Altered dendritic distribution of dopamine D2 receptors and reduction in mitochondrial number in parvalbumin-containing interneurons in the medial prefrontal cortex of cannabinoid-1 (CB1) receptor knockout mice

The prelimbic prefrontal cortex (PL) is a brain region integral to complex behaviors that are highly influenced by cannabinoids and by dopamine D2 receptor (D2R)-mediated regulation of fast-firing parvalbumin-containing interneurons. We have recently shown that constitutive deletion of the cannabinoid-1 receptor (CB1R) greatly reduces parvalbumin levels in these neurons. The effects of CB1R deletion on PL parvalbumin interneurons may be ascribed to loss of CB1R-mediated retrograde signaling on mesocortical dopamine transmission, and, in turn, altered expression and/or subcellular distribution of D2R in the PL. Furthermore, diminished parvalbumin expression could indicate metabolic changes in fast-firing interneurons that may be reflected in changes in mitochondrial density in this population. We therefore comparatively examined electron microscopic dual labeling of D2R and parvalbumin in CB1 (-/-) and CB1 (+/+) mice to test the hypothesis that absence of CB1R produces changes in D2R localization and mitochondrial distribution in parvalbumin-containing interneurons of the PL. CB1 (-/-) mice had a significantly lower density of cytoplasmic D2R-immunogold particles in medium parvalbumin-labeled dendrites and a concomitant increase in the density of these particles in small dendrites. These dendrites received both excitatory and inhibitory-type synapses from unlabeled terminals and contained many mitochondria, whose numbers were significantly reduced in CB1 (-/-) mice. Non-parvalbumin dendrites showed no between-group differences in either D2R distribution or mitochondrial number. These results suggest that cannabinoid signaling provides an important determinant of dendritic D2 receptor distribution and mitochondrial availability in fast-spiking interneurons.

The influences of reproductive status and acute stress on the levels of phosphorylated delta opioid receptor immunoreactivity in rat hippocampus

In the hippocampus, ovarian hormones and sex can alter the trafficking of delta opioid receptors (DORs) and the proportion of DORs that colocalize with the stress hormone, corticotropin releasing factor. Here, we assessed the effects of acute immobilization stress (AIS) and sex on the phosphorylation of DORs in the rat hippocampus. We first localized an antibody to phosphorylated DOR (pDOR) at the SER363 carboxy-terminal residue, and demonstrated its response to an opioid agonist. By light microscopy, pDOR-immunoreactivity (ir) was located predominantly in CA2/CA3a pyramidal cell apical dendrites and in interneurons in CA1-3 stratum oriens and the dentate hilus. By electron microscopy, pDOR-ir primarily was located in somata and dendrites, associated with endomembranes, or in dendritic spines. pDOR-ir was less frequently found in mossy fibers terminals. Quantitative light microscopy revealed a significant increase in pDOR-ir in the CA2/CA3a region of male rats 1h following an injection of the opioid agonist morphine (20mg/kg, I.P). To look at the effects of stress on pDOR, we compared pDOR-ir in males and cycling females after AIS. The level of pDOR-ir in stratum radiatum of CA2/CA3a was increased in control estrus (elevated estrogen and progesterone) females compared to proestrus and diestrus females and males. However, immediately following 30min of AIS, no significant differences in pDOR levels were seen across estrous cycle phase or sex. These findings suggest that hippocampal levels of phosphorylated DORs vary with estrous cycle phase and that acute stress may dampen the differential effects of hormones on DOR activation in females.

Angiotensin II slow-pressor hypertension enhances NMDA currents and NOX2-dependent superoxide production in hypothalamic paraventricular neurons

Adaptive changes in glutamatergic signaling within the hypothalamic paraventricular nucleus (PVN) may play a role in the neurohumoral dysfunction underlying the hypertension induced by "slow-pressor" ANG II infusion. We hypothesized that these adaptive changes alter production of gp91phox NADPH oxidase (NOX)-derived reactive oxygen species (ROS) or nitric oxide (NO), resulting in enhanced glutamatergic signaling in the PVN. Electron microscopic immunolabeling showed colocalization of NOX2 and N-methyl-D-aspartate receptor (NMDAR) NR1 subunits in PVN dendrites, an effect enhanced (+48%, P < 0.05 vs. saline) in mice receiving ANG II (600 ng·kg⁻¹·min⁻¹ sc). Isolated PVN cells or spinally projecting PVN neurons from ANG II-infused mice had increased levels of ROS at baseline (+40 ± 5% and +57.6 ± 7.7%, P < 0.01 vs. saline) and after NMDA (+24 ± 7% and +17 ± 5.5%, P < 0.01 and P < 0.05 vs. saline). In contrast, ANG II infusion suppressed NO production in PVN cells at baseline (-29.1 ± 5.2%, P < 0.05 vs. saline) and after NMDA (-18.9 ± 2%, P < 0.01 vs. saline), an effect counteracted by NOX inhibition. In whole cell recording of unlabeled and spinally labeled PVN neurons in slices, NMDA induced a larger inward current in ANG II than in saline groups (+79 ± 24% and +82.9 ± 6.6%, P < 0.01 vs. saline), which was reversed by the ROS scavenger MnTBAP and the NO donor S-nitroso-N-acetylpenicillamine (P > 0.05 vs. control). These findings suggest that slow-pressor ANG II increases the association of NR1 with NOX2 in dendrites of PVN neurons, resulting in enhanced NOX-derived ROS and reduced NO during glutamatergic activity. The resulting enhancement of NMDAR activity may contribute to the neurohumoral dysfunction underlying the development of slow-pressor ANG II hypertension.

