High-affinity neurotensin receptors in the rat nucleus accumbens: subcellular targeting and relation to endogenous ligand

Neurotensin is present in selective mesolimbic dopaminergic projections to the nucleus accumbens (NAc) shell but also is synthesized locally in this region and in the motor-associated NAc core. We examined the electron microscopic immunolabeling of the high-affinity neurotensin receptor (NTR) and neurotensin in these subdivisions of rat NAc to determine the sites for receptor activation and potential regional differences in distribution. Throughout the NAc, NTR immunoreactivity was localized discretely within both neurons and glia. NTR-labeled neuronal profiles were mainly axons and axon terminals with diverse synaptic structures, which resembled dopaminergic and glutamatergic afferents, as well as collaterals of inhibitory projection neurons. These terminals had a significantly higher numerical density in the NAc core than in the shell but were prevalent in both regions, suggesting involvement in both motor and limbic functions. In each region, neurotensin was detected in a few NTR-immunoreactive axon terminals and in terminals that formed symmetric, inhibitory type synapses with NTR-labeled somata and dendrites. The NTR labeling, however, was not seen within these synapses and, instead, was localized to segments of dendritic and glial plasma membranes often near excitatory type synapses. Neuronal NTR immunoreactivity also was associated with cytoplasmic tubulovesicles and nuclear membranes. Our results suggests that, in the NAc shell and core, NTR is targeted mainly to presynaptic sites, playing a role in the regulated secretion and/or retrograde signaling in diverse, neurotransmitter-specific neurons. The findings also support a volume mode of neurotensin actions, specifically affecting excitatory transmission through activation of not only axonal but also dendritic and glial NTR.

Vesicular acetylcholine transporter in the rat nucleus accumbens shell: subcellular distribution and association with mu-opioid receptors

Cholinergic interneurons in the nucleus accumbens shell (AcbSh) are implicated in the reinforcing behaviors that develop in response to opiates active at mu-opioid receptors (MOR). We examined the electron microscopic immunocytochemical localization of the vesicular acetylcholine transporter (VAChT) and MOR to determine the functional sites for storage and release of acetylcholine (ACh), and potential interactions involving MOR in this region of rat brain. VAChT was primarily localized to membranes of small synaptic vesicles in axon terminals. Less than 10% of the VAChT-labeled terminals were MOR-immunoreactive. In contrast, 35% of the cholinergic terminals formed symmetric or punctate synapses with dendrites showing an extrasynaptic plasmalemmal distribution of MOR. Membranes of tubulovesicles in other selective dendrites were also VAChT-labeled, and almost half of these dendrites displayed plasmalemmal MOR immunoreactivity. The VAChT-labeled dendritic tubulovesicles often apposed unlabeled axon terminals that formed symmetric synapses. Our results indicate that in the AcbSh MOR agonists can modulate the release of ACh from vesicular storage sites in axon terminals as well as in dendrites where the released ACh may serve an autoregulatory function involving inhibitory afferents. These results also suggest, however, that many of the dendrites of spiny projection neurons in the AcbSh are dually influenced by ACh and opiates active at MOR, thus providing a cellular substrate for ACh in the reinforcement of opiates.

Targeting of serotonin 1A receptors to dopaminergic neurons within the parabrachial subdivision of the ventral tegmental area in rat brain

Serotonin (5-hydroxytryptamine [5-HT]) modulates dopamine-related cognitive functions and motor activity through activation of selective receptor subtypes including 5-HT1A. Potential targets for these 5-HT1A-mediated actions of 5-HT include mesocortical and mesolimbic dopaminergic neurons having partially segregated distribution in the parabrachial and paranigral subdivisions of the ventral tegmental area (VTA), respectively. We therefore examined the ultrastructural immunocytochemical localization of the 5-HT1A receptor in the parabrachial (VTApb) and paranigral (VTApn) subdivisions of rat VTA, to determine 1) the functional sites for receptor activation, and 2) the cellular associations between this receptor and dopaminergic neurons identified by their tyrosine hydroxylase (TH) content. In each region, 5-HT1A immunoreactivity was mainly observed in somatodendritic profiles, but it was also present in small unmyelinated axons and in a few axon terminals and glia, suggesting a role for 5-HT1A receptors in presynaptic and glial functions, as well as postsynaptic neuronal activation, in VTA. In somatodendritic profiles, 5-HT1A gold particles were mainly localized to tubulovesicles presumed to be smooth endoplasmic reticulum. In addition, however, in distal dendrites receiving multiple inputs the receptor was targeted to selective postsynaptic junctions, or more randomly distributed on nonsynaptic portions of the plasma membrane. Of the 5-HT1A-labeled dendrites, 64% in VTApb and 44% in VTApn contained TH. These findings suggest a reserve of cytoplasmic 5-HT1A receptors that are mobilized to functional postsynaptic sites on the plasma membrane by afferent input to distal dendrites in the VTA. They also indicate that 5-HT1A activation may affect a larger population of dopaminergic neurons in VTApb compared with VTApn, thus having a potentially greater impact on cognitive functions modulated by mesocortical dopaminergic neurons.

