The hypotension evoked by visceral nociception is mediated by delta opioid receptors in the periaqueductal gray

The contribution of periaqueductal gray in the regulation of physiological and pathological behaviors

Periaqueductal gray (PAG), an integration center for neuronal signals, is located in the midbrain and regulates multiple physiological and pathological behaviors, including pain, defensive and aggressive behaviors, anxiety and depression, cardiovascular response, respiration, and sleep-wake behaviors. Due to the different neuroanatomical connections and functional characteristics of the four functional columns of PAG, different subregions of PAG synergistically regulate various instinctual behaviors. In the current review, we summarized the role and possible neurobiological mechanism of different subregions of PAG in the regulation of pain, defensive and aggressive behaviors, anxiety, and depression from the perspective of the up-down neuronal circuits of PAG. Furthermore, we proposed the potential clinical applications of PAG. Knowledge of these aspects will give us a better understanding of the key role of PAG in physiological and pathological behaviors and provide directions for future clinical treatments.

Differential cardiovascular responses to cutaneous afferent subtypes in a nociceptive intersegmental spinal reflex

Electrical stimulation to segmental dorsal cutaneous nerves (DCNs) activates a nociceptive sensorimotor reflex and the same afferent stimulation also evokes blood pressure (BP) and heart rate (HR) responses in rats. To investigate the relationship between those cardiovascular responses and the activation of nociceptive afferents, we analyzed BP and HR responses to electrical stimulations at each DCN from T6 to L1 at 0.5 mA to activate A-fiber alone or 5 mA to activate both A- and C-fibers at different frequencies. Evoked cardiovascular responses showed a decrease and then an increase in BP and an increase and then a plateau in HR. Segmentally, both cardiovascular responses tended to be larger when evoked from the more rostral DCNs. Stimulation frequency had a larger effect on cardiovascular responses than the rostrocaudal level of the DCN input. Stimulation strength showed a large effect on BP changes dependent on C-fibers whereas HR changes were dependent on A-fibers. Additional A-fiber activation by stimulating up to 4 adjacent DCNs concurrently, but only at 0.5 mA, affected HR but not BP. These data support that cutaneous nociceptive afferent subtypes preferentially contribute to different cardiovascular responses, A-fibers to HR and C-fibers to BP, with temporal (stimulation frequency) and spatial (rostrocaudal level) dynamics.

Differential responses of neurons in the rat caudal ventrolateral medulla to visceral and somatic noxious stimuli and their alterations in colitis

Visceral and somatic types of pain have been reported to manifest crucial differences not only in the experience, but also in their peripheral and central processing. However, the precise neuronal mechanisms that responsible for the modality-specific transmission of pain signals, especially at the supraspinal level, remain unclear. Very little is known also about the potential involvement of such mechanisms in the development of viscero-somatic hyperalgesia. Therefore, in the present study performed on urethane-anesthetized adult male Wistar rats we examined responses of neurons in the caudal ventrolateral medulla (CVLM)-the first site for supraspinal processing of both internal and external pain signals-to visceral (colorectal distension, CRD) and somatic (squeezing of the tail) noxious stimulations and evaluated alterations in response properties of these cells after the induction of colitis. It has been found out that the CVLM of healthy control rats, along with harboring of cells excited by both stimulations (23.7%), contained neurons that were activated by either visceral (31.9%) or somatic noxious stimuli (44.4%). In inflamed animals, the percentages of the visceral and somatic nociceptive cells were decreased (to 18.3% and 34.3%, correspondingly) and the number of bimodal neurons was increased (up to 47.4%); these alterations were associated with substantially enhanced responses of both the modality-specific and convergent CVLM neurons not only to CRD, but also to squeezing of the tail. Under these conditions, visceral and somatic pain stimuli induced similar changes in arterial blood pressure and respiratory rate, whereas in the absence of intestinal inflammation noxious CRD and tail stimulation evoked predominantly divergent autonomic reactions. The data obtained can benefit to a deeper understanding of the neuronal mechanisms that underlie differential supraspinal processing of visceral and somatic noxious stimuli and can potentially contribute to the realization of specific cardiovascular and respiratory accompaniments inherent to a particular type of pain. Therewith, results of the study elucidate colitis-induced alterations in these mechanisms, which may be responsible for the combined development of visceral hypersensitivity and somatic hyperalgesia.

