Mu-opioid receptor is located on the plasma membrane of dendrites that receive asymmetric synapses from axon terminals containing leucine-enkephalin in the rat nucleus locus coeruleus

We have recently shown, by using immunoelectron microscopy, that the mu-opioid receptor (mu OR) is prominently distributed within noradrenergic perikarya and dendrites of the nucleus locus coeruleus (LC), many of which receive excitatory-type (i.e., asymmetric) synaptic contacts from unlabeled axon terminals. To characterize further the neurotransmitter present in these afferent terminals, we examined in the present study the ultrastructural localization of an antipeptide sequence unique to the mu OR in sections that were also dually labeled for the opioid peptide leucine-enkephalin (L-ENK). Immunogold-silver labeling for mu OR was localized to extrasynaptic portions of the plasma membranes of perikarya and dendrites. The mu OR-labeled dendrites were usually postsynaptic to axon terminals containing heterogeneous types of synaptic vesicles and forming asymmetric synaptic specializations characteristic of excitatory-type synapses. The majority of these were immunolabeled for the endogenous opioid peptide L-ENK. Some mu OR-labeled dendrites received synaptic contacts from unlabeled axon terminals in fields containing L-ENK immunoreactivity. In such cases, the mu OR-labeled dendrites were in proximity to L-ENK axon terminals that contained intense peroxidase labeling within large dense core vesicles along the perimeter of the axoplasm. These results indicate that L-ENK may be released by exocytosis from the dense core vesicles and diffuse within the extracellular space to reach mu OR sites on the postsynaptic dendrite or dendrites of other neighboring neurons. The present study also reveals that unlabeled terminals apposed to mu OR-labeled dendrites may contain other opioid peptides, such as methionine-enkephalin. These data demonstrate several sites where endogenous opioid peptides may interact with mu OR receptive sites in the LC and may provide an anatomical substrate for the LC's involvement in mechanisms of opiate dependence and withdrawal.

Ultrastructural immunocytochemical localization of mu opioid receptors and Leu5-enkephalin in the patch compartment of the rat caudate-putamen nucleus

To delineate the cellular sites for the motor effects of opiates acting at the mu opioid receptor (MOR) in the rat caudate-putamen nucleus, we examined the ultrastructural immunogold and immunoperoxidase labeling of an antipeptide antiserum specific for the MOR. We also combined these labeling methods to examine the subcellular relationship between the MOR and the endogenous opioid peptide, Leu5-enkephalin (LE). By light microscopy, MOR-labeling was seen in a heterogeneous patchy distribution. Electron microscopic analysis of these patches showed that more than 80% of the total neuronal profiles (n = 1,586) containing MOR-like immunoreactivity (MOR-IR) were dendrites and dendritic spines. The remaining labeled profiles included a few perikarya and many axon terminals. MOR-IR was predominantly localized to extrasynaptic plasma membranes of dendrites, and to both synaptic vesicles and plasma membranes in terminals. Ten percent of the total MOR-labeled terminals (n = 272) formed asymmetric synapses with unlabeled or MOR-labeled dendritic spines. Terminals containing LE-IR formed synapses, in almost equal proportions, on MOR-labeled dendrites and dendritic spines, while over 80% of the unlabeled terminals formed synapses on MOR-labeled dendritic spines. Moreover, colocalization of MOR- and LE-IR was often seen in both dendrites and terminals. These results indicate that in patch compartments of the caudate-putamen nucleus, the MOR is mainly involved in extrasynaptic modulation of spiny neurons, including those that contain LE. In addition, the findings provide a cellular basis for presynaptic opioid modulation of neurotransmitter release through MOR located on axon terminals.

