Functional interactions between delta- and mu-opioid receptors in rat thermoregulation

Effect of different concentrations of sufentanil combined with ropivacaine on epidural fever during labor: A single-center retrospective study

Labor epidural analgesia (LEA) is associated with increased maternal body temperature; however, the responsible mechanism is unknown. Recent studies suggest that changes in EA affect the incidence of fever and that epidural sufentanil supplementation enhances analgesia and reduces the amount of local anesthetic. The aim of this study was to evaluate the effect of different concentrations of sufentanil combined with ropivacaine on intrapartum fever during delivery. We performed a retrospective study comparing maternal fever rates in patients receiving labor analgesia between December 2018 and January 2019. Each patient receiving different concentrations of sufentanil in their EA received either proposal H (0.08% ropivacaine + 0.4 µg/mL sufentanil) or proposal L (0.08% ropivacaine + 0.2 µg/mL sufentanil), with the same nulliparous status. The primary outcome of this study was the incidence of intrapartum maternal fever, which was defined as any temperature ≥ 38°C during labor using Fisher exact test. Secondary outcome measures included visual analog scale (VAS) pain scores, birth events, and neonatal outcomes. We observed a perinatal fever incidence rate of 11.7% in the group receiving proposal L, while the incidence rate was 19.8% in the group receiving proposal H (P = .001). Five hours after administration, the average body temperature of the puerpera decreased significantly in the proposal L group compared with proposal H group. In addition, treatment with 0.2 µg/mL sufentanil provided satisfactory pain relief during labor, shortened the first stage of labor and total labor time, reduced oxytocin use, and had no significant adverse effects on neonatal outcomes. EA may increase the risk of intrapartum epidural-associated fever. Compared with the 0.4 µg/mL sufentanil group, the 0.2 µg/mL sufentanil group can provide better analgesia and improve maternal fever. These retrospective results highlighted the importance of prospective and mechanistic studies of maternal fever associated with intraspinal analgesia.

Inhibition of endothelin A receptor by a novel, selective receptor antagonist enhances morphine-induced analgesia: Possible functional interaction of dimerized endothelin A and μ-opioid receptors

BACKGROUND

The misuse of opioids has led to an epidemic in recent times. The endothelin A receptor (ETAR) has recently attracted attention as a novel therapeutic target to enhance opioid analgesia. We hypothesized that endothelin A receptors may affect pain mechanisms by heterodimerization with μ opioid receptors. We examined the mechanisms of ETAR-mediated pain and the potential therapeutic effects of an ETAR antagonist, Compound-E, as an agent for analgesia.

METHODS

Real-time in vitro effect of Compound-E on morphine response was assessed in HEK293 cells expressing both endothelin A and μ opioid receptors through CellKey™ and cADDis cAMP assays. Endothelin A/μ opioid receptor dimerization was assessed by immunoprecipitation and live cell imaging. The in vivo effect of Compound-E was evaluated using a morphine analgesia mouse model that observed escape response behavior, body temperature, and locomotor activity.

RESULTS

In CellKey™ and cAMP assays, pretreatment of cells with endothelin-1 attenuated morphine-induced responses. These responses were improved by Compound-E, but not by BQ-123 nor by bosentan, an ETAR and endothelin B receptor antagonist. Dimerization of ETARs and μ opioid receptors was confirmed by Western blot and total internal reflection fluorescence microscopy in live cells. In vivo, Compound-E potentiated and prolonged the analgesic effects of morphine, enhanced hypothermia, and increased locomotor activity compared to morphine alone.

CONCLUSION

The results suggest that attenuation by endothelin-1 of morphine analgesia may be caused by dimerization of Endothelin A/μ opioid receptors. The novel ETAR antagonist Compound-E could be an effective adjunct to reduce opioid use.

Activation of Kappa Opioid Receptor Regulates the Hypothermic Response to Calorie Restriction and Limits Body Weight Loss

