G. Blanc

Topographical distribution of dopaminergic innervation and dopaminergic receptors of the anterior cerebral cortex of the rat

The quantitative topographical distribution of the dopaminergic innervation and the DA-sensitive adenylate cyclase were estimated in the anterior cerebral cortex of the rat. The high affinity uptake of [3H]DA and endogenous levels of DA were used as markers of the dopaminergic innervation. [3H]DA uptake, DA levels and DA-sensitive adenylate cyclase were estimated in microdiscs of tissues punched out from frozen serial frontal slices. The uptake of [3H]DA was measured on sucrose homogenates prepared from such microdiscs. The ventral part of the frontal cortex contained the highest DA concentration and DA-sensitive adenylate cyclase activity; the other structures rich in DA and in DA receptors were the cingular (close to the corpus callsoum) and the rhinal cortices. All of these cortical areas were rich in [3H]DA uptake sites. However, curiously, the dorsal part of the frontal cortex, which contained only moderate amounts of DA and of DA-sensitive adenylate cyclase, presented the highest number of [3H]DA uptake sites. Nevertheless, the uptake of [3H]DA in this region decreased by 60% after bilateral electrolytical lesions of the ventral tegmental area (A10 group). The parietal cortex was practically devoid of dopaminergic innervation and of DA-sensitive adenylate cyclase. The activity of the DA-sensitive adenylate cyclase in the frontal, cingular and rhinal cortices was 10-fold higher than that found in the striatum when compared to their respective DA levels.

Blockade of Prefronto‐cortical α1‐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 α1 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 α1‐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. It is suggested that the blockade of cortical d‐adrenergic receptors facilitates locally dopaminergic D1 transmission. This latter effect may counteract the increased locomotor activity induced by the application of D‐amphetamine into the nucleus accumbens.

Opposite changes in dopamine utilization in the nucleus accumbens and the frontal cortex after electrolytic lesion of the median raphe in the rat

As revealed by the changes in dihydroxyphenylacetic acid (DOPAC) levels and in the DOPAC/Dopamine (DA) ratio, DA utilization was markedly enhanced in the nucleus accumbens and reduced in the prefrontal cortex of rats five days after the electrolytic lesions of the median raphe. These opposite effects were not seen any more seventeen days after the injection. These results suggest that neurones originating from the median raphe and projecting to the ventral tegmental area exert an opposite effect on the activity of DA cells innervating the nucleus accumbens and on those projecting to the prefrontal cortex.

Reduction of dopamine utilization in the prefrontal cortex but not in the nucleus accumbens after selective destruction of noradrenergic fibers innervating the ventral tegmental area in the rat

The present study was made to determine the role of the noradrenergic (NA) neurons which innervate the ventral tegmental area (VTA) in the regulation of VTA dopaminergic (DA) neurons projecting to the prefrontal cortex and the nucleus accumbens. For this purpose, a 6-hydroxydopamine lesion was made in benztropine pretreated rats medially just above the decussatio of the pedunculus cerebellaris superior in order to specifically destroy the NA fibers innervating the VTA without affecting those projecting to the prefrontal cortex. Seven days later the ratio of DOPAC and DA levels was estimated in the prefrontal cortex and the nucleus accumbens and used as an index of DA utilization. In the lesioned rats the DOPAC/DA ratio was significantly decreased in the prefrontal cortex but not in the nucleus accumbens. These results suggest that the NA neurons which innervate the VTA exert a specific tonic excitatory influence on the mesocortico-prefrontal DA neurons.

Differential effects of a two-minute open-field session on dopamine utilization in the frontal cortices of BALB/c and C57 BL/6 mice

Abstract BALB/c and C57 BL/6 mice were introduced for 2 min in an open field. The locomotor activity of the BALB/c was significantly lower than that of C57 BL/6. This situation enhanced dihydroxyphenylacetic acid (DOPAC) levels and the DOPAC/dopamine (DA) ratio in the frontal cortex of the BALB/c but not in that of the C57 BL/6 mice.