Deletion of the NMDA-NR1 receptor subunit gene in the mouse nucleus accumbens attenuates apomorphine-induced dopamine D1 receptor trafficking and acoustic startle behavior

The nucleus accumbens (Acb) contains subpopulations of neurons defined by their receptor content and potential involvement in sensorimotor gating and other behaviors that are dysfunctional in schizophrenia. In Acb neurons, the NMDA NR1 (NR1) subunit is coexpressed not only with the dopamine D1 receptor (D1R), but also with the µ-opioid receptor (µ-OR), which mediates certain behaviors that are adversely impacted by schizophrenia. The NMDA-NR1 subunit has been suggested to play a role in the D1R trafficking and behavioral dysfunctions resulting from systemic administration of apomorphine, a D1R and dopamine D2 receptor agonist that impacts prepulse inhibition to auditory-evoked startle (AS). Together, this evidence suggests that the NMDA receptor may regulate D1R trafficking in Acb neurons, including those expressing µ-OR, in animals exposed to auditory startle and apomorphine. We tested this hypothesis by combining spatial-temporal gene deletion technology, dual labeling immunocytochemistry, and behavioral analysis. Deleting NR1 in Acb neurons prevented the increase in the dendritic density of plasma membrane D1Rs in single D1R and dual (D1R and µ-OR) labeled dendrites in the Acb in response to apomorphine and AS. Deleting NR1 also attenuated the decrease in AS induced by apomorphine. In the absence of apomorphine and startle, deletion of Acb NR1 diminished social interaction, without affecting novel object recognition, or open field activity. These results suggest that NR1 expression in the Acb is essential for apomorphine-induced D1R surface trafficking, as well as auditory startle and social behaviors that are impaired in multiple psychiatric disorders.

The impact of adolescent social isolation on dopamine D2 and cannabinoid CB1 receptors in the adult rat prefrontal cortex

Adolescent experiences of social deprivation result in profound and enduring perturbations in adult behavior, including impaired sensorimotor gating. The behavioral deficits induced by adolescent social isolation in rats can be ameliorated by antipsychotic drugs blocking dopamine D2 receptors in the prefrontal cortex (PFC) or by chronic administration of a cannabinoid CB1 receptor antagonist. The patterning and abundance of D2 receptors in the PFC evolves concurrently with CB1 receptors through the period of adolescence. This evidence suggests that mature expression and/or surface distribution of D2 and CB1 receptors may be influenced by the adolescent social environment. We tested this hypothesis using electron microscopic immunolabeling to compare the distribution of CB1 and D2 receptors in the PFC of adult male Sprague-Dawley rats that were isolated or socially reared throughout the adolescent transition period. Prepulse inhibition (PPI) of acoustic startle was assessed as a measure of sensorimotor gating. Social isolation reduced PPI and selectively decreased dendritic D2 immunogold labeling in the PFC. However, the decrease was only evident in dendrites that were not contacted by axon terminals containing CB1. There was no apparent change in the expression of CB1 or D2 receptors in presynaptic terminals. The D2 deficit therefore may be tempered by local CB1-mediated retrograde signaling. This suggests a biological mechanism whereby the adolescent social environment can persistently influence cortical dopaminergic activity and resultant behavior.

Distribution of angiotensin type 1a receptor-containing cells in the brains of bacterial artificial chromosome transgenic mice

