The pharmacology of adrenergic neuronal responses in the cerebral cortex: evidence for excitatory alpha- and inhibitory beta-receptors

A Photoactivatable Norepinephrine for Probing Adrenergic Neural Circuits

Norepinephrine (NE) is a critical neuromodulator that mediates a wide range of behavior and neurophysiology, including attention, arousal, plasticity, and memory consolidation. A major source of NE is the brainstem nucleus the locus coeruleus (LC), which sends widespread projections throughout the central nervous system (CNS). Efforts to dissect this complex noradrenergic circuitry have driven the development of many tools that detect endogenous NE or modulate widespread NE release via LC activation and inhibition. While these tools have enabled research that elucidates physiological roles of NE, additional tools to probe these circuits with a higher degree of spatial precision could enable a finer delineation of function. Here, we describe the synthesis and chemical properties of a photo-activatable NE, [Ru(bpy) 2 (PMe 3 )(NE)]PF 6 (RuBi-NE). We validate the one-photon (1P) release of NE using whole-cell patch clamp electrophysiology in acute mouse brain slices containing the LC. We show that a 10 ms pulse of blue light, in the presence of RuBi-NE, briefly modulates the firing rate of LC neurons via α-2 adrenergic receptors. The development of a photo-activatable NE that can be released with light in the visible spectrum provides a new tool for fine-grained mapping of complex noradrenergic circuits, as well as the ability to probe how NE acts on non-neuronal cells in the CNS.

Adrenergic Modulation of Visually-Guided Behavior

Iontophoretic application of norepinephrine (NE) into the primary visual cortex (V1) in vivo reduces spontaneous and evoked activity, without changing the functional selectivity of cortical units. One possible consequence of this phenomenon is that adrenergic receptors (ARs) regulate the signal-to-noise ratio (SNR) of neural responses in this circuit. However, despite such strong inhibitory action of NE on neuronal firing patterns in V1, its specific action on visual behavior has not been studied. Furthermore, the majority of observations regarding cortical NE from in vivo recordings have been performed in anesthetized animals and have not been tested behaviorally. Here, we describe how micro-infusion of AR agonists/antagonists into mouse V1 influences visually-guided behavior at different contrasts and spatial frequencies. We found that cortical activation of α₁- and β-AR produced a substantial reduction in visual discrimination performance at high contrasts and low spatial frequencies, consistent with a divisive effect. This reduction was reversible and was accompanied by a rise in escape latencies as well as an increase in the group averaged choice variance as a function of stimulus contrast. We conclude that pharmacological activation of cortical AR regulates visual perception and adaptive behavior through a divisive gain control of visual responses.

RGD/TAT-functionalized chitosan-graft-PEI-PEG gene nanovector for sustained delivery of NT-3 for potential application in neural regeneration

In this study, we prepared a multifunctional gene delivery nanovector containing a chitosan (CS) backbone and polyethylenimine (PEI) arms with arginine-glycine-aspartate (RGD)/twin-arginine translocation (TAT) conjugated via polyethylene glycol (PEG). Branched PEI, with a molecular weight of 2000 Da, was used to achieve a balance between biocompatibility and transfection efficiency, whereas RGD/TAT peptides were conjugated for enhanced targeting ability and cellular uptake. Synthesis of the copolymers was confirmed by characterizing the chemical structure with ¹H nuclear magnetic resonance and Fourier Transform Infrared Spectroscopy (FTIR). The nanovector was biocompatible with cells and showed excellent capability for DNA condensation; the resulting complexes with DNA were well-formed, and possessed small particle size and reasonable positive charge. Higher gene transfection efficiency, compared to that achieved with PEI (25 kDa), was confirmed in tumor (HeLa cells) and normal cells (293T and NIH 3T3 cells). More importantly, the cells transfected with the chitosan-graft-PEI-PEG/pCMV-EGFP-Ntf3 complex produced sustained neurotrophin-3 with a linear increase in cumulative concentration, which induced neuronal differentiation of neural stem cell and promoted neurite outgrowth. These findings suggested that our multifunctional copolymers might be ideal nanovectors for engineering cells via gene transfection, and could potentially be applied in tumor therapy and regenerative medicine.

STATEMENT OF SIGNIFICANCE

We successfully prepared a multifunctional gene delivery nanovector containing branched PEI with a molecular weight of 2000 Da to balance between biocompatibility and transfection efficiency, and RGD/TAT peptides for enhanced targeting ability and cellular uptake. The well-formed CPPP/DNA complexes of small particle size and reasonable positive charges potentially enhanced gene transfection in both tumor and normal cells. More importantly, the CPPP/pCMV-EGFP-Ntf3 complex-transfected 293T cells could produce sustained NT-3 with a constant ratio, which induced neuron differentiation of NSC and promoted neurite outgrowth. Therefore, our study provided an effective strategy for producing neurotrophins by engineering cells with gene delivery, which deserved wide investigation and potential application in regenerative medicine.

Does Global Astrocytic Calcium Signaling Participate in Awake Brain State Transitions and Neuronal Circuit Function?

We continuously need to adapt to changing conditions within our surrounding environment, and our brain needs to quickly shift between resting and working activity states in order to allow appropriate behaviors. These global state shifts are intimately linked to the brain-wide release of the neuromodulators, noradrenaline and acetylcholine. Astrocytes have emerged as a new player participating in the regulation of brain activity, and have recently been implicated in brain state shifts. Astrocytes display global Ca²⁺ signaling in response to activation of the noradrenergic system, but whether astrocytic Ca²⁺ signaling is causative or correlative for shifts in brain state and neural activity patterns is not known. Here we review the current available literature on astrocytic Ca²⁺ signaling in awake animals in order to explore the role of astrocytic signaling in brain state shifts. Furthermore, we look at the development and availability of innovative new methodological tools that are opening up for new ways of visualizing and perturbing astrocyte activity in awake behaving animals. With these new tools at hand, the field of astrocyte research will likely be able to elucidate the causal and mechanistic roles of astrocytes in complex behaviors within a very near future.

Estradiol modulation of phenylephrine-induced excitatory responses in ventromedial hypothalamic neurons of female rats

Estrogens act within the ventromedial nucleus of the hypothalamus (VMN) to facilitate lordosis behavior. Estradiol treatment in vivo induces alpha(1b)-adrenoreceptor mRNA and increases the density of alpha(1B)-adrenoreceptor binding in the hypothalamus. Activation of hypothalamic alpha(1)-adrenoceptors also facilitates estrogen-dependent lordosis. To investigate the cellular mechanisms of adrenergic effects on VMN neurons, whole-cell patch-clamp recordings were carried out on hypothalamic slices from control and estradiol-treated female rats. In control slices, bath application of the alpha(1)-agonist phenylephrine (PHE; 10 microM) depolarized 10 of 25 neurons (40%), hyperpolarized three neurons (12%), and had no effect on 12 neurons (48%). The depolarization was associated with decreased membrane conductance, and this current had a reversal potential close to the K(+) equilibrium potential. The alpha(1b)-receptor antagonist chloroethylclonidine (10 microM) blocked the depolarization produced by PHE in all cells. From estradiol-treated rats, significantly more neurons in slices depolarized (71%) and fewer neurons showed no response (17%) to PHE. PHE-induced depolarizations were significantly attenuated with 4-aminopyridine (5 mM) but unaffected by tetraethylammonium chloride (20 mM) or blockers of Na(+) and Ca(2+) channels. These data indicate that alpha(1)-adrenoceptors depolarize VMN neurons by reducing membrane conductance for K(+). Estradiol amplifies alpha(1b)-adrenergic signaling by increasing the proportion of VMN neurons that respond to stimulation of alpha(1b)-adrenergic receptors, which is expected in turn to promote lordosis.

