Activity-regulated cytoskeleton-associated protein Arc is targeted to dendrites and coexpressed with mu-opioid receptors in postnatal rat caudate-putamen nucleus

Dendritic expression of the activity-regulated cytoskeleton-associated protein (Arc) is dramatically enhanced by increased synaptic activity in adult brain. We used immunocytochemical electron microscopy to determine whether the subcellular localization of Arc in developing dendrites corresponds to the peak period of synaptogenesis in the postnatal rat caudate-putamen nucleus (CPN). The distribution was compared with that of mu-opioid receptors (MORs), whose localization in dendritic spines closely parallels excitatory synapse formation during postnatal development (Wang et al. [2003] Neuroscience 118:695-708). Sections were processed for immunocytochemical detection of antisera against Arc or MORs at the beginning (postnatal day 15; P15) and the end (P30) of the peak period of synaptogenesis in rat CPN. At P15, immunolabeling for Arc showed a punctate distribution in the cytoplasm of dendritic shafts, some of which was associated with polyribosomes. In some spiny dendrites, Arc immunoreactivity was more intensely localized in putative spines than in their parental dendrites, whereas, in other spiny dendrites, Arc labeling was restricted in the shafts. Many dendritic shafts and spines also showed immunoreactivity for MORs, although dually labeled spines were less numerous than the shafts. At P30, the proportion of singly and dually labeled spines significantly increased from 2.0% to 7.5% and from 9.5% to 21%, respectively. Arc labeling in spines was more detectable beneath the postsynaptic density or at extrasynaptic sites on the plasma membrane. Our results suggest a correlation between Arc expression in dendritic spines during postnatal development and the onset of synaptogenesis in opioid-responsive neurons in the rat CPN.

Electron microscopic dual labeling of high-affinity neurotensin and dopamine D2 receptors in the rat nucleus accumbens shell

The dopamine D2 receptor (D2R) in the nucleus accumbens (NAc) shell is implicated in schizophrenia and in psychostimulant-induced drug-seeking behavior, both of which are affected by activation of the functionally opposed high-affinity neurotensin receptor (NTS1). To determine the functionally relevant sites, we examined the dual electron microscopic immunocytochemical localization of D2R and NTS1 in the NAc shell of rat brain. Immunolabeling for each receptor was seen in association with cytoplasmic organelles, or more rarely, on the plasma membrane of both axonal and somatodendritic profiles. Some of the axonal and many of the dendritic processes colocalized the two receptors. The dually labeled axon terminals often formed symmetric synapses or appositional contacts with unlabeled dendritic profiles. The morphology of these terminals suggests that they contain either inhibitory amino acids or dopamine. Other axonal profiles expressing exclusively NTS1 or D2R were without synaptic specializations or formed asymmetric, excitatory-type synapses mainly on unlabeled dendritic spines. In addition, however, several D2R-immunoreactive terminals were observed presynaptic to dendrites containing NTS1. The somatodendritic profiles immunolabeled for NTS1 and/or D2R had morphological features typical of inhibitory spiny projection neurons in the NAc. These results suggest that activation of NTS1 and D2R can dually modulate transmitter release from the same or separate phenotypically distinct axon terminals in the NAc shell. These presynaptic receptors as well as the postsynaptic NTS1 distribution in neurons that also contain or receive input from terminals containing D2R may mediate the opposing actions of neurotensin and dopamine in the NAc.

Receptor targeting in medullary nuclei mediating baroreceptor reflexes

The sympathoinhibitory component of the baroreceptor reflex prominently involves glutamatergic visceral afferents terminating in the nuclei of the solitary tract (NTS) and C1 adrenergic neurons of the rostral ventrolateral medulla (RVLM). As reviewed, we have used electron microscopic immunocytochemical dual labeling in these regions to precisely analyze (1) the cellular sites for synergistic or opposing responses attributed to activation of different receptor subtypes on single neurons and (2) interactions involving monoaminergic neurons identified by their content of neurotransmitter synthesizing enzymes, vesicular monoamine transporter, and frequent coexpression of endogenous opioid peptides. The summarized results provide important cellular substrates for N-methyl-D-aspartate (NMDA)-mediated glutamatergic transmission and activation of either serotoninergic (5-HT2A), adrenergic (alpha 2A), or mu- or delta opioid receptors within the baroreceptor reflex circuit.

Angiotensin II subtype 1A (AT1A) receptors in the rat sensory vagal complex: subcellular localization and association with endogenous angiotensin

Angiotensin II (Ang II) type 1 (AT1) receptors are prevalent in the sensory vagal complex including the nucleus tractus solitarii (NTS) and area postrema, each of which has been implicated in the central cardiovascular effects produced by Ang II. In rodents, these actions prominently involve the AT1A receptor. Thus, we examined the electron microscopic dual immunolabeling of antisera recognizing the AT1A receptor and Ang II to determine interactive sites in the sensory vagal complex of rat brain. In both the area postrema and adjacent dorsomedial NTS, many somatodendritic profiles were dually labeled for the AT1A receptor and Ang II. In these profiles, AT1A receptor-immunoreactivity was often seen in the cytoplasm beneath labeled portions of the plasma membrane and in endosome-like granules as well as Golgi lamellae and outer nuclear membranes. In addition, AT1A receptor labeling was detected on the plasma membrane and in association with cytoplasmic membranes in many small axons and axon terminals. These terminals were morphologically heterogeneous containing multiple types of vesicles and forming either inhibitory- or excitatory-type synapses. In the area postrema, AT1A receptor labeling also was detected in many non-neuronal cells including glia, capillary endothelial cells and perivascular fibroblasts that were less prevalent in the NTS. We conclude that in the rat sensory vagal complex, AT1A receptors are strategically positioned for involvement in modulation of the postsynaptic excitability and intracrine hormone-like effects of Ang II. In addition, these receptors have distributions consistent with diverse roles in regulation of transmitter release, regional blood flow and/or vascular permeability.

