J. Glowinski

Selective activation of the mesocortical DA system by stress

IT is now well established that an important system of dopaminergic (DA) neurones innervates various parts of the cerebral cortex in the rat and other species1–3. In contrast to noradrenergic (NA) terminals which are widely distributed in this structure, the DA terminals are mainly confined to deep layers, particularly in the frontal, the cingular and the entorhinal areas4,5. The results of lesion studies demonstrated that the terminal endings in the frontal cortex originate from the A10 group of DA cell bodies localised in the mesencephalon6–8. This group also contains the cell bodies of the classical mesolimbic DA system projecting to the tuberculum olfactorium, the nucleus accumbens, the nucleus of the stria terminalis, and the amydgala9. The DA terminals found in the cingulate and entorhinal areas of cortex may originate mainly from the A9 group of DA neurones6–8. This group gives rise to the well known nigrostriatal DA system which is implicated in extrapyramidal processes. Its degeneration is in part responsible for some of the symptoms seen in Parkinsonian patients. Little is yet known about the functions of the mesocortical and mesolimbic DA pathways. Electrocoagulation or 6-hydroxydopamine (6-OHDA)-induced lesions of the ventral tegmental area, containing the A10 group, produce a syndrome characterised by “locomotor hyperactivity, serious impairment in tests requiring inhibition of a previously learned response, facilitation of approach learning and of active avoidance and hypoemotivity”10,11. Various workers have suggested that the antipsychotic effects of neuroleptics are in part related to the blockade of postsynaptic DA receptors localised in areas innervated by the mesolimbic and mesocortical DA systems12,13. It seems important to establish whether neurones of these two DA systems correspond to an homogeneous population of cells with similar functional characteristics. We have therefore explored this problem in the rat by examining the reactivity of the mesocortical and mesolimbic DA pathways as well as that of nigrostriatal DA system to stress induced by electric foot shocks. Our results suggest that the mesocortical DA system is selectively activated by this stress.

Dopaminergic Terminals in the Rat Cortex

The destruction of ascending noradreniergic pathways by bilateral microinjections of 6-hydroxydopamnine made laterally to the pedunculus cerebellaris superior completely abolished the in vitro synthesis of [3H]norepinephrine from L-[3H]tyrosine in slices and in synaptosomes of the rat cortex. However, normal [3H]dopamine synthesis could still be observed in both cortical preparations from animals with lesions. These results provide the first biochemical support for the existence of dopaminergic terminals independent of noradrenergic terminals in the rat cortex.

Hippocampo‐prefrontal cortex pathway: Anatomical and electrophysiological characteristics

The hippocampus, the prefrontal cortex, and interconnected neural circuits are implicated in several aspects of cognitive and memory processes. The present review is dedicated to the description of the anatomo‐functional characteristics of the hippocampo‐prefrontal pathway and related neuronal circuits in the rat. This pathway, which originates from the hippocampal CA1/subiculum fields, innervates the prelimbic/medial orbital areas of the prefrontal cortex (PL/MO). Its synaptic influence on cortical pyramidal neurons consists in an early monosynaptic excitation followed by an inhibition and, in some cases, a late excitation. These later effects are likely due to the subsequent activation of the local cortical network. PL/MO areas and the CA1/subiculum both send projections to the nucleus accumbens, a region of the ventral striatum which is particularly implicated in goal‐directed behavior. Therefore, emphasis is placed on respective projections from PL/MO areas and from the CA1/subiculum on the “core” and the “shell” regions of the nucleus accumbens, as well as on their interconnected circuits. Signals which are directed to the prefrontal cortex through these circuits might modulate hippocampo‐prefrontal inputs. Finally, the direct and/or indirect relationships of the hippocampus, prefrontal cortex, and nucleus accumbens with the ventral tegmental area/substantia nigra pars compacta complex (VTA/SNC) (where dopamine neurons are located) will also be described, because these neurons are known to modulate synaptic transmission and plasticity in their target structures and to play a fundamental role in motivational processes. Hippocampus 10:411–419, 2000

Blockade by benzodiazepines of the selective high increase in dopamine turnover induced by stress in mesocortical dopaminergic neurons of the rat

The effects of electrical foot shock on the activity of the ascending dopaminergic neurons were estimated in the rat by measuring the changes in DOPAC and DA levels in discrete brain areas. DOPAC and DA levels were estimated with a radioenzymatic method in microdiscs of tissues punched out from serial frontal sections of the brain. A marked rise in the ratio of DOPAC/DA levels resulting from an increase of DOPAC and a decrease of DA levels was found in the cerebral frontal cortex at the end of a 20 min stress. The effect was less pronounced in stress of shorter duration from 3 to 10 min and was only related to a reduction of DA levels. Using the DOPAC/DA ratio as an index of the activity of the neurons, the mesocortical dopaminergic neurons were found to be selectively activated under stress since this ratio was increased in the frontal and cingular cortices but not in limbic structures such as the septum, the amygdala and the nucleus accumbens or in the striatum. Finally, pretreatment of the rats with diazepam (5 mg/kg i.p.) or chlordiazepoxide (10 mg/kg i.p.) prevented the increase in the DOPAC/DA ratio in the frontal cerebral cortex of rats submitted to the 20 min stress.