Preferential cytoplasmic localization of delta-opioid receptors in rat striatal patches: comparison with plasmalemmal mu-opioid receptors

The activation of delta-opioid receptors (DORs) in the caudate-putamen nucleus (CPN) produces regionally distinct changes in motor functions, many of which are also influenced by opioids active at micro-opioid receptors (MORs). These actions most likely occur in MOR-enriched patch compartments in the CPN. To determine the functional sites for DOR activation and potential interactions involving MOR in these regions, immunoperoxidase and immunogold-silver labeling methods were applied reversibly for the ultrastructural localization of DOR and MOR in single rat brain sections containing patches of the CPN. DOR immunoreactivity was commonly seen within the cytoplasm of spiny and aspiny neurons, many of which also expressed MOR. In dendrites and spines, DOR labeling was preferentially localized to membranes of the smooth endoplasmic reticulum and spine apparatus, whereas MOR showed a prominent plasmalemmal distribution. DOR- and/or MOR-labeled spines received asymmetric, excitatory synapses, some of which showed notable perforations, suggesting the involvement of these receptors in activity-dependent synaptic plasticity. DORs were more frequently detected than were MORs within axon terminals that formed either asymmetric synapses with spine heads or symmetric synapses with spine necks. Our results suggest that in striatal patches, DORs, often in cooperation with MORs, play a direct modulatory role in controlling the postsynaptic excitability of spines, whereas presynaptic neurotransmitter release onto spines is mainly influenced by DOR activation. In comparison with MOR, the prevalent association of DOR with cytoplasmic organelles that are involved in intracellular trafficking of cell surface proteins suggests major differences in availability of these receptors to extracellular opioids.

Subcellular localization of alpha-2A-adrenergic receptors in the rat medial nucleus tractus solitarius: regional targeting and relationship with catecholamine neurons

alpha-2A-adrenergic receptor (alpha2A-AR) agonists modulate diverse autonomic functions. These actions are believed to involve functionally specialized, second-order neurons in catecholamine-containing portions of the medial nucleus tractus solitarius (mNTS) at both intermediate (NTSi) and caudal (NTSc) levels. However, the cellular mechanisms subserving alpha2A-AR-mediated actions within the mNTS have yet to be established. Immunocytochemistry was employed to examine the subcellular distribution of alpha2A-AR in both the intermediate and caudal mNTS and its association with cells containing the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH). Quantitative regional comparison using immunogold showed that this receptor was distributed differentially to dendrites (NTSi, 46%; NTSc, 31%) and glia (NTSi, 29%; NTSc, 48%) at different levels of the NTS. Somata, axons, and terminals less frequently contained alpha2A-AR. The subcellular distribution of alpha2A-AR relative to catecholaminergic neurons also was similar within both subregions. Approximately 50% of alpha2A-AR-labeled somata also contained TH. In somatic profiles, alpha2A-AR labeling was often found in the cytosol and in association with endoplasmic reticulum and Golgi complexes, sites of receptor synthesis and trafficking. Approximately 20% of alpha2A-AR-immunoreactive dendrites also contained TH, where the receptor was often found on extrasynaptic portions of the plasma membrane near unlabeled terminals, some of which made symmetric contacts. However, TH-labeled terminals and dendrites usually were detected in the neuropil at a short distance (<10 microm) from alpha2A-AR-labeled neurons. alpha2A-AR-labeled glia frequently apposed unlabeled dendrites and terminals and were often located near TH-immunoreactive dendrites. These results indicate that, within the mNTS, alpha2A-AR is involved in a variety of autonomic processes, including postsynaptic modulation of mostly noncatecholaminergic dendrites, as well as influencing glia functions.