Serotonin transporters (SERTs) within the rat nucleus of the solitary tract: subcellular distribution and relation to 5HT2A receptors

Serotonergic transmission is terminated by serotonin transporter (SERT)-mediated uptake following activation of serotonin receptors, several subtypes of which are present in the medial nucleus of the solitary tract (mNTS) at the area postrema level. In this region, serotonin (5HT) is a major modulator of the baroreceptor reflex and also affects gastric motility. This serotonin is derived from multiple sources including local neurons and inputs from raphe and visceral vagal afferents. To determine the relevant functional sites for serotonin uptake in the mNTS, we examined the electron microscopic localization of SERTs using both immunoperoxidase and immunogold labeling in rat brain. In addition, we combined these methods for dual labeling of SERTs and 5HT2A receptors to detect whether the SERT in this region was located near or at a distance from the sites of activation of these G-protein coupled receptors. Intensive SERT immunolabeling was seen on plasma membranes of axons and morphologically heterogeneous axon terminals that formed symmetric or asymmetric synapses on dendrites without detectable 5HT2A immunoreactivity (IR). 5HT2A-IR was, however, located in other nearby neuronal and glial profiles, some of which apposed intensively SERT-labeled terminals or terminals containing lower intensity of SERT immunolabeling. In somatodendritic profiles, co-expression of SERT and 5HT2A receptor immunolabeling was seen near synapses and Golgi lamellae. Our results suggest that in the mNTS 5HT activates 5HT2A receptors at a distance from SERT-mediated uptake sites in diverse cell types including some that express both 5HT2A receptors and SERTs.

Ultrastructural localization of Leu5-enkephalin immunoreactivity in mesocortical neurons and their input terminals in rat ventral tegmental area

Enkephalin (ENK) immunoreactivity is widely distributed in the ventral tegmental area (VTA), where endogenous ENK and dynorphin opioid peptides are known to have opposing actions in reward, stress, cognition, and fear-related behaviors. Many neurons in the VTA give rise to mesocortical projections terminating in the medial prefrontal cortex (mPFC), and these projections have been implicated to varying extents in all these functions. To determine whether there is a synaptic basis for ENK and/or dynorphin modulation of cortically projecting neurons within the VTA, we combined retrograde tract-tracing from the mPFC with dual immunocytochemical-labeling electron microscopy in the rat VTA. The retrograde tracer Fluorogold (FG) was microinjected into mPFC. At optimal survival periods, sections through the VTA were processed for immunolabeling of anti-FG and a Leu(5)-ENK antibody recognizing both ENK and dynorphin peptides. Over 26% of the retrogradely labeled neuronal somatodendritic profiles (n = 177) were contacted by ENK-immunoreactive axonal profiles including small axons and axon terminals. The axon terminals varied in their subcellular distribution of ENK immunoreactivity and also differed in forming either inhibitory-type (symmetric) or excitatory-type (asymmetric) synapses. Many of the axonal profiles also were apposed to FG-labeled somata or dendrites without forming recognizable synapses. Approximately one-third of the mesocortical neuronal perikarya also showed sparsely distributed somatodendritic ENK-immunoreactivity. Our results provide ultrastructural evidence that ENK and possibly dynorphin in the rat VTA have distributions consistent with involvement in diverse physiological actions affecting the output of mesocortical neurons, some of which also contain one or both peptides.

Ultrastructural localization of ?-opioid receptors in the rat caudate-putamen nucleus during postnatal development: Relation to synaptogenesis

During development, delta-opioid receptors (DORs) in the rat caudate-putamen nucleus (CPN) appear later than mu-opioid receptors (MORs), whose developmental pattern specifically relates to synaptogenesis. We used electron microscopic immunocytochemistry to determine whether there are also age-related changes in subcellular localization of DORs in the rat CPN. Sections from postnatal day (P) 0-P30 and adult dorsomedial CPN were immunogold-silver labeled to examine the plasmalemmal and cytoplasmic distribution of these receptors. In addition, immunoperoxidase labeling was used to determine the numerical density of synapses relative to DOR-labeled profiles. Immunolabeling for DOR was undetectable at P0, light at P5, and dense from P10 onward. The labeling during P5-P10 was mainly localized in somatodendritic profiles but also was readily seen in axon terminals, most of which formed asymmetric synapses with dendrites. From P15, a few immunogold particles were seen in contact with postsynaptic densities in spines, and the proportion of these particles significantly increased in P30 and adult CPN. Other particles were localized in the cytoplasm of dendrites and terminals without significant age-related changes. Stereological analysis showed that compared with labeled dendritic shafts and spines, labeled axon terminals have a closer correlation with synapse formation. These results are in marked contrast with MORs, which show an age-related increase in association with dendritic plasma membrane and a good correlation in the developmental pattern of MOR-labeled spines with synapse formation (Wang et al. [2003] Neuroscience 118:695-708). Together, our results suggest receptor-type specific roles for endogenous opioids acting at both pre- and postsynaptic sides in the developing CPN.