mu Opiate receptor immunoreactivity in rat central nervous system

Immunoreactivity corresponding to the C-terminus of the rat mu opiate receptor can be detected by light microscopy in fiber- and terminal-like patterns in a number of rat brain and spinal cord regions, and in immunoreactive perikarya in several of these regions. Especially abundant fiber- and terminal-like patterns were localized to superficial layers of the spinal cord dorsal horn and nucleus caudalis of the spinal tract of the trigeminal, the nucleus of the solitary tract, nucleus ambiguous, locus coeruleus, interpeduncular nucleus, medial aspect of the lateral habenular nucleus, presumed "striasomes" of the caudate-putamen and nucleus accumbens. Moderate fiber and terminal densities were found in the ventral tegmental area, more medial aspects of the thalamus and hypothalamus, and several amygdaloid nuclei. Immunostained perikarya were prominent in the nucleus accumbens and also observed in the middle layers of the cerebral cortex, septum and diagonal band, preoptic area, medial thalamic and habenular nuclei, locus coeruleus, nucleus ambiguous, nucleus of the solitary tract, trigeminal nucleus caudalis, and spinal cord substantia gelatinosa zones. Many of these localizations correspond well with the previously-determined autoradiographic distributions of mu opiate receptor ligand binding, and with reports of mu opiate receptor immunoreactivity determined using other antisera. Electron microscopic immunohistochemical studies reveal details of the membrane distribution of the mu receptor in nucleus accumbens, caudate/putamen, locus coeruleus, and spinal cord. These results suggest largely neuronal and largely extrasynaptic distributions of mu receptors that show differential patterns of perikaryal, dendritic, and/or axonal immunostaining in different central nervous system zones. Identification of these distributions adds substantially to data identifying the cellular localization of the principal opiate receptor involved in both analgesic and addictive processes.

Distinct cellular compartment of cyclin-dependent kinase 5 (Cdk5) and neuron-specific Cdk5 activator protein (p35nck5a) in the developing rat cerebellum

We have elucidated the spatial and temporal localization of Cdk5 and p35nck5a in the developing rat postnatal cerebellum. Both proteins were highly expressed in cell bodies of post mitotic and immature neurons. The localization of Cdk5 in cellular compartment was changed from cell body to the axon in development. On the other hand, p35nck5a was always expressed in the cell body throughout cerebellum development. The Cdk5 kinase activity was correlated with the expression of p35nck5a rather than that of Cdk5. These results indicate that p35nck5a is a physiological activator of Cdk5 in immature neurons and further suggest that Cdk5 has another function in mature neurons.

Ultrastructural localization of mu-opioid receptors in the superficial layers of the rat cervical spinal cord: extrasynaptic localization and proximity to Leu5-enkephalin

Many of the analgesic effects of opiate drugs and of endogenous opioid ligands, such as Leu5-enkephalin (LE) are thought to be mediated in part by mu-opioid receptors (MOR) in the dorsal horn of the spinal cord. To establish the cellular sites for the spinally mediated analgesic effects of MOR activation and the potential anatomical substrates for interactions with LE, we examined the ultrastructural localization of MOR and LE immunoreactivities in the adult rat cervical spinal cord (C3-C5). Anti-MOR sera recognizing the carboxyl terminal domain of MOR was localized using immunoperoxidase and immunogold-silver methods. mu-opioid receptor-like immunoreactivity (MOR-LI) was observed mainly in the superficial layers of the dorsal horn. Electron microscopy of this region revealed that small unmyelinated axons and axon terminals constituted 48% (91/189) and 15% (28/189), respectively, while dendrites comprised 36% (68/189) of the total population of neuronal profiles containing the MOR. MOR-LI was localized mainly along extrasynaptic portions of the plasma membrane in both axons and dendrites. In sections dually labeled for MOR and LE, 21% (14/68) of the dendrites containing MOR-LI closely apposed or received synaptic contact from axon terminals exhibiting LE reaction product. The results provide the first ultrastructural evidence that within the dorsal horn of the spinal cord, LE, as well as exogenous opiates may alter both axonal release of neurotransmitters and postsynaptic responsiveness of target neurons to afferent input through activation of extrasynaptic MOR.

Ultrastructural evidence for prominent distribution of the mu-opioid receptor at extrasynaptic sites on noradrenergic dendrites in the rat nucleus locus coeruleus

