Synaptic localization of kappa opioid receptors in guinea pig neostriatum

Blockade of stimulant-induced preprodynorphin mRNA expression in the striatal matrix by serotonin depletion

Cocaine and methamphetamine (METH) induce preprodynorphin (PPD) mRNA expression in the striatum. Cocaine induces PPD expression in both the patch and matrix compartments of the rostral striatum, whereas METH induces PPD expression in the patch compartment of the rostral striatum. In middle striatum, both stimulants increase PPD expression in the patch and matrix compartments. METH and cocaine treatment also increase extracellular serotonin (5-HT). Several studies have shown that 5-HT receptors are present on striatonigral neurons that express PPD mRNA, and that 5-HT is a positive regulator of striatal neuropeptide expression. The current study examined whether 5-HT plays a role in the patch/matrix expression of PPD mRNA induced by cocaine and METH in striatum. Male Sprague-Dawley rats were treated with p-chloroamphetamine (PCA; 8 mg/kg, i.p), a serotonin neurotoxin, 1 week prior to cocaine (30 mg/kg, i.p) and METH (15 mg/kg, s.c.) treatment. The 80% loss of 5-HT induced by PCA-pretreatment blocked cocaine-induced PPD expression in the rostral matrix compartment. Cocaine- and METH-induced PPD expression in the rostral patch compartment was unaffected by PCA-pretreatment. PCA-pretreatment also decreased both cocaine- and METH-induced PPD expression in the matrix, but not patch of middle striatum. PCA-induced 5-HT depletion did not affect stimulant-induced increases in PPT mRNA expression in the striatum. These data suggest that 5-HT plays a role in stimulant-induced PPD expression in the matrix compartment of rostral and middle striatum. Thus, 5-HT innervation may play a critical role in basal ganglia function.

The kappa opioid receptor and dynorphin co-localize in vasopressin magnocellular neurosecretory neurons in guinea-pig hypothalamus

The relationship between the cloned kappa opioid receptor, dynorphin, and the neurohypophysial hormones vasopressin and oxytocin was analysed in the guinea-pig hypothalamic magnocellular neurosecretory neurons. This analysis was performed in order to understand better which population of neuroendocrine neurons in the guinea-pig is modulated by kappa opioid receptors and its endogenous ligand dynorphin. Extensive co-localization was observed between kappa opioid receptor immunoreactivity and preprodynorphin immunoreactivity in neuronal cell bodies in the paraventricular and supraoptic nuclei. Cells positive for either the kappa opioid receptor or both the kappa opioid receptor and preprodynorphin were restricted to the vasopressin expressing neuronal population and not found in the oxytocin expressing neuronal population. The kappa opioid receptor and dynorphin were examined in the posterior pituitary and both were found to be extensively distributed. Staining for the kappa opioid receptor and dynorphin B co-localized in posterior pituitary. In addition, immunogold electron microscopy confirmed that kappa opioid receptor and dynorphin B immunoreactivity were found in the same nerve terminals. Ultrastructural analysis also revealed that kappa opioid receptor immunoreactivity was associated with both nerve terminals and pituicytes. Within nerve terminals, kappa opioid receptor immunoreactivity was often associated with large secretory vesicles and rarely associated with the plasma membrane. Our data suggest that the cloned kappa opioid receptor may directly modulate the release of vasopressin but not oxytocin in guinea-pig hypothalamic magnocellular neurosecretory neurons and posterior pituitary. Furthermore, we propose that this receptor is an autoreceptor in this system because our results demonstrate a high degree of co-localization between kappa opioid receptor and dynorphin peptide immunoreactivity in magnocellular nerve terminals.