Postnatal development of mu-opioid receptors in the rat caudate-putamen nucleus parallels asymmetric synapse formation

The mu-opioid receptor (MOR) in the caudate-putamen nucleus (CPN) appears early during prenatal development, and shows a patch-like distribution throughout the postnatal period and adulthood. In the adult rat CPN, neurons in patch compartments receive glutamatergic excitatory input mainly from the cortex through synapses onto spines, many of which express MORs. Thus, MOR expression in spines may be related to corticostriatal synaptogenesis. We used electron microscopic immunocytochemistry to determine potential age-dependent changes in the distribution pattern of MOR during postnatal synaptogenesis in the rat CPN. Immunogold-silver labeling revealed that the dendritic plasmalemmal density of MOR at postnatal day (P) 0 was significantly lower than, but after P10 was similar to, that of adult. In contrast, such age-dependent changes were not observed in axon terminals. Stereological analysis of immunoperoxidase labeling for MOR showed a good correlation in the developmental numerical densities of synapses with MOR-labeled spines and those of total asymmetric axospinous synapses, linear correlation coefficient r=0.99. Synapses with MOR-labeled dendrites, however, had a low correlation with axodendritic synapses (r=0.61), and synapses with MOR-labeled terminals showed no correlation with axospinous and axodendritic synapses (r=0.19). These results provide ultrastructural evidence that the targeting of MOR on the plasma membrane of dendrites and spines parallels the peak period of synaptogenesis during the third postnatal week in the rat CPN. Thus, the postnatal spatiotemporal expression pattern of MOR appears to match the functional maturation of corticostriatal glutamate transmission.

Morphine acutely regulates opioid receptor trafficking selectively in dendrites of nucleus accumbens neurons

Morphine stimulates the internalization of mu-opioid receptors (MORs) in transfected cell models to a lesser degree than opioid peptides and other analgesic drugs, such as methadone, and previous studies have reported that morphine does not produce a detectable redistribution of MORs in neural tissue after either acute or chronic administration. Nevertheless, morphine produces profound physiological effects, raising the question of whether receptor trafficking plays any role in the in vivo actions of morphine. We investigated the effects of opiate drugs on recombinant and native opioid receptors in the nucleus accumbens, which plays an important role in mediating the behavioral effects of opiate drugs. Morphine and methadone differed in their effects on the internalization of epitope-tagged MORs in cell bodies, introduced by viral gene transfer and imaged by fluorescence microscopy. A mutation of the cytoplasmic tail that confers morphine-induced internalization in cultured cells had a similar effect on receptor trafficking in nucleus accumbens cell bodies. Surprisingly, in contrast to its failure to affect MOR distribution detectably in cell bodies, acute morphine administration produced a pronounced change in MOR distribution visualized in the processes of the same neurons. A similar effect of acute morphine administration was observed for endogenously expressed MORs by immunoelectron microscopy; the acute administration of morphine increased the density of MORs associated with internal membrane structures specifically in dendrites. These results provide the first evidence that morphine regulates the distribution of MORs in neuronal processes, suggesting that "compartment-selective" membrane trafficking represents a previously unanticipated type of opioid receptor regulation contributing to the in vivo effects of opiate drugs on a physiologically relevant population of CNS neurons.

Identification of a novel membrane-associated protein expressed in neurons of the squid and rodent nervous systems

In a previous communication, we reported the isolation of a novel cDNA clone (pA6) from a library constructed from squid axonal mRNAs. The partial cDNA clone contained a unique open reading frame that encoded 84 amino acids and was complementary to a moderately abundant mRNA approximately 550-600 nucleotides in length [Chun et al., J. Neurosci. Res. 49 (1997) 144-153]. In this report, we identify the pA6 gene product, and characterize its expression in the squid and rodent brain. Results of immunoblot analyses conducted in squid, using a polyclonal antibody raised against a synthetic peptide corresponding to the C-terminus of the putative protein, established the presence of two pA6 immunoreactive proteins of approximately 14 kDa and 26 kDa in size. In contrast, mouse brain contained only a single 26-kDa immunoreactive species. In both the squid and mouse brain, the expression of pA6 appears highly selective, being detected in certain neurons but not in non-neuronal cells, as judged by both in situ hybridization and immunocytochemistry. Findings derived from light microscopic, double-label immunohistofluorescence studies indicate that pA6 protein co-localizes with prohibitin, a mitochondrial marker protein. Consistent with these results, electron microscopy localized pA6 immunoreactivity to several membrane compartments to include the outer membrane of mitochondria, as well as to the smooth endoplasmic reticulum and tubulovesicles in dendrites, axons, and axon terminals of neurons in the rat brain. Taken together, these findings indicate that pA6 is a novel, membrane-associated protein, which is expressed in the distal structural/functional domains of neurons in both the invertebrate and vertebrate nervous systems.

