Receptor autoradiography of mu and delta opioid peptide receptors in spontaneously hypertensive rats

Behavioral neuroscience of psychological pain

Pain is a common word used to refer to a wide range of physical and mental states sharing hedonic aversive value. Three types of pain are distinguished in this article: Physical pain, an aversive state related to actual or potential injury and disease; social pain, an aversive emotion associated to social exclusion; and psychological pain, a negative emotion induced by incentive loss. This review centers on psychological pain as studied in nonhuman animals. After covering issues of terminology, the article briefly discusses the daily-life significance of psychological pain and then centers on a discussion of the results originating from two procedures involving incentive loss: successive negative contrast-the unexpected devaluation of a reward-and appetitive extinction-the unexpected omission of a reward. The evidence reviewed points to substantial commonalities, but also some differences and interactions between physical and psychological pains. This evidence is discussed in relation to behavioral, pharmacological, neurobiological, and genetic factors that contribute to the multidimensional experience of psychological pain.

The anterior cingulate cortex and pain processing

The neural network that contributes to the suffering which accompanies persistent pain states involves a number of brain regions. Of primary interest is the contribution of the cingulate cortex in processing the affective component of pain. The purpose of this review is to summarize recent data obtained using novel behavioral paradigms in animals based on measuring escape and/or avoidance of a noxious stimulus. These paradigms have successfully been used to study the nature of the neuroanatomical and neurochemical contributions of the anterior cingulate cortex (ACC) to higher order pain processing in rodents.

Similarities in hypothalamic and mesocorticolimbic circuits regulating the overconsumption of food and alcohol

Historically, studies of food intake regulation started with the hypothalamus and gradually expanded to mesocorticolimbic regions, while studies of drug use began with mesocorticolimbic regions and now include the hypothalamus. As research on ingestive behavior has progressed, it has uncovered more and more similarities between the regulation of palatable food and drug intake. It has also identified specific neurochemicals involved in palatable food and drug intake. Hypothalamic orexigenic neurochemicals specifically involved in controlling fat ingestion, including galanin, enkephalin, orexin and melanin-concentrating hormone, show positive feedback with this macronutrient, with these peptides both increasing fat intake and being further stimulated by its intake. This positive relationship offers some explanation for why foods high in fat are so often overconsumed. Research in Bart Hoebel's laboratory in conjunction with our own has shown that consumption of ethanol, a drug of abuse that also contains calories, is similarly driven by these neurochemical systems involved in fat intake, consistent with evidence closely relating fat and ethanol consumption. Both fat and ethanol intake are also regulated by dopamine and acetylcholine acting in mesocorticolimbic nuclei. This close relationship of fat and ethanol is likely driven in part by circulating lipids, which are increased by fat and ethanol intake, known to increase expression and levels of the neurochemicals, and found to promote further intake of fat and ethanol. Compellingly, recent studies suggest that these systems may already be dysregulated in animals prone to consuming excess fat or ethanol, even before they have ever been exposed to these substances. Further understanding of these systems involved in consummatory behavior will allow researchers to develop effective therapies for the treatment of overeating as well as drug abuse.

Morphine-Induced Analgesia, Hypotension, and Bradycardia Are Enhanced in Hypertensive Rats

Several studies have emphasized an opioidergic link between the central regulation of cardiovascular function and acute noninflammatory pain. By contrast, relatively few studies have investigated the relationships between opioids, hypertension, and inflammatory pain. We used the formalin model of acute inflammatory pain to compare morphine antinociception among spontaneously hypertensive (SHR) rats, their genetic normotensive controls, Wistar-Kyoto (WKY) rats, and Sprague-Dawley (SD) rats. Measures of nociception included both behavioral and cardiovascular end-points (increased mean arterial blood pressure and heart rate). Morphine (3.0 mg/kg subcutaneously) produced greater hypotension and bradycardia in SHR than in WKY or SD rats. We next administered formalin (5%; 50 microL) and observed greater nociception during both Phase 1 and Phase 2 in SHR controls than in WKY controls. The morphine-treated groups did not differ, suggesting that morphine attenuates hypersensitivity to formalin pain in the SHR. Morphine inhibited edema but not paw hyperthermia to a greater degree in SHR, whereas Phase 1 remifentanil produced a relatively shorter delay in the onset of Phase 2 in SHR. We suggest that the presentation of essential hypertension be considered when opioid regimens are planned both during surgery (to minimize cardiovascular complications) and during the postoperative period (to optimize analgesic effects).

