Contribution of an α1-adrenergic receptor subtype to the expression of the “ventral tegmental area syndrome”

Nicotine-induced enhancement of a sensory reinforcer in adult rats: antagonist pretreatment effects

RATIONALE AND OBJECTIVES

The reinforcement-enhancing effect (REE) of nicotine refers to the drug's ability to enhance the strength of other primary and conditioned reinforcers. The main aim was to investigate neuropharmacological mechanisms underlying nicotine's strengthening of a primary visual reinforcer (i.e., a light cue), using a subcutaneous (SC) dose previously shown to provide plasma nicotine levels associated with habitual smoking.

METHODS

Adult male rats pressed an "active" lever to illuminate a brief cue light during daily 60-min sessions. Rats that showed a clear REE were tested with systemically administered pretreatment drugs followed by nicotine (0.1 mg/kg SC) or saline challenge, in within-subject counterbalanced designs. Pretreatments were mecamylamine (nicotinic, 0.1-1 mg/kg SC), SCH 39166 (D1-like dopaminergic, 0.003-0.2 mg/kg SC), naloxone (opioid, 1 and 5 mg/kg SC), prazosin (alpha1-adrenergic antagonist, 1 and 2 mg/kg IP), rimonabant (CB1 cannabinoid inverse agonist, 3 mg/kg IP), sulpiride (D2-like dopaminergic antagonist, 40 mg/kg SC), or propranolol (beta-adrenergic antagonist, 10 mg/kg IP).

RESULTS

The nicotine REE was abolished by three antagonists at doses that did not impact motor output, i.e., mecamylamine (1 mg/kg), SCH 39166 (0.01 and 0.03 mg/kg), and naloxone (5 mg/kg). Prazosin and rimonabant both attenuated the nicotine REE, but rimonabant also suppressed responding more generally. The nicotine REE was not significantly altered by sulpiride or propranolol.

CONCLUSIONS

In adult male rats, the reinforcement-enhancing effect of low-dose nicotine depends on nicotinic receptor stimulation and on neurotransmission via D1/D5 dopaminergic, opioid, alpha1-adrenergic, and CB1 cannabinoid receptors.

Blocking alpha2A adrenoceptors, but not dopamine receptors, augments bupropion-induced hypophagia in rats

OBJECTIVE

Anti-obesity drugs have adverse effects which limit their use, creating a need for novel anti-obesity compounds. We studied effects of dopamine (DA) and norepinephrine (NE) reuptake inhibitor bupropion (BUP), alone and after blocking α1- or α2-adrenoceptors (AR), D1/5, D2/3, or D4 receptors, to determine which receptors act downstream of BUP.

DESIGN AND METHODS

Effects on caloric intake, meal patterning and locomotion were assessed, using an automated weighing system and telemetry in male rats with 18-h access to Western Human style diet.

RESULTS

BUP (30 mg/kg) induced hypophagia by reducing meal size and postponing meal initiation. WB4101 (α1-AR; 2 mg/kg) and imiloxan (α2B-AR; 5 mg/kg) attenuated BUP's effect on meal size, while WB4101 and BRL 44408 (α2A/D-AR; 2 mg/kg) counteracted effect on meal initiation. Atipamezole (α2-AR; 1 mg/kg) and imiloxan further postponed initiation of meals. SKF 83566 (D1/5; 0.3 mg/kg), raclopride (D2/3; 0.5 mg/kg) and to a lesser extent FAUC 213 (D4; 0.5 mg/kg), attenuated BUP-induced hypophagia. BUP stimulated locomotion, which was blocked by all antagonists, except FAUC 213 or BRL 44408.

CONCLUSIONS

Alpha1-, α2A/D- and α2B-ARs, and DA receptors underlie BUP's effects on size and initiation of meals, while blocking pre-synaptic α2-ARs enhanced BUP-induced hypophagia. An inverse agonist of (pre-synaptic) α2A-ARs could enhance BUP-induced anorexia and treat eating disorders and obesity.

D1-dopamine and α1-adrenergic receptors co-localize in dendrites of the rat prefrontal cortex

Functional interactions between dopaminergic and noradrenergic systems occur in many brain areas, including the prefrontal cortex (PFC). Biochemical, electrophysiological and behavioral data indicate crosstalk between D1 dopamine receptor (D1R) and α1-adrenergic receptor (α1AR) signaling in the PFC. However, it is unknown whether these interactions occur within the same neurons, or between neurons expressing either receptor. In this study, we used electron microscopy immunocytochemistry to demonstrate that D1Rs and α1ARs co-localize in rat PFC neuronal elements, most prominently in dendrites (60-70%), but also significantly in axon terminals, unmyelinated axons and spines (∼20-30%). Our data also showed that the ratio of plasma membrane-bound to intracellular α1ARs is significantly reduced in D1R-expressing dendrites. Similar results were obtained using either a pan-α1AR or a selective α1bAR antibody to label noradrenergic receptors. Thus, these results demonstrate that D1Rs and α1ARs co-localize in PFC dendrites, thereby suggesting that the catecholaminergic effects on PFC function may be driven, at least in part, by cell-autonomous D1R-α1AR interactions.

Effect of Block of α1-adrenoceptors on Overall Motor Activity but not on Spatial Cognition in the Object-Position Recognition Task

Prazosin, an α1-adrenoceptor antagonist, is well known for its depressant effect on motivation and motor activity, while it has no effect on retention of spatial behavior in several tasks, e.g. in the Morris water maze and radial arm maze. The role of α1-adrenoceptors in operant tasks with stimulus-controlled behavior has not yet been tested. The present study investigated the effect of prazosin on the modulation of overall motor activity and on cognitive performance in a spatial operant task called object-position recognition task, where operant behavior (lever pressing) was controlled by spatial stimuli displayed on a computer screen. This task has been previously showed to be hippocampal-dependent. Pre-test injection of prazosin at the dose of 3 mg/kg decreased the responding rate, while it did not affect the recognition of object’s position. In conclusion, we validated the new cognitive test with a drug with known pharmacological effects on behavior and confirmed the depressant effect of prazosin on motor activity and no effect on retrieval of spatial memory in the hippocampal-dependent operant task.