Ultrastructural localization of serotonin 2A and N-methyl-D-aspartate receptors in somata and dendrites of single neurons within rat dorsal motor nucleus of the vagus

Both glutamate and serotonin are potent modulators of autonomic functions involving the nucleus of the solitary tract (NTS) and the dorsal motor nucleus of the vagus (DMNV) at the level of the area postrema. Moreover, many of the dendrites in this NTS region express both N-methyl-D-aspartate (NMDA) and serotonin (5HT) 2A receptors, and some of these dendrites may arise from the adjacent DMNV. Thus, single neurons in DMNV may also express both receptors. To test this hypothesis, we used electron microscopic immunocytochemistry for dual localization of the essential R1 subunit of the NMDA receptor (NR1) and the 5HT2A receptor in rat intermediate DMNV, a region serving mainly gastrointestinal functions. Gold particles representing NR1 and peroxidase reaction product for 5HT2A receptors were seen in the cytoplasm, as well as on distinct segments of the plasma membrane of many dendrites. Of the NR1-labeled dendrites, 31% (254/814) also contained 5HT2A immunoreactivity; among the 5HT2A-labeled dendrites, 52% (254/485) expressed NR1. The 5HT2A labeling was also present in numerous small unmyelinated axons, axon terminals, and glial processes. These profiles were largely without NR1 immunoreactivity, although NR1 was detected in some of the dendrites postsynaptic to 5HT2A-labeled terminals. Our results suggest that calcium entry through NMDA channels and 5HT2A receptor activation may dramatically affect postsynaptic excitability of single neurons in the DMNV. In addition, the findings also indicate that the 5HT2A receptor is strategically positioned for involvement in modulation of the presynaptic release of neurotransmitters affecting the postsynaptic activity of DMNV neurons responsive to NMDA activation.

Regional and subcellular compartmentation of the dopamine transporter and tyrosine hydroxylase in the rat ventral pallidum

The ventral pallidum (VP) is a major intermediary in the prefrontal cortical circuitry regulating sensorimotor gating and locomotor behavior, both of which are potently modulated by catecholamines. The VP catecholaminergic innervation is derived from midbrain dopaminergic neurons that differ in expression levels of the dopamine transporter (DAT) and from brainstem noradrenergic neurons without DAT. The preferentially low level of DAT in dopaminergic terminals in the prefrontal cortex and in striatal regions projecting more extensively to the VP medial (VPm) compared with VP lateral (VPl) compartment suggests possible region-specific differences in VP axonal distribution of DAT. To test this hypothesis, we examined the electron microscopic localization of DAT and the catecholamine-synthesizing enzyme, tyrosine hydroxylase (TH), in the VPm and VPl of rat brain. In both regions, DAT and TH were localized primarily in small unmyelinated axons and morphologically heterogeneous axon terminals. DAT-immunogold particles were few in number, but mostly located on the plasma membrane. In contrast, TH immunoreactivity was distributed in the cytoplasm of individual profiles, many of which were without detectable DAT. In comparison with TH, the mean area density of DAT-labeled axons was low throughout the VP. The mean area density of DAT-immunogold axon terminals, however, was significantly higher in VPl than in VPm, whereas that of TH-labeled axons was higher in VPm than in VPl. This dissociation suggests that, compared to the VPl, the VPm receives the greatest input from catecholaminergic afferents that are either nondopaminergic or characterized by having low levels or less terminal distributions of DAT.

Alpha-2A-adrenergic receptors are present on neurons in the central nucleus of the amygdala that project to the dorsal vagal complex in the rat

The descending pathway between the central nucleus of the amygdala (CeA) and the dorsal vagal complex (DVC) is an important substrate for autonomic functions associated with emotion. Activity in this circuit is crucially modulated by catecholamines and agonists of the alpha-2A-adrenergic receptor (alpha(2A)-AR), which relieve cardiovascular and gastrointestinal symptoms associated with experience of aversive stimuli. The subcellular distribution of alpha(2A)-AR within the CeA, however, has not been characterized. It is also not known if any alpha(2A)-AR-expressing neurons in the CeA project to the dorsal vagal complex. In order to address these questions, we examined the immunocytochemical labeling of alpha(2A)-AR in the CeA of rats receiving microinjection of the retrograde tracer fluorogold (FG) into the dorsal vagal complex at the level of the area postrema, an area involved in cardiorespiratory and gastrointestinal functions. Of all alpha(2A)-AR-labeled profiles in the CeA, the majority were either dendrites (42%) or somata (24%). alpha(2A)-AR labeling was often present on the plasmalemma in dendrites and was mainly found in endosome-like organelles in somata. Of all alpha(2A)-AR immunoreactive somata, 62% also contained immunolabeling for FG and 23% of all dendrites also showed labeling for the retrograde tracer. The intracellular distribution of alpha(2A)-AR did not differ in somata or dendrites with or without detectable FG. The remaining singly labeled alpha(2A)-AR profiles consisted of axons (11%), axon terminals (12%), and glial processes (13%). In numerous instances, alpha(2A)-AR-labeled glia or axon terminals were apposed to DVC projecting neurons. Together, this evidence suggests that the principal site for alpha(2A)-AR activation is at extrasynaptic sites on dendrites of CeA neurons, many of which project to the DVC and also show endosomal receptor labeling. In addition, these results indicate that activation of alpha(2A)-AR in the CeA may influence the activity of DVC projecting neurons through indirect mechanisms, including changes in presynaptic transmitter release or glial function. These results suggest that alpha(2A)-AR agonists in the CeA may modulate numerous processes including stress-evoked autonomic reactions and feeding behavior.

