Investigating the role of the hypothalamic supraoptic nucleus in nociception in the rat

How Sex Hormones Affect Migraine: An Interdisciplinary Preclinical Research Panel Review

Sex hormones and migraine are closely interlinked. Women report higher levels of migraine symptoms during periods of sex hormone fluctuation, particularly during puberty, pregnancy, and perimenopause. Ovarian steroids, such as estrogen and progesterone, exert complex effects on the peripheral and central nervous systems, including pain, a variety of special sensory and autonomic functions, and affective processing. A panel of basic scientists, when challenged to explain what was known about how sex hormones affect the nervous system, focused on two hormones: estrogen and oxytocin. Notably, other hormones, such as progesterone, testosterone, and vasopressin, are less well studied but are also highlighted in this review. When discussing what new therapeutic agent might be an alternative to hormone therapy and menopause replacement therapy for migraine treatment, the panel pointed to oxytocin delivered as a nasal spray. Overall, the conclusion was that progress in the preclinical study of hormones on the nervous system has been challenging and slow, that there remain substantial gaps in our understanding of the complex roles sex hormones play in migraine, and that opportunities remain for improved or novel therapeutic agents. Manipulation of sex hormones, perhaps through biochemical modifications where its positive effects are selected for and side effects are minimized, remains a theoretical goal, one that might have an impact on migraine disease and other symptoms of menopause. This review is a call to action for increased interest and funding for preclinical research on sex hormones, their metabolites, and their receptors. Interdisciplinary research, perhaps facilitated by a collaborative communication network or panel, is a possible strategy to achieve this goal.

Protective effect of HCN2-induced SON sensitization on chronic visceral hypersensitivity in neonatal-CRD rat model

Abnormal brain-gut interactions contribute to the development of chronic visceral hypersensitivity (CVH), which is the pivotal feature of irritable bowel syndrome (IBS). Despite the consensus with respect to the vital role of hyperpolarization-activated cyclic nucleotide-gated 2 (HCN2) channels in promoting painful symptoms in the peripheral nervous system, we identified that the upregulation of HCN2 in supraoptic nucleus (SON) was involved in the modulation of CVH in rat model of neonatal colorectal distention (n-CRD). Specifically, colorectal distention (CRD) upregulated the expression of c-Fos in SON in adult CVH rats, indicating the involvement of SON sensitazation in visceral sensation. Moreover, the administration of ZD7288 (the pan-HCN channel inhibitor) rather than 8-Br-cAMP (the non-specific HCN channel agonist) aggravated the CVH symptoms and reduced the phosphorylation level of CaMKII-CREB cascade. Together, the findings indicated that the upregulation of supraoptic HCN2 contributed to the sensitization of SON, which had protective effects on the modulation of CVH with the involvement of CaMKII-CREB cascade in n-CRD rat model.

Oxytocin in the periaqueductal gray mainly comes form the hypothalamic supraoptic nucleus to participate in pain modulation

Oxytocin (OXT) that effects the nociception process is mainly synthesized and secreted in the hypothalamic supraoptic nucleus (SON). Although the periaqueductal gray (PAG) hardly synthesizes OXT, OXT in PAG also plays a role in pain regulation. The communication investigates whether OXT in the PAG comes from SON to influence pain modulation. RT-PCR was used to analyze OXT mRNA expression and radioimmunoassay to measure OXT concentration. The results showed that (1) pain stimulation enhanced OXT mRNA expression in the SON at 10 min (268.1 ± 39.2%, p < 0.001) and 20 min (135.4±37.9%, p < 0.05) treatment and did not change in the PAG; (2) OXT level increase in SON perfusion liquid during pain stimulation [236.7±22.1% at 10 min (p < 0.001), 223.1±12.4% at 20 min (p<0.001), 56.1 ± 15.7% at 30 min (p < 0.01) and 11.2±14.2% at 40 min] was earlier than that in PAG perfusion liquid [17.8±9.7% at 10 min, 375.6±35.1% at 20 min (p <  0.001), 123.2±17.7% at 30 min (p <  0.001) and 52.7±22.4% at 40 min (p < 0.05)]; (3) SON excitation (L-glutamate sodium microinjection) induced OXT level increase in PAG perfusion liquid in a dose-dependent manner; (4) the bilateral SON cauterization completely controlled and the right SON cauterization partly reversed the pain stimulation induced-OXT concentration increase in PAG perfusion liquid. The data suggested that OXT in PAG came from SON, which might influence the pain process.