In the central nervous system, angiotensin II (AngII) binds to angiotensin type 1 receptors (AT(1)Rs) to affect autonomic and endocrine functions as well as learning and memory. However, understanding the function of cells containing AT(1)Rs has been restricted by limited availability of specific antisera, difficulties discriminating AT(1)R-immunoreactive cells in many brain regions and, the identification of AT(1)R-containing neurons for physiological and molecular studies. Here, we demonstrate that an Agtr1a bacterial artificial chromosome (BAC) transgenic mouse line that expresses type A AT(1)Rs (AT1aRs) identified by enhanced green fluorescent protein (EGFP) overcomes these shortcomings. Throughout the brain, AT1aR-EGFP was detected in the nuclei and cytoplasm of cells, most of which were neurons. EGFP often extended into dendritic processes and could be identified either natively or with immunolabeling of GFP. The distribution of AT1aR-EGFP cells in brain closely corresponded to that reported for AngII binding and AT1aR protein and mRNA. In particular, AT1aR-EGFP cells were in autonomic regions (e.g., hypothalamic paraventricular nucleus, central nucleus of the amygdala, parabrachial nucleus, nuclei of the solitary tract and rostral ventrolateral medulla) and in regions involved in electrolyte and fluid balance (i.e., subfornical organ) and learning and memory (i.e., cerebral cortex and hippocampus). Additionally, dual label electron microscopic studies in select brain areas demonstrate that cells containing AT1aR-EGFP colocalize with AT(1)R-immunoreactivity. Assessment of AngII-induced free radical production in isolated EGFP cells demonstrated feasibility of studies investigating AT1aR signaling ex vivo. These findings support the utility of Agtr1a BAC transgenic reporter mice for future studies understanding the role of AT(1)R-containing cells in brain function.

Cannabinoid-1 receptors in the mouse ventral pallidum are targeted to axonal profiles expressing functionally opposed opioid peptides and contacting N-acylphosphatidylethanolamine-hydrolyzing phospholipase D terminals

The ventral pallidum (VP) is a major recipient of inhibitory projections from nucleus accumbens (Acb) neurons that differentially express the reward (enkephalin) and aversion (dynorphin)-associated opioid peptides. The cannabinoid-1 receptor (CB1R) is present in Acb neurons expressing each of these peptides, but its location in the VP is not known. To address this question, we used electron microscopic dual immunolabeling of the CB1R and either dynorphin 1-8 (Dyn) or Met(5)-enkephalin (ME) in the VP of C57BL/6J mice, a species in which CB1R gene deletion produces a reward deficit. We also used similar methods to determine the relationship between the CB1R and N-acylphosphatidylethanolamine (NAPE)-hydrolyzing phospholipase D (NAPE-PLD), an anandamide-synthesizing enzyme located presynaptically in other limbic brain regions. CB1R-immunogold was principally localized to cytoplasmic endomembranes and synaptic or extrasynaptic plasma membranes of axonal profiles, but was also affiliated with postsynaptic membrane specializations in dendrites. The axonal profiles included many single CB1R-labeled axon terminals as well as terminals containing CB1R-immunogold and either Dyn or ME immunoreactivity. Dually labeled terminals comprised 26% of all Dyn- and 17% of all ME-labeled axon terminals. Both single- and dual-labeled terminals formed mainly inhibitory-type synapses, but almost 16% of these terminals formed excitatory synapses. Approximately 60% of the CB1R-labeled axonal profiles opposed or converged with axon terminals containing NAPE-PLD immunoreactivity. We conclude that CB1Rs in the mouse VP have subcellular distributions consistent with on demand activation by endocannabinoids that can regulate the release of functionally opposed opioid peptides and also modulate inhibitory and excitatory transmission.

Cannabinoid modulation of the dopaminergic circuitry: implications for limbic and striatal output

Cannabinoid modulation of dopaminergic transmission is suggested by the ability of delta9-tetrahydrocanabinoid to affect motor and motivated behaviors in a manner similar to that produced by pharmacological manipulation of the nigrostriatal and mesocorticolimbic dopamine systems. These behavioral effects as well as analogous effects of endocannabinoids are largely mediated through the cannabinoid type 1 receptor (CB1R). This receptor is located within the substantia nigra and ventral tegmental area, which respectively house the somata of nigrostriatal and mesocorticolimbic dopaminergic neurons. The CB1R is also abundantly expressed in brain regions targeted by the efferent terminals of these dopaminergic neurons. In this review we present the accumulating anatomical and electrophysiological evidence indicating that in each of these systems cannabinoids modulate dopamine transmission largely if not exclusively through indirect mechanisms. The summarized mechanisms include presynaptic release of amino acid transmitters onto midbrain dopamine neurons and onto both cortical and striatal neurons that express dopamine D1-like or D2-like receptors functionally affiliated with the CB1 receptor. The review concludes with a consideration of the psychiatric and neurological implications of cannabinoid modulation of dopamine transmission within these networks.

