en Behavioral Neuroscience Behavioral Neuroscience Original Original <p dir="ltr" style="text-align: justify;"><strong>Introduction:</strong> A close interaction exists between the brain opioid and serotonin (5-HT)&nbsp;neurotransmitter systems. Brain neurotransmitter 5-HT plays an important role in the regulation&nbsp;of reward-related processing. However, a few studies have investigated the potential role of&nbsp;5-HT2A receptors in this behavior. Therefore, the aim of the present study was to assess the<br> influence of morphine and Conditioned Place Preference (CPP) on the density of 5-HT2A receptor&nbsp;in neurons of rat hippocampal formation.<br> <strong>Methods:</strong> Morphine (10 mg/kg, IP) was injected in male Wistar rats for 7 consecutive days&nbsp;(intervention group), but control rats received just normal saline (1 mL/kg, IP). We used a&nbsp;hotplate test of analgesia to assess induction of tolerance to analgesic effects of morphine on days&nbsp;1 and 8 of injections. Later, two groups of rats were sacrificed one day after 7 days of injections,&nbsp;their whole brains removed, and the striatum and PFC immediately dissected. Then, the NR1&nbsp;gene expression was examined with a semi-quantitative RT-PCR method.<br> <strong>Results:</strong> Our data showed that the maximum response was obtained with 2.5 mg/kg of morphine.&nbsp;The density of 5-HT2A receptor in different areas of the hippocampus increased significantly at&nbsp;sham-morphine and CPP groups (P<0.05). On the other hand, the CPP groups had more 5-HT2A&nbsp;receptors than sham-morphine groups and also the sham-morphine groups had more 5-HT2A&nbsp;receptors than the control groups.<br> <strong>Conclusion:</strong> We concluded that the phenomenon of conditioned place preference induced by&nbsp;morphine can cause a significant increase in the number of serotonin 5-HT2A receptors in neurons&nbsp;of all areas of hippocampus.</p> Conditioned place preference, Morphine, 5-HT2A receptors, Hippocampus, Rats 249 258 http://bcn.iums.ac.ir/browse.php?a_code=A-10-93-4&slc_lang=en&sid=1 Rabie Mohammadi 1370031947532846007925 1370031947532846007925 No Department of Anatomy, Neuroscience Research Center, Faculty of Medicine, Golestan University of Medical Sciences, Gorgan, Iran. Mehrdad Jahanshahi mejahanshahi@yahoo.com 1370031947532846007926 1370031947532846007926 Yes Department of Anatomy, Neuroscience Research Center, Faculty of Medicine, Golestan University of Medical Sciences, Gorgan, Iran. Seyed Behnamedin Jameie 1370031947532846007927 1370031947532846007927 No Department of Anatomy, School of Medicine, Iran University of Medical Sciences, Tehran, Iran