Noradrenergic inputs mediate state dependence of auditory responses in the avian song system

Norepinephrine (NE) plays a complex role in the behavioral state-dependent regulation of sensory processing. However, the role of forebrain NE action in modulating high-order sensory activity has not been directly addressed. In this study, we take advantage of the discrete, feedforward organization of the avian song system to identify a site and mechanism of NE action underlying state-dependent modulation of sensory processing. We have developed an experimental paradigm in which brief arousal repeatedly suppresses song system auditory responsiveness. Using pharmacological manipulations in vivo, we show that infusion of alpha-adrenergic antagonists into the NIf (nucleus interfacialis of the nidopallium), an auditory forebrain area, blocks this state-dependent modulation. We also demonstrate dose-dependent enhancement and suppression of song system auditory response properties by NE and adrenergic agonists. Our results demonstrate that noradrenergic release in a single forebrain area is a mechanism underlying behavioral state-dependent regulation of auditory processing in a neural system specialized for vocal learning.

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.

Norepinephrine exhibits two distinct profiles of action on sensory cortical neuron responses to excitatory synaptic stimuli

Located within the central gray of the caudal pons, the locus coeruleus (LC) is the sole source of norepinephrine (NE) projections to the forebrain. NE is released both tonically and phasically from axonal varicosities in LC efferent target circuits. NE has been shown to produce a diverse set of actions, including suppression of spontaneous and stimulus evoked discharge, augmentation of synaptically evoked excitation, and inhibition and gating of otherwise subthreshold synaptic inputs. Utilizing an extracellular in vitro tissue slice preparation and microiontophoretic techniques, the dose-dependent actions of NE on glutamate-evoked discharges of layer II/III and layer V sensory cortical neurons were investigated. Noradrenergic effects were further examined in terms of cell and adrenoceptor specificity. The results indicate two exclusive modulatory actions of NE: 1) ejection current-dependent suppression of glutamate evoked discharge, and 2) ejection current-dependent facilitation of glutamate-evoked discharge followed by suppression of the maximal facilitated response. These effects were observed in both normal and low Ca(2+) / high Mg(2+) bathing media, suggesting a postsynaptic site for NE's actions. The facilitation of glutamate evoked discharge was selectively mimicked by the alpha-1 agonist, phenylephrine, whereas the dose-dependent suppression was mimicked by the beta-agonist isoproterenol. These results suggest that the suppressant and facilitating actions of NE are mediated by beta and alpha-1 receptors, respectively. In general, these results are consistent with previous demonstrations of NE modulatory actions on central neurons, but indicate that in the cerebral cortex these effects are both cell- and receptor-specific.

Expression of alpha(1b) adrenoceptor mRNA in corticotropin-releasing hormone-containing cells of the rat hypothalamus and its regulation by corticosterone

Considerable evidence supports a role for brainstem adrenergic and noradrenergic inputs to corticotropin-releasing hormone (CRH) cells of the hypothalamic paraventricular nucleus (PVN), in the control of hypothalamic-pituitary-adrenocortical (HPA) axis function. However, little is known about specific adrenoceptor (ADR) subtypes in CRH-containing cells of the PVN. Here we demonstrate, using dual in situ hybridization, that mRNA encoding alpha(1b) ADR is colocalized with CRH in the rat PVN. Furthermore, we confirm that these alpha(1b) ADR mRNA-containing cells are stress-responsive, by colocalization with c-fos mRNA after restraint, swim, or immune stress. To determine whether expression of alpha(1b) ADR mRNA is influenced by circulating glucocorticoids, male rats underwent bilateral adrenalectomy (ADX) or sham surgery, and were killed after 1, 3, 7, or 14 d. In situ hybridization revealed levels of alpha(1b) ADR mRNA were increased in the PVN 7 and 14 d after ADX, but were not altered in the hippocampus, amygdala, or dorsal raphe. Additional rats underwent ADX or sham surgery and received a corticosterone pellet (10 or 50 mg) or placebo for 7 d. Corticosterone replacement (10 mg) reduced the ADX-induced increase in PVN alpha(1b) ADR mRNA to control levels, whereas 50 mg of corticosterone replacement resulted in a decrease in PVN alpha(1b) ADR mRNA as compared with all other groups. Furthermore, levels of plasma corticosterone were significantly correlated (inverse relationship) with alpha(1b) ADR mRNA in the PVN. We conclude that alpha(1b) ADR mRNA is expressed in CRH-containing, stress-responsive cells of the PVN and is highly sensitive to circulating levels of corticosterone. Because activation of the alpha(1B) adrenoceptor is predominantly excitatory within the brain, we predict that this receptor plays an important role in facilitation of the HPA axis response.

Expression of alpha(1b) adrenoceptor mRNA in corticotropin-releasing hormone-containing cells of the rat hypothalamus and its regulation by corticosterone

Considerable evidence supports a role for brainstem adrenergic and noradrenergic inputs to corticotropin-releasing hormone (CRH) cells of the hypothalamic paraventricular nucleus (PVN), in the control of hypothalamic-pituitary-adrenocortical (HPA) axis function. However, little is known about specific adrenoceptor (ADR) subtypes in CRH-containing cells of the PVN. Here we demonstrate, using dual in situ hybridization, that mRNA encoding alpha(1b) ADR is colocalized with CRH in the rat PVN. Furthermore, we confirm that these alpha(1b) ADR mRNA-containing cells are stress-responsive, by colocalization with c-fos mRNA after restraint, swim, or immune stress. To determine whether expression of alpha(1b) ADR mRNA is influenced by circulating glucocorticoids, male rats underwent bilateral adrenalectomy (ADX) or sham surgery, and were killed after 1, 3, 7, or 14 d. In situ hybridization revealed levels of alpha(1b) ADR mRNA were increased in the PVN 7 and 14 d after ADX, but were not altered in the hippocampus, amygdala, or dorsal raphe. Additional rats underwent ADX or sham surgery and received a corticosterone pellet (10 or 50 mg) or placebo for 7 d. Corticosterone replacement (10 mg) reduced the ADX-induced increase in PVN alpha(1b) ADR mRNA to control levels, whereas 50 mg of corticosterone replacement resulted in a decrease in PVN alpha(1b) ADR mRNA as compared with all other groups. Furthermore, levels of plasma corticosterone were significantly correlated (inverse relationship) with alpha(1b) ADR mRNA in the PVN. We conclude that alpha(1b) ADR mRNA is expressed in CRH-containing, stress-responsive cells of the PVN and is highly sensitive to circulating levels of corticosterone. Because activation of the alpha(1B) adrenoceptor is predominantly excitatory within the brain, we predict that this receptor plays an important role in facilitation of the HPA axis response.

