Molecular Studies on the Protective Effect of Nicotine in Adult-Onset Hypothyroidism-Induced Impairment of Long-Term Potentiation K.H. Alzoubi, 1 A.M. Aleisa, 2 and K.A. Alkadhi 1* ABSTRACT: We have recently shown that chronic nicotine treatment reverses hypothyroidism-induced learning and memory impairment. Chronic nicotine treatment also reverses the hypothyroidism-induced impairment of long-term potentiation (LTP). Analysis of LTP associated key signaling molecules revealed that chronic nicotine treatment pre- vented the hypothyroidism-induced reduction of the basal phosphotrans- ferase activity of CaMKII and protein levels of P-CaMKII. In addition, the failure of high frequency stimulation to increase the levels of P- CaMKII in hypothyroid rats was reversed by nicotine treatment, suggest- ing that the neuroprotective effect of nicotine during hypothyroidism involved activation of CaMKII. Furthermore, chronic nicotine treatment reverses the hypothyroidism-induced elevated phosphatase activity and protein levels of calcineurin, a phosphatase that regulates CaMKII acti- vation. We conclude that the neuroprotective effects of nicotine in adult-onset hypothyroidism may result from restoration of CaMKII and calcineurin activity. V V C 2006 Wiley-Liss, Inc. KEY WORDS: anesthetized rat; hippocampus; CaMKII; calcineurin; calmodulin; neurogranin; PKCg INTRODUCTION Nicotine enhances cognitive function both in clinical experiments (Levin et al., 1998) and animal studies (Levin et al., 1990). Nicotine also affects synaptic plasticity. In hippocampal slice experiments, it has been shown that both acute and chronic administration of nicotine low- ers the threshold of induction of long-term potentiation (LTP; (Fujii et al., 1999). Furthermore, nicotine alleviates memory impairment asso- ciated with many health conditions including mental stress (Aleisa et al., 2006b), aging (e.g., White and Levin, 2004), Alzheimer’s disease (Wilson et al., 1995; Newhouse et al., 2001), schizophrenia (Levin et al., 1996b), brain lesions (e.g., Levin et al., 1993b), attention deficit hyperactivity disorder (Levin et al., 1998), and Parkinson’s disease (New- house et al., 1997; Maggio et al., 1998). In addition, it has been reported that the incidence of Alzheimer’s and Parkinson’s diseases is lower in smokers compared with that in nonsmokers (Fratiglioni and Wang, 2000). Nicotine also reverses the inhibitory effect of proinflam- matory cytokines on LTP (Curran and O’Connor, 2003). Hypothyroidism causes a wide range of central nervous system (CNS) dysfunctions, including impair- ment of cognition (Gerges et al., 2004b) and synaptic plasticity in adult onset (Gerges et al., 2001; Gerges and Alkadhi, 2004; Alzoubi et al., 2005; Gerges et al., 2005) as well as in developmental hypothyroidism (e.g., Sui et al., 2005). The expression of LTP, a cellular correlate of learning and memory (Kandel, 2001), is initiated when gluta- mate, released by high frequency stimulation (high-fre- quency stimulation (HFS)), activates NMDA receptors on the postsynaptic membrane, causing elevation of in- tracellular calcium levels (Malenka et al., 1988). This transient increase of intracellular calcium leads to disso- ciation of the neurogranin–calmodulin complex (Geren- dasy and Sutcliffe, 1997; Krucker et al., 2002). The free calmodulin forms a calcium–calmodulin complex that binds to and activates CaMKII, triggering autophospho- rylation at the Thr-286 residue (Wang and Kelly, 1995). The newly phosphorylated CaMKII (P-CaMKII) activates a variety of substrates important in LTP expres- sion, including synapsin-1 and a-amino-3-hydroxy-5- methyl-4-isoxazole propionic acid (AMPA) glutamate receptor (Nayak et al., 1996; Barria et al., 1997). The activation of these substrates by P-CaMKII is sus- tained, even when calcium returns to basal levels, until P-CaMKII is dephosphorylated by protein phosphatases including calcineurin (Fukunaga and Miyamoto, 2000). Therefore, postsynaptic CaMKII activity is necessary and sufficient to generate and maintain LTP in area CA1 of the hippocampus (e.g., Malenka et al., 1989; Giese et al., 1998). Recently, we reported that hypothyr- oidism reduces the basal levels and activity of P-CaM- KII and blocks HFS-induced P-CaMKII activation in the CA1 area of the hippocampus, which may, at least partially, explain hypothyroidism-induced impairment of LTP (Gerges et al., 2005). In the present study, we have examined the role of signaling molecules involved in the nicotine-induced antagonism of the inhibitory effect of hypothyroidism on LTP. MATERIALS AND METHODS All animal experiments were carried out in accord- ance with the NIH Guide for Care and Use of Labo- ratory Animals and were approved by the University 1 Department of Pharmacological and Pharmaceutical Sciences, College of Pharmacy, University of Houston, Houston, Texas 77204; 2 College of Pharmacy, King Saud University, Riyadh, Saudi Arabia *Correspondence to: Karim A. Alkadhi, Department of Pharmacological and Pharmaceutical Sciences, University of Houston, Houston, TX 77204- 5515, USA. E-mail: kalkadhi@uh.edu Accepted for publication 7 July 2006 DOI 10.1002/hipo.20217 Published online 8 August 2006 in Wiley InterScience (www.interscience. wiley.com). HIPPOCAMPUS 16:861–874 (2006) V V C 2006 WILEY-LISS, INC.