Sensitization of dopamine-stimulated adenylyl cyclase in the striatum of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated rhesus monkeys and patients with idiopathic Parkinson's disease.

Dopamine-stimulated adenylyl cyclase activity was measured in striatal homogenates of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated rhesus monkeys and humans with idiopathic Parkinson's disease and compared with the activity in control tissue. No differences between parkinsonian and control tissue were found in the presence of 20 mM NaCl. However, when 120 mM NaCl was included in the assay medium, a significantly higher increase in the Vmax of dopamine-stimulated adenylyl cyclase activity was observed in the caudate of MPTP-parkinsonian rhesus monkeys and the putamen of patients with idiopathic Parkinson's disease. No such sensitization was seen in the MPTP-treated rhesus putamen or human Parkinson's disease caudate tissue. A role of D2 receptors in this sensitization could be ruled out by the concomitant use of the D2 antagonist l-sulpiride and by [3H]spiperone saturation analysis of the D2 receptor density, which was found at control level in the caudate tissue of MPTP-treated rhesus monkeys. Similarly, on the basis of saturation binding with the D1 selective ligand 125I-SCH 23982, there was no difference in caudate nucleus D1 receptor densities between control and MPTP-treated monkeys. Our results point to a region-specific functional sensitization of D1 receptors as a consequence of severe dopaminergic denervation of the striatum and suggest the possibility of a therapeutic potential of a D1 agonist with full intrinsic activity in Parkinson's disease.

Biochemical and pharmacological studies on pramipexole, a potent and selective dopamine D2 receptor agonist.

Pramipexole (SND 919; 2-amino-4,5,6,7-tetrahydro-6-propyl-amino-benzthiazole- dihydrochloride) was tested for its agonistic activity at pre- and postsynaptic dopamine (DA) receptors. L-Dihydroxyphenylalanine (L-dopa) accumulation in the rat striatum and limbic system and the alpha-methyltyrosine-induced reduction of DA were inhibited. Both effects were fully antagonized by haloperidol but not by the selective DA D1 receptor antagonist SCH 23390. Pramipexole decreased the levels of DA metabolites dose dependently, whereas striatal DA levels remained unchanged. In mice, pramipexole (0.001-1 mg/kg s.c.) reduced exploratory locomotor activity. In rats with unilateral striatal lesions, only weak ipsilateral rotation was produced by pramipexole at the highest dose. However, in rats with unilateral lesions of the medial forebrain bundle, pramipexole potently induced contralateral circling (ED50 0.026 mg/kg s.c.). In the N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkey model, pramipexole also had potent stimulatory effects. Finally, in haloperidol-sensitized monkeys, the substance did not elicit dyskinesia/dystonia when given alone, but rather inhibited those symptoms which had been induced by haloperidol (ED50 0.116 mg/kg i.m.). It is concluded that pramipexole has therapeutic potential for schizophrenic patients, as a result of its autoreceptor agonistic effects and its weak effects at normosensitive postsynaptic DA receptors. Furthermore, its potent stimulatory effects in DA-depleted animals suggest a possible use in the treatment of Parkinson's disease.

Effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine on the regional distribution of brain monoamines in the rhesus monkey.