Basolateral amygdala noradrenergic influences on memory storage are mediated by an interaction between beta- and alpha1-adrenoceptors

Extensive evidence indicates that norepinephrine modulates memory storage through an activation of beta-adrenoceptors in the basolateral nucleus of the amygdala (BLA). Recent findings suggest that the effects of beta-adrenergic activation on memory storage are influenced by alpha1-adrenoceptor stimulation. Pharmacological findings indicate that activation of postsynaptic alpha1-adrenoceptors potentiates beta-adrenoceptor-mediated activation of cAMP formation. The present study examined whether inactivation of alpha1-adrenoceptors in the BLA would alter the dose-response effects on memory storage of intra-BLA infusions of a beta-adrenoceptor agonist, as well as that of a synthetic cAMP analog. Male Sprague Dawley rats received bilateral microinfusions into the BLA of either the beta-adrenoceptor agonist clenbuterol (3-3000 pmol in 0.2 microliter) or 8-bromoadenosine 3':5'-cyclic monophosphate (8-bromo-cAMP) (0.2-7 nmol in 0.2 microliter) alone or together with the alpha1-adrenoceptor antagonist prazosin (0.2 nmol) immediately after training in an inhibitory avoidance task. Retention was tested 48 hr later. Clenbuterol induced a dose-dependent enhancement of retention, and prazosin attenuated the dose-response effects of clenbuterol. Posttraining intra-BLA infusions of 8-bromo-cAMP also induced a dose-dependent enhancement of retention latencies. However, concurrent infusion of prazosin did not alter the dose-response effects of 8-bromo-cAMP. These findings are consistent with the view that alpha1-adrenoceptors affect memory storage by modulating beta-adrenoceptor activation in the BLA. Moreover, these findings are consistent with those of pharmacological studies indicating that beta-adrenoceptors modulate memory storage by a direct coupling to adenylate cyclase, whereas alpha1-receptors act indirectly by influencing the beta-adrenoceptor-mediated influence on cAMP formation.

Basolateral amygdala noradrenergic influences on memory storage are mediated by an interaction between beta- and alpha1-adrenoceptors

Extensive evidence indicates that norepinephrine modulates memory storage through an activation of beta-adrenoceptors in the basolateral nucleus of the amygdala (BLA). Recent findings suggest that the effects of beta-adrenergic activation on memory storage are influenced by alpha1-adrenoceptor stimulation. Pharmacological findings indicate that activation of postsynaptic alpha1-adrenoceptors potentiates beta-adrenoceptor-mediated activation of cAMP formation. The present study examined whether inactivation of alpha1-adrenoceptors in the BLA would alter the dose-response effects on memory storage of intra-BLA infusions of a beta-adrenoceptor agonist, as well as that of a synthetic cAMP analog. Male Sprague Dawley rats received bilateral microinfusions into the BLA of either the beta-adrenoceptor agonist clenbuterol (3-3000 pmol in 0.2 microliter) or 8-bromoadenosine 3':5'-cyclic monophosphate (8-bromo-cAMP) (0.2-7 nmol in 0.2 microliter) alone or together with the alpha1-adrenoceptor antagonist prazosin (0.2 nmol) immediately after training in an inhibitory avoidance task. Retention was tested 48 hr later. Clenbuterol induced a dose-dependent enhancement of retention, and prazosin attenuated the dose-response effects of clenbuterol. Posttraining intra-BLA infusions of 8-bromo-cAMP also induced a dose-dependent enhancement of retention latencies. However, concurrent infusion of prazosin did not alter the dose-response effects of 8-bromo-cAMP. These findings are consistent with the view that alpha1-adrenoceptors affect memory storage by modulating beta-adrenoceptor activation in the BLA. Moreover, these findings are consistent with those of pharmacological studies indicating that beta-adrenoceptors modulate memory storage by a direct coupling to adenylate cyclase, whereas alpha1-receptors act indirectly by influencing the beta-adrenoceptor-mediated influence on cAMP formation.

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.

Noradrenergic excitation and inhibition of GABAergic cell types in rat frontal cortex

Noradrenaline (NA) from the locus coeruleus and GABA from intracortical nonpyramidal cells exert strong influences on cortical activity. To assess possible interaction between the two, the effects of noradrenergic agonists on spontaneous GABAergic IPSCs as well as on the activity of identified GABAergic cell types were investigated by in vitro whole-cell recordings from the frontal cortex of 18- to 22-d-old rats. NA (3-50 microM) and an alpha-adrenergic agonist, 6-fluoronorepinephrine (FNE; 30-50 microM), induced an increase of IPSC frequency in pyramidal cells, but a beta-adrenergic agonist did not. This increase was reduced by tetrodotoxin, bicuculline, and alpha-adrenergic antagonists, suggesting that GABAergic cells are excited via alpha-adrenoceptors. Fast-spiking or late-spiking cells were depolarized by application of NA or FNE, but none demonstrated spike firings. The former morphologically included common multipolar cells with extended axonal arborizations as well as chandelier cells, and the latter neurogliaform cells. Most somatostatin-immunoreactive regular or burst-spiking cells, including Martinotti cells and wide arbor cells, were depolarized and accompanied by spike firing. In a few cases this was preceded by hyperpolarization. Cholecystokinin-immunoreactive regular or burst-spiking nonpyramidal cells, including large basket cells, were affected heterogeneously: depolarization, hyperpolarization followed by depolarization, or hyperpolarization resulted. The findings suggest that, similar to the effects of acetylcholine, the excitability of cortical GABAergic cell types is differentially regulated by NA and that NA actions are similar to cholinergic ones in some GABAergic cell types but not in others.

Noradrenergic excitation and inhibition of GABAergic cell types in rat frontal cortex

Noradrenaline (NA) from the locus coeruleus and GABA from intracortical nonpyramidal cells exert strong influences on cortical activity. To assess possible interaction between the two, the effects of noradrenergic agonists on spontaneous GABAergic IPSCs as well as on the activity of identified GABAergic cell types were investigated by in vitro whole-cell recordings from the frontal cortex of 18- to 22-d-old rats. NA (3-50 microM) and an alpha-adrenergic agonist, 6-fluoronorepinephrine (FNE; 30-50 microM), induced an increase of IPSC frequency in pyramidal cells, but a beta-adrenergic agonist did not. This increase was reduced by tetrodotoxin, bicuculline, and alpha-adrenergic antagonists, suggesting that GABAergic cells are excited via alpha-adrenoceptors. Fast-spiking or late-spiking cells were depolarized by application of NA or FNE, but none demonstrated spike firings. The former morphologically included common multipolar cells with extended axonal arborizations as well as chandelier cells, and the latter neurogliaform cells. Most somatostatin-immunoreactive regular or burst-spiking cells, including Martinotti cells and wide arbor cells, were depolarized and accompanied by spike firing. In a few cases this was preceded by hyperpolarization. Cholecystokinin-immunoreactive regular or burst-spiking nonpyramidal cells, including large basket cells, were affected heterogeneously: depolarization, hyperpolarization followed by depolarization, or hyperpolarization resulted. The findings suggest that, similar to the effects of acetylcholine, the excitability of cortical GABAergic cell types is differentially regulated by NA and that NA actions are similar to cholinergic ones in some GABAergic cell types but not in others.

