The effect of medial hypothalamus lesions on pain control

Dopamine release and negative valence gated by inhibitory neurons in the laterodorsal tegmental nucleus

GABAergic neurons in the laterodorsal tegmental nucleus (LDTGABA) encode aversion by directly inhibiting mesolimbic dopamine (DA). Yet, the detailed cellular and circuit mechanisms by which these cells relay unpleasant stimuli to DA neurons and regulate behavioral output remain largely unclear. Here, we show that LDTGABA neurons bidirectionally respond to rewarding and aversive stimuli in mice. Activation of LDTGABA neurons promotes aversion and reduces DA release in the lateral nucleus accumbens. Furthermore, we identified two molecularly distinct LDTGABA cell populations. Somatostatin-expressing (Sst+) LDTGABA neurons indirectly regulate the mesolimbic DA system by disinhibiting excitatory hypothalamic neurons. In contrast, Reelin-expressing LDTGABA neurons directly inhibit downstream DA neurons. The identification of separate GABAergic subpopulations in a single brainstem nucleus that relay unpleasant stimuli to the mesolimbic DA system through direct and indirect projections is critical for establishing a circuit-level understanding of how negative valence is encoded in the mammalian brain.

Efficacy of relaxation therapy as an effective nursing intervention for post-operative pain relief in patients undergoing abdominal surgery: A systematic review and meta-analysis

This systematic review and meta-analysis was conducted with the aim of assessing the efficacy of relaxation techniques for pain relief in patients undergoing abdominal surgery. The electronic search of the PubMed, Scopus, Cochrane Central Register of Controlled Trials (CENTRAL) and Google Scholar databases was performed for studies in the English language published up to May, 2019. A total of 12 studies were included in the review and 7 in the meta-analysis. In total, 4 relaxation techniques were utilized in the included studies: Jaw relaxation, Benson's relaxation, progressive muscle relaxation (PMR) and systematic relaxation. Of the 12 included, 10 studies demonstrated statistically significant pain relief in the relaxation group as compared to the controls. The data of 422 patients in the relaxation group and 424 patients in the control group were pooled for a meta-analysis, which indicated that patients undergoing abdominal surgery had significantly greater pain relief following relaxation therapy as compared to the controls [random: standardized mean difference (SMD), −1.15; 95% CI, −2.04 to −0.26; P<0.00001). The overall quality of the studies was not high. On the whole, despite trials demonstrating the benefits of relaxation therapy for immediate pain relief in patients post-abdominal surgery, there is lack of high-quality scientific evidence substantiating its routine use. There is a need for more robust randomized control trials (RCTs) utilizing standardized relaxation protocols to provide further evidence on this subject.

Bidirectional selection for high and low stress-induced analgesia affects G-protein activity

Mice selected for high (HA) and low (LA) swim stress-induced analgesia (SSIA) are a unique model for studying the genetic background of this phenomenon. HA and LA miceshow substantial differences in the magnitude of the antinociceptive response to stress and when treated with exogenous opioids. However, the direct cause underplaying this distinctive feature has not yet been identified. The current study was designed to investigate the possibility that disturbances in G-protein signaling could explain the divergent response to opioid agonists. Supraspinal and spinal opioid sensitivity was assessed in vivo with intraperitoneal morphine and subsequent thermal stimulus exposure. The level of opioid receptor-mediated G-protein activation was investigated by means of DAMGO and morphine-stimulated [³⁵S]GTPγS assay in the brain and spinal cord homogenates from HA and LA mice. Morphine (3-249 μmol/kg, i.p) was over 6 - and 3 - times more potent in HA than LA mice in the hot plate and tail-flick assays, respectively. Additionally, HA mice showed elevated β - endorphin levels in the brain. Enhanced efficacy of agonist-stimulated [³⁵S]GTPγS binding was detected in opioid receptor-rich limbic regions of HA mice like the hypothalamus and hippocampus. Increased G-protein activity also emerged in the thalamus, periaqueductal gray matter and prefrontal cortex. In conclusion, the magnitude of the antinociceptive response to opioids in HA and LA mice is correlated with alterations in G-protein activation in brain regions responsible for integration and descending modulation of nociceptive information as well as at sites governing the emotional response to stressful stimuli.

Ventromedial and medial preoptic hypothalamic ibotenic acid lesions potentiate systemic morphine analgesia in female, but not male rats

Sex differences in systemic morphine analgesia occur with male rodents displaying significantly greater analgesic magnitudes and potencies than females. Neonatal androgenization, and to a lesser degree, adult ovariectomy enhance systemic morphine analgesia in female rats, implicating both organizational and activational effects of gonadal hormones. The neuroanatomical circuits sensitive to sex-related hormones by which females display a smaller opiate analgesic effect is not clear, but the ventromedial (VMH) and medial preoptic (MPOA) hypothalamic nuclei are critical in the monitoring of estradiol and other sex hormone levels. To assess the contribution of these nuclei to sex and adult gonadectomy differences in systemic morphine analgesia, intact male, intact female and adult ovariectomized (OVEX) female rats received bilateral saline (SAL) or ibotenic acid (IBO) microinjections into either the VMH or MPOA. Following surgeries, baseline tail-flick latencies over 120 minutes (min) were assessed over 4 days in all nine groups with intact females tested in the estrus phase of their cycle. All animals then received an ascending series of morphine (1.0, 2.5, 5.0, 7.5, 10.0mg/kg) injections 30min prior to the tail-flick test time course with 8-12 day inter-injection intervals between doses. Baseline latencies failed to differ between SAL-treated intact males and females, but were significantly higher in SAL-treated OVEX females. Both VMH IBO and MPOA IBO lesions increased baseline latencies in intact male and female rats, but not in OVEX females. SAL-treated intact males (ED(50)=4.0mg/kg) and SAL-treated OVEX females (ED(50)=3.5mg/kg) displayed significantly greater potencies of systemic morphine analgesia than SAL-treated intact females (ED(50)=6.3mg/kg), confirming previous gender and gonadectomy differences. Neither VMH IBO (ED(50)=3.7 mg/kg) nor MPOA IBO (ED(50)=4.1mg/kg) males differed from SAL-treated males in the potency of systemic morphine analgesia. In contrast, VMH IBO (ED(50)=4.1mg/kg) and MPOA IBO (ED(50)=3.5mg/kg) intact females displayed significantly greater potencies in systemic morphine analgesia than SAL-treated intact females. However, VMH IBO OVEX (ED(50)=3.5mg/kg) and MPOA IBO OVEX (ED(50)=3.9 mg/kg) failed to differ from SAL-treated OVEX females in the potency of systemic morphine analgesia. The magnitudes of systemic morphine analgesia as measured by Maximum Percentage Effect values displayed similar patterns, but lesser degrees, of effects. These data suggest that VMH and MPOA nuclei act to tonically inhibit endogenous pain-inhibitory circuits in the intact female, but not intact male brain, and that removal of circulating gonadal hormones by OVEX and/or excitotoxic destruction of these estrogen receptor accumulating nuclei disinhibit the female analgesic response to systemic morphine.