Norepinephrine drives persistent activity in prefrontal cortex via synergistic α1 and α2 adrenoceptors

Optimal norepinephrine levels in the prefrontal cortex (PFC) increase delay-related firing and enhance working memory, whereas stress-related or pathologically high levels of norepinephrine are believed to inhibit working memory via α1 adrenoceptors. However, it has been shown that activation of Gq-coupled and phospholipase C-linked receptors can induce persistent firing, a cellular correlate of working memory, in cortical pyramidal neurons. Therefore, despite its importance in stress and cognition, the exact role of norepinephrine in modulating PFC activity remains elusive. Using electrophysiology and optogenetics, we report here that norepinephrine induces persistent firing in pyramidal neurons of the PFC independent of recurrent fast synaptic excitation. This persistent excitatory effect involves presynaptic α1 adrenoceptors facilitating glutamate release and subsequent activation of postsynaptic mGluR5 receptors, and is enhanced by postsynaptic α2 adrenoceptors inhibiting HCN channel activity. Activation of α2 adrenoceptors or inhibition of HCN channels also enhances cholinergic persistent responses in pyramidal neurons, providing a mechanism of crosstalk between noradrenergic and cholinergic inputs. The present study describes a novel cellular basis for the noradrenergic control of cortical information processing and supports a synergistic combination of intrinsic and network mechanisms for the expression of mnemonic properties in pyramidal neurons.

Inverse agonism at α2A adrenoceptors augments the hypophagic effect of sibutramine in rats

Because the use of monoamine reuptake inhibitors as weight-reducing agents is limited by adverse effects, novel antiobesity drugs are needed. We studied acute effects of the noradrenaline (NA) and serotonin (5-HT) reuptake inhibitor sibutramine (SIB), alone and after pretreatment with α1- and α2-adrenoceptor (AR), and 5-HT1/2/7, 5-HT1B and 5-HT2C receptor antagonists in order to determine which ARs and 5-HT receptors act downstream of SIB on feeding and locomotion. Acute effects on caloric and water intake, meal microstructure and locomotion were assessed, using an automated weighing system and telemetry in male rats with restricted 18-h access to Western style diet. SIB 3 mg/kg reduced meal size and frequency, which suggests enhanced within- and postmeal satiety. Imiloxan (α2B-AR), WB4101 (α1-AR), SB-224289 (5-HT1B), and modestly BRL 44408 (α2A/D-AR) attenuated SIB's effect on meal size, suggesting that α2B- and α1-ARs and 5-HT1B receptors mediate within-meal satiety, with a modest role for α2A/D-ARs. Only prazosin (α1/2B/2C-AR) counteracted SIB's effect on meal frequency. At 3 mg/kg, SIB modestly increased locomotion. This effect was blocked by metergoline (5-HT1/2/7), WB4101 (α1-AR), and RX821002 (α2-AR). Interestingly, the α2-AR antagonists atipamezole and RX821002 enhanced SIB's effect on caloric intake, probably due to inverse agonistic actions at α2A-autoreceptors that further enhanced release of NA that regulates caloric intake. Thus, an inverse agonist of presynaptic α2A-ARs might beneficially enhance SIB's weight-reducing effect and offer novel treatment for obesity. All in all, the present data supports the ARs and 5-HT receptors involved in the effects of SIB on different aspects of caloric intake and locomotion.

Uncoupling between noradrenergic and serotonergic neurons as a molecular basis of stable changes in behavior induced by repeated drugs of abuse

A challenge in drug dependence is to delineate long-term behavioral and neurochemical modifications induced by drugs of abuse. In rodents, drugs of abuse induce locomotor hyperactivity, and repeating injections enhance this response. This effect, called behavioral sensitization, persists many months after the last administration, thus mimicking long-term sensitivity to drugs observed in human addicts. Although addictive properties of drugs of abuse are generally considered to be mediated by an increased release of dopamine in the ventral striatum, recent pharmacological and genetic experiments indicate an implication of alpha1b-adrenergic receptors in behavioral and rewarding responses to psychostimulants and opiates. Later on, it was shown that not only noradrenergic but also serotonergic systems, through 5-HT(2A) receptors, were controlling behavioral effects of drugs of abuse. More recently, experiments performed in animals knockout for alpha1b-adrenergic or 5-HT(2A) receptors indicated that noradrenergic and serotonergic neurons, besides their activating effects, inhibit each other by means of the stimulation of alpha1b-adrenergic and 5-HT(2A) receptors and that this mutual inhibition vanishes in wild type mice with repeated injections of psychostimulants, opiates or alcohol. Uncoupling induced by repeated treatments with drugs of abuse installs a stable sensitization of noradrenergic and serotonergic neurons, thus explaining an increased reactivity of dopaminergic neurons and behavioral sensitization. We propose that noradrenergic/serotonergic uncoupling is a common stable neurochemical consequence of repeated drugs of abuse which may also occur during chronic stressful situations and facilitate the onset of mental illness. Drug consumption would facilitate an artificial re-coupling of these neurons, thus bringing a temporary relief.

Structural and functional models of depression: from sub-types to substrates

OBJECTIVE

To present a functional model of depression facilitating research and clinical understanding.

METHOD

The authors conducted a systematic literature search and reviewed articles pertaining to the neurochemistry and pathophysiology of depressive disorders, focusing on the contribution made by the principal monoamines to three differing depressive structural sub-types (i.e. psychotic, melancholic and non-melancholic).

RESULTS

We suggest that the three structural depressive subtypes appear functionally underpinned by differential contributions of serotonergic, noradrenergic and dopaminergic neurotransmitters, so influencing phenotypic distinction (our structural model) and allowing an aetiological model to be derived with treatment specificity implications.

CONCLUSION

The functional model logically iterates with the structural model of depression and provides a useful framework for conceptualizing the depressive disorders. This model provides a logic for distinguishing between principal depressive subtypes, pursuing their functional underpinnings and explaining treatment differential effects across the three sub-types.

