Early age-related cognitive impairment in mice lacking cannabinoid CB1 receptors

Abstract

The molecular mechanisms contributing to the normal age-related decline of cognitive functions or to pathological learning and memory impairment are largely unknown. We demonstrate here that young mice (6-7 weeks) with a genetic deletion of the cannabinoid CB1 receptor performed as well as WT mice, or often better, in a number of learning and memory paradigms, including animal models of skill-learning, partner recognition, and operant conditioning. In contrast, the performance of mature mice (3-5 months) lacking CB1 receptors was much worse than that of age-matched WT animals. In most tests, these mice performed at the same level as old animals (14-17 months), suggesting that the decline in cognitive functions is accelerated in the absence of CB1 receptors. This rapid decline in CB1-deficient animals is accompanied by a loss of neurons in the CA1 and CA3 regions of the hippocampus.

Fig. 1.

Open-field test. The activity of young, mature, and old mice was evaluated in the open field in a dimly lit environment. Both the horizontal and the vertical activity showed an age-dependent decrease in both genotypes; this decrease was significant in the old age group in Cnr1^+/+ animals and in mature and old Cnr1^–/– mice. In young animals, we found a small reduction of vertical, but not horizontal, activity in Cnr1^–/– mice. Each column represents the mean (+SEM), n = 10. *, P < 0.05; **, P < 0.01 compared with the young age group (Student–Newman–Keuls test). Y, young; M, mature; O, old.

Fig. 2.

Skill-learning on a rotarod. Time spent on the rotating rod is presented as a function of trials. Symbols represent the mean value (+SEM) of 8–10 animals. The inflexion point of a sigmoid curve derived from the Hill equation indicates the performance of the task acquisition. Young and mature Cnr1^+/+ animals show a similar performance. Old Cnr1^+/+ mice learn the task less well, and they show a lower maximal performance. Young Cnr1^–/– mice also readily learn the task, whereas mature and old Cnr1^–/– mice show a similarly poor performance in this test.

Fig. 3.

Partner recognition test with animals from C57BL/6J (A) and CD1 (B) genetic backgrounds. Reduction in the duration of exploratory social contacts between the first and second trials is indicated as a function of intertrial time. Symbols represents the mean value (+SEM) of 8–10 animals. Sign of recognition is a significant difference in social time between the first and second presentation (Student's paired t test). The duration of social memory was ≈8 h in young and mature Cnr1^+/+ mice but only 1 h in old Cnr1^+/+ animals. Young Cnr1^–/– animals were even able to recognize their partner 16 h after the first presentation. The maximum duration of recognition is 1 h in mature mice and <1hinold Cnr1^–/– mice. *, P < 0.05; **, P < 0.01; ***, P < 0.001, first vs. second presentation (Student's paired t test).

Fig. 4.

Operant conditioning. Performance is expressed as percent maximum performance as a function of time. Symbols represent the mean value (±SEM) of 8–13 animals. The sigmoid curve derived from the Hill equation indicates the learning performance during the repetitions. Young and mature Cnr1^+/+ mice showed an almost identical performance, whereas old Cnr1^+/+ animals learned the task more slowly. Cnr1^–/– mice also readily learned the task, whereas mature and old Cnr1^–/– animals showed a similarly low performance in this model.

Fig. 5.

Neuronal cell densities in the hippocampus. The density of neurons in the CA3 regions was reduced in Cnr1^–/– mice of all age groups. Significantly lower neuronal densities were also observed in the CA1 region of mature and old Cnr1^–/– mice. In this region, the neuronal loss seemed to progress with age. Bars represent mean neuronal density expressed as the number of neurons per mm² (+SEM), n = 4–8; *, P < 0.05; **, P < 0.01; ***, P < 0.001, Cnr1^–/– vs. Cnr1^+/+ mice (Student's unpaired t test).