Selective amino acids changes in the medial and lateral preoptic area in the formalin test in rats

A combination of microdialysis in freely moving rats and capillary zone electrophoresis coupled to laser induced fluorescence detection was used to measure extracellular concentrations of amino acid neurotransmitters in different hypothalamic areas during noxious stimulation. Arginine, glutamate and aspartate were monitored every 30 s before and after a s.c. injection of formalin (5%, 50 microl) or saline (0.9%) in the right hind paw. In the medial and lateral preoptic area, calcium and nerve impulse dependent increases of arginine, glutamate and aspartate were observed during the first 2 min after formalin injection. However, amino acid changes were not detected in the lateral hypothalamus or in the ventromedial nucleus when compared with pre-injection levels or with the levels from animals injected with saline in the hind paw. Flinching behavior was also scored during the first 10 min following the formalin or saline injection. Flinching frequency was maximum at minute 2 after formalin injection, whereas saline injection did not elicited any flinching behavior. These results show that nociceptive stimulation induces rapid and differential amino acids changes in discrete areas of the hypothalamus that can be associated with pain-related behavior.

Electron microscopic examination of the endomorphin 2-like immunoreactive neurons in the rat hypothalamus

Endomorphins are endogenous opioid peptides with high affinity and selectivity for the mu-opioid receptor. In the present study, we examined the morphology of the endomorphin 2-like immunoreactive (EM2-LI) neurons in the hypothalamus at the light and electron microscopic levels. At the light microscopic level, EM2-LI neurons were found mostly distributed in the regions between the dorsomedial and ventromedial hypothalamic nuclei and the region near the third ventricle. At the electron microscopic level, EM2-LI perikarya could be divided into two groups. Type I perikarya contained relatively undeveloped endoplasmic reticulum and Golgi apparatus while type II perikarya contained well-developed rough-surfaced endoplasmic reticulum and Golgi apparatus. Both type I and type II neurons contained numerous EM2-LI dense-cored vesicles. Type II perikarya and dendrites received synapses and showed immunoreactivity in the endoplasmic reticulum and Golgi apparatus. EM2-LI axon terminals formed synapses with both immunonegative and immunopositive dendrites. In some cases, the axon terminals contained both immunonegative and immunopositive dense-cored vesicles. EM2-LI neurons often had synaptic relationships with neurons containing immunonegative dense-cored vesicles. Myelinated axon shafts containing EM2-LI were also found. This first demonstration of the ultrastructure and synaptic relationships of EM2-LI neurons in the hypothalamus provides morphological evidence that suggests (1) endomorphin 2-containing neurons modulate physiological function through synaptic relationships; (2) endomorphin 2 may coexist with other neurotransmitters in the same neurons; and (3) endomorphin 2-containing neurons could modulate other endomorphin 2-containing neurons as well as those containing other neurotransmitters.

Trigeminal nociception-induced cerebral Fos expression in the conscious rat

Little is known about trigeminal nociception-induced cerebral activity and involvement of cerebral structures in pathogenesis of trigeminovascular headaches such as migraine. Neuroimaging has demonstrated cortical, hypothalamic and brainstem activation during the attack and after abolition with sumatriptan. This has led to the conclusion that the dorsal raphe and locus coeruleus may initiate events that generate migraneous headache. Using a conscious rat model of trigeminal nociception and cerebral Fos expression as histochemical markers of neuronal activity, we characterized the pattern of brain activity after noxious trigeminal stimulation with capsaicin (250 and 1000 nm). A significantly increased Fos immunoreactivity was found in the trigeminal nucleus caudalis (layers I and II), the area postrema, the nucleus of the solitary tract, the parvicellular reticular nucleus, the locus coeruleus, the parabrachial nucleus and the raphe nuclei. In addition, the ventrolateral periaqueductal grey, the intralaminar thalamic and various hypothalamic areas, showed an enhanced Fos expression after the intracisternal administration of capsaicin. Other responding areas were the amygdala, the upper lip and forelimb regions of the primary somatosensory cortex, and the insula. Many of these areas participate in (anti)-nociception, although we cannot exclude the possibility that in conscious animals the pain-associated physiological and behavioural responses that are an intrinsic and necessary part of coping with pain have generated the increased Fos expression. Trigeminal stimulation-induced locus coeruleus, dorsal raphe and hypothalamic activation are opposed to a suggested pathogenic role of these nuclei in migraine and cluster headache, respectively.