Cellular sites for dynorphin activation of kappa-opioid receptors in the rat nucleus accumbens shell

The nucleus accumbens (Acb) is prominently involved in the aversive behavioral aspects of kappa-opioid receptor (KOR) agonists, including its endogenous ligand dynorphin (Dyn). We examined the ultrastructural immunoperoxidase localization of KOR and immunogold labeling of Dyn to determine the major cellular sites for KOR activation in this region. Of 851 KOR-labeled structures sampled from a total area of 10,457 microm2, 63% were small axons and morphologically heterogenous axon terminals, 31% of which apposed Dyn-labeled terminals or also contained Dyn. Sixty-eight percent of the KOR-containing axon terminals formed punctate-symmetric or appositional contacts with unlabeled dendrites and spines, many of which received convergent input from terminals that formed asymmetric synapses. Excitatory-type terminals that formed asymmetric synapses with dendritic spines comprised 21% of the KOR-immunoreactive profiles. Dendritic spines within the neuropil were the major nonaxonal structures that contained KOR immunoreactivity. These spines also received excitatory-type synapses from unlabeled terminals and were apposed by Dyn-containing terminals. These results provide ultrastructural evidence that in the Acb shell (AcbSh), KOR agonists play a primary role in regulating the presynaptic release of Dyn and other neuromodulators that influence the output of spiny neurons via changes in the presynaptic release of or the postsynaptic responses to excitatory amino acids. The cellular distribution of KOR complements those described previously for the reward-associated mu- and delta-opioid receptors in the Acb shell.

Cellular sites for dynorphin activation of kappa-opioid receptors in the rat nucleus accumbens shell

The nucleus accumbens (Acb) is prominently involved in the aversive behavioral aspects of kappa-opioid receptor (KOR) agonists, including its endogenous ligand dynorphin (Dyn). We examined the ultrastructural immunoperoxidase localization of KOR and immunogold labeling of Dyn to determine the major cellular sites for KOR activation in this region. Of 851 KOR-labeled structures sampled from a total area of 10,457 microm2, 63% were small axons and morphologically heterogenous axon terminals, 31% of which apposed Dyn-labeled terminals or also contained Dyn. Sixty-eight percent of the KOR-containing axon terminals formed punctate-symmetric or appositional contacts with unlabeled dendrites and spines, many of which received convergent input from terminals that formed asymmetric synapses. Excitatory-type terminals that formed asymmetric synapses with dendritic spines comprised 21% of the KOR-immunoreactive profiles. Dendritic spines within the neuropil were the major nonaxonal structures that contained KOR immunoreactivity. These spines also received excitatory-type synapses from unlabeled terminals and were apposed by Dyn-containing terminals. These results provide ultrastructural evidence that in the Acb shell (AcbSh), KOR agonists play a primary role in regulating the presynaptic release of Dyn and other neuromodulators that influence the output of spiny neurons via changes in the presynaptic release of or the postsynaptic responses to excitatory amino acids. The cellular distribution of KOR complements those described previously for the reward-associated mu- and delta-opioid receptors in the Acb shell.

Complementarity of radioautographic and immunohistochemical techniques for localizing neuroreceptors at the light and electron microscopy level

To assess relationships between neuropeptide-binding sites and receptor proteins in rat brain, the distribution of radioautographically labeled somatostatin and neurotensin-binding sites was compared to that of immunolabeled sst2A and NTRH receptor subtypes, respectively. By light microscopy, immunoreactive sst2A receptors were either confined to neuronal perikarya and dendrites or diffusely distributed in tissue. By electron microscopy, areas expressing somatodendritic sst2A receptors displayed only low proportions of membrane-associated, as compared to intracellular, receptors. Conversely, regions displaying diffuse sst2A labeling exhibited higher proportions of membrane-associated than intracellular receptors. Furthermore, the former showed only low levels of radioautographically labeled somatostatin-binding sites whereas the latter contained high densities of somatostatin-binding suggesting that membrane-associated receptors are preferentially recognized by the radioligand. In the case of NTRH receptors, there was a close correspondence between the light microscopic distribution of NTRH immunoreactivity and that of labeled neurotensin-binding sites. Within the substantia nigra, the bulk of immuno- and autoradiographically labeled receptors were associated with the cell bodies and dendrites of presumptive DA neurons. By electron microscopy, both markers were detected inside as well as on the surface of labeled neurons. At the level of the plasma membrane, their distribution was highly correlated and characterized by a lack of enrichment at the level of synaptic junctions and by a homogeneous distribution along the remaining neuronal surface, in conformity with the hypothesis of an extra-synaptic action of this neuropeptide. Inside labeled dendrites, there was a proportionally higher content of immunoreactive than radiolabeled receptors. Some of the immunolabeled receptors not recognized by the radioligand were found in endosome-like organelles suggesting that, as in the case of sst2A receptors, they may have undergone endocytosis subsequent to binding to the endogenous peptide.

