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

Differential brain ADRA2A and ADRA2C gene expression and epigenetic regulation in schizophrenia. Effect of antipsychotic drug treatment

Postsynaptic α_2A-adrenoceptor density is enhanced in the dorsolateral prefrontal cortex (DLPFC) of antipsychotic-treated schizophrenia subjects. This alteration might be due to transcriptional activation, and could be regulated by epigenetic mechanisms such as histone posttranslational modifications (PTMs). The aim of this study was to evaluate ADRA2A and ADRA2C gene expression (codifying for α₂-adrenoceptor subtypes), and permissive and repressive histone PTMs at gene promoter regions in the DLPFC of subjects with schizophrenia and matched controls (n = 24 pairs). We studied the effect of antipsychotic (AP) treatment in AP-free (n = 12) and AP-treated (n = 12) subgroups of schizophrenia subjects and in rats acutely and chronically treated with typical and atypical antipsychotics. ADRA2A mRNA expression was selectively upregulated in AP-treated schizophrenia subjects (+93%) whereas ADRA2C mRNA expression was upregulated in all schizophrenia subjects (+53%) regardless of antipsychotic treatment. Acute and chronic clozapine treatment in rats did not alter brain cortex Adra2a mRNA expression but increased Adra2c mRNA expression. Both ADRA2A and ADRA2C promoter regions showed epigenetic modification by histone methylation and acetylation in human DLPFC. The upregulation of ADRA2A expression in AP-treated schizophrenia subjects might be related to observed bivalent chromatin at ADRA2A promoter region in schizophrenia (depicted by increased permissive H3K4me3 and repressive H3K27me3) and could be triggered by the enhanced H4K16ac at ADRA2A promoter. In conclusion, epigenetic predisposition differentially modulated ADRA2A and ADRA2C mRNA expression in DLPFC of schizophrenia subjects.

Corticotropin releasing factor and norepinephrine related circuitry changes in the bed nucleus of the stria terminalis in stress and alcohol and substance use disorders

Alcohol Use Disorder (AUD) affects around 14.5 million individuals in the United States, with Substance Use Disorder (SUD) affecting an additional 8.3 million individuals. Relapse is a major barrier to effective long-term treatment of this illness with stress often described as a key trigger for a person with AUD or SUD to relapse during a period of abstinence. Two signaling molecules, norepinephrine (NE) and corticotropin releasing factor (CRF), are released during the stress response, and also play important roles in reward behaviors and the addiction process. Within the addiction literature, one brain region in which there has been increasing research focus in recent years is the bed nucleus of the stria terminalis (BNST). The BNST is a limbic structure with numerous cytoarchitecturally and functionally different subregions that has been implicated in drug-seeking behaviors and stress responses. This review focuses on drug and stress-related neurocircuitry changes in the BNST, particularly within the CRF and NE systems, with an emphasis on differences and similarities between the major dorsal and ventral BNST subregions.

GPCR dimerization in brainstem nuclei contributes to the development of hypertension

BACKGROUND AND PURPOSE

μ-Opioid receptors, pro-opiomelanocortin and pro-enkephalin are highly expressed in the nucleus tractus solitarii (NTS) and μ receptor agonists given to the NTS dose-dependently increased BP. However, the molecular mechanisms of this process remain unclear. In vitro, μ receptors heterodimerize with α2A -adrenoceptors. We hypothesized that α2A -adrenoceptor agonists would lose their depressor effects when their receptors heterodimerize in the NTS with μ receptors.

EXPERIMENTAL APPROACH

We microinjected μ-opioid agonists and antagonists into the NTS of rats and measured changes in BP. Formation of μ receptor/α2A -adrenoceptor heterodimers was assessed with immunofluorescence and co-immunoprecipitation methods, along with proximity ligation assays.

KEY RESULTS

Immunofluorescence staining revealed colocalization of α2A -adrenoceptors and μ receptors in NTS neurons. Co-immunoprecipitation revealed interactions between α2A -adrenoceptors and μ receptors. In situ proximity ligation assays confirmed the presence of μ receptor/α2A -adrenoceptor heterodimers in the NTS. Higher levels of endogenous endomorphin-1 and μ receptor/α2A -adrenoceptor heterodimers were found in the NTS of hypertensive rats, than in normotensive rats. Microinjection of the μ receptor agonist [D-Ala(2) , MePhe(4) , Gly(5) -ol]-enkephalin (DAMGO), but not that of the α2A -adrenoceptor agonist guanfacine, into the NTS of normotensive rats increased μ receptor/α2A -adrenoceptor heterodimer formation and BP elevation. The NO-dependent BP-lowering effect of α2A -adrenoceptor agonists was blunted following increased formation of μ receptor/α2A -adrenoceptor heterodimers in the NTS of hypertensive rats and DAMGO-treated normotensive rats.

CONCLUSIONS AND IMPLICATIONS

Increases in endogenous μ receptor agonists in the NTS induced μ receptor/α2A -adrenoceptor heterodimer formation and reduced the NO-dependent depressor effect of α2A -adrenoceptor agonists. This process could contribute to the pathogenesis of hypertension.