Distribution of alpha 1a-, alpha 1b- and alpha 1d-adrenergic receptor mRNA in the rat brain and spinal cord

The technique of in situ hybridization with specific ribonucleotide probes was used to determine the distribution patterns of mRNA encoding the alpha 1a-, alpha 1b- and alpha 1d-adrenoceptor (AR) subtypes in rat brain and spinal cord. The expression pattern of alpha 1a-AR mRNA has not been reported previously, and was found to be widespread throughout the rat central nervous system. High levels were found in regions of the olfactory system, several hypothalamic nuclei, and regions of the brainstem and spinal cord, particularly in areas related to motor function. Regions expressing moderate levels of mRNA for this receptor were the septum, bed nucleus of the stria terminalis, cerebral cortex, amygdala, cerebellum and pineal gland. Low expression levels were detected in the hippocampal formation. Most nuclei in the basal ganglia and thalamus expressed extremely low or undetectable levels of alpha 1a-AR mRNA. The expression patterns of the alpha 1b- and alpha 1d-AR mRNAs were similar to those described using oligonucleotide probes in earlier studies. High expression of alpha 1b-AR mRNA was noted in the pineal gland, most thalamic nuclei, lateral nucleus of the amygdala and dorsal and median raphe nuclei. Moderate expression levels were noted throughout the cerebral cortex, and in some olfactory, septal, and brainstem regions. The distribution of alpha 1d-AR mRNA was the most discrete of the three receptors examined. Expression was strong in the olfactory bulb, cerebral cortex, hippocampus, reticular thalamic nucleus, regions of the amygdala, motor nuclei of the brainstem, inferior olivary complex and spinal cord. Comparison of the distributions of the alpha 1a-, alpha 1b- and alpha 1d-AR mRNA suggests unique functional roles for each of these receptors.

Noradrenergic mechanisms in stress and anxiety: I. Preclinical studies

There is considerable preclinical evidence for a relationship between noradrenergic brain systems and behaviors associated with stress and anxiety. The majority of noradrenergic neurons are located in the locus coeruleus (pons), with projections throughout the cerebral cortex and multiple subcortical areas, including hippocampus, amygdala, thalamus, and hypothalamus. This neuroanatomical formation of the noradrenergic system makes it well suited to rapidly and globally modulate brain function in response to changes in the environment, as occurs during the presentation of stressors. Stress exposure is associated with an increase in firing of the locus coeruleus and with associated increased release and turnover of norepinephrine in brain regions which receive noradrenergic innervation. Increased firing of the locus coeruleus is also associated with behavioral manifestations of fear, such as arched back and piloerection in the cat. Exposure to chronic stress results in long-term alterations in locus coeruleus firing and norepinephrine release in target brain regions of the locus coeruleus. Norepinephrine is also involved in neural mechanisms such as sensitization and fear conditioning, which are associated with stress. These findings are relevant to an understanding of psychiatric disorders, such as panic disorder and post-traumatic stress disorder (PTSD), the symptoms of which have been hypothesized to be related to alterations in noradrenergic function.

Noradrenergic enhancement of GABA-induced input resistance changes in layer V regular spiking pyramidal neurons of rat somatosensory cortex

Previous in vivo studies have shown that microiontophoretic application of norepinephrine (NE) and isoproterenol (ISO) can enhance gamma-aminobutyric acid (GABA)-induced depressant responses of rat somatosensory cortical neurons. In the present investigation we have examined the transmembrane electrophysiological events which are associated with interactions between NE and GABA in layer V pyramidal neurons of rat barrel field cortex. Intracellular recordings were made from electrophysiologically identified cells in a superfused cortical tissue slice preparation before, during and after bath or microdrop application of GABA, NE and ISO, alone or in combination. GABA application produced a small depolarization from resting membrane potential associated with a reduction (22%) in input resistance. NE and ISO (10-100 microM) also produced in some cases small membrane depolarizations (1-4 mV) but little concomitant changes in input resistance. Simultaneous application of NE with GABA potentiated amino acid-induced changes in input resistance in 4 cases and antagonized (n = 4) or had no effect (n = 4) on GABA-associated membrane events in 8 other cases. When the alpha-blocker, phentolamine (20 microM), was added to the medium, NE-induced enhancement of the GABA response was observed in 3 of 5 cases (60%), suggesting both, a beta-adrenergic mediation and a possible alpha-receptor masking of this noradrenergic-potentiating action. Consistent with this interpretation was the finding that the beta-agonist, ISO (10-100 microM), produced net increases in GABA-induced input resistance changes in 64% of cases tested (9 of 14). The potentiating effect of NE and ISO was mimicked by the adenyl cyclase activator, forskolin (n = 2), and a membrane permeant analog of cyclic-AMP, 8-bromo-cyclic AMP (n = 3); and could also be demonstrated when the GABAA agonist muscimol (0.5-1 microM) was substituted for GABA. The reversal potential for GABA and GABA + NE remained the same. These findings suggest that previous demonstrations of NE-potentiating effects on GABA inhibition may be mediated by beta-receptor/cyclic-AMP-linked actions on mechanisms which regulate GABAA receptor-induced membrane conductance changes.

Effects of disulfiram, phenoxybenzamine and propranolol on the behaviors evoked by apomorphine and amphetamine in adult cats

The aim of this work was to study the role that the noradrenergic system could play in the mechanism of production of the behaviors evoked by parenteral injection of apomorphine and amphetamine in adult cats. Ten cats were injected s.c. with 2 mg/kg of apomorphine and 2.5 mg/kg of amphetamine in separate sessions. The behaviors were recorded, until control conditions were again attained. In a second stage, disulfiram was administered ip., followed by apomorphine and amphetamine in the same doses as cited above. The effects on behaviors produced by disulfiram and those of apomorphine and amphetamine were recorded by three independent observers. Comparisons of the pre- and post-disulfiram behavioral results were analyzed with the help of the non-parametric Wilcoxon signed rank test. In another group of ten cats a similar procedure was carried on employing the alpha and beta noradrenergic blocking agents, phenoxybenzamine and propranolol. The noradrenergic blocking drugs, especially disulfiram and phenoxybenzamine produced by themselves a decrease in motility, in alertness and an increase in indifference and inappetence. Apomorphine and amphetamine administered after the blocking drugs showed slight behavioural modifications, reflection most of them the changes produced by the three blocking drugs. It is concluded that probably the nor-adrenergic system could be involved in the hypomotility elicited by amphetamine. NA is not involved in the induction of the other behaviors evoked by apomorphine and amphetamine.