IMPLICATIONS

Presentation of essential hypertension should be considered when opioid regimens are planned both during surgery (to minimize cardiovascular complications) and during the postoperative period (to optimize analgesic effects).

The effect of CNS opioid on autonomic nervous and cardiovascular responses in diet-induced obese rats

The intracerebroventricular (i.c.v.) infusion of beta-endorphin can cause either a decrease in blood pressure in normal rats or an increase in obese rats. Diet-induced obesity is associated with an increase of hypothalamic mu opioid receptors. Since beta-endorphins act by opioid receptors, we investigated the effect of CNS mu as well as kappa opioid receptor agonist and antagonist on mean blood pressure (MAP), heart rate (HR) and renal sympathetic nerve activity (RSNA) in male Wistar rats fed either a high fat (HF) (40% fat by weight) or a regular low fat (control) (4% fat by weight) diet. After a 12-week-feeding period the animals were implanted with i.c.v. cannulas and 3-5 days later they were anesthetized and instrumented to record MAP, HR and RSNA. HF rats have higher MAP and the i.c.v. injection of a mu opioid agonist (DAMGO) initially decreased the MAP and then increased MAP, HR and RSNA in the normal animals. The increase was greater in HF animals. The i.c.v. injection of the mu antagonist (beta-FNA) resulted in a significantly greater decrease in MAP in HF animals. beta-FNA increased the RSNA in the HF rats but decreased it in the normal rats. The kappa agonist (dynorphin) decreased MAP in normal rats followed by a return to baseline, but not in HF rats. The kappa antagonist, nor-binaltorphimine (N-BP), increased MAP and RSNA in normal rats and to a lesser extent in HF rats. These findings suggest that rats given a high fat diet have higher blood pressures and a greater mu opioid-mediated responsiveness with a greater mu opioid-mediated autonomic tone. Additionally there is a decreased kappa responsiveness and tone in the HF rats. Both these changes, increased mu and decreased kappa responsiveness could strongly contribute to the increased blood pressure in obese animals.

High fat feeding is associated with increased blood pressure, sympathetic nerve activity and hypothalamic mu opioid receptors

Obesity and high fat diets are associated with an increased prevalence of diabetes, cardiovascular disease, and hypertension. However, the mechanism(s) linking obesity and high fat diet to these metabolic and cardiovascular disorders are not fully elucidated. Leptin stimulates the formation of pro-opiomelanocortin and its products. The stimulation of the central nervous system (CNS) opioids and their receptors is associated with an increase in cardiovascular dynamics. In this study we hypothesized that obesity changed the CNS opioids and their receptors that could play a role in altered cardiovascular and autonomic nervous regulation in obesity. Male Wistar rats were fed either a high fat (HF) or regular chow (control) diet. After 12 weeks, rats were anesthetized and instrumented to record mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA). A blood sample was collected and plasma glucose, insulin, leptin, beta-endorphins were measured. The brains were subsequently processed for immunohistochemistry and in situ hybridization. The HF rats were larger and had a greater percentage of body fat. Leptin and insulin levels were also higher in the HF animals. Basal MAP and RSNA were significantly higher in HF rats. Additionally, immunohistochemistry and in situ hybridization demonstrated that HF rats had increased hypothalamus mu opioid receptors compared to controls. These studies suggest that HF feeding is associated with increased body fat, plasma leptin, insulin, and hypothalamic mu opioid receptors. The increased mu opioid receptors may contribute to the higher MAP and RSNA observed in HF animals.

