Involvement of dopamine neurons in the regulation of beta-adrenergic receptor sensitivity in rat prefrontal cortex

Neurochemical interaction between dopaminergic and noradrenergic neurons in the medial prefrontal cortex

Growing evidence indicates that there is an interaction between the transmission of dopamine (DA) and norepinephrine (NE) in the noradrenergic and dopaminergic projections that converge in the medial prefrontal cortex (mPFC). The effects of the noradrenergic alpha1 and alpha2 receptors and the NE transporters on the DA outflow and those of the dopaminergic D1 and D2 receptors on NE release in the mPFC were investigated. Local infusions of NE (90, 150, and 300 nM) into the mPFC increased the extracellular release of DA in anesthetized rats. The alpha1 receptor antagonist (10 microM prazosin), but not the alpha2 receptor antagonist (100 microM piperoxan), blocked the NE-induced increase of DA in the mPFC. In addition, local infusion of alpha1 receptor agonist (10 microM phenylephrine) enhanced DA release in the mPFC. Local application of DA in different concentrations into the mPFC increased extracellular NE levels. Intra-mPFC infusion of a D1 receptor antagonist (10 nM SCH23390), inhibited the DA-induced increase of NE; this did not happen with a D2 receptor antagonist (1 nM eticlopride). Local administration of a selective NE uptake inhibitor (1 microM desmethylimipramine) into the mPFC increased the outflows of both DA and NE in the mPFC. However, co-infusion of DMI and prazosin blunted, but did not totally abolish, the DMI-increase in the extracellular levels of DA and NE. These results suggest that in the mPFC, 1) extracellular NE could enhance DA release by activating the alpha1 receptors; and 2) extracellular DA increased the extracellular levels of NE by activating the D1 receptors.

Noradrenergic lesions differentially alter the expression of two subtypes of low Km cAMP-sensitive phosphodiesterase type 4 (PDE4A and PDE4B) in rat brain

This study examined the effects of selective, central noradrenergic dennervation with 6-hydroxydopamine (6-OHDA) on the expression of type 4 phosphodiesterases (PDE4). Twenty-one days following i.c.v. injection of 6-OHDA (200 microg) hypothalamus, neostriatum, and cerebellum were dissected. Infusion of 6-OHDA reduced norepinephrine (NE) content in all the brain areas examined (to 17%, 76% and 16% of sham-operated controls in hypothalamus, striatum, and cerebellum, respectively). 6-OHDA injections also reduced dopamine levels in hypothalamus (53%) and neostriatum (68%). Administration of desipramine (20 mg/kg, i.p.) 30 min prior to 6-OHDA injection protected neostriatal and cerebellar noradrenergic neurons NE levels (110-122% of the control levels). Desipramine partially attenuated the 6-OHDA-mediated decrease in NE content of hypothalamus, but had little or no effect on either striatal or hypothalamic dopamine (DA) levels. Western blot analysis using a PDE4A-selective antibody revealed three major bands (109 kDa PDE4A5, 102 kDa PDE4AX and 76 kDa PDE4A1) in hypothalamus and striatum. Infusion of 6-OHDA decreased the expression of PDE4A5 and PDE4AX but not of PDE4A1 in hypothalamus, as determined by quantitative Western blotting. Pretreatment of rats with desipramine attenuated the 6-OHDA-induced down-regulation of PDE4A5 and PDE4AX bands in hypothalamus. The PDE4B selective antibody K118 labels 5 major bands in all the brain regions studied. One hundred kDa PDE4B3, 86 kDa PDE4B2 and a 78 kDa PDE4B band was identified using recombinant proteins. Treatment of rats with 6-OHDA resulted in a 52% decrease in the PDE4B3 and 58% decrease in 78 kDa PDE4B variant in hypothalamus; administration of desipramine attenuated the 6-OHDA-induced down-regulation of both PDE4B variants. Neither 6-OHDA nor desipramine altered striatal PDE4A or PDE4B isozymes. In contrast, cerebellar PDE4B3 variant is up-regulated by 6-OHDA treatment and were partially normalized to control values by desipramine pretreatment. These data demonstrate that PDE4 subtypes are differentially regulated by presynaptic noradrenergic activity and may play an important role in the maintaining homeostasis of noradrenergic signal transduction in rat brain.

