The differential effect of morphine and b-endorphin administered intracerebroventricularly on pERK and pCaMK-II expression induced by various nociceptive stimuli in mice brains Young-Jun Seo, Min-Soo Kwon, Hee-Woo Choi, Jeong-Eun Jang, Jin-Koo Lee, Jun-Sub Jung, Soo-Hyun Park, Hong-Won Suh * Department of Pharmacology and Institute of Natural Medicine, College of Medicine, Hallym University, 1 Okcheon-Dong, Chuncheon, Gangwon-Do 200-702, South Korea Received 17 August 2007; accepted 24 January 2008 Available online 21 March 2008 Abstract The present study was performed to characterize the differential molecular mechanisms of morphine and b-endorphin which are injected intracerebroventiricularly in mice. In the immunoblot assay, the increases of phosphorylated extracellular signal-regulated protein kinase (pERK) as well as phosphorylated calcium/calmodulin-dependent protein kinase IIa (pCaMK-IIa) expression induced by noxious stimuli were attenuated by intracerebroventricular (i.c.v.) b-endorphin pretreatment in the hypothalamus, but not by i.c.v. morphine pretreatment. In addition to these immunoblot results, immunohistochemical study also showed that the attenuation of pERK or pCaMK-IIa immunoreactivity elicited by i.c.v. pretreatment of b-endorphin mainly occurred in the paraventricular nucleus of the hypothalamus (PVN). We also investigated the effect of morphine and b-endorphin on pERK and pCaMK-IIa expression in the locus coeruleus (LC). I.c.v. injection of morphine significantly increased pERK as well as pCaMK-IIa expression in the locus coeruleus, while b-endorphin increased only pCaMK-IIa in the LC. In addition, b-endorphin significantly attenuated pERK expression induced by SP i.t. injection. These results suggest that the antinociceptive effects of sup- raspinally administered morphine and b-endorphin are involved with differentially intracellular signal transduction molecules- pERK, pCaMK-IIa in the PVN and the LC. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: b-Endorphin; Morphine; Phosphor-CaMK-II; Phosphor-ERK; Formalin; Substance P 1. Introduction b-Endorphin is an endogenous opioid peptide synthe- sized by the cells in the arcuate nucleus of the hypothal- amus and the nucleus tractus solitarius in the brain stem (Bloom et al., 1978; Bronstein et al., 1992) and which contain neurons projecting widely throughout the brain. Although b-endorphin was known to have moderate affinity for endogenous l/d-opioid receptors and about 10-fold lower affinity for j-opioid receptors (Raynor et al., 1994), it was suggested that the analgesic effect of b-endorphin administered i.c.v. may be mediated by the activation of l- or pupative e-opioid receptor (Sun et al., 2003); for review see reference (Tseng, 2001). Several lines of evidence have supported the antinoci- ceptive effect of e-receptor activation. Suh and Tseng (1990b) have reported the absence of cross-tolerance between supraspinally injected morphine and b-endor- phin in the tail-flick test in mice. In addition, it was also demonstrated that b-endorphin and morphine activate different sites of the pain modulatory system in the supraspinal regions (Tseng and Wang, 1992), which 0143-4179/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.npep.2008.01.003 * Corresponding author. Tel.: +82 33 248 2614; fax: +82 33 248 2612. E-mail address: hwsuh@hallym.ac.kr (H.-W. Suh). www.elsevier.com/locate/npep Available online at www.sciencedirect.com Neuropeptides 42 (2008) 319–330 Neuropeptides