Ultrastructural localization of enkephalin and mu-opioid receptors in the rat ventral tegmental area

Enkephalins are endogenous ligands for opioid receptors whose activation potently modulates the output of mesocorticolimbic dopaminergic neurons within the ventral tegmental area. Many of the reinforcing effects of enkephalins in the mesocorticolimbic system are mediated by mu-opioid receptors. To determine the sites for Leu(5)-enkephalin activation of mu-opioid receptors in the ventral tegmental area, we examined the dual electron microscopic immunocytochemical localization of their respective antigens in this region of rat brain. Enkephalin immunoperoxidase reaction product and mu-opioid receptor immunogold-silver labeling showed similar cellular and subcellular distribution in both the paranigral and parabrachial subdivisions of the ventral tegmental area. Enkephalin immunoreactivity was mainly localized in small unmyelinated axons (50.4%) and in axon terminals (40.4%). The majority of these terminals formed symmetric, inhibitory-type synapses, many of which were on dendrites expressing plasmalemmal mu-opioid receptors. Appositional contacts were also often seen between axons or terminals that were differentially labeled for the two antigens. In addition, some of the enkephalin-labeled terminals and a few somatodendritic profiles showed a plasmalemmal or vesicular localization of mu-opioid receptors. Our results indicate that dendritic targets of inhibitory terminals, as well as nearby axon terminals, are potential sites for enkephalin activation of mu-opioid receptors throughout the ventral tegmental area. Moreover, co-localization of enkephalin and mu-opioid receptors in selective neuronal profiles may indicate an autoregulatory role for these receptors or their internalization along with the bound ligand in this brain region.

Electron microscopic immunolabeling of transporters and receptors identifies transmitter-specific functional sites envisioned in Cajal's neuron

Neuronal arborizations that were so elegantly demonstrated in the early drawings of Santiago Ramón y Cajal can now be viewed by high resolution electron microscopic immunocytochemical localization of vesicular and plasmalemmal neurotransmitter transporters and receptors. The subcellular distribution of these proteins confers both chemical selectivity and functional specificity to the dendritic and axonal arborizations described by Cajal. This is illustrated by central dopaminergic and cholinergic neurons. Dopamine terminals in the striatum and ventral pallidum, as well as dendrites of midbrain dopaminergic neurons in the ventral tegmental area and substantia nigra express the plasmalemmal dopamine transporter (DAT) and the vesicular monoamine transporter (VMAT2). In forebrain regions, the dopamine D2 receptor (D2R) autoreceptor is localized to dopamine terminals, but also is targeted to pre- and postsynaptic neuronal profiles at a distance from the dopamine terminals. In somata and dendrites of the midbrain dopaminergic neurons, D2R labeling is expressed in most dendrites that contain VMAT2 storage vesicles, as well as in both excitatory and inhibitory afferents. Together, these observations indicate that dopamine is stored in and released from vesicles in both dendrities and axons, and may activate either local or more distant receptors through volume transmission. By analogy, the vesicular acetylcholine transporter (VachT) is similarly localized to the membranes of axon terminals and tubulovesicles in dendrities in the mesopontine tegmental cholinergic nuclei, suggesting that there also may be release of acetylcholine from both dendrities and axons. These results identify chemically selective functional sites for neuronal signaling envisioned by Cajal and redefined by modern technology.

Region-specific targeting of dopamine D2-receptors and somatodendritic vesicular monoamine transporter 2 (VMAT2) within ventral tegmental area subdivisions

Throughout the ventral tegmental area (VTA), dopamine is packaged within subcellular organelles by the vesicular monoamine transporter-2 (VMAT2). Somatodendritically released dopamine in this region binds to the D2 receptor (D2R) to modulate ongoing neurotransmission. Although autoregulation of mesocortical dopaminergic neurons in the parabrachial VTA (PB-VTA) is known to be less efficacious than that of mesolimbic dopaminergic neurons in the paranigral (PN-VTA), the cellular basis for this regional heterogeneity is not known. For this reason, we used electron microscopic immunocytochemistry to determine the subcellular localization of the dopamine storage vesicles (identified by the presence of VMAT2) in relation to the D2R in these VTA subdivisions. In both regions, D2R immunoreactivity was principally located on extrasynaptic dendritic plasma membranes near excitatory-type synapses. Equivalent percentages (72 and 74%) of the D2R-labeled dendrites in each region contained VMAT2-immunoreactive tubulovesicles. Of the total VMAT2-labeled dendrites, however, a significantly lower percentage in the PB-VTA (26%) than in the PN-VTA (38%) contained D2R labeling. In contrast, a significantly higher number of VMAT2 immunogold-silver deposits was seen within individual dendrites in the PB-VTA than in PN-VTA. In both regions, D2R immunoreactivity was also detected in VMAT2-negative axon terminals that formed synapses on dendrites containing VMAT2. Our results are the first to demonstrate that within VTA neurons and their afferents the D2R is strategically positioned for activation by dopamine released from dendritic storage vesicles. These findings also suggest that the potential for D2R activation may affect the expression levels of VMAT2 in VTA dendrites.

