Sex Differences and Role of Gonadal Hormones on Glutamate LevelAfter Spinal Cord Injury in Rats: A Microdialysis Study.

INTRODUCTION: Sex differences in outcomes of Spinal Cord Injury (SCI) suggest a sex-hormone-mediated effect on post-SCI pathological events, including glutamate excitotoxicity. This study aimed to investigate the importance of gonadal hormones on glutamate release subsequent to SCI in rats. METHODS: After laminectomy at T8-T9, an electrolytic lesion was applied to the spinothalamic tracts of male and female rats. Using spinal microdialysis, we assessed glutamate levels at the site of lesion in both intact and gonadectomized rats for 4 hours. In this way, we examined the sex differences in the glutamate concentrations. RESULTS: The peak retention time of glutamate level was 10.6 min and spinal glutamate concentration reached a maximum level 40 min following SCI. In male SCI rats, gonadectomy caused a significant elevation of glutamate level (P<0.001) following injury which was maximum 40 min post-SCI as well. However, no significant alterations were seen in gonadectomized female rats. CONCLUSION: The significant differences in glutamate levels between both intact and gonadectomized SCI male and female rats show the sex-hormone-related mechanisms underlying the molecular events in the second phase of SCI. It seems that the role of male gonadal hormones to prevent glutamate excitotoxicity is more prominent. The exact mechanisms of these hormones on the functional recovery after SCI should be clarified in further studies.

Estradiol attenuates spinal cord injury-related central pain by decreasing glutamate levels in thalamic VPL nucleus in male rats.

Central neuropathic pain (CNP) is a complicated medical problem that involves both the spinal and supraspinal regions of the central nervous system. Estrogen, a neuroprotective agent, has been considered a possible candidate for CNP treatment. In this study, we examined the effects of a single dose of 17ß-estradiol on glutamate levels in the ventral posterolateral (VPL) nucleus of the rat thalamus. Furthermore, we determined whether there was a correlation between glutamate levels and neuropathic pain induced by unilateral electrolytic spinothalamic tract (STT) lesion. STT lesioning was performed in male Wistar rats at the T8-T9 vertebrae; rats were then administered 17ß-estradiol (4 mg/kg, i.p.) 30 min after injury. Glutamate samples were collected using a microdialysis probe and quantified by high performance liquid chromatography. Mechanical allodynia (MA) and thermal hyperalgesia (TH) thresholds were measured pre-injury and 7, 14, and 28 days post-injury. We found that STT lesion significantly increased glutamate levels in the ipsilateral VPL nucleus 14 and 28 days post-injury; this was accompanied by allodynia and hyperalgesia in the hind paws of the rats. Administering 17ß-estradiol to the rats decreased glutamate levels in the ipsilateral VPL nucleus and significantly increased MA and TH thresholds. These results suggest that glutamate in the VPL nucleus of the thalamus is involved in the pathology of neuropathic pain after STT injury; furthermore, 17ß-estradiol may attenuate this neuropathic pain by decreasing glutamate levels.

Cannabinoid receptors in the basolateral amygdala are involved in the potentiation of morphine rewarding properties in the acquisition, but not expression of conditioned place preference in rats.

Several studies show the role of the basolateral amygdala (BLA) in drug-seeking, relapse and the brain׳s emotional systems. Several lines of evidence indicate a functional interaction between opioid and endogenous cannabinoid systems. In the present study, we investigated the role of intra-BLA cannabinoid CB1 receptors in the potentiation, acquisition and expression of morphine-induced conditioned place preference (CPP). One-hundred and forty-two adult male Wistar rats weighing 230-280g were bilaterally implanted by two separate cannulae into the BLA. The CPP paradigm was done, and conditioning score and locomotor activity were recorded by Ethovision software. Results showed that intra-BLA administration of different doses of WIN55,212-2 (1, 2 and 4mmol/0.3µl DMSO) as a cannabinoid receptor agonist during the conditioning phase induced place preference in animals that received the ineffective (2mg/kg) dose of morphine compared to respective control group in saline-treated animals. On the other hand, intra-BLA injection of the cannabinoid CB1 receptor antagonist AM251 (45 and 90µmol/0.3µl DMSO) during the 3-day conditioning phase reduced morphine-induced CPP. Furthermore, microinjection of both AM251 (15, 45 and 90µmol) and WIN55,212-2 (1-4mmol), into the BLA had no effect on the expression of morphine (5mg/kg)-induced CPP. Our findings suggest that cannabinoid CB1 receptors in the BLA are involved in the development of reward-related behaviors and they can potentiate the rewarding effects of morphine. It seems that the glutamatergic projection from the BLA to the nucleus accumbens and reward-related learning in the hippocampus may be involved in the acquisition and expression of opioid reward-related behaviors in rats.

Streptozotocin-induced diabetes affects the development and maintenance of morphine reward in rats.

Diabetes mellitus is a chronic illness that has been associated with the decrease of insulin in type I diabetes. Insulin has an impact not only on the direct control of food intake and plasma glucose levels, but also on brain pathways associated with reward. It affects brain reward pathways through regulation of the dopamine (DA) transporter (DAT). Moreover, it has been found to affect the ability of drugs that target the DA system. In the present study, the effects of streptozotocin (STZ)-induced diabetes on the acquisition (development) and maintenance of morphine-induced conditioned place preference (CPP) were investigated in rats. Forty adult male Albino Wistar rats were used in these experiments. For induction of diabetes, STZ was administered at a dose of 60 mg/kg. After seven days, the CPP paradigm was done; conditioning score and locomotor activity were recorded by Ethovision software. The results showed that diabetes significantly increased the magnitude of conditioning scores, acquisition of morphine-induced CPP in compared to naive animals (P<0.05). Moreover, in the diabetic group, there were significant differences among conditioning scores in the post-conditioning phase and the last four days (7th-10th), but these differences elongated up to 10 days after the CPP protocol while the extinction period was eight days in the naive group. Our findings indicated that the magnitude and maintenance of morphine rewarding properties have been changed in STZ-induced diabetic animals. It seems that a level of insulin and their receptors are involved in the development and maintenance of morphine-induced CPP in the rats.