Selective distribution of mu-opioid receptors in C1 adrenergic neurons and their afferents

Regularly swimming exercise modifies opioidergic neuromodulation in rostral ventrolateral medulla in hypertensive rats

Moderate exercise reduces arterial pressure (AP) and heart rate (HR) in spontaneously hypertensive rats (SHR) and changes neurotransmission in medullary areas involved in cardiovascular regulation. We investigated if regularly swimming exercise (SW) affects the cardiovascular adjustments mediated by opioidergic neuromodulation in the RVLM in SHR and Wistar-Kyoto (WKY) rats. Rats were submitted to 6 wks of SW. The day after the last exercise bout, α-chloralose-anesthetized rats underwent a cannulation of the femoral artery for AP and HR recordings, and Doppler flow probes were placed around the lower abdominal aorta and superior mesenteric artery. Bilateral injection of endomorphin-2 (EM-2, 0.4 mmol/L, 60 nL) into the RVLM increased MAP in SW-SHR (20 ± 4 mmHg, N = 6), which was lower than in sedentary (SED)-SHR (35 ± 4 mmHg, N = 6). The increase in MAP in SW-SHR induced by EM-2 into the RVLM was similar in SED- and SW-WKY. Naloxone (0.5 mmol/L, 60 nL) injected into the RVLM evoked an enhanced hypotension in SW-SHR (-66 ± 8 mmHg, N = 6) compared to SED-SHR (-25 ± 3 mmHg, N = 6), which was similar in SED- and SW-WKY. No significant changes were observed in HR after EM-2 or naloxone injections into the RVLM. Changes in hindquarter and mesenteric conductances evoked by EM-2 or naloxone injections into the RVLM in SW- or SED-SHR were not different. Mu Opioid Receptor expression by Western blotting was reduced in SW-SHR than in SED-SHR and SW-WKY. Therefore, regularly SW alters the opioidergic neuromodulation in the RVLM in SHR and modifies the mu opioid receptor expression in this medullary area.

Activation of µ-opioid receptors in the rostral ventrolateral medulla blocks the sympathetic counterregulatory response to glucoprivation

Activation of neurons in the rostral ventrolateral medulla (RVLM) following glucoprivation initiates sympathoadrenal activation, adrenaline release, and increased glucose production. Here, we aimed to determine the role of RVLM µ-opioid receptors in the counterregulatory response to systemic glucoprivation. Experiments were performed in pentobarbital sodium anesthetized male Sprague-Dawley rats ( n = 30). Bilateral activation of RVLM µ-opioid receptors with [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO) (8 mM, 50 nl) depressed adrenal sympathetic nerve activity for ~60 min ( n = 6; Δ49.9 ± 5.8%, P < 0.05). The counterregulatory response to glucoprivation (measured by adrenal sympathetic efferent nerve activity) induced by 2-deoxyglucose (2-DG) ( n = 6; Δ63.6 ± 16.5%, P < 0.05) was completely blocked 60 min after DAMGO microinjections ( n = 6; Δ10.2 ± 3.5%, P < 0.05). Furthermore, DAMGO pretreatment attenuated the increase in blood glucose levels after 2-DG infusion ( n = 6; 6.1 ± 0.7mmol/l vs. baseline 5.2 ± 0.3mmol/l, P > 0.05) compared with 2-DG alone ( n = 6; 7.6 ± 0.4mmol/l vs. baseline 6.0 ± 0.4mmol/l, P < 0.05). Thus, activation of RVLM µ-opioid receptors attenuated the neural efferent response to glucoprivation and reduced glucose production.

Comparison of propofol-hydromorphone and propofol-dexmedetomidine in patients with intubation after maxillofacial plastic surgery

Objective

To compare the sedation and analgesic effects between propofol–hydromorphone and propofol–dexmedetomidine in patients with postoperative intubation after maxillofacial plastic surgery.

Methods

Forty-two patients undertaking maxillofacial plastic surgery with intubation were randomly assigned into propofol plus hydromorphone (P–H) group or propofol plus dexmedetomidine (P–D) group, receiving intravenous infusion of P–H or P–D, respectively. Cerebral state index, Ramsay sedation score, arterial blood gas analysis, and physiology indices were recorded before admission (T0), 30 minutes (T1), 1 hour (T2), 2 hours (T3), 6 hours (T4), and 12 hours after admission (T5) to intensive care unit, and 10 minutes after extubation (T6). Blood interleukin-6 was measured with enzyme-linked immunosorbent assay.