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.

Increased dopamine and norepinephrine release in medial prefrontal cortex induced by acute and chronic stress: effects of diazepam

We have examined the effects of diazepam on the stress-induced increase in extracellular dopamine and norepinephrine in the medial prefrontal cortex using in vivo microdialysis. In naive rats, acute tail pressure (30 min) elicited an increase in the concentrations of dopamine and norepinephrine in extracellular fluid of medial prefrontal cortex (+54 and +50%, respectively). Diazepam (2.5 mg/kg, i.p.) decreased the basal concentration of extracellular dopamine and norepinephrine. Diazepam also attenuated the stress-evoked increase in the absolute concentrations of extracellular dopamine (+17%), but did not alter the stress-induced increase in norepinephrine (+41%). However, when the drug-induced decrease in basal dopamine and norepinephrine concentration was taken into account, the stress-induced net increase in dopamine above the new baseline was equivalent to that obtained in vehicle pretreated rats, whereas the net increase in norepinephrine was almost twice that obtained in control subjects. In rats previously exposed to chronic cold (three to four weeks at 5 degrees C), tail pressure again produced an increase in the concentrations of dopamine and norepinephrine in the medial prefrontal cortex (+42% and +92%, respectively). However, in these chronically stressed rats, diazepam no longer decreased basal dopamine or norepinephrine in extracellular fluid, nor did it affect the stress-induced increase in the concentrations of these catecholamines. These data indicate that diazepam has complex effects on the extracellular concentrations of dopamine and norepinephrine which vary depending upon whether the rat is undisturbed or stressed during the period of drug exposure as well as the rat's prior history of exposure to stress. Moreover, these data raise questions regarding the role of catecholamines in the mechanism by which diazepam exerts its anxiolytic properties.

In vivo evidence that nonneuronal beta-adrenoceptors as well as dopamine receptors contribute to cyclic AMP efflux in rat striatum

We applied in vivo microdialysis to assess the effects of dopaminergic and beta-adrenergic receptor stimulation on cyclic AMP efflux in rat striatum under chloral hydrate anesthesia. Dopamine (up to 1 mM) infused for 20 min through the probe did not increase cyclic AMP, whereas both the selective dopamine D1 agonist SKF 38393 and D2 antagonist sulpiride produced modest increases. It is interesting that the beta-adrenoceptor agonist isoproterenol produced a marked increase (204.7% of basal level at 1 mM) which was antagonized by the beta-adrenoceptor antagonist propranolol. Pretreatment with a glial selective metabolic inhibitor, fluorocitrate (1 mM), by a 5-h infusion through the probe attenuated basal cyclic AMP efflux by 30.3% and significantly blocked the response to isoproterenol. By contrast, striatal injection of a neurotoxin, kainic acid (2.5 micrograms), 2 days before the dialysis experiment did not affect basal cyclic AMP or the response to isoproterenol, but blocked the response to SKF 38393. These data demonstrate the beta-adrenoceptors as well as dopamine receptors contribute to cyclic AMP efflux in rat striatum in vivo. They also suggest that basal and beta-adrenoceptor-stimulated cyclic AMP efflux are substantially dependent on intact glial cells.