Effect of VMH lesion on sucrose-fed analgesia in formalin pain

The ingestion of sucrose (ad libitum) produces an immediate analgesic response to phasic noxious stimuli. The underlying mechanism for the analgesic effect of sucrose is attributed to its palatability, which mediates analgesia probably by the release of beta-endorphin in the hypothalamus. The present study was designed to explore the role of ventromedial hypothalamus in the mediation of sucrose-fed analgesia. Adult male albino rats each received (20%) sucrose solution orally through a separate bottle until they had ingested 4-5 ml. Their behavioral responses to tonic noxious stimulus in a formalin test were studied in pre- and postsucrose-fed rats of control and in the VMH lesion groups. The average pain rating of a 60-min session significantly (p < 0.01) decreased after sucrose feeding in control rats, from 1.94 +/- 0.13 to 1.45 +/- 0.14, but sucrose feeding by the VMH lesion rats did not alter their tonic nociceptive response from a 1.70 +/- 0.07 presucrose-fed state to a 1.71 +/- 0.08 postsucrose-fed state. VMH lesion per se did not alter the nociceptive response in comparison with controls. The results suggest that sucrose feeding produces analgesia to tonic noxious stimulus, which is abolished by lesion of the VMH, thereby indicating a significant role of VMH in sucrose-fed analgesia.

Effect of VMH lesion on sucrose-Fed nociceptive responses

An initial analgesia followed by hyperalgesia to phasic noxious stimuli occurs after ingestion of sucrose ad libitum. However, the mechanism underlying hyperalgesia is not known. The present study was designed to explore the role of VMH in the mediation of the hyperalgesic effect of sucrose ingestion. Adult male albino rats received sucrose solution (20% p.o.) in addition to laboratory food pellets and tap water ad libitum. Their behavioural responses to various phasic and tonic noxious stimuli were recorded after 6, 12 and 48 h during pre and post-sucrose fed states in both the control and VMH lesion groups of rats. Sucrose feeding to control rats significantly reduced the tail flick latency (TFL) and threshold of vocalization during stimulus (SV) and after discharge (VA) indicating hyperalgesia, while the threshold of tail flick remained unaffected. The average pain rating during the formalin test (tonic pain) decreased significantly indicating analgesia. VMH lesion decreased the latency (mean +/- SD) for tail flick (11.26 +/- 4.65 from 15.61 +/- 5.12 s), threshold (median) for tail flick (0.04 from 0.08 mA), vocalization during stimulus (0.05 from 0.1 mA) and vocalization after discharge (0.15 from 0.2 mA), while the tonic pain rating increased, thereby suggesting a hyperalgesic state. However, sucrose feeding to lesioned rats neither potentiated nor attenuated their hyperalgesia. The results suggest that sucrose feeding for 6-48 h ad libitum produces hyperalgesia to phasic noxious and analgesia to tonic noxious stimuli, while VMH lesion produces hyperalgesia to both phasic and tonic noxious stimuli. Secondly, sucrose ingestion by VMH lesion rats does not affect their responses to pain, suggesting the possible role of VMH in the mediation of sucrose-fed nociceptive responses.

Prostaglandin E(2) has antinociceptive effect through EP(1) receptor in the ventromedial hypothalamus in rats

The effects of microinjection of prostaglandin E(2) (PGE(2)) (50 fg-50 ng/0.2 microl) into the ventromedial hypothalamus (VMH) on nociception were studied using a hot-plate test in rats. Microinjection of PGE(2) (5-500 pg and 50 ng/0.2 microl) into the VMH significantly prolonged the paw-withdrawal latency on a hot plate 5 and 10 min after injection, respectively. Maximal prolongation was obtained 5 min after the injection of PGE(2) at 5 pg. Subsequently, to determine whether the PGE(2) receptor subtype EP(1) is involved in the PGE(2)-induced antinociceptive effect in the VMH, we observed the changes in nociception after intraVMH microinjection of SC19220, an EP(1) receptor antagonist, and 17-phenyl-omega-trinor PGE(2), an EP(1) receptor agonist. Simultaneous injection of SC19220 (150 ng) with PGE(2) (500 pg) into the VMH blocked the PGE(2)-induced prolongation of the paw-withdrawal latency. Moreover, an intraVMH microinjection of 17-phenyl-omega-trinor PGE(2) (500 pg) prolonged it. These results indicate that PGE(2) in the VMH has antinociceptive effect through its actions on EP(1) receptors in rats.

The opposing effects of interleukin -1 beta microinjected into the preoptic hypothalamus and the ventromedial hypothalamus on nociceptive behavior in rats

The effects of microinjections of recombinant human interleukin-1 beta (rhIL-1 beta) into the hypothalamus and neighboring basal forebrain on nociceptive behavior were studied using a hot-plate test in rats. The microinjection of rhIL-1 beta at doses between 5 pg/kg and 50 pg/kg into the medial part of the preoptic area (MPO) reduced the paw-withdrawal latency. The maximal reduction was obtained 30 min after the injection of rhIL-1 beta at 20 pg/kg. RhIL-1 beta (20 pg/kg)-induced hyperalgesia was completely blocked by the simultaneous injection of IL-1 receptor antagonist (IL-1ra, 20 ng/kg), Na salicylate (200 ng/kg) or alpha-melanocyte-stimulating hormone alpha-MSH, 20 ng/kg). The intra-MPO injection of rhIL-1 beta at doses of less than 5 pg/kg or more than 50 pg/kg (up to 2 ng/kg) into the paraventricular nucleus, the lateral hypothalamic area and the septal nucleus had no effect on nociception. The microinjection rhIL-1 beta (20 pg/kg-50 pg/kg) into the ventromedial hypothalamus produced a prolongation of the paw-withdrawal latency. A maximal prolongation was obtained 10 min after the injection of rhIL-1 beta at 50 pg/kg. This reaction was also blocked by the simultaneous injection of IL-1ra (50 ng/kg) and Na salicylate (500 ng/kg). These findings indicate that IL-1 beta in the MPO and the VMH produces hyperalgesia and analgesia, respectively, while, in addition, both effects are mediated by IL-1 receptors and the synthesis of prostaglandins.

