Gerald J. LaHoste

Cognitive neuroscience of attention deficit hyperactivity disorder and hyperkinetic disorder

Currently, diagnoses of attention deficit hyperactivity disorder (ADHD) and hyperkinetic disorder (HKD) are made on the basis of phenomenology, but information is accumulating from the neurosciences about the biological bases of these disorders. Recent studies addressing the neuropsychology, neuroanatomy, neurochemistry, and molecular biology of ADHD/HKD document abnormalities in well-defined neuroanatomical networks and neurochemical pathways. Magnetic resonance imaging (MRI) studies have shown that some regions of the frontal lobes (anterior superior and inferior) and basal ganglia (caudate nucleus and globus pallidus) are about 10% smaller in ADHD groups than in control groups of children, and molecular genetic studies have shown that diagnosis of ADHD is associated with polymorphisms in some dopamine genes (the dopamine D4 receptor gene and the dopamine transporter gene).

RGS9–2 Negatively Modulates l-3,4-Dihydroxyphenylalanine-Induced Dyskinesia in Experimental Parkinson's Disease

Chronic l-dopa treatment of Parkinsons disease (PD) often leads to debilitating involuntary movements, termed l-dopa-induced dyskinesia (LID), mediated by dopamine (DA) receptors. RGS9–2 is a GTPase accelerating protein that inhibits DA D2 receptor-activated G proteins. Herein, we assess the functional role of RGS9–2 on LID. In monkeys, Western blot analysis of striatal extracts shows that RGS9–2 levels are not altered by MPTP-induced DA denervation and/or chronic l-dopa administration. In MPTP monkeys with LID, striatal RGS9–2 overexpression – achieved by viral vector injection into the striatum – diminishes the involuntary movement intensity without lessening the anti-parkinsonian effects of the D1/D2 receptor agonist l-dopa. In contrasts, in these animals, striatal RGS9–2 overexpression diminishes both the involuntary movement intensity and the anti-parkinsonian effects of the D2/D3 receptor agonist ropinirole. In unilaterally 6-OHDA-lesioned rats with LID, we show that the time course of viral vector-mediated striatal RGS9–2 overexpression parallels the time course of improvement of l-dopa-induced involuntary movements. We also find that unilateral 6-OHDA-lesioned RGS9−/− mice are more susceptible to l-dopa-induced involuntary movements than unilateral 6-OHDA-lesioned RGS9+/+ mice, albeit the rotational behavior – taken as an index of the anti-parkinsonian response – is similar between the two groups of mice. Together, these findings suggest that RGS9–2 plays a pivotal role in LID pathophysiology. However, the findings also suggest that increasing RGS9–2 expression and/or function in PD patients may only be a suitable therapeutic strategy to control involuntary movements induced by nonselective DA agonist such as l-dopa.

Sleep deprivation impairs hippocampus‐mediated contextual learning but not amygdala‐mediated cued learning in rats

Prolonged sleep deprivation results in cognitive deficits. In rats, for example, sleep deprivation impairs spatial learning and hippocampal long‐term potentiation. We tested the effects of sleep deprivation on learning in a Pavlovian fear conditioning paradigm, choosing a sleep deprivation paradigm in which REM sleep was completely prevented and non‐REM sleep was strongly decreased. During conditioning, rats were given footshocks, either alone or paired with a tone, and tested 24 h later for freezing responses to the conditioning context, and to the tone in a novel environment. Whereas control animals had robust contextual learning in both background and foreground contextual conditioning paradigms, 72 h of sleep deprivation before conditioning dramatically impaired both types of contextual learning (by more than 50%) without affecting cued learning. Increasing the number of footshocks did not overcome the sleep deprivation‐induced deficit. The results provide behavioural evidence that REM/non‐REM sleep deprivation has neuroanatomically selective actions, differentially interfering with the neural systems underlying contextual learning (i.e. the hippocampus) and cued learning (i.e. the amygdala), and support the involvement of the hippocampus in both foreground and background contextual conditioning.

Nigral D1 and striatal D2 receptors mediate the behavioral effects of dopamine agonists

