Presynaptic localization of mu-opioid receptor-like immunoreactivity in retinal axon terminals within the terminal nuclei of the accessory optic tract: a light and electron microscope study in the rat

Effect of morphine on cholecystokinin and mu-opioid receptor-like immunoreactivities in rat spinal dorsal horn neurons after peripheral axotomy and inflammation

In order to further investigate the interaction between the octapeptide cholecystokinin and opioid analgesia in the spinal cord we used double-colour immunofluorescence to examine the anatomical distribution of cholecystokinin and mu-opioid receptors in the dorsal horn, as well as the effect of morphine on cholecystokinin- and mu-opioid receptor-like immunoreactivities following peripheral nerve injury and inflammation. Mu-opioid receptor-like immunoreactivity was present in 65.6% of cholecystokinin-positive neurons in laminae I and II of rat spinal cord. Conversely, 40.4% of mu-opioid receptor-positive neurons contained cholecystokinin-like immunoreactivity. Systemic application of morphine (1, 3 or 10 mg/kg; i.v.) after sciatic nerve section significantly, but reversibly, decreased mu-Opioid receptor-like immunoreactivity in the medial half of lamina II in segment L5 of the ipsilateral dorsal horn, and cholecystokinin-like immunoreactivity was also markedly reduced in the same region. These effects were dose- and time-dependent and could be prevented by naloxone preadministration. In contrast, no significant change in the pattern of distribution or intensity of mu-opioid receptor- and cholecystokinin-like immunoreactivities was observed in intact rats or during peripheral inflammation. These results provide a cellular basis for the interaction of mu-opioid receptors and cholecystokinin at the spinal level by showing a high degree of co-existence of these two molecules in local interneurons, and also show that morphine can induce rapid and short lasting effects on mu-opioid receptors after peripheral nerve injury. The results contribute to our understanding of how endogenous cholecystokinin reduces the analgesic effect of morphine.

Electrophysiological effects of opioid receptor activation on Syrian hamster suprachiasmatic nucleus neurones in vitro

Entrainment of the dominant circadian pacemaker localised to the hypothalamic suprachiasmatic nuclei (SCN) is mediated partially via the indirect retino-geniculo-hypothalamic projection to the SCN, which is presumed to utilise enkephalin and other neurotransmitters, to modulate circadian rhythmicity. In the present study, we have investigated electrophysiologically the currently unknown functional effects of enkephalin, and another opioid receptor agonist morphine, on hamster SCN neuronal activity in vitro. Basal or N-methyl-D-aspartate-evoked firing rates of SCN neurones were generally unresponsive (86%) to the opioid receptor agonists leucine-enkephalin, methionine-enkephalin, or morphine. Washout of the enkephalins or morphine resulted in a rebound excitatory response ("withdrawal activation") in 39% of neurones tested. Withdrawal activation was also elicited by administration of the opioid receptor antagonist naloxone, following pre-exposure to morphine, in 59% of neurones tested. These withdrawal responses were blocked or attenuated by the alpha2-adrenoceptor agonist clonidine, results which suggest a functional interaction exists between opioid receptors and alpha2-adrenoceptors in the SCN. Our observations show that opioid receptor agonists are largely devoid of actions on normal hamster SCN circadian pacemaker activity, while the occurrence of withdrawal responses may have implications on circadian function during withdrawal from opiate abuse.

Localization and regulation of the delta-opioid receptor in dorsal root ganglia and spinal cord of the rat and monkey: evidence for association with the membrane of large dense-core vesicles

Using immunohistochemistry and immunoelectron microscopy, the localization and regulation of delta-opioid receptor-like immunoreactivity were studied in dorsal root ganglia and spinal cord of normal rat and monkey, and after peripheral axotomy. Delta-opioid receptor-like immunoreactivity was observed in many small dorsal root ganglion neurons, and in the rat most of them contained substance P and calcitonin gene-related peptide. At the ultrastructural level, delta-opioid receptor-like immunoreactivity was localized in the Golgi complex, on the membrane of the large dense-core vesicles and on the membrane of and/or inside a type of large vesicle with an interior of low electron density. The latter vesicles were often in contact with multivesicular bodies. In the superficial dorsal horn of the spinal cord, most delta-opioid receptor-positive nerve fibers contain substance P and/or calcitonin gene-related peptide, both in rat and monkey. Also, in these nerve endings delta-opioid receptor-like immunoreactivity was found on the membrane of large dense-core vesicles and on the membrane of, or in, the lucent vesicles. Occasionally, delta-opioid receptor-like immunoreactivity was observed on the plasmalemma of the terminals, particularly when the vesicles were in exocytotic contact with the plasmalemma. Peripheral axotomy induced a decrease in delta-opioid receptor-like immunoreactivity both in cell bodies in the dorsal root ganglia and in terminals in the dorsal horn. These data suggest that the delta-opioid receptor may be a constituent of the membrane of large dense-core vesicles storing and releasing neuropeptides. It is suggested that upon exocytotic release of substance P and calcitonin gene-related peptide from large dense-core vesicles, there is a transient modification of the surface of the primary afferent terminals which leads to exposure of the receptor protein so that enkephalin released from adjacent terminals can activate the receptor. The decrease in delta-opioid receptors after axotomy indicates that delta-opioid receptor-mediated inhibitory effects are attenuated at the spinal level both in the rat and monkey.