The initial fall in arterial pressure evoked by endotoxin is mediated by the ventrolateral periaqueductal gray

This study tested the hypothesis that the initial fall in arterial pressure evoked by lipopolysaccharide (LPS) is mediated by the ventrolateral column of the midbrain periaqueductal gray region (vlPAG). To test this hypothesis, the local anaesthetic lidocaine (2%; 0.1 μL, 0.2 μL or 1.0 μL), the delta opioid receptor antagonist naltrindole (2 nmol) or saline was microinjected into the vlPAG of isoflurane-anaesthetized rats bilaterally and LPS (1 mg/kg) or saline was administered intravenously 2 min later. Both lidocaine and naltrindole inhibited LPS-evoked hypotension significantly but did not affect arterial pressure in saline-treated control animals. Neither lidocaine nor naltrindole altered heart rate significantly in either LPS-treated or control animals. Microinjection of lidocaine or naltrindole into the dorsolateral PAG was ineffective. These data indicate that the vlPAG plays an important role in the initiation of endotoxic hypotension and further show that delta opioid receptors in the vlPAG participate in the response.

Adverse Drug Reactions in the Intensive Care Unit

Adverse drug reactions (ADRs) are undesirable effects of medications used in normal doses [1]. ADRs can occur during treatment in an intensive care unit (ICU) or result in ICU admissions. A meta-analysis of 4139 studies suggests the incidence of ADRs among hospitalized patients is 17% [2]. Because of underreporting and misdiagnosis, the incidence of ADRs may be much higher and has been reported to be as high as 36% [3]. Critically ill patients are at especially high risk because of medical complexity, numerous high-alert medications, complex and often challenging drug dosing and medication regimens, and opportunity for error related to the distractions of the ICU environment [4]. Table 1 summarizes the ADRs included in this chapter.

Hypovolemic hemorrhage induces Fos expression in the rat hypothalamus: Evidence for involvement of the lateral hypothalamus in the decompensatory phase of hemorrhage

This study tested the hypothesis that the hypothalamus participates in the decompensatory phase of hemorrhage by measuring Fos immunoreactivity and by inhibiting neuronal activity in selected hypothalamic nuclei with lidocaine or cobalt chloride. Previously, we reported that inactivation of the arcuate nucleus inhibited, but did not fully prevent, the fall in arterial pressure evoked by hypotensive hemorrhage. Here, we report that hemorrhage (2.2 ml/100g body weight over 20 min) induced Fos expression in a high percentage of cells in the paraventricular, supraoptic and arcuate nuclei of the hypothalamus as shown previously. Lower densities of Fos immunoreactive cells were also found in the medial preoptic area (mPOA), anterior hypothalamus, lateral hypothalamus (LH), dorsomedial hypothalamus, ventromedial hypothalamus (VMH) and posterior hypothalamus. Bilateral injection of lidocaine (2%; 0.1 μl or 0.3 μl) or cobalt chloride (5mM; 0.3 μl) into the tuberal portion of the LH immediately before hemorrhage was initiated reduced the magnitude of hemorrhagic hypotension and bradycardia significantly. Lidocaine injection into the VMH also attenuated the fall in arterial pressure and heart rate evoked by hemorrhage although inactivation of the mPOA or rostral LH was ineffective. These findings indicate that hemorrhage activates neurons throughout much of the hypothalamus and that a relatively broad area of the hypothalamus, extending from the arcuate nucleus laterally through the caudal VMH and tuberal LH, plays an important role in the decompensatory phase of hemorrhage.