Results

There was no significant difference in arterial blood gas analysis, oxygen saturation, mean arterial pressure, and respiratory rate between two groups at all time-points (P>0.05). The changes of heart rate (at T4, T5, and T6), cerebral state index (T1, T2, T3, T4, and T5), and Ramsay score (at T3) in P–H group were significantly different from that in P–D group (P<0.05). The plasma interleukin-6 at T4 in P–H group was significantly lower than that in P–D group (P<0.05).

Conclusion

The P–H approach takes advantages over P–D approach in relieving the pain and discomfort, reducing the overstimulation of sympathetic nerve and the stress level, and enhancing the tolerance of postoperative intubation after maxillofacial plastic surgery.

Remote intrathecal morphine preconditioning confers cardioprotection via spinal cord nitric oxide/cyclic guanosine monophosphate/protein kinase G pathway

BACKGROUND

Remote intrathecal morphine preconditioning (RMPC) induces cardioprotection, but the underlying mechanisms of this effect is unknown. The aim of this study was to investigate the role of spinal cord nitric oxide/cyclic guanosine monophosphate/protein kinase G (NO/cGMP/PKG) signal pathway in the cardioprotection of RMPC in rats.

MATERIALS AND METHODS

Anesthetized, open chest, male Sprague-Dawley rats were assigned to one of eight treatment groups 3 d after intrathecal catheter placement. Before ischemia and reperfusion, RMPC received intrathecal morphine (3 μg/kg) by three cycles of 5-min infusions interspersed with 5-min infusion free periods. Intrathecally administrated a nonspecific nitric oxide synthase (NOS) inhibitor Nω-Nitro-L-arginine methyl ester (30 nmol), a specific guanylate cyclase inhibitor oxadiazolo [4,3-a] quinoxalin-1-one (11 nmol) and PKG inhibitor KT-5823 (20 pmol) 10 min before RMPC was used to evaluate the role of NO/cGMP/PKG of spinal cord. Ischemia and reperfusion injury were then induced by 30 min of left coronary artery occlusion, followed by 120 min of reperfusion. Infarct size, as a percentage of the area at risk, was determined by 2,3,5-triphenyltetrazolium staining. The content of cyclic guanosine monophosphate in the thoracic spinal cord was determined by radioimmunity protocol; the contents of nitric oxide and activity of NOS in the thoracic spinal cord were determined by nitrate reductase reduction and colorimetric methods; the expression of neuronal NOS (nNOS) and PKG in the thoracic spinal cord were determined by immunohistochemical and Western blotting method; the expression of nNOS messenger RNA was determined by reverse transcription-polymerase chain reaction method.

RESULTS

RMPC group markedly reduced the infarct size compared with the control group. However, the cardioprotection of RMPC could be abolished by pretreatment with Nω-Nitro-L-arginine methyl ester, Oxadiazolo [4,3-a] quinoxalin-1-one, and KT-5823. RMPC enhanced nitric oxide , NOS, and cyclic guanosine monophosphate levels in the spinal cord. Meanwhile, RMPC increased PKG and nNOS protein or messenger RNA expression in the spinal cord.

CONCLUSIONS

Spinal cord NO/cGMP/PKG signaling pathway mediates RMPC-induced cardioprotective effect.

Cannabinoid receptor 1 signaling in cardiovascular regulating nuclei in the brainstem: A review

Cannabinoids elicit complex hemodynamic responses in experimental animals that involve both peripheral and central sites. Centrally administered cannabinoids have been shown to predominantly cause pressor response. However, very little is known about the mechanism of the cannabinoid receptor 1 (CB₁R)-centrally evoked pressor response. In this review, we provided an overview of the contemporary knowledge regarding the cannabinoids centrally elicited cardiovascular responses and the possible underlying signaling mechanisms. The current review focuses on the rostral ventrolateral medulla (RVLM) as the primary brainstem nucleus implicated in CB₁R-evoked pressor response.

