Hypophysectomy produces analgesia and paraventricular lesions have no effect on formalin-induced pain

Influence of hypophysectomy, ovariectomy and gonadectomy on postoperative hypersensitivity in rats

Surgical procedures lead to profound and sustained (up to 1–2 weeks) activation of the pituitary gland, resulting in changes in endocrine function. Questions remain on whether activation of the pituitary influences the threshold and development time-course of postoperative pain. To address these questions, we evaluated postoperative hypersensitivity in female and male rats with ablated pituitary and gonadal hormone productions via hypophysectomy, ovariectomy and gonadectomy, respectively. Plantar incision, a model of acute postoperative pain, or sham operation was performed on rat hind paws. Hypophysectomy, ovariectomy and gonadectomy were achieved by surgical disconnection of pituitary, ovaries and testicles, respectively. Postoperative thermal and mechanical hypersensitivity were monitored for 7 days post incision. Hypophysectomy on female and male rats produced statistically similar thermal and mechanical postoperative hypersensitivity thresholds and time-courses as compared to intact estrous female and male rats. Moreover, ovariectomy and gonadectomy did not significantly change postoperative hypersensitivity observed in control female and male animals. Our experiments demonstrate that hypophysectomy, ovariectomy and gonadectomy do not significantly impact postoperative hypersensitivity observed in normal female and male animals. These data suggest that surgery-induced changes in the endocrine system via activation of pituitary and subsequently gonadal tissues have little impact on the threshold and development of postoperative pain in female and male rats.

The role of hypothalamo-hypophyseal-adrenocortical system hormones in controlling pain sensitivity

The present review addresses analysis of data demonstrating the role of the hypothalamo-hypophyseal-adrenocortical axis (HHACA) in controlling pain sensitivity. Experiments on rats have demonstrated the analgesic effects of exogenous hormones of all components of the HHACA - corticotropin-releasing hormone (CRH), adrenocorticotropic hormone (ACTH), and glucocorticoids - in the same models, and have also shown that the opioid and non-opioid mechanisms contribute to the development of the analgesia induced by these hormones. Endogenous glucocorticoids are involved in the development of analgesia mediated by non-opioid mechanisms. Along with the non-opioid mechanisms associated with endogenous glucocorticoids, the analgesic effect of ACTH can be mediated by the opioid mechanism. Unlike the situation with ACTH, the analgesic effect of CRH is mediated exclusively by non-opioid mechanisms, one of which is associated with HHACA hormones, while the other, appearing only on systemic administration, is not associated with these hormones. The actions of glucocorticoids on pain are mediated by neurons in the central gray matter of the midbrain.

Prolactin response to formalin is related to the acute nociceptive response and it is attenuated by combined application of different stressors

Subcutaneous injection of diluted formalin (0.2 ml of 4% solution/100 g BW) can influence the increase of plasma epinephrine levels in rats exposed to exteroceptive (handling, immobilization), as well as to interoceptive stressors (insulin-induced hypoglycemia), without having any effect on norepinephrine release. In the present studies, the effect of the above-mentioned stressors has been investigated on formalin-induced prolactin (PRL) and corticosterone secretion. Administrations of formalin via chronically implanted subcutaneous cannula into the hind limb without handling induce an immediate increase in both plasma PRL and corticosterone levels. While PRL concentration reaches its peak value within 5 min then returns to the basal level by the end of the 30th min, corticosterone level also starts to rise immediately after formalin administration reaching its highest concentration within 15-30 min, but it remains at this high level during the next 60 min, then it declines and returns to the pre-injection level. Application of formalin to animals exposed to different heterotypic stressors (like handling or insulin-induced hypoglycemia) produces an attenuated PRL response, while plasma corticosterone levels induced by the same nociceptive component remained unchanged. Combinations of formalin injection with immobilization also show an attenuated PRL response. The present data indicate that plasma PRL response to formalin is related to its acute nociceptive phase, and application of different stressors prior to formalin injection significantly attenuate plasma PRL levels, while it does not influence corticosterone responses.

