Cellular and subcellular distribution of alpha 2A-adrenergic receptors in the visual cortex of neonatal and adult rats

Distribution of the Noradrenaline Innervation and Adrenoceptors in the Macaque Monkey Thalamus

Noradrenaline (NA) in the thalamus has important roles in physiological, pharmacological, and pathological neuromodulation. In this work, a complete characterization of NA axons and Alpha adrenoceptors distributions is provided.

NA axons, revealed by immunohistochemistry against the synthesizing enzyme and the NA transporter, are present in all thalamic nuclei. The most densely innervated ones are the midline nuclei, intralaminar nuclei (paracentral and parafascicular), and the medial sector of the mediodorsal nucleus (MDm). The ventral motor nuclei and most somatosensory relay nuclei receive a moderate NA innervation. The pulvinar complex receives a heterogeneous innervation. The lateral geniculate nucleus (GL) has the lowest NA innervation.

Alpha adrenoceptors were analyzed by in vitro quantitative autoradiography. Alpha-1 receptor densities are higher than Alpha-2 densities. Overall, axonal densities and Alpha adrenoceptor densities coincide; although some mismatches were identified. The nuclei with the highest Alpha-1 values are MDm, the parvocellular part of the ventral posterior medial nucleus, medial pulvinar, and midline nuclei. The nucleus with the lowest Alpha-1 receptor density is GL. Alpha-2 receptor densities are highest in the lateral dorsal, centromedian, medial and inferior pulvinar, and midline nuclei.

These results suggest a role for NA in modulating thalamic involvement in consciousness, limbic, cognitive, and executive functions.

GANEing traction: The broad applicability of NE hotspots to diverse cognitive and arousal phenomena

The GANE (glutamate amplifies noradrenergic effects) model proposes that local glutamate-norepinephrine interactions enable "winner-take-more" effects in perception and memory under arousal. A diverse range of commentaries addressed both the nature of this "hotspot" feedback mechanism and its implications in a variety of psychological domains, inspiring exciting avenues for future research.

The α2A -adrenoceptor suppresses excitatory synaptic transmission to both excitatory and inhibitory neurons in layer 4 barrel cortex

KEY POINTS

The effects of noradrenaline on excitatory synaptic transmission to regular spiking (excitatory) cells as well as regular spiking non-pyramidal and fast spiking (both inhibitory) cells in cortical layer 4 were studied in thalamocortical slice preparations, focusing on vertical input from thalamus and layer 2/3 in the mouse barrel cortex. Excitatory synaptic responses were suppressed by noradrenaline. However, currents induced by iontophoretically applied glutamate were not suppressed. Further, paired pulse ratio and coefficient of variation analysis indicated the site of action was presynaptic. Pharmacological studies indicated that the suppression was mediated by the α_2- adrenoceptor. Consistent with this, involvement of α_2A -adrenoceptor activation in the synaptic suppression in excitatory and inhibitory cells was confirmed by the use of α_2A -adrenoceptor knockout mice.

ABSTRACT

The mammalian neocortex is widely innervated by noradrenergic (NA) fibres from the locus coeruleus. To determine the effects of NA on vertical synaptic inputs to layer 4 (L4) cells from the ventrobasal thalamus and layer 2/3 (L2/3), thalamocortical slices were prepared and whole-cell recordings were made from L4 cells. Excitatory synaptic responses were evoked by electrical stimulation of the thalamus or L2/3 immediately above. Recorded cells were identified as regular spiking, regular spiking non-pyramidal or fast spiking cells through their firing patterns in response to current injections. NA suppressed (∼50% of control) excitatory vertical inputs to all cell types in a dose-dependent manner. The presynaptic site of action of NA was suggested by three independent studies. First, responses caused by iontophoretically applied glutamate were not suppressed by NA. Second, the paired pulse ratio was increased during NA suppression. Finally, a coefficient of variation (CV) analysis was performed and the resultant diagonal alignment of the ratio of CV^-2 plotted against the ratio of the amplitude of postsynaptic responses suggests a presynaptic mechanism for the suppression. Experiments with phenylephrine (an α₁ -agonist), prazosin (an α₁ -antagonist), yohimbine (an α₂ -antagonist) and propranolol (a β-antagonist) indicated that suppression was mediated by the α₂ -adrenoceptor. To determine whether the α_2A -adrenoceptor subtype was involved, α_2A -adrenoceptor knockout mice were used. NA failed to suppress EPSCs in all cell types, suggesting an involvement of the α_2A -adrenoceptor. Altogether, we concluded that NA suppresses vertical excitatory synaptic connections in L4 excitatory and inhibitory cells through the presynaptic α_2A -adrenoceptor.

Cortical adrenoceptor expression, function and adaptation under conditions of cannabinoid receptor deletion

A neurochemical target at which cannabinoids interact to have global effects on behavior is brain noradrenergic circuitry. Acute and repeated administration of a cannabinoid receptor synthetic agonist is capable of increasing multiple indices of noradrenergic activity. This includes cannabinoid-induced 1) increases in norepinephrine (NE) release in the medial prefrontal cortex (mPFC); 2) desensitization of cortical α2-adrenoceptor-mediated effects; 3) activation of c-Fos in brainstem locus coeruleus (LC) noradrenergic neurons; and 4) increases in anxiety-like behaviors. In the present study, we sought to examine adaptations in adrenoceptor expression and function under conditions of cannabinoid receptor type 1 (CB1r) deletion using knockout (KO) mice and compare these to wild type (WT) controls. Electrophysiological analysis of α2-adrenoceptor-mediated responses in mPFC slices in WT mice showed a clonidine-induced α2-adrenoceptor-mediated increase in mPFC cell excitability coupled with an increase in input resistance. In contrast, CB1r KO mice showed an α2-adrenoceptor-mediated decrease in mPFC cell excitability. We then examined protein expression levels of α2- and β1-adrenoceptor subtypes in the mPFC as well as TH expression in the locus coeruleus (LC) of mice deficient in CB1r. Both α2- and β1-adrenoceptors exhibited a significant decrease in expression levels in CB1r KO mice when compared to WT in the mPFC, while a significant increase in TH was observed in the LC. To better define whether the same cortical neurons express α2A-adrenoceptor and CB1r in mPFC, we utilized high-resolution immunoelectron microscopy. We localized α2A-adrenoceptors in a knock-in mouse that expressed a hemoagglutinin (HA) tag downstream of the α2A-adrenoceptor promoter. Although the α2A-adrenoceptor was often identified pre-synaptically, we observed co-localization of CB1r with α2-adrenoceptors post-synaptically in the same mPFC neurons. Finally, using receptor binding, we confirmed prior results showing that α2A-adrenoceptor is unchanged in mPFC following acute or chronic exposure to the synthetic cannabinoid receptor agonist, WIN 55,212-2, but is increased, following chronic treatment followed by a period of abstinence. Taken together, these data provide convergent lines of evidence indicating cannabinoid regulation of the cortical adrenergic system.

