Prolactin induces yawning and the stretch-yawning syndrome in young adult male rats.
Herein we report that subcutaneous injection of low doses of ovine prolactin (oPRL) induce yawning in young adult male rats. The most effective dose of oPRL in evoking yawning was 0.25 microgram/kg body weight (5.2 yawns/60 min at 1000 hr vs 0.3 in control animals). Doses of 0.025, 0.05, 2.5, 25, and 250 micrograms/kg were less effective. Interestingly, yawning in response to oPRL changes over the course of one circadian cycle with highest frequency at 1600 hr (11 yawns/80 min vs 2 yawns/80 min in animals injected with boiled oPRL). The onset of yawning in most oPRL-treated rats began approximately 40 min after oPRL injection, whereas with apomorphine the latency to the response was about 10 min. These results indicate that oPRL in addition to other hypophysial peptides such as ACTH and MSH can stimulate yawning. It is proposed that PRL after initial activation of the nigrostriatal dopamine system secondarily induces yawning by inhibition of this system via an autoreceptor-mediated negative feedback mechanism. This may explain the long latency to the response.