Mammals maintain a nearly constant core body temperature (T_b) by balancing heat production and heat dissipation. This comes at a high metabolic cost that is sustainable if adequate calorie intake is maintained. When nutrients are scarce or experimentally reduced such as during calorie restriction (CR), endotherms can reduce energy expenditure by lowering T_b [1-6]. This adaptive response conserves energy, limiting the loss of body weight due to low calorie intake [7-10]. Here we show that this response is regulated by the kappa opioid receptor (KOR). CR is associated with increased hypothalamic levels of the endogenous opioid Leu-enkephalin, which is derived from the KOR agonist precursor dynorphin [11]. Pharmacological inhibition of KOR, but not of the delta or the mu opioid receptor subtypes, fully blocked CR-induced hypothermia and increased weight loss during CR independent of calorie intake. Similar results were seen with DIO mice subjected to CR. In contrast, inhibiting KOR did not change T_b in animals fed ad libitum (AL). Chemogenetic inhibition of KOR neurons in the hypothalamic preoptic area reduced the CR-induced hypothermia, whereas chemogenetic activation of prodynorphin-expressing neurons in the arcuate or the parabrachial nucleus lowered T_b. These data indicate that KOR signaling is a pivotal regulator of energy homeostasis and can affect body weight during dieting by modulating T_b and energy expenditure.

Entanglement between thermoregulation and nociception in the rat: the case of morphine

In thermoneutral conditions, rats display cyclic variations of the vasomotion of the tail and paws, the most widely used target organs in current acute or chronic animal models of pain. Systemic morphine elicits their vasoconstriction followed by hyperthermia in a naloxone-reversible and dose-dependent fashion. The dose-response curves were steep with ED50 in the 0.5-1 mg/kg range. Given the pivotal functional role of the rostral ventromedial medulla (RVM) in nociception and the rostral medullary raphe (rMR) in thermoregulation, two largely overlapping brain regions, the RVM/rMR was blocked by muscimol: it suppressed the effects of morphine. "On-" and "off-" neurons recorded in the RVM/rMR are activated and inhibited by thermal nociceptive stimuli, respectively. They are also implicated in regulating the cyclic variations of the vasomotion of the tail and paws seen in thermoneutral conditions. Morphine elicited abrupt inhibition and activation of the firing of on- and off-cells recorded in the RVM/rMR. By using a model that takes into account the power of the radiant heat source, initial skin temperature, core body temperature, and peripheral nerve conduction distance, one can argue that the morphine-induced increase of reaction time is mainly related to the morphine-induced vasoconstriction. This statement was confirmed by analyzing in psychophysical terms the tail-flick response to random variations of noxious radiant heat. Although the increase of a reaction time to radiant heat is generally interpreted in terms of analgesia, the present data question the validity of using such an approach to build a pain index.

Effects of opioids, cannabinoids, and vanilloids on body temperature

Cannabinoid and opioid drugs produce marked changes in body temperature. Recent findings have extended our knowledge about the thermoregulatory effects of cannabinoids and opioids, particularly as related to delta opioid receptors, endogenous systems, and transient receptor potential (TRP) channels. Although delta opioid receptors were originally thought to play only a minor role in thermoregulation compared to mu and kappa opioid receptors, their activation has been shown to produce hypothermia in multiple species. Endogenous opioids and cannabinoids also regulate body temperature. Mu and kappa opioid receptors are thought to be in tonic balance, with mu and kappa receptor activation producing hyperthermia and hypothermia, respectively. A particularly intense research focus is TRP channels, where TRPV1 channel activation produces hypothermia whereas TRPA1 and TRPM8 channel activation causes hyperthermia. The marked hyperthermia produced by TRPV1 channel antagonists suggests these warm channels tonically control body temperature. A better understanding of the roles of cannabinoid, opioid, and TRP systems in thermoregulation may have broad clinical implications and provide insights into interactions among neurotransmitter systems involved in thermoregulation.

Effects of opioids, cannabinoids, and vanilloids on body temperature

Cannabinoid and opioid drugs produce marked changes in body temperature. Recent findings have extended our knowledge about the thermoregulatory effects of cannabinoids and opioids, particularly as related to delta opioid receptors, endogenous systems, and transient receptor potential (TRP) channels. Although delta opioid receptors were originally thought to play only a minor role in thermoregulation compared to mu and kappa opioid receptors, their activation has been shown to produce hypothermia in multiple species. Endogenous opioids and cannabinoids also regulate body temperature. Mu and kappa opioid receptors are thought to be in tonic balance, with mu and kappa receptor activation producing hyperthermia and hypothermia, respectively. A particularly intense research focus is TRP channels, where TRPV1 channel activation produces hypothermia whereas TRPA1 and TRPM8 channel activation causes hyperthermia. The marked hyperthermia produced by TRPV1 channel antagonists suggests these warm channels tonically control body temperature. A better understanding of the roles of cannabinoid, opioid, and TRP systems in thermoregulation may have broad clinical implications and provide insights into interactions among neurotransmitter systems involved in thermoregulation.