The effects of methylphenidate on resting-state striatal, thalamic and global functional connectivity in healthy adults

By blocking dopamine and norepinephrine transporters, methylphenidate affects cognitive performance and regional brain activation in healthy individuals as well as those with neuropsychiatric disorders. Resting-state connectivity evaluates the functional integrity of a network of brain regions. Here, we examined how methylphenidate effects resting-state functional connectivity of the dorsal striatum and thalamus, areas each with dense dopaminergic and noradrenergic innervations, as well as global cerebral connectivity. We administered a single, oral dose (45 mg) to 24 healthy adults and compared resting-state connectivity to 24 demographically matched adults who did not receive any medication. The results showed that methylphenidate alters seed-based and global connectivity between the thalamus/dorsal striatum with primary motor cortex, amygdala/hippocampus and frontal executive areas (p < 0.05, corrected). Specifically, while methylphenidate at this dosage enhances connectivity to the motor cortex and memory circuits, it dampens prefrontal cortical connectivity perhaps by increasing catecholaminergic signalling past the 'optimal' level. These findings advance our understanding of a critical aspect of the multifaceted effects of methylphenidate on brain functions. The results may also facilitate future studies of the aetiology and treatment of neurological and psychiatric disorders that implicate catecholaminergic dysfunction.

Synaptic and extrasynaptic location of the receptor tyrosine kinase met during postnatal development in the mouse neocortex and hippocampus

MET, a replicated autism risk gene, encodes a pleiotropic receptor tyrosine kinase implicated in multiple cellular processes during development and following injury. Previous studies suggest that Met modulates excitatory synapse development in the neocortex and hippocampus, although the underlying mechanism is unknown. The peak of Met expression corresponds to the period of process outgrowth and synaptogenesis, with robust expression in hippocampal and neocortical neuropil. Resolving whether neuropil expression represents presynaptic, postsynaptic or glial localization provides insight into potential mechanisms of Met action. The subcellular distribution of Met was characterized using complementary ultrastructural, in situ proximity ligation assay (PLA), and biochemical approaches. At postnatal day (P) 7, immunoelectron microscopy revealed near-equivalent proportions of Met-immunoreactive pre- (axons and terminals) and postsynaptic (dendritic shafts and spines) profiles in the stratum radiatum in the hippocampal CA1 region. Staining was typically in elements in which the corresponding pre- or postsynaptic apposition was unlabeled. By P21, Met-immunoreactive presynaptic profiles predominated and ~20% of Met-expressing profiles were glial. A different distribution of Met-immunoreactive profiles was observed in layer V of somatosensory cortex: Met-labeled spines were rare and a smaller proportion of glial profiles expressed Met. Strikingly, Met-immunoreactive presynaptic profiles predominated over postsynaptic profiles as early as P7. PLA analysis of neurons in vitro and biochemical analysis of tissue subsynaptic fractions confirmed the localization of Met in specific synaptic subcompartments. The study demonstrates that Met is enriched at synapses during development and its activation may modulate synapse formation and stability through both pre- and postsynaptic mechanisms.

Norepinephrine: a neuromodulator that boosts the function of multiple cell types to optimize CNS performance

Norepinephrine (NE) is a neuromodulator that in multiple ways regulates the activity of neuronal and non-neuronal cells. NE participates in the rapid modulation of cortical circuits and cellular energy metabolism, and on a slower time scale in neuroplasticity and inflammation. Of the multiple sources of NE in the brain, the locus coeruleus (LC) plays a major role in noradrenergic signaling. Processes from the LC primarily release NE over widespread brain regions via non-junctional varicosities. We here review the actions of NE in astrocytes, microglial cells, and neurons based on the idea that the overarching effect of signaling from the LC is to maximize brain power, which is accomplished via an orchestrated cellular response involving most, if not all cell types in CNS.

Extrasynaptic exocytosis and its mechanisms: a source of molecules mediating volume transmission in the nervous system

We review the evidence of exocytosis from extrasynaptic sites in the soma, dendrites, and axonal varicosities of central and peripheral neurons of vertebrates and invertebrates, with emphasis on somatic exocytosis, and how it contributes to signaling in the nervous system. The finding of secretory vesicles in extrasynaptic sites of neurons, the presence of signaling molecules (namely transmitters or peptides) in the extracellular space outside synaptic clefts, and the mismatch between exocytosis sites and the location of receptors for these molecules in neurons and glial cells, have long suggested that in addition to synaptic communication, transmitters are released, and act extrasynaptically. The catalog of these molecules includes low molecular weight transmitters such as monoamines, acetylcholine, glutamate, gama-aminobutiric acid (GABA), adenosine-5-triphosphate (ATP), and a list of peptides including substance P, brain-derived neurotrophic factor (BDNF), and oxytocin. By comparing the mechanisms of extrasynaptic exocytosis of different signaling molecules by various neuron types we show that it is a widespread mechanism for communication in the nervous system that uses certain common mechanisms, which are different from those of synaptic exocytosis but similar to those of exocytosis from excitable endocrine cells. Somatic exocytosis has been measured directly in different neuron types. It starts after high-frequency electrical activity or long experimental depolarizations and may continue for several minutes after the end of stimulation. Activation of L-type calcium channels, calcium release from intracellular stores and vesicle transport towards the plasma membrane couple excitation and exocytosis from small clear or large dense core vesicles in release sites lacking postsynaptic counterparts. The presence of synaptic and extrasynaptic exocytosis endows individual neurons with a wide variety of time- and space-dependent communication possibilities. Extrasynaptic exocytosis may be the major source of signaling molecules producing volume transmission and by doing so may be part of a long duration signaling mode in the nervous system.