Endothelin 1-dependent neurovascular dysfunction in chronic intermittent hypoxia

Obstructive sleep apnea, a condition resulting in chronic intermittent hypoxia (CIH), is an independent risk factor for stroke and dementia, but the mechanisms of the effect are unknown. We tested the hypothesis that CIH increases cerebrovascular risk by altering critical mechanisms regulating cerebral blood flow thereby lowering cerebrovascular reserves. Male C57Bl6/J mice were subjected to CIH (10% O(2) for 90 seconds/room air for 90 seconds; during sleep hours) or sham treatment for 35 days. Somatosensory cortex blood flow was assessed by laser Doppler flowmetry in anesthetized mice equipped with a cranial window. CIH increased mean arterial pressure (from 74±2 to 83±3 mm Hg; P<0.05) and attenuated the blood flow increase produced by neural activity (whisker stimulation; -39±2%; P<0.05) or neocortical application of endothelium-dependent vasodilators (acetylcholine response: -41±3%; P<0.05). The cerebrovascular dysfunction was associated with oxidative stress in cerebral resistance arterioles and was abrogated by free radical scavenging or NADPH oxidase inhibition. Furthermore, cerebrovascular dysfunction and free radical increase were not observed in mice lacking the NOX2 subunit of NADPH oxidase. CIH markedly increased endothelin 1 in cerebral blood vessels, whereas cerebrovascular dysfunction and oxidative stress were abrogated by neocortical application of the endothelin type A receptor antagonist BQ123. These data demonstrate for the first time that CIH alters key regulatory mechanisms of the cerebral circulation through endothelin 1 and NADPH oxidase-derived radicals. The ensuing cerebrovascular dysfunction may increase stroke risk in patients with sleep apnea by reducing cerebrovascular reserves and increasing the brain's susceptibility to cerebral ischemia.

Angiotensin II type 2 receptor-coupled nitric oxide production modulates free radical availability and voltage-gated Ca2+ currents in NTS neurons

The medial region of the nucleus tractus solitarius (mNTS) is a key brain stem site controlling cardiovascular function, wherein ANG II modulates neuronal L-type Ca(2+) currents via activation of ANG II type 1 receptors (AT(1)R) and production of reactive oxygen species (ROS). ANG II type 2 receptors (AT(2)R) induce production of nitric oxide (NO), which may interact with ROS and modulate AT(1)R signaling. We sought to determine whether AT(2)R-mediated NO production occurs in mNTS neurons and, if so, to elucidate the NO source and the functional interaction with AT(1)R-induced ROS or Ca(2+) influx. Electron microscopic (EM) immunolabeling showed that AT(2)R and neuronal NO synthase (nNOS) are coexpressed in neuronal somata and dendrites receiving synapses in the mNTS. In the presence of the AT(1)R antagonist losartan, ANG II increased NO production in isolated mNTS neurons, an effect blocked by the AT(2)R antagonist PD123319, but not the angiotensin (1-7) antagonist D-Ala. Studies in mNTS neurons of nNOS-null or endothelial NOS (eNOS)-null mice established nNOS as the source of NO. ANG II-induced ROS production was enhanced by PD123319, the NOS inhibitor N(G)-nitro-l-arginine (LNNA), or in nNOS-null mice. Moreover, in the presence of losartan, ANG II reduced voltage-gated L-type Ca(2+) current, an effect blocked by PD123319 or LNNA. We conclude that AT(2)R are closely associated and functionally coupled with nNOS in mNTS neurons. The resulting NO production antagonizes AT(1)R-mediated ROS and dampens L-type Ca(2+) currents. The ensuing signaling changes in the NTS may counteract the deleterious effects of AT(1)R on cardiovascular function.

The link between angiotensin II-mediated anxiety and mood disorders with NADPH oxidase-induced oxidative stress

The renin-angiotensin system (RAS) and its active peptide angiotensin II (AngII) have major involvements not only in hypertension but also in mood and anxiety disorders. Substantial evidence supports the notion that AngII acts as a neuromodulator in the brain. In this review, we provide an overview of the link between the RAS and anxiety or mood disorders, and focus on recent advances in the understanding of AngII-linked, NADPH oxidase-derived oxidative stress in the central nervous system, which may underlie pathogenesis of mood and anxiety disorders.

Quinpirole elicits differential in vivo changes in the pre- and postsynaptic distributions of dopamine D₂ receptors in mouse striatum: relation to cannabinoid-1 (CB₁) receptor targeting

RATIONALE

The nucleus accumbens (Acb) shell and caudate-putamen nucleus (CPu) are respectively implicated in the motivational and motor effects of dopamine, which are mediated in part through dopamine D₂-like receptors (D₂Rs) and modulated by activation of the cannabinoid-1 receptor (CB₁R). The dopamine D(₂/D3) receptor agonist, quinpirole elicits internalization of D₂Rs in isolated cells; however, dendritic and axonal targeting of D₂Rs may be highly influenced by circuit-dependent changes in vivo and potentially influenced by endogenous CB₁R activation.

OBJECTIVE

We sought to determine whether quinpirole alters the surface/cytoplasmic partitioning of D₂Rs in striatal neurons in vivo.

METHODS

To address this question, we examined the electron microscopic immunolabeling of D₂ and CB₁ receptors in the Acb shell and CPu of male mice at 1 h following a single subcutaneous injection of quinpirole (0.5 mg/kg) or saline, a time point when quinpirole reduced locomotor activity.

