Bombesin decreases yawning in a high-yawning subline of Sprague–Dawley rats Marilu ´ Dı ´az-Romero, Jose ´ Ramo ´n Eguı ´bar*, Alejandro Moyaho Instituto de Fisiologı ´a, Beneme ´rita Universidad Auto ´noma de Puebla, Apdo. Postal 406, Puebla, Pue. 72000, Me ´xico Received 20 November 2000; received in revised form 5 July 2001; accepted 31 July 2001 Abstract This study analysed the effect of the intracerebroventricular administration of bombesin (BN) at doses of 0.001, 0.005, 0.1 and 1.0 mg/2 ml 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. D 2002 Elsevier Science Inc. All rights reserved. Keywords: Inbred strains; Rats; Stress; Penile erection; Principal component analysis; Scratching; Stretching; Yawning; Grooming 1. Introduction The effects of the central administration of bombesin (BN) have been documented for a variety of behaviors (Kulkosky et al., 1982; Cowan et al., 1985). Some of the most conspicuous are vigorous scratching (Brown et al., 1977), excessive grooming and wet-dog shaking (Gmerek and Cowan, 1981). BN also stimulates dose-dependent elevations in plasma adrenocorticotrophic hormone (ACTH), corticosterone, catecholamines and glucose (Brown et al., 1979, 1988; Gunion et al., 1989; Plamondon and Merali, 1993, 1997; Malendowicz and Nussdorfer, 1995). These findings have led to suggest that BN-like peptides play a role in the mediation of the stress response (Kent et al., 1998). The behavioral reactions to mild stress, however, include not only excessive grooming but also other behaviors such as yawning (Delius, 1967, 1988; Dourish and Cooper, 1990). Yet, there are few reports on the relation between BN and yawning (Kulkosky et al., 1988) which could indicate whether BN has different behavioral correlates from other peptides involved in the stress response. Indeed, peptides such as ACTH, whose link to stress is well established (Akil and Moreno, 1995), elicit grooming and yawning (Gispen and Isaacson, 1981; Argio- las and Melis, 1998). The lack of such studies is partly because yawning occurs spontaneously at a very low frequency, making it difficult to appreciate little variations that may otherwise, indicate meaningful changes. We have in our laboratory two strains of Sprague– Dawley rats that were selectively bred for high-yawning (HY) and low-yawning (LY) frequency (Urba ´-Holmgren et al., 1990). The strains also differ in other behaviors. For example, HY rats circulate in an open field arena and groom in a novel environment more than LY rats (Moyaho et al., 1995; Eguı ´bar and Moyaho, 1997). The fine structure of water-induced grooming is also different between both strains: LY rats show sequences that are more stereotyped than those of HY rats (Moyaho et al., 1995). In addition, HY rats yawn more than LY rats after the administration of cholinesterase inhibitors and dopa- 0091-3057/01/$ – see front matter D 2002 Elsevier Science Inc. All rights reserved. PII:S0091-3057(01)00631-1 * Corresponding author. Tel.: +52-2-244-8811; fax: +52-2-233-4511. E-mail address: jeguibar@siu.buap.mx (J.R. Eguı ´bar). www.elsevier.com/locate/pharmbiochembeh Pharmacology, Biochemistry and Behavior 71 (2002) 103– 109