Behavioural Brain Research 174 (2006) 78–85 Research report Habituation of the head-shake response induces changes in brain matrix metalloproteinases-3 (MMP-3) and -9 John W. Wright a,b,c,∗ , Starla E. Meighan b , Eric S. Murphy d , Kelby L. Holtfreter a,c , Christopher J. Davis a,b , Mikel L. Olson a , Caroline C. Benoist b , Kalyani Muhunthan b , Joseph W. Harding a,b,c a Department of Psychology, Washington State University, PO Box 644820, Pullman, WA 99164-4820, United States b Department of Veterinary and Comparative Anatomy, Pharmacology and Physiology, Washington State University, Pullman, WA 99164-4820, United States c Programs in Neuroscience and Biotechnology, Washington State University, Pullman, WA 99164-4820, United States d Department of Psychology, University of Alaska Anchorage, 3211 Providence Drive, Anchorage, AK 99508, United States Received 26 July 2005; received in revised form 5 July 2006; accepted 6 July 2006 Available online 17 August 2006 Abstract Habituation is defined as a decrease in responsiveness to a repeatedly presented stimulus. The head-shake response (HSR) demonstrates several fundamental properties of habituation including sensitivity to the frequency and intensity of stimulation, and spontaneous recovery. This response shows behavioral plasticity; however the neural plasticity presumed to underlie this behavioral phenomenon has only recently been investigated. The present study initially compared male and female rats and noted equivalent habituation and spontaneous recovery. A second experiment utilized female rats to test the hypothesis that habituation induces changes in neural plasticity. At inter-session intervals (ISIs) of 5min, 2, 6, and 24 h following HSR habituation independent groups of rats received a second habituation experience, then tissue samples were immediately collected from hippocampal, prefrontal and piriform cortices, and cerebellum. Western blots indicated significant elevations in the expression of matrix metalloproteinase-3 (MMP-3) in hippocampal, prefrontal and piriform cortices at a delay interval of 2 h, and in the prefrontal cortex at 24h in habituated rats. Increases in active and pro MMP-9 activity were measured by zymography in the hippocampus of habituated rats over yoked controls. Decreases in active MMP-9 activity were seen in the prefrontal cortex, and in pro MMP-9 in the piriform cortex, of habituated as compared with yoked control rats. No changes in MMP-3 or MMP-9 were observed in the cerebellum, and no changes in MMP-2 were seen in any of the four structures examined. These results suggest that habituation of the HSR produced elevations in MMP-3 expression in three of the four structures presently examined, accompanied by increased MMP-9 activity in the hippocampus and decreases in the prefrontal cortex. However, cues present in the test environment appear to have provoked elevations in MMP-3 and -9 independent of those accompanying habituation. © 2006 Elsevier B.V. All rights reserved. Keywords: Nonassociative learning; Habituation; Head-shake response; Matrix metalloproteinases; MMP-3; MMP-9; Neural plasticity 1. Introduction Nonassociative learning includes the phenomena of habitu- ation, dishabituation, and sensitization and is considered to be the simplest form of learning. Habituation refers to a decrease in responding (as related to frequency, magnitude, or intensity) to a stimulus repeatedly presented, or presented for a prolonged period of time [23,34,45,54,55]. Habituation has been doc- ∗ Corresponding author. Tel.: +1 509 335 2329; fax: +1 509 335 5043. E-mail address: wrightjw@wsu.edu (J.W. Wright). umented across many species and response systems ranging from the gill-withdrawal reflex in Aplysia [8] and tap with- drawal or chemotaxic response in the nematode Caenorhab- ditis elegans [4,47], to acoustic startle response in rats and mice [42,50], schedules of reinforcement in operant condition- ing [33,39] and feeding in humans [15]. Although the neural mechanism(s) underlying habituation has not been identified [1,11,19,21,43,53,58], the hippocampus has been implicated in the control of inhibitory processes, particularly habituation [14,25,29,44,53]. In support of this notion bilateral hippocam- pectomy in rats has been shown to interfere with habituation to familiar objects in an open field object recognition task 0166-4328/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.bbr.2006.07.006