Adrenoceptors in brain: cellular gene expression and effects on astrocytic metabolism and [Ca(2+)]i

Recent in vivo studies have established astrocytes as a major target for locus coeruleus activation (Bekar et al., 2008), renewing interest in cell culture studies on noradrenergic effects on astrocytes in primary cultures and calling for additional information about the expression of adrenoceptor subtypes on different types of brain cells. In the present communication, mRNA expression of alpha(1)-, alpha(2)- and beta-adrenergic receptors and their subtypes was determined in freshly isolated, cell marker-defined populations of astrocytes, NG2-positive cells, microglia, endothelial cells, and Thy1-positive neurons (mainly glutamatergic projection neurons) in murine cerebral cortex. Immediately after dissection of frontal, parietal and occipital cortex of 10-12-week-old transgenic mice, which combined each cell-type marker with a specific fluorescent signal, the tissue was digested, triturated and centrifuged, yielding a solution of dissociated cells of all types, which were separated by fluorescence-activated cell sorting (FACS). mRNA expression in each cell fraction was determined by microarray analysis. alpha(1A)-Receptors were unequivocally expressed in astrocytes and NG2-positive cells, but absent in other cell types, and alpha(1B)-receptors were not expressed in any cell population. Among alpha(2)-receptors only alpha(2A)-receptors were expressed, unequivocally in astrocytes and NG-positive cells, tentatively in microglia and questionably in Thy1-positive neurons and endothelial cells. beta(1)-Receptors were unequivocally expressed in astrocytes, tentatively in microglia, and questionably in neurons and endothelial cells, whereas beta(2)-adrenergic receptors showed tentative expression in neurons and astrocytes and unequivocal expression in other cell types. This distribution was supported by immunochemical data and its relevance established by previous studies in well-differentiated primary cultures of mouse astrocytes, showing that stimulation of alpha(2)-adrenoceptors increases glycogen formation and oxidative metabolism, the latter by a mechanism depending on intramitochondrial Ca(2+), whereas alpha(1)-adrenoceptor stimulation enhances glutamate uptake, and beta-adrenoceptor activation causes glycogenolysis and increased Na(+), K(+)-ATPase activity. The Ca(2+)- and cAMP-mediated association between energy-consuming and energy-yielding processes is emphasized.

Chronic intermittent hypoxia reduces neurokinin-1 (NK(1)) receptor density in small dendrites of non-catecholaminergic neurons in mouse nucleus tractus solitarius

Chronic intermittent hypoxia (CIH) is a frequent concomitant of sleep apnea, which can increase sympathetic nerve activity through mechanisms involving chemoreceptor inputs to the commissural nucleus of the solitary tract (cNTS). These chemosensory inputs co-store glutamate and substance P (SP), an endogenous ligand for neurokinin-1 (NK(1)) receptors. Acute hypoxia results in internalization of NK(1) receptors, suggesting that CIH also may affect the subcellular distribution of NK(1) receptors in subpopulations of cNTS neurons, some of which may express tyrosine hydroxylase, the rate-limiting enzyme for catecholamine synthesis (TH). To test this hypothesis, we examined dual immunolabeling for the NK(1) receptor and TH in the cNTS of male mice subjected to 10days or 35days of CIH or intermittent air. Electron microscopy revealed that NK(1) receptors and TH were almost exclusively localized within separate somatodendritic profiles in cNTS of control mice. In dendrites, immunogold particles identifying NK(1) receptors were prevalent in the cytoplasm and on the plasmalemmal surface. Compared with controls, CIH produced a significant region-specific decrease in the cytoplasmic (10 and 35days, P<0.05, unpaired Student t-test) and extrasynaptic plasmalemmal (35days, P<0.01, unpaired Student t-test) density of NK(1) immunogold particles exclusively in small (<0.1microm) dendrites without TH immunoreactivity. These results suggest that CIH produces a duration-dependent reduction in the availability of NK(1) receptors preferentially in small dendrites of non-catecholaminergic neurons in the cNTS. The implications of our findings are discussed with respect to their potential involvement in the slowly developing hypertension seen in sleep apnea patients.