Presence of dopaminergic terminals and absence of dopaminergic cell bodies in the cerebral cortex of the cat

Abstract Dopamine (DA) and noradrenaline (NA) levels were estimated in the suprasylvian gyrus and the cerebellum of the cat and compared to those found in the cerebral and cerebellar cortex of the rat. As in the rat, DA was undetectable in the cerebellum and represented about 30% of the total catecholamine (CA) levels in the cerebral cortex of the cat. The uptake of [3H]DA was estimated in homogenates of the suprasylvian cortex, the caudate nucleus and the cerebellum of the cat. In the suprasylvian cortex the [3H]-amine uptake was partially reduced by desipramine alone or benztropine alone and more extensively inhibited in the presence of both inhibitors. In the cerebellum, the desipramine resistant [3H]DA uptake was insensitive to benztropine; conversely the benztropine resistant [3H]DA uptake was insensitive to desipramine in the caudate nucleus. The synthesis of [3H]CA from [3H]tyrosine was estimated in slices of the suprasylvian gyrus and of the cerebellar cortex of the cat and was compared to that obtained in cerebral and cerebellar slices of the rat. In both species, the [3H]DA/[3H]NA ratio was higher in the cerebral cortex than in the cerebellum. All these data suggest that catecholaminergic innervation in the cerebral cortex of the cat can be attributed, as in the rat, to dopaminergic as well as noradrenergic terminals. In further experiments, [3H]DA uptake was estimated in homogenates of the chronically isolated suprasylvian gyrus and compared to that of the contralateral side. [3H]DA uptake was markedly reduced 27 days after the operation and the residual [3H]amine uptake was not sensitive to benztropine or desipramine. Finally, [3H]CA synthesis from [3H]tyrosine was no more detectable in slices of the chronically isolated suprasylvian gyrus (27 days). These data reveal that both types of catecholaminergic terminals degenerate in the chronically isolated area and suggest the absence of dopaminergic interneurons in the cat neocortex. As a working hypothesis, it is suggested that the intermediary ascending catecholaminergic bundle is responsible for the dopaminergic innervation of the cerebral cortex. There is now little doubt that a dopaminergic system innervates the cerebral cortex of the rat. In this structure, after the selective degeneration of the ascending noradrenergic pathways induced by local 6-hydroxydopamine lesions, we have successively demonstrated: (1) the occurrence of an appreciable amount of dopamine (DA)25; (2) a persistent [3H]DA synthesis from [3H]tyrosine,in vivo as well asin vitro in slices or purified synaptosomal preparations24; (3) the existence of a specific DA uptake system sensitive to benztropine (a known inhibitor of DA transport in dopaminergic terminals)23; (4) the histochemical visualization of a network of fine green fluorescent fibers (Bergeret al., unpublished observations). Complementary results recently obtained by others support these findings. A DA sensitive adenyl cyclase has been demonstrated in cortical tissues of the rat11. Dopaminergic terminals have also been identified in some areas of the cerebral cortex in unlesioned animals using combined histochemical and pharmacological methods9,15. Catecholaminergic nerve terminals have also been observed in the cerebral cortex of the cat. It has generally been assumed that most of these terminals arise from noradrenergic neurons located in the locus coeruleus and in the subcoeruleus1,17,20. However, even after severing all the ascending catecholaminergic bundles in the cat some catecholamine fluorescence still persisted in the neocortex17. In this work we present biochemical evidence for the occurrence of both dopaminergic and noradrenergic terminals in the neocortex of the cat. Moreover, it is suggested that these terminals do not belong to cortical catecholaminergic interneurons, since both the synthesis of catecholamines (CA) and the specific uptake systems for DA and noradrenaline (NA) were no longer detectable in the chronically isolated cortical gyrus in which all afferent fibers have degenerated6.