Release into ventriculo-cisternal perfusate of beta-endorphin- and Met-enkephalin-immunoreactivity: effects of electrical stimulation in the arcuate nucleus and periaqueductal gray of the rat

To examine the resting and evoked release of the endogenous opioid peptides beta-endorphin and Met-enkephalin from brain, we examined the levels of the respective immunoreactivities in the lateral ventricle-cisterna magna perfusate of the halothane-anesthetized rat. Ten Hz but not 100 Hz stimulation in the arcuate nucleus (ARC) of the hypothalamus released beta-endorphin immunoreactivity (beta-EPir) to the perfusate, whereas 100 Hz but not 10 Hz stimulation in the periaqueductal gray (PAG) of the mid brain released Met-enkephalin immunoreactivity (MEir). MEir was not released by stimulation in ARC and beta-EPir was not released by stimulation in PAG. Characterization of the released beta-EPir and MEir by high performance liquid chromatography showed that authentic beta-endorphin and Met-enkephalin were the major constituents of beta-EPir and MEir, respectively. Systemic administration of the dopaminergic antagonist haloperidol increased plasma, but not perfusate levels of beta-EPir. Both the opioid antagonist naloxone and the NMDA antagonist MK-801 failed to affect beta-EPir or MEir release. ARC and PAG stimulated inhibited a nociceptive reflex (tail-dip in 52.5 degrees C water), and naloxone did not reliably reverse this inhibition. These data support the previously suggested possibility of opioid mediation of stimulation induced analgesia, although we were unable to confirm the theory by naloxone reversibility in this study. Furthermore, the data support the assumption that measurement of opioid peptides in cerebrospinal fluid is a relevant approach in research aimed at elucidating the physiological and pathophysiological roles of endogenous opioid peptides.

Induction of c-Fos immunoreactivity in the rat forebrain by conditioned and unconditioned aversive stimuli

The protein product of the c-fos proto-oncogene was immunocytochemically localized in forebrain regions of adult male Lewis rats subjected to a physically aversive footshock stimulus or a Pavlovian-conditioned, non-aversive, auditory stimulus. Animals receiving the conditioned stimulus were first conditioned by repeatedly pairing electric footshock, the unconditioned stimulus (US), with an auditory cue, the conditioned stimulus (CS). These animals were later tested with the CS in the absence of the US, a procedure which, like footshock itself, suppresses immune function. In animals exposed to the conditioned or unconditioned stressor, c-Fos was strongly expressed in cells of the paraventricular nuclei (PVN) of the hypothalamus, some of which contain corticotropin-releasing hormone (CRH), and other forebrain areas directly associated with autonomic function, the ventral lateral septal nuclei (LSV), the medial amygdaloid nuclei (AME), the sensorimotor cortex, the basal ganglia and thalamic nuclei. Control animals exhibited very little or no c-Fos in the above areas. The identified forebrain nuclei can now be targeted for further study aimed at elucidating their role in stress-induced immune alteration.

Descending pain inhibitory system involved in acupuncture analgesia

The descending pain inhibitory system (DPIS) associated with acupuncture analgesia (AA), caused by low frequency stimulation of an acupuncture point, was identified by the results of lesion and stimulation procedures previously determined to differentiate the afferent and efferent paths in rats. The DPIS starts in the posterior arcuate nucleus and descends to the hypothalamic ventromedian nucleus (HVM) from whence it divides into two pathways: one path, the serotonin mediated path, descends through the ventral periaqueductal central gray (V-PAG) and then to the raphe magnus (RM). The other, the noradrenaline mediated path, descends through the reticuloparagigantocellular nucleus (NRPG) and part of the reticulogigantocellular nucleus (NRGC). The afferent and efferent paths are both present in the RM and NRGC, and were separately identified by means of the analgesia (SPA) produced by stimulation of the separate regions in AA responders and nonresponders, because SPA of these regions in nonresponders produced only efferent pathway mediated analgesia.

Arcuate nucleus projections to brainstem regions which modulate nociception

Anterograde tracing studies were conducted in order to identify efferents from the arcuate nucleus, which contains the hypothalamic opiocortin neuronal pool. Phaseolus vulgaris leucoagglutinin (PHA-L) was stereotaxically iontophoresed into the arcuate nucleus and the terminal fields emanating from the labelled perikarya were identified immunocytochemically. PHA-L-immunoreactive (-ir) fibers were identified in nucleus accumbens, lateral septal nucleus, bed nucleus of the stria terminalis, medial and lateral preoptic areas, anterior hypothalamus, amygdaloid complex, lateral hypothalamus, paraventricular nucleus, zona incerta, dorsal hypothalamus, periventricular gray, medial thalamus and medial habenula. In the brainstem, arcuate terminals were identified in the periaqueductal gray (PAG), dorsal raphe nucleus (DRN), nucleus raphe magnus (NRM), nucleus raphe pallidus, locus coeruleus, parabrachial nucleus, nucleus reticularis gigantocellularis pars alpha, nucleus tractus solitarius and dorsal motor nucleus of the vagus nerve. Dual immunostaining was used to identify the neurochemical content of neurons in arcuate terminal fields in the brainstem. Arcuate fiber terminals established putative contacts with serotonergic neurons in the ventrolateral PAG, DRN and NRM and with noradrenergic neurons in periventricular gray, PAG and locus coeruleus. In the PAG, arcuate fibers terminated in areas with neurons immunoreactive to substance P, neurotensin, enkephalin and cholecystokinin (CCK) and putative contacts were identified with CCK-ir cells. This study provides neuroanatomical evidence that putative opiocortin neurons in the arcuate nucleus influence a descending system which modulates nociception.