RESULTS

Many neuronal profiles throughout the striatum of both treatment groups expressed the D₂R and/or CB₁R. As compared with saline, quinpirole-injected mice showed a significant region-specific decrease in the plasmalemmal and increase in the cytoplasmic density of D₂R-immunogold particles in postsynaptic dendrites without CB₁R-immunolabeling in the Acb shell. However, quinpirole produced a significant increase in the plasmalemmal density of D₂R immunogold in CB₁R negative axons in both the Acb shell and CPu.

CONCLUSIONS

Our results provide in vivo evidence for agonist-induced D₂R trafficking that is inversely related to CB₁R distribution in postsynaptic neurons of Acb shell and in presynaptic axons in this region and in the CPu.

Insights to drug addiction derived from ultrastructural views of the mesocorticolimbic system

Drugs of abuse increase the release of dopamine from mesocorticolimbic neurons in the ventral tegmental area. Thus, insights into the cytoarchitecture and the synaptic circuitry affecting the activity of dopaminergic neurons in this area are fundamental for understanding the commonalities produced by mechanistically distinct drugs of abuse. Electron microscopic immunolabeling has provided these insights and also shown the critical relationships between the dopaminergic axon terminals and their targeted neurons in the prefrontal cortex and in the both the dorsal and ventral striatum. These brain regions are among those where dopamine and associated neurotransmitters are most implicated in the transition from recreational to compulsive consumption of reinforcing drugs. Thus, the synaptic circuitry and drug-induced plasticity occurring in the ventral tegmental area and in dopamine-targeted regions are reviewed, as both are essential for understanding the long-lasting changes produced by addictive substances.

Acetylcholine α7 nicotinic and dopamine D2 receptors are targeted to many of the same postsynaptic dendrites and astrocytes in the rodent prefrontal cortex

The alpha-7 nicotinic acetylcholine receptor (α7nAChR) and the dopamine D(2) receptor (D(2) R) are both implicated in attentional processes and cognition, mediated in part through the prefrontal cortex (PFC). We examined the dual electron microscopic immunolabeling of α7nAChR and either D(2) R or the vesicular acetylcholine transporter (VAChT) in rodent PFC to assess convergent functional activation sites. Immunoreactivity (ir) for α7nAChR and/or D(2) R was seen in the same as well as separate neuronal and glial profiles. At least half of the dually labeled profiles were somata and dendrites, while most labeled axon terminals expressed only D(2) R-ir. The D(2) R-labeled terminals were without synaptic specializations or formed inhibitory or excitatory-type synapses with somatodendritic profiles, some of which expressed the α7nAChR and/or D(2) R. Astrocytic glial processes comprised the majority of nonsomatodendritic α7nAChR or α7nAChR and D(2) R-labeled profiles. Glial processes containing α7nAChR-ir were frequently located near VAChT-labeled terminals and also showed perisynaptic and perivascular associations. We conclude that in rodent PFC α7nACh and D(2) R activation can dually modulate (1) postsynaptic dendritic responses within the same or separate but synaptically linked neurons in which the D(2) R has the predominately presynaptic distribution, and (2) astrocytic signaling that may be crucial for synaptic transmission and functional hyperemia.

Cannabinoid-1 receptor gene deletion has a compartment-specific affect on the dendritic and axonal availability of μ-opioid receptors and on dopamine axons in the mouse nucleus accumbens

Cannabinoid-type 1 (CB1) receptors are implicated in μ-opioid receptor (μ-OR)-dependent reward ascribed partially to mesolimbic dopamine release in the nucleus accumbens (Acb) shell. Thus, CB1 receptor gene deletion may preferentially alter the availability of μ-ORs and/or dopamine innervation in this brain region, which is functionally distinct from the motor-associated Acb core. To test this hypothesis, we examined the electron microscopic immunolabeling of the μ-OR and the dopamine-synthesizing enzyme, tyrosine hydroxylase (TH) in Acb shell, and core of adult C57BL/6J wild-type (WT) and CB1-knock-out (KO) mice. The μ-OR-immunogold particles were observed in the cytoplasm and on the plasmalemma in dendrites, dendritic spines, and axon terminals throughout the Acb. Compared to WT, the Acb shell of CB1-KO mice showed a lower cytoplasmic density of μ-ORs in dendrites and fewer μ-OR labeled, but not unlabeled, dendritic spines. In this region, the CB1-KO's had a significantly enhanced plasmalemmal density of μ-OR-immunogold in axon terminals, 70% of which formed excitatory-type synapses. However, the number of both μ-OR-labeled terminals and TH-labeled small varicosities was significantly reduced in the Acb shell of CB1-KO's. These adaptations were not seen in the Acb core, where CB1-KO's had a preferentially lower dendritic plasmalemmal and total spine density of μ-OR immunogold. Our results indicate that constitutive deletion of the CB1 receptor gene has a major impact on the pre and postsynaptic availability of μ-ORs at axospinous synapses and on the dopamine innervation of the Acb shell as well as the dendritic surface expression of μ-ORs in Acb core of mature rodents.