Development of cortical and nigro-neostriatal dopaminergic systems after destruction of central noradrenergic neurones in foetal or neonatal rats

Abstract Foetal or newborn rats were injected in various ways with 6-hydroxydopamine (6-OHDA). The effects of these treatments on the development of mesocortical and nigro-neostriatal dopaminergic and noradrenergic pathways were examined. Either foetal (day 17 of gestation) or one-day-old animals were injected intraperitoneally with 6-OHDA. The drug was also administered by the intracisternal route one day after birth, or locally in the brain stem using a stereotaxic procedure in 4-day-old animals. All rats were sacrificed at the age of 40 days and various structures were dissected to measure catecholamine (CA) content, or [3H]CA synthesis from l -[3,5-3H]tyrosine in slices. The cerebral cortex of rats injected with 6-OHDA during their foetal periods was examined to determine the uptake of [3H]dopamine ([3H]-DA) and the sensitivity of uptake to desipramine and benztropine. The two 6-OHDA intraperitoneal treatments as well as the local intracerebral injection of the drug abolished the noradrenergic innervation of the cerebral cortex and the hippocampus. In contrast, the dopaminergic innervastion of the cerebral cortex was not reduced. Normal DA levels, a high [3H]DA synthesis in the absence of [3H]noradrenaline ([3H]NA) formation and a selective uptake of [3H]DA that was insensitive to desipramine but sensitive to benztropine were observed in this structure. The local intracerebral 6-OHDA treatment selectively prevented the development of the dorsal noradrenergic pathway, but had no effect on NA levels in the hypothalamus, the neostriatum and peripheral organs. The 6-OHDA intraperitoneal treatments slightly reduced NA levels in the hypothalamus but the effects were less pronounced in the neostriatum. In contrast to intracerebral and intraperitoneal treatments, intracisternal administration of 6-OHDA affected both the noradrenergic and the dopaminergic systems; the dopaminergic pathways being less severely affected. All 6-OHDA treatments, except the intracisternal injection, markedly increased the NA content in the brain stem. This effect was associated with a pronounced increase of [3H]DA synthesis from l -[3,5-3H]tyrosine. In contrast, in adult rats, 6-OHDA local destruction of the noradrenergic ascending pathways was not followed, 48 days later, by an increase in NA levels in the brain stem even though the destruction of the dorsal noradrenergic pathways was as extensive as that observed after similar lesions made in newborn rats. The intracisternal injection of 6-OHDA induced a partial destruction of the cortical dopaminergic terminals but the synthesis of [3H]DA from l -[3,5-3H]tyrosine in cortical slices was identical to that observed following a treatment which similarly destroyed the cortical noradrenergic terminals, but completely respected the integrity of the dopaminergic innervation. This maintained synthesis was interpreted as being the result of hyperactivity of the remaining dopaminergic terminals. The present study demonstrates that the development of the mesocortical dopaminergic systems is not impaired by the absence of cortical noradrenergic innervation. Furthermore, selective destruction of the noradrenergic ascending pathways can be achieved at birth or in the foetus. Such animal models should make studies of the respective functional properties of the cortical dopaminergic and noradrenergic systems possible.

Partial protection by desmethylimipramine of the mesocortical dopamine neurones from the neurotoxic effect of 6-hydroxydopamine injected in ventral mesencephalic tegmentum. The role of noradrenergic innervation

As shown in the rat by estimation of dopamine (DA) and noradrenaline (NA) levels, bilateral 6-hydroxydopamine (6-OHDA, 4 micrograms/microliter) lesions made in the ventral mesencephalic tegmentum (VMT) destroy both ascending DA and NA neurones. Pretreatment of rats with desmethylimipramine (DMI, 30 mg/kg, i.p.), 30 min before microinjection of 6-OHDA into the VMT partially prevented the destruction of the DA neurones innervating the prefrontal and cingulate cortices but not those innervating subcortical structures (nucleus accumbens, olfactory tubercles, septum). Results obtained from the prefrontal cortex of rats with extensive lesions of the ascending NA neurones performed 15 days prior to the 6-OHDA lesions of the VMT in the presence of DMI, imply that NA innervation of the VMT seems to be required for DMI to protect the cortical DA neurons from the neurotoxic effect of 6-OHDA.