Physiological studies have indicated that agonists at the mu-opioid receptor (mu OR), such as morphine or the endogenous peptide methionine5-enkephalin, can markedly decrease the spontaneous activity of noradrenergic neurons in the locus coeruleus (LC). Messenger RNA and protein for mu OR are also densely expressed by LC neurons. During opiate withdrawal, increased discharge rates of LC neurons coincide with the expression of behavioral features associated with the opiate withdrawal syndrome. To better define the cellular sites for the physiological activation of mu OR in the LC and its relation to afferent terminals, we examined the ultrastructural localization of mu OR immunoreactivity in sections dually labeled for the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH). Immunogold-silver labeling for mu OR (i-mu OR) was localized to parasynaptic and extrasynaptic portions of the plasma membranes of perikarya and dendrites, many of which also contained immunolabeling for TH. The dendrites containing exclusively i-mu OR were more numerous in the rostral pole of the LC. The i-mu OR in dendrites with and without detectable TH immunoreactivity were usually postsynaptic to unlabeled axon terminals containing heterogeneous types of synaptic vesicles and forming asymmetric synaptic specializations characteristic of excitatory-type synapses. These results provide the first direct ultrastructural evidence that mu OR is strategically localized to modulate the postsynaptic excitatory responses of catecholamine-containing neurons in the LC.

Calcineurin inhibitors, FK506 and cyclosporin A, suppress the NMDA receptor-mediated potentials and LTP, but not depotentiation in the rat hippocampus

The effects of FK506, a Ca2+/calmodulin-dependent phosphatase 2B (calcineurin) inhibitor, on the NMDA receptor-mediated potentials and synaptic plasticity were investigated in the CA1 region of the rat hippocampus. Bath application of FK506 (50 microM) produced a 45% inhibition on the NMDA receptor-mediated potentials. FK506 also inhibited the induction of long-term potentiation (LTP), but had no effect on the depotentiation in the CA1 hippocampus. Cyclosporin A (100 microM), another calcineurin inhibitor, mimicked the effects of FK506 on the NMDA responses and synaptic plasticity. These results suggest that FK506 inhibits the activity of NMDA receptors via the involvement of calcineurin. The differential effects of FK506 on LTP and depotentiation may attribute to the partial inhibition on the activity of NMDA receptors and the subsequent attenuation of intracellular Ca2+ increase.

Ultrastructural immunocytochemical localization of mu-opioid receptors in rat nucleus accumbens: extrasynaptic plasmalemmal distribution and association with Leu5-enkephalin

mu-Opioid receptors and their endogenous ligands, including Leu5-enkephalin (LE), are distributed abundantly in the nucleus accumbens (NAC), a region implicated in mechanisms of opiate reinforcement. We used immunoperoxidase and/or immunogold-silver methods to define ultrastructural sites for functions ascribed to mu-opioid receptors and potential sites for activation by LE in the NAC. An antipeptide antibody raised against an 18 amino acid sequence of the cloned mu-opioid receptor (MOR) C terminus showed that MOR-like immunoreactivity (MOR-LI) was localized predominantly to extrasynaptic sites along neuronal plasma membranes. The majority of neuronal profiles containing MOR-LI were dendrites and dendritic spines. The dendritic plasma membranes immunolabeled for MOR were near sites of synaptic input from LE-labeled terminals and other unlabeled terminals forming either inhibitory or excitatory type synapses. Unmyelinated axons and axon terminals were also intensely but less frequently immunoreactive for MOR. Observed sites for potential axonal associations with LE included coexistence of MOR and LE within the same terminal, as well as close appositions between differentially labeled axons. Astrocytic processes rarely contained detectable MOR-LI, but also were sometimes observed in apposition to LE-labeled terminals. We conclude that in the rat NAC, MOR is localized prominently to extrasynaptic neuronal and more rarely to glial plasma membranes that are readily accessible to released LE and possibly other opioid peptides and opiate drugs. The close affiliation of MOR with spines receiving excitatory synapses and dendrites receiving inhibitory synapses provides the first direct morphological evidence that MOR selectively modulates postsynaptic responses to cortical and other afferents.

Immunosuppressant FK506 prevents mossy fiber sprouting induced by kindling stimulation

Kindling stimulation induces expansive growth of the axons of the dentate granule cells, the mossy fiber, into several areas of the hippocampus. An intraperitoneal injection of the immunosuppressant drug FK506, which is a specific inhibitor of Ca(2+)-calmodulin dependent phosphatase, calcineurin, prevented the full development of kindling as well as mossy fiber sprouting. The results show a correlation between mossy fiber sprouting and the development of kindling. The results suggest also that calcineurin may have a promoting role in mossy fiber sprouting and subsequent synaptogenesis.

