Opioid receptor binding in rat spinal cord

Mu-opioid receptor activation in live cells

Interaction of the mu-opioid receptor (MOP) with selected ligands was investigated in live cells using advanced imaging by confocal laser scanning microscopy integrated with fluorescence correlation spectroscopy and fluorescence cross-correlation spectroscopy. In PC12 cells stably transformed to express the fluorescently labeled MOP-enhanced green fluorescent protein construct, two pools of MOP were identified that could be discriminated by differences in their lateral mobility in the cell membrane. The majority of MOP receptors (80+/-10%) were characterized by a diffusion coefficient D(MOP,1) = (4+/-2) x 10(-11) m(2) s(-1), compared with the slowly moving fraction, D(MOP,2) = (4+/-2) x 10(-12) m(2) s(-1). On stimulation with selected agonists ([D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin, enkephalin-heptapeptide Tyr-Gly-Gly-Phe-Met-Arg-Phe, morphine, and methadone), surface density of the MOP decreased, whereas the lateral mobility increased. In contrast, antagonists (naloxone and naltrexone) "froze" the receptor in the membrane, i.e., increased MOP surface density and decreased lateral mobility. Agonist activation was also accompanied by pronounced changes in the dynamics of plasma membrane lipids, as revealed by the general lipid marker 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate dye. The results provide new information about MOP activation in live cells at the molecular level, with a special focus on the dynamics of the intricate interplay between this receptor and the surrounding lipids.

Postnatal development of multiple opioid receptors in the spinal cord and development of spinal morphine analgesia

The postnatal ontogeny of mu, delta and kappa opioid receptor binding sites in the spinal cord of rat pups at various postnatal days was determined using in vitro autoradiographical methods. The functional effect of spinal morphine was also assessed using in vivo electrophysiological methods in rats at P14, P21 and adults (P56). Both mu and kappa opioid receptor binding-sites are present from P0 and spread relatively diffusely throughout the spinal cord. Overall binding peaks at P7 and subsequently decreases to adult levels with the mu opioid receptor binding sites regressing to become denser in the superficial dorsal horn. delta Opioid receptor binding was first seen at P7, and no distinction between superficial and deeper laminae was seen. In the adult, the relative proportions of the opiate receptors in the superficial dorsal horn are 63%, 22% and 15%, for mu, delta and kappa receptor binding sites, respectively. C-fibre evoked dorsal horn neuronal responses recorded from anaesthetized rat pups were highly sensitive to spinal morphine at P21, (EC50 0.005 microgram), compared to the adult (EC50 0.9 microgram). However, the EC50 (0.2 microgram) at P14 was greater than at P21 despite the fact that mu receptor binding was greater at P14. Opioid receptor binding is developmentally regulated and undergoes substantial postnatal reorganization. However, the number of mu receptor binding sites appears not to be the only determinant of functional sensitivity to spinal morphine. Other factors, such as coupling of the receptors are likely to be important.

Differential involvement of opioid receptors in intrathecal butorphanol-induced analgesia: compared to morphine

The present experiments were performed to investigate the differential involvement of the opioid receptor subtypes in the antinociception of intrathecal (IT) butorphanol compared to IT morphine. A single dose (26 nmol) of IT nor-binaltorphimine (nor-BNI), beta-funaltrexamine (beta-FNA), and naltrindole (NTI) demonstrated a significant attenuation in the overall antinociception of IT butorphanol (52 nmol) or IT morphine (26 nmol). However, IT butorphanol elicits thermal antinociceptive effect through kappa > delta > or = mu, whereas morphine acts on mu >delta >> kappa. These results indicate that the antinociceptive effect of both IT butorphanol and IT morphine are mediated through mu, delta, and kappa opioid receptors in different relative orders.