Efferent projections of C3 adrenergic neurons in the rat central nervous system

C3 neurons constitute one of three known adrenergic nuclei in the rat central nervous system (CNS). While the adrenergic C1 cell group has been extensively characterized both physiologically and anatomically, the C3 nucleus has remained relatively obscure. This study employed a lentiviral tracing technique that expresses green fluorescent protein behind a promoter selective to noradrenergic and adrenergic neurons. Microinjection of this virus into the C3 nucleus enabled the selective tracing of C3 efferents throughout the rat CNS, thus revealing the anatomical framework of C3 projections. C3 terminal fields were observed in over 40 different CNS nuclei, spanning all levels of the spinal cord, as well as various medullary, mesencephalic, hypothalamic, thalamic, and telencephalic nuclei. The highest densities of C3 axon varicosities were observed in Lamina X and the intermediolateral cell column of the thoracic spinal cord, as well as the dorsomedial medulla (both commissural and medial nuclei of the solitary tract, area postrema, and the dorsal motor nucleus of the vagus), ventrolateral periaqueductal gray, dorsal parabrachial nucleus, periventricular and rhomboid nuclei of the thalamus, and paraventricular and periventricular nuclei of the hypothalamus. In addition, moderate and sparse projections were observed in many catecholaminergic and serotonergic nuclei, as well as the area anterior and ventral to the third ventricle, Lamina X of the cervical, lumbar, and sacral spinal cord, and various hypothalamic and telencephalic nuclei. The anatomical map of C3 projections detailed in this survey hopes to lay the first steps toward developing a functional framework for this nucleus.

Daily voluntary exercise alters the cardiovascular response to hemorrhage in conscious male rats

The present study tested the hypothesis that voluntary wheel-exercised rats would better tolerate severe hemorrhage (HEM) compared to age matched sedentary (SED) controls. Conscious rats housed with (EX, n = 8) or without (SED, n = 8) a running wheel for 6 weeks underwent a 30% total blood volume HEM over 15 min and were euthanized 90 min later and brain tissue was processed for Fos-like immunoreactivity (FLI). Both EX and SED groups displayed typical responses to HEM (initial tachycardia followed by decreased HR and MAP) but at the end of HEM, mean arterial pressure (93 ± 6 vs 58 ± 3 mm Hg) and heart rate (316 ± 17 vs. 247 ± 22 bpm,) were higher in the EX vs. SED animals and 60 min following the end of HEM, HR remained significantly elevated in the EX vs SED animals. The altered HR response to HEM in the EX animals was linked to a significant difference in sympatho-vagal drive identified by heart rate variability analysis and an augmented baroreflex response to hypotension tested in a separate group of animals (n = 4-5/group). In many of the brain regions analyzed, EX rats displayed lower levels of FLI compared to SED rats. Significantly lower levels of FLI in the EX vs SED rats were identified in the middle and caudal external lateral subnucleus of the lateral parabrachial nucleus and the dorsal cap of the hypothalamic paraventricular nucleus. These results suggest that enhanced tolerance to HEM following daily exercise may result from an EX-induced reduction in excitation or exaggerated inhibition in central circuits involved in autonomic control.

Transcutaneous electrical nerve stimulation at both high and low frequencies activates ventrolateral periaqueductal grey to decrease mechanical hyperalgesia in arthritic rats

Transcutaneous electric nerve stimulation (TENS) is widely used for the treatment of pain. TENS produces an opioid-mediated antinociception that utilizes the rostroventromedial medulla (RVM). Similarly, antinociception evoked from the periaqueductal grey (PAG) is opioid-mediated and includes a relay in the RVM. Therefore, we investigated whether the ventrolateral or dorsolateral PAG mediates antinociception produced by TENS in rats. Paw and knee joint mechanical withdrawal thresholds were assessed before and after knee joint inflammation (3% kaolin/carrageenan), and after TENS stimulation (active or sham). Cobalt chloride (CoCl(2); 5 mM) or vehicle was microinjected into the ventrolateral periaqueductal grey (vlPAG) or dorsolateral periaqueductal grey (dlPAG) prior to treatment with TENS. Either high (100 Hz) or low (4 Hz) frequency TENS was then applied to the inflamed knee for 20 min. Active TENS significantly increased withdrawal thresholds of the paw and knee joint in the group microinjected with vehicle when compared to thresholds prior to TENS (P<0.001) or to sham TENS (P<0.001). The increases in withdrawal thresholds normally observed after TENS were prevented by microinjection of CoCl(2) into the vlPAG, but not the dlPAG prior to TENS and were significantly lower than controls treated with TENS (P<0.001). In a separate group of animals, microinjection of CoCl(2) into the vlPAG temporarily reversed the decreased mechanical withdrawal threshold suggesting a role for the vlPAG in the facilitation of joint pain. No significant difference was observed for dlPAG. We hypothesize that the effects of TENS are mediated through the vlPAG that sends projections through the RVM to the spinal cord to produce an opioid-mediated analgesia.

