Methylphenidate affects striatal dopamine differently in an animal model for attention-deficit/hyperactivity disorder--the spontaneously hypertensive rat.

The spontaneously hypertensive rat (SHR) is used as a model for attention-deficit/hyperactivity disorder (ADHD) because it has behavioural characteristics (hyperactivity, impulsiveness, poorly sustained attention) similar to those of ADHD. ADHD children have been shown to have reduced striatal activation in certain tasks. SHR have reduced striatal dopamine release in response to electrical stimulation. The present study set out to investigate possible long-term effects of methylphenidate treatment on dopaminergic function in striatal slices of SHR compared to their normotensive Wistar-Kyoto (WKY) control rats. Methylphenidate treatment (3 mg/kg daily for 14 days) did not normalize the decreased electrically-stimulated release of [(3)H]dopamine from SHR caudate-putamen slices nor did it affect postsynaptic D(2) receptor function. However, the second electrical stimulus caused a relatively greater release of [(3)H]dopamine from caudate-putamen slices of methylphenidate-treated SHR than from vehicle-treated SHR, suggesting that presynaptic mechanisms controlling dopamine release had been altered. Interestingly, [(3)H]dopamine release from WKY caudate-putamen slices in response to D(2) autoreceptor blockade by the antagonist, sulpiride, was selectively increased by methylphenidate treatment. This effect was not seen in SHR possibly because D(2) autoreceptor function had already been up-regulated. The results show that methylphenidate is unable to enhance D(2) autoreceptor function in SHR.

Kindled seizures do not affect adenosinergic inhibition of DA or ACh release in rat accumbens or PFC.

Epileptic seizures are thought to terminate largely as a result of the extracellular accumulation of the purinergic neuromodulator, adenosine, released by discharging neurons. However, the postictal surge in extracellular adenosine and its widespread inhibitory effects are limited in time to only a few minutes and cannot directly account for increased resistance to seizures and the complex behavioural and motivational effects that may persist for hours or days after a seizure. The present study examined whether kindled seizures might alter the sensitivity or efficacy of inhibitory presynaptic adenosine receptors, and thereby induce more enduring changes in downstream transmitter systems. Rats were kindled in the amygdala of the dominant cerebral hemisphere, contralateral to the preferred direction of rotation, and their brains were removed either 2 h or 28 days after completion of kindling. Inhibition of electrically stimulated release of dopamine (DA) and acetylcholine (ACh) by the A1 adenosine-receptor agonist, R-phenylisopropyladenosine (R-PIA) was then measured in the prefrontal cortex (PFC) and nucleus accumbens. R-PIA (1.0 microM) inhibited [1H]DA release from PFC and nucleus accumbens tissue, and [14C]ACh release from nucleus accumbens tissue, but release was unaffected by prior kindling, regardless of the intervening interval. These results do not support suggestions that DA or ACh might mediate the effects of seizure-induced changes in purinergic inhibitory tone so as to cause long-term shifts in seizure threshold and postictal behavior.

Lack of interaction between alpha 2-adrenoceptors and dopamine D2-receptors in mediating their inhibitory effects on [3H]dopamine release from rat nucleus accumbens slices.

The alpha 2-adrenoceptor agonist, UK14304, dose-dependently inhibited the electrically stimulated release of dopamine (DA) from rat nucleus accumbens slices. This effect was antagonized by idazoxan, confirming that it was an alpha 2-adrenoceptor mediated effect. There was no evidence of endogenous activation of noradrenergic receptors suggesting that the alpha 2-adrenoceptor agonist was not acting presynaptically to inhibit noradrenaline release. An in vitro superfusion technique was used to investigate whether there was any interaction between alpha 2-adrenoceptors and DA D2-receptors in mediating their inhibitory effects on [3H]DA release from rat nucleus accumbens slices. alpha 2-Adrenoceptors and DA D2-receptors interact with similar second messenger systems and it was considered that they may compete for a common pool of G-proteins. The inhibitory effects of the alpha 2-adrenoceptor agonist, UK14304, and the DA receptor agonists, quinpirole, apomorphine and pergolide were not independent. However, there was no evidence of any interaction between UK14304 and the DA D2-receptor antagonists, sulpiride or haloperidol, suggesting that the two receptors do not compete for a common pool of G-proteins in mediating their inhibitory effects on DA release.

Regional distribution of monoamines and dopamine D1- and D2-receptors in the striatum of the rat.

Dopamine (DA) D1- and D2-receptor densities were determined in 18 discrete areas of the caudate-putamen-globus pallidus of male Wistar rats and compared to local DA concentrations. All three parameters were found to decrease caudally. The globus pallidus was distinguished by the low concentration of DA and its receptors and high noradrenaline (NA) content. While there were no mediolateral differences in DA or DA D1-receptors, a clear mediolateral gradient was observed for DA D2-receptors which extended over several sections of the brain. The ratio of DA D1- to D2-receptors was significantly higher in the dorsal than in the ventral areas of the mediolateral and caudal striatum. This is the first report of clear dorsoventral differences in parameters relating to DA activity in the striatum. These findings may be of particular significance in understanding the functional dichotomy between the dorsal and ventral striatum.

