The dopamine transporter: comparative ultrastructure of dopaminergic axons in limbic and motor compartments of the nucleus accumbens

The dopamine transporter (DAT) regulates extracellular dopamine concentrations, transports neurotoxins, and acts as a substrate for cocaine reinforcement. These functions are known to differ in the limbic-associated shell and motor-associated core compartments of the nucleus accumbens (NAc). Previous studies have shown differential expression of DAT in the NAc shell and core but were limited in resolution to the regional level. Thus, it is not known whether there are differences in the amount, subcellular localization, or plasmalemmal targeting of DAT within individual dopaminergic axons in the two regions. We used high-resolution electron microscopic immunocytochemistry to investigate these possibilities. We show that in both the shell and core, DAT immunogold labeling is present in tyrosine hydroxylase-immunoreactive varicose axons that form symmetric synapses. Within these labeled axons, most DAT gold particles are located on extrasynaptic plasma membranes, but some are associated with intracellular membranes. Dopaminergic axons in the shell contain lower mean densities of both total DAT gold particles (per square micron) and plasmalemmal DAT gold particles (per micron) than those in the core. Within labeled axons in the NAc shell and core, however, there are no detectable differences in the subcellullar distribution of DAT or the percentage of total DAT gold particles that are located on plasma membranes. These studies are the first to examine and compare the subcellular localization of DAT in the NAc shell and core. As a result, they identify intrinsic, cell-specific differences in the expression of DAT within dopaminergic axons in these functionally distinct striatal compartments.

Inducible expression of N-methyl-D-aspartate receptor, and delta and mu opioid receptor messenger RNAs and protein in the NT2-N human cell line

Retinoic acid treatment of NT-era2/cl.D1 (NT2) cells, a human teratocarcinoma cell line, yields 95% pure cultures of terminally differentiated neuronal cells. Concomitant with their terminal differentiation into neurons, NT2 cells are induced by retinoic acid to express neuronal N-methyl-D-aspartate receptor channels, which are fully functional. We determined the effects of retinoic acid-induced differentiation of NT2 cells on the levels of N-methyl-D-aspartate, delta opioid and mu opioid receptor messenger RNAs. RNA levels were measured using quantitative solution hybridization assays. The riboprobes were complementary to major portions of the coding regions of the N-methyl-D-aspartate, delta opioid and mu opioid receptor complementary DNAs. After four weeks of exposure to 10 microM retinoic acid, followed by four weeks of treatment with mitotic inhibitors (1 microM of cytosine arabinoside, 10 microM of fluorodeoxyuridine and 10 microM of uridine) the levels of N-methyl-D-aspartate receptor messenger RNA in differentiated NT2-N cells increased 10-fold, delta opioid receptor messenger RNA increased three-fold, and mu opioid receptor messenger RNA increased four-fold. Northern blot analysis revealed two transcripts for the N-methyl-D-aspartate receptor messenger RNA (4.2 and 4.4 kb) and two transcripts for delta opioid receptor messenger RNA (7.0 and 11.0 kb). To determine whether the increases in messenger RNAs were accompanied by an increased synthesis of the respective proteins, we examined the immunoperoxidase localization of N-methyl-D-aspartate receptor and delta opioid receptor antisera. N-Methyl-D-aspartate receptor-like immunoreactivity was seen within the cell bodies as well as on the processes of the retinoic acid-differentiated cells. Although delta opioid receptor-like immunoreactivity was detected within the soma of isolated cells prior to retinoic acid treatment, the apparent number of these labelled cells and their ramified processes were markedly enhanced following retinoic acid differentiation. These results demonstrate parallels between the inducible expression of the N-methyl-D-aspartate and opioid receptor messenger RNAs and proteins during the acquisition of the fully differentiated neuronal phenotype in cultured NT2 cells. Retinoic acid-differentiated NT2 cells express increased levels for the N-methyl-D-aspartate, delta opioid and mu opioid receptor messenger RNAs, providing the opportunity to study the interactions among these receptor systems in human terminally differentiated neuronal cells in culture.