Loss of glutamatergic pyramidal neurons in frontal and temporal cortex resulting from attenuation of FGFR1 signaling is associated with spontaneous hyperactivity in mice

Fibroblast growth factor receptor (FGFR) gene products (Fgfr1, Fgfr2, Fgfr3) are widely expressed by embryonic neural progenitor cells throughout the CNS, yet their functional role in cerebral cortical development is still unclear. To understand whether the FGF pathways play a role in cortical development, we attenuated FGFR signaling by expressing a tyrosine kinase domain-deficient Fgfr1 (tFgfr1) gene construct during embryonic brain development. Mice carrying the tFgfr1 transgene under the control of the Otx1 gene promoter have decreased thickness of the cerebral cortex in frontal and temporal areas because of decreased number of pyramidal neurons and disorganization of pyramidal cell dendritic architecture. These alterations may be, in part, attributable to decreased genesis of T-Brain-1-positive early glutamatergic neurons and, in part, to a failure to maintain radial glia fibers in medial prefrontal and temporal areas of the cortical plate. No changes were detected in cortical GABAergic interneurons, including Cajal-Retzius cells or in the basal ganglia. Behaviorally, tFgfr1 transgenic mice displayed spontaneous and persistent locomotor hyperactivity that apparently was not attributable to alterations in subcortical monoaminergic systems, because transgenic animals responded to both amphetamine and guanfacine, an alpha2A adrenergic receptor agonist. We conclude that FGF tyrosine kinase signaling may be required for the genesis and growth of pyramidal neurons in frontal and temporal cortical areas, and that alterations in cortical development attributable to disrupted FGF signaling are critical for the inhibitory regulation of motor behavior.

Emerging evidence for a central epinephrine-innervated alpha 1-adrenergic system that regulates behavioral activation and is impaired in depression

Currently, most basic and clinical research on depression is focused on either central serotonergic, noradrenergic, or dopaminergic neurotransmission as affected by various etiological and predisposing factors. Recent evidence suggests that there is another system that consists of a subset of brain alpha(1B)-adrenoceptors innervated primarily by brain epinephrine (EPI) that potentially modulates the above three monoamine systems in parallel and plays a critical role in depression. The present review covers the evidence for this system and includes findings that brain alpha(1)-adrenoceptors are instrumental in behavioral activation, are located near the major monoamine cell groups or target areas, receive EPI as their neurotransmitter, are impaired or inhibited in depressed patients or after stress in animal models, and are restored by a number of antidepressants. This "EPI-alpha(1) system" may therefore represent a new target system for this disorder.

alpha 1d Adrenoceptor signaling is required for stimulus induced locomotor activity

alpha 1 Adrenergic receptors mediate a variety of physiological responses and have been well studied in the cardiovascular and peripheral nervous system. However, their role in the central nervous system remains ill defined because of the lack of highly specific ligands to the alpha1 receptor subtypes. Here, we have employed gene targeting to elucidate the role of alpha 1d receptors in vivo. In addition to disrupting function, the insertion of the lacZ gene into the alpha 1d receptor locus enabled the specific identification of cells expressing the alpha 1d gene. These cells are localized in the cortex, hippocampus, olfactory bulb, dorsal geniculate and ventral posterolateral nuclei of the thalamus. Behaviorally, the alpha 1d(-/-) mice show normal locomotor activity during the subjective day, or resting phase of their cycle. However, during subjective night, or active phase, wheel-running activity is significantly reduced in mutant mice. Furthermore, these mice show a reduction in exploratory rearing behavior in a novel cage environment. Lastly, alpha 1d(-/-) mice show reduced hyperlocomotion after acute amphetamine administration. Together, these data reveal the functional importance of alpha 1d adrenoceptors in mediating a variety of stimulus-induced changes in locomotor behaviors. While the sensitivity of noradrenergic neurons to environmental stimuli has been well documented, our data demonstrate that at least some of these post-synaptic responses are mediated by alpha 1d adrenergic receptors.

Alpha1b-adrenergic receptors control locomotor and rewarding effects of psychostimulants and opiates

Drugs of abuse, such as psychostimulants and opiates, are generally considered as exerting their locomotor and rewarding effects through an increased dopaminergic transmission in the nucleus accumbens. Noradrenergic transmission may also be implicated because most psychostimulants increase norepinephrine (NE) release, and numerous studies have indicated interactions between noradrenergic and dopaminergic neurons through alpha1-adrenergic receptors. However, analysis of the effects of psychostimulants after either destruction of noradrenergic neurons or pharmacological blockade of alpha1-adrenergic receptors led to conflicting results. Here we show that the locomotor hyperactivities induced by d-amphetamine (1-3 mg/kg), cocaine (5-20 mg/kg), or morphine (5-10 mg/kg) in mice lacking the alpha1b subtype of adrenergic receptors were dramatically decreased when compared with wild-type littermates. Moreover, behavioral sensitizations induced by d-amphetamine (1-2 mg/kg), cocaine (5-15 mg/kg), or morphine (7.5 mg/kg) were also decreased in knock-out mice when compared with wild-type. Ruling out a neurological deficit in knock-out mice, both strains reacted similarly to novelty, to intraperitoneal saline, or to the administration of scopolamine (1 mg/kg), an anti-muscarinic agent. Finally, rewarding properties could not be observed in knock-out mice in an oral preference test (cocaine and morphine) and conditioned place preference (morphine) paradigm. Because catecholamine tissue levels, autoradiography of D1 and D2 dopaminergic receptors, and of dopamine reuptake sites and locomotor response to a D1 agonist showed that basal dopaminergic transmission was similar in knock-out and wild-type mice, our data indicate a critical role of alpha1b-adrenergic receptors and noradrenergic transmission in the vulnerability to addiction.