Electron microscopic distribution of mu opioid receptors on noradrenergic neurons of the locus coeruleus

The distribution of mu opioid receptors was examined by light and electron microscopic autoradiography in the locus coeruleus of the rat following in vitro labelling with the iodinated agonist [125I]FK-33824. At the light microscopic level, specific mu opioid binding sites were concentrated over the perikarya and dendrites of neurons that were tyrosine hydroxylase-immunopositive in adjacent sections. Accordingly, both the number of tyrosine hydroxylase-immunoreactive neurons and the density of labelled mu receptors decreased markedly throughout the rostrocaudal extent of the nucleus following treatment with the catecholaminergic neurotoxin 6-hydroxydopamine. By electron microscopy, specifically labelled receptors were detected both inside and on the surface of locus coeruleus neurons. Intracellular sites were found by resolution circle analysis to be highly concentrated within the endoplasmic reticulum and Golgi apparatus, suggesting that the ligand recognizes both glycosylated and preglycosylated forms of receptor. The remainder were found mainly over the cytoplasmic matrix or intracytoplasmic vesicles, and were attributed to newly synthesized or recycled receptors in transit. Cell surface receptors were present over both dendritic and perikaryal membranes of noradrenergic cells. These were most highly concentrated opposite abutting axon terminals, suggesting the existence of receptor 'hot spots' at sites of putative endogenous ligand release. However, only a small proportion of these sites was associated with synaptic specializations. Furthermore, an important contingent was detected opposite non-axonal elements, such as dendrites and glial cells, suggesting that mu opioid ligands act mainly parasynaptically on locus coeruleus neurons. Finally, approximately 5% of labelled receptors were associated with axoglial interfaces, indicating that a minor action of mu opioids in the locus may be presynaptic and/or glial.

Presynaptic and postsynaptic subcellular localization of substance P receptor immunoreactivity in the neostriatum of the rat and rhesus monkey (Macaca mulatta)

The substance P receptor (SPR) gene is expressed at high levels in basal ganglia, but the paucity of information about localization of the encoded receptor protein has limited our understanding of this peptide's involvement in cellular and subcellular mechanisms in this region. Morphological evidence in the rodent striatum indicates that SPRs are expressed in postsynaptic neuronal elements, while pharmacological studies suggest the existence of presynaptic SPRs in this structure. We have examined the issue of subcellular distribution of this receptor protein in rat and primate neostriatal tissue, employing an antiserum raised against SPR. Electron microscopic analysis revealed that SPR immunoreactivity is present in presynaptic and postsynaptic neuronal elements in both species. In agreement with earlier studies, SPR immunoreactivity was found predominantly in perikarya and dendrites of a small subset of striatal neurons, the large and medium-sized aspiny interneurons. In addition, a small but significant proportion of the immunoreaction product was localized in presynaptic profiles, both in axons and axon terminals. The majority of SPR immunoreactive boutons formed asymmetric synapses with dendrites and dendritic spines. The association of SPRs with asymmetric synapses provides a morphological substrate for peptidergic modulation of excitatory neurotransmission of extrastriatal origin. A minor proportion of immunolabeled axons established symmetric synaptic junctions with unlabeled dendrites. The presence of SPRs in these synapses suggests a presynaptic peptidergic modulation of intrinsic striatal transmitter systems. The observations in this study also indicate that SPR mediates a complex combination of postsynaptic and presynaptic effects on acetylcholine release in the mammalian striatum.