Decreased parvalbumin immunoreactivity in the cortex and striatum of mice lacking the CB1 receptor

Cortical and striatal regions of the brain contain high levels of the cannabinoid-1 (CB1) receptor, the central neuronal mediator of activity-dependent synaptic plasticity evoked by endocannabinoids. The expression levels of parvalbumin, a calcium-binding protein found in fast-spiking interneurons of both regions, may be controlled in part by synaptic activity during critical periods of development. However, there is currently no evidence that CB1 receptor expression affects parvalbumin levels in either cortical or striatal interneurons. To assess this possibility, we examined parvalbumin immunoreactivity in the dorsolateral striatum, primary motor cortex (M1), and prefrontal cortex (PFC) of CB1 knockout and wild-type C57/BL6 mice. Quantitative densitometry showed a significant decrease in parvalbumin immunoreactivity within individual neurons in each of these regions of CB1 knockout mice relative to controls. A significantly lower density (number of cells per unit area) of parvalbumin-labeled neurons was observed in the striatum, but not the cortical regions of CB1 knockout mice. These findings suggest that CB1 receptor deletion may elicit a compensatory mechanism for network homeostasis affecting parvalbumin-containing cortical and striatal interneurons.

Enhanced dendritic availability of μ-opioid receptors in inhibitory neurons of the extended amygdala in mice deficient in the corticotropin-releasing factor-1 receptor

Activation of the corticotropin-releasing factor-1 (CRF-1) receptor in the anterolateral BNST (BSTal), a key subdivision of the extended amygdala, elicits opiate-seeking behavior exacerbated by stress. However, it is unknown whether the presence of CRF-1 affects expression of the μ-opioid receptor (μ-OR) in the many GABAergic BSTal neurons implicated in the stress response. We hypothesized that deletion of the CRF-1 receptor gene would alter the density and/or subcellular distribution of μ-ORs in GABAergic neurons of the BSTal. We used electron microscopy to quantitatively examine μ-OR immunogold and γ-aminobutyric acid (GABA) immunoperoxidase labeling in the BSTal of CRFr-1 knockout (KO) compared to wild-type (WT) mice. To assess regional specificity, we examined μ-OR distribution in dorsal striatum. The μ-ORs in each region were predominantly localized in dendrites, many of which were GABA-immunoreactive. Significantly, more cytoplasmic μ-OR gold particles per dendritic area were observed selectively in GABA-containing dendrites of the BSTal, but not of the dorsal striatum, in KO compared to WT mice. In both regions, however, significantly fewer GABA-immunoreactive axon terminals were present in KO compared to WT mice. Our results suggest that the absence of CRF-1 results in enhanced expression and/or dendritic trafficking of μ-ORs in inhibitory BSTal neurons. They also suggest that the expression of CRF-1 is a critical determinant of the availability of GABA in functionally diverse brain regions. These findings underscore the complex interplay between CRF, opioid, and GABA systems in limbic and striatal regions and have implications for the role of CRF-1 in influencing the pharmacological effects of opiates active at μ-ORs.

Differential glutamate AMPA-receptor plasticity in subpopulations of VTA neurons in the presence or absence of residual cocaine: implications for the development of addiction

Cocaine-induced plasticity of mesocorticolimbic dopamine (DA) neurons, originating in the ventral tegmental area (VTA), persists in the absence of cocaine and may contribute to both drug-craving and relapse. Glutamate AMPA receptors (AMPARs) in these neurons are implicated in this plasticity. However, there is no ultrastructural evidence that the absence of cocaine following repeated administrations affects the critical surface/synaptic availability of AMPAR GluR1 subunits in either DA or non-DA, putative GABAergic neurons within the VTA. To assess this, we used electron microscopic immunolabeling in the VTA of adult male mice sacrificed at 30 min or 72 h after receiving the final of six (15 mg/kg) cocaine injections, a dosing paradigm that resulted in development of locomotor sensitization. At each time point, both cocaine- and saline-injected mice showed AMPAR GluR1 immunogold labeling in somatodendritic profiles, many of which contained immunoperoxidase labeling for the DA-synthesizing enzyme, tyrosine hydroxylase (TH). At 30 min after the last injection, when cocaine was systemically present, only the non-TH labeled dendrites showed a significant increase in the synaptic/plasmalemmal density of GluR1 immunogold particles. At 72 h, when systemic cocaine was depleted, synaptic GluR1 labeling was greatly enhanced in TH-containing dendrites throughout the VTA and in non-TH dendrites of the limbic-associated paranigral VTA. Our results demonstrate that systemic cocaine produces GluR1 trafficking specifically in non-DA neurons of the VTA, which may subsequently contribute to the abstinent-induced enhancement of AMPA receptor synaptic transmission in mesocorticolimbic DA neurons leading to heightened drug seeking behavior.