C1 catecholamine neurons form local circuit synaptic connections within the rostroventrolateral medulla of rat

C1 catecholamine neurons reside within the rostroventrolateral medulla (RVLM), an area that plays an integral role in blood pressure regulation through reticulospinal projections to sympathetic preganglionic neurons in the thoracic spinal cord. In a previous investigation we mapped the efferent projections of C1 neurons, documenting supraspinal projections to cell groups in the preautonomic network that contribute to the control of cardiovascular function. Light microscopic study also revealed putative local circuit connections within RVLM. In this investigation we tested the hypothesis that RVLM C1 neurons elaborate a local circuit synaptic network that permits communication between C1 neurons giving rise to supraspinal and reticulospinal projections. A replication defective lentivirus vector that expresses enhanced green fluorescent protein (EGFP) under the control of a synthetic dopamine beta hydroxylase (DβH) promoter was used to label C1 neurons and their processes. Confocal fluorescence microscopy demonstrated thin varicose axons immunopositive for EGFP and tyrosine hydroxylase that formed close appositions to C1 somata and dendrites throughout the rostrocaudal extent of the C1 area. Dual-labeled electron microscopic analysis revealed axosomatic, axodendritic and axospinous synaptic contacts with C1 and non-C1 neurons with a distribution recapitulating that observed in the light microscopic analysis. Labeled boutons were large, contained light axoplasm, lucent spherical vesicles, and formed asymmetric synaptic contacts. Collectively these data demonstrate that C1 neurons form a synaptic network within the C1 area that may function to coordinate activity among projection-specific subpopulations of neurons. The data also suggest that the boundaries of RVLM should be defined on the basis of function criteria rather than the C1 phenotype of neurons.

Adrenaline modulates on the respiratory network development

Adrenaline regulates respiratory network, however, adrenergic contribution to the developing respiratory center has not well studied. Adrenaline application on embryonic day 17 medulla-spinal cord block preparations abolished non-respiratory activity and enhanced respiratory frequency. Phentolamine application on neonatal brainstem-spinal cord preparations that produced stable neonatal respiration resulted in respiratory destabilization. In E19 rat, adrenaline switched from enhancement to depression of the respiratory rhythm. Adrenaline modulated GABAergic synaptic transmission to respiratory neurons in late developmental stage. These results suggest that the involvement of central adrenergic modulation on the respiratory network maturation.

Adrenaline contributes to prenatal respiratory maturation in rat medulla–spinal cord preparation

Adrenaline is a potent respiratory regulator. However, adrenergic contribution to the developing respiratory center has not been studied extensively. Adrenaline application on embryonic day 17 medulla-spinal cord block preparations abolished non-respiratory activity and enhanced respiratory frequency. Phentolamine application on neonatal blocks that produced stable neonatal respiration resulted in respiratory destabilization. These results suggest that central adrenergic modulation is involved in fetal respiratory development and maintenance of stable respiration.

Redistribution of mu-opioid receptors in C1 adrenergic neurons following chronic administration of morphine

Neurons in the rostral ventrolateral medulla (RVLM) are involved in both tonic and reflex control of sympathetic outflow. Many of these neurons express the adrenaline-synthesizing enzyme phenylethanolamine N-methyltransferase (PNMT), and are designated C1 neurons. C1 neurons that contain mu-opioid receptors (MORs) are hyperpolarized by MOR activation and are activated during morphine withdrawal. The present study examined the subcellular distribution of the cloned MOR, MOR1, in rat C1 neurons following chronic morphine treatment, using RVLM sections that were dually labeled for PNMT-immunoperoxidase and MOR1-immunogold. Electron microscopic analysis of the subcellular distribution of MOR1 revealed a lower abundance of plasma membrane-associated MOR1 in C1 dendrites of rats treated with morphine, compared to placebo-treated controls, only in distal dendrites. There were no differences in the size of dual-labeled dendrites between treatment groups or in the overall density of MOR1 within PNMT immunoreactive dendrites between treatment groups. These results suggest that chronic morphine treatment leads to a decreased presence of MOR1 at the cell surface, without a significant reduction in cytoplasmic receptor density. These observations suggest that chronic morphine produces a selective internalization of MOR1 in C1 neurons, without apparent changes in receptor synthesis or trafficking. The reduction of accessible MORs on these neurons may be a mechanism for tolerance with regard to autonomic responses to opioid administration and may facilitate the profound sympathetic hyperactivity that occurs during acute opioid withdrawal.

