ULTRA-LOW DOSE NALTREXONE POTENTIATES THE ANTICONVULSANT EFFECT OF LOW DOSE MORPHINE ON CLONIC SEIZURES H. HONAR, a K. RIAZI, a H. HOMAYOUN, a H. SADEGHIPOUR, a N. RASHIDI, b M. R. EBRAHIMKHANI, a N. MIRAZI c AND A. R. DEHPOUR a * a Department of Pharmacology, School of Medicine, Tehran Univer- sity of Medical Sciences, P.O. Box 13145-784, Tehran, Iran b Department of Biology, School of Science, University of Tehran, Tehran, Iran c Department of Biology, School of Science, Bu Ali Sina University, Hamadan, Iran Abstract—Significant potentiation of analgesic effects of opi- oids can be achieved through selective blockade of their stimulatory effects on intracellular signaling pathways by ultra-low doses of opioid receptor antagonists. However, the generality and specificity of this interaction is not well un- derstood. The bimodal modulation of pentylenetetrazole- induced seizure threshold by opioids provide a model to assess the potential usefulness of this approach in seizure disorders and to examine the differential mechanisms in- volved in opioid anti- (morphine at 0.5–3 mg/kg) versus pro- convulsant (20 –100 mg/kg) effects. Systemic administration of ultra-low doses of naltrexone (100 fg/kg–10 ng/kg) signifi- cantly potentiated the anticonvulsant effect of morphine at 0.5 mg/kg while higher degrees of opioid receptor antago- nism blocked this effect. Moreover, inhibition of opioid- induced excitatory signaling by naltrexone (1 ng/kg) un- masked a strong anticonvulsant effect for very low doses of morphine (1 ng/kg–100 g/kg), suggesting that a presumed inhibitory component of opioid receptor signaling can exert strong seizure-protective effects even at very low levels of opioid receptor activation. However, ultra-low dose naltrex- one could not increase the maximal anticonvulsant effect of morphine (1–3 mg/kg), possibly due to a ceiling effect. The proconvulsant effects of morphine on seizure threshold were minimally altered by ultra-low doses of naltrexone while be- ing completely blocked by a higher dose (1 mg/kg) of the antagonist. The present data suggest that ultra-low doses of opioid receptor antagonists may provide a potent strategy to modulate seizure susceptibility, especially in conjunction with very low doses of opioids. © 2004 IBRO. Published by Elsevier Ltd. All rights reserved. Key words: excitatory opioid receptor, ultra-low dose naltrex- one, pentylenetetrazole, clonic seizure threshold, mice. Receptors of opioid peptides belong to the super family of G-protein-coupled receptors. These receptors exert their diverse effects through activation of guanosine triphosphate-binding protein and effectors such as adenyl- ate cyclase and ion channels (Childers, 1991; Kieffer, 1995). Opioids exert a wide range of their effects, including their analgesic properties, through coupling to inhibitory G i /G o proteins, leading to decreased neuronal cyclic AMP levels, decreased Ca ++ conductance, shortening of action potential duration (APD) and decreased neurotransmitter release (Williams et al., 2001, for review). However, some recent data suggest that opioids may also have direct stimulatory effects on intracellular signaling mechanisms including stimulation of adenylyl cyclase, increasing cal- cium influx, prolongation of APD and increased neuronal excitability (Chen et al., 1988; Shen and Crain, 1989; Crain and Shen, 1990; Gintzler and Xu, 1991; Cruciani et al., 1993; Mao et al., 1994; Smart and Lambert, 1996). According to Crain and colleagues (Shen and Crain 1997; Crain and Shen, 1995, 2000) very low doses of opioids can selectively activate an excitatory G s protein- coupled signaling pathway that may account for the men- tioned stimulatory effects of opioids. It has been suggested that ultra-low doses of an opioid receptor antagonist can selectively block this sensitive excitatory mechanisms and unmask a potent inhibitory tone that is induced by opioids at concentrations much lower than their minimal effective doses in antinociception tests. Accordingly, evidence has been provided by several basic and clinical groups in favor of the significant potentiation of opioid-induced antinoci- ception by ultra-low doses of opioid receptor antagonists (Levine et al., 1988; Crain and Shen, 1995; Gan et al., 1997; Powell et al., 2002; Cruciani et al., 2003). These putative excitatory mechanisms may also be responsible, at least partly, for the “paradoxical” phenomenon of opioid- induced hyperalgesia that is shown to be reversible by ultra-low doses of opioid antagonists (Wiesenfeld-Hallin et al., 1991; Crain and Shen, 2001). In addition, pretreatment with ultra-low doses of opioid receptor antagonists can inhibit the development of tolerance to morphine-induced analgesia. This effect is suggested to be through blockade of a switch from early dominant inhibitory to later dominant excitatory signaling by chronic opioid treatment (Powell et al., 2002; Low et al., 2003; Wang et al., 2003). While it is not certain to what extent the reported coupling of opioid receptors to G s second messenger system in pain-related pathways may be observed in other neural tissues, the excitatory pathway activated by very low doses of opioids may be of functional significance beyond the antinocicep- tion paradigms. One important field for investigation would be the mod- ulatory effects of opioids on seizure susceptibility. There is *Corresponding author. Tel: +98-21-611-2802; fax: +98-21-640-2569. E-mail address: dehpour@yahoo.com (A. R. Dehpour). Abbreviations: APD, action potential duration; NTX, naltrexone; PTZ, pentylenetetrazole. Neuroscience 129 (2004) 733–742 0306-4522/04$30.00+0.00 © 2004 IBRO. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.neuroscience.2004.08.029 733