Enhancement of laminar FosB expression in frontal cortex of rats receiving long chronic clozapine administration

The frontal cortex (FrC) and cingulate cortex (CgC) are critical sites for normal cognitive function, as well as cognitive dysfunction in schizophrenia. Thus, modulation of synaptic transmission within these cortical areas may, in part, account for the therapeutic actions of antipsychotic drugs such as haloperidol and clozapine. FosB and DeltaFosB are immediate-early gene (IEG) products sensitive to changes in response to chronic neuroleptic drug administration. We quantitatively examine whether there are light microscopic regional and/or laminar variations in FosB or DeltaFosB in the FrC or CgC of normal adult rats, or animals receiving 6 months administration of either drinking water clozapine, or depot haloperidol. Only animals receiving chronic haloperidol developed vacuous chewing movements, the equivalent of tardive dyskinesia in humans. In control animals, the deep and superficial layers of the FrC showed a higher area density of FosB, but not DeltaFosB immunoreactive cells than the medial layers of FrC or any of the CgC layers. In animals receiving clozapine, but not haloperidol there was increase in the area density of FosB immunoreactive neurons in all FrC layers, but the major increase occurs in medial layers. These findings suggest that FosB expression identifies those FrC neurons that are most active during normal waking behaviors and are further activated following chronic administration of atypical neuroleptics without motor side effects. The results also indicate that the actions of clozapine are attributed in large part to modulation of the output of frontal cortical pyramidal neurons residing in the medial layers.

FosB in rat striatum: normal regional distribution and enhanced expression after 6-month haloperidol administration

Subcortical motor nuclei show differential expression of FosB immediate early gene products and specifically deltaFosB after short (8, 19, or 21 days) chronic exposure to typical and atypical neuroleptics represented by haloperidol and clozapine, respectively. We quantitatively examined whether there are light microscopic regional variations in area density of FosB or the truncated deltaFosB in several motor-related nuclei of adult rats receiving vehicle or long chronic (6 months) administration of either depot haloperidol or clozapine in their drinking water. In control animals the dorsomedial and ventromedial caudate-putamen nucleus (CPN) had a significantly higher density of FosB-immunoreactive cells than the dorsolateral and ventrolateral regions. The nucleus accumbens (NAc) core also serving motor functions had a higher basal expression than the limbic shell region in control animals. The mediolateral gradient in area density of FosB-labeled cells was maintained in animals receiving either haloperidol or clozapine. In animals receiving haloperidol, but not clozapine, however, there was a regionally selective increase in the area density of only FosB-immunoreactive neurons in the dorsolateral and ventrolateral CPN and in both the core and shell of the NAc. Only the animals receiving chronic haloperidol showed vacuous chewing movements, the animal equivalent of tardive dyskinesia in humans. Our results suggest that, whereas the medial striatal neurons are activated under basal conditions, long chronic haloperidol induced FosB expression more exclusively in the lateral CPN and NAc core, implicating these regions specifically in the motor side effects of this drug.

Ultrastructural localization of the CB1 cannabinoid receptor in mu-opioid receptor patches of the rat Caudate putamen nucleus

Cannabinoids and opioids are widely consumed drugs of abuse that produce motor depression, in part via respective activation of the cannabinoid subtype 1 receptor (CB1R) and the mu-opioid receptor (muOR), in the striatal circuitry originating in the caudate putamen nucleus (CPN). Thus, the CB1R and muOR may show similar targeting in the CPN. To test this hypothesis, we examined the electron microscopic immunocytochemical labeling of CB1R and muOR in CPN patches of rat brain. Of the CB1R-labeled profiles, 34% (588) were dendrites, presumably arising from spiny as well as aspiny-type somata, which also contained CB1R immunoreactivity. In dendrites, CB1R often was localized to nonsynaptic and synaptic plasma membranes, particularly near asymmetric excitatory-type junctions. Almost one-half of the CB1R-labeled dendrites contained muOR immunoreactivity, whereas only 20% of all muOR-labeled dendrites expressed CB1R. Axons and axon terminals as well as abundant glial processes also showed plasmalemmal CB1R and were mainly without muOR immunoreactivity. Many CB1R-labeled axon terminals were small and without recognizable synaptic junctions, but a few also formed asymmetric, or more rarely symmetric, synapses. The CB1R-labeled glial processes were often perivascular or perisynaptic, surrounding asymmetric excitatory-type axospinous synapses. Our results show that in CPN patches CB1R and muOR are targeted strategically to some of the same postsynaptic neurons, which may account for certain similarities in motor function. Furthermore, they also provide evidence that CB1R may play a major role in the modulation of presynaptic transmitter release and glial functions that are unaffected in large part by opioids active at muOR in CPN.