Immunogold localization of the dopamine transporter: an ultrastructural study of the rat ventral tegmental area

The dopamine transporter (DAT) plays an important role in the plasmalemmal reuptake of dopamine and, thus, in the termination of normal dopaminergic neurotransmission. DAT is also a major binding site for cocaine and other stimulants, the psychoactive effects of which are associated primarily with the inhibition of dopamine reuptake within mesocorticolimbic dopaminergic neurons. We used electron microscopy with an anti-peptide antiserum directed against the N-terminal domain of DAT to determine the subcellular localization of this transporter in the rat ventral tegmental area (VTA), the region that contains the cell bodies and dendrites of these dopaminergic neurons. We show that in the VTA, almost 95% of the DAT immunogold-labeled profiles are neuronal perikarya and dendrites, and the remainder are unmyelinated axons. Within perikarya and large proximal dendrites, almost all of the DAT immunogold particles are associated with intracellular membranes, including saccules of Golgi and cytoplasmic tubulovesicles. In contrast, within medium- to small-diameter dendrites and unmyelinated axons, most of the DAT gold particles are located on plasma membranes. In dually labeled tissue, peroxidase reaction product for the catecholamine-synthesizing enzyme tyrosine hydroxylase is present in DAT-immunoreactive profiles. These findings suggest that intermediate and distal dendrites are both the primary sites of dopamine reuptake and the principal targets of cocaine and related psychostimulants within dopaminergic neurons in the VTA.

Dual ultrastructural localization of mu-opioid receptors and NMDA-type glutamate receptors in the shell of the rat nucleus accumbens

The effectiveness of NMDA antagonists in modulating the motor and motivational effects of opiates is attributed, in part, to functional associations involving NMDA receptors and micro-opioid receptors (MORs) in the shell of the nucleus accumbens (Acb). To determine the subcellular sites for potential functional interactions between opiate ligands and NMDA receptors in this region, we examined the ultrastructural localization of antipeptide antisera against MOR and the R1 subunit of the NMDA receptor in the Acb shell of the adult rat brain. MOR-like immunoreactivity (MOR-LI) was seen primarily in dendrites, whereas NMDAR1-like immunoreactivity (NMDAR1-LI) was detected more often in axon terminals forming asymmetric synapses. In these profiles, MOR labeling was localized mainly to extrasynaptic plasma membranes, whereas NMDAR1-LI was associated with both synaptic and extrasynaptic sites. Of 307 MOR-labeled processes, 17.9% of the dendrites and 9.4% of the axon terminals also contained NMDAR1-LI. In addition, 24.7% of the dendrites containing only MOR-LI were apposed to NMDAR1-labeled axons or terminals. We conclude that in the shell of the Acb, the output of single neurons can be dually modulated by (1) activation of MOR and NMDA receptors in the same dendrites or (2) combined activation of presynaptic NMDA receptors in afferents contacting dendrites containing MOR. In addition, the colocalization of MOR and NMDAR1 in certain axon terminals in the Acb suggests their dual involvement in the presynaptic release of neurotransmitters in this region.

Vesicular monoamine transporter-2: immunogold localization in striatal axons and terminals

The vesicular monoamine transporter-2 (VMAT2) mediates the reserpine-sensitive neuronal uptake of monoamines into vesicles and other intracellular organelles. Accordingly, this transporter is expressed at high levels in regions that contain a dense monoamine innervation, such as the rat dorsolateral striatum. We used ultrastructural immunocytochemistry in this region to show that immunogold labeling for VMAT2 is present in varicose axonal processes, many of which also contain the catecholamine-synthesizing enzyme tyrosine-hydroxylase. Within these mainly dopaminergic processes, VMAT2 was associated with small synaptic vesicles (SSVs) and more rarely with large dense-core vesicles or tubulovesicles. These findings suggest that SSVs are the major organelles involved in the storage and release of dopamine in the dorsolateral striatum.

Immunogold localization of the dopamine transporter: an ultrastructural study of the rat ventral tegmental area

The dopamine transporter (DAT) plays an important role in the plasmalemmal reuptake of dopamine and, thus, in the termination of normal dopaminergic neurotransmission. DAT is also a major binding site for cocaine and other stimulants, the psychoactive effects of which are associated primarily with the inhibition of dopamine reuptake within mesocorticolimbic dopaminergic neurons. We used electron microscopy with an anti-peptide antiserum directed against the N-terminal domain of DAT to determine the subcellular localization of this transporter in the rat ventral tegmental area (VTA), the region that contains the cell bodies and dendrites of these dopaminergic neurons. We show that in the VTA, almost 95% of the DAT immunogold-labeled profiles are neuronal perikarya and dendrites, and the remainder are unmyelinated axons. Within perikarya and large proximal dendrites, almost all of the DAT immunogold particles are associated with intracellular membranes, including saccules of Golgi and cytoplasmic tubulovesicles. In contrast, within medium- to small-diameter dendrites and unmyelinated axons, most of the DAT gold particles are located on plasma membranes. In dually labeled tissue, peroxidase reaction product for the catecholamine-synthesizing enzyme tyrosine hydroxylase is present in DAT-immunoreactive profiles. These findings suggest that intermediate and distal dendrites are both the primary sites of dopamine reuptake and the principal targets of cocaine and related psychostimulants within dopaminergic neurons in the VTA.