FK506, a Ca2+/calmodulin-dependent phosphatase inhibitor, inhibits the induction of long-term potentiation in the rat hippocampus

The effects of FK506, a Ca2+/calmodulin-dependent phosphatase 2B (calcineurin) inhibitor, on the synaptic potentials and long-term potentiation (LTP) were investigated, using extracellular recordings in the CA1 region of adult rat hippocampal slices. Bath application of FK506 (1-50 mu M) produced a dose-dependent and reversible inhibition on the field excitatory postsynaptic potentials. FK506 also showed an inhibitory effect on the induction of LTP evoked by theta-burst stimulation. Analyses of the fiber volley and paired-pulse facilitation revealed that FK506-induced effects were based on postsynaptic mechanisms. These results demonstrate that FK506 inhibits both the synaptic transmission and the LTP induction, and suggest that calcineurin plays a promoting role in the LTP processes in the hippocampus of adult rats.

Effects of neurotensin on neurons in the rat central amygdaloid nucleus in vitro

The effects of neurotensin (NT) on neurons in the central amygdaloid nucleus (ACe) were investigated in rat brain slice preparations by adding the peptide to the perfusing medium. Of 115 ACe neurons, 69 cells (60%) showed excitatory responses and 10 cells (9%) showed inhibitory responses to application of NT. The excitatory response to NT was observed in a dose-dependent manner and the threshold concentration was approximately 3 x 10(-9) M. The excitatory effects of NT persisted under blockade of synaptic transmission. The NT fragment neurotensin 8-13 and the NT analogue neuromedin N showed effects similar to those of NT, whereas the NT fragment neurotensin 1-8 had no effect on ACe neurons. Of 43 neurons in the septal nucleus, 8 cells (19%) and 3 cells (7%) showed excitatory and inhibitory responses, respectively, to NT. The results suggest that NT exerts a potent excitatory effect on ACe neurons through a direct action on specific receptors, in which NT may play a role in amygdala-relevant functions.

Induction of Fos expression following anodal polarization in rat brain

Expression of c-fos immunoreactivity was investigated in rat brain after unilateral application of a weak anodal direct current (anodal polarization) to the sensorimotor cortex of rats. Increases in Fosimmunopositive neurons were observed transiently in the neocortex, cingulate cortex, piriform cortex, and hippocampal formation, which were ipsilateral to the polarization, as a function of the duration and intensity of the current applied. It is likely that anodal polarization enhances the neuronal activities in the cortex dependent on polarization paradigms.

Increase in the calcium level following anodal polarization in the rat brain

The accumulation of calcium ions (Ca) was examined in the rat brain by means of 45Ca autoradiography following the application of a weak anodal direct current to the surface of the sensorimotor cortex. Repetition of the anodal polarization with 3.0 microA for 30 min caused more Ca to accumulate in the cerebral cortex. The degree and extent of accumulation was greater in the hemisphere ipsilateral to the polarization than in the other. Accumulation was also noted in the hippocampus and thalamus. Ca accumulation was detected after 24 h and it remained virtually constant up to 72 h after the last polarization. The results suggest that a long-lasting disturbance of Ca homeostasis is involved in the cortical plastic changes seen following anodal polarization.

Co-localization of c-fos protein and protein kinase C gamma in the rat brain following anodal polarization

The expression of protein kinase c gamma (PKC gamma) and c-fos protein was examined by means of double labeling in the rat brain in relation to the molecular mechanism of central plastic changes associated with anodal polarization. Under normal, non-polarized condition, approximately 75% of all fos positive neurons in the neocortex were immunopositive for PKC gamma. Conversely, nearly all PKC gamma positive neurons were fos immunopositive. Although both pyramidal and non-pyramidal neurons express both types of protein, the pyramidal cell type represents the vast majority. An anodal direct current of 3.0 microA for 30 min to the surface of the left sensorimotor cortex resulted in a pronounced increase in the intensity of immunoreactivity for both PKC gamma and c-fos protein ipsilateral to the polarization. Approximately, 91% of fos positive neurons in the polarized neocortex was also intensely immunoreactive for PKY gamma. The high degree of codistribution of both transduction proteins in specific neurons following anodal polarization suggests the functional connection between PKY gamma activation and c-fos expression in polarization phenomenon.

