The CCK 2 agonist BC264 reverses freezing behavior habituation in PVG hooded rats on repeated exposures to a cat J.M. Farook a,b, * , C.S. McLachlan b , Y.Z. Zhu b , L. Lee c , S.M. Moochhala c , P.T.-H. Wong b a Department of Pharmacology, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78229, USA b Department of Pharmacology, Faculty of Medicine, National University of Singapore, 18 Medical Drive, Kent Ridge, 17597 Singapore, Singapore c Defense Medical Research Institute, Defense Science and Technology Agency, 18 Medical Drive, #01-06 MD2, NUS, 119260 Singapore, Singapore Received 6 October 2003; received in revised form 3 November 2003; accepted 4 November 2003 Abstract Our previous studies (NeuroReport 12 (2001) 2717) showed that PVG hooded and not Sprague – Dawley (SD) rats exhibit remarkable freezing behavior on exposure to a cat in the cat freezing test apparatus. In the present study, we further examined the differences between these two strains of rats in response to repeated daily exposure to a cat in the cat freezing test apparatus. Freezing behavior habituation was observed in both PVG hooded (days 5–7) and SD rats (days 3–7). A selective CCK 2 agonist (BC264, 0.3 mg/kg, i.p.) on day 8 reversed habituated freezing behavior and locomotor activity in PVG hooded rats, but not in SD rats. These results suggest that CCK 2 receptors mediate habituation to an anxiety-inducing stimulus in PVG hooded rats and further suggest that differential expression of these CCK 2 receptors underlies this strain difference. q 2003 Elsevier Ireland Ltd. All rights reserved. Keywords: Behavior; Rats; PVG hooded and Sprague–Dawley; Freezing; Habituation It is well known that anxiety-related behavior of rodents is remarkably influenced by strain differences [8,17]. Previous studies conducted in our laboratory [6] showed that PVG hooded rats exhibit more anxiety-related behavior than Sprague–Dawley (SD) rats on exposure to a cat in the cat freezing test apparatus. These two strains seemed appro- priate for investigating the anxiety-related behaviors. Various naturalistic models have been developed to test anxiety in rodents, such as the social interaction tests, predator confrontations (odor, sound or presence), and conspecific confrontations [2,8]. Rats that spent most of their time hiding on exposure to a cat collar showed gradual habituation over repeated daily exposures [16]; however, the habituated hiding behavior was reversed by administer- ing the anxiolytic drug midazolam (0.375 mg/kg). On the elevated plus-maze, these rats showed increased anxiety- like behavior after exposure to the cat collar and this increase in anxiety response was absent in habituated rats, suggesting that habituation of hiding reflects decreased odor-induced anxiety. In the brain, cholecystokinin (CCK) has been described to act as a central neurotransmitter or neuromodulator and to be involved in anxiety [4]. CCK exerts its action in the central nervous system (CNS) mainly through two different G-protein coupled receptors, CCK 1 and CCK 2 . CCK 2 agonists increase the anxiety response to predatory stimuli, whereas CCK 2 antagonists reduce such a response [7]. Endogenous CCK is co-localized with dopamine (DA) in certain mesolimbic dopaminergic neurons [9,10] and extensive studies have shown a functional relationship between CCK peptides and the dopaminergic system. BC264, a selective CCK 2 agonist, increased grooming behavior in male Wistar rats and this grooming behavior was antagonized by DA 1 antagonists emphasizing the existence of a link between peptidergic (CCK) and dopaminergic systems [13]. The aim of the present work was to determine whether anxiety habituation occurs in either the PVG hooded or SD rats as well as to determine the involvement of CCK pathways in anxiety habituation by using a CCK 2 agonist (BC264 at a dose of 0.3 mg/kg, i.p.) which has been shown to increase attention and motivation through the release of DA [14]. Male SD (Laboratory Animal Center, National Univer- sity of Singapore) and PVG hooded rats (Animal Resources 0304-3940/03/$ - see front matter q 2003 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.neulet.2003.11.010 Neuroscience Letters 355 (2004) 205–208 www.elsevier.com/locate/neulet * Corresponding author. Department of Pharmacology, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78229, USA. Tel.: þ 1-210-567-0106; fax: þ1-210-567-4303. E-mail address: farook@uthscsa.edu (J.M. Farook).