Increased dopamine D2 receptor-mediated inhibition of [14C]acetylcholine release in the dorsomedial part of the nucleus accumbens.

Dopamine (DA) D2 receptor-mediated inhibition of the K+-stimulated release of [14C]acetylcholine (ACh) from prelabeled rat dorsomedial nucleus accumbens slices was found to be 1.7 times greater than that observed in dorsorostral and ventromedial slices. This observation is consistent with the 1.9 fold higher DA D2 receptor density found in the dorsomedial area. In contrast, there were no differences in the DA D2 receptor-mediated effects on [3H]DA release in these areas. In addition, DA D2 receptor-mediated effects on [3H]DA and [14C]ACh release could not be demonstrated in the ventrorostral part of the nucleus accumbens consistent with the fact that DA D2 receptors were barely detectable in this area. The results suggest that cholinergic terminals in the dorsomedial part of the nucleus accumbens are under greater inhibitory DA control than in other areas of the nucleus accumbens.

Effect of selective noradrenergic denervation on noradrenaline content and [3H]dopamine release in rat nucleus accumbens slices.

DSP4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine) treatment (50 mg/kg i.p., 10 days previously) significantly decreased the noradrenaline (NA) content of the rostral part of the nucleus accumbens. The medial and caudal areas were not affected. The nucleus accumbens appears to receive noradrenergic innervation predominantly from subcoeruleus nuclei of the pons-medulla while the locus coeruleus neurons project to the rostral area. The isoproterenol-induced enhancement of the K+-evoked release of [3H]dopamine (DA) was not affected by DSP4 treatment. Noradrenergic denervation does not appear to have been sufficient to cause up-regulation of postsynaptic beta-adrenoceptors.

Regional distribution of monoamines in the nucleus accumbens of the rat.

Monoamine concentrations were low in the rostral area of the nucleus accumbens. Their distributions were not identical. Differences were observed in the medial area. DA concentrations were high in both medial and caudal areas. Noradrenaline (NA) and serotonin (5-HT) concentrations were considerably lower than the dopamine (DA) concentration. The NA concentration was highest in the caudal area of the nucleus accumbens and the (5-HT) concentration was highest in the ventrocaudal area. There was a rostrocaudal decrease in the 3,4-dihydroxyphenylacetic acid (DOPAC)/DA and 5-hydroxyindole-3-acetic acid (5-HIAA)/5-HT ratios. Uptake of [3H]DA and [14C]choline was lowest in the rostral area. The K+-stimulated release of [14C]acetylcholine (ACh) was also lowest rostrally, but there was no rostrocaudal difference in the K+-stimulated release of [3H]DA. These results provide further evidence of the heterogeneity of the nucleus accumbens.

Effect of ethanol on [3H]dopamine release in rat nucleus accumbens and striatal slices.

Ethanol (10-200 mM) transiently increased tritium overflow from superfused rat nucleus accumbens slices previously incubated with [3H]dopamine (DA) and [14C]choline. The effect was greater in striatal tissue and did not appear to be a non-specific membrane effect since [14C]acetylcholine (ACh) release was not affected. Lack of antagonism by picrotoxin suggested that gamma-aminobutyric acid (GABA) receptors were not involved. Calcium was not a requirement and the DA uptake blocker, nomifensine, was without effect. Ethanol appeared to be causing [3H]DA release into the cytoplasm. K+ -stimulated release of [3H]DA and [14C]ACh from nucleus accumbens and striatal slices was not affected. Clonidine-mediated inhibition of the K+-evoked release of [3H]DA remained unaltered. Ethanol attenuated the isoproterenol-induced enhancement of [3H]DA release. Ethanol therefore appeared to interact with components of the DA terminal causing a transient increase in the release of neurotransmitter without impairing K+-evoked release but apparently interfering with the isoproterenol-induced effect.

Effect of chronic antidepressant treatment on noradrenergic modulation of [3H]dopamine release from rat nucleus accumbens and striatal slices.

Unlike desipramine, a potent blocker of noradrenaline uptake, the antidepressant drugs, mianserin and citalopram did not, after chronic administration (28 days), attenuate the clonidine-induced inhibition of [3H]dopamine (DA) release from rat nucleus accumbens slices. Mianserin, like desipramine, failed to alter the isoproterenol-induced enhancement of [3H]DA release from rat nucleus accumbens and striatal slices. These findings suggest that antidepressant action does not necessarily involve desensitization of central alpha 2- or beta-adrenoceptors which influence neuronal transmission.

Effects of combined administration of L-tryptophan and tricyclic antidepressants on alpha 2- and beta-adrenoceptors and monoamine levels in rat brain.

In order to test whether co-administration of a serotonin precursor with antidepressant drugs could potentiate the effects of the antidepressants on monoamines or adrenoceptors in rat brain, L-tryptophan (20 mg/kg) was administered to rats daily for 7 or 15 days, either alone or in combination with desipramine (10 mg/kg) or amitriptyline (10 mg/kg). After treatment with L-tryptophan for 7 days, increases were observed in rat hypothalamic and frontal cortex 5-hydroxy-3-indoleacetic acid levels as well as in hypothalamic dopamine and nucleus accumbens 3,4-dihydroxyphenylacetic acid levels. After 15 days, hippocampal beta-adrenoceptor density was found to be decreased. There was no evidence of potentiation of desipramine or amitriptyline action when L-tryptophan was administered in combination with the antidepressants. On the contrary, the antidepressants appeared to interact with L-tryptophan to reduce its effects.