mu-Opioid receptors are localized to extrasynaptic plasma membranes of GABAergic neurons and their targets in the rat nucleus accumbens

The activation of mu-opioid receptors in the nucleus accumbens (Acb) produces changes in locomotor and rewarding responses that are believed to involve neurons, including local gamma-aminobutyric acid (GABA)ergic neurons. We combined immunogold-silver detection of an antipeptide antiserum against the cloned mu-opioid receptor (MOR) and immunoperoxidase labeling of an antibody against GABA to determine the cellular basis for the proposed opioid modulation of GABAergic neurons in the rat Acb. MOR-like immunoreactivity (MOR-LI) was localized prominently to plasma membranes of neurons having morphological features of both spiny and aspiny cells, many of which contained GABA. Of 351 examples of profiles that contained MOR-LI and GABA labeling, 65% were dendrites. In these dendrites, MOR-LI was seen mainly along extrasynaptic portions of the plasma membrane apposed to unlabeled terminals and/or glial processes. Dually labeled dendrites often received convergent input from GABAergic terminals and/or from unlabeled terminals forming asymmetric excitatory-type synapses. Of all profiles that contained both MOR and GABA immunoreactivity, 28% were axon terminals. MOR-containing GABAergic terminals and terminals separately labeled for MOR or GABA formed synapses with unlabeled dendrites and also with dendrites containing MOR or GABA. Our results indicate that MOR agonists could modulate the activity of GABA neurons in the Acb via receptors located mainly at extrasynaptic sites on dendritic plasma membranes. MOR ligands also could alter the release of GABA onto target dendrites that contain GABA and/or respond to opiate stimulation.

Ultrastructural localization and comparative distribution of nitric oxide synthase and N-methyl-D-aspartate receptors in the shell of the rat nucleus accumbens

Nitric oxide (NO), the diffusible gas formed by nitric oxide synthase (NOS) has been implicated in the enhanced locomotor activity attributed mainly to increased dopamine release in the shell of the nucleus accumbens (Acb). Furthermore, the release of both NO and dopamine are known to be altered by agonists of N-methyl-D-aspartate (NMDA) type glutamate receptors in this region. We examined the cellular sites of NO synthesis and the sites of potential relevancy for functional associations between neurons containing NOS and the NMDA receptor in the shell of the Acb. This was achieved by dual ultrastructural immunogold and immunoperoxidase labeling of antisera raised against the brain form of NOS and the NMDAR1 subunit of the NMDA receptor in this region of rat brain. NOS-like immunoreactivity (NOS-LI) was seen throughout the cytoplasm of isolated medium-large somata, aspiny dendrites and axon terminals. In 217 NOS-labeled profiles, NMDAR1-like immunoreactivity (NMDAR1-LI) was colocalized in 17% of somata and dendrites. Additionally, 35% of NOS-labeled dendrites apposed glial processes containing NMDAR1-LI, and 29% apposed axon terminals containing NMDARI-LI. NOS-labeled terminals more rarely colocalized NMDAR1 or apposed NMDAR1-labeled glial processes or dendrites. These results provide anatomical evidence that, in the shell of the Acb, NMDA receptors are localized so as to directly modulate the output of neurons producing NO as well as to influence other neurons and glia having the greatest access to the released gas.

