I. J. Mitchell

Neural mechanisms underlying Parkinsonian symptoms based upon regional uptake of 2-deoxyglucose in monkeys exposed to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

The 2-deoxyglucose metabolic mapping technique has been used to investigate the neural mechanisms which underlie the symptoms of Parkinsonism in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine primate model of Parkinsons disease. In six cynomolgus monkeys, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine was either (a) administered intravenously to induce generalized Parkinsonism, or (b) infused into one carotid artery to induce unilateral Parkinsonism. Post-mortem examination revealed profound cell loss from the substantia nigra, pars compacta either bilaterally or unilaterally in the two groups, respectively. In addition, there was pathological involvement of the ventral tegmental area and locus coeruleus in animals receiving intravenous 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. 2-Deoxyglucose autoradiography revealed widespread changes in 2-deoxyglucose uptake in the brains of parkinsonian animals when compared to controls. Most of these changes were in basal ganglia and related structures and were qualitatively similar in the two groups of experimental animals. Prominent increases in 2-deoxyglucose uptake were observed in the lateral segment of the globus pallidus (24-27%), the ventral anterior and ventral lateral nuclei of the thalamus (14-22%) and the nucleus tegmenti pedunculopontinus of the caudal midbrain (17-69%). A profound decrease (17-26%) in 2-deoxyglucose uptake was observed in the subthalamic nucleus. We propose these data to indicate that in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced Parkinsonism there is the following pattern of abnormal neuronal activity in basal ganglia circuitry: (i) increased activity in the projection from the putamen to the lateral segment of the globus pallidus; (ii) decreased activity in the projection from the putamen to the medial segment of the globus pallidus; (iii) decreased activity in the projection from the lateral segment of the globus pallidus to the subthalamic nucleus; (iv) increased activity in the projection from the subthalamic nucleus to the globus pallidus; and (v) increased activity in neurons of the medial segment of the globus pallidus projecting to the ventral anterior/ventral lateral thalamus and the pedunculopontine nucleus. These results are compared to the 2-deoxyglucose uptake findings in previous studies from this laboratory in hemiballism and hemichorea in the monkey. The central importance of the subthalamic nucleus in all three conditions is proposed, and supportive evidence for the excitatory nature of subthalamic efferent fibres is adduced.

Sites of the neurotoxic action of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in the macaque monkey include the ventral tegmental area and the locus coeruleus

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) produces a profound parkinsonian state when systemically administered in monkeys and man. Previous studies have shown MPTP to be toxic to only the dopamine (DA) cells in the substantia nigra pars compacta and not to other catecholamine (CA)-containing cells. The data presented here suggest that MPTP also has a specific neurotoxic effect on the DA-containing cells of the ventral tegmental area and the noradrenaline-containing cells of the locus coeruleus in macaque monkeys with a moderate-to-severe parkinsonian syndrome. The results suggest that MPTP-induced parkinsonism in the monkey more closely replicates the neurochemical changes seen in idiopathic Parkinsons disease than previously thought.

On the role of enkephalin cotransmission in the GABAergic striatal efferents to the globus pallidus

In the MPTP-treated primate model of Parkinsons disease, loss of dopaminergic afferents to the striatum leads to increased activity in striatal efferents to the external segment of the globus pallidus. This pathway utilizes both GABA and enkephalin as cotransmitters. Little is known regarding either the role of this cotransmission in the generation of parkinsonian symptoms or of the nature of any functional interaction between GABA and enkephalin. We have investigated the roles played by enkephalin and GABA in mediating parkinsonian symptoms by injection the GABAA antagonist bicuculline and the broad spectrum opioid antagonist naloxone directly into the globus pallidus in the reserpine-treated rat model of parkinsonism. Injections of bicuculline, but not naloxone, had marked antiparkinsonian effects. However, naloxone attenuated the antiparkinsonian effects of bicuculline. We interpret these findings as suggesting that increased GABAergic transmission in the globus pallidus is responsible for the generation of parkinsonian symptoms in the reserpine-treated rat. However, overactive enkephalinergic transmission is not responsible for the generation of symptoms and appears to act to reduce the effects of increased GABAergic transmission. In complementary studies in vitro, we have demonstrated a potential mechanism for this negative interaction. Met-enkephalin (3-10 microM) reduced depolarization-evoked release of GABA from terminals in slices prepared from rat globus pallidus (IC50, 0.38 microM). A better comprehension of the mechanisms by which enkephalin and other peptides modulate the action of amino acid transmitters in the basal ganglia is critical to the understanding of the neural processes underlying basal ganglia function and movement disorders.

Regional brain uptake of 2-deoxyglucose in N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)—induced parkinsonism in the macaque monkey

This preliminary report describes application of the 2-deoxyglucose (2DG) autoradiographic technique to study regional changes in brain metabolism in experimental parkinsonism, induced in the monkey by administration of the neurotoxin MPTP. In one monkey, rendered severely parkinsonian by MPTP, there was a marked increase in the uptake of 2DG in the globus pallidus (both medial and lateral segments) and in the ventral anterior and ventral lateral thalamic nuclei, in comparison to non-parkinsonian animals. Increased uptake of 2DG in the globus pallidus may reflect increased activity of striatopallidal synapses secondary to loss of nigrostriatal dopaminergic neurones. The findings are in sharp contrast to our observations on regional brain metabolism in experimental choreiform dyskinesia in the monkey.