Changes of opioid binding density in the rat spinal cord following unilateral dorsal rhizotomy

Mu, delta and kappa opioid receptors in the vertebrate spinal cord mediate the potent antinociceptive effects of opioid agonists administered onto the spinal cord. The present experiments were conducted to determine the effect of unilateral dorsal rhizotomy on mu, delta and kappa spinal opioid binding sites. Measurements of opioid binding were made at 1, 2, 4 or 8 days after rhizotomy and comparisons were made to intact animals. The changes in mu, delta and kappa opioid binding sites were determined by receptor autoradiography using the highly selective radioligands [3H]sufentanil, [3H]DPDPE and [3H]U69593, respectively. Within autoradiograms of each spinal cord, three regions on each side of the spinal cord were targeted for densitometric analysis: laminae I-II (medial), V (lateral) and X. When effects of unilateral rhizotomy within animals were assessed by comparison of the density of binding on the side ipsilateral to the rhizotomy to the contralateral side, decreases in the binding of all three radioligands were observed in laminae I-II on the side of the spinal cord ipsilateral to the rhizotomy at 2-8 days postlesion. A significant reduction in binding was also noted for mu and delta sites in lamina V after 8 days and for delta binding in lamina X at 2 and 4 days on the side ipsilateral to the rhizotomy. However, when densities of binding sites were compared with the corresponding regions in control, it was clear that dorsal rhizotomy resulted in significant changes in opioid binding on both sides of the spinal cord; changes differed for each type of opioid binding site. On the contralateral side of the spinal cord, rhizotomy caused a significant decrease of mu opioid sites 1 day after the lesion and showed partial recovery by day 8. Delta opioid sites were also significantly decreased as early as 1 day postlesion, but did not recover. Kappa opioid sites did not change at 1 day after the rhizotomy but increased on day 2, decreased on day 4 and fully recovered 8 days after rhizotomy. The present results support the hypothesis that a significant proportion of spinal mu, delta and kappa opioid binding sites are present on the central terminations of primary afferents. Finally the present data are the first to report a contralateral effect of the unilateral rhizotomy on spinal opioid binding sites. The contralateral changes in binding were specific to the type of opioid site examined, time after the surgery and region of the spinal cord examined.

Characterization of opioid binding sites in rat spinal cord

Binding sites were characterized in rat whole spinal cord crude membrane preparations using selective labelling techniques with multiple methods of mathematical analysis of experimental curves. Mathematical analysis of single [3H]-[D-Ala2,MePhe4,Gly-ol5] enkephalin (DAGO) saturation curves suggested binding of the [3H]-ligand at one site, while displacement curves of low concentrations of [3H]-DAGO with selective mu-ligands indicated the presence of high- and low-affinity sites. All the [3H]-DAGO curves processed simultaneously by LIGAND analysis showed the presence of high (27%) and low (73%) affinity components, with a total Bmax of 3.19 pmol/g tissue. Eighty percent of [3H]-[D-Ala2,D-Leu5] enkephalin (DADLE) binding was displaced by DAGO with high affinity, indicating that a high percentage of [3H]-DADLE binding was at mu-sites. Saturation curves of [3H]-DADLE after inhibition of mu-sites by unlabelled DAGO (delta-sites) were monophasic with non-linear fitting analysis and the Bmax was 0.90 pmol/g tissue. Most mathematical analysis of single saturation curves of [3H]-(-)-bremazocine indicated binding at more than one site. DAGO, DADLE, U-69,593 and PD 117302 displaced 0.15 nM of [3H]-(-)-bremazocine biphasically: the percentages of displacement calculated with the non-linear fitting program were respectively 50 (mu-sites), 64 ((mu + delta)-sites), 18 and 25 (kappa-sites). Haloperidol displaced [3H]-(-)-bremazocine only at microM concentrations. suggesting no binding at sigma-sites. In the presence of 225 nM of DAGO, DADLE displaced only 21% of [3H]-(-)-bremazocine 0.15 nM binding (delta-sites). Most mathematical analysis of saturation curves of [3H]-(-)-bremazocine, after inhibition of binding at mu- and delta-sites by DAGO and DADLE, still indicated binding at more than one site and the selective kappa-ligands U-69,593 and PD 117302 displaced [3H]-(-)-bremazocine in these experimental conditions. LIGAND analysis of saturation and inhibition curves of [3H]-(-)-bremazocine by U-69,593 and PD 117302 showed the presence of high (43%) and low (57%) affinity components, with a total Bmax of 2,73 pmol/g tissue. Thus in rat spinal cord there are at least two mu-sites bound by [3H]-DAGO which amount together to approximately 47% of total opioid sites, delta-sites bound by [3H]-DADLE amounting to approximately 13%, kappa-sites and other unknown sites (possibly a kappa-subtype) bound by [3H]-(-)-bremazocine, which together are approximately 40% of total opioid sites.