Ultrastructural immunocytochemical localization of the dopamine D2 receptor within GABAergic neurons of the rat striatum

Classical antipsychotics, which block dopamine (DA) D2 receptors, showing intrastriatal variation in their effectiveness in modulating GABAergic function. To determine the cellular basis for such differences, we examined the electron microscopic immunocytochemical labeling of D2 receptors and GABA in the dorsolateral caudate-putamen (CPn) and the nucleus accumbens (Acb) shell. In both regions, peroxidase reaction product and gold-silver deposits representing D2 receptor immunoreactivity (D2-IR) and GABA immunoreactivity (GABA-IR), respectively, were detected in dendrites and perikarya having characteristics of either spiny projection neurons or aspiny interneurons. Some perikarya in both regions are dually labeled with D2-IR and GABA-IR. Neurons axon terminals in each region also contained one or both markers. However, there were notable regional differences in the immunolabeling patterns. In the CPn, D2-IR was more commonly seen in dendrites/spines than in axon terminals, and proportionally more dendrites were dually labeled than in the Acb. In the Acb shell, D2-IR was detected with similar frequency in terminals and dendrites/spines, but more terminals co-localized D2-IR and GABA-IR in this region compared with the CPn. These results provide the first ultrastructural evidence for direct D2-mediated effects of DA on striatal GABAergic neurons. They further suggest that modulation of GABAergic neurons by DA acting at D2 receptors may be relatively more postsynaptic in the CPn, but more presynaptic in the Acb shell.

Detection of delta opioid receptor and N-methyl-D-aspartate receptor-like immunoreactivity in retinoic acid-differentiated neuroblastoma x glioma (NG108-15) cells

NG108-15 neuroblastoma cells differentiated with 0.1 M of all-trans retinoic acid (RA) were processed for immunohistochemical analysis using polyclonal antisera against the delta opioid receptor (DOR) and the N-Methyl-D-Aspartate receptor (NMDAR1) to determine the cellular sites for possible functional associations between DOR and NMDAR1 receptors. In this study, 6 days of RA treatment resulted in prominent morphological differentiation characterized by the appearance of numerous axon- and dendrite-like processes and formation of networks between the cell clusters. An immunocytochemical approach allowed the demonstration of antibody concentration-dependent differences, not evident in ligand binding studies, in the distribution of DOR and NMDA receptor protein between cell soma and processes. RA-differentiated cultures showed positive DOR-like immunostaining (DOR-LI) throughout the cell bodies as well as on the newly acquired processes. In contrast, NMDAR1-like immunoreactivity (NMDAR1-LI) in the RA-treated cells was detected in the cell soma and processes only with the higher concentration of the antiserum. With the lower concentration of the antibody the NMDAR1-LI was not detected in the processes and was limited to a punctuate subcellular distribution in the soma. The DOR-LI pattern of distribution in NG108-15 cells differentiated with RA appeared to be consistent with the DOR-LI detected in the CNS. The NMDAR1-LI distribution in these cells is similar to brain tissue with respect to its presence on the newly acquired processes. However, it differed from brain in that a much higher abundance of NMDAR1 receptors was observed in the cell soma. This differential distribution of DOR and NMDAR1 receptors in the RA-treated NG108-15 cells could provide a basis for future studies of drug-induced changes in these two receptors.

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.

Ultrastructural immunocytochemical localization of the N-methyl-D-aspartate receptor and tyrosine hydroxylase in the shell of the rat nucleus accumbens

The N-methyl-D-aspartate (NMDA)-type glutamate receptors in the shell region of the nucleus accumbens (ACB) have been implicated in the modulation of dopamine release and in amphetamine-induced neurotoxicity. We used electron microscopic immunocyto-chemistry to determine the anatomical sites for NMDA-mediated effects of glutamate and for their potential interactions with dopaminergic afferents identified by the presence of tyrosine hydroxylase (TH) in this region of the rat brain. Immunogold and immunoperoxidase methods were used to localize antisera against the R1 subunit of the NMDA receptor (NMDAR1) alone or combined with TH. In single labeling experiments, approximately half of the NMDAR1-like immunoreactivity (NMDAR1-LI) was localized to extrasynaptic plasma membranes of neuronal processes, many (92 out of 215) of which were dendrites, and only 33 out of 215 were unmyelinated axons or terminals. Surprisingly, the neuronal labeling of NMDAR1 was almost equaled by that seen in astrocytic processes (88 out of 215). Dual labeling for TH and NMDAR1 was rarely observed and was only seen in axons. However, in favorable planes of section, NMDAR1 was noted along intervaricose segments of axons in which TH was more readily seen in the varicosity. This differential intra-axonal distribution suggests an underestimation of dual labeling in single coronal sections through unmyelinated axons and terminals. The TH-immunoreactive terminals were more often seen apposed to NMDA-immunoreactive astrocytic processes and dendrites. These results provide the first ultrastructural evidence for presynaptic modulation of dopamine release by NMDA receptors in the shell of the nucleus accumbens. They also indicate that NMDA receptors modulate postsynaptic neurons receiving input from the dopaminergic afferents and suggest a previously unsuspected functional association involving glial NMDA receptors and dopaminergic afferents in this brain region.