Localization of mu-opioid receptor-like immunoreactivity in the central components of the vagus nerve: a light and electron microscope study in the rat

mu-Opioid receptor, the opioid receptor that shows the highest affinity for morphine, appears to induce a variety of side-effects, at least partly, directly through the mu-opioid receptor on neurons constituting the autonomic part of the vagus nerve. Thus, in the present study, location of mu-opioid receptor-like immunoreactivity in the central components of the autonomic part of the vagus nerve was examined in the rat. The intense immunoreactivity was observed light microscopically in the neuropil of the commissural subnucleus and the dorsal part of the medial subnucleus of the nucleus of the solitary tract, and in the neuropil of the rostral half of the ambiguus nucleus. The immunoreactivity was moderate in the neuropil of the rostral and lateral subnuclei and ventral part of the medial subnucleus of the nucleus of the solitary tract, and weak in the neuropil of the dorsal motor nucleus of the vagus nerve. In the nodose ganglion, many neurons of various sizes (17-48 microns in soma diameter) showed moderate immunoreactivity. After unilateral vagotomy at a level proximal to the nodose ganglion, the immunoreactivity in the ipsilateral ambiguus nucleus was apparently reduced within 48 h of the operation, and completely disappeared by the seventh day after the operation. In the nucleus of the solitary tract and dorsal motor nucleus of the vagus nerve, the reduction of immunoreactivity after the ganglionectomy was detectable on the fourth day after the operation, and became readily apparent by the seventh day after the operation; the immunoreactivity, none the less, still remained on the 10th day after the operation. Electron microscopically, the immunoreactivity in the ambiguus nucleus was seen mainly on dendritic profiles and additionally on somatic ones; no immunoreactivity was detected in axonal profiles. The immunoreactivity in the dorsal motor nucleus of the vagus nerve was observed only on dendritic profiles. The immunoreactivity in the nucleus of the solitary tract was seen on axonal and dendritic profiles, but not on somatic profiles. The immunoreactive axon terminals in the nucleus of the solitary tract were filled with spherical synaptic vesicles and made asymmetric synapses with dendritic profiles. The results indicate that the mu-opioid receptor in the central components of the autonomic part of the vagus nerve is located on dendrites and cell bodies of efferent neurons in the ambiguus, on dendrites of efferent neurons in the dorsal motor nucleus, and on axons which arise from nodose ganglion neurons and terminate in the nucleus of the solitary tract. The receptors on these structures may constitute the targets of enkephalin-containing and beta-endorphin-containing afferent axons arising from brainstem neurons. The receptors on the axon terminals of nodose ganglion neurons may be involved in regulation of the release of neurotransmitters and/or neuromodulators.

Immunohistochemical localization of mu-opioid receptors in the central nervous system of the rat

Of the three major types of opioid receptors ( mu, delta, kappa) in the nervous system, mu-opioid receptor shows the highest affinity for morphine that exerts powerful effects on nociceptive, autonomic, and psychological functions. So far, at least two isoforms of mu-opioid receptors have been cloned from rat brain. The present study attempted to examine immunohistochemically the distribution of mu-opioid receptors in the rat central nervous system with two kinds of antibodies to recently cloned mu-opioid receptors (MOR1 and MOR1B). One antibody recognized a specific site for MOR1, and the other bound to a common site for MOR1 and MOR1B. Intense MOR1-like immunoreactivity (LI) was seen in the 'patch' areas and subcallosal streak in the striatum, medial habenular nucleus, medial terminal nucleus of the accessory optic tract, interpeduncular nucleus, median raphe nucleus, parabrachial nuclei, locus coeruleus, ambiguous nucleus, nucleus of the solitary tract, and laminae I and II of the medullary and spinal dorsal horns. Many other regions, including the cerebral cortex, amygdala, thalamus, and hypothalamus, also contained many neuronal elements with MOR1-LI. The distribution pattern of the immunoreactivity revealed with the antibody to the common site for MOR1 and MOR1B (MOR1/1B-LI) was almost the same as that of MOR1-LI. Both MOR1-LI and MOR1/1B-LI were primarily located in neuronal cell bodies and dendrites. However, the immunoreactivities were observed in the accessory optic tract, fasciculus retroflexus, solitary tract, and primary afferent fibers in the superficial layers of the medullary and spinal dorsal horns. The presynaptic location of MOR1-LI and MOR1/1B-LI was confirmed by lesion experiments: Enucleation, placing a lesion in the medial habenular nucleus, removal of the nodose ganglion, or dorsal rhizotomy resulted in a clear reduction of the immunoreactivities, respectively, in the nuclei of the accessory optic tract, some subnuclei of the interpeduncular nucleus, nucleus of the solitary tract, or laminae I and II of the spinal dorsal horn. The results indicate that the mu-opioid receptors are widely distributed in the brain and spinal cord, mainly postsynaptically and occasionally presynaptically. Opioids, including morphine, may inhibit the excitation of neurons via the postsynaptic mu-opioid receptors, and also suppress the release of neurotransmitters and/or neuromodulators from axon terminals through the presynaptic mu-opioid receptors.