Self-stimulation and catecholamines. III. Effect of imposed or self-stimulation in the area ventralis tegmenti on catecholamine utilization in the rat brain

Abstract Changes in NE or DA utilization were estimated in various structures of the rat brain after imposed or self-stimulation of the area ventralis tegmenti. In the first two experiments, rats were implanted bilaterally with electrodes and were stimulated with current characteristics similar to those used during self-stimulation in previous training sessions. The imposed stimulation was applied for 1 h, either 1 h after α-methyl-p-tyrosine (α-MpT) injection (150 mg/kg i.p.) or immediately after bis(1-methyl-4-homopiperazinyl-thiocarbonyl disulfide) (FLA-63) (40 mg/kg i.p.). NE utilization was accelerated significantly in both experiments in the brain stem, hypothalamus, hippocampus and cortex. DA utilization in the striatum was not accelerated by imposed stimulation in α-MpT-treated rats. In the third experiment, the effect of self-stimulation on [3H]CA utilization was directly examined by measuring the rate of disappearance of the 3H-amines after labeling of CA stores. [3H]DA was introduced through a chronic cannula implanted in the cisterna magna, and rats were allowed to self-stimulate bilaterally 5 min later for 1 h. Self-stimulation accelerated the utilization of [3H]DA in the tuberculum olfactorium but also in the hypothalamus and the hippocampus. [3H]DA levels were not significantly changed in either the brain stem or in the cortex. [3H]NE utilization as well as NE turnover were markedly enhanced in the brain stem, hypothalamus, hippocampus and cortex. Imposed or self-stimulation of the area ventralis tegmenti activated the utilization and turnover of NE in structures innervated by both the ventral and dorsal ascending noradrenergic pathways. Dorsal noradrenergic neurons are probably activated by an interneuronal mechanism. The dopamine limbic system also appeared to be directly activated by self-stimulation. The respective role of noradrenergic and dopaminergic neurons in self-stimulation of the area ventralis tegmenti is discussed.

Autoradiographic identification of D1 dopamine receptors labelled with [3H]dopamine: distribution, regulation and relationship to coupling.

On the basis of experiments made on striatal membranes, Leff and Creese [Molec. Pharmac. (1985) 27, 184-192] have proposed that tritiated dopamine binds to a high-affinity agonist state of D1 dopamine receptors (D1h) which adopt this conformation when they are associated with the GTP-binding protein involved in the transduction process. Quantitative autoradiography was thus used to look for the distribution of these D1h sites in the rat brain and to compare it with that of D1 receptors labelled with [3H]7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benz aze pine [( 3H]SCH23390), a D1 antagonist. The effects of unilateral 6-hydroxydopamine lesion of the ascending dopamine pathways on the density of [3H]dopamine D1h and [3H]SCH23390 binding sites in the striatum and the nucleus accumbens were also analysed. In the striatum, when D2 receptors were blocked by spiroperidol (20 nM), [3H]dopamine was found to bind specifically to dopamine receptors of the D1 type. Complementary experiments made with dopamine uptake blockers indicated that high-affinity dopamine uptake sites were not labelled by [3H]dopamine under our experimental conditions. The anatomical distribution of [3H]dopamine D1h binding sites was found to be markedly different from that of [3H]SCH23390 binding sites. This was particularly the case in the substantia nigra, some amygdaloid nuclei and the prefrontal cortex--structures in which the ratios between [3H]SCH23390 and [3H]dopamine binding sites were more than seven-fold higher than that observed in the striatum. [3H]SCH23390 binding was not significantly affected in either the striatum or the nucleus accumbens six weeks after a complete unilateral destruction of ascending dopamine pathways. In contrast, a marked decrease in [3H]dopamine D1h binding sites was found in both structures, but this effect was lower in the medioventral (-60%) than in the laterodorsal (-81%) part of the striatum, even though dopamine denervation was uniform throughout the structure. Preincubation of the sections with dopamine (0.5 microM) led to a partial recovery (+126%) in the lesioned striatum and an increase of [3H]dopamine labelling in the control striatum (+68%). This suggest that the presence of dopamine stabilizes the D1h state of D1 receptors. The absence or low amount of dopamine, either due to dopamine denervation or naturally occurring (prefrontal cortex), would then impair the [3H]dopamine D1h binding. In addition, a lower coupling of D1 receptors with adenylate cyclase was observed in the substantia nigra when compared to that in the striatum: this may explain the relatively weak [3H]dopamine binding in the substantia nigra.(ABSTRACT TRUNCATED AT 400 WORDS)