Comparison of response characteristics of cutaneous mechanoreceptors in normal and neuropathic Sprague-Dawley and spontaneously hypertensive rats

The activity of single myelinated afferents was recorded from dorsal roots L4-5 in normal Spontaneously Hypertensive rats (SHR) and animals that developed mechanical hypersensitivity following ischemic injury to the sciatic nerve. Control and neuropathic SHRs had significantly higher paw withdrawal threshold to mechanical stimulation than control and neuropathic Sprague-Dawleys (SD). In the SHR rats the mechanical response properties of afferents conducting through the injury site were similar to normals and many of the afferents not conducting through the injury site had spontaneous activity. The only significant difference between the two strains was a faster conduction velocity in afferents recorded from SHR than SD rats. Thus, the behavioral hyposensitivity and less development of mechanical allodynia of SHR rats, compared to SD is not due to differences in the properties of myelinated afferents, but probably involves differences in central inhibitory mechanisms in the two strains.

Possible impact of genetic differences on the development of neuropathic pain-like behaviors after unilateral sciatic nerve ischemic injury in rats

The development of neuropathic-like behaviors following unilateral ischemic injury to the sciatic nerve was examined and compared in four rat strains: Sprague--Dawley (SD), Wistar--Kyoto (WK), spontaneously hypertensive (SHR) and Dark--Agouti (DA). We have also compared two sub-strains of SD rats supplied from two different vendors (SD-BK and SD-DK). The responses to mechanical, heat or cold stimuli of both hind paws were measured before and regularly after injury for up to 10 weeks. Spontaneous paw lifting and changes in paw posture after nerve injury were also examined. Significant differences in basal sensitivity to mechanical or heat stimulation were seen among the four rat strain studied with SHR and DA rats being less sensitive than the SD and WK rats. All strains of rats developed bilateral mechanical allodynia and ipsilateral heat hyperalgesia after photochemically-induced nerve ischemia, but the time-course and magnitude of the responses were significantly different among the strains. Again, the SHR and DA were found to be least susceptible to the development of abnormal pain-like responses. Cold allodynia occurred only in WK and SD-BK. SD-DK rats on the other hand developed more severe mechanical allodynia than SD-BK. SHR and DA rats showed less deficits in paw posture after nerve injury whereas spontaneous pain lifting, a measure of possible spontaneous pain, was comparable among all strains. Light microscopic study of the injured sciatic nerve showed comparable nerve damage in SHR, WK and two sub-strains of SD rats. The DA rats however exhibited reduced area of intraneural damage. Finally, electronmicroscopic examination revealed that damage to both myelinated and unmyelinated fibers occurred in this model in all strains. These results showed that normal sensitivity and the development of pain-like response after partial nerve injury differ substantially among different strains of rats, supporting the emerging concept that genetic factors affect pain sensitivity under normal conditions and after nerve injury. The apparent resistance of DA rats to nerve ischemia, however, may suggest that genetic factors not directly related to pain modulation also play a role in the diverse outcomes. Our results indicate that sub-strains of rats also showed variable development of neuropathic pain-like behaviors to both the modality and magnitude of the effect. Thus, controlling sub-strains is also important in experimental studies of neuropathic pain in rats.

Genetic differences in morphine sensitivity, tolerance and withdrawal in rats

Significant genetic differences in the endogenous opioid system and in response to a variety of noxious stimuli are present in rodents. We now compared the response to noxious heat with the hot plate test, morphine sensitivity and the development of tolerance and dependence to morphine in spontaneously hypertensive (SHR), Wistar-Kyoto (WK) and Sprague-Dawley (SD) rats. Significant differences were observed in basal nociception among the three strains, where SHRs were hypoalgesic compared to WK and SD. The antinociceptive effect of morphine varied among strains (SD>SHR>WK) as did the rate of tolerance development (10 mg/kg morphine 2/day for 4 days) where WK>SD=SHR. SHR rats developed hyperalgesia following morphine administration during the course of tolerance development. Furthermore, although naloxone (2 mg/kg) precipitated withdrawal symptoms in all tolerant rats, the panorama of symptoms varied among the three strains. Thus, there are significant genetic differences in a variety of effect of opiates.