Cellular localisation of enkephalin gene expression in MPTP-treated cynomolgus monkeys

Cellular sites of enkephalin gene expression were investigated using the technique of in situ hybridization in the normal striatum and in the denervated striatum of monkeys depleted of dopamine by pretreatment with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Animals received MPTP by either (a) intravenous injection to induce generalized parkinsonism, or (b) infusion into one carotid artery to induce unilateral parkinsonism. The animals which received systemic injections of MPTP were found to have an essentially total loss of nigral dopamine cells whereas the intracarotid MPTP treatment was found to destroy approximately 95% of the dopamine neurons in the ipsilateral substantia nigra. A double-stranded cDNA probe encoding the human preproenkephalin (PPE) gene was isotopically labelled with 35S and used to detect PPE mRNA within striatal tissue sections. Application of this radiolabelled cDNA probe to lightly fixed striatal sections from both groups of animals revealed an increase in expression of PPE mRNA within denervated striatal enkephalinergic neurons relative to control tissue. An increase in the number of detectable enkephalinergic mRNA-positive neurons relative to control tissue was also noted. These results suggest that the nigral dopaminergic neurons tonically inhibit PPE gene expression in the striatum.

Functional implications of kappa opioid receptor-mediated modulation of glutamate transmission in the output regions of the basal ganglia in rodent and primate models of Parkinson's disease.

Parkinsons disease is characterized by an increased excitatory amino acid transmission in the internal segment of the globus pallidus and the substantia nigra pars reticulata. The effects of the kappa receptor agonist enadoline (CI-977) on glutamate transmission were investigated in vitro. Enadoline reduced the K(+)-evoked release of glutamate from slices of substantia nigra in a concentration-dependent manner (maximum effect: 78% inhibition at 200 microM). This effect was blocked by the selective kappa receptor antagonist nor-binaltorphimine. The endogenous ligand for kappa receptors is thought to be dynorphin. Dynorphin released from terminals of striato-pallidal and striato-nigral pathways might thus act as an endogenous modulatory agent on glutamatergic transmission in the basal ganglia. In vivo experiments were carried out in rodent and primate models of Parkinsons disease to assess the potential of manipulating kappa receptors as a potential treatment for Parkinsons disease. Enadoline reduced reserpine-induced akinesia when injected in the entopeduncular nucleus of the rat. Similarly, injections of CI-977 in the internal segment of globus pallidus (GPi) of the MPTP-treated marmoset alleviated parkinsonian symptoms and allowed the animal to recover its locomotor activity. This suggest that reducing the overactive glutamatergic transmission in the output regions of the basal ganglia by activating kappa receptors might potentially form the basis of a novel anti-parkinsonian therapy.

Neural mechanisms mediating 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced parkinsonism in the monkey: Relative contributions of the striatopallidal and striatonigral pathways as suggested by 2-deoxyglucose uptake

The neural mechanisms which mediate parkinsonian symptoms have been investigated in the monkey by application of the 2-deoxyglucose (2-DG) metabolic mapping technique to animals rendered parkinsonian by systemic administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The results show that 2-DG uptake was dramatically increased in the globus pallidus, but not the substantia nigra pars reticulata, in parkinsonian monkeys compared to controls. This observation has been interpreted as indicating increased synaptic activity in the putaminopallidal, but not the caudatonigral, pathway which suggests a relatively greater involvement of the putaminopallidal pathway in relation to the motor manifestations of parkinsonism.

Common neural mechanisms in experimental chorea and hemiballismus in the monkey. Evidence from 2-deoxyglucose autoradiography.

The autoradiographic 2-deoxyglucose uptake technique was used to visualise local cerebral metabolic activity in the monkey in recently developed models of chorea and hemiballismus. Unilateral dyskinesia was induced by injection of a gamma-aminobutyric acid antagonist into the corpus striatum (in the case of chorea) or subthalamic nucleus (in the case of hemiballismus). Patterns of 2-deoxyglucose uptake suggest that during both forms of experimental dyskinesia the subthalamic nucleus and its projection to the globus pallidus are abnormally hypoactive.

In defence of optical density ratios in 2-deoxyglucose autoradiography.

The use of optical density ratios to describe changes in [14C]2-deoxyglucose uptake in neuroanatomical mapping experiments has recently been criticized. It has been argued that a fixed ratio of tissue isotope concentration does not yield a constant optical density ratio but is dependent on the exposure time and the absolute amounts of isotope used. Here it is demonstrated that such variations in optical density ratios are due to an artifact in calculating the optical density ratio, which can easily be corrected provided that the film is not approaching saturation and not due to the non-linearity of an exposure-density curve as has previously been suggested.

A semi-quantitative atlas of 5-hydroxytryptamine-1 receptors in the primate brain

The regional distribution of 5-hydroxytryptamine-1 receptors in the primate brain was studied by semi-quantitative autoradiographic analysis of tritiated ligand binding. Areas showing the highest density of 5-hydroxytryptamine-1 receptors (greater than 200 fmol [3H]5-hydroxytryptamine bound per mg tissue), included the cerebral cortex (laminae I-II), claustrum, posterior cell group of the basal nucleus of Meynert, the infracommissural part of the globus pallidus, cortical amygdaloid nucleus, hippocampal formation (CA1-subiculum region, the anterior CA2, CA3 and CA4 regions and the molecular layer of the dentate gyrus), thalamic nuclei (parafascicular, parataenial, paraventricular and superior central lateral nuclei), substantia nigra pars reticulata, dorsal raphe nucleus and choroid plexus. The distribution of 5-hydroxytryptamine-1 receptors is compared to the distribution of both 5-hydroxytryptamine receptors and terminal fields of serotonergic projections as previously described in subprimates.