c-Fos expression mediated by N-methyl-D-aspartate receptors following anodal polarization in the rat brain

c-Fos protein-like immunoreactivity (IR) was investigated in the rat brain following an application of weak anodal direct current to the surface of the unilateral sensorimotor cortex in an attempt to elucidate the cellular and molecular bases of central plasticity. Anodal polarization resulted in a massive increase in c-Fos protein-like IR in neurons of the cingulate, piriform, frontoparietal cortices, and hippocampus ipsilateral to the polarization. The effects were dependent upon the duration and intensity of currents applied. The time-dependent induction of c-Fos protein-like IR was maximal at 1 h, became weaker by 6 h, and almost returned to the baseline within 24 h following polarization. When MK-801 [(+)-5-methyl-10,11-di-hydro-5H- dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate], a noncompetitive antagonist for N-methyl-D-aspartate (NMDA) receptors, was injected intraperitoneally, the induction of this nuclear protein was reduced or completely blocked in both hemispheres, except around the polarized point itself, as a function of the time and dosage. These results suggest that the proto-oncogene c-fos is rapidly and transiently activated in the brain following anodal polarization and this activation is mediated by NMDA receptors.

Inhibition of neurons in the rat medial amygdaloid nucleus in vitro by somatostatin

Effects of somatostatin (SRIF) on neurons in the medial amygdaloid nucleus were investigated in rat brain slice preparations, using extracellular recordings. Following bath application of SRIF at 10(-7) - 10(-6) M, 63 of 81 (78%) medial amygdala neurons showed an inhibitory response. The inhibitory effect of SRIF was dose dependent, and the threshold concentration was approximately 10(-9) M. The inhibitory response to SRIF persisted during synaptic blockade in two-thirds of neurons tested. The inhibitory effect of SRIF was reduced by picrotoxin, a GABAA receptor antagonist, in one-third of neurons. These results suggest that SRIF exerts an inhibitory effect on medial amygdala neurons through either a direct action on SRIF receptors or a GABAergic synaptic involvement.

Detection of DNA damage induced by apoptosis in the rat brain following incomplete ischemia

To clarify the pathogenesis and molecular basis of ischemia-related nerve cell death, we examined the occurrence of DNA fragmentation as a hallmark of apoptotic cell death following incomplete ischemia in the rat brain by means of in situ end labeling of fragmented DNA. Incomplete ischemia was produced by permanently occluding one carotid artery, while temporarily occluding the other. The condensed nuclei of ischemic neurons in the neocortex, and in the subiculum and CA1 area of the hippocampus were positively stained 24 h and 3 days following vessel occlusion, respectively, and their morphology was typically apoptotic. The ischemic neurons with condensed nuclei gradually increased in number and were clearly stained for fragmented DNA in these areas. The labeled nuclei in the neocortex became pyknotic 72 h later, and in the hippocampus 7 days later incomplete ischemia. After attaining a peak, the number of labeled nuclei decreased with the duration of recovery in all areas. These results suggest that an apoptotic process plays, at least primarily, a role in the degeneration of neurons associated with incomplete forebrain ischemia in rat.

Dopamine transporter cysteine mutants: second extracellular loop cysteines are required for transporter expression

Studies with thiol-modifying reagents have suggested that cysteines might play important roles in the function of the dopamine transporter (DAT). To identify DAT cysteines with important thiol groups, we have studied six mutant dopamine transporters in which cysteines were replaced by alanines. Substitutions of cysteines assigned to the DAT's second putative extracellular loop--positions 180 and 189--dramatically decreased the expression of the mutant transporters. Substitutions at positions 90, 242, 305, and 345 had no significant effect in decreasing dopamine uptake, MPP+ uptake, or cocaine analogue binding. Immunostaining COS cells transfected with Cys180 and Cys189 to Ala mutants revealed reduced membrane staining and prominent staining in perinuclear regions consistent with Golgi apparatus. These results suggest that cysteines i the DAT second extracellular loop may provide sulfide residues crucial to full transporter expression, at least in part, through interference with membrane insertion. Conceivably, they might also provide the targets for the influences of thiol-modifying reagents in modifying the function of the wild-type DAT expressed in striatal membranes.