Ultrastructural study of substance P receptors in the dorsal horn of the rat spinal cord using monoclonal anti-complementary peptide antibody

A monoclonal antibody directed against a peptide (PS5) specified by RNA complementary to the mRNA coding for substance P (SP), was used to label SP receptors in the rat spinal cord as demonstrated by light and electron microscopy. An immunocytochemical method (avidin-biotin-peroxidase) was used on vibratome sections from rats perfused with paraformaldehyde. Immunoreactivity was observed principally in the two superficial layers of the dorsal horn, in lamina X and the region of motoneurons. The labeling was absent when the antibody was preincubated with the complementary peptide (PS5) used as immunogen. Competition between the anti-complementary peptide antibody and different ligands was tested by preincubation of tissue sections with the ligand in the presence of peptidase inhibitors before addition of the antibody. A specific agonist (SP) or antagonist (spantide, RP 67580) at 10(-6)M led to total absence of labeling. These results indicate that under our experimental conditions, the anti-complementary peptide antibody recognizes a SP binding site in the rat spinal cord. Electron microscopic study of the two superficial laminae of the dorsal horn showed that immunolabeling was mainly localized extracellularly at apposing neuronal plasma membranes. It was mostly associated with axodendritic or axosomatic appositions. Occasionally labeling was observed between two axon terminals. In all cases, these appositions were non-junctional. Generally, neuronal processes involved in these appositions did not contain large granular vesicles. These observations suggest that SP may act in a diffuse, nonsynaptic manner probably on targets distant from SP release sites.

Postsynaptic and extrasynaptic localization of kappa-opioid receptor in selected brain areas of young rat and chick using an anti-receptor monoclonal antibody

kappa-opioid receptors were visualized by light and electron microscopical immunohistochemistry in young rat and chick brains, using a monoclonal antibody KA8 (IgG1, kappa) raised against a kappa-opioid receptor preparation from frog brain, which recognizes selectively the kappa-type receptor with preference for the kappa-2 subtype. The most pronounced kappa-opioid receptor-like immunoreactivity was observed in the hypothalamic nuclei of the rat brain and in the chick optic tectum, in regions where the functional significance of kappa-opioid receptors is well documented. Both neurons and glia were stained, the former on both somata and dendrites. At the ultrastructural level, the receptor-like immunoreactivity was similar in both species. Immunoprecipitate decorated the inner surface of the plasma membrane of glial cells, neuronal somata and dendrites, in a discontinuous arrangement. In the cytoplasm, labelling was associated with ribosomes, polyribosomes and rough endoplasmic reticulum membranes but not with Golgi cisternae. In the neuropil, the immunoprecipitate was observed along the dendritic microtubules and was also associated with postsynaptic sites. Nuclei and axons were devoid of label and immunoreactivity was never visible presynaptically. Our findings indicate that the antibody used in the present study marks various forms of the kappa-opioid receptor protein including those synthesised in ribosomes, transported along dendritic microtubules and incorporated into postsynaptic and non-synaptic membranes. The antibody also recognizes glial opioid receptors. The observed subcellular distribution appears to be conserved in phylogenetically distant species.

Neuropeptide Y and dynorphin-immunoreactive large dense-core vesicles are strategically localized for presynaptic modulation in the hippocampal formation and substantia nigra

Neuropeptide Y (NPY) and dynorphin elicit regionally selective presynaptic modulation in the hippocampal formation and the pars reticulata of the substantia nigra, respectively. We examined potential anatomical substrates for their presynaptic modulation by determining the distribution and size of large (80-120 nm) dense-core vesicles (DCVs), organelles previously shown to be immunoreactive for each peptide. Throughout the hippocampal formation, NPY-immunoreactive DCVs were located primarily in axon terminals and were more sparingly distributed in dendrites. In comparison with other portions of the hippocampal formation, NPY-labeled DCVs were most abundant in axons and terminals of the CA1 region. The DCVs in the CA1 region of the hippocampus also more frequently had larger mean cross-sectional diameters when located along portions of the terminal in contact with unlabeled axons. In both the CA1 region of the hippocampus and the dentate gyrus, NPY-labeled DCVs in contact with portions of the axonal membrane apposed to astrocytes also were larger than those located more centrally in the axon terminal. Dynorphin-immunoreactive DCVs in axon terminals of the substantia nigra were significantly larger when found near portions of the axonal membrane in contact not only with other axons and astrocytic processes, but also occasionally with postsynaptic dendrites. The parallels between diameters of DCVs and known selectivity of NPY for presynaptic modulation in the CA1 region of the hippocampus suggest a direct correlation between the size and distribution of immunoreactive DCVs and their sites of exocytotic release.(ABSTRACT TRUNCATED AT 250 WORDS)