Mu opioid receptors in rat ventral medulla: effects of endomorphin-1 on phrenic nerve activity

Anatomical and in vitro studies suggest that mu opioid receptors (MOR) on pre-Bötzinger complex neurons are responsible for opioid induced respiratory depression (Grey et al., Science 286 (1999) 1566). However, mu opioid agonists injected in vivo, in other regions of the ventral respiratory group (VRG), produce respiratory depression, suggesting that opioids are widely distributed in the VRG. We therefore re-examined the distribution of the MOR in the ventral medulla and found MOR-immunoreactive neurons and terminals in all subdivisions of the VRG. Furthermore, we determined, in rats, the effects of a MOR agonist (endomorphin-1, 10 mM, 60 nl, unilateral), microinjected into different subdivisions of the VRG, on phrenic nerve activity. Endomorphin-1 produced changes in phrenic nerve frequency and amplitude, throughout the VRG. Unexpectedly, endomorphin-1 microinjected into the Bötzinger and pre-Bötzinger complexes consistently increased phrenic nerve frequency. These results support the widespread distribution of MOR in the VRG and also indicate that endomorphin-1, a postulated endogenous ligand, may differentially regulate respiration.

Endogenous opiates and behavior: 2001

This paper is the twenty-fourth installment of the annual review of research concerning the opiate system. It summarizes papers published during 2001 that studied the behavioral effects of the opiate peptides and antagonists. The particular topics covered this year include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology(Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Baroreceptor reflex pathways and neurotransmitters: 10 years on

The central nervous system plays a critical role in the management of blood flow to the tissues and its return to the heart and lungs. This is achieved by a complex interplay of neural efferent pathways, humoral mechanisms and afferent pathways. In this review, we focus on recent progress (within the past 10 years) that has been made in the sympathetic control of arterial blood pressure with a special emphasis on the role of baroreceptor mechanisms and central neurotransmitters. In particular, we focus on new features since 1991, such as neurotransmission in the nucleus tractus solitarius, the role of neurons in the most caudal part of the ventrolateral medulla oblongata and the increasing understanding of the exquisite control of different sympathetic pathways by different neurotransmitter systems.

C1 adrenergic neurons are contacted by presynaptic profiles containing DELTA-opioid receptor immunoreactivity

Ligands of the delta-opioid receptor tonically influence sympathetic outflow. Some of the actions of delta-opioid receptor agonists may be mediated through C1 adrenergic neurons in the rostral ventrolateral medulla. The goal of this study was to determine whether C1 adrenergic neurons or their afferents contain delta-opioid receptors. Single sections through the rostral ventrolateral medulla were labeled for delta-opioid receptor using the immunoperoxidase method and the epinephrine synthesizing enzyme phenylethanolamine N-methyltransferase (PNMT) using the immunogold method, and examined at the light and electron microscopic level. Few ( approximately 5% of 903) profiles dually labeled for PNMT and delta-opioid receptor were detected; most of these were dendrites with diameters < 1.5 microm. delta-Opioid receptor immunoreactivity was affiliated with multivesicular bodies in dually labeled perikarya, whereas delta-opioid receptor immunoperoxidase labeling appeared as isolated clusters within both singly and dually labeled dendrites. The majority ( approximately 83% of 338) of delta-opioid receptor-immunoreactive profiles were axons and axon terminals. delta-Opioid receptor-immunoreactive terminals averaged 0.75 microm in diameter, contained numerous large dense-core vesicles and usually formed appositions or asymmetric (excitatory-type) synapses with their targets. The majority (>50% of 250) of delta-opioid receptor-immunoreactive axons and axon terminals contacted PNMT-immunoreactive profiles. Most of the contacts formed by delta-opioid receptor-immunoreactive profiles ( approximately 75% of 132) were on single-labeled PNMT-immunoreactive dendrites with diameters <1.5 microm. The prominent localization of delta-opioid receptors to dense-core vesicle-rich presynaptic profiles suggests that delta-opioid receptor activation by endogenous or exogenous agonists may modulate neuropeptide release. Furthermore, the presence of delta-opioid receptors on axon terminals that form excitatory-type synapses with PNMT-immunoreactive dendrites suggests that delta-opioid receptor ligands may modulate afferent activity to C1 adrenergic neurons. The observation that some PNMT-immunoreactive neurons contain delta-opioid receptor immunoreactivity associated with multivesicular bodies and other intracellular organelles suggests that some C1 adrenergic neurons may present, endocytose and/or recycle delta-opioid receptors.

