Deep brain stimulation of the accumbens increases dopamine,serotonin, and noradrenaline in the prefrontal cortex

Deep brain stimulation (DBS) of the nucleus accumbens (NAc) is effective in treatment‐refractory obsessive‐compulsive disorder and major depressive disorder. However, little is known about the neurobiological mechanisms underlying the rapid and effective changes of DBS. One of the hypotheses is that DBS modulates activity of monoamine neurotransmitters. In this study, we evaluated the effects of DBS in the NAc core on the extracellular concentration of monoaminergic neurotransmitters in the medial (mPFC) and orbital prefrontal cortex (OFC). Freely moving rats were bilaterally stimulated in the NAc core for 2 h while dopamine, serotonin, and noradrenaline were measured using in vivo microdialysis in the mPFC and the OFC. We report rapid increases in the release of dopamine and serotonin to a maximum of 177% and 127% in the mPFC and an increase up to 171% and 166% for dopamine and noradrenaline in the OFC after onset of stimulation in the NAc core. These results provide further evidence for the distal effects of DBS and corroborate previous clinical and pre‐clinical findings of altered neuronal activity in prefrontal areas.     

Stimulus-specific plasticity of prefrontal cortex dopamine neurotransmission

Future planning and behavioral modification is thought to require experience-dependent plasticity in neuronal circuits involving the prefrontal cortex, nucleus accumbens and amygdala. Dopamine has been implicated in such plasticity; however, the nature of the adaptive response of dopamine systems to emotionally salient experiences is poorly understood. We determined whether the dopaminergic response to a given stimulus changes after the first exposure to that stimulus and whether this alteration is stimulus specific. Dopamine release was measured in the prefrontal cortex and the nucleus accumbens in response to two aversive but qualitatively distinct stimuli, physical restraint and electrical microstimulation of basolateral amygdala. In the prefrontal cortex, the first exposure to restraint or amygdala stimulation produced similar increases in dopamine release. The second exposure to restraint resulted in an attenuated response (- 36%) whereas the second exposure to amygdala stimulation produced a potentiated response (+ 110%). Cross-modal potentiation of response occurred with both stimuli. These adaptive changes were specific to the prefrontal cortex and were not observed in the nucleus accumbens. These findings demonstrate that prefrontal cortical dopamine output adapts after a single exposure to stimuli with emotional salience. The direction of this adaptation, however, is not uniform and depends on the nature of the stimulus.     

Differential alpha-mediated inhibition of dopamine and noradrenaline release in the parietal and occipital cortex following noradrenaline transporter blockade

Parietal and occipital cortices, while densely innervated by noradrenalin 2 (NA) projections, possess a comparatively sparse dopamine 2 (DA) innervation, even sparser than the prefrontal cortex. We previously reported that reboxetine and desipramine, two selective norepinephrine transporter (NET) blockers, at doses that maximally increase DA in the prefrontal cortex, do not increase DA in the parietal and occipital cortices. In the present study, we performed a full dose-response study of the effect of systemic reboxetine and desipramine on DA and NA in dialysates from the parietal and occipital cortices. Seven doses of reboxetine (0.1, 0.25, 0.5, 1.0, 2.5, 5.0 and 10 mg/kg) and four doses of desipramine (0.25, 1.0, 2.5 and 5.0 mg/kg) were tested. Reboxetine and desipramine differentially affected dialysate DA as compared with NA. Reboxetine increased DA maximally by about 100% after doses of 0.25-0.5 mg/kg and showed a bell-shaped dose-response function in both areas; desipramine did not affect DA in the parietal cortex and increased it in the occipital cortex only at 2.5 mg/kg. NA was maximally increased by 275% by 0.5-2.5 mg/kg reboxetine and by about 300% by 5.0 mg/kg desipramine with a more linear dose-response curve. The mechanism of peculiar dose-response function of dialysate DA after reboxetine and desipramine was further investigated by testing the effect of drugs on dialysate DA and NA under alpha(2) receptor blockade. Under local perfusion of the occipital cortex with idazoxan, an otherwise ineffective dose of reboxetine and desipramine (5 mg/kg) became effective in raising extracellular DA. In contrast, the effect of reboxetine on NA was potentiated, while that of desipramine was not affected. These results suggest that, in the parietal and occipital cortices, extracellular NA, raised by NET blockade, exerts a preferential inhibitory influence on DA release by acting on local alpha(2) receptors, thus accounting for the bell-shaped feature of the dose-response function of drugs on dialysate DA in these areas.     

