Anxiogenic effects of activation of NK-1 receptors of the dorsal periaqueductal gray as assessed by the elevated plus-maze, ultrasound vocalizations and tail-flick tests Gabriel S. Bassi, Manoel J. Nobre, Joa ˜o E. de Arau ´ jo, Marcus L. Branda ˜o * Instituto de Neurocie ˆncias & Comportamento-INeC, Campus USP, 14040-901, Ribeira ˜o Preto, SP, Brazil Laborato ´ rio de Psicobiologia, FFCLRP, Campus USP, Avenida Bandeirantes 3900, 14049-901 Ribeira ˜o Preto, SP, Brazil Received 30 June 2007; accepted 15 September 2007 Available online 5 November 2007 Abstract Ultrasound vocalizations (USVs) known as 22 kHz are usual components of the defensive responses of rats exposed to threat- ening conditions. The amount of emission of 22 kHz USVs depends on the intensity of the aversive stimuli. While moderate fear causes an anxiolytic-sensitive enhancement of the defensive responses, high fear tended to reduce the defensive performance of the animals to aversive stimuli. The dorsal periaqueductal gray (dPAG) is an important vocal center and a crucial structure for the expression of defensive responses. Substance P (SP) is involved in the modulation of the defensive response at this midbrain level, but the type of neurokinin receptors involved in this action is not completely understood. In this study we examined whether local injections of the selective NK-1 agonist SAR-MET-SP (10–100 pmol/0.2 lL) into the dPAG (i) cause anxiogenic effects in the ele- vated plus-maze (EPM) (Exp. I), (ii) influence the novelty-induced 22 kHz USVs recorded within the frequency range of 20–26 kHz (Exp. II) and (iii) change the nociceptive reactivity to heat applied to the rat’s tail (Exp III). The data obtained showed that SAR- MET-SP elicited significant ‘‘anxiety-like’’ behaviors, as revealed by the decrease in the number of entries into and time spent onto the open arms of the EPM. These anxiogenic effects were accompanied with antinociception and disruption of the novelty-induced increase in the number and duration of 22 kHz USVs. These findings are in agreement with the notion that NK-1 receptors of the dPAG may be an important neurochemical target for new selective drugs aimed at the control of pathological anxiety states. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Elevated plus-maze; Ultrasound vocalizations; NK-1 receptors; Dorsal PAG 1. Introduction Ultrasound vocalizations (USVs) termed as 22 kHz, have been considered to be important components of the defensive behavior repertoire in rats and a reliable measure of anxiety in laboratory animals (Blanchard et al., 1991; Commissaris et al., 2000; Brudzynski and Holland, 2005; Tomazini et al., 2006). Such vocaliza- tions may, therefore, be relevant for intraspecies signal- ing or warning in case of potential danger (Brudzynski and Chiu, 1995; Brudzynski and Holland, 2005). Recently, it has been demonstrated that animals submit- ted to moderate or intense fear present distinct behav- ioral reactivity when tested in the elevated plus-maze or in novelty tests (Silva and Branda ˜o, 2000; Tomazini et al., 2006; Bassi et al., 2007). A growing amount of evidence has demonstrated that the activation of some mesencephalic structures, particularly the dorsal periaqueductal gray (dPAG), a central structure of the so-called brain aversion system, produces several behavioral and somatic responses 0143-4179/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.npep.2007.09.004 * Corresponding author. Address: Laborato ´rio de Psicobiologia, FFCLRP- Campus USP, 14040-901 Ribeira ˜o Preto, SP, Brazil. Tel.: +55 (016) 3602 3638; fax: +55 (016) 3633 0619. E-mail address: mbrandao@usp.br (M.L. Branda ˜o). www.elsevier.com/locate/npep Available online at www.sciencedirect.com Neuropeptides 41 (2007) 365–374 Neuropeptides