Oxytocin in hypothalamic supraoptic nucleus is transferred to the caudate nucleus to influence pain modulation

Oxytocin (OXT), which is synthesized and secreted in the hypothalamic supraoptic nucleus (SON), is the most important bioactive substance in SON regulating pain process. Our previous study has pointed that OXT in the caudate nucleus (CdN) plays a role in pain modulation. The communication was designed to investigate the source of OXT in the rat CdN during pain process using the methods of push-pull perfusion and radioimmunoassay. The results showed that (1) pain stimulation increased the OXT concentration in the CdN perfusion liquid; (2) SON cauterization inhibited the increase of OXT concentration in CdN perfusion liquid induced by the pain stimulation, which role in both sides of SON cauterization was stronger than that in one side of SON cauterization; and (3) SON microinjection of l-glutamate sodium, which excited the SON neurons, increased OXT concentration in the CdN perfusion liquid. The data suggested that OXT in the CdN was influenced by SON during pain process, i.e., OXT in the SON might be transferred to the CdN to influence pain modulation.

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 interaction between the oxytocin and pain modulation in headache patients

Oxytocin (OXT), a nonapeptide hormone of posterior pituitary, reaches the central nervous system from systemic blood circulation with a difficulty because of the blood-brain barrier (BBB). The interest has been expressed in the use of the nasal route for delivery of OXT to the brain directly, exploiting the olfactory pathway. Our previous study has demonstrated that OXT in the central nervous system rather than the blood circulation plays an important role in rat pain modulation. The communication tried to investigate the interaction between the OXT and pain modulation in Chinese patients with headache to understand the OXT effect on human pain modulation. The results showed that (1) intranasal OXT could relieve the human headache in a dose-dependent manner; (2) OXT concentration in both plasma and cerebrospinal fluid (CSF) increased significantly in headache patients in relation with the pain level; and (3) there was a positive relationship between plasma and CSF OXT concentration in headache patients. The data suggested that intranasal OXT, which was delivered to the central nervous system through olfactory region, could treat human headache and OXT might be a potential drug of headache relief by intranasal administration.

Oxytocin, but not arginine vasopressin is involving in the antinociceptive role of hypothalamic supraoptic nucleus

Our previous study has demonstrated that the hypothalamic supraoptic nucleus (SON) plays a role in pain modulation. Oxytocin (OXT) and arginine vasopressin (AVP) are the important hormones synthesized and secreted by the SON. The experiment was designed to investigate which hormone was relating with the antinociceptive role of the SON in the rat. The results showed that (1) microinjection of L-glutamate sodium into the SON increased OXT and AVP concentrations in the SON perfusion liquid, (2) pain stimulation induces OXT, but not AVP release in the SON, and (3) intraventricular injection (pre-treatment) with OXT antiserum could inhibit the pain threshold increase induced by SON injection of L-glutamate sodium, but administration of AVP antiserum did not influence the antinociceptive role of SON stimulation. The data suggested that the antinociceptive role of the SON relates to OXT rather than AVP.

Norepinephrine regulates arginine vasopressin secretion in hypothalamic paraventricular nucleus relating with pain modulation

Our previous study has pointed that arginine vasopressin (AVP) and norepinephrine (NA) are two most important bioactive substances that play a role in hypothalamic paraventricular nucleus (PVN) regulating pain process. The communication was designed to investigate the interaction between AVP and NA in the rat PVN during the pain process. We used the potassium iontophoresis inducing tail-flick to test the pain threshold, PVN push-pull perfusion to collect the samples, high performance chromatography (HPLC) to determine the NA concentration and radioimmunoassay (RIA) to measure the AVP concentration. The results showed that (1) pain stimulation increased both NA and AVP concentrations in the PVN perfusion liquid; (2) PVN administration of l-glutamate sodium increased AVP, not NA concentration in the PVN perfusion liquid; (3) AVP or d(CH(2))(5)Tyr(Et)DAVP (AVP-receptor antagonist) neither changed pain threshold, nor influenced NA concentration in the PVN perfusion liquid; (4) Microinjection of NA into PVN could increase pain threshold in a dose-dependent manner, while PVN administration with phentolamine (alpha-receptor antagonist), not propranolol (beta-receptor antagonist) decreased pain threshold; (5) Administration of NA increased AVP concentration, while phentolamine, not propranolol decreased AVP concentration in the PVN perfusion liquid. These data suggested that it is through alpha-receptor rather than beta-receptor, NA induced PVN secretion of AVP that was delivered to the related brain regions to participate in pain modulation.