α2-Adrenergic Receptors Are Present in Normal Human Conjunctiva

PURPOSE

Despite the passage of medications, including antiglaucoma drugs, through the ocular surface, and despite the increasing relevance of neurogenic inflammation in the ocular surface, the presence of some neuroreceptors in the conjunctiva has not been ascertained. This study describes the presence of alpha2-adrenergic receptors in normal human conjunctiva.

METHODS

Immunofluorescence microscopy, electrophoresis, and Western blot analyses were done in human conjunctival biopsies and rat control tissues. Antibodies against alpha2-adrenergic receptor subtypes alpha2A, alpha2B, and alpha2C were used.

RESULTS

Immunoreactivity for alpha2A- and alpha2B-, but not alpha2C-adrenergic receptors was evenly distributed in epithelial cells of human conjunctiva cryosections. Immunoreactive bands were detected for the three alpha2-adrenergic receptor subtypes: a major band of 48-50 kDa and fellow bands of 65-67 kDa.

CONCLUSIONS

Normal human conjunctival epithelial cells express alpha2A-, alpha2B-, and alpha2C-adrenergic receptors. Further studies to determine the functional implications of these receptors in ocular surface homeostasis are warranted.

alpha2A-adrenergic receptors heterosynaptically regulate glutamatergic transmission in the bed nucleus of the stria terminalis

Stress is a major driving force in reinstatement of drug-seeking behavior. The bed nucleus of the stria terminalis (BNST) has been identified as a key brain region in this behavior, and receives a dense input of the stress-neurotransmitter norepinephrine through the ventral noradrenergic bundle. Activation of alpha(2)-adrenergic receptors (alpha(2)-ARs) in the BNST blocks stress-induced reinstatement of drug-seeking, indicating a potentially important role for these receptors. Currently, it is unclear how alpha(2)-AR agonists elicit this behavioral action, or through which alpha(2)-AR subtype. Activation of alpha(2)-ARs decreases glutamatergic transmission in the BNST, an effect which is nearly absent in the alpha(2A)-AR knockout mouse. Here, we take advantage of a knock-in mouse in which a hemagglutinin-tagged alpha(2A)-AR was inserted into the endogenous locus, along with the alpha(2A)-AR selective agonist guanfacine, to further study the role of the alpha(2A)-AR subtype in modulation of neurotransmission in the BNST. Using immunohistochemistry, we find that alpha(2A)-ARs are highly expressed in the BNST, and that this expression is more similar in distribution to the vesicular glutamate transporters than to either norepinephrine transporter or tyrosine hydroxylase positive terminals. Using whole cell patch-clamp recordings, we show that guanfacine causes a depression of evoked excitatory and, to a more limited extent, inhibitory fast synaptic transmission. In total, these data support a prominent heterosynaptic role for alpha(2A)-ARs in modulating fast synaptic transmission in the BNST.

Sex differences in the subcellular distribution of angiotensin type 1 receptors and NADPH oxidase subunits in the dendrites of C1 neurons in the rat rostral ventrolateral medulla

The rostral ventrolateral medulla (RVLM), a region critical for the tonic and reflex control of arterial pressure, contains a group of adrenergic (C1) neurons that project to the spinal cord and directly modulate pre-ganglionic sympathetic neurons. Epidemiological data suggest that there are gender differences in the regulation of blood pressure. One factor that could be involved is angiotensin II signaling and the associated production of reactive oxygen species (ROS) by NADPH oxidase, which is emerging as an important molecular substrate for central autonomic regulation and dysregulation. In this study dual electron microscopic immunolabeling was used to examine the subcellular distribution of the angiotensin type 1 (AT(1)) receptor and two NADPH oxidase subunits (p47 and p22) in C1 dendritic processes, in tissue from male, proestrus (high estrogen) and diestrus (low estrogen) female rats. Female dendrites displayed significantly more AT(1) labeling and significantly less p47 labeling than males. While elevations in AT(1) labeling primarily resulted from higher levels of receptor on the plasma membrane, p47 labeling was reduced both on the plasma membrane and in the cytoplasm. Across the estrous cycle, proestrus females displayed significantly higher levels of AT(1) labeling than diestrus females, which resulted exclusively from plasma membrane density differences. In contrast, p47 labeling did not change across the estrous cycle, indicating that ROS production might reflect AT(1) receptor membrane density. No significant differences in p22 labeling were observed. These findings demonstrate that both sex and hormonal levels can selectively affect the expression and subcellular distribution of components of the angiotensin II signaling pathway within C1 RVLM neurons. Such effects could reflect differences in the capacity for ROS production, potentially influencing short term excitability and long term gene expression in a cell group which is critically involved in blood pressure regulation, potentially contributing to gender differences in the risk of cardiovascular disease.