Genetic variation in immediate and delayed postictal refractory period in rats

We determined postictal refractoriness in Sprague-Dawley rats by comparing lengths of two suprathreshold ECS seizures given 15 s to 24 h apart. A bimodal (immediate and delayed) decrease in seizure duration was found, suggesting ECS alters mechanisms of seizure termination. Since adenosine is implicated in seizure termination, we determined immediate (30 s) and delayed (24 h) postictal ECS refractoriness in Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats which vary in adenosine properties and initial ECS seizure length. At 30 s, the decrease in seizures did not differ between WKY (-44%) and SHR (-36%) rats. At 24 h, SHR rats showed no change while the WKY rats showed a 20% decrease in seizure length (P < 0.01). These two strains also differed in the ability of the adenosine antagonist caffeine (50 mg/kg, i.p.) to prolong ECS seizures (no change for WKY, +13% for SHR, P < 0.001). The results suggest immediate and delayed postictal refractoriness are subject to genetic variation and may depend on central adenosine mechanisms.

Autoradiographic comparison of [3H]DPDPE and [3H]DSLET binding: evidence for distinct delta 1 and delta 2 opioid receptor populations in rat brain

The delta opioid ligands, [3H]DPDPE (delta 1) and [3H]DSLET (delta 2) were used in quantitative autoradiographic experiments to ascertain whether separate populations of delta opioid subtypes could be identified in rat brain. Densitometric image analysis showed a general similarity in delta 1 and delta 2 distributions. However, statistically significant differences in binding levels were observed in anatomically discrete regions. Examples of these regions and their delta 2/delta 1 ratio(s) are: dorsomedial hypothalamus (9.3), ventromedial hypothalamus (4.9), superior colliculis (2.7), medial division of bed nucleus stria terminalis (1.6-3.0), external cortex of the inferior colliculis (2.1), amygdaloid nuclei (1.5-2.1), cingulate cortex (1.8), CA1, CA2, and CA3 regions of Ammon's horn (1.6-2.0), dentate gyrus (1.7), laminar VI of the frontal, forelimb, hindlimb and parietal cortices (1.3-1.8), nucleus accumbens (1.4) and caudate/putamen (1.3). These findings provide evidence supporting the existence of distinct delta 1 and delta 2 opioid receptors.

Postsynaptic and extrasynaptic localization of kappa-opioid receptor in selected brain areas of young rat and chick using an anti-receptor monoclonal antibody

kappa-opioid receptors were visualized by light and electron microscopical immunohistochemistry in young rat and chick brains, using a monoclonal antibody KA8 (IgG1, kappa) raised against a kappa-opioid receptor preparation from frog brain, which recognizes selectively the kappa-type receptor with preference for the kappa-2 subtype. The most pronounced kappa-opioid receptor-like immunoreactivity was observed in the hypothalamic nuclei of the rat brain and in the chick optic tectum, in regions where the functional significance of kappa-opioid receptors is well documented. Both neurons and glia were stained, the former on both somata and dendrites. At the ultrastructural level, the receptor-like immunoreactivity was similar in both species. Immunoprecipitate decorated the inner surface of the plasma membrane of glial cells, neuronal somata and dendrites, in a discontinuous arrangement. In the cytoplasm, labelling was associated with ribosomes, polyribosomes and rough endoplasmic reticulum membranes but not with Golgi cisternae. In the neuropil, the immunoprecipitate was observed along the dendritic microtubules and was also associated with postsynaptic sites. Nuclei and axons were devoid of label and immunoreactivity was never visible presynaptically. Our findings indicate that the antibody used in the present study marks various forms of the kappa-opioid receptor protein including those synthesised in ribosomes, transported along dendritic microtubules and incorporated into postsynaptic and non-synaptic membranes. The antibody also recognizes glial opioid receptors. The observed subcellular distribution appears to be conserved in phylogenetically distant species.