Acute and chronic cocaine differentially alter the subcellular distribution of AMPA GluR1 subunits in region-specific neurons within the mouse ventral tegmental area

Cocaine administration increases AMPA GluR1 expression and receptor-mediated activation of the ventral tegmental area (VTA). Functionality is determined, however, by surface availability of these receptors in transmitter- and VTA-region-specific neurons, which may also be affected by cocaine. To test this hypothesis, we used electron microscopic immunolabeling of AMPA GluR1 subunits and tyrosine hydroxylase (TH), the enzyme needed for dopamine synthesis, in the cortical-associated parabrachial (PB) and in the limbic-associated paranigral (PN) VTA of adult male C57BL/6 mice receiving either a single injection (acute) or repeated escalating-doses for 14 days (chronic) of cocaine. Acute cocaine resulted in opposing VTA-region-specific changes in TH-containing dopaminergic dendrites. TH-labeled dendrites within the PB VTA showed increased cytoplasmic GluR1 immunogold particle density consistent with decreased AMPA receptor-mediated glutamatergic transmission. Conversely, TH-labeled dendrites within the PN VTA showed greater surface expression of GluR1 with increases in both synaptic and plasmalemmal GluR1 immunogold density after a single injection of cocaine. These changes diminished in both VTA subregions after chronic cocaine administration. In contrast, non-TH-containing, presumably GABAergic dendrites showed VTA-region-specific changes only after repeated cocaine administration such that synaptic GluR1 decreased in the PB, but increased in the PN VTA. Taken together, these findings provide ultrastructural evidence suggesting that chronic cocaine not only reverses the respective depression and facilitation of mesocortical (PB) and mesolimbic (PN) dopaminergic neurons elicited by acute cocaine, but also differentially affects synaptic availability of these receptors in non-dopaminergic neurons of each region. These adaptations may contribute to increased cocaine seeking/relapse and decreased reward that is reported with chronic cocaine use.

Spatial and intracellular relationships between the alpha7 nicotinic acetylcholine receptor and the vesicular acetylcholine transporter in the prefrontal cortex of rat and mouse

The alpha 7 subunit of the nicotinic acetylcholine receptor (alpha7nAChR) is expressed in the prefrontal cortex (PFC), a brain region where these receptors are implicated in cognitive function and in the pathophysiology of schizophrenia. Activation of this receptor is dependent on release of acetylcholine (ACh) from axon terminals that contain the vesicular acetylcholine transporter (VAChT). Since rat and mouse models are widely used for studies of specific abnormalities in schizophrenia, we sought to determine the subcellular location of the alpha7nAChR with respect to VAChT storage vesicles in axon terminals in the PFC in both species. For this, we used dual electron microscopic immunogold and immunoperoxidase labeling of antisera raised against the alpha7nAChR and VAChT. In both species, the alpha7nAChR-immunoreactivity ((-)ir) was principally identified within dendrites and dendritic spines, receptive to axon terminals forming asymmetric excitatory-type synapses, but lacking detectable alpha7nAChR or VAChT-ir. Quantitative analysis of the rat PFC revealed that of alpha7nAChR-labeled neuronal profiles, 65% (299/463) were postsynaptic structures (dendrites and dendritic spine) and only 22% (104/463) were axon terminals or small unmyelinated axons. In contrast, VAChT was principally localized to varicose vesicle-filled axonal profiles, without recognized synaptic specializations (n=240). Of the alpha7nAChR-labeled axons, 47% (37/79) also contained VAChT, suggesting that ACh release is autoregulated through the presynaptic alpha7nAChR. The VAChT-labeled terminals rarely formed synapses, but frequently apposed alpha7nAChR-containing neuronal profiles. These results suggest that in rodent PFC, the alpha7nAChR plays a major role in modulation of the postsynaptic excitation in spiny dendrites in contact with VAChT containing axons.

Angiotensin II type 2 receptors have a major somatodendritic distribution in vasopressin-containing neurons in the mouse hypothalamic paraventricular nucleus