Adult rat barrel cortex plasticity occurs at 1 week but not at 1 day after vibrissectomy as demonstrated by the 2-deoxyglucose method

Stimulation of a single facial vibrissa in rats receiving [14C]2-deoxyglucose leads to increased local cerebral glucose utilization in the corresponding contralateral barrel of lamina IV of the first somatosensory cortex (SmI). In the adult rat, the metabolic representation of such a barrel enlarges 2 months after removal of all other vibrissal follicles but enlargement is prevented by prior removal of SmI norepinephrine. Here, the early time course of such enlargement and how this was affected by cortical norepinephrine manipulations were examined in adult rats. One day after total vibrissal follicle removal with sparing of the central (C3) vibrissa, neither the areal extent nor absolute glucose utilization in the stimulated, spared C3 cortical barrel were changed. However, 7 days after follicle removal, the spared C3 barrel was enlarged by 41%, although absolute glucose utilization remained constant. This delayed onset of enlargement is compatible with either a structural or neurochemical change in barrel circuitry following vibrissal deafferentation. With ipsilateral locus coeruleus lesions but intact vibrissae, there was progressive enlargement of stimulated C3 barrel areas with increasing cortical norepinephrine depletion (r = 0.864) suggesting a suppressive effect of norepinephrine on activity spread in barrels with intact vibrissal afferents. Previously shown blockade of chronic (2 month) vibrissectomy-induced barrel enlargement by norephinephrine depletion suggested an additional effect on plasticity. Even though acute (1 day) follicle removal here produced no change in spared C3 barrel area, addition of norepinephrine depletion produced a surprising 40% decrease in barrel area. Thus, barrel plasticity assessed by 2-deoxyglucose reflects a complex interaction between barrel metabolic activity and the extent of vibrissal and noradrenergic afferent input.

Actions of norepinephrine in the cerebral cortex and thalamus: implications for function of the central noradrenergic system

Norepinephrine (NE) has potent and long-lasting ionic effects on cortical and thalamic neurons. In cortical pyramidal cells, activation of beta-adrenergic receptors results in an enhanced excitability and responsiveness to depolarizing inputs. This enhanced excitability is expressed as a reduction in spike frequency adaptation and is mediated by a marked suppression of a slow Ca(++)-activated potassium current known as IAHP. In the thalamus, application of NE results in the suppression of ongoing rhythmic burst activity and a switch to the single spike firing mode of action potential generation. This effect is mediated through an alpha 1-adrenergic suppression of a resting leak potassium current, IKL, and through a beta-adrenoceptor-mediated enhancement of the hyperpolarization activated cation current Ih. Together with the actions of other neuromodulatory neurotransmitters (i.e., acetylcholine, histamine, serotonin) these effects facilitate the switch of these neurons from a state of rhythmic oscillation and low excitability during drowsiness and slow-wave sleep to a state of increased excitability and responsiveness during periods of waking, attentiveness and cognition.

Iontophoresis of norepinephrine onto neurons of the pigeon's lateral geniculate nucleus: characterization of an inhibitory response

A group of neurons in the pigeon's lateral geniculate equivalent nucleus (LGNe) shows associative enhancement of their response to light during visually conditioned heart rate change. The source of the relevant unconditioned stimulus input to LGNe for this enhancement has been identified as the locus coeruleus (LC). Thus, we have begun to examine neurotransmitters synthesized in LC for possible involvement in associative modification of neuronal discharge in LGNe. As a first step, we have examined the responses of LGNe neurons to iontophoretic application of norepinephrine (NE) and identified the receptor mediating one response class. The majority of neurons in LGNe show inhibition of maintained activity in response to iontophoretic application of NE or its agonists. The potency of the NE agonists is alpha-methyl NE greater than epinephrine greater than NE greater than phenylephrine greater than isoproterenol. This profile is characteristic of an alpha 2-adrenoceptor. The alpha 2-agonist clonidine also induces inhibition of maintained activity. The alpha 2-antagonists WB-4101 and yohimbine block the NE-inhibition while the alpha 1-antagonist prazosin and beta-antagonist sotalol do not. It is thus suggested that the receptor mediating the NE-inhibition of maintained activity has the characteristics of an alpha 2-adrenoceptor.

Effects of iontophoretically applied monoamines on somatosensory cortical neurons of unanesthetized rats

The response of somatosensory cortical neurons to iontophoretic applications of monoamines was studied in unanesthetized rats. The animal's head was held in a stereotaxic apparatus by means of a painless head-restraining system implanted 8 days prior to the recording sessions. The electrodes consisted of a recording micropipette attached to a multibarreled iontophoresis micropipette. The electrode penetrations were reconstructed on camera lucida drawings of frontal brain sections. The percentage of cortical neurons responding to application of monoamines were 76% after noradrenaline, 58% after dopamine and 66% after serotonin. The differences observed among percentages of responses seemingly correlate with the relative abundance of terminal axons and receptors for each of the three monoamines in the somatosensory cortex. The vast majority of the responding neurons were inhibited by monoamines and this inhibitory effect was independent of the level of spontaneous activity. The depressant effect of the monoamines on glutamate and acetylcholine-evoked responses supports a modulatory role for these substances. Serotonin was the most potent, followed by noradrenaline and dopamine. The present study shows that when the influence of anesthesia is eliminated, the predominant effect of monoamines on cortical first somatosensory neurons is one of inhibition. These findings contrast with results obtained under some anesthetic conditions, as well as under in vitro conditions.

Alpha-adrenergic influences on neuronal responses to visceral afferent input in the nucleus tractus solitarius

Studies were made of the effects of the alpha 1-adrenoceptor agonist methoxamine on spontaneous and synaptically evoked activity in the solitary tract nucleus in isolated, perfused slices of the brain of the rat and those effects were compared to the effects of the alpha 2-agonist clonidine. Methoxamine had no effect on the spike activity of 7 out of 8 spontaneously firing neurones, with no response to electrical stimulation of the solitary tract. Among neurones that responded to tract stimulation, those which were otherwise silent (N = 38) showed a decrease of responsiveness to tract input during the infusion of methoxamine, whereas those with spontaneous activity (N = 10) showed mostly increases of both firing rate and responsiveness. The effects of activation of alpha 2-receptors on responsiveness to tract stimulation were the opposite of previously demonstrated effects of activation of alpha 2-receptors, and these opposing effects could be demonstrated in the same neurone. These results suggest that the transmission of visceral sensory information within the nucleus tractus solitarius may be controlled by opposing alpha 1- and alpha 2-receptor-mediated neural systems.