In an attempt to define neurochemically the part played by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) as a potential Parkinson's disease-inducing neurotoxin, we measured the tissue concentrations of the monoamines dopamine, noradrenaline and serotonin in 45 brain regions in nine rhesus monkeys (Macaca mulatta) receiving repeated intramuscular injections of a total amount of 2.1-7.5 mg/kg MPTP-HCl. Four monkeys treated with MPTP during a period of one to five weeks developed permanent Parkinsonism, and five animals receiving the neurotoxin during a period of two to seven months remained asymptomatic. We found that, compared with the distribution pattern established in the brain of seven normal (drug-free) rhesus monkeys, in the MPTP-treated monkeys none of the three major brain monoamine neuron systems was completely resistant to the neurotoxin. In addition, each brain monoamine had a characteristic regional pattern of MPTP-induced changes. As expected, the most significant alterations were found within the nigrostriatal dopamine system, i.e. profound dopamine loss in caudate nucleus, putamen and substantia nigra. However, many extrastriatal regions of the subcortex and brainstem also suffered significant loss of dopamine, with the noradrenaline loss in the regionally subdivided brainstem being less widespread, and the serotonin levels least affected. Thus, in subcortex/brainstem the ranking order of sensitivity to MPTP was: dopamine greater than noradrenaline much greater than serotonin. In the cerebral (neo- and limbic) cortex, all three monoamine neuron systems suffered widespread statistically significant losses. The ranking order of MPTP sensitivity of the cortical monoamines was: noradrenaline greater than serotonin greater than dopamine. In the cerebellar cortex, dopamine and noradrenaline concentrations were significantly reduced, whereas the serotonin level remained unchanged. A remarkable observation was that many of the subcortical and cortical changes found in the symptomatic monkeys were also found in the asymptomatic animals. Our data are compatible with several possible mechanisms by which MPTP may have produced the observed patterns of monoamine loss in the brain of the rhesus monkey. Our study demonstrates that in the rhesus monkey MPTP mimicked, in addition to the profound striatal dopamine loss, some of the extrastriatal dopamine, noradrenaline and serotonin changes often seen in the brain of patients with idiopathic Parkinson's disease. However, using our treatment regimen, we have not been able to reproduce in the rhesus monkey the inter-regional pattern of striatal dopamine loss typical of idiopathic Parkinson's disease, i.e. a significantly greater loss of dopamine in the putamen compared with the caudate nucleus.

Extrastriatal dopamine in symptomatic and asymptomatic rhesus monkeys treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP).

We analyzed in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated rhesus monkeys, with and without parkinsonian symptoms, the regional changes in dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and tyrosine hydroxylase activity (TH). Symptomatic monkeys had wide-spread DA loss in subcortical and cortical regions. However, the magnitude of this MPTP-induced DA reduction was markedly smaller than the DA loss in the caudate nucleus and putamen, where only less than 1% DA remained. No comparable loss of DA was found in the subcortical extrastriatal regions of asymptomatic MPTP monkeys, despite more than 90% DA loss in the striatal nuclei. The most pronounced difference in DA levels between the symptomatic and the asymptomatic group was observed in nucleus accumbens, nucleus of the stria terminalis, ventral tegmental area, globus pallidus and the cingulate gyrus. Levels of DOPAC and TH activity paralleled the behavior of DA. In contrast, the concentration of HVA was reduced in many brain regions of both symptomatic and asymptomatic monkeys. These effects of MPTP on extrastriatal DA levels in the rhesus monkey are compared with DA and HVA changes in the brain of patients with Parkinson's disease, and their possible contributory role for the production, by MPTP, of a permanent parkinsonian condition is discussed.

The neurotoxin MPTP does not reproduce in the rhesus monkey the interregional pattern of striatal dopamine loss typical of human idiopathic Parkinson's disease.

Using high-pressure liquid chromatography with electrochemical detection, we measured dopamine (DA) and homovanillic acid (HVA) in caudate nucleus, putamen and substantia nigra in 4 untreated rhesus monkeys and 4 monkeys with permanent parkinsonism produced by repeated injections of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP; total dose: 2.1-6.45 mg/kg, i.m.). MPTP consistently produced a severe striatal and nigral loss of DA and HVA and an increase in the ratio 'HVA/DA'. In this respect, MPTP mimicked the changes found in human Parkinson's disease (PD). However, MPTP lowered the DA in caudate (-99.6%) to the same degree as in putamen (-99.5%). This is in contrast to idiopathic PD where the caudate is significantly less affected by DA loss (-84%) than the putamen (-98%). Thus, in our rhesus monkeys MPTP failed to reproduce the interregional caudate-putamen gradient characteristic of idiopathic PD. The DA pattern produced by MPTP was similar to the DA loss in caudate (-98%) and putamen (-99%) observed in patients with postencephalitic parkinsonism.

Pharmacology of WEB 1881-FU, a central cholinergic agent, which enhances cognition and cerebral metabolism.

