Kenneth G. Lloyd

Biochemical Evidence of Dysfunction of Brain Neurotransmitters in the Lesch-Nyhan Syndrome

Different brain regions were removed post mortem from three patients with the Lesch-Nyhan syndrome and were examined for alterations in hypoxanthine-guanine phosphoribosyl transferase (HGPRT), adenine phosphoribosyl transferase, and biochemical indexes of norepinephrine, dopamine, serotonin, gamma-aminobutyric acid (GABA), and acetylcholine neuron function, as compared with age-matched controls. The level of HGPRT activity in the material from patients with the Lesch-Nyhan syndrome was less than 1 per cent of control levels, whereas adenyl phosphoribosyl transferase was not significantly altered. All biochemical aspects of the function of dopamine-neuron terminals in the striatum (except dihydroxyphenylacetic acid levels) were decreased to 10 to 30 per cent of the control values. Serotonin and 5-hydroxyindoleacetic acid levels were increased, striatal choline acetyltransferase levels were low, and striatal glutamic acid decarboxylase and guanylate cyclase activities were unaltered. The disruption of the balance between the functions of GABA, dopamine, and acetylcholine neurons in the extrapyramidal system probably accounts for some of the symptoms observed in the Lesch-Nyhan syndrome (e.g., choreoathetosis).

Cortical modulation of striatal function

The effect of bilateral section of the corticostriatal projections or of selective bilateral ablation of the frontal cortex on behavioral and biochemical parameters related to striatal function were investigated in the rat. Either lesion almost completely prevented the cataleptogenic action of haloperidol: this effect was observed as soon as 3 days and lasted for at least 3 months after surgery, paralleling a reduction in striatal glutamate uptake. Also, such lesions enhanced the apomorphine-induced stereotyped behavior (as measured 21 days after surgery). In the striatum, dopamine, dihydroxyphenylacetic acid, acetylcholine and substance P levels as well as choline acetyltransferase and glutamic acid decarboxylase activities were unaffected 10 or 21 days after either type of lesion. In the substantia nigra, substance P levels were unchanged 10 days following suction of the frontal cortex, but glutamic acid decarboxylase was reduced at 21 days postsurgery. Cortical lesions only partially prevented the reduction in striatal acetylcholine concentrations and did not affect the increase in striatal dihydroxyphenylacetic acid caused by haloperidol. Finally, lesions of the corticostriatal pathways failed to affect the apomorphine-induced increase in striatal acetylcholine levels, reduction of the potassium (20 mM) evoked [3H]acetylcholine release in striatal slices preloaded with [3H]choline and decrease of striatal dihydroxyphenylacetic acid concentrations. These findings indicate that the frontal cortex influences extrapyramidal function by a mechanism which--in behavioral terms--is antagonistic to dopamine-mediated events. As indicated by the biochemical data, this mechanism does not involve changes in striatal dopaminergic and cholinergic neuron activity. This mechanism may utilize: (1) corticostriatal glutamatergic neurons as suggested by the reduction in striatal glutamate uptake following lesions; and (2) GABAergic pathways as suggested by the reduction of nigral glutamic acid decarboxylase activity as well as by the finding that GABA receptor agonists reinstate haloperidol-induced catalepsy.

Enhancing GABAergic transmission reverses the aversive state in rats induced by electrical stimulation of the periaqueductal grey region

In a proposed rat model for anxiety (electrical stimulation of the periaqueductal grey region), progabide (a GABA agonist) and diazepam both increased the latency to escape to a safe compartment and also the current needed to induce the escape response (escape threshold). Furthermore, the effects of progabide and diazepam were greater than additive in their actions on the escape response as when given together in normally subliminal doses, the combination exerted a marked anti-aversive effect. These actions of the drugs alone or in combination could not be explained by non-specific motor effects. Blockade of GABA receptors by bicuculline greatly reduced or abolished the action of progabide and diazepam (single administration). Sodium valproate, which indirectly augments GABAergic transmission, also increased the escape latency and escape threshold whereas, in contrast, diphenylhydantoin accentuated the aversive effects of stimulation of the periaqueductal grey. Haloperidol increased the escape latency and threshold but not other signs of distress following central stimulation (vocalization, jumping) which were effectively blocked by progabide and diazepam. The action of haloperidol was completely explicable by an interference with motor mechanisms. These results are interpreted as an indication that GABA agonists have an anti-aversive action in this proposed rat model for anxiety and, furthermore, that GABA receptors at least partially mediate the actions of benzodiazepines in this model.

CNS Compensation to Dopamine Neuron Loss in Parkinson’s Disease

Postmortem studies in brains from parkinsonian patients consistently reveal a minimum loss of 75% of the nigrostriatal dopamine neurons. This indicates that over a prolonged period, before Parkinsons disease is clinically evident, there is a physiological compensation for the slow loss of dopamine neurons (i.e. compensated stage of Parkinsons disease). Only when the dopamine neuron loss is sufficiently severe (greater than 75% of nigrostriatal dopamine neurons) does the disease become clinically evident (decompensated state). Postmortem examination of Parkinsons disease brains and study of animal models indicate that the following mechanisms may contribute to this CNS compensation: 1) A decrease in striatal cholinergic activity, in an attempt to maintain a critical DA:ACh balance; and 2) A decrease in activity of GABA neurons in the striatum and substantia nigra, resulting in an increased firing rate of nigral dopamine cells. These mechanisms allow the brain to readjust to the initial dopamine cell loss in Parkinsons disease.

Alterations in 3H-GABA binding in Huntington's chorea.