Expression of material mRNA in the hypothalamus and frontal cortex in a rat model of fatal hypothermia

Diagnosis of fatal hypothermia is considered to be difficult in forensic practice because of the lack of any specific pathological findings. The mechanism that induces abnormal behavior such as undressing or hiding during the state of hypothermia has not been clarified. In order to solve these problems, we made a rat model of fatal hypothermia and investigated the expression of some mRNA within the hypothalamus and the frontal cortex. The expression of aldehyde dehydrogenase 6 family, member A1 (ALDH6A1), cocaine- and amphetamine-regulated transcript peptide (CARTPT), desmin (DES), heat shock 70kDa protein 4 (HSPA4), serotonin receptor 2A (HTR2A), opioid receptor, delta 1 (OPRD1) and transthyretin (TTR) supposedly related to fatal hypothermia was determined using quantitative real-time PCR. The expression of OPRD1 in the hypothalamus of fatal hypothermia was significantly increased, while the expression of TTR within the frontal cortex was significantly decreased compared to that in the control. These findings suggest that OPRD1 and TTR may be involved in thermoregulation at a low ambient temperature.

Methionine-enkephalin modulation of hydrogen peroxide (H2O2) release by rat peritoneal macrophages involves different types of opioid receptors

We investigated the involvement of specific types of opioid receptors in methionine-enkephalin (MET)-induced modulation of hydrogen peroxide (H2O2) release by rat macrophages primed with sub-optimal concentrations of phorbol myristate acetate (PMA). Peritoneal macrophages in vitro treated with different concentrations of MET were tested for H2O2 release in phenol red assay. In the antagonistic study macrophages were treated with MET and one opioid receptor antagonist, or combination of MET and two or three opioid receptor antagonists. MET decreased H2O2 release in eight individual macrophage samples, and increased it in 10 samples. The increase of H2O2 release induced by MET in macrophages was blocked with combination of opioid receptor antagonists specific delta1,2 and mu receptors, as well as with combination of antagonists specific for delta1,2 and kappa opioid receptors. MET-induced decrease of the H2O2 release in macrophages was prevented by opioid receptor antagonists specific for delta1,2 or mu receptors, and also with combination of two or three opioid receptor antagonists. MET-induced enhancement of H2O2 release was mediated via delta1 or delta2 opioid receptor subtypes, or by mu-kappa opioid receptor functional interactions, while MET-induced suppression involved functional interactions between delta1 and mu, delta2 and mu, or delta1 and kappa opioid receptors. It is possible that individual differences in basal or induced macrophage capacity to produce H2O2 might shape the repertoire of opioid receptors expression and in that way pre-determine the direction of MET-induced changes after the in vitro treatment.

Modulation of delta opioid-evoked hypothermia in rats by WAY 100635 and fluoxetine

Delta opioid receptor and 5-hydroxytryptamine (5-HT) interactions in rats were investigated using the endpoint of hypothermia. The intraperitoneal (i.p.) administration of SNC-80, a delta opioid agonist (35 mg/kg, i.p.), produced a significant hypothermia. For combined administration, SNC-80-evoked hypothermia was (1) abolished by pre-treatment with naltrindole (5 mg/kg, i.p.); (2) attenuated by pre-treatment with WAY 100635 (1 mg/kg, s.c.), a 5-HT1A antagonist; and (3) enhanced by pre-treatment with non-hypothermic doses of fluoxetine (2.5, 5 and 10 mg/kg, i.p.). The present data reveal that 5-HT1A receptor activation mediates a significant proportion of the hypothermic response to delta opioid receptor activation and that a 5-HT uptake blockade potentiates delta receptor-induced hypothermia.

Brain delta2 opioid receptors mediate SNC-80-evoked hypothermia in rats

Despite insights into an increasingly significant role for delta opioid receptors in thermoregulation, it is unclear whether delta receptors located in the brain or periphery play the more critical role in body temperature regulation. Moreover, it is not entirely clear which delta receptor phenotype, delta1 or delta2, mediates the hypothermic actions of delta agonists. Because SNC-80 distributes into central and peripheral compartments and produces rapid hypothermia following systemic injection, the nonpeptide delta agonist is particularly useful in discriminating the site of action of delta receptor-mediated hypothermia. To determine the locus and phenotype of delta receptor which mediates SNC-80-induced hypothermia, we injected SNC-80 and phenotype selective delta antagonists to male Sprague-Dawley rats. SNC-80 (10-50 mg/kg, im) evoked hypothermia that peaked 30 min post-injection. Naltrexone (5 mg/kg, sc), an opioid antagonist, or naltrindole (5 mg/kg, sc), a delta antagonist, blocked the hypothermic response to SNC-80 (35 mg/kg, im). The hypothermia caused by SNC-80 (35 mg/kg, im) was blocked by a delta2 antagonist, naltriben (2.5 mg/kg, sc), but was not affected by BNTX (5 and 10 mg/kg, sc), a delta1 antagonist. The administration of naltriben (10 microg/rat, icv) 30 min before SNC-80 (35 mg/kg, im) prevented SNC-80-evoked hypothermia. In contrast, methylnaltrexone (5 mg/kg, sc), a peripherally restricted opioid antagonist, did not affect the hypothermia caused by SNC-80. The present data demonstrate that selective activation of brain delta2 receptors is a major mechanism of SNC-80-evoked hypothermia in rats.

