Sustained δ1-Opioid Receptor Stimulation Down Regulates δ2-Opioid Receptor Responses

Abstract

Deo, Shekhar., Sustained δ1-opioid receptor stimulation down regulates δ2-opioid receptor responses. Master of Science (Integrative Physiology), May 2005, 49 pp., 2 tables, 8 figures, references, 25 titles. Ultra-low doses of methionine-enkephalin-arginine-phenylalanine (MEAP) improve vagal transmission (vagotonic) and decrease the heart rate by the stimulating δ1-opioid receptors (OR) within the sinoatrial (SA) node. Higher doses of MEAP (5 nmol/min) acting on δ2-opioid receptors interrupt vagal transmission (vagolytic) and reduce the decline in heart rate. Repeated occlusion of the SA node artery produced a vagotonic response during a subsequent prolonged ischemia that was reversed by selective δ1-opioid receptor antagonist, 7-benzylidenaltrexone (BNTX). 2-Methyl-4aα-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12aα-octahydro-quinolino[2,3,3-g]isoquinoline (TAN-67); a selective δ1-OR agonist has been used experimentally to mimic ischemic preconditioning in the heart. The following study was designed to test the hypothesis that sustained δ1-opioid receptor stimulation reduces the δ2-opioid receptor response. The cervical vagi were isolated and ligated and the right vagus was stimulated at frequencies chosen to produce a two-step decline in heart rate of about 25 and 50 beats per min (bpm). A microdialysis probe was introduced into the SA node such that the porous window was in the nodal interstitium. In study one, the selective δ2-OR agonist, deltorphin II was introduced (1.67 nmol/min) into the dialysis inflow to evaluate the δ2-OR response before and after the infusion of TAN-67 (1.67 nmol/min). The vagolytic effect (% inhibition) of deltorphin after TAN-67 infusion was significantly reduced during both low (76% to 22%) and high (80% to 21%) frequency vagal stimulation. In study two, BNTX (1.67 nmol/min), a selective δ1-OR antagonist was mixed with TAN-67 (1.67 nmol/min) in an equimolar ratio and introduced into the dialysate inflow. By blocking the effects of TAN-67, BNTX preserved the deltorphin response suggesting that the earlier attrition of the response was mediated by δ1-receptor activity. In study three, TAN-67 was omitted to perform a time control study. Unexpectedly, some loss of δ2-response was apparent in the absence of added TAN-67, So study 4 was designed in which BNTX (1.67 nmol/min) was infused for the period of time similar to that of saline in study 3. Like study 2, BNTX prevented the loss of the deltorphin response, suggesting that the attenuated response seen in study 3 was also mediated by δ1-activity. Two additional studies were conducted to determine the effects of TAN-67 alone and the duration of the experiment without prior exposure to deltorphin. Another time control was conducted in which the initial deltorphin evaluation prior to the vehicle treatment was omitted. In that study (Study 5) a very robust vagolytic response was observed when deltorphin was first tested after 2.5 hours. In study six, TAN-67 was substituted for saline and the subsequent δ2-evaluation at 2.5 hour was eroded compared to that in study 5. When BNTX was added to deltorphin after erosion of the vagolytic response, the vagolytic response was not restored. This observation support the conclusion that sustained δ1-stimulation desensitizes or down regulates δ2-mediated responses through a heterologous mechanism. Collectively the six studies support the conclusion that the loss of δ2-mediated vagolytic response was a result of reduction in the number of δ2-receptors mediated by sustained δ1-receptor stimulation.