Abstract 3 H-GABA binding was measured in the caudate nucleus, putamen, parietal cortex and cerebellar cortex of control patients and patients with Huntingtons chorea. The density of 3 H-GABA binding in the parietal cortex was similar in both patient groups. In the striatal regions there was a significant large (70–80 percent) decrease in the density of GABA binding which is consistent with the severe atrophy and cell loss observed in these regions. In contrast the membranes prepared from the cerebellar cortex exhibited both an increased binding capacity (at 25 nM 3 H-GABA) and an apparently increased affinity (decreased KD) for 3 H-GABA. The decreased capacity of the striatum to bind 3 H-GABA may partially explain the reported lack of clinical efficacy of GABAmimetic compounds in Huntingtons chorea.

The effects ofN-methyl-d-aspartate and kainate lesions of the rat striatum on striatal ornithine decar☐ylase activity and polyamine levels

The intrastriatal injection of N-methyl-D-aspartate (NMDA) (250 nmol) produced a delayed and marked increase in striatal ornithine decarboxylase (ODC) activity and putrescine levels which peaked 6-15 h following the injection of NMDA. Striatal ODC activity subsequently returned to normal values while putrescine levels remained significantly elevated for up to 4 days following the lesion. NMDA produced an early and progressive decline in striatal spermine and spermidine levels, preceding the increase in ODC activity, with a maximum effect 2 h following injection. Spermidine levels returned to normal 6 h post-NMDA infusion, and subsequently increased to above normal levels 36 h and 4 days after the infusion of NMDA. This late increase in striatal spermidine levels paralleled an increase in the binding of the glial cell/macrophage marker [3H]PK 11195. Spermine levels tended to return to normal values 6 h after the injection of NMDA but may be further depressed at later intervals (15 h to 4 days). The intrastriatal injection of saline also resulted in a delayed increase in striatal ODC activity and putrescine levels, but these changes were minor compared to those produced by NMDA. Intrastriatal saline injection provoked no consistent change in striatal spermine or spermidine levels. The changes in polyamine metabolism produced by the intrastriatal injection of kainic acid (4 nmol) were only analysed at 6 and 15 h following injection but were qualitatively similar to those produced by NMDA although perhaps following a slightly more delayed time-course.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of chronic neuroleptic or L-DOPA administration on GABA levels in the rat substantia nigra

Abstract Male albino rats were administered daily with haloperidol, clozapine or L-DOPA and sacrificed 18 hours after the last dose of the drug. Acutely haloperidol (5mg/kg, i.p.) greatly lowered nigral GABA levels whereas after 167 daily doses the nigral GABA levels were not significantly different from controls, but were significantly increased as compared with the acutely treated animals. In contrast, acute L-DOPA (2 × 100mg, p.o.) greatly raised nigral GABA levels whereas after chronic L-DOPA (167 days) nigral GABA levels were not significantly different from controls and were significantly lower as compared with the animals receiving the acute treatment. Clozapine (20 mg/kg, i.p. either acutely or chronically) did not have as marked an effect on nigral GABA levels as did haloperidol. Of these various drug regimens only chronic L-DOPA significantly affected nigral GAD activity, producing a moderate decrease.

The effect of different types of cortical lesions on drug-induced catalepsy in rats: A pharmacological analysis

The effect of bilateral lesions of various cortical areas on neuroleptic- and non-neuroleptic-induced catalepsy was studied in the rat. Ablation of the frontal or parietal cortex led to a marked decrease in haloperidol catalepsy, whereas lesions of the occipital cortex, or of the olfactory bulbs did not affect catalepsy, when measured 3 weeks post lesion in all cases. The frontal cortex lesions also diminished the cataleptic state induced by cis-flupenthixol, (+)-butaclamol and chlorpromazine, but failed to affect that induced by reserpine, tetrabenazine or morphine. Four months after the frontal lesions, haloperidol catalepsy was at control values, and a rebound phenomenon was observed at 10 months post lesion. The homeostatic mechanisms seem to have stabilized at 18 months post lesion when haloperidol catalepsy had returned to control levels.

The behavioural effects of systemically administered kainic acid: a pharmacological analysis.

Abstract Systemic injection of kainic acid (KA), a powerful neuroexcitant and structural analogue of glutamate, induced a complex behaviour in the rat characterized by early “wet-dog-shakes” (WDS and delayed convulsions. 1) The WDS syndrome was antagonized by serotonin blockers (mianserin and cyproheptadine) and by GABAmimetic agents, which decrease serotonergic transmission; in contrast, WDS were potentiated by compounds which enhance serotonin-mediated events (fluoxetine, fenfluramine, imipramine and tranylcypromine) as well as by blockade of GABA receptors (bicuculline). In addition, WDS were antagonized by haloperidol (which possesses some anti-serotonin properties) whereas KA potentiated haloperidol-induced catalepsy, an effect which was blocked by cyproheptadine. This suggests that KA induces WDS and potentiates catalepsy via an increase in serotoninergic function. 2) KA induced convulsions were antagonized by GABAmimetic agents, in agreement with their broad anticonvulsant spectrum; in contrast, mianserine and cyproheptadine did not affect or even potentiated seizures. Thus KA seizures respond differently to pharmacological treatment than do WDS, and may me related to the nwuro-excitatory action of KA.

Akinesia after locally applied morphine near the nucleus raphe pontis of the rat

Morphine (10 micrograms/microliter) was injected into different sites within the pontine brainstem in different groups of rats with indwelling guide cannulae. The rats were tested for the persistence of abnormal postures as a measure for the degree of akinesia. Local morphine was most effective in inducing akinesia at a site identical or close to the nucleus raphe pontis. Midline injections were effective in contrast to bilateral injections. Fenfluramine, a serotonin releaser, strongly potentiated morphine akinesia.