Cortical alpha 1-adrenergic regulation of acute and sensitized morphine locomotor effects

The role of alpha1-adrenergic transmission was tested on locomotor effects of acute or repeated morphine (5 mg/kg, i.p.) administration. Prazosin, an alpha1-adrenergic antagonist, administered 30 min before morphine, either systemically (0.5 mg/kg, i.p.) or locally and bilaterally into the prefrontal cortex (200 pmol/side) reduced the stimulatory influence of morphine on locomotion. The progressive increase of the locomotor response induced by repeated morphine injections was blocked by a prazosin pretreatment but not the behavioral sensitization on the test day. These data suggest that blockade of cortical alpha1-adrenergic receptors reduces the expression of acute and sensitized locomotor responses to morphine, but does not prevent the induction of behavioral sensitization.

Galpha(olf) levels are regulated by receptor usage and control dopamine and adenosine action in the striatum

In the striatum, dopamine D(1) and adenosine A(2A) receptors stimulate the production of cAMP, which is involved in neuromodulation and long-lasting changes in gene expression and synaptic function. Positive coupling of receptors to adenylyl cyclase can be mediated through the ubiquitous GTP-binding protein Galpha(S) subunit or through the olfactory isoform, Galpha(olf), which predominates in the striatum. In this study, using double in situ hybridization, we show that virtually all striatal efferent neurons, identified by the expression of preproenkephalin A, substance P, or D(1) receptor mRNA, contained high amounts of Galpha(olf) mRNA and undetectable levels of Galpha(s) mRNA. In contrast, the large cholinergic interneurons contained both Galpha(olf) and Galpha(s) transcripts. To assess the functional relationship between dopamine or adenosine receptors and G-proteins, we examined G-protein levels in the striatum of D(1) and A(2A) receptor knock-out mice. A selective increase in Galpha(olf) protein was observed in these animals, without change in mRNA levels. Conversely, Galpha(olf) levels were decreased in animals lacking a functional dopamine transporter. These results indicate that Galpha(olf) protein levels are regulated through D(1) and A(2A) receptor usage. To determine the functional consequences of changes in Galpha(olf) levels, we used heterozygous Galpha(olf) knock-out mice, which possess half of the normal Galpha(olf) levels. In these animals, the locomotor effects of amphetamine and caffeine, two psychostimulant drugs that affect dopamine and adenosine signaling, respectively, were markedly reduced. Together, these results identify Galpha(olf) as a critical and regulated component of both dopamine and adenosine signaling.

Abnormal presynaptic catecholamine regulation in a hyperactive SNAP-25-deficient mouse mutant

The consequences of a reduction in the presynaptic protein, SNAP-25, were investigated to determine the neurochemical basis of the marked hyperlocomotor activity in coloboma (Cm/+) mice. SNAP-25 is part of the minimal presynaptic machinery necessary for exocytotic neurotransmitter release. Reserpine treatment was used to deplete vesicular stores of catecholamines. Coloboma mice were more sensitive to the effects of reserpine than control mice. However, presynaptic regulation of dopamine (DA) release, as assessed by low-dose apomorphine challenge, was intact. There were region-specific reductions in in vivo tyrosine hydroxylation and the DA metabolites homovanillic acid (HVA) and 3,4-dihydroxyphenylacetic acid (DOPAC) in the striatum and nucleus accumbens of Cm/+ mice. While hyperactivity is often associated with changes in DA concentration, norepinephrine (NE) concentration was significantly increased in the striatum and nucleus accumbens of the hyperactive mutant. The increase in NE may regulate the hyperactivity in these mice, as suggested by current hypotheses of the mechanisms underlying attention-deficit hyperactivity disorder (ADHD) and Tourette's syndrome.

Evidence for a modulatory effect of sulbutiamine on glutamatergic and dopaminergic cortical transmissions in the rat brain

Chronic treatment of rats by sulbutiamine induced no change in density of N-methyl-D-aspartate (NMDA) and (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors in the cingular cortex, but a significant decrease of the kainate binding sites, as measured by quantitative autoradiography. In the same treated animals, an increase of D1 dopaminergic (DA) binding sites was measured both in the prefrontal and the cingular cortex, while no modification of the D2 binding sites was detected. Furthermore, an acute sulbutiamine administration induced a decrease of kainate binding sites but no change of the density of D1 and D2 DA receptors. Acute sulbutiamine injection led to a decrease of the DA levels in the prefrontal cortex and 3,4-dihydroxyphenylacetic acid levels in both the cingular and the prefrontal cortex. These observations are discussed in terms of a modulatory effect of sulbutiamine on both dopaminergic and glutamatergic cortical transmissions.

Aboulia: neurobehavioural dysfunction of dopaminergic system?

Recent advances in the understanding of the neural substrates of goal-directed behaviour have created new interest in unlocking the mystery behind those disorders that are characterized by poverty of thought and action. In this review, various studies will be considered which proffer converging evidence that the dopaminergic brain circuitry running from ventral tegmental areas in the midbrain, via nucleus accumbens in the forebrain, to the frontal cortex, tends to produce aboulia when its restitutive function fails. Such aboulic deficits occur in various neurological and psychiatric disorders in which they have profound implications for the patients' management, rehabilitation and social interactions. We begin by examining the consequences of dopamine agonism and antagonism in pre-clinical studies and draw on the inferences that can be made from studies in humans. We then go on to discuss aboulic features in neuropsychiatric conditions, focusing on clinical manifestation, animal models, abnormal dopamine activity and pharmacological interventions.