Amphetamine‐induced release of dopamine in primate prefrontal cortex and striatum: striking differences in magnitude and timecourse

The psychostimulant amphetamine (AMPH) is frequently used to increase catecholamine levels in attention disorders and positron emission tomography imaging studies. Despite the fact that most radiotracers for positron emission tomography studies are characterized in non‐human primates (NHPs), data on regional differences of the effect of AMPH in NHPs are very limited. This study examined the impact of AMPH on extracellular dopamine (DA) levels in the medial prefrontal cortex and the caudate of NHPs using microdialysis. In addition to differences in magnitude, we observed striking differences in the temporal profile of extracellular DA levels between these regions that can likely be attributed to differences in the regulation of dopamine uptake and biosynthesis. The present data suggest that cortical DA levels may remain elevated longer than in the caudate which may contribute to the clinical profile of the actions of AMPH.

     

Stimulation of prefrontal cortex at physiologically relevant frequencies inhibits dopamine release in the nucleus accumbens

The prefrontal cortex (PFC) is thought to provide an excitatory influence on the output of mesoaccumbens dopamine neurons. The evidence for this influence primarily arises from findings in the rat that chemical or high-intensity and high-frequency (60-200 Hz) electrical stimulations of PFC increase burst activity of midbrain dopamine neurons, and augment terminal release of dopamine in the nucleus accumbens. However, PFC neurons in animals that are engaged in PFC-dependent cognitive tasks increase their firing frequency from a baseline of 1-3 Hz to 7-10 Hz, suggesting that the commonly used high-frequency stimulation parameters of the PFC may not be relevant to the behavioral states that are associated with PFC activation. We investigated the influence of PFC activation at lower physiologically relevant frequencies on the release of dopamine in the nucleus accumbens. Using rapid (5-min) microdialysis measures of extracellular dopamine in the nucleus accumbens, we found that although PFC stimulation at 60 Hz produces the expected increases in accumbal dopamine release, the same amplitude of PFC stimulation at 10 Hz significantly decreased these levels. These results indicate that activation of PFC, at frequencies that are associated with increased cognitive demand on this region, inhibits the mesoaccumbens dopamine system.     

The mGlu2/3 receptor agonist, LY354740, blocks immobilization-induced increases in noradrenaline and dopamine release in the rat medial prefrontal cortex

The metabotropic glutamate (mGlu2/3) receptor agonist, LY354740, exhibits anxiolytic-like properties in a number of rodent models. The present study utilized in vivo microdialysis to examine the effects of LY354740 on extracellular monoamine levels in the medial prefrontal cortex (mPFC) of animals subjected to 30 min immobilization stress. Immobilization stress significantly elevated extracellular levels of noradrenaline (NA) and dopamine (DA) in the mPFC, while systemic administration of LY354740 (30 mg/kg, s.c.) significantly attenuated immobilization-induced increases in both NA and DA. Reverse-dialysis of LY354740 (30 microm) into the mPFC significantly attenuated immobilization-induced increases in NA, but not DA without affecting basal levels of either amine. In separate studies in the presence of citalopram (1 microm; reverse dialysis into the mPFC), systemic administration of LY354740 attenuated immobilization-induced increases in NA and DA, but had no effect on serotonin (5-HT) levels. Co-administration of the selective mGlu2/3 receptor antagonist, LY341495, partially or fully reversed the attenuation in NA and DA levels produced by LY354740, respectively. Taken together, these data suggest that LY354740 may produce anti-stress actions, in part, by blocking stress-related increases in catecholamines in the mPFC via mGlu2/3 receptor stimulation.     