Neurokinin 1 receptor distribution in cholinergic neurons and targets of substance P terminals in the rat nucleus accumbens

Substance P (SP) is the major endogenous ligand for neurokinin 1 (NK1) receptors and, together with acetylcholine, has an important role in motivated behaviors involving the limbic shell and motor core of the nucleus accumbens (NAc). To determine the functional sites for SP activation of NK-1 receptors and potential interactions with cholinergic neurons in these regions, the authors examined the electron microscopic immunocytochemical localization either of antisera against the NK1 receptor or of the NK1 receptor and either 1) SP or 2) the vesicular acetylcholine transporter (VAchT) in rat NAc. In both the NAc shell and core, NK1 receptor labeling was localized mainly to somatic and dendritic plasma membranes and nearby endosomal organelles in aspiny neurons. In sections through the ventromedial shell that were processed for NK1/SP labeling, 46% of the NK1-immunoreactive dendrites (n = 603 dendrites) showed symmetric or appositional contacts with SP-containing terminals. These terminals and several others that formed symmetric synapses also occasionally were immunoreactive for NK1 receptors. Analysis of the shell region for NK1/VAchT labeling showed that 61% of the total immunoreactive dendrites (n = 534 dendrites) contained NK1 receptors without VAchT, 29% contained both products, and 10% contained VAchT only. Many of the labeled somata and dendrites also received synaptic contact from VAchT-containing terminals. These findings suggest that, in the NAc, NK1 receptors are recycled through endosomal compartments and play a role in modulating mainly the postsynaptic responses, but also the presynaptic release, of SP and/or inhibitory neurotransmitters onto aspiny interneurons, some of which are cholinergic.

Ultrastructural localization of nitrotyrosine within the caudate-putamen nucleus and the globus pallidus of normal rat brain

Nitration of protein tyrosine residues by nitric oxide (NO)-derived reactive species results in the production of stable nitrotyrosine (NT) moieties that are immunochemically detectable in many regions of normal brain and enriched in those areas containing constitutive nitric oxide synthase (cNOS). These include the caudate-putamen nucleus (CPN) and the globus pallidus, which receives major inhibitory input from the CPN. To determine the functional sites for NT production in these critical motor nuclei, we examined the electron microscopic immunocytochemical localization of NT and cNOS in rat brain. In the CPN, NT was localized to the somata and dendrites of cNOS-containing interneurons and spiny neurons, some of which received input from cNOS-labeled terminals. The NT immunoreactivity was most prevalent on outer mitochondrial membranes and nearby segments of the plasma membranes in dendrites and within asymmetric synapses on dendritic spines. In the CPN and globus pallidus, there was also a prominent labeling of NT in astrocytic processes, small axons, and tubulovesicles and/or synaptic vesicles in axon terminals. These terminals formed mainly asymmetric synapses in the CPN and inhibitory-type synapses in the globus pallidus where they often apposed cNOS-containing terminals that also formed asymmetric, excitatory-type synapses. Our results suggest that NT is generated by mechanisms requiring the dual actions of excitatory transmitters and NO derived either from interneurons in the CPN or from excitatory afferents in the globus pallidus. The findings also implicate NT in the physiological actions of NO within the striatal circuitry and, particularly, in striatopallidal neurons severely affected in Huntington's disease.

mu-opioid receptors are present in vagal afferents and their dendritic targets in the medial nucleus tractus solitarius

