Differential patterns of c-fos mRNA expression in the amygdaloid nuclei induced by chemical somatic and visceral noxious stimuli in rats Takayuki Nakagawa, Akiko Katsuya, Sachi Tanimoto, Junki Yamamoto, Yosuke Yamauchi, Masabumi Minami, Masamichi Satoh * Department of Molecular Pharmacology, Graduate School of Pharmaceutical Sciences, Kyoto University, Kyoto 606-8501, Japan Received 26 March 2003; received in revised form 9 April 2003; accepted 10 April 2003 Abstract Pain includes a negative affective component, although the neural system is unclear. The amygdala including the lateral (La), basolateral (BL) and central (Ce) nuclei is thought to play a key role in emotional responses. In this study, we analyzed the c-fos mRNA expression, as a marker of neuronal activity, induced by two types of pain, chemical somatic and visceral noxious stimuli, in each amygdaloid nucleus in unanesthetized rats. We found that intraplantar injection of formalin as a chemical somatic noxious stimulus increased c-fos mRNA expression in the La and BL, but not Ce. On the other hand, intraperitoneal injection of acetic acid as a chemical visceral noxious stimulus induced it highly in the Ce, moderately in La and hardly in BL. These results suggest that distinct amygdaloid nuclei are activated by chemical somatic and visceral noxious stimuli, which might differentially contribute to emotional responses by them. q 2003 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Visceral pain; Somatic pain; c-fos mRNA; Central nucleus of amygdala; Basolateral nucleus of amygdala; Formalin; Acetic acid Painful stimuli evoke pain sensation as well as its unpleasant emotional feelings. Although the neural systems of the sensory component of pain have been extensively studied, those of the negative affective component are less clear. The amygdala is a forebrain structure composed of several distinct nuclei including the lateral (La), basolateral (BL) and central (Ce) nuclei, and is thought to be a key neural substrate underlying emotional responses such as anxiety, fear and depression [7]. Electrophysiological studies indicated that painful stimuli activated the amygda- loid neurons in rats [3,14]. Lesions of the amygdala decreased emotional-related pain reactions such as post- stress analgesia and shock-induced hyperalgesia without affecting the basal nociceptive threshold [6]. These findings suggest that the amygdala could be involved in receiving, integrating and encoding pain information, all of which are considered to contribute to emotional responses. Intraplantar (i.pl.) injection of formalin into the hind-paw evokes nociceptive behaviors such as lifting, licking, shaking or biting. These nociceptive responses are accompanied by inflammation, and caused by cutaneous as well as deep somatic stimuli. On the other hand, intraperitoneal (i.p.) injection of acetic acid produces inflammation of the wall of the abdominal cavity and evokes a sustained writhing behavior caused by visceral stimuli. Both of these methods have been extensively used as well established animal models for chemical somatic and visceral pain, respectively, in rodents. The induction of immediate early c-fos gene is frequently used as an indirect method of gauging neural activation, and its distribution in response to noxious stimulation is generally correlated with the nociceptive pathways mapped by electrophysiological and tract-tracing methods [8]. In this study, we investigated the distribution of the activated amygdaloid neurons by i.pl. injection of formalin as a chemical somatic noxious stimulus and i.p. injection of acetic acid as a chemical visceral noxious stimulus to unanesthetized rats by using in situ hybridization analysis of c-fos mRNA induction. Male Sprague–Dawley rats weighing 180–220 g were used. They were kept at a constant ambient temperature of 24 ^ 1 8C under a 12 h light/dark cycle with free access to food and water. For i.pl. injection of formalin, each rat was placed in a Plexiglass cylinder 30 cm in diameter and 50 cm in height to acclimatize it to the experimental environment 0304-3940/03/$ - see front matter q 2003 Elsevier Science Ireland Ltd. All rights reserved. doi:10.1016/S0304-3940(03)00465-8 Neuroscience Letters 344 (2003) 197–200 www.elsevier.com/locate/neulet * Corresponding author. Tel.: þ 81-75-753-4526; fax: þ81-75-753-4586. E-mail address: msatoh@pharm.kyoto-u.ac.jp (M. Satoh).