Glutamatergic Control of Dopamine Release in the Rat Striatum: Evidence for Presynaptic N-Methyl-D-Aspartate Receptors on Dopaminergic Nerve Terminals

Abstract: The N‐methyl‐D‐aspartate (NMDA) receptor‐mediated regulation of the release of newly synthesized [3H]dopamine ([3H]DA) was studied in vitro, both on rat striatal slices using a new microsuperfusion device and on rat striatal synaptosomes. Under Mg2+‐free medium conditions, the NMDA (5 × 10−5M)‐evoked release of [3H]DA from slices was found to be partly insensitive to tetrodotoxin (TTX). This TTX‐resistant stimulatory effect of NMDA was blocked by either Mg2+ (10−3M) or the noncompetitive antagonist MK‐801 (10−6M). In addition, the TTX‐resistant NMDA‐evoked response could be potentiated by glycine (10−6M) in the presence of strychnine (10−6M). The coapplication of NMDA (5 × 10−5M) and glycine (10−6M) stimulated the release of [3H]DA from striatal synaptosomes. This effect was blocked by Mg2+ (10−3M) or MK‐801 (10−5M). These results indicate that some of the NMDA receptors involved in the facilitation of DA release are located on DA nerve terminals. These presynaptic receptors exhibit pharmacological properties similar to those described in electro‐physiological studies for postsynaptic NMDA receptors.

Gap junctions in cultured astrocytes: Single-channel currents and characterization of channel-forming protein

Currents from gap junction channels were recorded from pairs of astrocytes in primary culture using the double whole-cell recording technique. In weakly coupled pairs, single-channel events could be resolved without pharmacological uncoupling treatment. Under these conditions, unitary conductance was 56 +/- 7 pS, and except for multiples of this value, no other level of conductance was observed consistently. To characterize the type of junctional protein constituting astrocyte gap junction channels, immunological and biochemical experiments were carried out on the same material. Specific cDNA probes for three connexins identified in mammals (Cx26, Cx32, and Cx43) showed that only Cx43 mRNA was expressed in cultured astrocytes. The presence of Cx43 protein in cultured astrocytes was demonstrated by immunoblotting, immunofluorescence, and immunogold labeling using anti-peptide antibodies specific to Cx43. These results strongly suggest that gap junctions in astrocytes have a 50-60 pS unitary conductance associated with channels composed of Cx43 protein.

PYRUVATE PROTECTS NEURONS AGAINST HYDROGEN PEROXIDE-INDUCED TOXICITY

Hydrogen peroxide (H2O2) is suspected to be involved in numerous brain pathologies such as neurodegenerative diseases or in acute injury such as ischemia or trauma. In this study, we examined the ability of pyruvate to improve the survival of cultured striatal neurons exposed for 30 min to H2O2, as estimated 24 hr later by the 3-[4,5-dimethylthiazol-2-yl]−2,5-diphenyltetrazoliumbromide assay. Pyruvate strongly protected neurons against both H2O2 added to the external medium and H2O2 endogenously produced through the redox cycling of the experimental quinone menadione. The neuroprotective effect of pyruvate appeared to result rather from the ability of α-ketoacids to undergo nonenzymatic decarboxylation in the presence of H2O2 than from an improvement of energy metabolism. Indeed, several other α-ketoacids, including α-ketobutyrate, which is not an energy substrate, reproduced the neuroprotective effect of pyruvate. In contrast, lactate, a neuronal energy substrate, did not protect neurons from H2O2. Optimal neuroprotection was achieved with relatively low concentrations of pyruvate (≤1 mm), whereas at high concentration (10 mm) pyruvate was ineffective. This paradox could result from the cytosolic acidification induced by the cotransport of pyruvate and protons into neurons. Indeed, cytosolic acidification both enhanced the H2O2-induced neurotoxicity and decreased the rate of pyruvate decarboxylation by H2O2. Together, these results indicate that pyruvate efficiently protects neurons against both exogenous and endogenous H2O2. Its low toxicity and its capacity to cross the blood–brain barrier open a new therapeutic perspective in brain pathologies in which H2O2is involved.