Noradrenaline transporter blockers raise extracellular dopamine in medial prefrontal but not parietal and occipital cortex: differences with mianserin and clozapine

This study compared the interaction between noradrenaline (NA) and dopamine (DA) mechanisms in the prefrontal (PFCX) and in the parietal (ParCX) and occipital (OccCX) cortex. The effect of reboxetine and desipramine, two NA transporter blockers, of mianserin, an antagonist of alpha2 and 5-HT2 receptors, and of clozapine, an atypical antipsychotic, on dialysate DA in the medial PFCX, ParCX and OccCX was studied. We also assessed the influence of a prior 6-hydroxydopamine (6-OHDA) lesion of the dorsal noradrenergic bundle (DNAB) on the effect of reboxetine and clozapine on dialysate DA in the PFCX and ParCX. Systemic administration of reboxetine and desipramine dose-dependently increased dialysate DA in the PFCX but not in the ParCX and OccCX. In contrast, mianserin and clozapine raised dialysate DA in the ParCX and OccCX to an even larger extent than in the PFCX. 6-OHDA lesions of DNAB abolished the increase of dialysate DA elicited by reboxetine in the PFCX and by clozapine both in the PFCX and in the ParCX. It is concluded that, although PFCX and ParCX/OccCX share the presence of a strong control of DA transmission by NA through alpha2 receptors, they differ in the extent to which DA is cleared from the extracellular compartment by uptake through the NA transporter. This process, although extensive in the PFCX, appears insignificant in the ParCX and OccCX, probably as a result of the higher ratio of NA to DA resulting in exclusion of DA from NA transporter.     

Environmental enrichment decreases cell surface expression of the dopamine transporter in rat medial prefrontal cortex

Rats raised in an enriched environmental condition (EC) exhibit a decreased (35%) maximal velocity (V(max)) of [3H]dopamine (DA) uptake in medial prefrontal cortex (mPFC) compared with rats raised in an impoverished condition (IC); however, no differences between EC and IC groups in V(max) for [3H]DA uptake were found in nucleus accumbens and striatum. Using biotinylation and immunoblotting techniques, the present study examined whether the brain region-specific decrease in DA transporter (DAT) function is the result of a reduction in DAT cell surface expression. In mPFC, nucleus accumbens and striatum, total DAT immunoreactivity was not different between EC and IC groups. Whereas no differences in cell surface expression of DAT were found in nucleus accumbens and striatum, DAT immunoreactivity in the biotinylated cell surface fraction of mPFC was decreased (39%) in EC compared with IC rats, consistent with the magnitude of the previously observed decrease in V(max) for [3H]DA uptake in mPFC in EC rats. These results suggest that the decrease in DAT cell surface expression in the mPFC may be responsible for decreased DAT function in the mPFC of EC compared with IC rats, and that there is plasticity in the regulatory mechanisms mediating DAT trafficking and function.     

Repeated exposure to cocaine alters the modulation of mesocorticolimbic glutamate transmission by medial prefrontal cortex Group II metabotropic glutamate receptors

Repeated cocaine exposure enhances glutamatergic output from the medial prefrontal cortex to subcortical brain regions. Loss of inhibitory control of cortical pyramidal neurons may partly account for this augmented cortical glutamate output. Recent research indicated that repeated cocaine exposure reduced the ability of cortical Group II metabotropic glutamate receptors to modulate behavioral and neurochemical responses to cocaine. Thus, experiments described below examined whether repeated cocaine exposure alters metabotropic glutamate receptor regulation of mesocorticolimbic glutamatergic transmission using in vivo microdialysis. Infusion of the Group II metabotropic glutamate receptor antagonist LY341495 into the medial prefrontal cortex enhanced glutamate release in this region, the nucleus accumbens and the ventral tegmental area in sensitized animals, compared to controls, following short-term withdrawal but not after long-term withdrawal. Additional studies demonstrated that vesicular (K(+)-evoked) and non-vesicular (cystine-evoked) glutamate release in the medial prefrontal cortex was enhanced in sensitized animals, compared to controls, that resulted in part from a reduction in Group II metabotropic glutamate receptor modulation of these pools of glutamate. In summary, these findings indicate that the expression of sensitization to cocaine is correlated with an altered modulation of mesocorticolimbic glutamatergic transmission via reduction of Group II metabotropic glutamate receptor function.     