Effects of 5-HT and alpha 1 adrenoceptor antagonists on kappa opioid-induced sedation

The kappa opioid agonists PD-117302 and U-50488 were found to produce dose-dependent reductions in spontaneous locomotor activity in mice. The magnitude of the response to a given dose of each kappa agonist was found to be clearly potentiated by pretreating the animals with either ketanserin (1 mg/kg) or prazosin (0.5 mg/kg). Pretreatment with the selective 5-HT2 receptor antagonist ritanserin given at a high dose (1 mg/kg), the nonselective 5-HT antagonist methysergide or the 5-HT synthesis inhibitor parachlorophenylalanine did not alter the magnitude of the response to the kappa agonist. These results suggest that 5-HT systems are not involved in the sedative effects of kappa opioid agonists and that the potentiating effect seen in animals pretreated with ketanserin is due to the alpha 1 blocking properties of this compound since the effect was mimicked by the alpha 1 antagonist prazosin.

Identification and localization of adrenergic receptors in cat visual cortex

The concentration and location of adrenergic receptors in cat visual cortex have been determined by radioligand binding techniques using [3H]prazosin (alpha 1-adrenergic receptors), [3H]yohimbine (alpha 2-adrenergic receptors) and [3H]dihydroalprenolol (beta-adrenergic receptors). Saturable high affinity binding sites for all of these ligands were found. The beta-adrenergic receptor population was resolved into beta 1- and beta 2-sites that were present in the ratio 35:65. The laminar distributions of the alpha 1-, alpha 2- and beta-adrenergic receptors were different. The alpha 1- and beta-adrenergic receptors were very similarly localized, being seen in upper layers (I, II and III) and lower layers (layers V and VI). The labelling in upper layers was greater than that in lower layers, more so for alpha 1-adrenergic receptors than beta-adrenergic receptors. alpha 2-Adrenergic receptors were seen in a single band that occupied layer II and III but did extend to the pial surface. These results indicate that the effect of norepinephrine on neuronal activity in cat visual cortex will depend upon the layer in which it is released. Our results provide a basis for further physiological studies of the role of norepinephrine in the processing of visual information.

Noradrenaline excites neurons in the guinea pig cerebellar vermis in vitro

Noradrenaline (NA) was applied to the solution bathing the cavy cerebellar vermis in vitro and the responses of 98 neurons were recorded extracellularly. Two thirds (23/35) of the responses were excitations and the remaining third were inhibitions. The lowest concentration of NA with which responses could be obtained was 10(-11) M NA. Responses were generally transient and occurred with a mean latency of 61 +/- 8 sec. The excitation was generally direct as most responses (9/11) survived synaptic blockade. The excitations were thought to be mediated by alpha 1 receptors because they could be mimicked by phenylephrine and antagonised by prazozin.

Electrophysiological actions of norepinephrine in rat lateral hypothalamus. I. Norepinephrine-induced modulation of LH neuronal responsiveness to afferent synaptic inputs and putative neurotransmitters

The present studies were conducted as part of an ongoing investigation of the effects of norepinephrine (NE) in neuronal circuits of the mammalian brain. In this report, we describe noradrenergic actions in the lateral hypothalamus (LH), an area which has been implicated in the central integration of cardiovascular regulatory mechanisms, fluid balance and ingestive behaviors. Microiontophoretically applied NE was interacted with extracellularly recorded responses of LH neurons to iontophoretically applied putative neurotransmitters gamma-aminobutyric acid (GABA), acetylcholine (ACh) and glutamate (Glu); and activation of known input pathways from the reticular thalamus (RT) and the lateral preoptic area (LPO). Peri-event histograms of cell responses were computed before, during and after NE microiontophoresis (5-50 nA) and used to quantitatively evaluate monoamine-induced effects on spontaneous and stimulus evoked activity of LH neurons. In 16 of 23 LH neurons, RT-stimulus-induced inhibition was markedly prolonged from a mean of 28.3 +/- 4.8 ms to 44.7 +/- 5.2 ms, during iontophoretic application of NE. In 22 of 38 LH cells, LPO-stimulus-induced excitatory responses were enhanced above control levels during NE administration. In further tests, inhibitory responses of LH cells to iontophoretic pulses of GABA were potentiated during NE administration in 69% (24 of 35) of the cases tested. ACh-induced excitation was potentiated in 9 of 21 cells. In 4 of these cases, otherwise subthreshold doses of ACh caused marked increases in cell firing during the period of NE administration. By contrast, Glu-evoked excitation was antagonized by NE iontophoresis in 65.5% (17 of 26) of LH cells tested. These findings indicate that, as in other noradrenergic target regions of the CNS, NE can facilitate synaptically mediated responses of LH neurons. Taken together these observations suggest that NE may play an important regulatory role in the synaptic transfer of information within LH circuits, and consequently exert considerable influence over the homeostatic functions mediated by this structure.

Noradrenaline- and vasoactive intestinal peptide-containing neuronal systems in neocortex: functional convergence with contrasting morphology

Neurotransmitter-specific anatomical techniques have provided a tool to define the morphological constraints within which a given neurotransmitter will exert its cellular actions. Biochemical and electrophysiological approaches have revealed the nature of these cellular actions for several neurotransmitters. Furthermore, by using purified preparations and tissue cultures a certain degree of resolution has been achieved by which the cell type, where a neurotransmitter's effect takes place, can be determined. In this article we review these aspects for noradrenaline and vasoactive intestinal peptide, two neurotransmitters of the cerebral cortex contained within neuronal systems that present strikingly different morphologies. Nevertheless, noradrenaline and vasoactive intestinal peptide share certain cellular actions and can interact synergistically. The experimental evidence accumulated to date indicates that noradrenaline- and vasoactive intestinal peptide-containing neurons can influence three general cell types of the cerebral cortex, i.e. (i) other neurons, (ii) astrocytes and (iii) cells of the vasculature. This diversity in cellular partners supports the notion that noradrenaline and vasoactive intestinal peptide can be released from neurons at conventional synapses as well as at extrasynaptic sites, thus suggesting the co-existence of two modes of release within the same neuron.

Differential effects of norepinephrine on hippocampal complex-spike and theta-neurons

The central noradrenergic system has long been postulated to modulate learning and memory. A brain structure known to be important in these functions is the hippocampus. Since the hippocampus receives a noradrenergic projection from the locus coeruleus, knowledge of norepinephrine's actions in the hippocampus may help determine its role in learning and memory. In the present study, the effects of norepinephrine were examined on two hippocampal cell types: complex-spike and theta-neurons. In the hippocampus, there is good evidence that complex-spike cells are pyramidal neurons, while theta-neurons are interneurons. Extracellular action potentials from hippocampal neurons were recorded using multibarrel glass micropipettes. Drugs were locally applied using pressure micro-ejection. Norepinephrine inhibited the spontaneous firing of complex-spike cells, while theta-neurons were excited. The inhibitory response of complex-spike neurons was mediated by an alpha 1-receptor. However, selective agonists for the alpha 2- and beta-noradrenergic receptors excited the complex-spike cells. The noradrenergic-induced excitatory response of theta-neurons was also mediated by alpha 2- and beta-receptors. This study provides evidence that locally applied norepinephrine produces different responses on two types of hippocampal neurons. Furthermore, these differential responses arise primarily from the activation of distinct populations of noradrenergic receptors.