Integrity of the mesocortical dopaminergic system is necessary for complete expression of in vivo hippocampal-prefrontal cortex long-term potentiation

The prefrontal cortex receives dopaminergic inputs from the ventral tegmental area and excitatory inputs from the hippocampus. Both afferent pathways target in close proximity dendritic spines of pyramidal cells in layer V-VI of the prefrontal cortex. In view of the prominent role of dopamine in cognitive functions we examined the effects of ventral tegmental area stimulation on the induction of long-term potentiation in the hippocampal-prefrontal cortex pathway of anesthetized rats. Stimulation of the ventral tegmental area at a frequency known to evoke dopamine overflow in the prefrontal cortex produces a long-lasting enhancement of the magnitude of the hippocampal-prefrontal cortex long-term potentiation. The role of dopamine was further examined by investigating the effects of prefrontocortical dopamine depletion induced by an electrolytic ventral tegmental area lesion. A significant correlation (r = 0.8; P < 0.001; n = 14) was obtained between cortical dopamine levels and cortical long-term potentiation amplitude, a depletion of more than 50% of cortical levels corresponding to a dramatic decrease in hippocampal-prefrontal cortex long-term potentiation. However, a recovery to normal long-term potentiation was observed 1 h after tetanic stimulation. In contrast to the effects on long-term potentiation, ventral tegmental area stimulation, when applied at low or high frequency, decreases the amplitude of the hippocampal-prefrontal cortex postsynaptic synaptic response. The present study demonstrates the importance of the integrity of the mesocortical dopaminergic system for long-term potentiation to occur in the hippocampal-prefrontal cortex pathway and suggests a frequency-dependent effect of dopamine on hippocampal-prefrontal cortex transmission.

Central alpha1-adrenoceptors: their role in the modulation of attention and memory formation

Adrenoceptors presently are classified into three main subclasses: alpha1-, alpha2-, and beta-receptors, each with three (perhaps more) subtypes. All three alpha1-adrenoceptor subtypes are present in rat brain. The purpose of this review is to assess the role of alpha1-adrenoceptors in the modulation of synaptic transmission and plasticity, as well as their ability to modulate higher cerebral functions, such as attentional and memory processes. However, since there are no truly subtype-specific agonists or antagonists available at present, it is virtually impossible to allocate a particular central effect to one or other of the subtypes. The activation of alpha1-adrenoceptors reduces the firing probability and glutamate release in the cornu ammonis of the hippocampus. Alpha1-Adrenoceptors may flexibly modulate weak and strong activation of the pyramidal neurones in the neocortex. Alpha1-Adrenoceptors play only a minor role in the modulation of long-term potentiation in the hippocampus, and may influence many brain functions also via non-neuronal mechanisms. since glial cells can express alpha1-adrenoceptors. At the behavioural level, the activation of alpha1-adrenoceptors promotes vigilance and influences working memory and behavioural activation, while having only a minor role in the modulation of long-term memory.

Stimulation of alpha-1 adrenergic receptors facilitates spatial learning in rats

The present experiments were designed to examine the effects of alpha-1 adrenergic stimulation and inhibition on memory encoding and to investigate whether the alpha-1 adrenergic and muscarinic cholinergic systems interact in the regulation of spatial navigation behavior in the Morris water maze test and we also studied the effects of D-cycloserine, a partial agonist at the glycine binding site on the N-methyl-D-aspartate (NMDA) receptor complex, on the performance of scopolamine-treated rats in this task. Pre-training subcutaneous administration of St-587 (a putative alpha-1 agonist) at 1000 micrograms kg-1 or 1500 micrograms kg-1 improved water maze navigation to a hidden platform. Prazosin (an alpha-1 antagonist), 300-2000 micrograms kg-1, did not significantly impair the spatial navigation performance. Pre-training administration of prazosin 1000 micrograms kg-1, but not 300 micrograms kg-1, slightly potentiated the deficit in water maze navigation seen after scopolamine (200 micrograms kg-1, pre-training intraperitoneal injection). Pre-training administration of St-587 at a dose 1500 micrograms kg-1, but not 500 micrograms kg-1, slightly ameliorated the scopolamine-induced (200 micrograms kg-1) impairment in performance of rats. Pre-training administration of prazosin at doses 300 or 1000 micrograms kg-1 or St-587 at doses 500 micrograms kg-1 or 1500 micrograms kg-1 did not have any significant influence on the scopolamine-induced (200 micrograms kg-1) increase of swimming speed. Furthermore, D-cycloserine at the dose of 300 micrograms kg-1 but not 1000 or 3000 micrograms kg-1 reversed the scopolamine (200 micrograms kg-1)-induced deficit in acquisition of the water maze task but not the increase in motor output (increased swimming speed). These results indicate that the stimulation of alpha-1 adrenoceptors may facilitate the encoding of new information. These findings suggest that alpha-1 adrenergic mechanisms do not participate or at least are not the most critical part of the noradrenergic system in the interaction between noradrenaline and muscarinic receptors in the modulation of learning and memory. In addition, these results suggest that D-cycloserine may be effective in alleviating states of central cholinergic hypofunction.

6-Hydroxydopamine lesion of locus coeruleus and the antiparkinsonian potential of NMDA-receptor antagonists in rats

Behavioral and neurochemical effects after bilateral 6-hydroxy-dopamine locus coeruleus- (LC) lesion were examined in rats and compared to sham-lesioned controls. Behavior after treatment with the antiakinetic drugs dizocilpine, amantadine, memantine or L-DOPA as well as joint treatment of these drugs with haloperidol were tested in an open field with holeboard and in an experimental chamber. Under saline spontaneous activity (open field with holeboard) and sniffing (experimental chamber) were reduced after lesion. Injection of the proparkinsonian drug haloperidol decreased sniffing in all rats but to a greater extent in LC-lesioned rats. In combination with haloperidol none of the tested drugs could completely compensate for the motor deficits induced by the lesion. Neurochemical data revealed a reduced content of noradrenaline in the prefrontal cortex and in the posterior striatum of LC-lesioned rats. These results indicate that loss of LC neurons intensifies parkinsonian symptoms induced by blockade of dopamine D2-receptors, and lowers the antiakinetic potential of dizocilpine, amantadine, memantine or L-DOPA.