The hypothalamic paraventricular nucleus (PVN) and angiotensin II (AngII) play critical roles in cardiovascular and neurohumoral regulation ascribed in part to vasopressin (VP) release. The AngII actions in the PVN are mediated largely through angiotensin II type 1 (AT1) receptors. However, there is indirect evidence that the functionally elusive central angiotensin II type 2 (AT2) receptors are also mediators of AngII signaling in the PVN. We used electron microscopic dual immunolabeling of antisera recognizing the AT2 receptor and VP to test the hypothesis that mouse PVN neurons expressing VP are among the cellular sites where this receptor has a subcellular distribution conducive to local activation. Immunoreactivity for the AT2 receptor was detected in somatodendritic profiles, of which approximately 60% of the somata and approximately 28% of the dendrites also contained VP. In comparison with somata and dendrites, axons, axon terminals, and glia less frequently contained the AT2 receptor. Somatic labeling for the AT2 receptor was often seen in the cytoplasm near the Golgi lamellae and other endomembrane structures implicated in receptor trafficking. AT2 receptor immunoreactivity in dendrites was commonly localized to cytoplasmic endomembranes, but was occasionally observed on extra- or peri-synaptic portions of the plasma membrane apposed by astrocytic processes or by unlabeled axon terminals. The labeled dendritic plasmalemmal segments containing AT2 receptors received asymmetric excitatory-type or more rarely symmetric inhibitory-type contacts from unlabeled axon terminals containing dense core vesicles, many of which are known to store neuropeptides. These results provide the first ultrastructural evidence that AT2 receptors in PVN neurons expressing VP and other neuromodulators are strategically positioned for surface activation by AngII and/or intracellular trafficking.

Subcellular plasticity of the corticotropin-releasing factor receptor in dendrites of the mouse bed nucleus of the stria terminalis following chronic opiate exposure

Chronic opiate administration alters the expression levels of the stress-responsive peptide, corticotropin-releasing factor (CRF), in the bed nucleus of the stria terminalis (BNST). This brain region contains CRF receptors that drive drug-seeking behavior exacerbated by stress. We used electron microscopy to quantitatively compare immunolabeling of the corticotropin-releasing factor receptor (CRFr) and CRF in the anterolateral bed nucleus of the stria terminalis (BSTal) of mice injected with saline or morphine in escalating doses for 14 days. We also compared the results with those in non-injected control mice. The tissue was processed for CRFr immunogold and CRF immunoperoxidase labeling. The non-injected controls had a significantly lower plasmalemmal density of CRFr immunogold particles in dendrites compared with mice receiving saline, but not those receiving morphine, injections. Compared with saline, however, mice receiving chronic morphine showed a significantly lower plasmalemmal, and greater cytoplasmic, density of CRFr immunogold in dendrites. Within the cytoplasmic compartment of somata and dendrites of the BSTal, the proportion of CRFr gold particles associated with mitochondria was three times as great in mice receiving morphine compared with saline. This subcellular distribution is consistent with morphine,- and CRFr-associated modulation of intracellular calcium release or oxidative stress. The between-group changes occurred without effect on the total number of dendritic CRFr immunogold particles, suggesting that chronic morphine enhances internalization or decreases delivery of the CRFr to the plasma membrane, a trafficking effect that is also affected by the stress of daily injections. In contrast, saline and morphine treatment groups showed no significant differences in the total number of CRF-immunoreactive axon terminals, or the frequency with which these terminals contacted CRFr-containing dendrites. This suggests that morphine does not influence axonal availability of CRF in the BSTal. The results have important implications for drug-associated adaptations in brain stress systems that may contribute to the motivation to continue drug use during dependence.

The neurokinin-3 (NK3) and the neurokinin-1 (NK1) receptors are differentially targeted to mesocortical and mesolimbic projection neurons and to neuronal nuclei in the rat ventral tegmental area

Tonic activation of neurokinin-3 (NK(3)) receptors in dopamine neurons of the ventral tegmental area (VTA) has been implicated in the pathophysiology of schizophrenia. This psychiatric disorder is associated with a dysfunctional activity in VTA projection neurons that can affect cognitive function at the level of the medial prefrontal cortex (mPFC) as well as motor and motivational states controlled in part by mesolimbic output to the nucleus accumbens (Acb). To determine the relevant sites for NK(3) receptor activation within this neuronal network, we used confocal and electron microscopy to examine NK(3) receptors (Cy5; immunogold) and retrograde labeling of fluorogold (FG, FITC; immunoperoxidase) in the VTA of rats receiving either Acb or mPFC injections of FG. Comparison was made with neurokinin-1 (NK(1)) receptors, which are also present, but less abundant then NK(3) receptors, in dopaminergic and GABAergic VTA neurons. There were no observable differences between NK(3) and NK(1) receptors in their primary locations in the cytoplasm and on the plasma membrane of VTA somata and dendrites with or without FG. Dendrites labeled with FG retrogradely transported from mPFC, however, contained more NK(3) or less NK(1) immunogold particles (plasmalemmal + cytoplasmic) then those retrogradely labeled following FG injection in the Acb. Moreover, only the NK(3) receptors were detected in neuronal nuclei in the VTA and in the nuclei of human HEK-293T NK(3)-transfected cells. The enrichment of NK(3) receptors in mesocortical projection neurons and nuclear distribution of these receptors may provide insight for understanding the selective antipsychotic effectiveness of NK(3) antagonists.