Met-enkephalin and preproenkephalin mRNA changes in the striatum of the nicotine abstinence mouse Raffaella Isola a , Hailing Zhang a , Anne-Marie Duchemin a , Gopi A. Tejwani b , Norton H. Neff a,b , Maria Hadjiconstantinou a,b, * a Department of Psychiatry, The Ohio State University School of Medicine and Public Health, 5034 Graves Hall, 333 West 10th Avenue, Columbus, OH 43210, USA b Department of Pharmacology, The Ohio State University School of Medicineand Public Health,5034 Graves Hall, 333 West 10th Avenue, Columbus, OH 43210, USA Received 27 December 2001; received in revised form 12 February 2002; accepted 1 March 2002 Abstract We studied the changes of met-enkephalin (Met-Enk) content and preproenkephalin (PPE) mRNA in the striatum in a mouse model of nicotine abstinence. Nicotine, 2 mg/kg, s.c., was administered four times daily for 14 days and Met-Enk and PPE mRNA evaluated at various times (4–96 h) following drug discontinuation. Met-Enk, assayed by radioimmu- noassay, was increased in the ventral (nucleus accumbens) but not dorsal (putamen/caudate) striatum, while PPE mRNA, assayed in whole striatum by Northern blotting was elevated. Both changes were seen early during withdrawal and lasted over 72 h. In situ hybridization revealed enhanced signal in the dorsal striatum, mostly laterally, and smaller increases in the rostral pole, core and shell of the nucleus accumbens. These observations indicate that during nicotine withdrawal, striatal enkephalinergic neurons undergo adaptative responses, which might contribute to the abstinence behavioral syndrome. q 2002 Elsevier Sciencce Ireland Ltd. All rights reserved. Keywords: Nicotine abstinence mouse model; Nicotine withdrawal; Met-enkephalin; Preproenkephalin; Ventral and dorsal striatum Nicotine in tobacco smoke is responsible for the pharma- cological actions of smoking and for its addictive effects. Although the dopaminergic mesolimbic system has been considered as the neural substrate for the motivational and reinforcing properties of nicotine, there is evidence that other neurotransmitter systems might be involved in the behavioral pharmacology of nicotine as well. A growing body of evidence has provided credence to the long specu- lated role of opioids in the neurobiology of drug addiction, including nicotine [12].Of particular interest are reports that nicotine alters the synthesis and release of b-endorphin [17], met-enkephalin (Met-Enk) [3,7,8,18,19] and dynorphin (Ref. [6]; manuscript in preparation). In this regard, we have reported that a single dose of nicotine changes the content of Met-Enk in the striatum of mice in a complex temporal manner [3]; an early decrease of the neuropeptide is followed by a prolonged biphasic increase. Based on the profile of Met-Enk and preproenkephalin (PPE) mRNA change, we have proposed that nicotine acting at nicotinic receptors enhances the synthesis and release of striatal Met- Enk. This nicotine/Met-Enk link is indirect as glutamate N- methyl-d-aspartate and alpha-amino-3-hydroxy-5-methyli- soxazole-4-propionic acid, as well as dopamine D2 recep- tors appear to be involved [10]. Repeated exposure to nicotine results in tolerance, sensi- tization, dependence and withdrawal. In rats, anxiogenic responses [1], somatic, opiate abstinence-like, signs [14], and decreased brain reward function [5] have been described as the behavioral sequelae of withdrawal follow- ing chronic continuous infusion of nicotine. As not all smokers receive nicotine continuously, we produced a mouse model of intermittent nicotine administration and abstinence [11]. Administration of nicotine, 2 mg/kg, s.c., four times daily for 14 days produced a mild and protracted somatic abstinence syndrome characterized by rearing, jumping, shakes, abdominal constrictions, chewing, facial tremor and scratching, signs often observed during opiate withdrawal. The resemblance of the somatic nicotine absti- nence syndrome to that of opiates, along with our observa- tions that acute nicotine alters Met-Enk and PPE mRNA Neuroscience Letters 325 (2002) 67–71 0304-3940/02/$ - see front matter q 2002 Elsevier Sciencce Ireland Ltd. All rights reserved. PII: S0304-3940(02)00240-9 www.elsevier.com/locate/neulet * Corresponding author. Tel.: 11-614-292-8191; fax: 11-614- 247-7088. E-mail address: neff.6@osu.edu (M. Hadjiconstantinou).