Bombesin decreases yawning in a high-yawning subline of Sprague-Dawley rats.

This study analysed the effect of the intracerebroventricular administration of bombesin (BN) at doses of 0.001, 0.005, 0.1 and 1.0 microg/2 microl on yawning, grooming and other behavioral correlates in two inbred strains of male rats. These were selected for high-yawning (HY) and low-yawning (LY) frequency, a difference that correlates with novelty-induced grooming. Grooming increased with BN in a strain-specific manner, and yawning decreased in HY rats. Principal component analysis (PCA) showed that rats' behaviors changed from yawning to grooming with BN. Such change differed between the strains. While the first principal component was dominated by grooming in both strains, the second principal component was dominated by stretching and penile erections in HY rats, and by scratching in LY rats. While LY rats spent more time in scratching both within and outside grooming bouts, HY rats tended to favour the latter category. An increment in mean duration of grooming bouts characterized the effect of the highest dose. These findings show that BN inhibits yawning and increases grooming, suggesting that this peptide enhances the initial response to novel environments. The study shows the importance of combining studies on inbred strains with appropriate multivariate methods to separate drug-induced behavioral patterns.