Diurnal variations in testosterone in plasma were studied in two inbred strains of mice, BALB/cBy and C57BL/6By. Blood was taken every 4 h over 24 h from male mice at 70 days of age using a lighting regimen of 12 h light to 12 h darkness (lights on 07.00–19.00 h). Values of testosterone in plasma were transformed to log(testosterone in ng/ml) to reduce inequality of variance between groups. In both strains, the distribution of pooled values over all times of day was bimodal, and bimodality was present at most times of day. Circadian variation was evaluated by dividing the transformed values into high and low modes at each time of day and testing for significant variation in the number of animals in each mode over time using the chi-squared test. Significant circadian variation was found in the BALB/cBy strain of mice but not in the C57BL/6By strain. The highest number of high mode cases for BALB/cBy mice was at 22.00 h and the lowest number of high mode cases was at 10.00 h. The log transformation and bimodality of these values are presented as biological expressions of blood levels of testosterone and of tissue responses to these levels in the male mouse. The strain difference in circadian variation may be related to reported circadian changes in behaviour and to possible genetic effects on sensitivity to environmental change or capacity to express circadian rhythms.

Although there is evidence which suggests that age and social environment are significant variables in all experiments dealing with stress in intact animals, there is relatively little information available on the manner in which these variables interact to influence the pituitary adrenal response to stress. Inbred mice of strain, C57BL/6J, of 3 different age groups (49, 255 and 720 days) were subjected to a brief immobilization stress 24 hours subsequent to regrouping them 1, 2 or 4 per cage. Plasma corticosterone concentration was measured by radioimmunoassay prior to and 15 minutes after immobilization or 60 minutes after treatment with ACTH. It was found that preimmobilization levels of corticosterone and increments in corticosterone in response to ACTH treatment were smaller in 255 day than in 49 or 720 day old mice and that preimmobilization corticosterone levels of control and recaged 49 and 255 day old mice were similar. In 49 day old mice, recaging increased the immobilization evoked increment in corticosterone, but in 255 and 720 day old mice recaging in groups or pairs did not change the immobilization evoked response. However, recaging of 720 day old mice in isolation resulted in a decrease in the immobilization evoked increment. Therefore, it appears that the act of transfer itself increased the pituitary adrenal function in the 49 day old mice while, in the oldest mice, isolation itself reduced pituitary adrenal activity. Finally, it appears that in the 255 day old mice, recaging is only a minor stress since after 24 hours′ there is no evidence of elevated steroid levels.

A genetic analysis of alcohol-induced modification of basal activity level was conducted in two strains of inbred mice, their reciprocal F1 hybrids, and seven of their recombinant inbred (RI) strains. Alcohol induced a progressive and persistent decline in basal activity level in all of the 11 strains examined. However, the degree and extent of decline in activity varied significantly with particular progenitor or RI strains. Based on this distinct response of the various strains, a strain distribution pattern (SDP) was obtained, followed by testing of congenic line, B6.C-H-16c. Results of the latter testing indicated that alcohol modification of basal activity is controlled by at least one locus designated Eam (ethanol activity modifier) with Eamh designating the high decrement in activity and similar to the C57BL/6By progenitor, and Eam1 designating the low decrement in activity and similar to the BALB/cBy progenitor. The locus is found on chromosome 4 (LG VIII) adjacent to the Exa locus which exerts a major influence on basal exploratory activity.

Recombinant inbred (RI) strains of mice, their progenitor strains, and reciprocal F1 hybrids were compared with regard to wheel-running activity. The resulting strain distribution pattern (SDP), backcross data, and testing with congenic lines suggested a genetic model with a major locus for wheel-running. Correlations of the SDP for wheel-running and SDPs for testosterone and corticosterone levels determined immediately after testing were low and nonsignificant. Thus, all 3 phenotypic traits measured exhibited genetic variation, but variation in wheel-running was unrelated to variation in corticosterone and testosterone levels following a wheel-running experience.