The dynamic relationship between mu and kappa opioid receptors in body temperature regulation

Previous studies demonstrated that intracerebroventricular (icv) injection of a kappa opioid receptor agonist decreased, and a mu agonist increased, body temperature (Tb) in rats. A dose-response study with the selective kappa antagonist nor-binaltorphimine (nor-BNI) showed that a low dose (1.25 nmol, icv) alone had no effect, although a high dose (25 nmol, icv) increased Tb. It was hypothesized that the hyperthermia induced by nor-BNI was the result of the antagonist blocking the kappa opioid receptor and releasing its inhibition of mu opioid receptor activity. To determine whether the Tb increase caused by nor-BNI was a mu receptor-mediated effect, we administered the selective mu antagonist CTAP (1.25 nmol, icv) 15 min after nor-BNI (25 nmol, icv) and measured rectal Tb in unrestrained rats. CTAP significantly antagonized the Tb increase induced by icv injection of nor-BNI. Injection of 5 or 10 nmol of CTAP alone significantly decreased the Tb, and 1.25 nmol of nor-BNI blocked that effect, indicating that the CTAP-induced hypothermia was kappa-mediated. The findings strongly suggest that mu antagonists, in blocking the basal hyperthermia mediated by mu receptors, can unmask the endogenous kappa receptor-mediated hypothermia, and that there is a tonic balance between mu and kappa opioid receptors that serves as a homeostatic mechanism for maintaining Tb.

Anti-exudative effects of opioid receptor agonists in a rat model of carrageenan-induced acute inflammation of the paw

We evaluated the anti-exudative effects (Evan's blue) of mu-, delta- and kappa-opioid receptor agonists in a rat model of carrageenan-induced acute inflammation. The contribution of different components was assessed after the administration of: cyclosporine A, capsaicin, 6-hydroxydopamine, compound 48/80, and specific histamine-receptor antagonists. The results show that the mu-opioid receptor agonists morphine and fentanyl and the delta-opioid receptor agonists DPDPE (enkephalin, [D-Pen(2,5)]) and SNC 80 ((+)-4-[(alpha R)-alpha((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N diethylbenzamide) decrease plasma extravasation in a dose-dependent manner, with a biphasic response. The effects were reversed by specific antagonists, and are predominantly mediated by peripheral opioid receptors. The integrity of sensory and sympathetic fibres is essential for the anti-exudative effects of fentanyl and DPDPE. Histamine and functional histamine H(2) and H(3) receptors are required for morphine and fentanyl (but not DPDPE) inhibition of plasma extravasation, suggesting different mechanism for mu- and delta-opioid receptor agonists. The present findings implicate multiple sites and mechanisms in the anti-exudative effects of exogenous opioids.

Peripheral kappa-opioid agonist, ICI 204448, evokes hypothermia in cold-exposed rats

ICI 204448, a selective kappa-opioid agonist with limited CNS access, can be used to discriminate central and peripheral opioid actions on physiological systems such as pain and thermoregulation. Therefore, we investigated the effect of ICI 204448 (2.5, 5, and 10 mg/kg, s.c.) on male Sprague-Dawley rats exposed to ambient temperatures of 5, 20, or 32 degrees C. ICI 204448 did not alter the body temperature of rats maintained at 20 or 32 degrees C. However, 5 and 10 mg/kg of ICI 204448 evoked significant hypothermia in rats exposed to 5 degrees C. The i.c.v. administration of nor-BNI, a kappa-opioid antagonist, did not affect the hypothermia produced by the systemic injection of ICI 204448. Thus, an involvement of brain kappa-opioid receptors in ICI 204448-evoked hypothermia is unlikely. The present data demonstrate for the first time that ICI 204448 produces hypothermia in cold-exposed rats and suggest that the role of peripheral kappa-opioid receptors in thermoregulation becomes more significant at cold ambient temperatures.