Noradrenergic pain modulation

Norepinephrine is involved in intrinsic control of pain. Main sources of norepinephrine are sympathetic nerves peripherally and noradrenergic brainstem nuclei A1-A7 centrally. Peripheral norepinephrine has little influence on pain in healthy tissues, whereas in injured tissues it has variable effects, including aggravation of pain. Its peripheral pronociceptive effect has been associated with injury-induced expression of novel noradrenergic receptors, sprouting of sympathetic nerve fibers, and pronociceptive changes in the ionic channel properties of primary afferent nociceptors, while an interaction with the immune system may contribute in part to peripheral antinociception induced by norepinephrine. In the spinal cord, norepinephrine released from descending pathways suppresses pain by inhibitory action on alpha-2A-adrenoceptors on central terminals of primary afferent nociceptors (presynaptic inhibition), by direct alpha-2-adrenergic action on pain-relay neurons (postsynaptic inhibition), and by alpha-1-adrenoceptor-mediated activation of inhibitory interneurons. Additionally, alpha-2C-adrenoceptors on axon terminals of excitatory interneurons of the spinal dorsal horn possibly contribute to spinal control of pain. At supraspinal levels, the pain modulatory effect by norepinephrine and noradrenergic receptors has varied depending on many factors such as the supraspinal site, the type of the adrenoceptor, the duration of the pain and pathophysiological condition. While in baseline conditions the noradrenergic system may have little effect, sustained pain induces noradrenergic feedback inhibition of pain. Noradrenergic systems may also contribute to top-down control of pain, such as induced by a change in the behavioral state. Following injury or inflammation, the central as well as peripheral noradrenergic system is subject to various plastic changes that influence its antinociceptive efficacy.

Pituitary-adrenocortical responses to persistent noxious stimuli in the awake rat: endogenous corticosterone does not reduce nociception in the formalin test

Although glucocorticoids inhibit inflammation and are used to treat painful inflammatory rheumatic diseases, the contribution, if any, of endogenous pituitary-adrenocortical activity to the control of pain remains unclear. We report that injection of dilute formalin into the hindpaw not only evokes inflammation and pain-related behavior, but it also increases ACTH and corticosterone to a greater extent than restraint and saline injection alone. This difference was particularly robust during the final periods of pain-related behavior in the formalin test, when the ACTH and corticosterone (B) levels in the restraint/saline control group had returned to normal. These results indicate that formalin-evoked increases in ACTH and B reflect nociceptive input, rather than the stress associated with handling. To test the hypothesis that the formalin-induced increase in corticosterone reduces pain and inflammation, we next evaluated the effect of adrenalectomy (to prevent activation of glucocorticoid receptors) or high-dose dexamethasone (to saturate glucocorticoid receptors) on nociceptive processing in the formalin test. Neither adrenalectomy nor dexamethasone changed behavioral or cardiovascular nociceptive responses. Furthermore, the increases in blood pressure and heart rate produced by formalin may not be mediated by adrenomedullary catecholamine release. In addition, we conclude that the nociceptive component of the formalin stimulus is sufficient to activate the pituitary-adrenocortical system in the awake rat, but that the resulting release of corticosterone does not feed back and reduce nociceptive processing.

Electrical stimulation of the cingulum bundle and surrounding cortical tissue reduces formalin-test pain in the rat

Surgical lesions of the cingulum bundle in humans produce marked decreases in severe pain associated with cancer, reflex sympathetic dystrophy and other forms of chronic pain. Similarly, a temporary block of the anterior cingulum bundle in the rat by microinjection of lidocaine produces significant decreases in formalin-pain and reduces autotomy following peripheral neurectomy. The present study explored the effect of electrical stimulation of the cingulum bundle/surrounding cortical tissue (CB/CT) on tonic pain in the rat. Experiment 1 examined changes in formalin-induced pain responses following a 2.5-min period (30 s/min for 5 min) of electrical stimulation of the CB/CT 15 min prior to the formalin injection. The stimulation produced a significant reduction of first-period and second-period pain responses. Experiment 2 examined changes in formalin-induced pain responses following a 2.5-min period (30 s/min for 5 min) of electrical stimulation of the CB/CT 20 min following the formalin injection. The stimulation produced a dramatic reduction in second-period pain responses which persisted for the duration of the 35-min post-stimulation test period. The fact that either electrical stimulation or surgical section of the CB/CT produces pain relief suggests that this region serves a complex role in pain processing. Since the cingulum bundle has major connections with all other structures of the limbic system, it is possible that electrical stimulation disrupts patterned activity in the system, which is known to play an especially important role in the affective-motivational dimension of pain.