Modulation of vigilance and behavioral activation by alpha-1 adrenoceptors in the rat

This study investigated the role of alpha-1 adrenergic receptors in the modulation of attention and behavioral activity by assessing the effects of alpha-1 adrenergic receptor stimulation or blockade on the performance of rats in tasks involving vigilance (sustained attention) and selective attention [five-choice serial reaction time (5-CSRT)]. Pretesting subcutaneous administration of St-587 (a putative alpha-1 agonist) at 100 micrograms/kg, but not at 300 or 1000 micrograms/kg, significantly improved the choice accuracy of rats in the 5-CSRT task (monitoring of visual stimuli), whereas prazosin (a prototype alpha-1 antagonist) at 300 micrograms/kg administered subcutaneously slightly impaired choice accuracy of the rats in this task. Prazosin at 100 micrograms/kg blocked the ability of St-587 at 100 micrograms/kg to improve choice accuracy. Furthermore, St-587 at 100 micrograms/kg significantly increased the number of trials completed and reduced the probability of premature responses, whereas prazosin at 300 micrograms/kg decreased the number of trials completed and the latency of animals to make correct responses in the task. Prazosin at 100 micrograms/kg blocked the effect of St-587 at 100 micrograms/kg in increasing the number of trials completed. However, prazosin at 100 micrograms/kg did not abolish the effect of St-587 in reducing the probability of premature responses. Because the effect of St-587 at 100 micrograms/kg in improving choice accuracy is rather modest, it is possible that when the 100- and 300-microgram/kg doses of St-587 were administered in a counterbalanced order, this effect could have been overlooked due to day-to-day variation. Thus, the present results suggest that stimulation of alpha-1 adrenergic receptors can facilitate vigilance.

alpha1B adrenoceptors in rat paraventricular nucleus overlap with, but do not mediate, the induction of c-Fos expression by osmotic or restraint stress

A role has been suggested for hypothalamic alpha1 adrenoceptors in the acute stress-induced activation of the hypothalamic-pituitary-adrenal axis. Using a polyclonal antiserum against the rat alpha1B adrenergic receptor protein, we have demonstrated alpha1B receptor immunoreactivity in neurons and especially in punctate cell processes in the rat paraventricular nucleus. The distribution of alpha1B receptor immunoreactivity overlapped in part with the distributions of c-Fos immunoreactivity induced in the paraventricular nucleus by either restraint stress or hypertonic saline administration. However, intraperitoneal pretreatment with the alpha1 receptor antagonist prazosin (0.5 or 5.0 mg/kg) failed to attenuate stress-induced c-Fos expression in the paraventricular nucleus. Prazosin also failed to attenuate the secretion of corticosterone following restraint stress. Thus, we conclude that neither acute secretory activity nor activation of gene transcriptional responses mediated by c-Fos in the hypothalamic pituitary adrenal axis following these stressors are dependent upon hypothalamic alpha1 adrenergic receptors.

Stimulation and blockade of alpha1 adrenoceptors affect behavioural activity, but not spatial working memory assessed by delayed non-matching to position task in rats

The present study investigated the role of alpha1 adrenergic receptors in the modulation of working memory and behavioural activity by assessing the effects of alpha1 adrenergic receptor stimulation or blockade on the performance of rats in a delayed non-matching to position task. St-587 (a putative agonist of alpha1 adrenoceptors) at a dose of 100 microg/kg slightly increased choice accuracy (per cent correct responses) of rats, but the effect was delay-independent which is interpreted as an improvement in choice accuracy and non-mnemonic (non-working memory) in character. Neither St-587 (300 or 1000 microg/kg) nor prazosin (a prototype antagonist of alpha1 adrenoceptors) (100 or 300 microg/kg) significantly affected the choice accuracy of rats in this task. Prazosin 300 microg/kg lengthened the latency for correct responses in the working memory task but did not affect food collection latencies. This combination of effects may reflect decreased motor output. St-587 300 and 1000 microg/kg, but not prazosin, increased food collection latencies in the working memory task. Thus, the present results suggest that alpha1 adrenergic receptors do not play any important role in spatial working memory as assessed using the delayed non-matching to position task, but that modulation of alpha1 adrenoceptors may affect motor activity and motivation in rats.

Role of D1 and alpha1 receptors in the enhanced locomotor response to dopamine D2-like receptor stimulation induced by repeated electroconvulsive shock

We previously reported that, in rats chronically treated with the antidepressant drug imipramine, the enhanced locomotor response to the D2-like receptor agonist quinpirole became less sensitive to the inhibitory effect of the D1 receptor antagonist SCH 23390 and more sensitive to the inhibitory effect of the alpha1 receptor antagonist prazosin. In this study, we show that in electroconvulsive shock-treated rats these antagonists behave in the opposite manner to that observed in imipramine-treated rats, with SCH 23390 being highly effective and prazosin ineffective in antagonizing the locomotor response to quinpirole. The possibility that these differences may reflect some of the clinical characteristics of these antidepressant treatments is discussed.

Anti-ataxic effects of TRH and its analogue, TA-0910, in Rolling mouse Nagoya by metabolic normalization of the ventral tegmental area

1. The mechanism of the anti-ataxic action of thyrotropin-releasing hormone (TRH) and its analogue. TA-0910, in the Rolling mouse Nagoya (RMN), an ataxic mutant mouse, has been investigated. 2. TRH (30 mg kg-1, i.p.) and TA-0910 (3 mg kg-1, i.p.) reduced the fall index (number of falls/spontaneous motor activity), an index of ataxia, 10-30 and 10-60 min after administration, respectively. 3. Relative local cerebral glucose utilization (LCGU) in the cerebellum and ventral tegmental area (VTA) of the rolling mouse was significantly smaller than that in normal animals. TRH (30 mg kg-1, i.p.) and TA-0910 (3 mg kg-1, i.p.) increased the relative LCGU value of the VTA but not of the cerebellum in rolling mice to the level of normal animals. 4. These results suggest that the ataxia of the rolling mouse may be due to dysfunction of the cerebellum and VTA, and that amelioration by TRH and TA-0910 could result from metabolic normalization of the VTA.