Hypotension-induced dopamine release in prefrontal cortex is mediated by local glutamatergic projections at the level of nerve terminals

In a previous study it was shown that nitroprusside-induced hypotension strongly enhances the release of dopamine (DA) in the prefrontal cortex (PFC). In the present study we have further investigated the mechanism involved in this effect. Glutamate receptor antagonists were infused into the ventral tegmental area (VTA) or PFC, while DA release was measured in the ipsilateral PFC and hypotension was applied by intravenous infusion of nitroprusside. Infusion into the VTA of neither a NMDA receptor antagonist (CPP), nor a non-NMDA antagonist (DNQX) affected the hypotension-induced increase of DA in the PFC. Intracortical infusion of CPP also failed to affect significantly, whereas local infusion of DNQX inhibited the hypotension-enhanced release of DA dose-dependently. The stimulation of DA release was relatively small in the VTA as well as in the nucleus accumbens when compared with the response in the PFC. It is concluded that DA released from mesocortical neurons can be modulated by two different mechanisms: first, by glutamate afferents to the VTA that modify the nerve-impulse flow of DA neurons; and, second, by glutamate afferents to the PFC that act at the level of the DA nerve terminals. The behaviour context (arousal or stress versus hypotension) determines which type of interaction predominates.     

Stimulation of group I mGlu receptors in the ventrotegmental area enhances extracellular dopamine in the rat medial prefrontal cortex

Group I mGlu receptors have been implicated in the control of brain dopamine release. However, the receptor subtype involved and the precise site of action have not been determined. In this study we show that (R,S)3,5-dihydroxyphenylglycine (DHPG; 6 and 60 nmol ICV), a selective group I mGlu receptor agonist, raised extracellular dopamine respectively by 176% and 243% of basal values in the medial prefrontal cortex as assessed by in vivo microdialysis in conscious rats. (R,S)2-chloro-5-hydroxyphenylglycine (60 nmol ICV), a selective mGlu5 receptor agonist, raised extracellular dopamine by 396% of basal values. Intra-VTA DHPG (0.6–6 nmol) mimicked ICV injection whereas intracortical infusion (1–1000 µmol/L) had no effect. DHPG-induced rise of extracellular dopamine was reversed by tetrodotoxin and by the selective mGlu1 and mGlu5 receptor antagonists 7(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate (CPCCOEt) and 2-methyl-6-(phenylethynyl)pyridine (MPEP) either ICV or into the ventrotegmental area (VTA), suggesting that neuronal release and both mGlu1 and mGlu5 receptors were involved. These results support the existence of functional mGlu1 and mGlu5 receptors in the VTA regulating the release of dopamine in the medial prefrontal cortex.     

Inhibition of stress- or anxiogenic-drug-induced increases in dopamine release in the rat prefrontal cortex by long-term treatment with antidepressant drugs

The effects of long-term treatment with imipramine or mirtazapine, two antidepressant drugs with different mechanisms of action, on the response of cortical dopaminergic neurons to foot-shock stress or to the anxiogenic drug FG7142 were evaluated in freely moving rats. As expected, foot shock induced a marked increase (+ 90%) in the extracellular concentration of dopamine in the prefrontal cortex of control rats. Chronic treatment with imipramine or mirtazapine inhibited or prevented, respectively, the effect of foot-shock stress on cortical dopamine output. Whereas acute administration of the anxiogenic drug FG7142 induced a significant increase (+ 60%) in cortical dopamine output in control rats, chronic treatment with imipramine or mirtazapine completely inhibited this effect. In contrast, the administration of a single dose of either antidepressant 40 min before foot shock, had no effect on the response of the cortical dopaminergic innervation to stress. These results show that long-term treatment with imipramine or mirtazapine inhibits the neurochemical changes elicited by stress or an anxiogenic drug with an efficacy similar to that of acute treatment with benzodiazepines. Given that episodes of anxiety or depression are often preceded by stressful events, modulation by antidepressants of the dopaminergic response to stress might be related to the anxiolytic and antidepressant effects of these drugs.     

Phenylephrine enhances glutamate release in the medial prefrontal cortex through interaction with N‐type Ca2+ channels and release machinery

α₁‐adrenoceptors (α₁‐ARs) stimulation has been found to enhance excitatory processes in many brain regions. A recent study in our laboratory showed that α₁‐ARs stimulation enhances glutamatergic transmission via both pre‐ and post‐synaptic mechanisms in layer V/VI pyramidal cells of the rat medial prefrontal cortex (mPFC). However, a number of pre‐synaptic mechanisms may contribute to α₁‐ARs‐induced enhancement of glutamate release. In this study, we blocked the possible post‐synaptic action mediated by α₁‐ARs to investigate how α₁‐ARs activation regulates pre‐synaptic glutamate release in layer V/VI pyramidal neurons of mPFC. We found that the α₁‐ARs agonist phenylephrine (Phe) induced a significant enhancement of glutamatergic transmission. The Phe‐induced potentiation was mediated by enhancing pre‐synaptic glutamate release probability and increasing the number of release vesicles via a protein kinase C‐dependent pathway. The mechanisms of Phe‐induced potentiation included interaction with both glutamate release machinery and N‐type Ca²⁺ channels, probably via a pre‐synaptic G_q/phospholipase C/protein kinase C pathway. Our results may provide a cellular and molecular mechanism that helps explain α₁‐ARs‐mediated influence on PFC cognitive functions.