Autoradiographic distribution of mu opioid receptors in the brains of Cu/Zn-superoxide dismutase mice

Superoxide dismutase (SOD) is an important free radical scavenging enzyme which dismutates the superoxide anion radical. We have evaluated the role of SOD in the regulation of opioid receptors by comparing the concentration of mu opioid receptors labeled with [3H]DAGO (Tyr-D-Ala-Gly-NMe-Phe-Gly-ol) in SOD-transgenic (SOD-Tg) mice and their non-transgenic (Non-Tg) littermates. SOD-Tg mice had higher maximal binding capacity (Bmax) in the shell division of the nucleus accumbens (NAc-shell) in comparison to Non-Tg littermates. There were no differences in Bmax in mu receptors in the core subdivision of the nucleus accumbens (NAc-core). There were no significant differences in receptor affinity (Kd) in either the NAc-shell or in the NAc-core. Moreover, there were no significant differences in either Bmax or Kd in the matrices nor in the patches of any of the striatal subdivisions. However, in a fashion similar to the situation in the NAc-shell, [3H]DAGO binding in the substantia nigra pars compacta (SNpc), the ventral tegmental area (VTA), and the ventral part of the central grey was significantly higher in the SOD-Tg mice in comparison to Non-Tg mice. The present results are discussed in terms of their support for a possible involvement of free radicals in the differences observed in various regions of the SOD-Tg and control mice, which differ in their ability to scavenge the superoxide anion.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic factors may influence the development of spinal reflex hyperexcitability following sciatic nerve section in the rat

We compared changes in the excitability of the hamstring flexor reflex evoked by electrical stimulation of the sciatic nerve before and after section of the peroneal, tibial and sural branches of the nerve in Sprague-Dawley (SD) and spontaneously hypertensive (SHR) rats. In both rat strains, the flexor reflex exhibited a biphasic hyperexcitability lasting over 120 min after nerve section. The initial brief, intense increase in reflex excitability was similar in SD and SHR rats. The second prolonged phase, which may represent hyperalgesia, was significantly greater in SHR than SD rats. These results indicate that genetic differences may have a role in the development of central sensitization.

Genetic factors influence the development of mechanical hypersensitivity, motor deficits and morphological damage after transient spinal cord ischemia in the rat

Genetic influence on the sensory, motor and histopathological outcome of transient spinal cord ischemia was studied in Sprague-Dawley (SD), spontaneously hypertensive (SH) and Wistar-Kyoto (WKY) rats. Ischemia caused minor motor deficits and no identifiable morphological damage in the spinal cord of the majority of SD rats after 1 min laser irradiation. However, severe mechanical hypersensitivity, characterized by an allodynia-like reaction to non-noxious tactile and pressure stimuli, was observed for several days in SD rats after spinal cord ischemia. SH rats exhibited less allodynia than SD rats, but developed profound motor deficits. WKY rats, the normotensive progenitor of SH rats, developed both severe mechanical allodynia and motor impairment after spinal cord ischemia. Histological examination revealed that there was significantly more extensive morphological damage in the epicenter of the irradiated segments of the spinal cord in SH and WKY rats than in SD rats after transient spinal cord ischemia. The results indicated that SH rats are more resistant to the development of central pain after central nervous system ischemia, but SH and WKY rats are more susceptible to neuronal damage than SD rats. Thus, genetic variability can lead to variable predisposition for the development of pain and spinal cord damage after ischemia. Such factors may underlie the wide variability in the occurrence of pain after central nervous system injury in humans.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.