Effects of selective dopamine depletion in medial prefrontal cortex on basal and evoked extracellular dopamine in neostriatum

In this study, we demonstrate that 6-hydroxydopamine (6-OHDA) can be used to produce a lesion of dopamine (DA) terminals in medial prefrontal cortex (mPFC) while sparing the noradrenergic innervation in this region. Furthermore, we determined the impact of these lesions on both extracellular DA in neostriatum, using in vivo microdialysis, and locomotor activity. Our results demonstrate that, whereas higher doses of 6-OHDA (> or = 4 micrograms) depleted both DA and norepinephrine (NE) in mPFC, 1 micrograms 6-OHDA produced a depletion of DA (-79%) without significantly affecting NE content (-13%). Selective depletion of DA content in mPFC did not alter basal levels of extracellular DA in neostriatum determined 14 days after the lesion. The lesion also did not alter the ability of acute tail pressure (30 min) to increase extracellular DA in neostriatum or to stimulate locomotor activity. Depletion of DA in mPFC did not alter the ability of d-amphetamine (1.5 mg/kg, i.p.) to increase intracellular DA in neostriatum. In contrast, the maximum amphetamine-induced increase in locomotor activity was attenuated in lesioned rats as compared with control rats (670 and 280 locomotor counts/15 min, respectively). These data suggest that in the intact system, DA terminals in mPFC do not regulate extracellular DA in neostriatum. In addition, these data confirm that DA terminals in mPFC can influence stimulant-induced locomotion.

Unilateral damage to the ventral tegmental area facilitates feeding induced by stimulation of the contralateral ventral tegmental area

Unilateral lesions of the ventral tegmental area (VTA) facilitated feeding induced by electrical stimulation of the homologous VTA tissue in the contralateral hemisphere. The lesions shifted the function relating latency to begin feeding to stimulation frequency to the left simultaneously causing a reduction of frequency threshold for feeding reaction. Facilitation of feeding was immediate (with a peak on the 2nd postlesion day) and in some animals persisted up to the end of the 2-week experimental period. No facilitation of VTA stimulation-induced feeding was found in the control animals in which comparable lesions were performed in the contralateral lateral hypothalamus or the antero-dorsal thalamus which suggests that the effect was site specific. Individual differences in the magnitude and duration of the facilitatory effect on feeding may be related to the variability in the medio-lateral localization of the lesions. The results are interpreted in terms of compensatory increase in the dopaminergic transmission and/or decrease of the GABA-ergic inhibitory tone in the contralateral hemisphere after unilateral lesion to the mesencephalic dopaminergic systems. A possible involvement of the noradrenergic transmission is also discussed.

Blockade of prefronto-cortical alpha 1-adrenergic receptors prevents locomotor hyperactivity induced by subcortical D-amphetamine injection

The stimulation of cortical dopaminergic D1 receptors can counteract the increased locomotor activity evoked by D-amphetamine application in the nucleus accumbens (Vezina et al., Eur. J. Neurosci., 3, 1001-1007, 1991). Moreover, an alpha 1 antagonist, prazosin, prevents the locomotor hyperactivity induced by electrolytic lesions of the ventral tegmental area (Trovero et al., Neuroscience, 47, 69-76, 1992). Attempts were thus made to see whether blockade of alpha 1-adrenergic receptors in the rat prefrontal cortex could reduce nucleus accumbens D-amphetamine-evoked locomotor activity. Rats implanted chronically and bilaterally with cannulae into the medial prefrontal cortex and the nucleus accumbens were used for this purpose and locomotor activity was monitored in circular corridors. Preliminary experiments indicated that intraperitoneal injection of prazosin (0.06 mg/kg) reduces the locomotor hyperactivity induced by the peripheral administration of D-amphetamine (0.75 mg/kg). This effect of prazosin was not observed when locomotor hyperactivity was obtained by an intraperitoneal injection of scopolamine (0.8 mg/kg). Bilateral nucleus accumbens injections of D-amphetamine (4.0 nmol/side) markedly increased locomotor activity, as estimated in a 30 min period. Prior (20 min) bilateral injections of either prazosin or WB-4101 (0.16 pmol) into the medial prefrontal cortex abolished the nucleus accumbens D-amphetamine-evoked response. The recovery of the nucleus accumbens D-amphetamine-evoked response was closely dependent on the amount of prazosin used, very prolonged inhibitory effects of the drug being seen with a high amount (> 4 days with 160 pmol). In contrast, whatever the amount of WB-4101 used (0.16-160 pmol), recovery occurred within 3 days.(ABSTRACT TRUNCATED AT 250 WORDS)

Prazosin modulates the changes in firing pattern and transmitter release induced by raclopride in the mesolimbic, but not in the nigrostriatal dopaminergic system

Most antipsychotic drugs are, in addition to being dopamine (DA) D2 receptor antagonists, also relatively potent alpha 1 adrenoceptor antagonists. Here, we have studied the effects of the selective DA D2 receptor antagonist raclopride, alone and in combination with the selective alpha 1 adrenoceptor antagonist, prazosin, on midbrain DA neurons utilizing extracellular single cell recording techniques. As a reference compound, haloperidol (0.05-1.6 mg/kg, i.v.), a potent antagonist at both DA D2 receptors and alpha 1 adrenoceptors, was included in the electrophysiological part of the study. In addition, in vivo voltammetry was used to measure extracellular DA concentrations in the nucleus accumbens (NAC) and the dorsolateral striatum (STR) in anesthetized, pargyline pretreated rats treated with the above drugs. Raclopride (10-5120 micrograms/kg, i.v.) induced a dose dependent increase in firing rate of DA neurons in the ventral tegmental area (VTA), that was significant already at 10 micrograms/kg, and in the substantia nigra-zone compacta (SN-ZC), that reached significance at 2560 micrograms/kg. Burst firing of DA neurons was also increased in the VTA at 40 micrograms/kg, as well as in the SN-ZC at 640 micrograms/kg. A low dose of raclopride (80 micrograms/kg, cumulated dose) induced a significant increase in extracellular DA concentrations in NAC to 490% and in STR to 220%. A high dose of raclopride (2560 micrograms/kg, cumulated dose) induced a 930% increase in extracellular DA concentrations in NAC, but only a 280% increase in STR. These data demonstrate that raclopride exerts a relatively selective action on mesolimbic DA neurons. Prazosin (0.3 mg/kg, i.v.) decreased burst firing of VTA, but not SN-ZC DA neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Noradrenaline decreases transmission of NMDA- and non-NMDA-receptor mediated monosynaptic EPSPs in rat prefrontal neurons in vitro