     

Regulation of nucleus accumbens dopamine release by the dorsal raphe nucleus in the rat

The effects of microinfusingl-glutamate, serotonin (5-HT), (±)-8-hydroxy-2-(di-N-propylamino) tetralin (8-OH DPAT; a 5-HT_1A agonist), and muscimol (a GABA_A agonist) into the dorsal raphe nucleus on the extracellular levels of 5-HT, dopamine (DA) and their metabolites in the nucleus accumbens were studied in unanesthetized, freely moving, adult male Wistar rats, using the technique of microdialysis coupled with small-bore HPLC. Administration of 0.75 gl-glutamate produced a 25–50% increase (P<0.05) in the extracellular levels of both 5-HT and DA. On the other hand, infusion of 8-OH DPAT and, to a lesser extent, 5-HT produced a significant (P<0.05) decrease in the extracellular levels of both 5-HT and DA. Muscimol (0.25 or 0.50 g) had little effect on the extracellular concentrations of 5-HT or DA following its administration. In general, the extracellular levels of the major metabolites of 5-HT and DA in the nucleus accumbens were not altered by microinfusion of any of the agents. The data indicate that (a) the 5-HT neurons projecting to the nucleus accumbens from the dorsal raphe nucleus can be activated by excitatory amino acid receptors and inhibited by stimulation of 5-HT_1A autoreceptors, and (b) the dorsal raphe nucleus 5-HT neuronal system may regulate the ventral tegmental area DA projection to the nucleus accumbens.Special issue dedicated to Dr. Morris H. Aprison     

In vivo evidence that constitutive activity of serotonin2C receptors in the medial prefrontal cortex participates in the control of dopamine release in the rat nucleus accumbens: differential effects of inverse agonist versus antagonist

Control of the mesoaccumbens dopamine (DA) pathway by central serotonin_2C receptors (5-HT_2CRs) involves different 5-HT_2CR populations located within multiple brain areas. Here, using in vivo microdialysis in halothane-anesthetized rats, we assessed the role of medial prefrontal cortex (mPFC) 5-HT_2CRs in the control of basal and activated accumbal DA outflow, to identify the modalities of their recruitment and the role of 5-HT_2CR constitutive activity. Intra-mPFC injection of the 5-HT_2CR inverse agonist SB 206553 (0.5 μg/0.2 μL), without effect by itself, decreased accumbal DA outflow induced by morphine (2.5–10 mg/kg, s.c.), haloperidol (0.01 mg/kg, s.c.) or GBR 12909 (2.5 mg/kg, i.p.). Conversely, intra-mPFC injection of the 5-HT_2CR antagonist SB 242084 (0.5 μg/0.2 μL), without effect by itself, decreased the effect of 10 mg/kg morphine, the only drug enhancing basal 5-HT outflow in the mPFC. The inhibitory effect of SB 206553 on 2.5 mg/kg morphine-stimulated DA outflow was suppressed by the concomitant intra-mPFC injection of SB 242084. Finally, changes of basal DA outflow induced by the 5-HT_2CR agonist Ro 60-0175 (3 mg/kg, i.p.) or SB 206553 (5 mg/kg, i.p.) were unaffected by intra-mPFC injection of SB 242084. These results, showing that 5-HT_2CR antagonist and inverse agonist behave differently in vivo, demonstrate that mPFC 5-HT_2CRs facilitate activated accumbal DA outflow and that 5-HT_2CR constitutive activity participates in this interaction.     

