters and can have important consequences for interpretation of drug effects. Longevity, and the effects of ageing can differ dramatically from one strain to another; drug effects can alter radically with increasing age and show strain (and individual) differences in their action. Sex can further complicate interpretation of results. Thus, non-cognitive factors may exert a major effect on results in cognitive testing, and strain-dependent effects may account for many conflicting results in the literature concerning mnemonic performance. Strain differences in particular must be identified and used to help identify fundamental effects on memory, rather than continue to be ignored and allowed to obscure interpretation of drug effects on cognitive processes.

Augmenting and reducing of visual evoked potentials in Roman high- and low-avoidance rats

Human and cat high sensation seekers tend to show increasing amplitudes (augmenting) of the P1 and N1 components of the visual evoked potential (VEP) to increasing intensities of light flash, whereas low sensation seekers show VEP reducing. Roman high-avoidance (RHA) and Roman low-avoidance (RLA) rats have behavioral traits comparable to human and cat high and low sensation seekers, respectively. RHA rats show greater exploration, activity, and aggression than do RLA rats. Rats of each Roman line and Wistar rats were anesthetized with chloral hydrate and maintained at a stable moderate anesthetic level. VEPs to each of five flash intensities were computer averaged for each rat. The slopes of P1 amplitudes as a function of flash intensity were significantly greater in the RHA than the RLA rats. RHA rats were clear augmenters; RLA rats had almost flat amplitude-intensity functions. The Wistar rats had slope functions that were similar to those of the RLA rats. This study shows that the relationship between sensation-seeking behavior and VEP augmenting and reducing has a heritable base and extends across species from human, cat, and rat. In addition, we demonstrate a rat model of this relationship that yields advantages of genetic homogeneity and a short generational time, and provides access to a wealth of behavioral data and experimental manipulations available for the rat.