The Effects of Different Concentrations of the α2-Adrenoceptor Agonist Medetomidine on Basal Excitatory Synaptic Transmission and Synaptic Plasticity in Hippocampal Slices of Adult Mice

BACKGROUND

α2-Adrenoceptor agonists are used frequently in human and veterinary anesthesia as sedative/analgesic drugs. However, they can impair cognition. Little is known about the concentration-dependent effects of α2-adrenoceptor agonists on synaptic plasticity, the neurophysiological basis of learning and memory. Therefore, we investigated the effects of different concentrations of medetomidine, an α2-adrenoceptor agonist, on basal excitatory synaptic transmission and on 2 forms of synaptic plasticity: paired-pulse facilitation (PPF) and long-term potentiation (LTP).

METHODS

Evoked field excitatory postsynaptic potentials were recorded in Schaffer fibers-CA1 pyramidal cell synapses of mouse hippocampal slices, and the initial field excitatory postsynaptic potentials slope was measured. For basal synaptic transmission and PPF, increasing concentrations of medetomidine (1-200 μM) were applied to each slice. For LTP experiments, individual slices were used for each tested concentration of medetomidine (0.1-0.4 μM), where LTP induction and LTP maintenance were measured.

RESULTS

The lower tested concentrations of medetomidine decreased LTP in a concentration-dependent manner, whereas greater concentrations were required to decrease fiber volley amplitude and basal excitatory synaptic transmission. PPF was only affected by the greatest concentration (200 μM).

CONCLUSIONS

Medetomidine decreased LTP in the mouse hippocampus, in accordance with the ability of medetomidine to induce memory deficits.

Noradrenalin and dopamine receptors both control cAMP-PKA signaling throughout the cerebral cortex

Noradrenergic fibers innervate the entire cerebral cortex, whereas the cortical distribution of dopaminergic fibers is more restricted. However, the relative functional impact of noradrenalin and dopamine receptors in various cortical regions is largely unknown. Using a specific genetic label, we first confirmed that noradrenergic fibers innervate the entire cortex whereas dopaminergic fibers were present in all layers of restricted medial and lateral areas but only in deep layers of other areas. Imaging of a genetically encoded sensor revealed that noradrenalin and dopamine widely activate PKA in cortical pyramidal neurons of frontal, parietal and occipital regions with scarce dopaminergic fibers. Responses to noradrenalin had higher amplitude, velocity and occurred at more than 10-fold lower dose than those elicited by dopamine, whose amplitude and velocity increased along the antero-posterior axis. The pharmacology of these responses was consistent with the involvement of Gs-coupled beta1 adrenergic and D1/D5 dopaminergic receptors, but the inhibition of both noradrenalin and dopamine responses by beta adrenergic antagonists was suggestive of the existence of beta1-D1/D5 heteromeric receptors. Responses also involved Gi-coupled alpha2 adrenergic and D2-like dopaminergic receptors that markedly reduced their amplitude and velocity and contributed to their cell-to-cell heterogeneity. Our results reveal that noradrenalin and dopamine receptors both control cAMP-PKA signaling throughout the cerebral cortex with moderate regional and laminar differences. These receptors can thus mediate widespread effects of both catecholamines, which are reportedly co-released by cortical noradrenergic fibers beyond the territory of dopaminergic fibers.

Revisiting enigmatic cortical calretinin-expressing interneurons

Cortical calretinin (CR)-expressing interneurons represent a heterogeneous subpopulation of about 10-30% of GABAergic interneurons, which altogether total ca. 12-20% of all cortical neurons. In the rodent neocortex, CR cells display different somatodendritic morphologies ranging from bipolar to multipolar but the bipolar cells and their variations dominate. They are also diverse at the molecular level as they were shown to express numerous neuropeptides in different combinations including vasoactive intestinal polypeptide (VIP), cholecystokinin (CCK), neurokinin B (NKB) corticotrophin releasing factor (CRF), enkephalin (Enk) but also neuropeptide Y (NPY) and somatostatin (SOM) to a lesser extent. CR-expressing interneurons exhibit different firing behaviors such as adapting, bursting or irregular. They mainly originate from the caudal ganglionic eminence (CGE) but a subpopulation also derives from the dorsal part of the medial ganglionic eminence (MGE). Cortical GABAergic CR-expressing interneurons can be divided in two main populations: VIP-bipolar interneurons deriving from the CGE and SOM-Martinotti-like interneurons originating in the dorsal MGE. Although bipolar cells account for the majority of CR-expressing interneurons, the roles they play in cortical neuronal circuits and in the more general metabolic physiology of the brain remained elusive and enigmatic. The aim of this review is, firstly, to provide a comprehensive view of the morphological, molecular and electrophysiological features defining this cell type. We will, secondly, also summarize what is known about their place in the cortical circuit, their modulation by subcortical afferents and the functional roles they might play in neuronal processing and energy metabolism.

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.

A conserved switch in sensory processing prepares developing neocortex for vision

Developing cortex generates endogenous activity that modulates the formation of functional units, but how this activity is altered to support mature function is poorly understood. Using recordings from the visual cortex of preterm human infants and neonatal rats, we report a "bursting" period of visual responsiveness during which the weak retinal output is amplified by endogenous network oscillations, enabling a primitive form of vision. This period ends shortly before delivery in humans and eye opening in rodents with an abrupt switch to the mature visual response. The switch is causally linked to the emergence of an activated state of continuous cortical activity dependent on the ascending neuromodulatory systems involved in arousal. This switch is sensory system specific but experience independent and also involves maturation of retinal processing. Thus, the early development of visual processing is governed by a conserved, intrinsic program that switches thalamocortical response properties in anticipation of patterned vision.

Ultrastructural localization of extranuclear progestin receptors in the rat hippocampal formation

Progesterone's effects on hippocampus-dependent behavior and synaptic connectivity maybe mediated through the progestin receptor (PR). Although estrogen induces PR mRNA and cytosolic PR in the hippocampus, nuclear PR immunoreactivity is undetectable by light microscopy, suggesting that PR is present at extranuclear sites. To determine whether this is the case, we used immunoelectron microscopy to examine PR distribution in the hippocampal formation of proestrus rats. Ultrastructural analysis revealed that PR labeling is present in extranuclear profiles throughout the CA1 and CA3 regions and dentate gyrus, and, in contrast to light microscopic findings, in nuclei of a few pyramidal and subgranular zone cells. Most neuronal PR labeling is extranuclear and is divided between pre- and postsynaptic compartments; approximately 30% of labeled profiles were axon terminals and 30% were dendrites and dendritic spines. In most laminae, except in CA3 stratum lucidum, about 15% of PR-immunoreactive profiles were unmyelinated axons. In stratum lucidum, where the mossy fiber axons course, more than 50% of PR-labeled profiles were axonal. The remaining 25% of PR-labeled profiles were glia, some resembling astrocytes. PR labeling is strongly dependent on estrogen priming, insofar as few PR-labeled profiles were detected in ovariectomized, oil-replaced females. Synapses formed by PR-labeled terminals were predominantly asymmetric, consistent with a role for progesterone in directly regulating excitatory transmission. These findings suggest that some of progesterone's actions in the hippocampal formation may be mediated by direct and rapid actions on extranuclear PRs and that PRs are well positioned to regulate progesterone-induced changes at synapses.