Collateralization of periaqueductal gray neurons to forebrain or diencephalon and to the medullary nucleus raphe magnus in the rat

Antinociceptive effects elicited from the midbrain may involve both ascending and descending projections from the periaqueductal gray and dorsal raphe nucleus. To investigate the relationship between these different efferent pathways in the rat, we performed a double-labeling study using two retrograde tracers, colloidal gold-coupled wheatgerm agglutinin-apo horseradish peroxidase and a fluorescent dye. One tracer was microinjected in the medullary nucleus raphe magnus; the second was injected into one of several regions rostral to the periaqueductal gray that have been implicated in nociceptive and antinociceptive processes. The results can be grouped into two categories. First, injections into the ventrobasal thalamus, lateral hypothalamus, amygdala, and cerebral cortex labeled neurons in the dorsal raphe nucleus but not in the periaqueductal gray. Up to 90% of these projection neurons were serotonin immunoreactive, and up to 17% were also retrogradely labeled from the nucleus raphe magnus. Second, only injections into the ventrobasal hypothalamus (which included the beta-endorphin-containing arcuate neurons) or into the medial thalamus labeled neurons in the periaqueductal gray itself. Injections into the medial thalamus, but not into the ventrobasal hypothalamus, also labeled neurons in the dorsal raphe nucleus. Up to 20% of the neurons retrogradely labeled from these regions were also retrogradely labeled from nucleus raphe magnus. The presence of large populations of rostrally projecting periaqueductal gray neurons that collateralize to the nucleus raphe magnus implies that activity in ascending projections necessarily accompanies any activation of the periaqueductal gray-nucleus raphe magnus pathway. Possibly, projections from the medial thalamus and medial hypothalamus mediate antinociceptive effects that complement descending inhibition. Finally, possible antidromic activation of these pathways must be considered when interpreting the results of electrical brain stimulation studies.

Stimulation of the hypothalamic paraventricular nucleus produces analgesia not mediated by vasopressin or endogenous opioids

The analgesic effect of electrical stimulation of the hypothalamic paraventricular nucleus (PVN) was studied. Additionally, the involvement of vasopressin and opioid peptides in this process was examined by comparing vasopressin-deficient (Brattleboro) and Long-Evans rats and by administering the opiate antagonist naloxone. Rats were chronically implanted with a stimulating electrode in the parvocellular (PVN-Pc) and magnocellular (PVN-Mg) divisions of the PVN. At least 10 days after surgery, the analgesic effects of PVN stimulation were examined in lightly anesthetized rats, using the tail-flick method, and in unanesthetized rats, using the hot-plate test. PVN stimulation produced marked analgesia in both tests. Current threshold for analgesia was lower from PVN-Pc than from PVN-Mg. Threshold did not differ significantly between Brattleboro and Long-Evans rats and was not affected by naloxone administration. The results indicate that the PVN is part of the brain's pain inhibitory system, and show that the analgesia induced by PVN stimulation is not mediated by either vasopressin or opioid peptides.

Anatomical mapping of brain sites involved in the antinociceptive effects of ketoprofen

Ketoprofen, a non-steroidal anti-inflammatory drug, has analgesic effects in animals and humans through a peripheral as well as a central action. This study was designed to determine which brain sites are involved in the central analgesic action of ketoprofen, by using the hot-plate test. Latencies to the first hindpaw lick were recorded in animals receiving local cerebral injections of ketoprofen (10 micrograms in 0.3 microliters) or a control solution (saline). Nineteen brain sites were tested. A significant analgesia was obtained in the 8 following sites: central gray, centro-medial nucleus of the thalamus, nucleus reuniens, dorso-lateral geniculate nucleus, medial geniculate nucleus, dorso-medial and ventro-medial nuclei of the hypothalamus, posterior hypothalamic nucleus and lateral vestibular nucleus. A slight analgesia, which did not reach significance, was observed in three structures: dorsal raphé, raphé magnus and ventral postero-medial thalamic nucleus. No analgesia was observed in other sites: centro-lateral nucleus of the thalamus, posterior thalamic nuclear group, parafascicular nucleus, bed nucleus of the stria terminalis, mesencephalic tegmentum, nucleus of the tractus solitarius, spinal trigeminal complex, and brainstem reticular formation. Therefore ketoprofen seems to be centrally active mostly at the level of several integrative and non-specific structures.

Opiocortin and catecholamine projections to raphe nuclei

Afferent projections to the nucleus raphe magnus (NRM) and dorsal raphe nucleus (DRN) were identified using retrograde transport of horseradish peroxidase conjugated wheat germ agglutinin (HRP-WGA). Neurons were labeled in important nociceptive regions including periaqueductal gray (PAG), arcuate nucleus, lateral hypothalamus and medial thalamic nuclei following both injections. We have immunocytochemically identified opiocortin/WGA neurons in the arcuate nucleus following NRM and DRN injections. Dual stained catecholamine/WGA perikarya were found in zona incerta, locus coeruleus, substantia nigra, nucleus tractus solitarius and adjacent A2, C2 and C3, lateral paragigantocellular reticular nucleus/C1 and lateral reticular nucleus/A1 following DRN injections and in zona incerta, substantia nigra, nucleus tractus solitarius/A2 and lateral reticular nucleus/A1 after NRM injections. These results provide further evidence for opiocortin and catecholamine modulation of analgesia.