Recombinant inbred strains, their progenitor strains, and their reciprocal F1 hybrids were tested for maze learning in the Lashley III maze and in the Y-water maze. The resulting pattern of strain distribution suggested that the genetic model provided by the RI strains is based on at least two, and possibly many more, loci. There was no evidence of maternal effects or heterosis. Although the BALB/cBy strain represented an extremely high scoring strain for the Lashley maze and a low scoring strain for the Y-maze, presence of albino recombinant inbred strains intermediate to extreme strains and nonsignificantly different from pigmented recombinant inbred strains suggested that the albino gene was not responsible for the observed performance differences.

Activity levels of COMT were measured in the frontal cortex, hippocampus, hypothalamus and amygdala of two strains of male mice, C57BL/6J and DBA/2J, at various ages between 2 and 30 months. Determinations were made in mice housed under normal conditions and in mice exposed to a form of mild stress, an open-field apparatus, for 5 minutes. There were no major significant differences between the two strains as a function of age. However, after the open-field experience, C57BL/6J mice appeared significantly more responsive to the environment, as interpreted by increases in COMT, than DBA/2J mice. Interpretation is offered that C57BL/6J mice possess a more labile norepinephrine system, in spite of aging, than DBA/2J mice.

Three age groups of DBA/2J and C57BL/6J mice (5, 10, and 16 mo.) were given reversal learning trials in a spatial water maze with water temperatures (incentive conditions) of 12, 22, 34, and 45°C. Swimming times to the choice point were used as the index of motivation while trials-to-criterion were used as a measure of learning. Age effects were nonsignificant. For both strains water temperatures of 12°C and 45°C resulted in increased swimming times relative to 34°C and 22°C, although the effect of incentive of water temperature was less pronounced in the DBA/2J strain. Increased swimming time (motivation) was not accompanied by a corresponding decrease in trials-to-criterion (learning). Data indicate the importance in water-maze experiments of separating measures of incentive and learning by the mouse for which effects of incentives on learning are of concern.

Two progenitor strains, BALB/cBy and C57BL/6By, their reciprocal F1 hybrids, and seven of their recombinant-inbred derived lines were used to examine the genetic basis of the response to thermal pain, and morphine analgesia at doses of 2.5, 5.0 and 10.0mg/kg. Both the latency of response to thermal pain and the analgesic response differed significantly among the various strains tested. Strong genetic determinants appear to control their responses. Analyses of the data did not permit clarification regarding the linkage of these determinants. Photoelectric activity cages were used to test the running response of the same strains to 12.5, 25 and 40 mg/kg morphine sulfate. The genetic determinants for running activity were different from those for analgesia. There is clear evidence for two or more loci controlling the behavior at 60 and 75 min after injection, but not enough information to define the loci involved.

The concentration of opiate receptors in the brains of mice was determined by means of a naloxone-binding assay. The strains of mice used in these experiments were C57BL/6By, BALB/cBy, their reciprocal F1 hybrids, and 7 recombinant-inbred strains derived by inbreeding from the F2 generation. These strains could be divided into 3 groups on the basis of the number of opiate receptors: high (CXBH); low (CXBK); and intermediate (all the other strains). The difference in stereospecific binding of naloxone reflects a difference in the total number of receptor sites rather than in the affinity for the drug. The recombinantinbred strains also differ in their analgesic response to morphine, as previously determined by the tail-flick assay. The differences in the number of opiate receptors are not enough to account for the genetic difference in analgesic responsiveness. Both these parameters appear to be under different genetic control, and at least 2 genetic determinants may be involved in regulating the level of opiate receptors.

Tube-dominance behavior differed significantly in C57BL/6By and BALB/cBy mice. Evaluation of these two inbred strains, and their recently developed recombinant inbred strains permitted the derivation of an hypothesis regarding possible genetic control of this behavior in mice. The results were analyzed by means of patristic matrices with accompanying phylogenetic tree analysis that marks the first occasion that such analysis has been used for a behavioral trait. Based on the data obtained and the statistical analyses and groupings of the various strains, the most satisfactory hypothesis that can be proposed at this time is one which assumes that dominance behavior, as measured in this experiment, is influenced genetically by at least 3 loci. Attempts to link these loci were not successful.