M. C. L. Lamm

Altered dopaminergic function in the prefrontal cortex, nucleus accumbens and caudate-putamen of an animal model of attention-deficit hyperactivity disorder — the spontaneously hypertensive rat

The spontaneously hypertensive rat (SHR) has been proposed as an animal model for Attention-Deficit Hyperactivity Disorder (ADHD). The behavioural problems of ADHD have been suggested to be secondary to altered reinforcement mechanisms resulting from dysfunction of the mesolimbic and mesocortical dopaminergic systems. The present study therefore investigated whether there are regional differences in dopamine (DA) and acetylcholine (ACh) release and DA D2-receptor function in SHR compared to their normotensive Wistar-Kyoto (WKY) controls. The DA D2-receptor agonist, quinpirole, caused significantly greater inhibition of DA release from caudate-putamen but not from nucleus accumbens or prefrontal cortex slices of SHR relative to WKY. DA D2-receptor blockade by the antagonist, sulpiride, caused a significantly greater increase in DA release from nucleus accumbens slices of SHR compared to WKY suggesting increased efficacy of DA autoreceptors at low endogenous agonist concentrations in the nucleus accumbens of SHR. The electrically-stimulated release of DA was significantly lower in caudate-putamen and prefrontal cortex slices of SHR than in slices of WKY. This could be attributed to increased autoreceptor-mediated inhibition of DA release in caudate-putamen slices but not in the prefrontal cortex. No difference was observed between SHR and WKY with respect to DA D2-receptor-mediated inhibition of ACh release from caudate-putamen or nucleus accumbens slices, suggesting that postsynaptic DA D2-receptor function is not altered in SHR relative to WKY.

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.

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.

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 γ-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.

δ-Aminolaevulinic Acid Uptake, Toxicity, and Effect on [14C]γ-Aminobutyric Acid Uptake into Neurons and Glia in Culture

δ‐Aminolaevulinic acid (ALA) uptake into neurons and glia in primary culture as well as ALA toxicity and its effects on γ‐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 μM. 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.

Regional distribution of dopamine D1 and D2 receptors in the nucleus accumbens of the rat

Dopamine (DA) D1 receptor density was found to be significantly lower in the rostral than in the medial and caudal areas of the nucleus accumbens. This roughly followed the distribution of DA terminals. The distribution of DA D2 receptors did not follow the DA terminal distribution. DA D2 receptor number was lower in the ventrorostral and higher in the dorsomedial area than in any other area of the nucleus accumbens.

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.

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.

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

Abstractδ-Aminolaevulinic acid (ALA) has been shown to be toxic to cultured neurons and glia at concentrations as low as 10 μM. 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 μM 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.

lack of interaction between α2-adrenoceptors and dopamine D2-receptors in mediating their inhibitory effects on [3H]dopamine release from rat nucleus accumbens slices

The α2-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 α2-adrenoceptor mediated effect. There was no evidence of endogenous activation of noradrenergic receptors suggesting that the α2-adrenoceptor agonist was not acting presynaptically to inhibit noradrenaline release. An in vitro superfusion technique was used to investigate wheher there was any interaction between α2-adrenoceptors and DA D2-receptors in mediating their inhibitory effects on [3H]DA release from rat nucleus accumbens slices. α2-Adrenoceptor 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 α2-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.