Spino-diencephalic relays through the parabrachial nucleus in the cat

Previous studies have shown that the spinal input to the parabrachial nucleus (PBN) in the cat is limited to certain portions of its lateral division 8,21,45. The purpose of the present study was to determine some of the output targets of PBN neurons located within this spinal terminal domain by means of single, double and triple light microscopic labeling strategies. Combinations of tracers included the retrograde transport of tritiated wheat germ agglutinin, wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) and Fluoro-Gold from the hypothalamus, amygdala or thalamus/zona incerta together with either the anterograde transport of WGA-HRP from the spinal cord or the degeneration of spinal terminals following spinal lesions. The results (summarized in Fig. 10) showed that the spinal terminal domain contains separable populations of neurons projecting to the thalamus/zona incerta and hypothalamus. Only a limited number of amygdala-projecting neurons was located in this domain. Evidence from several laboratories supports the conclusion that these potential spino-diencephalic relays are involved somehow in nociception. More information is needed, however, regarding differences in the response properties of these separable populations of spinal-recipient neurons before more specific hypotheses concerning the precise nature of their nociceptive functions can be formulated.

Ultrastructural Characterization of Adrenocorticotrope Hormone (ACTH) Immunoreactive Fibres in the Mesencephalic Central Grey Substance of the Rat

The fine structural localization of fibres immunoreactive for the adrenocorticotrope hormone (ACTH) was studied in the mesencephalic central grey substance (MCG) of the male Wistar rat. Light microscopically, varicose ACTH-immunoreactive fibres were found throughout the MCG in a dorsal, lateral and ventral, periventricular position. Electron microscopically, the immunoreactivity was most prominent in the direct vicinity of electron-dense secretory granules in axonal varicosities, and, although to a lower degree, around other cytoplasmic organelles such as electron-lucent synaptic vesicles, mitochondria and microtubules. With serial section analysis two types of ACTH-immunoreactive varicosity were discerned. The first type is large, contains many, small electron-lucent synaptic vesicles, that are located in the vicinity of a morphologically well-defined synaptic contact. In this type of varicosity, large dense-core secretory granules are scarce. Immunoreactivity is low or absent, particularly near the active zone. The second type is strongly immunoreactive. It always contains many large, dense-core secretory granules; electron-lucent vesicles are rare. The smaller varicosities of this type never make synaptic contacts, but a few of the larger varicosities have synaptic contacts with dendrites of MCG cells.

Medial hypothalamic stimulation suppresses nociceptive spinal dorsal horn neurons but not the tail-flick reflex in the rat

This study investigated the potential analgesic effects of medial hypothalamic stimulation (HS) on a measure of nocifensive behavior (tail-flick test (TF] in awake rats, and potential inhibitory effects of identical HS on spinal dorsal horn neuronal responses to noxious skin heating in the same animals anesthetized with sodium pentobarbital. Sixty-five male Sprague-Dawley rats implanted with a bipolar stimulation electrode in histologically verified medial hypothalamic sites were tested behaviorally for TF suppression during HS (100 ms trains at 100 Hz, 3/s, 100-1100 microA) in 2-4 consecutive weekly test sessions. Thirty-three of these rats were then used in electrophysiological experiments to record responses of 36 dorsal horn units to noxious skin heating (48-54 degrees C, 10 s/2 min) of the hindfoot pad in the absence of and during HS. Behaviorally, 31/65 rats had no TF suppression at the highest HS intensity tested (1100 microA), 24/65 rats exhibited aversive behavior or motor activity which disallowed reliable TF testing, and only 10/65 rats showed TF suppression in at least one test session. In electrophysiological experiments, the heat-evoked responses of 25/36 dorsal horn units were inhibited to at least 50% of control during HS. The responses of 11 units remained at 65-100% of the control responses during HS of up to 1100 microA. In rats demonstrating TF suppression, 4/7 units were inhibited. In rats with no TF suppression, 10/15 units were inhibited, and in rats showing aversive behavior, 11/14 units were inhibited by HS. These data indicate that although HS suppresses spinal nociceptive neurons, it does not cause reliable TF suppression in unanesthetized rats and bring into question the often-held assumption that stimulation-evoked descending inhibition of spinal nociceptive neurons implies behavioral analgesia.

Ventromedial and dorsomedial hypothalamic syndromes in the weanling rat: is the "center" concept really outmoded?

This report juxtaposes findings from weanling rats with precise lesions in the ventromedial (VMNL rats) to data of weanling rats with lesions in the dorsomedial (DMNL) hypothalamic nuclei. Despite the proximity of the two nuclei their destruction produces opposite effects in most cases but similar responses in other parameters. Absolute and relative food intake are normal in VMNL rats yet they become obese in the face of normal body weight gains. DMNL rats show both reduced absolute food intake and body weight but normal relative food intake and body composition. Both VMNL and DMNL cause reduced linear growth and running wheel activity. DMNL rats defend their lower body weight set point against various challenges and maintain normal body composition. Organ growth in both absolute and relative terms is reduced in VMNL rats. In DMNL rats relative organ growth is normal. Pancreatic growth, protein/pancreas and content and concentrations of several pancreatic enzymes are normal in DMNL but reduced in VMNL rats. Mean 24-hour plasma growth hormone (GH) and corticosterone (B) levels are reduced and insulin levels are greatly elevated in VMNL rats; prolactin (PRL) levels are normal. In DMNL rats, GH, B, insulin and somatomedin activity are normal but PRL is elevated. Circadian rhythms of GH, insulin and triiodothyronine are normal in DMNL rats but B levels are disrupted, as they are in VMNL rats. Glucose incorporation and oxidation in adipose tissue of VMNL rats are enhanced in VMNL rats but normal in DMNL rats. Gluconeogenesis in VMNL rats is enhanced as early as 4 hours post-operatively; in DMNL rats it is normal at this time and several weeks thereafter. Basal lipolysis in epididymal fat pads is elevated in both VMNL and DMNL rats but epinephrine-stimulated lipolysis is elevated in VMNL and decreased in DMNL rats. Both VMNL and DMNL rats show normal basal and epinephrine-stimulated lipolysis in interscapular brown adipose tissue. Several hepatic enzymes are normal in DMNL and depressed in VMNL rats. The above data suggest that the DMN and its circuitry are part of an "organismic" set point system with a "true" body weight and no fat set point, as seems to be the case in the VMNL rat.

