Comprehensive Behavioral Analysis of Male Ox1r (-/-) Mice Showed Implication of Orexin Receptor-1 in Mood, Anxiety, and Social Behavior

Abstract

Neuropeptides orexin A and orexin B, which are exclusively produced by neurons in the lateral hypothalamic area, play an important role in the regulation of a wide range of behaviors and homeostatic processes, including regulation of sleep/wakefulness states and energy homeostasis. The orexin system has close anatomical and functional relationships with systems that regulate the autonomic nervous system, emotion, mood, the reward system, and sleep/wakefulness states. Recent pharmacological studies using selective antagonists have suggested that orexin receptor-1 (OX1R) is involved in physiological processes that regulate emotion, the reward system, and autonomic nervous system. Here, we examined Ox1r (-/-) mice with a comprehensive behavioral test battery to screen additional OX1R functions. Ox1r (-/-) mice showed increased anxiety-like behavior, altered depression-like behavior, slightly decreased spontaneous locomotor activity, reduced social interaction, increased startle response, and decreased prepulse inhibition. These results suggest that OX1R plays roles in social behavior and sensory motor gating in addition to roles in mood and anxiety.

Keywords: anxiety; emotion; hypocretin; orexin; orexin receptor 1 (OX1R); social behavior.

Figure 1

Normal general health, neurological and motor function and nociception in Ox1r^−/− mice. No significant differences were found between genotypes in the body weight (A), body temperature (B) and grip strength (C). A significant reduction in the wire hang latency was observed in Ox1r^−/− mice (D). No significant difference in the latency to fall off in the rotarod test was observed in Ox1r^−/− and wild-type mice (E). Hot plate latency was lower in Ox1r^−/− mice (F). Asterisk Indicates a significant difference from wild-type mice (p < 0.05).

Figure 2

Altered depression-like behavior in Ox1r^−/− mice. (A) A significant decrease in the immobility time (A₁) on day 2 was found in Ox1r^−/− mice compared with wild-type mice, while there was no significant difference in the distance traveled (A₂) between the genotypes in the Porsolt forced swim test. (B) Increased immobility time was observed in Ox1r^−/− mice compared with wild-type mice in the tail suspension test.

Figure 3

Decreased locomotor activity and increased anxiety-like behavior in Ox1r^−/− mice. (A) In the light/dark transition test, no significant differences between Ox1r^−/− and wild-type mice were found in number of transitions (A₂), time spent in the light chamber (A₃) and latency to enter the light chamber (A₄). The distance traveled in the dark chamber was significantly shorter in Ox1r^−/− mice (A₁). (B) In the elevated plus maze test, all the behavioral measures, including distance traveled (B₁), number of arm entries (B₂), percentage of entries into open arms (B₃), and percentage of time spent in open arms (B₄) were significantly decreased in Ox1r^−/− mice compared with wild-type mice. (C) In the open field test, there were no significant differences between Ox1r^−/− and wild-type mice in the total distance traveled (C₁), vertical activity (C₂), time spent in the center (C₃) and stereotypic counts (C₄). Asterisk Indicates a significant difference from wild-type mice (p < 0.05).

Figure 4

Decreased prepulse inhibition of Ox1r^−/− mice. Increased startle response (A) and decreased prepulse inhibition (B) were found in mutant mice.

Figure 5

Abnormal social behavior in Ox1r^−/− mice. Social interaction test in a novel environment (A). Total duration of contact (A₁), number of contacts (A₂), total duration of active contact (A₃), and mean duration of contact (A₄) were not significantly different between each genotype. Reduction of distance traveled was seen in mutant mice (A₅). In the sociability test (B), control mice, but not mutant mice, spent significantly longer time in the chamber with a novel conspecific (stranger 1) than in the empty side (B₁) and significantly longer time around the cage containing stranger 1 than that around the empty cage (B₂). In social novelty test, wild-type control mice tended to spend longer time around the cage containing stranger 2 than that containing stranger 1, but Ox1r^−/− mice did not show this tendency (B₃). Ox1r^−/− mice showed significantly shorter time spent around the cage with stranger 2 when compared with control mice (B₄). In a 24-h home cage social interaction test (C), mean number of objects (C₁) and activity level (C₂) were lower in Ox1r^−/− mice than control mice.