Group II metabotropic glutamate receptors in anxiety circuitry: correspondence of physiological response and subcellular distribution

Activation of group II metabotropic glutamate receptors (mGluR2/3) in the amygdala plays a critical role in the regulation of fear and anxiety states. Previous studies using nonselective agonists have suggested this action can result from activation of either pre- or postsynaptic mGluR2/3. Here, we have used a combination of whole-cell patch clamp recording with highly selective agonists (LY354740 and LY379268) and immunoelectron microscopy to examine structure-function relationships for mGluR2/3 in the basolateral amygdala (BLA) and bed nucleus of the stria terminalis (BNST). Stimulation of mGluR2/3 evoked a direct, TTX-insensitive membrane hyperpolarization in all BLA projection neurons tested, but only about half of BNST neurons. The membrane hyperpolarization was mediated by activation of an outward potassium current or blockade of a tonically active inward I(h) current in different groups of BLA neurons. In both regions, mGluR2/3 caused a long-lasting reduction of glutamate release from presynaptic afferent terminals even at concentrations that failed to elicit a direct postsynaptic response. The localization of mGluR2/3 differed regionally, with postsynaptic labeling significantly more common in BLA than BNST, corresponding to the strength of postsynaptic responses recorded there. Our results demonstrate a complex role for mGluR2/3 receptors in modulating anxiety circuitry, including direct inhibition and reduction of excitatory drive. The combination of direct inhibition of projection neurons within the BLA and suppression of excitatory neurotransmission in the BNST may be responsible for the anxiolytic actions of group II mGluR agonists.

Subcellular distributions of adenosine A1 and A2A receptors in the rat dorsomedial nucleus of the solitary tract at the level of the area postrema

Adenosine A1 and A2A receptors mediate distinct cardiovascular components of defense reactions that are ascribed, in part, to opposing actions within the nucleus tractus solitarius. To assess the cellular sites of relevance to these actions, we examined the light and electron microscopic immunolabeling of adenosine A1 and A2A receptors in the rat dorsomedial nucleus of the solitary tract at the level of the area postrema (dmNTS-AP), a region crucial for cardiovascular regulation involving vagal baroreceptor afferents. Immunoreactivity for each receptor was independently localized to distinct segments of plasma membranes and endomembranes in somatodendritic, axonal, and glial profiles. The dendritic labeling for each receptor also was detected within and near asymmetric, excitatory-type synapses. Of all peroxidase labeled profiles exclusive of somata, approximately 58% were A1- and 39% were A2A-labeled dendrites. Dendrites and astrocytic glia were the profiles that most often expressed both subtypes of adenosine receptors. The axonal labeling for A2A receptors was seen mainly in unmyelinated axons, whereas the A1 receptors were prominently localized within axon terminals. These terminals often formed single or multisynaptic excitatory-type junctions or single symmetric synapses on dendrites, a few of which expressed A1 and A2A receptors. These results provide the first ultrastructural evidence that A1 and A2A receptors have distributions conductive to their dual involvement in modulating the output of single neurons and glial function in the dmNTS-AP, where the predominate presynaptic effects of adenosine are mediated through A1 receptors.

Subcellular localization of nicotinamide adenine dinucleotide phosphate oxidase subunits in neurons and astroglia of the rat medial nucleus tractus solitarius: relationship with tyrosine hydroxylase immunoreactive neurons

Superoxide produced by the enzyme nicotinamide adenine dinucleotide phosphate (NADPH) oxidase mediates crucial intracellular signaling cascades in the medial nucleus of the solitary tract (mNTS), a brain region populated by catecholaminergic neurons, as well as astroglia that play an important role in autonomic function. The mechanisms mediating NADPH oxidase (phagocyte oxidase) activity in the neural regulation of cardiovascular processes are incompletely understood, however the subcellular localization of superoxide produced by the enzyme is likely to be an important regulatory factor. We used immunogold electron microscopy to determine the phenotypic and subcellular localization of the NADPH oxidase subunits p47(phox), gp91(phox,) and p22(phox) in the mNTS in rats. The mNTS contains a large population of neurons that synthesize catecholamines. Significantly, catecholaminergic signaling can be modulated by redox reactions. Therefore, the relationship of NADPH oxidase subunit labeled neurons or glia with respect to catecholaminergic neurons was also determined by dual labeling for the superoxide producing enzyme and tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine biosynthesis. In the mNTS, NADPH oxidase subunits were present primarily in somatodendritic processes and astrocytes, some of which also contained TH, or were contacted by TH-labeled axons, respectively. Immunogold quantification of NADPH oxidase subunit localization showed that p47(phox) and gp91(phox) were present on the surface membrane, as well as vesicular organelles characteristic of calcium storing smooth endoplasmic reticula in dendritic and astroglial processes. These results indicate that NADPH oxidase assembly and consequent superoxide formation are likely to occur near the plasmalemma, as well as on vesicular organelles associated with intracellular calcium storage within mNTS neurons and glia. Thus, NADPH oxidase-derived superoxide may participate in intracellular signaling pathways linked to calcium regulation in diverse mNTS cell types. Moreover, NADPH oxidase-derived superoxide in neurons and glia may directly or indirectly modulate catecholaminergic neuron activity in the mNTS.