Ligands of the mu-opiate receptor (MOR) are known to influence many functions that involve vagal afferent input to the nucleus tractus solitarius (NTS), including cardiopulmonary responses, gastrointestinal activity, and cortical arousal. The current study sought to determine whether a cellular substrate exists for direct modulation of vagal afferents and/or their neuronal targets in the NTS by ligands of the MOR. Anterograde tracing of vagal afferents arising from the nodose ganglion was achieved with biotinylated dextran amine (BDA), and the MOR was detected by using antipeptide MOR antiserum. The medial subdivision of the intermediate NTS was examined by electron microscopy for the presence of peroxidase-labeled, BDA-containing vagal afferents and immunogold MOR labeling. MOR was present in both presynaptic axon terminals and at postsynaptic sites, primarily dendrites. In dendrites, MOR immunogold particles usually were located along extrasynaptic portions of the plasma membrane. Of 173 observed BDA-labeled vagal afferent axon terminals, 33% contained immunogold labeling for MOR within the axon terminal. Many of these BDA-labeled terminals formed asymmetric, excitatory-type synapses with dendrites, some of which contained MOR immunogold labeling. MORs were present in 19% of the dendrites contacted by BDA-labeled terminals but were present rarely in both the vagal afferent and its dendritic target. Together, these results suggest that MOR ligands modulate either the presynaptic release from or the postsynaptic responses to largely separate populations of vagal afferents in the intermediate NTS. These results provide a cellular substrate for direct actions of MOR ligands on primary visceral afferents and their second-order neuronal targets in NTS.

Rostrocaudal variation in targeting of N-methyl-D-aspartate and mu-opioid receptors in the rat medial nucleus of the solitary tract

N-methyl-D-aspartate (NMDA) receptors and mu-opioid receptors (MOR) have been implicated in gustatory and cardiorespiratory visceral reflexes, respectively involving second order sensory neurons in rostral and intermediate portions of the medial nucleus of the solitary tract (mNTS). To determine whether there are cellular sites suggesting functional interaction involving NMDA receptors and MOR in these regions, we examined their ultrastructural immunocytochemical localization by using antisera recognizing the functional subunit of NMDA receptors (NR1) or MOR in rat brain. In both mNTS subdivisions, NR1 labeling was prominently seen along membranes of cytoplasmic organelles in somata and large dendrites, as well as on asymmetric postsynaptic densities in small dendrites and dendritic spines. Many of these profiles also contained MOR immunoreactivity that was mainly distributed along extrasynaptic plasma membranes. Quantitative regional comparison showed that dendrites composed 64% (167 of 261) and 35% (137 of 390) of the dually labeled structures in the rostral and intermediate mNTS, respectively. In contrast, only 11% (28 of 261) of the total dually labeled profiles in the rostral, but 46% (180 of 390) of those in the intermediate mNTS were axon terminals. Many of the terminals containing NR1 and/or MOR were large and formed asymmetric synapses with multiple targets, resembling those features of known visceral afferents. Our results suggest that opioids, active at MOR in mNTS, modulate excitatory visceral reflexes involving mainly postsynaptic NMDA receptors in the rostral region. In addition, they suggest that similar mechanisms exist in the intermediate mNTS, where both NMDA receptors and MOR may differentially regulate the presynaptic release of glutamate from the visceral afferents.

Presynaptic dopamine D(4) receptor localization in the rat nucleus accumbens shell

Dopamine D(4) receptors in the nucleus accumbens shell (AcbSh) are thought to play a key role in mediating the locomotor and sensitizing affects of psychostimulants, as well as the therapeutic efficacy of atypical antipsychotic drugs. We used electron microscopic immunocytochemistry to determine the functional sites for endogenous and exogenous D(4) receptor activation in this region. Of 1,090 D(4) receptor-labeled profiles observed in the AcbSh of rat brain, 65% were axons and axon terminals, while 22% were dendrites and dendritic spines. Within axons and terminals, D(4) receptor immunoreactivity was localized to segments of the plasma membrane and membranes of nearby vesicles. The axon terminals were morphologically heterogenous, varying in size and content of either all small synaptic vesicles (ssv), or ssv and large dense-core vesicles. The labeled terminals occasionally formed asymmetric excitatory-type axospinous synapses, but the majority were without recognizable synaptic specializations. In a separate series of tissue sections that were processed for dual-labeling of the D(4) receptor and the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH), 56% of all observed associations were appositions between differentially labeled axonal profiles, and 17% were terminals that contained immunoreactivity for both antigens. Dendritic spines containing D(4) receptor-labeling also received convergent input from TH-immunoreactive terminals and unlabeled terminals forming asymmetric synapses. These results provide the first ultrastructural evidence for a major presynaptic, and a more minor postsynaptic, involvement of D(4) receptors in dopaminergic modulation of excitatory transmission in the AcbSh.

