The opioid and serotonergic systems are closely involved in pain processing and mood disorders. The aim of this study was to assess the influence of systemic morphine on cerebral serotonin 2A receptor (5-HT 2A ) binding in dogs using SPECT with the 5-HT 2A radioligand 123 I-5I-R91150. Methods: 5-HT 2A binding was estimated with and without morphine pretreatment in 8 dogs. The 5-HT 2A binding indices in the frontal, parietal, temporal, and occipital cortex and in the subcortical region were obtained by semiquantification. Results: A significantly decreased 5-HT 2A binding index was found in the morphine group for the right (morphine, 1.41 6 0.06; control, 1.52 6 0.10) and left (morphine, 1.44 6 0.08; control, 1.55 6 0.11) frontal cortices, with P 5 0.012 and P 5 0.040, respectively. No significant differences were noted for the other regions. Conclusion: Morphine decreased the frontocortical 5-HT 2A availability, confirming an interaction between the 5-HTergic and the opioid systems. Whether this interaction is caused by decreased receptor density due to direct internalization or is the result of indirect actions, such as increased endogenous serotonin release, remains to be elucidated.Key Words: serotonin (5-HT); 5-HT 2A ; morphine; SPECT; frontal cortex Nucl Med 2012; 53:1969 53: -1973 53: DOI: 10.2967 It is well documented that there is a close interaction between the brain opioid and serotonin (5-hydroxytryptamine [5-HT]) neurotransmitter systems (1-3). Activation of the m-opioid receptor system leads to 5-HTergic changes, including increased release of synaptic 5-HT, as was shown in several brain regions and in the spinal cord in various in vivo microdialysis studies (1,2). Furthermore, pharmacologic and behavioral studies pointed out that the 5-HTergic system is implicated in spinal pain transmission, modulation, and control and particularly in the mediation of opioid analgesia (3,4). Morphine-induced analgesia is thought to rely in part on the indirect activation of descending inhibitory 5-HTergic pathways (3). Consequently, antidepressants that inhibit 5-HT reuptake, such as fluvoxamine and fluoxetine, are also known to have analgesic properties, especially in chronic pain states (5,6).
JBeside its role in pain modulation (7-9), 5-HT 2A receptor (5-HT 2A ) gained interest over the last few decades due to its involvement in several neurologic disorders in humans, including anxiety (10), depression (11), schizophrenia (12), and obsessive-compulsive disorder (13). More specifically, atypical antipsychotic drugs acting as 5-HT 2A antagonists and inverse agonists were seen to be effective in reducing the symptoms of these diseases (10,14). Because the opioid system has been shown to be involved in several of these mood disorders as well (15), the interaction between m-opioid receptors and 5-HT 2A at an emotional or cognitive level gained interest, partly with regard to common opioid side effects such as tolerance and addiction that are thought to have a 5-HTergic component as well. Consequently, several in v...