Cellular and subcellular localization of androgen receptor immunoreactivity relative to C1 adrenergic neurons in the rostral ventrolateral medulla of male and female rats

In male and female rats, high androgen levels can increase blood pressure. The C1 area of the rostral ventrolateral medulla (RVLM), which is crucial for blood pressure regulation, contains estrogen receptors (ERs) in pre- and postsynaptic neuronal compartments and is modulated by estrogens (Wang et al. [2006] Brain Res 1094:163-178). In this study, the cellular and subcellular localization of androgen receptors (ARs) in the C1 area was examined in sections from male, proestrus (high estrogen) and diestrus (low estrogen) female rat brains that were immunocytochemically labeled for AR and tyrosine hydroxylase (TH). By light and electron microscopy, AR-labeled nuclei were scattered among TH-labeled somata in the RVLM; significantly more AR-labeled nuclei were seen males compared to females. Electron microscopy revealed that extranuclear AR-immunoreactivity (ir) was in similar profile types in male and female rats. AR-ir was almost exclusively in myelinated and unmyelinated axons and in glia. Rarely, AR-ir was in axon terminals that contacted TH-containing dendrites. AR-labeled axon terminals had large diameters and contained numerous dense-core vesicles, resembling peptide-containing hypothalamic or solitary tract inputs. No nuclear or extranuclear AR-ir was found in TH-labeled perikarya and dendrites although a few non-TH- labeled dendrites contained AR-ir. Qualitatively, more axonal profiles appeared to be present in males compared to females. These studies suggest that, unlike ERs, ARs in male and female rats are almost exclusively positioned on afferents and glia, suggesting that androgens modulate RVLM C1 neurons, and thus blood pressure, through presynaptic and glial signaling.

Comparison of the maturation of the adrenergic and serotonergic neurotransmitter systems in the brain: implications for differential drug effects on juveniles and adults

Our understanding of the development of neurotransmitter systems in the central nervous system has increased greatly over the past three decades and it has become apparent that drug effects on the developing nervous system may differ considerably from effects on the mature nervous system. Recently it has become clear there are significant differences in the effectiveness of antidepressant drug classes in children and adolescents compared to adults. Whereas the selective serotonin reuptake inhibitors are effective in treating all ages from children to adults, the tricyclic antidepressants, many of which inhibit norepinephrine reuptake, have been shown to be ineffective in treating children and adolescents even though they are effective in adults. We review here the development of the noradrenergic and serotonergic nervous systems, both in terms of neurotransmitter system markers and function. Both of these neurotransmitter systems are primary targets of antidepressant medications as well as of central nervous system stimulants. It is clear from a comparison of their development that the serotonin system reaches maturity much earlier than the norepinephrine system. We suggest this may help explain the differences in response to antidepressants in children and adolescents compared to adults. In addition, these differences suggest that drugs acting preferentially on either neurotransmitter system may impact the normal course of CNS development at different time points. Consideration of such differences in the development of neurotransmitter systems may be of significance in optimizing treatments for a variety of centrally mediated disorders.

Ultrastructural localization of estrogen receptor beta immunoreactivity in the rat hippocampal formation

Several lines of evidence indicate that estrogen affects hippocampal synaptic plasticity through rapid nongenomic mechanisms, possibly by binding to plasma membrane estrogen receptors (ERs). We have previously shown that ERalpha immunoreactivity (ir) is in select interneuron nuclei and in several extranuclear locations, including dendritic spines and axon terminals, within the rat hippocampal formation (Milner et al., [2001] J Comp Neurol 429:355). The present study sought to determine the cellular and subcellular locations of ERbeta-ir. Coronal hippocampal sections from diestrus rats were immunolabeled with antibodies to ERbeta and examined by light and electron microscopy. By light microscopy, ERbeta-ir was primarily in the perikarya and proximal dendrites of pyramidal and granule cells. ERbeta-ir was also in a few nonprincipal cells and scattered nuclei in the ventral subiculum and CA3 region. Ultrastructural analysis revealed ERbeta-ir at several extranuclear sites in all hippocampal subregions. ERbeta-ir was affiliated with cytoplasmic organelles, especially endomembranes and mitochondria, and with plasma membranes primarily of principal cell perikarya and proximal dendrites. ERbeta-ir was in dendritic spines, many arising from pyramidal and granule cell dendrites. In both dendritic shafts and spines, ERbeta-ir was near the perisynaptic zone adjacent to synapses formed by unlabeled terminals. ERbeta-ir was in preterminal axons and axon terminals, associated with clusters of small, synaptic vesicles. ERbeta-labeled terminals formed both asymmetric and symmetric synapses with dendrites. ERbeta-ir also was detected in glial profiles. The cellular and subcellular localization of ERbeta-ir was generally similar to that of ERalpha, except that ERbeta was more extensively found at extranuclear sites. These results suggest that ERbeta may serve primarily as a nongenomic transducer of estrogen actions in the hippocampal formation.

High-Frequency Stimulation Together with Adrenoceptor Activation Facilitates the Maintenance of Long-Term Potentiation at Visual Cortical Inhibitory Synapses

Long-term potentiation (LTP) at inhibitory synapses of rat visual cortex requires firing of presynaptic cells for maintenance, at least at a low frequency. We examined the roles of adrenoceptors in this LTP maintenance. Although high-frequency stimulation (HFS) failed to produce LTP in normal Ca2+ medium, it produced pathway-specific LTP with addition of noradrenaline to the medium soon after HFS. However, this LTP disappeared after washout of noradrenaline. HFS applied during noradrenaline application produced LTP persisting even after washout, indicating that HFS together with adrenoceptor activation makes the adrenergic facilitation enduring. After washout, LTP was produced further by HFS of the conditioned, but not the unconditioned, pathway by the first HFS. Pharmacological examination demonstrated that alpha2 and beta, but not alpha1, receptors facilitated LTP maintenance synergistically. Bath application, but not postsynaptic loading, of either the adenylyl cyclase activator forskolin or the protein kinase C (PKC) activator phorbol ester facilitated LTP maintenance. These results suggest that adrenergic facilitation of LTP maintenance is mediated by presynaptic adrenoceptors via a subfamily of adenylyl cyclases stimulated by Gsalpha, Gibetagamma, and PKC. Thus, it is likely that the activity of noradrenergic afferents takes part in the control of LTP duration at visual cortical inhibitory synapses.

In vivo regulation of dopamine and noradrenaline release by alpha2A-adrenoceptors in the mouse nucleus accumbens

The present study investigated the role of alpha2A-adrenoceptor (alpha2A-AR) subtype in the regulation of noradrenaline (NA) and dopamine (DA) release in the nucleus accumbens (NAc). The effect of locally infused and systemically injected alpha2-AR agonist, dexmedetomidine (DMT), and alpha2-AR antagonist, atipamezole, on NA and DA release was investigated in alpha2A-AR knockout and control mice by using in vivo microdialysis. In addition, we compared the drug effects on DA and NA release in the NAc to their effect on locomotor activity. Baseline NA and DA concentrations did not differ between genotypes. Local infusion of DMT decreased, in a concentration-dependent manner, NA, but not DA, levels in the control mice. However, systemic injection of DMT decreased both NA and DA levels in the control mice. In both cases DMT had no effects on transmitter release in alpha2A-AR knockout mice. Our results suggest that alpha2-ARs regulate the release of NA, but not DA, at the terminal level in the NAc. However, alpha2-ARs regulate DA release in the NAc indirectly by their effect on DA neurones in the ventral tegmental area via an unknown mechanism. In both cases the regulation is mediated by alpha2A-adrenoceptor subtype. Also the modulation of locomotor activity by alpha2-AR agonist and antagonist seems to be mediated via alpha2A-adrenoceptors.