Neuron specific alpha-adrenergic receptor expression in human cerebellum: implications for emerging cerebellar roles in neurologic disease

Recent data suggest novel functional roles for cerebellar involvement in a number of neurologic diseases. Function of cerebellar neurons is known to be modulated by norepinephrine and adrenergic receptors. The distribution of adrenergic receptor subtypes has been described in experimental animals, but corroboration of such studies in the human cerebellum, necessary for drug treatment, is still lacking. In the present work we studied cell-specific localizations of alpha1 adrenergic receptor subtype mRNA (alpha 1a, alpha 1b, alpha 1d), and alpha2 adrenergic receptor subtype mRNA (alpha 2a, alpha 2b, alpha 2c) by in situ hybridization on cryostat sections of human cerebellum (cortical layers and dentate nucleus). We observed unique neuron-specific alpha1 adrenergic receptor and alpha2 adrenergic receptor subtype distribution in human cerebellum. The cerebellar cortex expresses mRNA encoding all six alpha adrenergic receptor subtypes, whereas dentate nucleus neurons express all subtype mRNAs, except alpha 2a adrenergic receptor mRNA. All Purkinje cells label strongly for alpha 2a and alpha 2b adrenergic receptor mRNA. Additionally, Purkinje cells of the anterior lobe vermis (lobules I to V) and uvula/tonsil (lobules IX/HIX) express alpha 1a and alpha 2c subtypes, and Purkinje cells in the ansiform lobule (lobule HVII) and uvula/tonsil express alpha 1b and alpha 2c adrenergic receptor subtypes. Basket cells show a strong signal for alpha 1a, moderate signal for alpha 2a and light label for alpha 2b adrenergic receptor mRNA. In stellate cells, besides a strong label of alpha 2a adrenergic receptor mRNA in all and moderate label of alpha 2b message in select stellate cells, the inner stellate cells are also moderately positive for alpha 1b adrenergic receptor mRNA. Granule and Golgi cells express high levels of alpha 2a and alpha 2b adrenergic receptor mRNAs. These data contribute new information regarding specific location of adrenergic receptor subtypes in human cerebellar neurons. We discuss our observations in terms of possible modulatory roles of adrenergic receptor subtypes in cerebellar neurons responding to sensory and autonomic input signals, and review species differences in cerebellar adrenergic receptor expression.

Angiotensin II AT-1A receptor immunolabeling in rat medial nucleus tractus solitarius neurons: subcellular targeting and relationships with catecholamines

The angiotensin II AT-1A receptor (AT-1A) is the major mediator of the hypertensive actions of angiotensin II (ANG II) in the medial nucleus of the solitary tract (mNTS). The localization of the AT-1A receptor at surface or intracellular sites is an important determinant of its signaling properties, including intercellular or intracrine communication. However, the spatial localization of this protein, particularly within small distal or intermediate size dendrites of mNTS neurons, is unknown. Within the mNTS, ANG II and catecholamines interact in the regulation of autonomic function; however, it is unknown if AT-1A receptors are present at functional sites in catecholamine containing dendrites, or are contacted by catecholamine containing axon terminals. We compared surface and intracellular distributions of the AT-1A receptor in dendritic processes from the mNTS using immunogold electron microscopy in conjunction with immunoperoxidase labeling for tyrosine hydroxylase (TH) and morphometric analysis. Collapsed across all AT-1A-labeled dendritic profiles, immunogold labeling was more frequent in intracellular sites as compared with the plasma membrane. Small (<0.6 microm) dendritic profiles contained a higher ratio of particles associated with the surface membrane when compared with larger profiles. Approximately 27% of all AT-1A receptor-labeled dendritic profiles also contained labeling for TH. Approximately 12% of dendritic profiles single labeled for the AT-1A receptor were contacted by TH containing axons or axon terminals. The present results provide the first quantitative demonstration of select plasmalemmal and intracellular localizations of AT-1A receptors in dendritic processes of mNTS neurons, including those containing TH, or contacted by catecholaminergic axon terminals. These results suggest that AT-1A receptors are positioned for modulation of catecholamine signaling in the mNTS.

Immunohistochemical visualization of corticotropin-releasing factor type 1 (CRF1) receptors in monkey brain

Corticotropin-releasing factor receptor type 1, CRF1, plays a prominent role in the hypothalamic-pituitary-adrenal (HPA) axis and is implicated in the autonomic and behavioral responses to stress. Dysregulation of the CRF system may underlie the pathophysiology of several disorders, including depression and anxiety. The distribution of CRF1 mRNA and CRF1 specific ligand binding has been reported by multiple groups in rodents using in situ hybridization and receptor autoradiography, respectively. More recently, somewhat conflicting rodent anti-CRF1 immunohistochemical studies were reported. In this study we report the generation of an antihuman CRF1 antiserum and provide the first immunohistochemical description of CRF1 distribution in a primate brain, that of the rhesus monkey. The specificity of anti-CRF-R1 antiserum R221 was demonstrated using transfected hCRF1-expressing HEK 293 cells and rhesus monkey pituitary. CRF1-immunoreactive neurons were widespread in the rhesus brain. CRF1 staining was associated with neuronal cell bodies and dendrites and was primarily intracellular, suggesting a high rate of receptor turnover or receptor sequestration. Anti-CRF1 immunoreactivity was most abundant in pituitary, cerebellum, and in portions of brain stem associated with sensorimotor function. CRF1 staining was also observed in cerebral cortex, basal forebrain, portions of the basal ganglia, and thalamus. Staining was relatively low in prefrontal cortex and in limbic areas, which may reflect masking of the N-terminal epitope. The distribution of CRF1 immunoreactivity is suggestive of roles in attentional processing as well as the processing of motor and sensory information.