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.

Immunolabeling of Mu opioid receptors in the rat nucleus of the solitary tract: extrasynaptic plasmalemmal localization and association with Leu5-enkephalin

Activation of the mu opioid receptor (MOR) by morphine within the caudal nucleus of the solitary tract (NTS) is known to mediate both cardiorespiratory and gastrointestinal responses. Leu5-enkephalin (LE), a potential endogenous ligand for MOR, is also present within neurons in this region. To determine the cellular sites for the visceral effects of MOR ligands, including LE, we used immunogold-silver and immunoperoxidase methods for light and electron microscopic localization of antisera against MOR (carboxyl terminal domain) and LE in the caudal NTS of rat brain. Light microscopy of coronal sections through the NTS at the level of the area postrema showed MOR-like immunoreactivity (MOR-LI) and LE labeling in punctate processes located within the subpostremal, dorsomedial and medial subnuclei. Electron microscopy of sections through the medial NTS at this level showed gold-silver particles identifying MOR-LI prominently distributed to the cytoplasmic side of the plasma membranes of axons and terminals. MOR labeled terminals formed mostly symmetric (inhibitory-type) synapses but sometimes showed multiple asymmetric junctions, characteristic of excitatory visceral afferents. MOR-LI was also present along extrasynaptic plasma membranes of dendrites receiving afferent input from unlabeled and LE-labeled terminals. We conclude that MOR ligands, possibly including LE, can act at extrasynaptic MORs on the plasma membranes of axons and dendrites in the caudal NTS to modulate the presynaptic release and postsynaptic responses of neurons. These are likely to include local inhibitory neurons and both gastric and cardiorespiratory afferents known to terminate in the subnuclei with the most intense MOR-LI.

Ultrastructural immunocytochemical localization of neurotensin and the dopamine D2 receptor in the rat nucleus accumbens

The neuroleptic-like effects of neurotensin (NT) are thought to be due to interactions with dopamine (DA) acting primarily at D2 receptors within the nucleus accumbens septi (Acb). Using electron microscopic dual labeling immunocytochemistry, we sought to demonstrate cellular substrates for functional interactions involving NT and DA D2 receptors in the adult rat Acb. Peroxidase reaction product representing D2 receptor-like immunoreactivity (D2-LI) was seen along membranes of Golgi lamellae and multivesicular bodies of perikarya containing immunogold labeling representing NT-LI. Dually labeled somata usually contained highly indented nuclei, a characteristic of aspiny neurons. Dendrites also occasionally colocalized the two immunomarkers. Other somata, dendrites, and all axon terminals were singly labeled with either NT-LI or D2-LI. In distinct sets of terminals, NT-LI was commonly associated with large, dense-cored vesicles, whereas D2-LI was found along the plasmalemma and over nearby small clear vesicles. Each type of terminal comprised approximately 20% of synaptic input to NT-immunoreactive dendrites. Similar proportions of terminals containing NT-LI or D2-LI contacted unlabeled (approximately 55%) or NT-labeled (approximately 35%) dendrites and, occasionally, were observed converging onto common dendrites. Terminals containing NT-LI or D2-LI also were often closely apposed. These findings provide the first ultrastructural evidence that: (1) NT and D2 receptors are colocalized in aspiny neurons and dendrites, (2) NT may produce a direct postsynaptic effect on neurons receiving input from terminals which are presynaptically modulated by DA via D2 receptors, and (3) NT and DA acting at D2 receptors may interact through presynaptic modulation of common axon terminals.

Input from central nucleus of the amygdala efferents to pericoerulear dendrites, some of which contain tyrosine hydroxylase immunoreactivity