Ultrastructural localization of the serotonin 2A receptor in dopaminergic neurons in the ventral tegmental area

Serotonin (5-hydroxytryptamine, 5-HT) 2A receptor antagonists are clinically effective antipsychotics that may differentially target mesocortical and mesolimbic dopaminergic neurons having partially segregated distribution in the parabrachial (PB) and paranigral (PN) ventral tegmental area (VTA). We examined the ultrastructural immunocytochemical localization of the 5-HT2A receptor in these subdivisions of rat VTA, to determine (1) the functional sites for receptor activation, and (2) cellular associations between the receptor and dopaminergic neurons identified by their content of tyrosine hydroxylase (TH). The mean area density of neuronal profiles containing 5-HT2A receptor labeling was not significantly different in the PB and PN VTA. In each region approximately 44% of the 5-HT2A labeled profiles were dendrites while the remainder were mainly axons. Dendritic 5-HT2A-immunoreactivity was often localized to membranous cytoplasmic organelles resembling smooth endoplasmic reticulum, and to more rarely to segments of the plasma membrane beneath contacts from unlabeled axon terminals. 5-HT2A labeling was also seen within the cytoplasm of a few axon initial segments and many small unmyelinated axons. Approximately 40% of the 5-HT2A-labeled dendritic profiles contained TH in either PB or PN VTA. Our results suggest that 5-HT2A receptors in VTA are largely cytoplasmic and play an equally important role in modulating dopaminergic neurons in PB and PN VTA. These results also implicate 5-HT2A receptors in the postsynaptic activation of non-dopaminergic neurons and possibly the presynaptic release from terminals of axons originating in, or passing through, these regions.

N-methyl-D-aspartate (NMDA) receptors in the ventral tegmental area: subcellular distribution and colocalization with 5-hydroxytryptamine(2A) receptors

Glutamate receptors of the N-methyl-D-aspartate (NMDA) subtype have been implicated in behavioral sensitization to psychostimulants and in psychotic behaviors involving excitation of ventral tegmental area (VTA) dopaminergic neurons. Antagonists of serotonin (5-hydroxytryptamine, 5-HT) receptors of the 5-HT(2A) subtype are potent antipsychotics that attenuate these NMDA-evoked responses. We examined the electron microscopic immunocytochemical localization of antisera against the NMDA R1 subunit (NMDAR1) and 5-HT(2A) receptors to determine potential sites for their dual activation in the rat paranigral and parabrachial VTA subdivisions that are distinguished, in part, by their respective striatolimbic and cortical projections. In both regions, NMDAR1 immunoreactivity was localized mainly to the cytoplasm of somata and dendrites, and was only occasionally seen near or within excitatory-type asymmetric synapses. Many of the NMDAR1-labeled somata and dendrites also expressed 5-HT(2A) receptors, having a similar, but largely non-overlapping, neuronal distribution. The mean area density of NMDAR1 and dually labeled dendritic profiles was significantly greater in the paranigral than in the parabrachial VTA. NMDAR1 was also present in small axons showing a similar regional difference in area density. No regional difference in area density was seen in dendrites or small axons containing only 5-HT(2A) receptors. Our results indicate that NMDA and 5-HT(2A) receptors in the VTA are transiently expressed on synaptic plasma membranes of single neurons showing widespread cytoplasmic distributions of each of the receptors. They also suggest a major role for NMDA receptors in modulating the output of paranigral neurons and the release of transmitters from axons passing through this region.

Dual ultrastructural localization of mu-opiate receptors and substance p in the dorsal horn

Opiates active at the mu-opiate receptor (MOR) produce antinociception, in part, through actions involving substance P (SP), a peptide present in both unmyelinated primary afferents and interneurons within the dorsal horn. We examined potential functional sites for interactions between SP and MOR by using dual electron microscopic immunocytochemical localization of antisera against SP and a sequence-specific antipeptide antibody against MOR in rat cervical spinal dorsal horn. The distribution was compared with that of the functionally analogous dorsal horn of the trigeminal nucleus caudalis. Many of the SP-immunoreactive terminals in the dorsal horn contacted dendrites that contain MOR (53% in trigeminal; 70% in cervical spinal cord). Conversely, within the cervical spinal dorsal horn 79% of the MOR-labeled dendrites that received any afferent input were contacted by at least one SP-containing axon or terminal. Although SP-immunoreactive dendrites were rare, many of these (48%) contained MOR, suggesting that the activity of SP-containing spinal interneurons may be regulated by MOR ligands. A few SP-labeled terminals also contained MOR (12% in trigeminal; 6% in cervical spinal cord). These data support the idea that MOR ligands produce antinociception primarily through modulation of postsynaptic second-order nociceptive neurons in the dorsal horns of spinal cord and spinal trigeminal nuclei, some of which contain SP. They also suggest, however, that in each region, MOR agonists can act presynaptically to control the release of SP and/or glutamate from afferent terminals. The post- and presynaptic MOR sites are likely to account for the potency of MOR agonists as analgesics.