The mediation of behavior by nigral and striatal dopamine (DA) D1 and D2 receptors was investigated in rats that had sustained extensive unilateral 6-hydroxydopamine-induced injury to ascending DA neurons. Selective D1 and D2 agonists and antagonists were injected directly into the DA-denervated substantia nigra pars reticula or the caudate-putamen via a chronically indwelling cannula. Contralateral rotation resulting from unilateral stimulation of supersensitive DA receptors was quantified over 46 min. Intrastriatal apomorphine (5 micrograms) or the selective D2 agonist quinpirole (5 micrograms), but not the selective D1 agonist (+/-)-SKF 38393 (15 micrograms), induced vigorous rotation. The rotation induced by intrastriatal quinpirole was greatly diminished by systemic administration of the selective D2 antagonist eticlopride (0.5 mg/kg, i.p.) and could not be enhanced by additional injection of intrastriatal (+/-)-SKF 38393. Intranigral administration of apomorphine or (+/-)-SKF 38393, but not quinpirole (same doses as above), elicited vigorous rotation. However, the rotation induced by intranigral (+/-)-SKF 38393 could not be blocked by systemic administration of the selective D1 antagonist SCH 23390 (0.5 mg/kg, s.c.), and was mimicked by intranigral (-)-SKF 38393 (15 micrograms), which exhibits 100-fold less activity than the dextrorotatory enantiomer at the D1 receptor. In order to circumvent the problem of this drugs apparent non-D1-mediated action when injected intranigrally, rotation was induced by systemic (+/-)-SKF 38393 (2.0 mg/kg, i.p.) 10 min after intranigral administration of selective antagonists. Intranigral SCH 23390 (10 micrograms), but not eticlopride (10 micrograms), powerfully antagonized the rotation induced by systemic (+/-)-SKF 38393. Conversely, rotation induced by systemic quinpirole (0.5 mg/kg, i.p.) was potently blocked by intrastriatal eticlopride but not SCH 23390. Rotation induced by systemic apomorphine (0.25 mg/kg, i.p.) was not attenuated by either antagonist alone, regardless of intracerebral injection site. The results indicate that both nigral D1 and striatal D2 receptors mediate the behavioral effects of DA agonists. These data may be useful in elucidating the mechanism(s) underlying the D1/D2 synergism observed in neurologically intact animals, as well as in understanding the action of drugs used in the treatment of Parkinsons disease.

Rhes, the Ras homolog enriched in striatum, is reduced under conditions of dopamine supersensitivity.

Striatal dopamine receptors become supersensitive when dopaminergic input is removed through either surgical denervation or pharmacological depletion. Although alterations such as increased D2 receptor binding and increased receptor-G protein coupling have been described in supersensitive striatal tissue, their roles in the mechanism of supersensitivity remain uncertain. The Ras Homolog Enriched in Striatum (Rhes) is expressed in brain areas that receive dopaminergic input, and here we test whether alterations in its expression accompany treatments that promote dopamine receptor supersensitivity in rats. Removal of dopamine input to the striatum by surgical denervation with 6-hydroxydopamine resulted in a decrease in rhes mRNA expression throughout striatum, as measured with quantitative in situ hybridization. The decrease was detected as early as two weeks and as late as seven months after surgery. Furthermore, a decrease in rhes mRNA was evident after repeated or acute reserpine treatment. Chronic daily injection of rats with the D2 antagonist eticlopride, which is known to up-regulate D2 receptors without inducing profound receptor supersensitivity, did not alter the expression of rhes mRNA in striatum. Thus, changes in rhes mRNA expression are strictly correlated with receptor supersensitivity, perhaps as a result of continuous removal of dopaminergic input. These findings suggest that rhes mRNA expression is maintained by dopamine and may play a role in determining normal dopamine receptor sensitivity.

Dopamine D2 receptor control of pallidal fos expression: comparisons between intact and 6-hydroxydopamine-treated hemispheres

Fos expression in the globus pallidus (GP) of rats was elicited by the D2 agonist quinpirole both ipsilateral and contralateral to a unilateral nigrostriatal 6-hydroxydopamine (6-OHDA) injection; however, the 6-OHDA-treated hemisphere was more sensitive to this effect. The quinpirole-induced GP Fos expression was antagonized in both hemispheres by the D2 antagonist eticlopride, but not by the D1 antagonist SCH 23390. In neurologically intact rats, the D1 agonist SKF 38393, which alone did not elicit pallidal Fos expression, augmented the quinpirole-induced Fos response. Thus, D1 agonists can synergize with D2 agonists in inducing GP c-fos; but the D2-stimulated induction does not depend on concurrent D1 agonism.

Cerebrocortical Fos expression following dopaminergic stimulation: D1/D2 synergism and its breakdown.

Using standard immunohistochemical techniques, we examined Fos expression in different areas and layers of cerebral cortex in rats following combined or separate stimulation of dopamine D1 and D2 receptors under normal conditions and following five days of reserpine (1 mg/kg/day), a treatment that causes a breakdown in requisite D1/D2 synergism. In normal animals, combined but not separate stimulation of D1 and D2 receptors elicited Fos expression in frontal and parietal, but not cingulate, cortex. Expression was highest in layer IV of primary somatosensory cortex; in frontal and secondary somatosensory cortex, Fos expression was lower and peaked in layer VI. Cortical Fos expression following amphetamine showed the same general pattern, and was blocked by either a selective D1 or D2 antagonist. Following reserpine treatment, stimulation of either D1 or D2 receptors gave rise to cortical Fos expression in patterns similar to each other and to combined D1/D2 stimulation in normal rats (except in frontal cortex in which separate D1 or D2 stimulation was unable to elicit Fos even following repeated reserpine treatment). The fact that cortical Fos expression was tightly associated with behavioral activation together with its laminar and areal distribution suggest that sensory input resulting from behavioral activation may be an important stimulus for this immediate-early gene response.