Nonsynaptic communication in the central nervous system

Classical synaptic functions are important and suitable to relatively fast and discretely localized processes, but the nonclassical receptorial functions may be providing revolutionary possibilities for dealing at the cellular level with many of the more interesting and seemingly intractable features of neural and cerebral activities. Although different forms of nonsynaptic communication (volume transmission) often appear in different studies, their importance to modulate and mediate various functions is still not completely recognized. To establish the existence and the importance of nonsynaptic communication in the nervous system, here we cite pieces of evidence for each step of the interneuronal communication in the nonsynaptic context including the release into the extracellular space (ECS) and the extrasynaptic receptors and transporters that mediate nonsynaptic functions. We are now faced with a multiplicity of chemical communication. The fact that transmitters can even be released from nonsynaptic varicosities without being coupled to frequency-coded neuronal activity and they are able to diffuse over large distances indicates that there is a complementary mechanism of interneuronal communication to classical synaptic transmission. Nonconventional mediators that are also important part of the nonsynaptic world will also be overviewed.

Cyclin-dependent kinase 11p110 and casein kinase 2 (CK2) inhibit the interaction between tyrosine hydroxylase and 14-3-3

Tyrosine hydroxylase (TH) is regulated by the reversible phosphorylation of serines 8, 19, 31 and 40. Upon initiation of this study, serine 19 was unique due to its requirement of 14-3-3 binding after phosphorylation for optimal enzyme activity, although it has been more recently demonstrated that phosphorylated serine 40 also binds 14-3-3. To identify proteins that interact with TH following phosphorylation of serine 19, this amino acid was mutated to alanine and THS19A was used as bait in a yeast two-hybrid system. From this, mouse-derived cyclin-dependent kinase 11 (CDK11)p110 was identified as an interacting partner with THS19A. The interaction was confirmed using human CDK11p110 cDNA in a mammalian system. Previous research has demonstrated that casein kinase 2 (CK2) interacts with CDK11p110, and both were observed to phosphorylate TH in vitro. In addition, CDK11p110 overexpression was observed to inhibit the interaction between TH and 14-3-3. A mechanism contributing to disruption of the interaction between TH and 14-3-3 may be due to CK2 phosphorylation of specific 14-3-3 isoforms, i.e. 14-3-3 tau. Collectively, these results imply that CDK11p110 and CK2 negatively regulate TH catecholamine biosynthetic activity since phosphoserine 19 of TH requires 14-3-3 binding for optimal enzyme activity and a decreased rate of dephosphorylation.

Alpha 2 adrenergic receptors on GABAergic, putative sleep-promoting basal forebrain neurons

The basal forebrain plays an important role in the modulation of cortical activity and sleep-wake states. Yet its role must be multivalent as lesions reportedly diminish cortical fast activity and also cortical slow activity along with slow wave sleep (SWS). Basal forebrain cholinergic vs. GABAergic cell groups could differentially influence these processes. By labelling recorded neurons with Neurobiotin (Nb) using the juxtacellular technique and identifying them by immunostaining, we previously found that whereas all cholinergic cells increased their firing, the majority of GABAergic neurons decreased their firing in association with evoked cortical activation in urethane-anaesthetized rats. Here, we examined the possibility that such GABAergic, cortical activation 'off' cells might bear alpha 2 adrenergic receptors (alpha2AR) through which noradrenaline (NA) could inhibit them during cortical activation. First using simple dual-immunostaining for glutamic acid decarboxylase (GAD) and the alpha2AAR, we found that the majority (approximately 60%) of GAD-immunopositive (GAD+) neurons through the magnocellular preoptic nucleus (MCPO) and substantia innominata (SI) were labelled for the alpha2AAR. Second, in urethane-anaesthetized rats, we examined whether Nb-labelled, GAD+ cortical activation 'off' neurons that discharged maximally in association with cortical slow wave activity, were immunopositive for alpha2AAR. We found that all the Nb+/GAD+'off' cells were labelled for the alpha2AAR. Such cells could be inhibited in association with cortical activation and waking when noradrenergic locus coeruleus (LC) neurons discharge and be disinhibited with cortical slow waves and SWS when these neurons become inactive. We thus propose that alpha2AR-bearing GABAergic basal forebrain neurons constitute sleep-active and sleep-promoting neurons.