Immunocytochemical distribution of gamma isoform of protein kinase C (PKC-gamma) following incomplete ischaemia

Immunocytochemical distribution of PKC-gamma was examined in rat brain in relation to molecular mechanisms of post-ischaemic neuronal modulation following incomplete ischaemia. Incomplete ischaemia was developed by either permanent occlusion of one common carotid artery (CA) or permanent occlusion of one CA with temporary occlusion of opposite CA. Unilateral CA (UCA) occlusion resulted in a pronounced increase in the intensity of staining and number of PKC-gamma positive neurons in the neocortex ipsilateral to the insult after 3 h. The effect was maximum at 6-12 h and was undetectable after 7 days. CA1 neurons showed an increase immunoreactivity (IR) after 1 day, reached to a peak by 3 days, then reduced to basal levels after 7 days. Bilateral CA (BCA) occlusion showed almost similar changes in the neocortex, but on both sides and short durated. The altered patterns of PKC-gamma IR in the neocortex and hippocampus following CA occlusion may reflect activation and/or down-regulation of PKC-gamma in ischaemic neurons. PKC-gamma may, therefore, potentially play an important role in the post-ischaemic modulation of synaptic efficacy in these neurons and in the neuronal damage following incomplete ischaemia.

Induction of N-methyl-D-asparate receptor mediated c-fos protein in the rat brain by incomplete ischaemia

The expression of c-fos protein was examined by means of immunocytochemistry in the rat brain following incomplete ischaemia, to elucidate the molecular mechanisms of post-ischaemic neuronal death and of the modulated neurotransmission of surviving neurons. Incomplete ischaemia was produced by permanent unilateral or bilateral common carotid artery (CCA) occlusion. After 1 h of unilateral occlusion, the level of c-fos protein-like nuclear immunoreactivity increased in cortical neurons ipsilateral to the insult, especially in cingulate and piriform cortices. The reactivity peaked at 3-6 h, and was undetectable after 3 days. A number of scattered immunostained neurons in the ipsilateral subiculum, CA 1 and dentate gyrus became visible after 1 day. The effect reached a peak between 1-3 days, then returned to basal levels by 7 days. Bilateral CCA occlusion showed a similar distribution of immunoreactivity, but on both hemispheres. Immunoreactive neurons were more numerous and intensely stained but more transient. The induction of c-fos was completely blocked or reduced by treatment with MK-801. Our results suggest that c-fos expression after CCA occlusion is NMDA receptor mediated, and that it has a specific role in neurons after ischaemic insult.

Repeated application of anodal direct current produces regional dominance in histamine-elicited cyclic AMP accumulation in rabbit cerebral cortex

A unilateral 30-min application of anodal direct current to the promotor cortex of rabbits was repeated 10 times, and cyclic AMP accumulation in response to histamine was investigated in slices of different cortical areas. Polarization with 1.0 microA decreased the cyclic AMP accumulation in the cortical area contralateral to the polarization, by which regional dominance in cyclic AMP accumulation was produced in the polarized cortex. In contrast, the regional difference in cyclic AMP accumulation was reversed when 10.0 or 30.0 microA was applied. The histamine-elicited accumulation of cyclic AMP was almost completely inhibited by the selective H2-receptor antagonist cimetidine. These results suggest that repeated anodal polarization regionally alters H2-receptor-mediated cyclic AMP generation in the cortex depending on the intensity of the polarizing currents and this pattern of cyclic AMP accumulation is responsible for the characteristic motor behavior induced by anodal polarization.

Anodal polarization induces protein kinase C gamma (PKC gamma)-like immunoreactivity in the rat cerebral cortex

Protein kinase C gamma (PKC gamma)-like immunoreactivity was examined in the rat brain, employing the monoclonal antibody 36G9 raised against purified PKC gamma, after an application of weak anodal direct current to the surface of the sensorimotor cortex. Anodal polarization with 3.0 microA for 30 min resulted in a pronounced increase in the number of PKC gamma-like-positive neurons in accordance with the intensity of PKC gamma-like immunostaining in the neocortex, cingulate cortex and piriform cortex ipsilateral to the polarization. The number of PKC gamma-like-positive neurons began to increase at 1 h after polarization, peaked at 3 h, and thereafter decreased to the control levels by 72 h. The increase in expression of PKC gamma-like immunoreactivity in specific areas of the cerebral cortex is suggested to serve as a basis for the long-lasting hyperexcitability in situ following anodal polarization.