The impact of stressors on immune and central neurotransmitter activity: bidirectional communication

Antigenic challenge may have broad ranging effects which include not only immunological changes, but also endocrine and central neurotransmitter repercussions, and may thus elicit profound behavioral sequelae. Commensurate with the notion that bidirectional communication exists between the immune and central nervous systems it has been demonstrated that manipulations which influence central neurotransmitter or endocrine activity provoke alterations of immune functioning, and conversely immunological alterations will affect central neurotransmitter and endocrine activity. It seems, as well, that environmental stressors may provoke marked alterations of the activity of each of these systems. Indeed, in several respects the variables that influence vulnerability to stressor-provoked neurotransmitter changes, likewise affect the immunological alterations engendered by stressors. Moreover, immunological challenges will affect central neurotransmitter functioning in much the same way as stressors provoke such effects. It is thought that immune derived products (including cytokines as well as peptide hormones) may act directly or indirectly to moderate neurotransmitter functioning, and centrally derived neurotransmitters and hormones may affect receptors present on lymphocytes. In accordance with earlier suggestions, it is maintained that the immune system may be acting as a sensory organ informing the brain of the presence of antigenic challenges, and the brain may interpret such challenge as a stressor, hence leading to behavioral alterations.

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.

Remote astrocytic response of prefrontal cortex is caused by the lesions in the nucleus basalis of Meynert, but not in the ventral tegmental area

The nucleus basalis of Meynert (nbM) was lesioned by injection of ibotenic acid, in 200 g male Wistar rats. The rats were killed 1, 3, 7 or 21 days after surgery, the brains were removed and the prefrontal cortices were subjected to immunohistochemical and Western blot analysis for the expression of glial fibrillary acidic protein (GFAP). In some rats, vehicle was injected into the nbM and in others 6-hydroxydopamine (6-OHDA) was injected into the ventral tegmental area (VTA). Quantitative Western blot analysis revealed significantly greater immunoreactivity for GFAP in the prefrontal cortex of nbM-lesioned rats. Immunohistochemical examination revealed fibrous and hypertrophic GFAP-positive astrocytes even one day after surgery, and this reaction was stronger at 3 days after surgery. After this peak, GFAP-immunoreactivity of the astrocytes decreased from 7 days to 21 days. In contrast, GFAP-positive astrocytes were not observed in the brains of vehicle-injected or VTA-lesioned rats, even 21 days after surgery. The present results indicate that cortical astrocytes respond to cholinergic deafferentation. In addition, our findings provide new insights into the abnormalities of cortical glial cells after cholinergic deafferentation in Alzheimer's disease.

Role of dopaminergic neurons in denervation-induced alpha 1-adrenergic up-regulation in the rat cerebral cortex

The density of [3H]prazosin binding to alpha 1-adrenoceptors in the rat cortex was measured after selective and mixed noradrenergic or dopaminergic lesions. DSP-4 produced a selective noradrenergic lesion and increased the density of alpha 1-adrenoceptors. 6-Hydroxydopamine produced a selective dopaminergic lesion (after desipramine protection of noradrenergic neurons) and a mixed noradrenergic and dopaminergic lesion that did not change the cortical alpha 1-adrenoceptor binding. On the basis of the results obtained, a hypothesis is put forward that the central dopaminergic system controls the denervation-induced cortical alpha 1-adrenoceptor up-regulation.

Striatal opiate mu-receptors are not located on dopamine nerve endings in the rat

In rat striatal slices, the autoradiographic analysis of [3H]naloxone binding allows one to define highly labelled patches corresponding to the striosomes and representing about 17% of the total striatal volume, surrounded by a poorly labelled zone, the matrix. Previous studies have shown that the density of these mu-opiate receptor binding sites is decreased by about 28% following destruction of the striatal dopamine innervation suggesting a partial localization of these receptors on dopamine presynaptic nerve endings. These results were confirmed but, in addition, we have shown that a chronic (30 days) blockade of dopamine transmission obtained by treatment of the animals with a long acting neuroleptic induces a similar decrease of mu binding sites. Further experiments made with D-Pen2,D-Pen5-[tyrosyl-3-5(n)-3H] enkephalin, a selective delta opiate receptor agonist, have revealed that the density of delta opiate binding sites is decreased (30%) in rats with striatal dopamine denervation but not in those treated with the long acting neuroleptic. These data indicate that part of these delta receptors is located on dopamine nerve terminals but are not in favour of the presence of mu receptors on these nerve terminals. The decrease in [3H]naloxone binding sites induced by prolonged interruption of dopamine transmission can be attributed to postsynaptic events.