The pharmacological effects of the novel compound WEB 1881 FU (4-amino-methyl-1-benzyl-pyrrolidine-2-one-fumarate) were investigated. The tests performed indicate that the compound has cytoprotective as well as metabolism and cognition enhancing and central cholinomimetic properties. The nootropic effects in all tests were more pronounced than those of piracetam, while the central cholinomimetic effects were generally weaker than those of directly acting cholinomimetic agents. However, the typical peripheral cholinergic side effects were not observed. From the results we believe that the stimulating effect of WEB 1881 FU upon the central cholinergic system is modulatory rather than direct. The combination of nootropic and cholinomimetic properties appears favorable for treatment of brain dysfunction in the elderly. Side effects are less serious than with other known cholinomimetics.

The dopamine autoreceptor agonist B-HT 920 stimulates denervated postsynaptic brain dopamine receptors in rodent and primate models of Parkinson's disease: a novel approach to treatment.

B-HT 920 (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo-[4,5-d]azepine), an agonist at alpha 2-adrenoceptors and at dopamine autoreceptors, was tested with respect to stimulation of postsynaptic brain dopamine receptors in mice, rats and rhesus monkeys. In mice B-HT 920 (0.2-20 mg/kg s.c.) injected 4 h after reserpine did not stimulate locomotor activity; this was in contrast to apomorphine (0.1-10 mg/kg s.c.) which elicited locomotor activity in a dose-dependent manner. However, B-HT 920 was effective in inducing locomotor activity when injected 12, 24 and 48 h after reserpine. This effect was dose-dependent and increased with the duration of reserpine pretreatment. In naive rats, B-HT 920 (0.02-2.0 mg/kg s.c.) only decreased exploratory activity and did not elicit stereotyped activity in doses up to 4 mg/kg s.c. This was in contrast to the stereotypy-inducing effect of apomorphine (2.0 and 4.0 mg/kg s.c.). In rats with unilateral striatal ibotenic acid lesion, B-HT 920 (0.2-2.0 mg/kg s.c.) was ineffective in producing significant ipsilateral rotation, whereas apomorphine (0.5-10.0 mg/kg s.c.) was very potent in this model. In rats with unilateral 6-OH-dopamine lesions of the medial forebrain bundle B-HT 920 elicited strong contralateral rotation in a dose-dependent manner (0.02-1.0 mg/kg s.c.). In this model B-HT 920 was equi-effective but long acting when compared with apomorphine. The contralateral rotation produced by B-HT 920 was antagonized by the D2-antagonist sulpiride but not by the D1-antagonist SCH 23390. In rhesus monkeys with severe parkinson-like symptoms induced by MPTP, B-HT 920 in doses of 10 micrograms/kg i.m. and higher restored normal behavior, resulting in complete relief of parkinson symptoms in all animals with 100 micrograms/kg i.m. It is concluded that the property of B-HT 920 to stimulate the 'denervated' supersensitive (reserpine, 6-OH-dopamine, MPTP) but not the normosensitive postsynaptic dopamine receptor in the striatum may represent a novel principle for a specific approach to dopamine substitution treatment of Parkinson's disease.

A method for a quantitative determination of changes in tissue volume as a result of perfusion fixation.

A method for quantifying changes in volume of tissue as a result of perfusion fixation is described. This is achieved by setting standards into the tissue in vivo and in situ by applying dyes to the intact tissue at known distances. Immediately after perfusion the distances of the dyes are macroscopically measured and the comparison between the corresponding distances before and after perfusion enables a quantification of the alteration of the tissue. The total error of this method is approximately 0.5-0.8% (linear).

Analysis of switching neurons within the thermoafferent system.

A total of 740 midbrain, thalamic, and hypothalamic neurons were tested as to their distribution and response/stimulus characteristics by means of thermal stimulation of the scrotal skin. The most frequent response type exhibited a basic discharge rate up to a threshold temperature and then switched to a maximum firing rate. The inverse behaviour was found in a low proportion. The temperature range in which the neurons switch from the 'low' to the 'high' state has been called the 'operating range' of the neuron. This study reveals, in contrast to former studies, that the 'operating range' is extremely narrow. This holds both for the individual neuron and for the populations studied. It is confirmed by simultaneous recordings and by cross correlation analysis that there is only one temperature threshold for all warm responsive neurons involved. It is concluded that according to the mean firing rate, temperature may only be discriminated as to below or above threshold. This means that a binary information is transmitted. For continuous temperature sensation and regulation either further neuronal types disposing of a continuous temperature/frequency characteristic or additional coding mechanisms have to be assumed. The major task of the switching neurons, integrating information from all over the body should be the generation of a trigger signal for warm defence.