Biochemical characterization and regional quantification of mu, delta and kappa opioid binding sites in rat spinal cord

The spinal cord contains mu, delta and kappa opioid receptors which mediate the antinociceptive effects of opioid agonists administered onto the spinal cord. In this study, we characterized the binding sites for highly-selective mu, delta and kappa opioid radioligands and quantified the distribution of opioid binding sites in rat lumbosacral spinal cord using autoradiography. In sections of rat brain mounted on glass slides, the mu ligand, [3H]sufentanil, bound with high affinity with an apparent Kd of 0.46 nM. The delta ligand, [3H]DPDPE [( D-Pen2.5]-enkephalin), bound with a Kd of 4.31 nM, and the kappa-ligand, [3H]U69593, bound with a Kd of 2.27 nM. Three regions of the spinal gray were targeted for quantification of binding sites by autoradiography. The data indicate that when considered as a percentage of the total opioid binding capacity within a region, the contribution of mu sites in laminae I-II was about 90%, with delta and kappa sites 7% and 3%, respectively. In lamina V, the mu sites comprised about 70% of the total opioid sites, with delta and kappa sites comprising 28% and 2%, respectively. In the area adjacent to the central canal, mu sites contributed about 65% of the total opioid sites followed by delta sites at 33% and kappa sites at 2% of total opioid sites. These results demonstrate a differential distribution of mu, delta and kappa binding sites with respect to the organization of the spinal gray matter. The preferential occurrence of all 3 opioid binding sites in the superficial dorsal horn is noteworthy since many fine caliber primary afferent fibers mediating nociception establish synaptic contact in this region.

Intrathecal administration of delta receptor agonists in the urethane anesthetized rat provokes an increase in arterial pressure via a non-opioid mechanism

Intrathecal administration of the delta receptor specific agonists Leu5-enkephalin (Leu-Enk; 300 nmol), Met5-enkephalin (Met-Enk; 300 nmol) and [D-Pen2,D-Pen5]enkephalin (DPDPE; 100 nmol) to the T2 or the T9 segment of the rat spinal cord provoked a transient (less than 5 min) increase (15-20 mm Hg) in arterial pressure. DPDPE, but not Leu-Enk or Met-Enk, also significantly increased heart rate by 30-35 bpm. Intravenous administration of 300 nmol of Leu-Enk mimicked the effects observed following intrathecal administration. The hypertensive effect of intrathecal and intravenous Leu-Enk administration was blocked by prior systemic administration (10 mg/kg) of the nicotinic ganglion blocker hexamethonium, suggesting that the effect was mediated via sympathetic activation. The increase in arterial pressure observed following intrathecal Leu-Enk administration was not blocked by either intrathecal (305 nmol) or intravenous (10 mg/kg) administration of the opiate receptor blocker naloxone, although naloxone did block the hypertension provoked by intravenous Leu-Enk administration. Moreover, intrathecal administration of Des-Tyr1-Leu-Enk (300 nmol), an enkephalin fragment devoid of opiate receptor activity, also increased arterial pressure. These results suggest that the hypertension elicited by intrathecal delta agonist administration was not mediated via an opioid mechanism.

Enkephalin-immunoreactive neuronal projections from the medulla oblongata to the intermediolateral cell column: relationship to substance P-immunoreactive neurons

The present study investigated the ventral medullary distribution of enkephalin-immunoreactive neurons that project to the intermediolateral cell column and the relationship of these neurons to substance P-immunoreactive neurons. Neurons that projected to the intermediolateral cell column were identified by the presence of rhodamine-labeled microspheres within the neuronal cell body after an injection of the microspheres into the intermediolateral cell column of the third thoracic spinal cord segment. Enkephalin- and substance P-immunoreactivities were identified by dual-color immunohisto-chemistry. Enkephalin-immunoreactive neurons that projected to the intermediolateral cell column were present in the raphe magnus, the nucleus reticularis magnocellularis pars alpha, the paragigantocellular reticular nucleus, and the parapyramidal region. These neurons were present throughout the rostrocaudal extent of each of these nuclei. However, in the raphe magnus the greatest number was present at more rostral levels of the nucleus. The morphology and distribution of enkephalin-immunoreactive neurons that projected to the intermediolateral cell column were similar to those of enkephalin-immunoreactive neurons that were not observed to contain rhodamine-labeled microspheres. Substance P- and enkephalin-immunoreactive neurons that projected to the intermediolateral cell column were present in similar distributions in each of the nuclei studied, except the raphe magnus. The raphe magnus contained more enkephalin- than substance P-immunoreactive neurons at rostral levels and more substance P-immunoreactive neurons than enkephalin-immunoreactive neurons at caudal levels. Coexistence of substance P- and enkephalin-immunoreactivities in ventral medullary neurons that projected to the intermediolateral cell column was rarely seen. These studies support the hypothesis that ventral medullary enkephalinergic neurons project to the intermediolateral cell column where they could act to modulate preganglionic sympathetic activity.