Cellular relations between mu-opioid receptive, GABAergic and reticulospinal neurons in the rostral ventrolateral medulla

Physiological studies have suggested that mu-opioid receptor (MOR) activation can both excite and inhibit reticulospinal neurons in the rostral ventrolateral medulla (RVL), possibly via influences on GABAergic neurons. Thus, to determine the cellular relationships of MORs to GABAergic neurons in the RVL, two experimental approaches were used. First, single sections through the RVL were labeled for MOR using immunoperoxidase detection and for GABA using immunogold detection and examined by electron microscopy. These studies revealed that MOR-immunoreactive (IR) terminals were smaller on average than GABA-IR terminals and formed both asymmetric and symmetric synapses, whereas GABA-IR terminals formed exclusively symmetric synapses. MOR and GABA immunoreactivities rarely co-localized. Interactions between axons and terminals containing MOR or GABA immunoreactivity were primarily: (1) direct appositions with each other; or (2) convergence onto a common dendritic target that sometimes contained either MOR or GABA immunoreactivity. Since the identity of these target dendrites mostly was unknown, a second study was designed to determine if they might be reticulospinal neurons. For this study, reticulospinal neurons were identified with a retrograde tracer and both MOR and GABA were localized in the same sections of the RVL. These studies revealed that numerous GABA-IR terminals formed symmetric synapses on the perikarya and proximal dendrites of reticulospinal neurons. In contrast, few MOR-IR terminals contacted reticulospinal perikarya and large dendrites although they were often found nearby. These results provide anatomical evidence that MOR activation by endogenous or exogenous agonists may indirectly alter GABAergic neurotransmission in the RVL either through presynaptic interactions between cells or through competing influences on postsynaptic targets.

Heterogeneous receptor distribution in autonomic neurons

The central nervous system components for baroreflex regulation of sympathetic outflow include specific sets of neurons in the brain and spinal cord. Critical nuclei containing sympathetic baroreceptive neurons are the nuclei of the solitary tract, regions of the caudal and rostral ventrolateral medulla, and the intermediolateral cell column in the spinal cord. While many other brain regions project to these nuclei, cells in these areas appear to form the minimal required pathway for baroreflex control of sympathetic outflow. Synaptic connections have been identified between cells in these nuclei that are consistent with a serial relay from baroreceptor afferents through the brain stem and to sympathetic preganglionic neurons in the spinal cord. In recent years, we have examined the distribution of receptor proteins in these neurons, with a focus on receptors that are most likely to modulate the activity of these cells. In three studies examining the distribution of different receptors on distinct neurons, each study found some type of heterogeneity in the distribution of each receptor within a particular type of neuron. This heterogeneity was seen with regard to the distribution of receptor protein within the dendritic tree of individual neurons, as well as between pre- and postsynaptic sites on the same cell. This heterogeneous distribution of receptors suggests that receptors undergo dendritic targeting within autonomic neurons. This receptor trafficking may be regulated by heterogeneous afferent input to autonomic neurons and could be changed under conditions where afferent activity is significantly altered.

Mu-opioid receptors are present in functionally identified sympathoexcitatory neurons in the rat rostral ventrolateral medulla

Agonists of the mu-opioid receptor (MOR) produce profound hypotension and sympathoinhibition when microinjected into the rostral ventrolateral medulla (RVL). These effects are likely to be mediated by the inhibition of adrenergic and other presympathetic vasomotor neurons located in the RVL. The present ultrastructural studies were designed to determine whether these vasomotor neurons, or their afferents, contain MORs. RVL bulbospinal barosensitive neurons were recorded in anesthetized rats and filled individually with biotinamide by using a juxtacellular labeling method. Biotinamide was visualized by using a peroxidase method and MOR was identified by using immunogold localization of an antipeptide antibody that recognizes the cloned MOR, MOR1. The subcellular relationship of MOR1 to RVL neurons with fast- or slow-conducting spinal axons was examined by electron microscopy. Fast- and slow-conducting cells were not morphologically distinguishable. Immunogold-labeling for MOR1 was found in all RVL bulbospinal barosensitive neurons examined (9 of 9). MOR1 was present in 52% of the dendrites from both types of cells and in approximately half of these dendrites the MOR1 was at nonsynaptic plasmalemmal sites. A smaller portion of biotinamide-labeled dendrites (16%) from both types of cells were contacted by MOR1-containing axons or axon terminals. Together, these results suggest that MOR agonists can directly influence the activity of all types of RVL sympathoexcitatory neurons and that MOR agonists may also influence the activity of afferent inputs to these cells. The heterogenous distribution of MORs within individual RVL neurons indicates that the receptor is selectively targeted to specific pre- and postsynaptic sites.