Brown adipose tissue thermogenesis contributes to fentanyl-evoked hyperthermia

Mu-opioid receptor activation increases body temperature and affects cardiovascular function. In the present study, fentanyl was administered intravenously [100 mug/kg (300 nmol/kg) iv] and intracerebroventricularly [3.4 mug (10 nmol) in 10 microl icv] in urethane-chloralose-anesthetized, artificially ventilated rats. Increases in brown adipose tissue (BAT) sympathetic nerve activity (SNA) (peak, +326% of control), BAT temperature (peak, +0.8 degrees C), renal SNA (peak, +146% of control), and heart rate (HR; peak, +32 beats/min) produced by intravenous fentanyl were abolished by premamillary transection of the neuraxis but were mimicked by intracerebroventricular administration of fentanyl, which also increased arterial pressure (AP; peak, +12 mmHg). Pretreatment with the opioid antagonist naloxone (100 nmol in 10 microl icv) eliminated the intracerebroventricular fentanyl-evoked responses. Microinjection of glycine (0.5 M, 60 nl) to inhibit local neurons in the rostral raphe pallidus (RPa) selectively reversed the intracerebroventricular fentanyl-evoked increases in BAT SNA and HR, while the fentanyl-evoked excitation in RSNA, the pressor responses, and the tachycardic responses were reversed by inhibition of neurons in the rostral ventrolateral medulla (RVLM). Prior inhibition of neurons in the dorsomedial hypothalamus eliminated the intracerebroventricular fentanyl-evoked increases in BAT SNA, BAT temperature, and HR, but not those in RSNA or AP. These results indicate that activation of central mu-opioid receptors with fentanyl can elicit BAT thermogenesis and cardiovascular stimulation through excitation of the sympathetic outflows to BAT, kidney, and heart. Activation of neurons in the rostral RPa and RVLM are essential for the increases in BAT thermogenesis and renal sympathoexcitation, respectively, induced by activation of central mu-opioid receptors. BAT thermogenesis could contribute to fentanyl-evoked hyperthermia, particularly in infants where BAT plays a significant role in thermoregulation.

Intrahypothalamic injection of deltorphin-II alters body temperature in rats

The present study investigated the effect of H-Try-d-Ala-Phe-Glu-Val-Gly-NH2 deltorphin-II, a selective delta-2 agonist, and [d-Pen(2),d-Pen(5)]enkephalin, a selective delta-1 agonist, on body temperature in the rat. Microinjected into the preoptic anterior hypothalamus (POAH), deltorphin-II (0.1-1 microg) produced an immediate dose-related hyperthermia following injection. Injection of the delta-2 antagonist naltriben into the preoptic anterior hypothalamus (1 microg, 30 min prior to deltorphin-II) significantly attenuated the deltorphin-II-induced hyperthermia. Microinjection of [d-Pen(2),d-Pen(5)]enkephalin into the POAH (0.1-3 microg) did not affect Tb. The data demonstrate that delta-2 receptors are involved in the mediation of Tb effects, and deltorphin-II exerts its action directly on thermosensitive cells of the preoptic anterior hypothalamus. Delta-1 opioid receptors do not appear to be involved in the control of body temperature.

Role of the nitric-oxide synthase isoforms during morphine-induced hyperthermia in rats

Recently, we demonstrated that the diffusible messenger molecule nitric oxide (NO) is involved in the hyperthermic response induced by morphine by using a nonselective nitric-oxide synthase inhibitor, N-nitro-L-arginine methyl ester. The present work extended these studies to include 7-nitroindazole (7-NI), an inhibitor specific for neuronal nitric-oxide synthase (nNOS), N(5)-(-iminoethyl)-L-ornithine (L-NIO), an inhibitor of endothelial NOS (eNOS), and aminoguanidine (AG), a potent inhibitor of inducible NOS (iNOS). A biotelemetry system was used in this study to measure the body temperature (Tb). A dose of 7-NI (5 or 10 mg/kg), which did not affect Tb by itself, blocked the hyperthermia induced by morphine in a dose-dependent manner (15 mg/kg i.p.). However, pretreatment with L-NIO (10-20 mg/kg) or with AG (50 mg/kg) failed to alter the hyperthermia induced by morphine. L-NIO (10-20 mg/kg) or AG (50 mg/kg) had no effect on Tb. These results suggest the involvement of nNOS in morphine-induced hyperthermia.