Light microscopic anterograde tracing studies indicate that neurons in the central nucleus of the amygdala (CNA) project to a region of the dorsal pontine tegmentum ventral to the superior cerebellar peduncle which contains noradrenergic dendrites of the nucleus locus coeruleus (LC). However, it has not been established whether the efferent terminals from the CNA target catecholamine-containing dendrites of the LC or dendrites of neurons from neighboring nuclei which may extend into this region. To examine this question, we combined immunoperoxidase labeling of the anterograde tracer biotinylated dextran amine (BDA) from the CNA with immunogold-silver labeling of the catecholamine-synthesizing enzyme tryrosine hydroxylase (TH) in the rostrolateral LC region of adult rats. By light microscopy, BDA-labeled processes were dense in the dorsal pons within the parabrachial nuclei as well as in the pericoerulear region immediately ventral to the superior cerebellar peduncle. Higher magnification revealed that BDA-labeled varicose fibers overlapped TH-labeled processes in this pericoerulear region. By electron microscopy, anterogradely labeled axon terminals contained small, clear as well as some large dense core vesicles and were commonly apposed to astrocytic processes along some portion of their plasmalemma. BDA-labeled terminals mainly formed symmetric type synaptic contacts characteristic of inhibitory transmitters. Of 250 BDA-labeled axon terminals examined where TH immunoreactivity was present in the neuropil, 81% contacted unlabeled and 19% contacted TH-labeled dendrites. Additionally, amygdala efferents were often apposed to unlabeled axon terminals forming asymmetric (excitatory type) synapses. These results demonstrate that amygdaloid efferents may directly alter the activity of catecholaminergic and non-catecholaminergic neurons in this pericoerulear region of the rat brain. Furthermore, our study suggests that CNA efferents may indirectly affect the activity of pericoerulear neurons through modulation of excitatory afferents. Amygdaloid projections to noradrenergic neurons may help integrate behavioral and visceral responses to threatening stimuli by influencing the widespread noradrenergic projections from the LC.

Pre- and postsynaptic sites for serotonin modulation of GABA-containing neurons in the shell region of the rat nucleus accumbens

The shell of the nucleus accumbens received a dense serotonergic innervation and contains abundant gamma-aminobutyric acid (GABA)-immunoreactive neurons. Moreover, serotonin (5-hydroxytryptamine: 5-HT) and GABA have been implicated in a variety of common motivational and motor-related functions partially ascribed in this brain area. We used immunoelectron microscopy of antisera directed against 5-HT and GABA in the same section of tissue to examine whether there were cellular substrates that might indicate more specific sites for functional interactions involving these transmitters in the shell region of the rat nucleus accumbens. Immunogold-silver labeling for GABA was localized to perikarya, dendrites, axons and axon terminals, whereas immunoperoxidase labeling for 5-HT was restricted to axons and axon terminals. Approximately half (187/366) of the 5-HT-immunoreactive axon terminals apposed or formed synaptic junctions with postsynaptic neurons. These junctions were mainly of the symmetric-type (83/187) characteristic of inhibitory transmitters, and were equally prevalent on dendrites with and without detectable gold-silver labeling for GABA. Of the 187 5-HT-labeled axon terminals with recognized synaptic contacts, 36% also showed convergence on a common dendrite with a GABA-labeled axon terminal. In addition, 5-HT- and GABA-immunoreactive axon terminals were commonly (83/366) identified in direct apposition to one another. Within a single plane of section, 41% of the apposed GABA-immunoreactive axon terminals formed symmetric-type junctions with dendrites or somata, whereas, the apposed 5-HT-labeled axon terminals rarely showed postsynaptic contacts. These results indicate that 5-HT-containing axon terminals may postsynaptically inhibit GABAergic neurons and their targets within the shell of the rat nucleus accumbens. Additionally, our results strongly suggest that, in this brain region, appositions between 5-HT and GABA axons and axon terminals may facilitate presynaptic interactions between these transmitter systems.

Ultrastructural localization of the vesicular monoamine transporter-2 in midbrain dopaminergic neurons: potential sites for somatodendritic storage and release of dopamine

Midbrain dopaminergic neurons are known to release dopamine from somata and/or dendrites located in the substantia nigra (SN) and the ventral tegmental area (VTA). There is considerable controversy, however, about the subcellular sites for somatodendritic dopamine storage in these regions. In the present study, we used dual-labeling electron microscopic immunocytochemistry to localize the vesicular monoamine transporter-2 (VMAT2), a novel marker for sites of intracellular monoamine storage, within identified dopaminergic (tyrosine hydroxylase-containing) neurons in the rat SN and VTA. In dopaminergic perikarya, immunogold labeling for VMAT2 was localized to the Golgi apparatus, tubulovesicles that resembled smooth endoplasmic reticulum (SER), and the limiting membranes of multivesicular bodies. In dopaminergic dendrites, VMAT2 was extensively localized to tubulovesicles that resembled saccules of SER, and less frequently localized to isolated small synaptic vesicles (SSVs) or large dense-core vesicles (DCVs). In rare cases, VMAT2-immunoreactive SSVs were clustered within the cytoplasm of an SN or a VTA dendrite. Dopaminergic dendrites in the VTA contained a significantly higher number of immunogold particles for VMAT2 per unit than those in the SN. Together, these observations support the proposal that dopamine is stored in and may be released from dendritic SSVs and DCVs, but suggest that the SER is the major site of dopamine storage within midbrain dopaminergic neurons. In addition, they provide new evidence that dopaminergic dendrites in the VTA may have greater potential for reserpine-sensitive storage and release of dopamine than those in the SN.