Transiently overexpressed alpha2-adrenoceptors and their control of DNA synthesis in the developing brain

During brain development, neurotransmitters act as trophic factors controlling the patterns of cell replication and differentiation. Alpha2-adrenoceptors (alpha2ARs) are transiently overexpressed in zones with high mitotic activity and we evaluated whether these receptors are linked to DNA synthesis in the perinatal rat brain. Acute administration of clonidine (2 mg/kg), an alpha2AR agonist, elicited dramatic decreases in DNA synthesis in the forebrain, brainstem, and cerebellum whether given on gestational day (GD) 21, or on postnatal days (PN) 1 or 8. However, alpha2AR blockade elicited by yohimbine (2.5 mg/kg) also resulted in decreased DNA synthesis on GD21 and PN8, albeit to a smaller extent than with clonidine. Yohimbine was able to blunt the effects of clonidine, verifying that both drugs are acting through the same receptor population. Because betaARs are also known to regulate DNA synthesis, we used propranolol (10 mg/kg) blockade of betaARs to evaluate whether the alpha2AR effects were mediated by presynaptic autoreceptors that regulate the release of norepinephrine and consequent betaAR responses; the effects of yohimbine were still discernible in the presence of propranolol. Accordingly, transiently overexpressed alpha2ARs in the developing brain participate in the control of DNA synthesis in a biphasic manner, with promotional actions at low, endogenous levels of stimulation, but inhibitory effects when stimulation is high. Effects on alpha2ARs are likely to contribute to long-term consequences of adrenergic agents used in obstetrics or neurotoxicants that affect adrenergic activity.

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.

α2-Adrenoceptor modulation of long-term potentiation elicited in vivo in rat occipital cortex

Pretreatment with the alpha(2)-adrenoceptor agonist clonidine (31.25, 62.5, or 125 microg/kg, i.p.) dose-dependently reduced long-term potentiation (LTP) elicited in vivo in the occipital cortex of anesthetized rats, whereas pretreatment with the alpha(2)-adrenoceptor antagonist yohimbine (0.133, 0.4, or 1.2 mg/kg, i.p.) increased neocortical LTP in a dose-dependent fashion. These effects could be related to the reported disruptive and facilitatory actions induced on memory formation by pretreatment with alpha(2)-adrenoceptor agonists and antagonists, respectively.

β-Adrenoceptor modulation of transiently overexpressed α2-adrenoceptors in brain and peripheral tissues: cellular mechanisms underlying the developmental toxicity of terbutaline

Terbutaline, a selective beta(2)-adrenoceptor (beta(2)AR) agonist, is widely used as a tocolytic to arrest preterm labor but recent studies indicate that excessive betaAR stimulation can alter the expression and function of other neurotransmitter receptors that are essential to fetal/neonatal development. In many immature tissues, alpha(2)-adrenergic receptors (alpha(2)ARs) are overexpressed and the receptors are thought to play a role in cell proliferation and architectural assembly. We evaluated whether betaAR agonists perturb the expression of alpha(2)ARs in central and peripheral tissues during various developmental stages in the fetal and neonatal rat. In peripheral tissues (heart, liver, kidney) administration of terbutaline (10mg/kg s.c. for 4 days) elicited decrements in alpha(2)AR expression only during a critical developmental window that differed for each tissue; terbutaline was more effective than isoproterenol, a mixed beta(1)/beta(2) agonist. Neonatal destruction of sympathetic nerves with 6-hydroxydopamine (6-OHDA) had a biphasic effect, initially reducing alpha(2)ARs but subsequently elevating receptor expression. In contrast to the effects in the periphery, terbutaline administration promoted alpha(2)AR expression in neonatal brain regions with effects preferential to males. As the rat is an altricial species, these results during late gestation and the early neonatal period indicate that betaAR input modulates alpha(2)AR expression during developmental stages in which betaAR tocolytics are likely to be used. Disruption of alpha(2)AR expression and function may therefore contribute to adverse effects that have been noted in the offspring of pregnant women treated with terbutaline.

Regional distribution of alpha(2C)-adrenoceptors in brain and spinal cord of control mice and transgenic mice overexpressing the alpha(2C)-subtype: an autoradiographic study with [(3)H]RX821002 and [(3)H]rauwolscine

Behavioral studies on gene-manipulated mice have started to elucidate the neurobiological functions of the alpha(2C)-adrenoceptor (AR) subtype. In this study, we applied quantitative receptor autoradiography to investigate the potential anatomical correlates of the observed functional effects of altered alpha(2C)-AR expression. Labeling of brain and spinal cord sections with the subtype non-selective alpha(2)-AR radioligand [(3)H]RX821002 and the alpha(2C)-AR-preferring ligand [(3)H]rauwolscine revealed distinct binding-site distribution patterns. In control mice, [(3)H]rauwolscine binding was most abundant in the olfactory tubercle, accumbens and caudate putamen nuclei, and in the CA1 field of the hippocampus. A mouse strain with overexpression of alpha(2C)-AR regulated by a gene-specific promoter showed approximately two- to four-fold increased levels of [(3)H]rauwolscine binding in these regions. In addition, dramatic increases in [(3)H]rauwolscine binding were seen in the nerve layer of the olfactory bulb, the molecular layer of the cerebellum, and the ventricular system of alpha(2C)-AR-overexpressing mice, representing "ectopic" alpha(2C)-AR expression. Competition-binding experiments with several alpha(2)-AR ligands confirmed the alpha(2C)-AR identity of these sites. Our results provide quantitative evidence of the predominance of the alpha(2A)-AR subtype in most regions of the mouse CNS, but also disclose the wide distribution of alpha(2C)-AR in the normal mouse brain, although at relatively low density, except in the ventral and dorsal striatum and the hippocampal CA1 area. alpha(2C)-AR are thus present in brain regions involved in the processing of sensory information and in the control of motor and emotion-related activities such as the accumbens and caudate putamen nuclei, the olfactory tubercle, the lateral septum, the hippocampus, the amygdala, and the frontal and somatosensory cortices. The current results may help in specifying an anatomical framework for the functional roles of the alpha(2A)- and alpha(2C)-AR subtypes in the mouse CNS.

