RESUMEN
The in vivo binding properties of cerebral opioid receptors were investigated in mice after intracerebroventricular (i.c.v.) injections of iodinated FK33-824 and [D.Ala2]deltorphin-I which behave in vitro as highly selective ligands possessing high affinity for mu and delta receptors, respectively. [125I]FK33-824 and [125I][D.Ala2] deltorphin-I exhibited similar diffusion kinetics after i.c.v. injection and bound specifically to sites characterized pharmacologically as mu and delta receptors respectively. Autoradiographic analysis revealed that after i.c.v. administration, concentrations of [125I]FK33-824 and [125I][D.Ala2]deltorphin-I remained higher in the circumventricular than in the deep structure of the brain and that specific sites labelled in vivo were differently distributed from those observed after in vitro labelling. FK33-824 was 250 times more analgesic than [D.Ala2]deltorphin-I in the tail-flick test and at doses producing a similar analgesia, [D.Ala2]deltorphin-I occupied a high proportion of mu receptors. Furthermore, analgesic effect of [D.Ala2]deltorphin-I was antagonized by pretreatment with naltrexone but not by naltrindole, a selective antagonist of delta-opioid receptors. These experiments reveal the localization of mu and delta opioid receptors reached after i.c.v. injection and provide evidence to support the suggestion that delta-opioid receptors contribute little or none to the supraspinal antinociception.
Asunto(s)
Analgésicos/metabolismo , Encéfalo/metabolismo , D-Ala(2),MePhe(4),Met(0)-ol-encefalina/metabolismo , Oligopéptidos/metabolismo , Receptores Opioides delta/metabolismo , Receptores Opioides mu/metabolismo , Secuencia de Aminoácidos , Analgésicos/administración & dosificación , Analgésicos/farmacocinética , Analgésicos/farmacología , Animales , Autorradiografía , Unión Competitiva , Encéfalo/efectos de los fármacos , D-Ala(2),MePhe(4),Met(0)-ol-encefalina/administración & dosificación , D-Ala(2),MePhe(4),Met(0)-ol-encefalina/farmacocinética , D-Ala(2),MePhe(4),Met(0)-ol-encefalina/farmacología , Difusión , Encefalina Ala(2)-MeFe(4)-Gli(5) , Encefalinas/farmacología , Inyecciones Intraventriculares , Masculino , Ratones , Datos de Secuencia Molecular , Naltrexona/análogos & derivados , Naltrexona/farmacología , Oligopéptidos/administración & dosificación , Oligopéptidos/farmacocinética , Oligopéptidos/farmacología , Dimensión del Dolor , Unión Proteica , Receptores Opioides delta/efectos de los fármacos , Receptores Opioides mu/efectos de los fármacosRESUMEN
The effect of the synthetic opioid agonist D-Ala2,N-Me-Phe4,Met(O)5-ol enkephalin (DAMME) on plasma growth hormone (GH) and prolactin (PRL) concentrations in Holstein heifer calves was investigated in this study. The possible site of action of DAMME was determined by pretreating calves with an opioid antagonist that crosses the blood-brain barrier poorly if at all (N-methyl levallorphan-methane sulphonate [MLM]) or one that crosses readily (naloxone [NAL]). All calves were assigned to one of three treatment groups: 1) pretreatment with saline, 2) pretreatment with NAL, or 3) pretreatment with MLM. All groups were injected with DAMME 30 min after pretreatments. Plasma PRL increased after injection of DAMME in calves pretreated with saline. Prolactin concentrations were not different before and after injection of DAMME in calves pretreated with either NAL or MLM. Plasma GH increased after injection of DAMME in saline- and MLM-pretreated calves but was unchanged in NAL-pretreated calves. These data show that peripherally administered DAMME increases plasma GH and PRL in Holstein heifer calves and suggest that DAMME mediates GH release through receptors located somewhere inside the blood-brain barrier, but it can induce PRL secretion at a site located outside the barrier.
Asunto(s)
Bovinos/metabolismo , D-Ala(2),MePhe(4),Met(0)-ol-encefalina/farmacología , Hormona del Crecimiento/sangre , Prolactina/sangre , Animales , Barrera Hematoencefálica , D-Ala(2),MePhe(4),Met(0)-ol-encefalina/administración & dosificación , D-Ala(2),MePhe(4),Met(0)-ol-encefalina/farmacocinética , Femenino , Inyecciones Intravenosas , Análisis de los Mínimos Cuadrados , Levalorfano/análogos & derivados , Levalorfano/farmacología , Naloxona/farmacocinética , Naloxona/farmacología , Antagonistas de Narcóticos/farmacología , Distribución AleatoriaRESUMEN
The ability of naltrexone but not methyl naltrexone to cross the blood-brain barrier (BBB) was used to provide a different approach for the demonstration that opiates can enter the brain. Cortical electroencephalographic (EEG) measurements were made in rats receiving peripheral (IP) injections of naltrexone or methyl naltrexone and morphine or an enkephalin analog [Tyr-D-Ala-Gly-MePhe-Met(O)-ol]. Naltrexone significantly blocked the EEG effects of morphine and the enkephalin analog, but methyl naltrexone failed to do so. The results provide biological evidence that an opiate peptide can cross the BBB to affect the activity of the brain.
Asunto(s)
Barrera Hematoencefálica , D-Ala(2),MePhe(4),Met(0)-ol-encefalina/farmacocinética , Morfina/farmacocinética , Secuencia de Aminoácidos , Animales , Barrera Hematoencefálica/efectos de los fármacos , D-Ala(2),MePhe(4),Met(0)-ol-encefalina/química , Electroencefalografía , Hormona Inhibidora de la Liberación de MSH/farmacología , Masculino , Datos de Secuencia Molecular , Naltrexona/análogos & derivados , Naltrexona/farmacología , Compuestos de Amonio Cuaternario , Ratas , Ratas EndogámicasRESUMEN
Previous studies have demonstrated that the LH response to naloxone changes during development but the reason(s) for this are unknown. In the present work we have investigated the possibility that variations in cell surface opioid receptor levels (determined in tissue slice/punches) or changes in the ability of opioids to enter the CNS might be responsible. Opioid binding data indicate that both [3H]naloxone and [3H]DAGO-labelled binding sites remain at low levels until 10 days of age after which there is a progressive rise to adult levels at 15 days ([3H]DAGO) and 21 days ([3H]naloxone). Although several peaks and nadirs were observed in this detailed profile of receptor ontogeny, no exact correlation with the time course of LH response to opioid drugs was found. In an adaption of the slice binding assay we are able to quantify drug penetration into the brain (ex vivo binding). Ex vivo binding studies of blood-brain barrier (BBB) ontogeny indicate that there are changes with age in the ability of opioid peptides, injected subcutaneously, to inhibit binding at the mu-receptor. FK 33-824 induced a reduction in [3H]DAGO binding in the mediobasal hypothalamus until 15 days of age. FK in older rats had no effect on [3H]DAGO binding suggesting that formation of the BBB is complete at this age. In contrast, FK injection reduced binding in the median eminence-arcuate nucleus area (outside BBB) until 30 days of age. Surprisingly, this area also became refractory to FK injection after this age.(ABSTRACT TRUNCATED AT 250 WORDS)