Analgesic effects of cyclazocine and morphine microinjected into the rat dorsomedial hypothalamus demonstrated by the bradykinin-induced flexor reflex test

The analgesic effect of cyclazocine microinjected into the diencephalon of rats was studied by using the bradykinin-induced flexor reflex test. The dorsal portion of the medial hypothalamic area was sensitive to cyclazocine with respect to the production of analgesia (ED50 2.6 micrograms/rat). Microinjections of morphine into the same portion also produced a dose-dependent analgesic effect (ED50 2.5 micrograms/rat). As naloxone (0.9 mg/kg s.c.) antagonized both these analgesic effects, the findings suggest that the dorsal portion of the medial hypothalamic area of the rat is one of the primary target sites of analgesic drugs.

Effects of hypothalamic lesions on central gray stimulation induced escape behavior and on withdrawal reactions in the rat

The effects of unilateral--medial or lateral--hypothalamic lesions were studied with regard to both switch-off (i.e., escape) responding induced by central gray (CG) stimulation and responsiveness of the same animals to peripherally-applied electrical or thermal stimulation. Medial hypothalamic (MH) lesions were found to reduce--but not abolish--the efficiency of CG stimulations applied on the side of the MH lesion, while the efficiency of contralaterally applied CG stimulations remained unchanged. Furthermore, such lesions enhanced the responsiveness to peripheral nociceptive stimuli on whatever body side they were applied. Lateral hypothalamic lesions were found to only slightly--or not at all--affect the efficiency of CG stimulations, while they enhanced the responsiveness to peripheral nociceptive stimuli on either side of the body. These results suggest that the hypothalamus does not exert a univocal control over both centrally induced escape behavior and peripherally induced withdrawal reactions.

Interactions between cardiovascular and pain regulatory systems

A review of pharmacological, neuroanatomical, electrophysiological, and behavioral data indicates that systems controlling cardiovascular function are closely coupled to systems modulating the perception of pain. This view is directly supported by experiments from our laboratory showing that activation of either the cardiopulmonary baroreceptor reflex arc or the sinoaortic baroreceptor reflex arc induces antinociception. The outcomes of studies using pharmacological treatments, peripheral nerve stimulation, peripheral nerve resection, and CNS lesions are also presented as a preliminary means of characterizing cardiovascular input to pain regulatory systems. The network formed by these systems is proposed to participate in the elaboration of adaptive responses to physical and psychological stressors at various levels of the neuroaxis, and possibly to participate in "diseases of adaptation." In particular, the present analysis suggests that the inhibition of pain brought about by elevations in either arterial or venous blood pressure may provide a form of psychophysiological relief under situations of stress and contribute to the development of essential hypertension in humans.

Regulation of body temperature and nociception induced by non-noxious stress in rat

The effects of 3 different non-noxious stressors on body temperature (Tb) were investigated in the rat: (1) loose restraint in cylinders, (2) removal of the rats from cylinders, exposure to a novel environment and replacement in cylinders, a stressor called here 'novelty', and (3) gentle holding of the rats by the nape of the neck. Loose restraint and 'novelty' produced hyperthermia. On the contrary, holding induced hypothermia. Hypophysectomy (HX) reduced basal Tb, abolished restraint hyperthermia and reduced both 'novelty' hyperthermia and holding hypothermia. Dexamethasone ( DEXA ) had no effect upon either restraint or novelty hyperthermia but reduced the hypothermia. Naloxone (Nx) produced a slight fall in basal Tb accounting for its reduction of restraint and 'novelty' hyperthermias ; it did not affect holding hypothermia. The inhibitory effects of HX suggest a participation of the pituitary in the hyperthermias ; the neurointermediate lobe would be involved as the hyperthermias were not affected by DEXA , which is known to block the stress-induced release of pituitary secretions from the anterior lobe but not from the neurointermediate lobe. In contrast, substances from the anterior lobe might participate in hypothermia due to holding since it is reduced by HX and DEXA . As to the effects of Nx, endogenous opioids would not be significantly involved in the thermic effects of the stressors used in this study; they might play, if any, only a minor role in the regulation of basal Tb. These results are compared with those previously obtained on nociception using the same non-noxious stressors. It emerges that, depending on the stressor, different types of association between thermoregulation and nociception may occur, i.e. hyperthermia with analgesia, hyperthermia with hyperalgesia and hypothermia with hyperalgesia.

Immunocytochemical localization of ACTH1-39 in the brainstem of the rat

The immunocytochemical distribution of ACTH1-39 (adrenocorticotropin hormone), a member of the opiocortin family of peptides, was examined in the brainstem of the rat. Immunoreactive ACTH fibers were localized in many serotonin-containing raphe nuclei and in the norepinephrine- and dopamine-containing regions throughout the brainstem. Brainstem areas involved in pain mediation contained immunoreactive ACTH fibers as well as several cranial nerve nuclei and nuclei associated with cardiovascular and respiratory functions.

Hyperalgesia induced by non-noxious stress in the rat

Stress-induced analgesia is well known but the reverse phenomenon is poorly documented. In this work, hyperalgesia is described following stressful but non-noxious manipulations consisting of inescapable holding or exposure to a novel environment. Hypophysectomy (HX) and dexamethasone enhanced 'holding' hyperalgesia. In contrast, 'novelty' hyperalgesia was reduced by HX and not modified by dexamethasone. So, pituitary factors may respectively compensate and take part in stress hyperalgesia. Thus pain can be modulated in opposite directions by different types of stimulation: hyperalgesia would predominate after moderate, anxiogenic stress and analgesia after noxious and/or intense stress.