Differential distribution of 5HT2A and NMDA receptors in single cells within the rat medial nucleus of the solitary tract

Activation of serotonin (5-hydroxytryptamine; 5HT) receptors of the 2A subtype (5HT2A) in the intermediate portion of the medial nucleus tractus solitarius (mNTS) produces marked hypotension and bradycardia. This portion of the mNTS receives major input from glutamatergic baroreceptor afferents. Thus, the cardiorespiratory effects of 5HT2A agonists may be attributed, in part, to interactions involving the glutamatergic target neurons, some of which express N-methyl-D-aspartate (NMDA) glutamate receptors. To determine the functional sites for activation of 5HT2A receptors and their relationship to NMDA receptors in this region, we used electron microscopic immunocytochemistry for the localization of antipeptide antisera selectively recognizing each receptor protein in the intermediate mNTS in rat brain. Of 1,052 5HT2A-labeled profiles, 38% were dendrites and dendritic spines, 27% were unmyelinated axons, 14% were axon terminals, and 11% were glial processes. These 5HT2A-labeled profiles frequently contained NR1 gold particles with dendrites comprising 68% of the total dual-labeled profiles. In dendrites, the 5HT2A immunoreactivity was localized to cytoplasmic organelles or discretely distributed on synaptic or extrasynaptic segments of the plasma membrane. In contrast, NR1 immunoreactivity was prominently localized to postsynaptic junctions and these were distinct from the 5HT2A receptor labeling when coexpressed in the same dendrites. Dendrites containing both receptors composed 56% (224/399) of the total 5HT2A-labeled dendrites and 34% (224/659) of the total NR1-labeled dendrites. Our results provide the first ultrastructural evidence that in the intermediate mNTS, 5HT2A receptor agonists may affect the postsynaptic excitability of many of the same neurons that show NMDA-evoked responses to glutamate.

Ultrastructural localization of neuropeptide Y Y1 receptors in the rat medial nucleus tractus solitarius: relationships with neuropeptide Y or catecholamine neurons

Neuropeptide Y (NPY) Y1 receptor (Y1-R) agonists influence cardiovascular regulation. These actions may involve NPY- and catecholamine-containing neurons in the medial nucleus of the solitary tract (mNTS), at the level of the area postrema. The cellular sites through which Y1-R agonists may interact with NPY and catecholamines in the mNTS, however, are not known. To determine potential sites of action for Y1-R agonists, and their relationship to NPY or catecholamines in the mNTS, we used electron microscopic immunocytochemistry for the detection of sequence-specific antipeptide antisera against Y1-R alone or in combination with antisera against NPY or the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH). Analyses were conducted in the rat mNTS, at the level of the area postrema. Y1-R was found mainly in small unmyelinated axons and axon terminals but also in some somata and dendrites as well as a small number of glia. Within axon terminals, labeling for Y1-R was often present on dense core vesicles and small synaptic vesicles as well as extrasynaptic areas of the plasmalemma. Some Y1-R-labeled terminals also contained NPY or TH, suggesting that agonists of Y1-R may influence the release of NPY or catecholamines in the mNTS. In addition, Y1-R was found in dendrites that received asymmetric excitatory-type synapses from unlabeled axon terminals. Some of these dendrites contained NPY or TH, which indicates that Y1-R may be targeted for functional activation within NPY- or catecholamine-expressing neurons in the mNTS. These results demonstrate that Y1-R is a presynaptic receptor in NPY- or catecholamine-containing axon terminals within the mNTS as well as a postsynaptic receptor on NPY- or catecholamine-containing neurons that are contacted by axon terminals that likely contain excitatory amino acid transmitters. Agonists of Y1-R in the mNTS may thus affect cardiovascular regulation by modulating NPY, catecholamine, and excitatory amino acid transmission.

Ultrastructural immunocytochemical localization of the dopamine D2 receptor and tyrosine hydroxylase in the rat ventral pallidum

The mesopallidal dopamine system plays a role in locomotor activity and reward. To understand the potential contribution of the dopamine D2 receptor (D2R) to the action of dopamine in the ventral pallidum (VP), we used electron microscopic immunocytochemistry to examine the cellular and subcellular localization of an antipeptide antiserum against the D2R in both ventromedial and dorsolateral VP compartments. In each region the majority of the total D2R-labeled profiles (n = 1,132) were axon terminals (55%) and small unmyelinated axons (27%). These terminals were often apposed to other axon terminals or dendrites and formed almost exclusively symmetric, inhibitory-type axodendritic synapses. Immunogold D2R labeling in axon terminals was seen on the plasmalemma and membranes of nearby synaptic vesicles. In ventral pallidal sections processed for dual detection of D2R peptide and the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH), D2R labeling was detected in a few axons and axon terminals containing TH immunoreactivity as well as in axons contacted by TH-labeled terminals. In most cases, however, the D2R-labeled profiles were located at a distance from small axons and terminals containing TH. Our results provide the first ultrastructural evidence that D2Rs in the two VP subterritories are strategically located for primary involvement in modulation of the presynaptic release of nondopaminergic inhibitory transmitters. They also suggest that in this region the presynaptic D2 receptors are 1) minimally involved in autoregulation of dopaminergic transmission, and 2) differentially activated by dopamine, depending in part on levels and distance from release sites.