Dendritic and axonal targeting of the vesicular acetylcholine transporter to membranous cytoplasmic organelles in laterodorsal and pedunculopontine tegmental nuclei

Autoregulation of cholinergic neurons in the laterodorsal tegmental (LDT) and pedunculopontine (PPT) nuclei has been implicated in many functions, most importantly in drug reinforcement and in the pathophysiology of schizophrenia. This autoregulation is attributed to the release of acetylcholine, but neither the storage or release sites are known. To determine these sites, we used electron microscopy for the immunocytochemical localization of antipeptide antiserum raised against the vesicular acetylcholine transporter (VAchT) that is responsible for the uptake of acetylcholine into storage vesicles. The cellular and subcellular distribution of VAchT was remarkably similar in the two regions by by using each of two methods, immunogold and immunoperoxidase. In both PPT and LDT nuclei, VAchT labeling was seen mainly on membranous organelles including the trans-Golgi network in many somata. VAchT-immunoreactive tubulovesicles resembling saccules of smooth endoplasmic reticulum were often seen near the plasma membrane in dendrites. The VAchT-containing dendrites comprised almost 50% of the labeled profiles (1027/2129) in PPT and LDT nuclei. The remaining VAchT-immunoreactive profiles were primarily small unmyelinated axons and axon terminals. In axon terminals, VAchT was densely localized to membranes of small synaptic vesicles. The VAchT-immunoreactive axon terminals formed either symmetric or asymmetric synapses. The postsynaptic targets of these axon terminals included dendrites that were with (36/110) or without (74/110) VAchT immunoreactivity. Our results suggest that dendrites, as well as axon terminals, have the potential for storage and release of acetylcholine in the LDT and PPT nuclei. The released acetylcholine is likely to play a major role in autoregulation of mesopontine cholinergic neurons.

Presence of NMDA-type glutamate receptors in cingulate corticostriatal terminals and their postsynaptic targets

The glutamatergic projection from the anterior cingulate cortex to the medial caudate-putamen nucleus (CPN) has been implicated in motor and cognitive functions, many of which are potently modulated by activation of N-methyl-D-aspartate subtype of glutamate receptors (NMDARs). To determine the functional sites for NMDAR activation within this circuitry, we combined anterograde transport of biotinylated dextran amine (BDA) from deep layers of the rat anterior cingulate cortex with immunogold labeling of NMDAR subunit, NMDAR1, in the dorsomedial CPN. BDA-containing axons were seen in patch-like clusters in a neuropil that showed more uniform immunogold-silver labeling for NMDAR1. Electron microscopy of these regions showed that BDA-labeling was present exclusively in axons and terminals, 23% (98 of 421) of which also contained NMDAR1-immunoreactivity (IR). BDA-labeled terminals often apposed NMDAR1-immunoreactive neuronal and glial profiles. These terminals also formed asymmetric excitatory-type synapses with dendritic spines. Of 155 anterogradely labeled axon terminals forming asymmetric synapses, 34% were with NMDAR1-labeled, and 66% with unlabeled dendritic spines. These results provide ultrastructural evidence for the involvement of NMDARs in presynaptic regulation of glutamate transmission, and in postsynaptic modulation of the excitability of spiny neurons in patch-like compartments of the dorsomedial CPN. These dual NMDAR-mediated actions are likely to play a major role in the acquisition of new behaviors and reward-related processes that have been associated with cortical input to the striatal patch compartments.