Dynorphin-immunoreactive terminals in the rat nucleus accumbens: cellular sites for modulation of target neurons and interactions with catecholamine afferents

Dynorphin facilitates conditioned place aversion and reduces locomotor activity through mechanisms potentially involving direct activation of target neurons or release of catecholamines from afferents in the nucleus accumbens. We examined the ultrastructural substrates underlying these actions by combining immunoperoxidase labeling for dynorphin 1-8 and immunogold silver labeling for the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH). The two markers were simultaneously visualized in single coronal sections through the rat nucleus accumbens. By light microscopy, dynorphin immunoreactivity was seen as patches of immunoreactive varicosities throughout all rostrocaudal levels of the nucleus accumbens. The dynorphin-immunoreactive terminals identified by electron microscopy ranged from 0.2 to 1.5 microns in cross-sectional diameter, contained numerous small (30-40 nm) clear vesicles, as well as one or more large (80-100 nm) dense core vesicles. From the dynorphin-immunoreactive terminals quantitatively examined in single sections, 74% (173/370) showed symmetric synaptic junctions mainly with large unlabeled dendrites. Of the dynorphin-immunoreactive terminals forming identifiable synapses, approximately 30% contacted more than one dendritic target. In addition, single dendrites frequently received convergent input from more than one dynorphin-labeled terminal. Irrespective of their dendritic associations, dynorphin-immunoreactive terminals also frequently showed close appositions with other axons and terminals; these included unlabeled (41%), TH-labeled (10%) or dynorphin-labeled axons (14%). In contrast to dynorphin-immunoreactive terminals, TH-labeled terminals formed primarily symmetric synapses with small dendrites and spines or lacked recognizable specializations in the plane of section analyzed. In some cases, single dendrites were postsynaptic to both dynorphin and TH-immunoreactive terminals. We conclude that dynorphin-immunoreactive terminals potently modulate, and most likely inhibit, target neurons in both subregions of the rat nucleus accumbens. This modulatory action could attenuate or potentiate incoming catecholamine signals on more distal dendrites of the accumbens neurons. The findings also suggest potential sites for presynaptic modulatory interactions involving dynorphin and catecholamine or other transmitters in apposed terminals.

Electron microscopic localization of photoaffinity-labelled delta opioid receptors in the neostriatum of the rat

The distribution of delta opioid receptors, selectively labelled in vitro with the photoaffinity probe monoiodo azido-DTLET ([D-Thr2,pN3Phe4, Leu5]enkephaly-Thr6), was analyzed by light and electron microscopic radioautography in sections from rat neostriatum. Preliminary experiments indicated that up to 65% of specific 125I-azido-DTLET binding to rat striatal sections was still detectable following prefixation of the brain with 0.5% glutaraldehyde. These experiments also showed that up to 20-30% of the specifically bound radioactivity was covalently linked following ultraviolet irradiation and was thereby retained in tissue during subsequent postfixation and dehydration steps. Accordingly, the topographic distribution of the covalently attached azido-DTLET molecules was similar to that seen in fresh frozen sections and characteristic of that previously described for delta sites. Light and electron microscopic examination of the label in prefixed, striatal sections irradiated with ultraviolet light revealed that a significant proportion of specifically bound 125I-azido-DTLET molecules was intraneuronal. Specifically, 16% of the labelled binding sites were found in dendrites, 12% in perikarya and 4% in axon terminals. These results suggest that an important proportion of delta opioid binding sites labelled in the neostriatum correspond to receptors that are undergoing synthesis, transport and/or recycling. They also imply that a major fraction of delta sites are associated with intrastriatal neurons, as opposed to afferent axons. Approximately 44% of the labelled binding sites were associated with neuronal plasma membranes. Although most of these were found at the level of axodendritic (20%) and dendrodendritic (7%) appositions, comparison of the labelling incidence of these two compartments with their frequency of occurrence in tissue suggested that delta sites are fairly widely dispersed along neuronal plasma membranes. Only a small proportion (smaller than that of mu or kappa sites labelled in the same region) was associated with synaptic specializations. These results support the concept that delta receptors correspond to molecular entities that are distinct from mu and kappa sites and suggest that delta ligands act primarily nonjunctionally on the plasma membrane of striatal neurons.