The effects of noradrenaline on pyramidal cells of layer V of the prefrontal cortex were examined in rat brain slices in vitro. Bath administration of noradrenaline (10 microM) reduced synaptic transmission of afferent inputs from layer 1. The decrease affected all the components of the evoked response and particularly the monosynaptic excitatory postsynaptic potential (EPSP) as evidenced by a reduction of its initial rising slope (mean slope: 71 +/- 11% of its control). Pharmacological dissociation of the NMDA- and non-NMDA-receptor components of the EPSP showed that noradrenaline reduced both (mean EPSP slopes were 71 +/- 8% and 73 +/- 10% of their control, respectively). Alpha 1-, but not alpha-2- or beta-adrenoceptor antagonists prevented the noradrenaline-induced decrease in synaptic efficacy. However, the effect of noradrenaline was not reproduced by alpha 1-adrenoceptor agonists. Lastly, noradrenaline acting through beta-adrenoceptors reduced the slow hyperpolarization that follows a train of action potentials.

Noradrenergic modulation of midbrain dopamine cell firing elicited by stimulation of the locus coeruleus in the rat

Electrical stimulation techniques were employed in the chloral hydrate anaesthetized male rat to evaluate if the pontine noradrenergic nucleus locus coeruleus can influence the activity of midbrain dopamine neurons in the ventral tegmental area and zona compacta, substantia nigra. Single-pulse locus coeruleus stimulation evoked an excitation, followed by an inhibition, of the electrical activity of single midbrain dopamine neurons. Neither of these responses were observed in animals pretreated with reserpine, implicating noradrenaline as a mediator. The alpha 1-adrenoceptor antagonist prazosin decreased the excitation, while other adrenoceptor antagonists were without general effect. Burst-type stimulation produced only a more long-lasting inhibition. The influence from the locus coeruleus on midbrain dopamine neurons could be important in behavioural situations involving novelty and reward, and might also be of importance for the actions of psychotropic drugs.

NE/DA interactions in prefrontal cortex and their possible roles as neuromodulators in schizophrenia

The monoaminergic innervation of the rat prefrontal cortex arises from well-defined mesencephalic nuclei, with noradrenergic (NE) neurons located in the locus coeruleus, dopaminergic (DA) neurons located in the ventral tegmental area, and serotonergic (5-HT) neurons originating in the raphe nuclei. Specific destruction of the NE bundle was found to induce morphological (i.e., sprouting) as well as metabolic (i.e., changes in rate of DA utilization) modifications of mesocortical DA neurons, suggesting that these two catecholaminergic systems have functional interactions within the prefrontal cortex. This was substantiated by experiments showing that DA afferents modulate the sensitivity of cortical post-synaptic beta-adrenergic receptors and that, reciprocally, NE neurons control the sensitivity of cortical D1 receptors. Behavioural and pharmacological data have further indicated that the stimulation of cortical alpha-1 adrenergic receptors inhibits cortical DA transmission at D1 receptors. Secondly, we have attempted to analyze how such interactions between neuromodulatory systems may be related to the development of mental diseases such as schizophrenia. On the basis of studies in the literature describing the effects produced by the ingestion of hallucinogenic drugs or data collected regarding REM sleep, it is postulated that two modes of brain functioning exist: analogical and cognitive. Each mode is characterized by differences in the relative activities of NE, DA and 5-HT neurons. At birth, during REM sleep, and following the ingestion of hallucinogens, the mode of brain functioning is essentially analogical; in contrast, both analogic and cognitive modes are postulated to coexist in the awake state. Oscillations between these two modes are under the control of monoaminergic systems on which an increase in cortical DA release favours the cognitive processing mode, whereas intermittent activations of NE neurons would switch the brain into the analogical mode of processing. It is proposed that schizophrenic patients with "positive" symptoms suffer from an abnormal preponderance of the analogical mode while awake, whereas "negative" symptoms are due to the excessive presence of the cognitive mode. Although pure biological deficits cannot be excluded, these dysfunctions could be related to the absence of particular environmental variables early in the development of these patients. This condition is probably required to establish normal regulatory control of monoaminergic neuronal activity.

In Vivo Partial Inactivation of Dopamine D1Receptors Induces Hypersensitivity of Cortical Dopamine-Sensitive Adenylate Cyclase: Permissive Role of ?1-Adrenergic Receptors

As shown by autoradiography, peripheral injections of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) induced a dose-dependent decrease of [3H]SCH 23390 and [3H]prazosin high-affinity binding sites in the rat prefrontal cortex. EEDQ showed similar efficacy in inactivating cortical and striatal dopamine (DA) D1 receptors, whereas prazosin-sensitive alpha 1-adrenergic receptors were more sensitive to the action of the alkylating agent, as for all doses of EEDQ tested (from 0.8 to 3 mg/kg, i.p.), the decrease in cortical [3H]SCH 23390 binding was less pronounced than that of [3H]prazosin. The effects of EEDQ on [3H]SCH 23390 binding and DA-sensitive adenylate cyclase activity were then simultaneously compared in individual rats. In the striatum, whatever the dose of EEDQ used, the decrease of DA-sensitive adenylate cyclase activity was always lower than that of D1 binding sites, suggesting the occurrence of a large proportion of spare D1 receptors. In the prefrontal cortex, a significant increase in DA-sensitive adenylate cyclase activity was observed in rats treated with a low dose of EEDQ (0.8 mg/kg), this effect being associated with a slight reduction in [3H]SCH 23390 binding sites (-20%). Parallel decreases in the enzyme activity and D1 binding sites were observed with higher doses. The EEDQ-induced supersensitivity of DA-sensitive adenylate cyclase did not occur in rats in which the decrease in [3H]prazosin binding sites was higher than 35%.(ABSTRACT TRUNCATED AT 250 WORDS)