Alpha-adrenergic stimulation of ERK phosphorylation in astrocytes is alpha(2)-specific and may be mediated by transactivation

The highly specific alpha(2)-adrenergic agonist, dexmedetomidine, has hypnotic-sedative, anesthetic-sparing and analgesic effects, and it protects neurons against ischemia. The alpha(1)-adrenergic agonist, phenylephrine, does not share dexmedetomidine's pharmacological properties, although both dexmedetomidine and phenylephrine increase free cytosolic Ca(2+) ([Ca(2+)](i)) in astrocytes, and most of dexmedetomidine's actions in the brain are exerted on postjunctional receptors. alpha(2)-Adrenergic receptors are abundant on astrocytes. Dexmedetomidine-mediated 'down-streamn' signal transduction was therefore investigated in primary cultures of mouse astrocytes and contrasted with that of phenylephrine. The cultures were incubated with dexmedetomidine concentrations known to be pharmacologically active and to act specifically on alpha(2)-adrenergic receptors (25-100 nM). ERK(1/2) phosphorylation was measured using specific antibodies. Peak increases of ERK(1/2) phosphorylation occurred at 50 nM dexmedetomidine, with less effect at higher concentrations. Phenylephrine caused ERK phosphorylation only at a concentration high enough to exert non subtype-specific effects (10 microM), and this effect was counteracted by the alpha(2)-adrenergic antagonist atipamezole. The phosphorylation of ERK was reduced by tyrphostin AG1478, an inhibitor of phosphorylation of the epidermal growth factor receptor (EGFR), and by heparin, which neutralizes heparin-binding epithelial growth factor (HB-EGF), suggesting the involvement of a transactivation process, in which alpha(2)-adrenergic stimulation leads to proteolytic shedding of HB-EGF (and perhaps other EGFR agonists) from transmembrane-spanning precursors.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

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

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

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

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

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

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

A role for alpha-synuclein in the regulation of dopamine biosynthesis

The alpha-synuclein gene is implicated in the pathogenesis of Parkinson's disease. Although alpha-synuclein function is uncertain, the protein has homology to the chaperone molecule 14-3-3. In addition, alpha-synuclein can bind to 14-3-3, and both alpha-synuclein and 14-3-3 bind to many of the same proteins. Because 14-3-3 binds to and activates tyrosine hydroxylase, the rate-limiting enzyme in dopamine (DA) biosynthesis, we explored whether alpha-synuclein also bound to tyrosine hydroxylase and influenced its activity. Immunoprecipitation revealed an interaction between alpha-synuclein and tyrosine hydroxylase in brain homogenates and MN9D dopaminergic cells. Colocalization of alpha-synuclein with tyrosine hydroxylase was confirmed by immunoelectron microscopy. To explore the consequences of the interaction, we measured the effect of recombinant alpha-synuclein on tyrosine hydroxylase activity in a cell-free system and observed a dose-dependent inhibition of tyrosine hydroxylase by alpha-synuclein. To measure the impact of alpha-synuclein on tyrosine hydroxylase in dopaminergic cells, we stably transfected MN9D cells with wild-type or A53T mutant alpha-synuclein. Overexpression of wild-type or A53T mutant alpha-synuclein did not significantly alter tyrosine hydroxylase protein levels in our stably transfected cells. However, overexpressing cell lines had significantly reduced tyrosine hydroxylase activity and a corresponding reduction in dopamine synthesis. The reduction in cellular dopamine levels was not caused by increased dopamine catabolism or dopamine efflux. These data suggest that alpha-synuclein plays a role in the regulation of dopamine biosynthesis, acting to reduce the activity of tyrosine hydroxylase. If so, a loss of soluble alpha-synuclein, by reduced expression or aggregation, could increase dopamine synthesis with an accompanying increase in reactive dopamine metabolites.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

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

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

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

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

Localization of agmatine in vasopressin and oxytocin neurons of the rat hypothalamic paraventricular and supraoptic nuclei

Agmatine (decarboxylated l-arginine), an endogenous ligand of imidazoline and alpha(2) adrenoreceptors, is particularly enriched in the rat hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei. The present study utilized light and electron microscopic immunocytochemical methods to determine the distribution and extent of colocalization of agmatine relative to subpopulations of vasopressin- (VP) and oxytocin- (OT) producing neurons in PVN and SON nuclei. By light microscopy, agmatine-immunoreactive perikarya were found in both the magnocellular and the parvocellular neuronal subdivisions of PVN and SON. Confocal and electron microscopy revealed that agmatine-immunoreactivity (I) within neuronal perikarya was associated with the nuclear membrane as well as mitochondria, Golgi complexes, endoplasmic reticula, and plasmalemma. Additionally, agmatine-I was identified in both axons and axonal terminals, which were enriched in large dense-core vesicles. Dual and triple immunocytochemical labeling experiments also demonstrated that agmatine coexists with VP or OT in most PVN and SON magnocellular neurons. Combinations of iontophoretic injections of Fluorogold into the dorsomedullary complex with immunocytochemical labeling revealed that many retrogradely labeled neurons in the parvocellular region of the PVN contained agmatine-I and either VP or OT. These findings provide evidence that agmatine may function as a modulator of both hypothalamically mediated neuroendocrine and autonomic responses.