Inhibition of Na+, K+-ATPase activity by delta-aminolevulinic acid.

delta-Aminolaevulinic acid (ALA) has been shown to be toxic to cultured neurons and glia at concentrations as low as 10 microM. In an attempt to elucidate the mechanism of toxicity, the effects of ALA on membrane ATPase activity were investigated. Exposure of neuron cultures to 1 mM ALA for 7 days caused a substantial decrease in both Na+, K+-ATPase and Mg2+-ATPase activities. At lower concentrations, ALA affected only the Na+, K+-component. ALA appeared to act directly, inhibiting Na+, K+-ATPase activity in rat brain cortex membrane preparations at 10 microM. Although this effect was slight, it may well represent the mechanism of action of ALA, since ouabain, a potent inhibitor of Na+, K+-ATPase activity, proved to be more toxic to cultured neurons than ALA. Furthermore, cardiac glycoside overdosage causes neurological disturbances which are very similar to those observed in the acute attack of porphyria.

Effects of delta-aminolaevulinic acid, porphobilinogen and structurally related amino acids on 2-deoxy-glucose uptake in cultured neurons.

The effects of delta-aminolaevulinic acid (ALA), porphobilinogen (PBG), gamma amino-butyric acid (GABA), muscimol, glutamic acid and kainic acid on [3H]2-deoxy-D-glucose uptake by cultured neurons were investigated. Exposure to the cultures for 4 days, to ALA at concentrations as low as 10 microM caused a significant, dose-dependent decrease in [3H] 2-deoxy-D-glucose uptake. Neither ALA nor PBG appeared to interfere directly with glucose transport into the neuron but 1 mM ALA caused an initial stimulation of [3H] 2-deoxy-D-glucose uptake which increased to a maximum after 4 hr and fell to below control values after 19 hr exposure. GABA and muscimol caused similar increases in [3H] 2-deoxy-D-glucose uptake but these values remained above control levels after 19 hr exposure. Glutamic acid and kainic acid caused an immediate increase in [3H] 2-deoxy-D-glucose uptake which declined to minimum values after 4 hr exposure. The effect of ALA on glucose utilization in neurons may be of particular relevance to patients with acute porphyria where a genetic lesion in neural haem and haemoprotein biosynthesis is postulated to occur. ALA appeared to be more toxic to the neurons than any of the other compounds tested, possibly causing a critical depletion of energy reserves and cell death.

[Mechanism of delta-aminolevulinic acid neurotoxicity]. / 'n Ondersoek na die meganisme van delta-aminolevuliniensuur-neurotoksisiteit.

The neurotoxicity of delta-aminolaevulinic acid (ALA), porphobilinogen (PBG), glutamic acid, gamma-aminobutyric acid and kainic acid was compared in order to investigate the possibility of a common neurotoxic mechanism. Only ALA (10 microM) and glutamic acid (1 mM) were toxic towards neurons in culture, as measured by cell survival after 5 days' exposure. 3H-kainic acid binds to striatal membranes with an equilibrium dissociation constant (KD) of 57 nM and the number of binding sites was found to be 20 pmol/g striatal tissue. However, neither of the porphyrin precursors could displace kainic acid from these binding sites. Kainic acid alone caused neuronal degeneration after intrastriatal administration, as determined by 3H-spiroperidol binding to striatal membranes. The porphyrin precursors ALA and PBG thus do not share a common neurotoxic mechanism with the well-known neurotoxin kainic acid.

delta-Aminolaevulinic acid uptake, toxicity, and effect on [14C]gamma-aminobutyric acid uptake into neurons and glia in culture.

delta-Aminolaevulinic acid (ALA) uptake into neurons and glia in primary culture as well as ALA toxicity and its effects on gamma-aminobutyric acid (GABA) uptake were examined. [4-14C]ALA uptake into neurons and glia was nonsaturable, partially Na+- and temperature-dependent, and appeared to comprise mainly diffusion into the cell. 2,4-Dinitrophenol caused some inhibition of [4-14C]ALA uptake whereas ouabain, KCN, or amino acids at 1 mM concentration were without effect. ALA (1 mM) caused a slight inhibition of [U-14C]GABA uptake into neurons (14%) and glia (9%), but was without effect at lower concentrations. It is unlikely that, in acute porphyria, ALA reaches sufficiently high levels in nervous tissue to interfere with the reuptake of GABA into neurons or glia. ALA was shown to be toxic, judged by the loss of cells, to both neurons and glia at concentrations as low as 10 microM. Such a concentration of ALA may be expected to occur in the CSF of porphyric patients in the acute attack. However, results obtained with dispersed cells in culture may not necessarily reflect the situation in vivo where the cell may have a far greater resistance to the effects of toxic agents.