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.

GABAergic neurons in rat nuclei of solitary tracts receive inhibitory-type synapses from amygdaloid efferents lacking detectable GABA-immunoreactivity

Gamma-aminobutyric acid (GABA) is a prominent inhibitory transmitter in both the central nucleus of the amygdala (Ce) and the medial nuclei of the solitary tracts (mNTS). These regions are reciprocally connected by anatomical pathways mediating the coordinated visceral responses to emotional stress. To further determine whether GABA is present in the amygdaloid efferents or their targets in the mNTS, we combined peroxidase labeling of Phaseolus vulgaris leucoagglutinin (PHA-L) or biotinylated dextran amine (BDA) anterogradely transported from the Ce with immunogold-silver detection of antibodies against GABA in the rat mNTS. By light microscopy, peroxidase labeling for either PHA-L or BDA was seen in varicose processes, whereas immunogold-silver labeling for GABA was detected in perikarya and processes throughout the rostrocaudal mNTS. The intermediate mNTS at the level of the area postrema, a region receiving mainly cardiorespiratory and gastric visceral afferents, were examined by electron microscopy. In this region, anterograde labeling was observed exclusively in unmyelinated axons and axon terminals. These terminals lacked detectable GABA-immunoreactivity, but formed symmetric synapses that are associated with inhibition. The targets of the anterogradely labeled terminals were medium-sized dendrites both with and without GABA-labeling. These dendrites often also received convergent input from terminals that were intensely GABA-immunoreactive. We conclude that visceral activation accompanying emotional response to stress is likely to involve inhibition of GABAergic neurons in the mNTS by non-GABA-containing amygdaloid efferents. Furthermore, our results indicate that the inhibition of these GABAergic neurons may be further augmented by release of GABA from other converging terminals in the mNTS.

Pyramidal neurons in rat prefrontal cortex show a complex synaptic response to single electrical stimulation of the locus coeruleus region: evidence for antidromic activation and GABAergic inhibition using in vivo intracellular recording and electron microscopy

Cognition and acquisition of novel motor skills and responses to emotional stimuli are thought to involve complex networking between pyramidal and local GABAergic neurons in the prefrontal cortex. There is increasing evidence for the involvement of cortical norepinephrine (NE) deriving from the nucleus locus coeruleus (LC) in these processes, with possible reciprocal influence via descending projections from the prefrontal cortex to the region of the LC. We used in vivo intracellular recording in rat prefrontal cortex to determine the synaptic responses of individual neurons to single electrical stimulation of the mesencephalic region including the nucleus LC. The most common response consisted of a late-IPSP alone or preceded by an EPSP. The presence of an early-IPSP following the EPSP was sometimes detected. Analysis of the voltage dependence revealed that the late-IPSP and early-IPSP were putative K(+)- and Cl- dependent, respectively. Synaptic events occurred following short delays and were inconsistent with the previously reported time for electrical activation of unmyelinated LC fibers. Moreover, systemic injection of the adrenergic antagonists propranolol (beta receptors), or prazosin (alpha 1 receptors), did not block synaptic responses to stimulation of the LC region. Finally, certain neurons were antidromically activated following electrical stimulation of this region of the dorsal pontine tegmentum. Taken together, these results suggest that the complex synaptic events in pyramidal neurons of the prefrontal cortex that are elicited by single electrical stimulation of the LC area are mainly due to antidromic activation of cortical efferents. Further insight into the chemical circuitry underlying these complex synaptic responses was provided by electron microscopic immunocytochemical analysis of the relations between the physiologically characterized neurons and either 1) GABA or 2) dopamine-beta-hydroxylase (DBH), a marker for noradrenergic terminals. GABA-immunoreactive terminals formed numerous direct symmetric synapses on somata and dendrites of pyramidal cells recorded and filled with lucifer yellow (LY). In contrast, in single sections, noradrenergic terminals immunoreactive for DBH rarely contacted LY-filled somata and dendrites. These results support the conclusion that IPSPs observed following single electrical stimulation of the LC region are mediated by GABA, with little involvement of NE. These IPSPs, arising from antidromic invasion of mPFC cells innervating the LC, may improve the signal-to-noise ratio and favor a better responsiveness of neighboring neurons to NE released in the mPFC.