Chronic eticlopride and dopamine denervation induce equal nonadditive increases in striatal D2 receptor density: Autoradiographic evidence against the dual mechanism hypothesis

In an attempt to resolve experimental discrepancies regarding the mode of action of D2 receptor regulation following denervation or chronic receptor blockade, rats with extensive unilateral destruction of the mesotelencephalic dopaminergic projections induced by intracerebral 6-hydroxydopamine were injected daily for 21 days with either saline or the potent, selective D2 antagonist eticlopride (0.5 mg/kg, i.p.). Four days after the last injection of eticlopride or saline, rats were killed, and brain sections through the caudate-putamen and nucleus accumbens septi were incubated with [3H]spiroperidol or (R)-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5- phenyl-1H-3-benzazepin-7-ol ([3H]SCH 23390) to assay D2 and D1 receptors, respectively. Autoradiographic analysis revealed that chronic eticlopride treatment increased the density of D2 sites in the intact hemisphere for all regions examined without further augmenting the already increased density of D2 receptors seen in the dopamine-denervated hemisphere. D2 receptor density was independent of functional sensitivity as evidenced by the fact that rats treated chronically with eticlopride rotated contralateral to the 6-hydroxydopamine lesion following systemic administration of the selective D2 agonist quinpirole during the neuroleptic wash-out period, despite the fact that D2 receptor binding was not significantly different in the left and right hemispheres of these subjects. D1 receptor density was not affected by eticlopride treatment but was significantly reduced reduced in the dopamine-denervated hemisphere. [3H]Mazindol labeling of high-affinity DA uptake sites indicated that the extent of dopamine denervation was greater than 97% in both saline- and eticlopride-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of dopamine in the maintenance and breakdown of D1/D2 synergism.

The neurochemical factors involved in the maintenance and breakdown of dopamine D1/D2 receptor synergism were investigated by giving rats various pharmacological treatments that diminish the ability of dopamine to interact with its D1 and/or D2 receptors. Following these treatments, rats were observed for the expression of stereotyped motor behavior in response to independent stimulation of D1 or D2 receptors. Independent D2-mediated responses were observed: (a) 2 h after the last of three daily reserpine (1 mg/kg) injections, (b) 48 h after bilateral 6-hydroxydopamine (6-OHDA) lesions of the mesostriatal pathways, (c) 24 h after a concentrated 48-h regimen (one injection/6 h) of eticlopride (0.5 mg/kg) or eticlopride+SCH 23390 (0.5 mg each), and (d) 2 h after a concentrated 48-h regimen (one injection/6 h) of alpha-methyl-p-tyrosine (alpha MPT; 100 mg/kg), but not after control treatments or a concentrated regimen of SCH 23390 alone. By contrast, independent D1-mediated responses were observed only after three daily reserpine injections or 48 h after bilateral 6-OHDA lesions. Independent D1-mediated stereotypy was not observed under control conditions or following a concentrated 48-h regimen of (a) SCH 23390 or eticlopride (0.5 mg/kg each) alone or in combination, (b) a high dose of SCH 23390 (1.0 mg/kg), (c) alpha MPT (100 mg/kg), or (d) alpha MPT (100 mg/kg)+SCH 23390 (1.0 mg/kg). Reserpine, bilateral 6-OHDA, and alpha MPT treatments produced striatal dopamine depletions of 96%, 92%, and 71%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid development of D1 and D2 dopamine receptor supersensitivity as indicated by striatal and pallidal Fos expression

Rats were injected with a single dose of the monoamine-depleting agent reserpine (5 mg/kg s.c.) or its vehicle. Twenty-four hours later rats were injected with either the selective D1 agonist SKF 38393 (5 mg/kg i.p.), the selective D2 agonist quinpirole (0.1 mg/kg i.p.) or saline, and perfused 2 h later. Fos-like immunoreactivity was visualized using a polyclonal antibody to the Fos protein and standard ABC methods. Reserpine rendered striatal D1 and D2 receptors supersensitive as indicated by 10- to 12-fold increases in striatal and pallidal Fos immunoreactivity. The short latency of the development of both D1 and D2 supersensitivity limits the candidate causative mechanisms to those that occur within hours of the initiating stimulus.