Modulation of p,p'-DDT-induced tremor by catecholaminergic agents

p,p'-DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane; 75 mg/kg) or corn oil was administered po to male Fischer 344N rats. Tremor was quantified 8 hr later by spectral analysis of whole body movements. The effect of sc injection of pharmacological challenges on the spectral profile of body movements was determined. The alpha antagonist phenoxybenzamine decreased the intensity of movements over most of the power spectra in animals exposed to DDT, but decreased spectral power only at lower frequencies in control subjects. The alpha-1 antagonist prazosin had similar effects in animals given DDT. The alpha-2 antagonist yohimbine and the beta antagonist propranolol produced lethality and increased the intensity of movements in animals administered DDT, without significantly affecting control animals. The alpha-2 agonist clonidine decreased the spectral profile over a wide range of frequencies in animals exposed to DDT, while depressing the spectral power of control animals only at higher frequencies. The dopamine antagonist haloperidol increased the intensity of movements in DDT-treated animals, without altering the spectral profile of controls. The dopamine agonist apomorphine induced stereotypy in control animals, but failed to significantly alter the power spectra in subjects given DDT. These data suggest a facilitatory and inhibitory role, respectively, for alpha-1 and alpha-2 receptors in the modulation of DDT-induced tremor. Dopamine and beta receptors may be involved in the tonic inhibition of tremor produced by DDT.

Responses of ventromedial hypothalamic neurons in vitro to norepinephrine: dependence on dose and receptor type

Application of norepinephrine (NE) at 12.5 microM in the bath surrounding hypothalamic slices from ovariectomized rats could evoke excitation, inhibition, or biphasic inhibition-excitation from single neurons in the ventromedial nucleus. Whether the rats were treated with estrogen or not did not alter the distribution of the type of neuronal responses to NE in vitro. Altering the composition of the bathing solution to achieve synaptic blockade did not abolish or alter the type of responses, indicating that all these types of NE responses, including both phases of the biphasic response, were mediated by postsynaptic receptors. Experiments with varying doses of NE showed that the inhibitory response could be evoked at doses lower than those required to evoke the excitatory response. The effective dose for 50% of the responsive neurons (ED50) was lower than 1.25 microM for inhibitions and higher than 5 microM for excitations. Using specific adrenergic receptor agonists and antagonists, it was found that the excitation and the inhibition were mediated, primarily, by alpha 1- and alpha 2-receptors, respectively. beta-Receptors played only a minor role, but might be related to both excitation and inhibition. Study with adrenergic agents further revealed that different types of adrenergic receptors co-localized not only in neurons showing the biphasic response, but also in a major portion of neurons showing monophasic excitation or inhibition. Because of the co-localization and the differential sensitivities to NE, alteration of the dose of NE or the ratio of excitatory/inhibitor receptors co-localized on a neuron should be able to reverse the type of a neuronal response to NE.

Changes in cortical beta-adrenergic receptor density and neuronal sensitivity to norepinephrine accompany morphine dependence and withdrawal

Radioligand binding experiments were carried out in conjunction with electrophysiological recordings in vivo in the parietal cortex in rats to assess changes in postsynaptic beta-adrenergic receptor function that result after chronic administration of morphine and during morphine withdrawal. Chronic treatment of rats with morphine for 14 days resulted in a 38% increase in the density of beta-adrenergic receptors in the parietal cortex, as measured by the binding of the specific antagonist [3H]dihydroalprenolol (DHA). In comparison, following withdrawal in the chronic morphine-treated animals, the number of specific [3H]DHA binding sites in this same cortical region was decreased 25%, when compared to saline-treated controls. These alterations in cortical beta-adrenergic receptor density were not accompanied by a significant change in the dissociation constant (Kd) for [3H]DHA or in the inhibitory constants (Ki) for the specific agonists norepinephrine and isoproterenol. Microiontophoretic testing revealed that the changes in beta-adrenergic receptor density found in parietal cortex after chronic morphine treatment and during morphine withdrawal were accompanied by a selective increase and decrease, respectively, in the sensitivity of cerebrocortical neurons in the same region to beta-adrenergic stimulation. These results suggest that changes in central adrenergic function might be related to the formation and/or expression of dependence on morphine.

The effects of catecholamines on electrical activity of neurons in the guinea pig supraoptic nucleus in vitro

The role of the central noradrenergic system in the supraoptic neuroendocrine regulation was investigated using slices of the guinea pig hypothalamus. Noradrenaline produced a complex membrane effect comprising two distinct depolarizations: one, associated with a moderate increase in input resistance and resulting in an augmentation of the spontaneous firing rate; the other, unaccompanied by a detectable change in input resistance and resulting in depression of the firing rate. The former depolarization was reproducible by applying specific alpha-agonist, phenylephrine, whereas the latter was induced by a beta-adrenergic agonist, isoproterenol. The actions of phenylephrine and isoproterenol were blocked by phentolamine and propranolol, respectively. Amplitude of the phenylephrine-induced depolarization was voltage-dependent with the estimated reversal potential of about - 115 mV and changed as a function of [K+]o. On the contrary, amplitude of the isoproterenol-induced depolarization was voltage-independent and was insensitive to changes in external concentrations of K+, Na+, Cl- and Ca2+. We conclude that catecholamines directly modulate the activity of supraoptic neurons through two functionally distinct adrenoceptive sites on neurosecretory cells. The activation of alpha-receptors may increase cellular excitability through suppression of membrane K+ conductance while the activation of beta-receptors would depress neuronal firings, possibly through some mechanism which is not directly linked to ionic channels.

Immunocytochemically defined astroglia from fetal, newborn and young adult rats express beta-adrenergic receptors in vitro

Autoradiography of radioligand binding was used to assess the expression of beta-adrenergic receptors (beta-AR) by immunocytochemically identified astroglia cultured from the cerebral cortices of rats 16 days in gestation through 28 days postnatal (DPN). Polygonal astroglia isolated from animals at each age examined were found to exhibit large numbers of beta-AR. In contrast, only low levels of beta-AR could be detected on process-bearing astroglia and fibroblasts. Quantitative analysis showed that there was an increase in the density of beta-AR on polygonal astroglia between 16 days in gestation and 1 DPN. This increase in beta-AR receptor density was present whether the cells were grown for long periods of time in culture (8-22 days) or for short periods of time in culture (1-5 days). The results also suggest that differences in the level of receptor expression between cells grown in short-term and long-term culture may be due in part to culture methodology.