Behavioral consequences of delta-opioid receptor activation in the periaqueductal gray of morphine tolerant rats

Chronic morphine administration shifts delta-opioid receptors (DORs) from the cytoplasm to the plasma membrane. Given that microinjection of morphine into the PAG produces antinociception, it is hypothesized that the movement of DORs to the membrane will allow antinociception to the DOR agonist deltorphin II as a way to compensate for morphine tolerance. Tolerance was induced by twice daily injections of morphine (5, 10, or 20 mg/kg, subcutaneous) for 3.5 days. Microinjection of deltorphin into the vPAG 6 hours after the last morphine injection produced a mild antinociception that did not vary in a consistent manner across morphine pretreatment doses or nociceptive tests. In contrast, deltorphin caused a decrease in activity in morphine tolerant rats that was associated with lying in the cage. The decrease in activity and change in behavior indicate that chronic morphine administration alters DORs in the vPAG. However, activation of these receptors does not appear to compensate for the decrease in antinociception caused by morphine tolerance.

Modulation of cardiovascular responses and neurotransmission during peripheral nociception following nNOS antagonism within the periaqueductal gray

Nitric oxide (NO) within the dorsal periaqueductal gray matter (dPAG) attenuated cardiovascular responses and changes in the concentrations of glutamate during both mechanical and thermal nociceptive stimulation [Ishide, T., Amer, A., Maher, T.J., Ally, A., 2005. Nitric oxide within periaqueductal gray modulates glutamatergic neurotransmission and cardiovascular responses during mechanical and thermal stimuli. Neurosci. Res. 51, 93-103]. Nitric oxide is synthesized from l-arginine via the enzyme, NO synthase (NOS), which exists in 3 isoforms: endothelial (eNOS), neuronal (nNOS), and inducible (iNOS). In this study, we examined the role of nNOS within the dPAG on cardiovascular responses and extracellular glutamate and GABA concentrations during mechanical and thermal nociception in anesthetized rats. The noxious mechanical stimulus was applied by a bilateral hindpaw pinch for 5 s that increased mean arterial pressure (MAP) and heart rate (HR) by 24+/-4 mm Hg and 41+/-7 bpm, respectively (n=10). Extracellular glutamate levels within the dPAG increased by 10.7+/-1.3 ng/mul while GABA concentrations decreased by 1.9+/-0.5 ng/microl. Bilateral microdialysis of a selective nNOS antagonist, 1-(2-trifluoromethylphenyl)-imidazole (TRIM; 10.0 microM), into the dPAG had no effect on MAP, HR, glutamate and GABA values (P>0.05) during a mechanical stimulation. In a separate set of experiments, a noxious thermal stimulus was generated by immersing the metatarsus of a hindpaw in a water-bath at 52 degrees C for 5 s (n=10). Glutamate, MAP, and HR increased by 14.6+/-2 ng/microl, 45+/-6 mm Hg, and 47+/-7 bpm, while GABA decreased by 2.1+/-0.6 ng/microl. Administration of TRIM into the dPAG significantly enhanced the cardiovascular responses and glutamate increases (P<0.05) but further attenuated GABA changes (P<0.05) during subsequent thermal nociception. These results demonstrate that nNOS within the dPAG plays a differential role in modulating cardiovascular responses and glutamatergic/GABAergic neurotransmission during thermal and mechanical nociception.