Thermoafferent signal processing in rats: an electrophysiological analysis of midbrain influences on thermoresponsive neurons in the ventrobasal thalamus.

The influence of electrical midbrain stimulation on thalamic cells which respond to scrotal skin warming with steep increases in firing rate was studied in anaesthetized rats. Square-wave stimuli of 200 microseconds duration and 30-250 microA intensity at 15 Hz were applied in trains lasting about 1 min. At low scrotal temperatures stimulation of the nucleus raphe dorsalis, the adjacent central grey or the median raphe nucleus was nearly always followed by a rise in thalamic activity. Inhibition occurred only in 4 of 75 experiments. Often midbrain stimulation caused neuronal responses similar to those following scrotal warming. Single stimulation experiments and the combination of electrical stimulation and electrolytic lesions indicated local interactions between excitatory and inhibitory neuronal networks in midbrain. Poststimulus latencies revealed polysynaptic midbrain/thalamic connections. Direct projections with no more than a few synaptic interruptions were seen in about 10% of the experiments. It is concluded that medial midbrain neurons influence the transmission of peripheral thermal information to the specific thalamus and contribute to the typical switching form of thalamic responses to scrotal warming.

Influence of cold adaptation on the activity of thermoresponsive neurons in thalamus and midbrain of the rat.

A group of 25 rats was adapted to cold by housing for five weeks at +3 degrees C. Using thermal stimuli of the scrotal skin, 53 recordings of warm-responsive thalamic and midbrain neurons were analyzed and compared with 84 control recordings from non-adapted rats. The activity of the analyzed neurons is characterized by a steep increase of firing rate above a certain temperature threshold zone. Between scrotal skin temperatures of 36 degrees and 38 degrees C the percentage of neurons with firing rates above the basal rate is higher, both in the thalamic and midbrain population of the cold adapted rats. It is concluded that after cold-adaptation the increase of firing rate starts on average at a lower temperature. The results are discussed in the context of findings of other authors on peripheral structures and on effector behaviour.

Neuronal responses in the rat's thalamus to scrotal heating.

Twenty-one single neurons from the rat's thalamus were recorded as they responded to scrotal skin thermal stimulation. Eighteen of them were warm and three inverse-warm cells. After the injection of Lidocain (Xylocain) into the scrotal skin, the response of each neuron was abolished. Eight neurons could be followed until the drug effect had disappeared; their responsiveness was fully restored. External application of Xylocain had the same effect as intracutaneous injection, apart from longer time intervals before the effect became apparent. From this, conclusions are drawn on the origin of the thalamic responses, and speculations are made on the functional role of warm and inverse-warm cells.

Thalamic burst firing--a neuronal code for temperature information$.

A total number of 840 neurons was extracellularly recorded from the rat's thalamus (ventrobasal complex, VB, and nucleus posterior, NPT) and from the midbrain (raphe nuclei, MB) while thermal stimuli were given to the hindlimbs, scrotal skin, and the tongue. Of all neurons found, 407 exhibited burst patterns; the highest percentage of burst cells was found in VB (50-85%) whereas only 1-015% of the NPt and MB neurons were burst cells. About one-half of all burst cells responded to peripheral temperature stimulation by changing their mean firing rate. The question dealt with in this paper is whether burst firing gives a more detailed information on thermal sensations than mean firing rate alone. Therefore, 40 burst cells (20 of which were thermo-responsive according to their mean firing rate) were further analyzed by evaluating several burst parameters such as burst frequency, number of spikes/burst and bursting index. However, when mean discharge rate did not respond to skin temperature, the burst parameters were not correlated with the stimulus, either. Hence it is concluded that the burst firing in the thalamic neurons investigated does not provide an additional detailed information of skin temperature.