Pharmacological characterization of the receptor mediating the adrenergic inhibition of responses to substance P in the cingulate cortex

The excitatory responses of neurones in the anterior cingulate cortex of the rat to iontophoretically applied substance P (SP) are reduced by noradrenaline (NA) applied iontophoretically or released from noradrenergic pathways. In order to determine the receptor involved in this inhibitory effect we have studied the effects of a number of receptor-specific adrenergic agonists and antagonists on responses of cingulate neurones to SP in rats anaesthetized with chloral hydrate. Low iontophoretic currents (0-15 nA) of NA, adrenaline and the beta-agonist, clenbuterol, all strongly reduced responses to SP. Isoprenaline was also effective but less consistently so, although problems were experienced with its iontophoretic release from micropipettes. The alpha 1-agonists, phenylephrine and methoxamine were also able to reduce responses to SP. However, this reduction required higher iontophoretic currents (15-60 nA) and was associated with depressant effects on baseline firing rate. The alpha 2-agonist clonidine was only weakly active at high currents and this too was associated with depression of baseline firing. Similar weak effects were noted with dopamine. The inhibitory effects of NA on SP responses were convincingly blocked or reversed by the beta-antagonist, practolol, but not by the alpha 1-antagonist, prazosin. The reduction of SP responses by phenylephrine was also blocked by practolol but unaffected by prazosin. Finally, reduction of SP excitations by activation of the coeruleocortical pathway was also blocked by practolol applied iontophoretically to the cortical cells. These results are consistent with the hypothesis that the effect of NA on SP responsiveness in the cingulate cortex is mediated by beta-adrenoreceptors.

Excitatory neuronal responses to dopamine in the cerebral cortex: involvement of D2 but not D1 dopamine receptors

The technique of microelectrophoresis was used to evaluate the relative contribution of D1 and D2 dopamine receptors towards the mediation of the excitatory response of single neurones to dopamine in the somatosensory cortex of the rat. The selective D1 dopamine receptor agonist, SKF 38393, failed to excite any of the cells to which it was applied. In contrast, the selective D2 dopamine receptor agonist, LY 171555, excited the majority of cells tested. The apparent potency of LY 171555 was significantly lower than that of dopamine. When the mobilities of SKF 38393 and LY 171555 were assessed by an in vitro method, they were found to be at least as great as those of dopamine and phenylephrine, suggesting that the lack of effect of SKF 38393 and the lower apparent potency of LY 171555 compared to dopamine reflect genuine biological phenomena. The alpha 1-adrenoceptor antagonist, prazosin, discriminated between excitatory responses to the alpha 1-adrenoceptor agonist, phenylephrine, and LY 171555: responses to phenylephrine were more susceptible to antagonism than were those to LY 171555. The dopamine receptor antagonist, haloperidol, produced the reverse discrimination: responses to LY 171555 were more affected than were those to phenylephrine. Neither antagonist reduced the response to the control agonist, acetylcholine. When applied continuously with low ejecting currents, LY 171555 antagonized the excitatory response to dopamine while the response to phenylephrine was relatively preserved. The response to acetylcholine was unaffected. When similarly applied, SKF 38393 had no selective action on the response to dopamine. 6 These results suggest that D2 dopamine receptors are involved in mediating the excitatory neuronal response to dopamine in the cerebral cortex, whereas DI dopamine receptors are unlikely to be involved. LY 171555 appears to act as a partial agonist at D2 dopamine receptors in this test system.

Monoamine, amino acid and cholinergic interactions in slices of rat cerebral cortex

Interactions of monoamine, amino acid and cholinergic transmitter systems were studied in slices of rat cerebral cortex using a superfusion procedure and measuring release of endogenous dopamine (DA), norepinephrine (NE), serotonin (5-HT), GABA, glutamate (GLU) and aspartate (ASP). Depolarizing concentrations of K+ were used to induce a Ca2+-dependent, Mg2+-inhibited release of the monoamines and amino acids. Submaximal release of the monoamines and amino acids was observed at 35 mM K+, which permitted studies of possible excitatory or inhibitory actions of the added agents. The 35 mM K+-stimulated, Ca2+-dependent release of GABA was inhibited 40, 30 and 25% by 100 microM NE, DA and 5-HT, respectively. The release of GLU was potentiated by NE and reduced by DA. Both DA and 5-HT inhibited the release of ASP. The Ca2+-dependent, K+-stimulated release of endogenous NE, DA and 5-HT was not altered by 100 microM GABA, GLU or ASP. However, 100 microM GLU did enhance the stimulated release of GABA. The cholinergic agonist, carbachol, enhanced the stimulated release of NE, 5-HT and GLU 10, 60 and 40%, respectively. On the other hand, carbachol attenuated the release of DA and GABA approximately 20%. One interpretation of the data is that the amino acid transmitter pathways in slices of the cerebral cortex of the rat can be controlled by monoaminergic and cholinergic systems while the monoamine afferents appear to have a cholinergic regulation but not a major direct amino acid transmitter influence.

Chronic cobalt-induced epilepsy: noradrenaline ionophoresis and adrenoceptor binding studies in the rat cerebral cortex

Several studies indicate that brain noradrenaline (NA) depletion facilitates the occurrence of epileptogenic syndromes in various animal models. In cobalt-induced epilepsy in the rat activity is associated with a cortical NA denervation. In order to search for cortical adrenoceptor modifications, inonophoretic studies and adrenoceptor binding assays were performed. At the period of maximal seizure activity, there was a significant supersensitivity of cortical neurons to the ionophoretic application of NA. An increase in the density of beta-adrenoceptor binding sites was observed. No modification in alpha 1- and alpha 2-adrenoceptor binding sites was found. This suggests that in cobalt-induced epilepsy there is a denervation supersensitivity which rests on a selective involvement of beta-adrenoceptors.

Effects of clonidine on neuronal firing evoked by a noxious stimulus

When norepinephrine was applied microiontophoretically to certain neurons in the pontine reticular formation of rats, it produced an increase in neuronal firing like that produced by noxious stimulation. Previous studies have shown that both noxious stimulus- and norepinephrine-evoked increases in neuronal firing are mediated by alpha-adrenoceptors. These neurons were unresponsive to non-noxious stimuli, suggesting that they might play a role in nociception. Microiontophoretic or systemic administration of the selective alpha 2-adrenoceptor agonist clonidine significantly attenuated noxious stimulus-evoked firing, but had little effect on firing evoked by norepinephrine. This effect of clonidine could be prevented by the alpha 2-adrenoceptor antagonists piperoxan and yohimbine. These antagonists, when given alone, increased noxious stimulus-evoked firing, but had no effect on firing evoked by norepinephrine. In contrast, the selective alpha 1-adrenoceptor antagonist ARC-239 (2-(2,4-(o-methoxyphenyl)-piperazin-1-yl)ethyl-4,4-dimethyl-1,3-(2 H,4) isoquinolindione dihydrochloride) attenuated both noxious stimulus- and norepinephrine-evoked firing. These data are consistent with the hypothesis that presynaptic alpha 2-adrenoceptors modulate the release of norepinephrine. Furthermore, these data suggest that the pontine reticular formation is one site at which clonidine could act to produce analgesia.