The posterior hypothalamus (PH) is an understudied but potentially powerful site for activating descending brainstem systems that modulate pain. Little is known about how forebrain structures modify either acute nociceptive or chronic neuropathic pain in the spinal cord dorsal horn, and whether such modulation is influenced by conditions such as sex and pain type. Using female adult rats, we demonstrated in an acute thermal pain model that: 1) stimulating the PH produced opposing modulation in the spinal cord dorsal horn;2) alpha2-adrenoceptors mediated antinociception;and 3) alpha1-adrenoceptors mediated opposing, concurrent pronociception. Antinociception predominates, but is attenuated by the pronociceptive effect. Our published data show that PH stimulation also produces robust antinociception in female (n = 11) rats in a model of neuropathic pain (the chronic constriction injury model, or CCI), and pilot data demonstrated PH-induced antinociception in male (n = 3) rats with CCI. We do not know whether the extent of PH stimulation (as determined by dose of PH stimulation) affects outcomes in either sex or pain type, or whether alpha- adrenoceptors function the same way based on animal sex or pain type. Deep brain stimulation in the PH is used clinically for severe headaches with success and with few side effects, but has not been used for other types of neuropathic pain. Given the magnitude of the problem of neuropathic pain and the potential pain relief from PH stimulation demonstrated in humans and rats, the PH may be an effective site for deep brain stimulation in humans for systemic types of pain. Therefore, the aim of this proposal is to extend our findings to examine dose of PH stimulating agent (carbachol), sex, and pain type (nociceptive or neuropathic) in activation of the alpha-adrenoceptor-mediated opposing response in the spinal cord dorsal horn. In two experiments, the PH will be stimulated with four doses of carbachol (62, 12, 250, 500 nmol) alone and with intrathecal injection (IT) of alpha-adrenoceptor antagonists. We will then examine the effect of sex, pain type and dose on the mechanisms of the alpha-mediated opposing response as measured by foot withdrawal latency (FWL). In all experiments, we will take blood samples from female Sprague-Dawley rats to measure serum estradiol and progesterone to establish the hormonal milieu at the time of the experiment. Using multivariate statistical analysis, we will determine whether PH stimulation produces different responses in alpha-adrenoceptor subtype-mediated nociceptive modulation in the nociceptive and CCI model, whether female responses differ from males, and whether the dose of the PH stimulating agent affects the outcome. Results will have an impact on the development of more effective individualized clinical treatments that reduce neuropathic pain and promote analgesia in female and male adults. Because the potential treatment has already been used for neurovascular headaches, our application done in whole animals provides a critical translational piece between cellular mechanisms and work done with human cluster headaches that may have different pain mechanisms than pain occurring elsewhere in the body.
This project aims to determine whether stimulating the posterior hypothalamus relieves nociceptive and neuropathic pain in female and male adult rats via the noradrenergic descending pain modulating system. The findings will define sex and pain type differences in responses to this pain modulation and will provide a foundation for establishing individualized clinical therapies that utilize PH activation. These clinical therapies may then be individualized based on gender and may help in relieving neuropathic pain that affects millions of adults.