In vivo presynaptic control of dopamine release in the cat caudate nucleus—II. Facilitatory or inhibitory influence ofl-glutamate

The local effects of various concentrations of L-glutamate (from 10(-8) M up to 10(-3) M) on the release of [3H]dopamine synthesized continuously from [3H]tyrosine were examined in the caudate nucleus of halothane-anaesthetized cats implanted with push-pull cannulae. When used at a concentration of 10(-8) M or 10(-7) M, L-glutamate stimulated the release of [3H]dopamine from nerve terminals of the nigrostriatal dopamine neurons. This effect was still observed in the presence of tetrodotoxin (5 X 10(-7) M) but it was antagonized by 2-amino 6-trifluoromethoxy benzothiazole (PK 26124) (10(-5) M), an antagonist dopamine nerve terminals. While no significant change in the release of [3H]dopamine was observed with 10(-6) M L-glutamate, higher concentrations (from 10(-5) M to 10(-3) M) of the amino acid produced a long-lasting reduction in the [3H]transmitter release. This latter effect was also antagonized by PK 26124 (10(-5) M) but, unlike that observed with 10(-8) M L-glutamate, it did not persist in the presence of tetrodotoxin (5 X 10(-7) M). On the contrary, a marked stimulation of the release of [3H]dopamine was seen in the presence of this neurotoxin. The reduction in the release of [3H]dopamine produced by 10(-4) M L-glutamate was also antagonized by bicuculline (10(-5) M) and moreover a marked stimulation of [3H]dopamine release took place in the presence of this gamma-aminobutyric acid (GABA) antagonist. Therefore, high concentrations of L-glutamate exerted an inhibitory presynaptic control on [3H]dopamine release which seemed to be indirect and mediated partly by GABAergic neurons. Since a sustained reduction in the spontaneous release of [3H]dopamine was seen in the presence of PK 26124, the corticostriatal glutamatergic neurons appeared to exert a tonic facilitatory presynaptic influence on dopamine release. This effect was important since it represented 40% of the tetrodotoxin-sensitive release of the [3H]transmitter. The direct (stimulatory) and indirect (inhibitory) presynaptic controls on dopamine release mediated by corticostriatal glutamatergic fibres are discussed in light of previous findings and of the anatomical organization of the caudate nucleus.

Relationship between the locomotor hyperactivity induced by A10 lesions and the destruction of the frontocortical dopaminergic innervation in the rat

Bilateral high frequency lesions of the ventral tegmental area (VTA) in the rat induce a behavioral syndrome characterized by a permanent locomotor hyperactivity and a reduction of attention capacities. The VTA contains the cell bodies of the mesocortical and mesolimbic dopaminergic (DA) systems but is also rich in serotoninergic (5-HT) fibers which originate from the raphe nuclei and innervate the forebrain. In order to establish possible correlation(s) between the destruction of specific aminergic system(s) and some of the behavioral effects of VTA lesions, rat locomotor activities were recorded and DA, 5-HT and norepinephrine (NE) were estimated in discrete areas of the forebrain using specific and sensitive radioenzymatic methods. VTA lesions greatly affected DA and 5-HT levels in the forebrain but only induced minor effects on cortical NE. No significant correlations were found between the changes in locomotor activity and the reduction of 5-HT levels in the parietal and rhinal cortices, the striatum and the hippocampus. On the other hand, a very good correlation was observed between the increase in locomotor activity and the decrease in DA content in the frontal cortex (r= −0.82,n= 20, P < 0.01). Although not as striking, a correlation was also found between the changes in locomotor activity and those of DA levels in the nucleus accumbens, a structure innervated by the mesolimbic DA system (r= −0.47,n= 24, P < 0.05). A comparison between changes in DA levels in the frontal cortex and the nucleus accumbens after VTA lesions suggested that cell bodies of the mesocortical and mesolimbic DA systems, although very close, are not the same. It cannot be excluded that the mesolimbic DA system plays a role in the ‘VTA syndrome’. However, it is clear that the disappearance of DA in the frontal cortex is critical for the development of the non-vicarious locomotor hyperactivity. This suggests that the dopaminergic neurons which innervate the frontal cortex exert an inhibitory role on locomotor behavior.

Regulation of dopamine release by presynaptic nicotinic receptors in rat striatal slices: Effect of nicotine in a low concentration

Abstract Rat striatal slices were continuously superfused with 3H-tyrosine to study the effect of a low concentration of nicotine on the spontaneous release of newly synthesized 3H-dopamine (3H-DA). Nicotine (10−6M) stimulated the calcium-dependent spontaneous release of 3H-DA. This effect was prevented by nicotinic blockers such as pempidine (10−5M) and d-tubocurarine (5×10−6M). The stimulatory effect of nicotine and its blockade by pempidine were still observed in the presence of tetrodotoxin (5×10−7M). This demonstrates for the first time that a low concentration of nicotine is effective in releasing DA by acting on presynaptic nicotinic receptors located on terminals of the nigro-striatal dopaminergic neurons.