The CA1 hippocampal serotonin alterations involved in anxiety-like behavior induced by sciatic nerve injury in rats

Abstract

Several clinical and experimental studies reported the anxiety as one of the neuropathic pain comorbidities; however, the mechanisms involved in this comorbidity are incompletely cleared. The current study investigated the consequence of pain induced by peripheral neuropathy on the serotonin (5-HT) level of the CA1 region of the hippocampus, which is known as a potential reason, for anxiety associated with neuropathic pain. In this manner, 72 male rats were inconstantly subdivided into three experimental groups as follows: control, sham, and chronic constriction injury (CCI). Neuropathic pain was initiated by the CCI of the sciatic nerve, and then, mechanical allodynia, thermal hyperalgesia, and anxiety-like behavior were evaluated using the von Frey filaments, radiant heat, open field test (OFT), and elevated plus maze (EPM) respectively. To investigate the probable mechanisms, the in vivo extracellular levels of 5-HT were assessed by microdialysis and using reverse-phase high-pressure liquid chromatography (HPLC) in the CA1 region of hippocampus on days 16 and 30 post-CCI. Our data suggested that CCI caused anxiety-like behavior in OFT and EPM test. 5-HT concentration in the CA1 region of the hippocampus significantly (F=43.8, p=0.000) reduced in CCI rats, when the pain threshold was minimum. Nevertheless, these alterations reversed while the pain threshold innate increased. Neuropathic pain, initiated by constriction of the sciatic nerve can induce anxiety-like behavior in rats. This effect accompanies the reduction in 5-HT concentration in the CA1 region of the hippocampus. When the pain spontaneously alleviated, 5-HT level increased and anxiety-like behavior relieved.