Cardiac baroreflex facilitation evoked by hypothalamus and prefrontal cortex stimulation: role of the nucleus tractus solitarius 5-HT2Areceptors

We previously showed that serotonin (5-HT2) receptor activation in the nucleus of the tractus solitarius (NTS) produced hypotension, bradycardia, and facilitation of the baroreflex bradycardia. Activation of the preoptic area (POA) of the hypothalamus, which is involved in shock-evoked passive behaviors, induces similar modifications. In addition, previous studies showed that blockade of the infralimbic (IL) part of the medial prefrontal cortex, which sends projections to POA, produced an inhibitory influence on the baroreflex cardiac response. Thus, to assess the possible implication of NTS 5-HT2receptors in passive cardiovascular responses, we analyzed in anesthetized rats the effects of NTS inhibition and NTS 5-HT2receptor blockade on the cardiovascular modifications induced by chemical (0.3 M d,l-homocysteic acid) and electrical (50 Hz, 150–200 μA) stimulation of IL or POA. Intra-NTS microinjections of muscimol, a GABAAreceptor agonist, prevented the decreases in blood pressure and heart rate normally evoked by IL or POA activation. In addition, we found that intra-NTS microinjection of R(+)-α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol, a specific 5-HT2Areceptor antagonist, did not affect the decreases in cardiovascular baseline parameters induced by IL or POA stimulation but prevented the facilitation of the aortic baroreflex bradycardia normally observed during IL (+65 and +60%) or POA (+70 and +69%) electrical and chemical stimulation, respectively. These results show that NTS 5-HT2Areceptors play a key role in the enhancement of the cardiac response of the baroreflex but not in the changes in basal heart rate and blood pressure induced by IL or POA stimulation.

ARD-353 [4-((2R,5S)-4-(R)-(4-diethylcarbamoylphenyl)(3-hydroxyphenyl)methyl)-2,5-dimethylpiperazin-1-ylmethyl)benzoic acid], a novel nonpeptide delta receptor agonist, reduces myocardial infarct size without central effects

A novel delta-receptor selective compound, ARD-353 [4-((2R,5S)-4-(R)-(4-diethylcarbamoylphenyl)(3-hydroxyphenyl)methyl)-2, 5-dimethylpiperazin-1-ylmethyl)benzoic acid], was evaluated for activity on infarct size in a rat model of acute myocardial infarction. ARD-353 was characterized as having delta receptor selectivity using radioligand binding and had no apparent selectivity between delta receptor subtypes as determined by [(3)H] cyclic [D-Pen(2),D-Pen(5)]enkephalin (delta(1)) and [(3)H]Deltorphin II (delta(2)) competition binding. ARD-353 also showed selective delta receptor agonist activity in mouse-isolated vas deferens. There was no evidence of any seizure-like convulsions when ARD-353 was administered to mice either i.v. or p.o., implying minimal penetration of the blood-brain barrier. ARD-353 decreased infarct size in a left anterior descending coronary artery (LAD) occlusion model of myocardial infarction. In animals pretreated with ARD-353 (i.v.) and then subjected to 30 min of LAD occlusion followed by 90 min of reperfusion, infarct size was reduced in a dose-dependent manner compared with vehicle-treated controls. The effects of ARD-353 on infarct size were blocked by the delta(1)-opioid selective antagonist 7-benzylidenenaltrexone, indicating a significant role for the delta(1)-opioid receptor in the cardioprotective mechanism of ARD-353. ARD-353 (0.3 mg/kg i.v.) produced significant protection when administered 5 min and 12 and 48 h before ischemic insult or when given immediately after the ischemic insult (at the start of reperfusion). Given the lack of central nervous system effects and beneficial efficacy in the rat model of myocardial ischemia, it is felt that ARD-353 is the first nonpeptide delta-receptor agonist with true potential for clinical use before surgically induced ischemia or in an emergency setting.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Muscimol injection into the lateral hypothalamus inhibits the hypotension and bradycardia caused by somato-visceral nociception

This study investigated whether the lateral hypothalamus (LH) contributes to the depressor response evoked by somato-visceral nociception. Lidocaine (2%; 0.1, 0.3 or 1.0 microl) or muscimol (0.34 nmol; 0.5 microl) was microinjected into the rostral LH of halothane-anesthetized rats bilaterally and somato-visceral nociception was induced 2 min later by injecting 5% acetic acid (0.5 ml) intraperitoneally. Lidocaine and muscimol inhibited the hypotension and bradycardia caused by somato-visceral nociception significantly without affecting cardiovascular function in normotensive animals.