Ultrastructural localization of calbindin-D28k and GABA in the matrix compartment of the rat caudate-putamen nuclei

The calcium binding protein, Calbindin-D28k, is known to be localized within spiny neurons of the matrix of the dorsal striatum, caudate-putamen nuclei. This compartment is also known to contain an abundance of GABAergic neurons and to receive extensive input from excitatory limbic and cortical afferents whose activation produces rapid influxes of calcium in neuronal targets. We used electron microscopic immunocytochemistry to examine a potential role for calbindin in GABAergic neurons in the caudate-putamen nuclei. Sections of striatal tissue from acrolein-fixed adult rat brains were dual-labeled using immunoperoxidase for the localization of rabbit anti-calbindin and immunogold-silver for the localization of rat anti-GABA antibodies. Calbindin-D28k and GABA were mainly co-localized in somata and large dendrites. The peroxidase reaction product for calbindin was diffusely distributed throughout the neuronal cytoplasm, but appeared more densely localized along asymmetric, excitatory-type, postsynaptic junctions of dendritic spines, as well as saccules of smooth endoplasmic reticulum near dendritic appositions. In contrast, the immunogold-silver labeling for GABA was largely restricted to perikarya and large dendrites. Axon terminals forming symmetric junctions were also sometimes dual-labeled for calbindin and GABA. However, the majority of the calbindin-immunoreactive terminals did not contain GABA and many formed asymmetric excitatory-type synapses with either unlabeled or calbindin-labeled dendritic spines. These results suggest that, in the striatal matrix, Calbindin-D28k contributes to the immobilization of calcium (i) in selectively activated postsynaptic spines of GABAergic and possibly non-GABAergic neurons and (ii) in terminals containing GABA as well as other excitatory and inhibitory transmitters. The extent to which calbindin is able to restrict the cytosolic increases in calcium to selective sites of utilization in these neurons may have important consequences for normal synaptic function and for neuroprotection against excitoxicity.

Enkephalin terminals form inhibitory-type synapses on neurons in the rat nucleus locus coeruleus that project to the medial prefrontal cortex

Norepinephrine-containing fibres in the medial prefrontal cortex derive from the locus coeruleus, a brainstem nucleus which also receives a dense innervation of enkephalin-immunoreactive axon terminals. We combined immunogold-silver labelling of retrogradely transported FluoroGold from the medial prefrontal cortex with immunoperoxidase detection of leucine5-enkephalin in the same section of tissue through the locus coeruleus of adult rats. This dual-labelling experiment was conducted to determine whether axon terminals containing lecuine5-enkephalin target neurons in the locus coeruleus that project to the frontal cortex and, if so, what are their morphological characteristics. By light microscopy, enkephalin-labelled processes overlapped FluoroGold retrogradely labelled neurons in the locus coeruleus. By electron microscopy, retrogradely labelled perikarya and dendrites were commonly enveloped by astrocytic processes and received few afferents in the plane of section examined. However, at sites unoccupied by glial processes, abundant afferent input could be identified. In addition, some FluoroGold-labelled perikarya and dendrites lacked this glial ensheathment but were more frequently apposed by axon terminals. Of 163 FluoroGold-labelled perikarya and dendrites examined where enkephalin immunoreactivity was present in the neuropil, 42% were contacted by enkephalin-immunoreactive axon terminals. The peroxidase-labelled enkephalin terminals as well as the unlabelled terminals often contained both small, clear and large dense core vesicles. Both labelled and unlabelled terminals also formed primary symmetric synapses characteristic of inhibitory transmitters with retrogradely labelled perikarya and proximal dendrites. At times, more than one enkephalin-labelled terminal was found to converge on a common retrogradely labelled perikarya or dendrite. These results demonstrate cellular sites where enkephalin-containing afferents may directly modulate and most likely inhibit the activity of cortically projecting neurons in the locus coeruleus.