Protective effects of the alpha 2-adrenoceptor antagonist, dexefaroxan, against degeneration of the basalocortical cholinergic system induced by cortical devascularization in the adult rat

It has been hypothesized [Colpaert, F.C., 1994. In: Briley, M., Marien, M. (Eds.), Noradrenergic Mechanisms in Parkinson's Disease. CRC Press, Boca Raton, FL, pp. 225-254] that a deficiency in the noradrenergic system originating from the locus coeruleus is a decisive factor in the progression of central neurodegenerative disorders including Alzheimer's disease, and that treatments which boost noradrenergic transmission (e.g. via blockade of alpha(2)-adrenoceptors) could provide both symptomatic and trophic benefits against the disease. Studies in the rat in vivo demonstrating that the selective alpha(2)-adrenoceptor antagonist dexefaroxan increases acetylcholine release in the cortex, improves measures of cognitive performance and protects against excitotoxin lesions, support this concept. As a further test of the hypothesis, we investigated the effect of dexefaroxan in a rat model of unilateral cortical devascularization that induces a loss of the cortical cholinergic terminal network and a retrograde degeneration of the cholinergic projections that originate in the nucleus basalis magnocellularis. Lesioned and sham-operated rats received a 28-day subcutaneous infusion of dexefaroxan (0.63 mg/rat/day) or vehicle, delivered by osmotic minipumps implanted on the day of the cortical devascularization procedure. In lesioned rats, the dexefaroxan treatment was associated with a significantly higher number and size of vesicular acetylcholine transporter-immunoreactive boutons in comparison to the vehicle treatment; this effect was most marked within cortical layer V. Dexefaroxan also significantly reduced the atrophy of cholinergic neurons within the nucleus basalis magnocellularis. Dexefaroxan had no observable effect on any of these parameters in sham-operated cohorts. These results show that systemically administered dexefaroxan mitigates cholinergic neuronal degeneration in vivo, and provide further evidence for a therapeutic potential of the drug in neurodegenerative diseases such as Alzheimer's disease, where central cholinergic function is progressively compromised.

Neuromodulatory transmitter systems in the cortex and their role in cortical plasticity

Cortical neuromodulatory transmitter systems refer to those classical neurotransmitters such as acetylcholine and monoamines, which share a number of common features. For instance, their centers are located in subcortical regions and send long projection axons to innervate the cortex. The same transmitter can either excite or inhibit cortical neurons depending on the composition of postsynaptic transmitter receptor subtypes. The overall functions of these transmitters are believed to serve as chemical bases of arousal, attention and motivation. The anatomy and physiology of neuromodulatory transmitter systems and their innervations in the cerebral cortex have been well characterized. In addition, ample evidence is available indicating that neuromodulatory transmitters also play roles in development and plasticity of the cortex. In this article, the anatomical organization and physiological function of each of the following neuromodulatory transmitters, acetylcholine, noradrenaline, serotonin, dopamine, and histamine, in the cortex will be described. The involvement of these transmitters in cortical plasticity will then be discussed. Available data suggest that neuromodulatory transmitters can modulate the excitability of cortical neurons, enhance the signal-to-noise ratio of cortical responses, and modify the threshold for activity-dependent synaptic modifications. Synaptic transmissions of these neuromodulatory transmitters are mediated via numerous subtype receptors, which are linked to multiple signal transduction mechanisms. Among the neuromodulatory transmitter receptor subtypes, cholinergic M(1), noradrenergic beta(1) and serotonergic 5-HT(2C) receptors appear to be more important than other receptor subtypes for cortical plasticity. In general, the contribution of neuromodulatory transmitter systems to cortical plasticity may be made through a facilitation of NMDA receptor-gated processes.

Prenatal cocaine exposure alters sensitivity to the effects of idazoxan in a distraction task

The present study was designed to test whether prenatal cocaine (COC) exposure alters sensitivity to the attentional effects of idazoxan (IDZ), an alpha-2 adrenergic antagonist that increases coeruleocortical NE activity. The task assessed subjects' ability to selectively attend to an unpredictable light cue and disregard olfactory distractors. IDZ increased commission errors specifically under conditions of distraction, an effect that was similar in the COC and control groups. In contrast, COC animals were significantly more sensitive than controls to the effects of IDZ on omission errors and nontrials. The pattern of effects suggests that the differential treatment response to IDZ on these latter measures resulted from an alteration in norepinephrine (NE)-modulated dopamine release in the COC animals, reflecting lasting changes in dopaminergic and/or noradrenergic systems as a result of the early cocaine exposure. Based on the behavioral measures that showed a differential response to IDZ in the COC animals, it seems likely that these changes may contribute to the alterations in sustained attention and arousal regulation that have been reported in both animals and humans exposed to cocaine in utero.

Noradrenergic innervation of the developing and mature visual and motor cortex of the rat brain: a light and electron microscopic immunocytochemical analysis

The noradrenergic (NA) innervation of the developing and adult visual and motor cortex of the rat was examined with light and electron microscopic immunocytochemistry by using antibodies against dopamine-beta-hydroxylase. At birth, NA fibers were present in both cortical areas, appearing as two tangential streams, one above and the other below the cortical plate. During the subsequent weeks, these two streams arborized gradually innervating all cortical layers. The adult pattern of distribution was attained by postnatal day 14, but the density of innervation, which was higher in the motor than in the visual cortex, appeared similar to the adult by the end of the third postnatal week. Electron microscopic analysis revealed that a low proportion of NA varicosities (the highest value was 12% in the adult motor cortex in single sections) were engaged in synaptic contact, throughout development, in both areas examined. The overwhelming majority of these synapses were symmetrical, involving predominantly small or medium dendrites. This evidence suggests that transmission by diffusion is the major mode of NA action in the developing and adult cerebral cortex. Noradrenaline released in the rare synaptic junctions may act mainly to reduce the activity of its cortical targets. The results altogether provide morphologic evidence for an involvement of noradrenaline in the development of the neocortex and, along with earlier data on the serotonergic system, indicate that the monoaminergic systems are endowed with a specific anatomic organization in various areas of the brain.

Effects of antisense to the (alpha)2A-adrenoceptors administered into the region of the locus ceruleus on behaviors in plus-maze and sexual behavior tests in sham-operated and castrated male rats

Clinical and experimental findings have implicated brain alpha2-adrenoceptors in the regulation of many physiological functions, including sexual activity and stress-related behavior. However, which subtypes of the three alpha2-adrenoceptors that have now been cloned (alpha2A, alpha2B, and alpha2C) are involved in these controls have yet to be established. Here, we investigated the contribution of alpha2A-adrenoceptors of the locus ceruleus, the principal source of brain noradrenaline, to exploratory and sexual behaviors. Using administration of antisense oligodeoxynucleotide to inhibit the receptor expression, we found that reductions in brainstem alpha2A-adrenoceptor mRNA levels and alpha2-adrenoceptor densities induced by antisense treatment were not accompanied by any changes in the major characteristics of male sexual activity, such as mount latencies and numbers of mounts. However, in sexual behavior tests, antisense-treated male rats had decreased numbers of rearings and thus have higher percentages of behaviors positively correlated with sexual activity. Besides, antisense-treated animals had decreased anxiety in plus-maze tests. The data demonstrate that inhibition of alpha2A-adrenoceptor expression in the region of the locus ceruleus has an anxiolytic-like effect and facilitates male's attention to female in sexual behavior test.