Alpha(2A)-adrenergic receptors are present in mu-opioid receptor containing neurons in rat medial nucleus tractus solitarius

Agonists of the alpha-2A-adrenergic- (alpha(2A)-AR) and the mu-opioid-receptor (muOR) jointly affect autonomic functions that are also disregulated in animals undergoing withdrawal from chronic administration of the muOR agonist morphine. Cardiovascular and gastrointestinal reflexes are mediated, in part, by the medial nucleus of the solitary tract (mNTS) at caudal (cNTS) and intermediate (iNTS) subregions. Together, this evidence suggests that alpha(2A)-AR and muOR may be colocalized within many of the same neuronal profiles in both the intermediate and caudal mNTS. In order to examine whether alpha(2A)-AR and muOR are present within common somata, dendrites, or axon terminals in the mNTS, we used electron microscopic immunocytochemistry for the detection of antisera against each receptor at intermediate and caudal levels of this brain region. Most of the dually labeled profiles were somata and dendrites. Of all dual-labeled profiles in the iNTS 49% were somata and were 47% dendrites, whereas in the cNTS 61% were somata and 32% were dendrites. Within dual-labeled profiles, the intracellular distribution of alpha(2A)-AR and muOR differed. MuOR was more frequently associated with the plasmalemma, whereas alpha(2A)-AR was often affiliated with vesicular organelles. Few axon terminals, and even fewer glia, contained both markers. We also frequently observed single-labeled alpha(2A)-AR glia that apposed exclusively muOR-containing dendrites or axon terminals. These findings indicate that somata and dendrites contain functional sites for convergent muOR and alpha(2A)-AR activation. In addition, each receptor is positioned for involvement in intercellular signaling between apposed neurons and glia. Activation of alpha(2A)-AR on muOR-containing somata or dendrites, or on glia apposed to muOR-containing neurons, may help to account for the efficacy of alpha(2A)-AR agonists in relieving some of the autonomic symptoms of opiate withdrawal.

Dopamine D2 receptors are present in prefrontal cortical afferents and their targets in patches of the rat caudate-putamen nucleus

Glutamatergic neurons within the deep layers of the prefrontal cortex and dopaminergic neurons of the substantia nigra pars compacta preferentially terminate in patch-like regions within the caudate putamen nucleus (CPN). Activation of dopamine D2 receptors is known to potently modulate striatal glutamatergic transmission and may play a role in reward-based motor learning. To determine the cellular substrate for D2-mediated regulation of prefrontal corticostriatal transmission in striatal patches, we combined anterograde transport of biotinylated dextran amine (BDA) with immunogold-silver labeling of a D2 receptor antipeptide antiserum in rat brain. Injections centered in deep layers of the dorsal part of the anterior cingulate cortex, one of the prefrontal cortical regions, produced varicose axonal BDA labeling in a patch-like distribution in the dorsomedial CPN. Electron microscopy showed that in these patch compartments, BDA labeling was present exclusively in axons and terminals (total number = 581), 9% of which contained detectable D2-like immunoreactivity. Thirty percent of the BDA-labeled terminals formed asymmetric excitatory synapses with dendritic spine heads, and the remainder were without recognizable junctions. The recipient spines were unlabeled or contained immunogold-silver particles for D2 receptors. A few of the D2-labeled spines also received convergent, often nonsynaptic contact from D2-labeled terminals resembling dopaminergic afferents. In addition, the corticostriatal terminals often apposed spiny and nonspiny neuronal profiles that contained D2 labeling. These results suggest that dopamine D2 receptors are strategically positioned for presynaptic and postsynaptic modulation of prefrontal corticostriatal excitation of spiny neurons in striatal patches. The findings have direct implications for D2-mediated control of reward-related motor learning.

Major coexpression of kappa-opioid receptors and the dopamine transporter in nucleus accumbens axonal profiles

The behavioral effects of psychostimulants, which are produced at least in part through inhibition of the dopamine transporter (DAT), are modulated by kappa-opioid receptors (KOR) in the nucleus accumbens (Acb). Using electron microscopic immunocytochemistry, we reveal that in the Acb KOR labeling is mainly, and DAT immunoreactivity is exclusively, presynaptic. From 400 KOR-labeled presynaptic structures, including axon terminals, intervaricosities, and small axons, 51% expressed DAT and 29% contacted another population of terminals exclusively labeled for DAT. Within axonal profiles that contained both antigens, DAT and KOR were prominently localized to plasma membrane segments that showed overlapping distributions of the respective immunogold-silver and immunoperoxidase markers. KOR labeling was also localized to membranes of small synaptic vesicles in terminals with or without DAT immunoreactivity. In addition, from 24 KOR-immunoreactive dendritic spines 42% received convergent input from DAT-containing varicosities and unlabeled terminals forming asymmetric, excitatory-type synapses. Our results provide the first ultrastructural evidence that in the Acb, KOR is localized to strategic sites for involvement in the direct presynaptic release and/or reuptake of dopamine. These data also suggest a role for KOR in the presynaptic modulation of other neurotransmitters and in the postsynaptic excitatory responses of single spiny neurons in the Acb. Dual actions on dopamine terminals and their targets in the Acb may account for KOR-mediated attenuation of drug reinforcement and sensitization.