Arginine vasopressin antinociception in the rat nucleus raphe magnus is involved in the endogenous opiate peptide and serotonin system

Arginine vasopressin (AVP) in the nucleus raphe magnus (NRM) has been implicated in antinociception. This communication was designed to investigate which neuropeptide and neurotransmitter are involved in AVP antinociception in the rat NRM. The results showed that (1) in the NRM perfuse liquid, pain stimulation could increase the concentrations of AVP, leucine-enkephalin (L-Ek), methionine-enkephalin (M-Ek), beta-endorphin (beta-Ep), serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA), but not change the concentrations of dynorphinA(1-13) (DynA(1-13)), oxytocin, achetylcholine, choline, gamma-aminobutyric acid, glutamate, dopamine, 3,4-dihydroxyphenylacetic acid, homovanilic acid, norepinephrine and epinephrine; (2) in the NRM perfuse liquid, AVP increased the concentrations of L-Ek, M-Ek, beta-Ep, DynA(1-13), 5-HT and 5-HIAA, but did not change the concentrations of oxytocin and the other studied neurotransmitters; (3) AVP antinociception in the NRM was attenuated by cypoheptadine (a 5-HT-receptor antagonist) or naloxone (an opiate receptor antagonist), but was not influenced by the other studied receptor antagonists. The data suggested that AVP antinociception in the NRM might be involved in endogenous opiate peptide and 5-HT system.

Endogenous opiate peptides in the spinal cord are involved in the analgesia of hypothalamic paraventricular nucleus in the rat

Many studies have shown that hypothalamic paraventricular nucleus (PVN) plays a role in pain process, and endogenous opiate peptide system in the spinal cord is involved in nociception. This communication was designed to study the relationship between PVN and endogenous opiate system in the spinal cord in the rat. The results showed that in both the thoracic and the lumber spinal cord, microinjection of 100 ng L-glutamate sodium into PVN could increase leucine-enkephalin (L-Ek), beta-endorphin (beta-Ep), dynorphinA(1-13) (DynA(1-13)) concentrations and PVN cauterization decreased L-Ek and beta-Ep concentrations. Pretreatment of the spinal cord with 5 microg naloxone, an opiate receptor antagonist could partly reverse the analgesia induced by microinjection of 100 ng L-glutamate sodium into PVN. The data suggested that PVN analgesia might be involved in the endogenous opiate peptide system in the spinal cord independently.

Effect of arginine vasopressin on acupuncture analgesia in the rat

Arginine vasopressin (AVP) has been proven to be involved in the process of pain regulation. This communication was designed to investigate the effect of AVP on acupuncture analgesia in the rat model. The results showed that intraventricular injection (icv) of AVP could enhance acupuncture analgesia in a dose-dependent manner, whereas icv of anti-AVP serum decreased acupuncture analgesia. However, neither intrathecal (ith) nor intravenous injection (iv) of AVP or anti-AVP serum could influence acupuncture analgesia. Electrical acupuncture of "Zusanli" points (St. 36) decreased AVP concentration in the hypothalamic paraventricular nucleus (PVN), and increased AVP concentration in the hypothalamic supraoptic nucleus (SON), periaqueductial gray (PAG), caudate nucleus (CdN) and raphe magnus nucleus (RMN), but did not change AVP concentration in the pituitary, spinal cord and plasma. The effect of AVP on acupuncture analgesia was partly reversed by pretreatment with naloxone, an opiate receptor antagonist. These data suggested that AVP in the brain played a role in the process of acupuncture analgesia in combination with the endogenous opiate peptide system.