Stimulation of 5-hydroxytryptamine (5-HT(2C) ) receptors in the ventrotegmental area inhibits stress-induced but not basal dopamine release in the rat prefrontal cortex

The present study investigated whether 5-HT(2C) receptors in the ventrotegmental area and prefrontal cortex regulate basal and stimulus-evoked dopamine release in the prefrontal cortex. Using the in vivo microdialysis technique in conscious rats, we studied the effect of a selective 5-HT(2C) receptor agonist, Ro60-0175, on basal and immobilization stress-induced dopamine release in the prefrontal cortex. Ro60-0175 intraperitoneally (2.5 mg/kg) and into the ventrotegmental area (10 microg/0.5 microL) completely antagonized the effect of stress on extracellular dopamine without altering basal levels. Infusion of 10 microm Ro60-0175 through the cortical probe had no significant effect on basal and stress-induced dopamine release. SB242084 (10 mg/kg), a selective antagonist of 5-HT(2C) receptors, significantly increased basal extracellular dopamine and completely prevented the effect of intraperitoneal and intraventrotegmental Ro60-0175 on the stress-induced rise of extracellular dopamine, but had no effect itself in stressed rats. The results show that Ro60-0175 suppresses cortical dopamine release induced by immobilization stress through the stimulation of 5-HT(2C) receptors in the ventrotegmental area. While confirming that endogenous 5-HT acting on 5-HT(2C) receptors tonically inhibit basal dopamine release in the prefrontal cortex, the present findings suggest that the stimulation of 5-HT(2C) receptors with an exogenous agonist preferentially inhibit stimulated release.     

Effect of subchronic lithium treatment on citalopram-induced increases in extracellular concentrations of serotonin in the medial prefrontal cortex

We investigated the effect of citalopram [a selective serotonin (5-HT) reuptake inhibitor; SSRI] and MKC-242 (a selective 5-HT1A agonist), following treatment with subchronic lithium (p.o., 1 week) on extracellular 5-HT concentrations in the medial prefrontal cortex (mPFC). Acute treatment with citalopram (3 and 30 mg/kg) led to significant increases in extracellular 5-HT concentrations. The subchronic lithium group showed significantly higher basal levels of extracellular 5-HT than normal diet controls. Acute citalopram (3 and 30 mg/kg) treatment together with subchronic lithium treatment showed significant increases in the extracellular 5-HT concentrations, compared with citalopram treatment alone. Acute MKC-242 (1 mg/kg) treatment showed significant decreases in extracellular 5-HT concentrations, in both the normal diet and lithium diet groups to the same extent. The addition of lithium did not change the effect of the 5-HT1A agonist on extracellular 5-HT concentrations. This study suggests that lithium augmentation of the antidepressant effect of SSRI is mediated by the additional increases in extracellular 5-HT concentrations following the co-administrations of lithium and SSRI.     

Alpha2-adrenoceptor mediated co-release of dopamine and noradrenaline from noradrenergic neurons in the cerebral cortex

Previous results suggest that extracellular dopamine (DA) in the rat cerebral cortex originates from dopaminergic and noradrenergic terminals. To further clarify this issue, dialysate DA, dihydroxyphenylacetic acid (DOPAC) and noradrenaline (NA) were measured both in the medial prefrontal cortex (mPFC) and in the occipital cortex (OCC), with dense and scarce dopaminergic projections, respectively. Moreover, the effect of the alpha2-adrenoceptor antagonist RS 79948 and the D2-receptor antagonist haloperidol on extracellular DA, DOPAC and NA was investigated. Extracellular DA and DOPAC concentrations in the OCC were 43% and 9%, respectively, those in the mPFC. Haloperidol (0.1 mg/kg i.p.) increased DA and DOPAC (by 35% and 150%, respectively) in the mPFC, but was ineffective in the OCC. In contrast, RS 79948 (1.5 mg/kg i.p.) increased NA, DA and DOPAC, both in the mPFC (by approximately 50%, 60% and 130%, respectively) and the OCC (by approximately 50%, 80% and 200%, respectively). Locally perfused, the DA transporter blocker GBR 12909 (10 micro m) was ineffective in either cortex, whereas desipramine (DMI, 100 micro m) markedly increased extracellular NA and DA in both cortices. The weak haloperidol effect on DA efflux was not enhanced after DA- and NA-transporter blockade, whereas after DMI, RS 79948 markedly increased extracellular NA, and especially DA and DOPAC in both cortices. The results support the hypothesis that most extracellular DA in the cortex is co-released with NA from noradrenergic terminals, such co-release being primarily controlled by alpha2-adrenoceptors.