Electrical stimulation of cerebellar fastigial nucleus protects rat brain, in vitro, from staurosporine-induced apoptosis

Electrical stimulation of the cerebellar fastigial nucleus (FN) elicits a prolonged ( approximately 10 days) and substantial (50-80%) protection against ischemic and excitotoxic injuries. The mechanism(s) of protection are unknown. We investigated whether FN stimulation directly protects brain cells against apoptotic cell death in an in vitro rat brain slice culture model. Rats were electrically stimulated in FN or, as control, the cerebellar dentate nucleus (DN). Coronal slices through the forebrain were explanted, exposed to staurosporine, harvested, and analyzed for caspase-3 activity by a fluorescence assay. FN, but not DN, stimulation significantly reduced staurosporine-induced caspase-3 activity by 39 +/- 7% at 3 h, 31 +/- 3% at 6 h and 26 +/- 4% at 10 h of incubation. Immunocytochemistry revealed FN-specific reductions in activated caspase-3 mainly in glial-like cells throughout the forebrain. FN stimulation also results in a 56.5% reduction in cytochrome c release upon staurosporine incubation. We conclude that neuroprotection elicited from FN stimulation can directly modify the sensitivity of brain cells to apoptotic stimuli and thereby suppress staurosporine induced apoptosis in adult rat brain slices. This model indicates that neuroprotection can be studied in vitro and provides new insight into the potential role of glial cells in ischemic protection of neurons induced by FN stimulation.

Plasmalemmal mu-opioid receptor distribution mainly in nondopaminergic neurons in the rat ventral tegmental area

Opiate-evoked reward and motivated behaviors reflect, in part, the enhanced release of dopamine produced by activation of the mu-opioid receptor (muOR) in the ventral tegmental area (VTA). We examined the functional sites for muOR activation and potential interactions with dopaminergic neurons within the rat VTA by using electron microscopy for the immunocytochemical localization of antipeptide antisera raised against muOR and tyrosine hydroxylase (TH), the synthesizing enzyme for catecholamines. The cellular and subcellular distribution of muOR was remarkably similar in the two major VTA subdivisions, the paranigral (VTApn) and parabrachial (VTApb) nuclei. In each region, somatodendritic profiles comprised over 50% of the labeled structures. MuOR immunolabeling was often seen at extrasynaptic/perisynaptic sites on dendritic plasma membranes, and 10% of these dendrites contained TH. MuOR-immunoreactivity was also localized to plasma membranes of axon terminals and small unmyelinated axons, none of which contained TH. The muOR-immunoreactive axon terminals formed either symmetric or asymmetric synapses that are typically associated with inhibitory and excitatory amino acid transmitters. Their targets included unlabeled (30%), muOR-labeled (25%), and TH-labeled (45%) dendrites. Our results suggest that muOR agonists in the VTA affect dopaminergic transmission mainly indirectly through changes in the postsynaptic responsivity and/or presynaptic release from neurons containing other neurotransmitters. They also indicate, however, that muOR agonists directly affect a small population of dopaminergic neurons expressing muOR on their dendrites in VTA and/or terminals in target regions.

Mu-opioid receptors in the ventral tegmental area are targeted to presynaptically and directly modulate mesocortical projection neurons

Mesocorticolimbic projections originating from dopaminergic and GABAergic neurons in the ventral tegmental area (VTA) play a critical role in opiate addiction. Activation of mu-opioid receptors (MOR), which are located mainly within inhibitory neurons in the VTA, results in enhanced dopaminergic transmission in target regions, including the medial prefrontal cortex (mPFC). We combined retrograde tract-tracing and electron microscopic immunocytochemistry to determine if neurons in the VTA that project to the mPFC contain MOR or receive input from MOR-containing terminals. Rats received unilateral injections of the retrograde tracer Fluoro-Gold (FG) into the mPFC. Tissue sections throughout the VTA were then processed for electron microscopic examination of FG and MOR. Immunoperoxidase labeling for FG was present in VTA cell bodies that contained immunogold-silver particles for MOR that often were contacted by profiles exclusively immunoreactive for MOR, including somata and axon terminals. The majority of dually labeled profiles were dendrites that received convergent input from unlabeled axon terminals forming either symmetric or asymmetric type synapses. Within retrogradely labeled cell bodies and proximal dendrites, MOR immunoreactivity was mainly sequestered within the cytoplasm. In contrast, distal retrogradely labeled dendrites contained MOR gold particles located along the plasma membranes. These data suggest that opiates active at MOR in the VTA modulate cortical activity through 1) presynaptic actions on MOR in terminals contacting mesocortical cell bodies, and 2) direct activation of MOR in distal dendrites of projection neurons.