Modulation of [3H]DAGO binding by substance P (SP) and SP fragments in the mouse brain and spinal cord via MU1 interactions

Binding of [3H]DAGO to fresh, frozen or beta-funaltrexamine (beta-FNA) pretreated membranes of mouse brain and spinal cord was extensively studied using substance P (SP) or SP fragments as potential competitors and/or modulators. The objective was to determine whether SP exerts its analgesic effect by interacting with mu opioid receptors. The affinity of DAGO was reduced and binding capacity was increased in the presence of SP or the N-terminal SP fragments SP(1-9) and SP(1-4) but not the C-terminal SP fragment SP(5-11). Because sub-nanomolar concentrations of SP or N-terminal SP fragments displaced [3H] DAGO binding to a minor but detectable degree, it is suggested that SP interacts with mu 1 sites through its N-terminus portion. The effect of SP on DAGO binding was less in the spinal cord compared to the rest of the brain. Modulation of DAGO binding by SP was enhanced in the brain after pretreatment of membranes with the narcotic antagonist beta-FNA. These results suggest a novel mechanism for the analgesic action of SP.

Depressor and bradycardic effects induced by spinal subarachnoid injection of D-Ala2-D-Leu5-enkephalin in rats

The effects of D-Ala2-D-Leu5-enkephalin (DADLE), a specific delta receptor agonist, on spinal control of cardiovascular function, were investigated by its intrathecal (i.th) injection into the spinal subarachnoid space at the T-9 level. In chloralose-anesthetized rats, DADLE (17.5, 35 and 70 nmol, i.th) caused dose-dependent hypotension and bradycardia. The mean maximal hypotension by 70 nmol of DADLE was -45 +/- 7 mmHg, with a bradycardia of -79 +/- 15 beats/min. These inhibitory cardiovascular effects were antagonized by the opiate antagonist naloxone (50 nmol, i.th.) given prior to DADLE. Intrathecal injection of DADLE also decreased splanchnic sympathetic nerve discharge (-46 +/- 5%). DADLE (70 nmol) given i.v. did not cause significant changes in mean arterial pressure (MAP) and heart rate (HR). Neither bilateral vagotomy nor pretreatment with atropine (0.2 mg/kg, i.v.) prevented the BP and HR effects of intrathecal injection of DADLE at a dose of 35 nmol. DADLE at this dose failed to produce significant alteration in the frequency of respiration and blood PaO2, PaCO2 and blood pH. In conscious rats, 140 nmol of DADLE (i.th.) did not produce any consistent changes in MAP and HR. These data suggest that intrathecal injection of DADLE inhibits central sympathetic activity, possibly at a spinal locus.

Simultaneous modulation of TRH and opioid receptor binding in rat spinal cord after chronic naloxone administration

[3H]MeTRH binds to crude membranes prepared from the spinal cord with high affinity, K = 3.13 (+/- 0.29) x 10(8) L/M. After chronic treatment with naloxone (2 mg-kg/h), the density of spinal opioid (mu) receptors increased 100% at the same time as the following changes in MeTRH binding were observed: the affinity of [3H]MeTRH decreased 58% (K = 1.32 (+/- 0.17) x 10(8) L/M; p less than 0.005), and binding capacity increased 71%, from 28 (+/- 4) x 10(-12) M/L or 56 fmol/mg protein to 48 (+/- 9) x 10(-12) M/L or 96 fmol/mg (p less than 0.05). The relative potencies of TRH and MeTRH in competition for the binding of [3H]MeTRH changed from a high IC50 ratio of 10 in control rats to a low IC50 ratio of about 3 only after naloxone treatment. The results of this study show for the first time that the opiate antagonist-induced upregulation of opioid receptors is associated with modulation in binding of TRH receptors.