Stress hyperalgesia in rats: an experimental animal model of anxiogenic hyperalgesia in human

Emotional, non-noxious stress consisting of inescapable holding or exposure to novelty were found to produce clearcut and reproducible hyperalgesia in the rat. The mechanisms of the two stress hyperalgesia appeared to be different: 1) hypophysectomy enhanced holding hyperalgesia but reduced novelty hyperalgesia, indicating that pituitary factors respectively compensate (through opioids ?) and participate (through ACTH ?) in stress hyperalgesia; 2) diazepam did not alter holding hyperalgesia but abolished novelty hyperalgesia. The two types of stress hyperalgesia might represent animal models of anxiogenic hyperalgesia observed in human: as for clinical anxieties, they are alleviated or not by diazepam.

Circulating opioids: possible physiological roles in central nervous function

Evidence is reviewed regarding the release of endorphins by such diverse conditions as stress, long distance running, acupuncture, sexual activity, suggestion and ritualistic dancing ceremonies. Additional evidence is cited regarding possible physiological roles of endorphins in antinociception, socialization, euphoria, some mental disorders, drive states and vegetative functions. The concentration of this latter type of evidence is on conditions during which endorphins seem to be exerting effects on a number of different systems together (for example, euphoria is almost always accompanied by analgesia), and the possibility is suggested that the activation of a number of functions together may be due to a global activation of opiate receptors throughout the CNS. A possible basis for this global activation arises from results from this laboratory indicating the presence of a blood-borne opioid hormone, secreted by the pituitary or by an endocrine gland under pituitary control, which is capable of passing from the blood into the CNS. This diffuse endorphinergic system, which is complementary to the well-established endorphinergic neuronal systems in the CNS, thus derives its property of global action on opiate receptors by the diffuse means by which the hormone reaches its target sites, i.e., by passing through the blood brain barrier. Thus, while each specific endorphin-mediated function can be activated by the activation of its respective neural pathway, it is proposed that the hormonal endorphinergic mechanism is activated to produce a global response provoked by conditions to which a more generalized response, including physiological and behavioural changes, is most appropriate.

The effect of pituitary removal on pain regulation in the rat

The effects of hypophysectomy (HX) on pain regulation in basal and in various stressful situations were investigated in the rat. Pain sensitivity was assessed by measuring the thresholds of 3 nociceptive reactions (tail withdrawal, vocalization, vocalization afterdischarge) following electrical stimulation of the tail. The completeness of HX and the integrity of hypothalamus were verified in each HX rat. (1) Baseline pain thresholds were lower in HX rats than in sham-operated animals; (2) naloxone (Nx) hyperalgesia was only slightly altered by HX; (3) different types of stress induced different types of changes in nociception i.e. analgesia or hyperalgesia. The influence of HX varied according to the stress: it increased hyperalgesia, reduced analgesia, or had no effect at all. These results indicate that in the rat: (i) the pituitary participates in the regulation of basal pain sensitivity, probably through analgesic factors; (ii) Nx hyperalgesia results essentially from an antagonism of endogenous opioids originating in the CNS and not in the pituitary; and (iii) the pain regulatory processes engaged in adaptation to stressful stimuli involve the CNS and the pituitary in variable proportions depending upon the nature of the stress.

Endogenous morphines and pain

The involvement of endogenous morphines (enkephalins and endorphins) in the regulation of pain is demonstrated by the following experimental evidence: (a) their analgesic activities; (b) their distribution in the central nervous systems; (c) the effects of their modifiers, especially of their antagonists, on nociceptive reactions and (or) on various types of analgesia; (d) rare modifications of their brain levels in pain and (or) analgesic states. Besides the well-known facts, the following items are particularly stressed: the functional roles of hypothalamic structures and of the pituitary, the effects of antagonists, the variety of analgesia following noxious and (or) stressful stimuli, genetic and environmental factors, endogenous antinociceptive substances other than opioids, relations with biogenic amines. As a whole, endogenous morphines apparently filter the particular important sensory input represented by nociception and control the reactions to pain, allowing for adjusted behaviour, if the stimuli are avoidable, or for prevention or at least delay of exhaustion if the stimuli are unavoidable.

Hypothalamic, other diencephalic, and telencephalic neurons that project to the dorsal midbrain

Neurons in the hypothalamus, other diencephalic regions, and the telencephalon which project to the mesencephalic central gray (CG) and the region lateral to it were demonstrated, in the rat, by the horseradish peroxidase retrograde neuroanatomical tracing method with diaminobenzidine and tetramethyl benzidine visualization reactions. The greatest concentrations of neurons that project to the dorsal mesencephalon were found in the ventromedial nucleus, particularly the anterior and ventrolateral subdivisions, in the dorsal premammillary nucleus, and in the zona incerta. Neurons that project to or lateral to the CG were also found in the laterocaudal hypothalamus, the dorsomedial hypothalamus, regions of the anterior hypothalamic area, specific areas of the cerebral cortex (32, 29, 8, 8A, 13, 14), and the central nucleus of the amygdala. Some neurons that project were also found in the preoptic area, septum, bed nucleus of the stria terminals, and the habenula. More neurons in the mediocaudal quadrant of the hypothalamus project to the mesencephalon than do those in laterocaudal, mediorostral, or laterorostral quadrants. More neurons in the medial than the lateral half, and more in the caudal than the rostral half of the hypothalamus project to the mesencephalon. More neurons project to the central gray, or the region lateral to it, at the levels of the superior colliculus, or intercollicular region, than at the level of the inferior colliculus. These descending connections to the midbrain, particularly from the hypothalamus and zona incerta, are probably components of neural networks that regulate nociception, certain neuroendocrine functions, sexual and other behaviors, and certain autonomic functions.