Hippocampal tyrosine kinase A receptors are restricted primarily to presynaptic vesicle clusters

Adult septohippocampal cholinergic neurons are dependent on trophic support for normal functioning and survival; these effects are largely mediated by the tyrosine kinase A receptor (TrkA), which binds its ligand, nerve growth factor (NGF), with high affinity. To determine the subcellular localization of TrkA within septohippocampal terminal fields, two rabbit polyclonal antisera to the extracellular domain of TrkA were localized immunocytochemically in rat dentate gyrus by light and electron microscopy. By light microscopy, TrkA immunoreactivity was found mostly in fine, varicose fibers primarily in the hilus and, to a lesser extent, in the granule cell and molecular layers. By electron microscopy, the central and infragranular regions of the hilus contained the highest densities of TrkA-immunoreactive profiles. Most TrkA-labeled profiles were axons (31% of 3,473), axon terminals (20%), and glia (38%); fewer were dendrites (6%), dendritic spines (5%), and granule cell and interneuron somata (<1%). TrkA immunolabeling in axons and axon terminals was discrete, often concentrated in patches of small synaptic vesicles that were adjacent to somatic and dendritic profiles. TrkA-labeled terminals formed both asymmetric and symmetric synapses, primarily with dendritic shafts and spines. TrkA-immunoreactive glial profiles frequently apposed terminals contacting dendritic spines. The findings that presynaptic profiles contain TrkA immunolabeling in sites of vesicle accumulation suggest that NGF binding to TrkA may influence transmitter release. The presence of TrkA immunoreactivity in somata, dendrites, and glia further suggests that cells within the dentate gyrus may take up NGF.

beta-adrenergic receptors primarily are located on the dendrites of granule cells and interneurons but also are found on astrocytes and a few presynaptic profiles in the rat dentate gyrus

In the rat dentate gyrus, beta-adrenergic receptor (beta-AR) activation is thought to be important in mediating the effects of norepinephrine (NE). beta-AR-immunoreactivity (beta-AR-I) was localized in this study by light and electron microscopy in the rat dentate gyrus by using two previously characterized antibodies to the beta-AR. By light microscopy, dense beta-AR-I was observed in the somata of granule cells and a few hilar interneurons. Diffuse and slightly granular beta-AR-I was found in all laminae, although it was most noticeable in the molecular layer. Ultrastructurally, the cytoplasm of granule cell and interneuronal perikarya (some of which contained parvalbumin immunoreactivity) contained beta-AR-I. beta-AR-I was associated primarily with the endoplasmic reticula; however, a few patches were observed near the plasmalemma. Quantitative analysis revealed that the greatest proportion of beta-AR-labeled profiles was found in the molecular layer. The majority of beta-AR-labeled profiles were either dendritic or astrocytic. In dendritic profiles, beta-AR-I was prominent near postsynaptic densities in large dendrites, many of which originated from granule cell somata. Moreover, some beta-AR-I was found in dendritic spines, sometimes affiliated with the spine apparati. Astrocytic profiles with beta-AR-I were commonly found next to unlabeled terminals which formed asymmetric (excitatory-type) synapses with dendritic spines. Additionally, beta-AR-I was observed in a few unmyelinated axons and axon terminals, many of which formed synapses with dendritic spines. Dual-labeling studies revealed that axons and axon terminals containing tyrosine hydroxylase (TH), the catecholamine synthesizing enzyme, often were near both neuronal and glial profiles containing beta-AR-I. These studies demonstrate that hippocampal beta-AR-I is localized: 1) principally in postsynaptic sites on granule cells and a few interneurons (some of which were basket cells); and 2) in glial processes. These observations add further support to the contention that beta-AR-activation modulates synaptic function through disparate pathways: directly, at either postsynaptic densities or presynaptic processes, or indirectly, through adjacent glial processes.

Regulation of monoamine receptors in the brain: dynamic changes during stress

Monoamine receptors are membrane-bound receptors that are coupled to G-proteins. Upon stimulation by agonists, they initiate a cascade of intracellular events that guide biochemical reactions of the cell. In the central nervous system, they undergo diverse regulatory processes, among which are receptor desensitization, internalization into the cell, and downregulation. These processes vary among different types of monoamine receptors. alpha 2-Adrenoceptors are often downregulated by agonists, and beta-adrenoceptors are internalized rapidly. Others, such as serotonin1A-receptors, are controlled tightly by steroid hormones. Expression of these receptors is reduced by the "stress hormones" glucocorticoids, whereas gonadal hormones such as testosterone can counterbalance the glucocorticoid effects. Because of this, the pattern of monoamine receptors in certain brain regions undergoes dynamic changes when there are elevated concentrations of agonists or when the hormonal milieu changes. Stress is a physiological situation accompanied by the high activity of brain monoaminergic systems and dramatic changes in peripheral hormones. Resulting alterations in monoamine receptors are considered to be in part responsible for changes in the behavior of an individual.

Ultrastructural localization of full-length trkB immunoreactivity in rat hippocampus suggests multiple roles in modulating activity-dependent synaptic plasticity

Neurotrophins acting at the trkB receptor have been shown to be important modulators of activity-dependent plasticity in the hippocampus, but the mechanisms underlying these effects are not yet well understood. To identify the cellular and subcellular targets of trkB ligands in the adult rat hippocampal formation, full-length trkB receptor immunoreactivity (trkB-IR) was localized using electron microscopy. trkB-IR was present in the glutamatergic pyramidal and granule cells. Labeling in these neurons appeared as discrete clusters and was primarily in axons, excitatory-type axon terminals, and dendritic spines and to a lesser extent in somata and dendritic shafts. trkB-IR was commonly found on the plasma membrane of dendritic spines, whereas in other subcellular regions trkB-IR was often intracellular. Labeling was strikingly dense within axon initial segments, suggesting extensive receptor trafficking. trkB-IR was not confined to pyramidal and granule cells. Dense trkB-IR was found in occasional interneuron axon initial segments, some axon terminals forming inhibitory-type synapses onto somata and dendritic shafts, and excitatory-type terminals likely to originate extrahippocampally. This suggests that trkB is contained in some GABAergic interneurons, neuromodulatory (e.g., cholinergic, dopaminergic, and noradrenergic) afferents, and/or glutamatergic afferents. These data indicate that full-length trkB receptor activation may modulate glutamatergic pathways of the trisynaptic circuit both presynaptically at axon terminals and initial segments and postsynaptically at dendritic spines and shafts. Signaling via catalytic trkB may also presynaptically affect inhibitory and modulatory neurons. A pan-trkB antibody labeled the same neuronal populations as the full-length-specific trkB antiserum, but the labels differed in density at various subcellular sites. These findings provide an ultrastructural foundation for further examining the mechanisms through which neurotrophins acting at trkB receptors contribute to synaptic plasticity.

Effects of noradrenaline on frequency tuning of auditory cortex neurons during wakefulness and slow-wave sleep

This study shows the effects of noradrenaline (NA) on receptive fields of auditory cortex neurons in awake animals; it is the first one to describe the effects of NA on neurons in sensory cortex, in different natural states of vigilance. The frequency receptive field of 250 auditory cortex neurons was determined before, during and after ionophoretic application of NA while recording the state of vigilance of unanaesthetized guinea-pigs. When NA significantly changed the spontaneous activity (85 out of 250 cells), the dominant effect was a decrease (61 out of 85 cells, 72%). When NA significantly changed the evoked activity (107 out of 250 cells), the dominant effect was also a decrease (84 out of 107 cells, 78%). During and after NA application, the signal-to-noise ratio (S/N, i.e. evoked/spontaneous activity) was unchanged, but the selectivity for pure-tone frequencies was enhanced. When the effects occurring in wakefulness and in slow-wave sleep (SWS) were compared, it appeared that the predominantly inhibitory effect of NA on spontaneous and evoked activity was present in both states. The S/N ratio was unchanged and the selectivity was increased in both states. However, during SWS, the percentage of cells inhibited by NA was lower, and the effects on the frequency selectivity were smaller than in wakefulness. In contrast, GABA produced similar inhibitory effects on spontaneous and on evoked activity during wakefulness and SWS. Comparisons with previous data obtained using the same protocol in urethane anaesthetized animals (Manunta & Edeline 1997) indicate that the effects of NA were qualitatively the same. Based on these results, we suggest that any hypothesis concerning the role of NA in cortical plasticity should take into account the fact that the predominantly inhibitory effects of NA lead to decrease the size of the receptive field.

