Repeated δ1-opioid receptor stimulation reduces δ2-opioid receptor responses in the SA node

Long-acting opioids and cardiovascular diseases: Help or hindrance!

Opioids are widely being used for chronic pain management, cough and diarrhea suppressants, anesthetic agents, and opioid de-addiction therapy. Opioid receptors, present in the central nervous system and peripheral tissues, are documented to regulate several cardiac functions through different signaling pathways. Long-acting opioids (LAO) have been successfully evaluated for their beneficial effects in various cardiovascular diseases viz. myocardial infarction, ischemic reperfusion injuries, atherosclerosis etc. However, on the other hand, several research studies pointed towards the harmful effects of LAOs which are mainly associated with QTc prolongation, torsade de pointes, ventricular arrhythmias, and cardiac arrest. This review shall familiarize readers with the benefits as well as the harmful effects of long-acting opioids in cardiovascular diseases. We have also provided an overview of cardiac opioid receptors, endogenous cardiac opioid peptides, and regulation of cardiovascular functions by central and cardiac opioid receptors.

Prospects for Creation of Cardioprotective and Antiarrhythmic Drugs Based on Opioid Receptor Agonists

It has now been demonstrated that the μ, δ1 , δ2 , and κ1 opioid receptor (OR) agonists represent the most promising group of opioids for the creation of drugs enhancing cardiac tolerance to the detrimental effects of ischemia/reperfusion (I/R). Opioids are able to prevent necrosis and apoptosis of cardiomyocytes during I/R and improve cardiac contractility in the reperfusion period. The OR agonists exert an infarct-reducing effect with prophylactic administration and prevent reperfusion-induced cardiomyocyte death when ischemic injury of heart has already occurred; that is, opioids can mimic preconditioning and postconditioning phenomena. Furthermore, opioids are also effective in preventing ischemia-induced arrhythmias.

Sympatholytic delta-2 opioid receptors moderate ganglionic vasomotor control

This study tested the hypothesis that enkephalin increases femoral vascular conductance via the delta-2 phenotype of the opioid receptor (DOR-2) within peripheral sympathetic ganglia. Graded pulses of methionine–enkephalin (ME) were administered (0.03–10 μg/kg) into the terminal aorta of anesthetized dogs proximal to lumbar arteries that perfuse vasomotor ganglia regulating femoral blood flow. Femoral vascular conductance increased sharply (ED50 = 2.6 × 10−9 mol/kg) accompanied by declines in arterial pressure and femoral vascular resistance. A dose-related increase in arterial pressure preceded each subsequent fall in pressure. The DOR-2 antagonist, naltriben (NTB), abrogated the hyperemic effect of ME (ID50 = 1.4 × 10−9 mol/kg). DOR-1 blockade (BNTX) was five-fold less effective. The hyperemic effect of ME was also enhanced when sympathetic activity was reflexly increased by bilateral carotid occlusion. The DOR-2 agonist, deltorphin II, produced exaggerated increases in conductance compared with ME that were also reduced by DOR-2 blockade. DOR-1 blockade eliminated the initial pressor responses, exaggerated the subsequent depressor response, increased baseline femoral conductance 10-fold and shifted the ME-mediated hyperemic threshold one dose lower from 0.3 to 0.1 μg/kg, providing indirect support for a competing DOR-1-mediated constriction. Extended exposure to DOR-1 blockade lowered the maximal ME increase in conductance by 30%, suggesting that BNTX reduces the available pool of DOR receptors. In summary, enkephalin mediates a robust hyperemic effect through sympatholytic ganglionic DOR-2 receptors and DOR-1 antagonist studies provide indirect evidence for constituent opposition from a proposed DOR-1-mediated sympathotonic constrictor pathway.

Repeated arterial occlusion, delta-opioid receptor (DOR) plasticity and vagal transmission within the sinoatrial node of the anesthetized dog

Brief interruptions in coronary blood flow precondition the heart, engage delta-opioid receptor (DOR) mechanisms and reduce the damage that typically accompanies subsequent longer coronary occlusions. Repeated short occlusions of the sinoatrial (SA) node artery progressively raised nodal methionine-enkephalin-arginine-phenylalanine (MEAP) and improved vagal transmission during subsequent long occlusions in anesthetized dogs. The DOR type-1 (DOR-1) antagonist, BNTX reversed the vagotonic effect. Higher doses of enkephalin interrupted vagal transmission through a DOR-2 mechanism. The current study tested whether the preconditioning (PC) protocol, the later occlusion or a combination of both was required for the vagotonic effect. The study also tested whether evolving vagotonic effects included withdrawal of competing DOR-2 vagolytic influences. Vagal transmission progressively improved during successive SA nodal artery occlusions. The vagotonic effect was absent in sham animals and after DOR-1 blockade. After completing the PC protocol, exogenously applied vagolytic doses of MEAP reduced vagal transmission under both normal and occluded conditions. The magnitude of these DOR-2 vagolytic effects was small compared to controls and repeated MEAP challenges rapidly eroded vagolytic responses further. Prior DOR-1 blockade did not alter the PC mediated, progressive loss of DOR-2 vagolytic responses. In conclusion, DOR-1 vagotonic responses evolved from signals earlier in the PC protocol and erosion of competing DOR-2 vagolytic responses may have contributed to an unmasking of vagotonic responses. The data support the hypothesis that PC and DOR-2 stimulation promote DOR trafficking, and down regulation of the vagolytic DOR-2 phenotype in favor of the vagotonic DOR-1 phenotype. DOR-1 blockade may accelerate the process by sequestering newly emerging receptors.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 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 (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 neurological 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).

The monosialosyl ganglioside GM-1 reduces the vagolytic efficacy of delta2-opioid receptor stimulation

The cardiac enkephalin, methionine-enkephalin-arginine-phenylalanine (MEAP), alters vagally induced bradycardia when introduced by microdialysis into the sinoatrial (SA) node. The responses to MEAP are bimodal; lower doses enhance bradycardia and higher doses suppress bradycardia. The opposing vagotonic and vagolytic effects are mediated, respectively, by delta(1) and delta(2) phenotypes of the same receptor. Stimulation of the delta(1) receptor reduced the subsequent delta(2) responses. Experiments were conducted to test the hypothesis that the delta-receptor interactions were mediated by the monosialosyl ganglioside GM-1. When the mixed agonist MEAP was evaluated after nodal GM-1 treatment, delta(1)-mediated vagotonic responses were enhanced, and delta(2)-mediated vagolytic responses were reduced. Prior treatment with the delta(1)-selective antagonist 7-benzylidenaltrexone (BNTX) failed to prevent attrition of the delta(2)-vagolytic response or restore it when added afterward. Thus the GM-1-mediated attrition was not mediated by delta(1) receptors or increased competition from delta(1)-mediated vagotonic responses. When GM-1 was omitted, deltorphin produced a similar but less robust loss in the vagolytic response. In contrast, however, to GM-1, the deltorphin-mediated attrition was prevented by pretreatment with BNTX, indicating that the decline in response after deltorphin alone was mediated by delta(1) receptors and that GM-1 effectively bypassed the receptor. Whether deltorphin has intrinsic delta(1) activity or causes the release of an endogenous delta(1)-agonist is unclear. When both GM-1 and deltorphin were omitted, the subsequent vagolytic response was more intense. Thus GM-1, deltorphin, and time all interact to modify subsequent delta(2)-mediated vagolytic responses. The data support the hypothesis that delta(1)-receptor stimulation may reduce delta(2)-vagolytic responses by stimulating the GM-1 synthesis.