Anatomical substrates for baroreflex sympathoinhibition in the rat

The fundamental neuronal substrates of the arterial baroreceptor reflex have been elucidated by combining anatomical, neurophysiological, and pharmacological approaches. A serial pathway between neurons located in the nuclei of the solitary tract (NTS), the caudal ventrolateral medulla (CVL), and the rostral ventrolateral medulla (RVL) plays a critical role in inhibition of sympathetic outflow following stimulation of baroreceptor afferents. In this paper, we summarize our studies using tract-tracing and electron microscopic immunocytochemistry to define the potential functional sites for synaptic transmission within this circuitry. The results are discussed as they relate to the literature showing: (1) baroreceptor afferents excite second-order neurons in NTS through the release of glutamate; (2) these NTS neurons in turn send excitatory projections to neurons in the CVL; (3) GABAergic CVL neurons directly inhibit RVL sympathoexcitatory neurons; and (4) activation of this NTS-->CVL-->RVL pathway leads to disfacilitation of sympathetic preganglionic neurons by promoting withdrawal of their tonic excitatory drive, which largely arises from neurons in the RVL. Baroreceptor control may also be regulated over direct reticulospinal pathways exemplified by a newly recognized sympathoinhibitory region of the medulla, the gigantocellular depressor area. This important autonomic reflex may also be influenced by parallel, multiple, and redundant networks.

Subcellular distribution of 5-hydroxytryptamine2A and N-methyl-D-aspartate receptors within single neurons in rat motor and limbic striatum

The dorsolateral caudate-putamen nucleus (CPN) and the nucleus accumbens (NAc) shell, respectively, are involved in many motor and limbic functions that are affected by activation of the 5-hydroxytryptamine2A receptor (5HT2AR) and the N-methyl-D-aspartate subtype of glutamate receptor (NMDAR). We examined the functional sites for 5HT2AR activation and potential interactions involving the NMDAR subunit NR1 (NMDAR1) within these striatal regions. For this examination, sequence-specific antipeptide antisera against these receptors were localized by electron microscopic dual-labeling immunocytochemistry in the rat brain. In the dorsolateral CPN and the NAc shell, the 5HT2AR-labeled profiles were mainly dendrites, but somata and axons were also immunoreactive. The neuronal somata contained round unindented nuclei that are typical of spiny striatal neurons, although few dendritic spines were 5HT2AR immunolabeled. In all neuronal profiles, the 5HT2AR labeling was primarily associated with cytoplasmic organelles and more rarely was localized to synaptic or nonsynaptic plasma membranes. Colocalization of 5HT2AR and NMDAR1 was seen primarily in somata and dendrites. Significantly greter numbers of 5HT2AR- or 5HT2AR- and NMDAR1-containing dendrites were seen in the dorsolateral CPN than in the NAc shell. As compared with 5HT2AR, NMDAR1 labeling was more often observed in dendritic spines, and these were also more numerous in the CPN. These results indicate that 5HT2A and NMDA receptors are coexpressed but differentially targeted in single spiny striatal neurons and are likely to play a major role in control of motor functions involving the dorsolateral CPN.

Localization of the delta-opioid receptor and dopamine transporter in the nucleus accumbens shell: implications for opiate and psychostimulant cross-sensitization

Opiate- and psychostimulant-induced modulation of dopamine transmission in the nucleus accumbens shell (AcbSh) is thought to play a key role in their potent reinforcing and locomotor effects. To investigate the cellular basis for potential functional interactions involving opiates active at the delta-opioid receptor (DOR) and psychostimulants that bind selectively to the dopamine transporter (DAT), we examined the electron microscopic localization of their respective antisera in rat AcbSh. DOR immunoperoxidase labeling was seen primarily, and DAT immunogold particles exclusively, in axon terminals. In these terminals, DOR immunoreactivity was prominently associated with discrete segments of the plasma membrane and the membranes of nearby small synaptic and large dense core vesicles. DAT immunogold particles were almost exclusively distributed along nonsynaptic axonal plasma membranes. Thirty-nine percent DOR-labeled profiles (221/566) either apposed DAT-immunoreactive terminals or also contained DAT. Of these 221 DOR-labeled profiles, 13% were axon terminals containing DAT and 15% were dendritic spines apposed to DAT-immunoreactive terminals. In contrast, 70% were morphologically heterogeneous axon terminals and small axons apposed to DAT-immunoreactive terminals. Our results indicate that DOR agonists in the AcbSh can directly modulate the release of dopamine, as well as postsynaptic responses in spiny neurons that receive dopaminergic input, but act principally to control the presynaptic secretion of other neurotransmitters whose release may influence or be influenced by extracellular dopamine. Thus, while opiates and psychostimulants mainly have differential sites of action, cross-sensitization of their addictive properties may occur through common neuronal targets.