p75NTR immunoreactivity in the rat dentate gyrus is mostly within presynaptic profiles but is also found in some astrocytic and postsynaptic profiles

To localize neurotrophin binding sites within the rat dentate gyrus, the distribution of low-affinity p75 neurotrophin receptor (p75NTR) immunoreactivity (IR) was examined by using antiserum raised against the cytoplasmic domain of the receptor. Semiquantitative electron microscopic examination of p75NTR-labeled sections showed that most p75NTR-labeled profiles were axons and axon terminals (72% from a total of 3,975); p75NTR-IR was observed throughout the extent of these structures and was not limited to the plasmalemmal surface. Axons and axon terminals containing p75NTR-IR were distributed in approximately equal proportions across the hilus, infragranular zone, and the inner, middle, and outer molecular layers; significantly fewer p75NTR-labeled profiles were observed in the granule cell layer. Axon terminals containing p75NTR-IR, which made synapses (296 of 552), formed equal proportions of symmetric and asymmetric synapses, primarily with the shafts and spines of dendrites. The remainder of the p75NTR-labeled terminals apposed unlabeled somata and dendrites without forming synapses in the single sections analyzed. In addition, p75NTR-IR was contained within some astrocytes (17.5% of 3,975) and dendritic shafts (3%) and spines (5%). Within dendritic spines, p75NTR-IR was most often associated with the plasmalemmal surface near postsynaptic densities; in dendritic shafts, p75NTR labeling was associated with microfilaments distant from the plasmalemma. Most p75NTR-labeled dendritic profiles were located in the molecular layer, and some originated from granule cells. Moreover, in some granule cell somata (<1% of 3,975), p75NTR-IR was associated with endosomes. The primary localization of p75NTR-IR to presynaptic structures in the dentate gyrus, presumably arising from medial septal/diagonal band neurons, agrees with previous reports. However, p75NTR-IR within some astrocytes, somata, and dendritic structures suggests that this receptor may also be involved in controlling local neurotrophin levels and possibly modulating the viability of local hippocampal cell populations.

Alpha2A-adrenergic receptors are primarily presynaptic heteroreceptors in the C1 area of the rat rostral ventrolateral medulla

The 2A subtype of the alpha-adrenergic receptor (alpha2A-AR) is necessary for the hypotensive effects of clonidine and other sympathoinhibitory adrenergic agonists. This hypotensive response appears to be due to the inhibition of sympathoexcitatory reticulospinal neurons found in the rostral ventrolateral medulla (RVL), including neurons of the C1 adrenergic cell group. The cellular mechanisms underlying this inhibition have not been established. Thus, this study examined the ultrastructural relationships between profiles containing alpha2AAR-immunoreactivity (alpha2AAR-I) and those containing the catecholamine synthesizing enzyme tyrosine hydroxylase (TH) to determine potential cellular substrates for alpha2A-AR inhibition of C1 neuron activity. Consistent with previous light microscopic studies, alpha2AAR-I was found in perikarya and large dendrites and the majority of these profiles also contained TH-labeling (approximately 70% of 140). However, alpha2AAR-I in these cells was primarily found within endosomes and Golgi complexes and in clusters associated with the endoplasmic reticula, probable sites for synthesis and/or trafficking of receptors. In contrast, most of the alpha2AAR-I profiles (n=646) in the RVL were axons and axon terminals (approximately 68%) which lacked TH immunoreactivity. alpha2AAR-labeled axons were small and unmyelinated and labeled terminals usually formed symmetric synapses on the shafts of catecholaminergic or unlabeled dendrites. Most of these alpha2AAR-labeled axons were found in close proximity to TH-labeled profiles and approximately one-fifth (17% of 408) of the alpha2AAR-labeled axons and axon terminals directly contacted TH-labeled profiles, mostly dendrites. These studies suggest that alpha2AARs in the C1 area of the RVL function primarily as heteroreceptors on presynaptic axons and terminals of non-catecholaminergic cells, some of which provide inhibitory synaptic input to C1 neurons. These receptors may be activated by catecholamines released either from the dendrites of C1 neurons or from the terminals of other catecholaminergic neurons via volume transmission.

Expression of alpha2A adrenoceptors during rat neocortical development

Norepinephrine has been suggested to play a neurotrophic role during development and is present in the brain as early as embryonic day (E) 12. We have recently demonstrated that the alpha2A adrenoceptor subtype is widely expressed during times of neuronal migration and differentiation throughout the developing brain. Here, we report the temporal and spatial expression pattern of alpha2A adrenoceptors in neocortex during late embryonic and early postnatal development using in situ hybridization and receptor autoradiography. Functional alpha2 receptors in embryonic rat cortex were also detected using agonist stimulated [35S]GTPgammaS autoradiography. Both alpha2A mRNA and protein expression were strongly increased by E19 and E20, respectively. The increased expression was in the cortical plate and intermediate and subventricular zones, corresponding to tiers of migrating and differentiating neurons. This transient up-regulation of alpha2A adrenoceptors was restricted to the lateral neocortex. At E20, functional alpha2 adrenoceptors were also detected in deep layers of lateral neocortex. During the first week of postnatal development, the expression of alpha2A mRNA and protein changed markedly, giving rise to a more mature pattern of anatomical distribution. The temporal and spatial distribution of alpha2A adrenoceptors in developing neocortex is consistent with expression of functional proteins on migrating and differentiating layer IV to II neurons. These findings suggest that alpha2A receptors may mediate a neurotrophic effect of norepinephrine during fetal cortical development. The early delineation of the lateral neocortex, which will develop into somatosensory and auditory cortices, suggests an intrinsic regulation of alpha2A mRNA expression.

Up-regulation of immunolabeled alpha2A-adrenoceptors, Gi coupling proteins, and regulatory receptor kinases in the prefrontal cortex of depressed suicides

Suicide and depression are associated with an increased density of alpha2-adrenoceptors (radioligand receptor binding) in specific regions of the human brain. The function of these inhibitory receptors involves various regulatory proteins (Gi coupling proteins and G protein-coupled receptor kinases, GRKs), which work in concert with the receptors. In this study we quantitated in parallel the levels of immunolabeled alpha2A-adrenoceptors and associated regulatory proteins in brains of suicide and depressed suicide victims. Specimens of the prefrontal cortex (Brodmann area 9) were collected from 51 suicide victims and 31 control subjects. Levels of alpha2A-adrenoceptors, Galphai1/2 proteins, and GRK 2/3 were assessed by immunoblotting techniques by using specific polyclonal antisera and the immunoreactive proteins were quantitated by densitometry. Increased levels of alpha2A-adrenoceptors (31-40%), Galphai1/2 proteins (42-63%), and membrane-associated GRK 2/3 (24-32%) were found in the prefrontal cortex of suicide victims and antidepressant-free depressed suicide victims. There were significant correlations between the levels of GRK 2/3 (dependent variable) and those of alpha2A-adrenoceptors and Galphai1/2 proteins (independent variables) in the same brain samples of suicide victims (r = 0.56, p = 0.008) and depressed suicide victims (r = 0.54, p = 0.041). Antemortem antidepressant treatment was associated with a significant reduction in the levels of Galphai1/2 proteins (32%), but with modest decreases in the levels of alpha2A-adrenoceptors (6%) and GRK 2/3 (18%) in brains of depressed suicide victims. The increased levels in concert of alpha2A-adrenoceptors, Galphai1/2 proteins, and GRK 2/3 in brains of depressed suicide victims support the existence of supersensitive alpha2A-adrenoceptors in subjects with major depression.

Preservation of ultrastructure and antigenicity for EM immunocytochemistry following intracellular recording and labeling of single cortical neurons in brain slices

Knowledge of the distribution of neurotransmitters, neuromodulators, and transmitter receptors operating at specific synaptic sites on cortical neurons is essential for understanding the precise mechanisms that underlie the dynamic properties of cortical microcircuitry. We report on a new combination of techniques for analyzing chemically-specified synaptic input to individual cortical neurons first electrophysiologically characterized in the in vitro brain slice preparation. We tested the feasibility of this approach by performing intracellular recordings and biocytin injections in guinea pig medial prefrontal cortex slices and then by performing dual preembedding immunocytochemistry in order to localize neuronal nitric oxide synthase relative to single biocytin-filled neurons. The recorded cell and nitric oxide synthase immunoreactivity were visualized by light and electron microscopy utilizing both peroxidase and silver intensified gold stains. Single neurons were also dually visualized with fluorescence for light microscopy and with silver intensified gold for electron microscopy. Our findings indicate that both antigenicity and ultrastructure can be well preserved in tissue first used for in vitro slice experiments. This combination of methods should be widely applicable for analyzing the subcellular distribution of neuronal molecules such as receptors, channels and enzymes on physiologically characterized mammalian neurons.

Ultrastructural evidence for combined action of noradrenaline and vasoactive intestinal polypeptide upon neurons, astrocytes, and blood vessels of the rat cerebral cortex

The intracortical organization of the noradrenaline (NA) and vasoactive intestinal polypeptide (VIP) systems provides ample opportunity for functional convergence, and accumulated evidence indicates that NA and VIP share certain cellular actions upon both neuronal and nonneuronal cortical elements. In the present study, a double immunolabeling method was combined with a silver-gold intensification procedure to examine the ultrastructural relationships of the NA coeruleocortical afferents and the intrinsic VIP neurons with three main constituents of the cortex: neurons, astrocytes, and blood vessels. Electron microscopy of singly or doubly labeled material indicated that NA and VIP boutons are engaged in a variety of anatomical relationships with both neuronal and nonneuronal elements. Dendritic shafts and perikarya of nonpyramidal neurons, some of which are VIP positive, receive combined NA and VIP synapses. A significant number of cortical microvessels are in intimate contact with NA or VIP profiles. NA axons often form perivascular loops, and VIP dendritic shafts of large diameter are frequently observed to bend around the vessel circumference. Serial section examination demonstrates that some NA boutons are directly apposed to the capillary wall at sites of glial end-feet discontinuities, whereas VIP boutons contact astrocytic sleeves of capillaries but never cross the perivascular astroglial barrier. Some VIP dendrites containing coated vesicles make intimate contact with the capillary basal lamina. Astrocytic perikarya, mainly in the supragranular layers, are also directly apposed to NA and/or VIP elements. These complex anatomical relationships provide a structural basis for the known interactions between NA and VIP in the control of cortical metabolism and function.

Differential timing for the appearance of neuronal and astrocytic beta-adrenergic receptors in the developing rat visual cortex as revealed by light and electron-microscopic immunocytochemistry

The developing cerebral cortex is likely to exhibit synaptic circuitries differing from those in adulthood, due to the asynchronous maturation of the various neurotransmitter systems. Two antisera directed against mammalian beta-adrenergic receptors (beta AR), beta AR248 and beta AR404, were used to characterize the laminar, cellular, and subcellular distributions of beta AR in postnatally developing visual cortex of rats. The antigenic sites were the receptor's third intracellular loop for beta AR248 and the C-terminus for beta AR404. During week 1, most of the beta AR404- and beta AR248-immunoreactive sites were dendritic. Morphologically identifiable synapses were rare, even in layer 1: yet, semiquantitative analysis revealed that beta AR404-immunoreactive synapses comprise half of those in layer 1. During week 2, the two antisera began to diverge in their immunoreactivity patterns. With beta AR248, there was an overall decline in immunoreactivity, while with beta AR404, there was an increase in immunoreactive sites, primarily due to labeled astrocytic processes that increased 200-fold in areal density by week 3. In contrast, the areal density of synaptic labeling by beta AR404 barely doubled, in spite of the 30-fold increase in areal density of synapses. These results suggest that beta AR undergo conformational changes during early postnatal periods, causing alterations in their relative antigenicity to the two antisera. Furthermore, the first 2 weeks appear to be characterized by modulation of earliest-formed synapses, and the subsequent phase is marked by addition of astrocytic responses that would be more diffuse temporally and spatially. Activation of beta AR is recognized to increase visually evoked activity relative to spontaneous activity. Moreover, astrocytic beta AR are documented to regulate extracellular concentrations of glutamate, ATP, and neurotrophic factors important for the formation of binocular connections. Thus, neuronal and astrocytic responses may, together and in tandem, facilitate strengthening of intracortical synaptic circuitry during early life.

Postnatal changes in the laminar and subcellular distribution of NMDA-R1 subunits in the cat visual cortex as revealed by immuno-electron microscopy

Although it is recognized that nearly all synapses in the cerebral cortex form postnatally, little is known about the emergence of molecules necessary to render these synapses functional. This study visualized the emergence of synaptically localized NMDA receptors by immuno-electron microscopic labeling of the receptor's obligatory subunit, NMDA-R1, in the developing cat visual cortex. Prior to eye-opening (postnatal day 2-10), NMDA-R1 immunoreactivity is already present within dendritic and growth cones, even though these profiles are devoid of synaptic specializations. This indicates that synthesis and incorporation of NMDA-R1 into plasma membranes are independent of form vision. During the next 2-3 weeks, i.e., preceding the onset of the critical period for ocular dominance plasticity (around the fourth week), NMDA-R1 immunoreactivity changes from a diffuse distribution within dendrites to a more discrete aggregation over postsynaptic densities of axo-spinous junctions. Such clustering of NMDA-R1 at synapses may be a prerequisite for stabilization and strengthening of synapses activated by visual stimulation during the critical period. Furthermore, only during the first several weeks, intensely NMDA-R1-immunoreactive neurons are present in the infragranular layers and the white matter. Enrichment of NMDA-R1 in the deep-layer neurons may reflect the neurons' supportive role in the development of cortical circuitry, serving as transient synaptic targets for geniculate and cortico-cortical afferents while these afferents 'wait' in the infragranular for their ultimate, life-long target neurons to become receptive in the upper layers.