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Biomedical subjects

P Mazzone

Publications and source records attributed to P Mazzone.

At least 55 records · Page 3Linked to original sources
PubMed1999

Electrophysiological and clinical desensitization to apomorphine administration in parkinsonian patients undergoing stereotaxic neurosurgery.

A decreased motor response after repeated doses of apomorphine is observed in severely affected Parkinson's disease patients. We simultaneously studied clinical symptoms and internal pallidus single unit activity in three parkinsonian patients underlying stereotaxic neurosurgery for deep brain stimulation. In each patient, two closely spaced doses of intraoperatory apomorphine were administered, while recording the same extracellular unit. The reduced clinical effect of the second administration was correlated to a lessened inhibition of the pallidal single unit recorded throughout the double administration. Our data support the proposition that fast postsynaptic desensitization to dopamine agonists may take place in the basal ganglia nuclei and play a role in the physiopathology of levodopa long-term treatment syndrome.

Apomorphine↗
PubMed1999

Direct demonstration of interhemispheric inhibition of the human motor cortex produced by transcranial magnetic stimulation.

Electromyographic (EMG) responses evoked in hand muscles by a magnetic test stimulus over the motor cortex can be suppressed if a conditioning stimulus is applied to the opposite hemisphere 6-30 ms earlier. In order to define the mechanism and the site of action of this inhibitory phenomenon, we recorded descending volleys produced by the test stimulus through high cervical, epidural electrodes implanted for pain relief in three conscious subjects. These could be compared with simultaneously recorded EMG responses in hand muscles. When the test stimulus was given on its own it evoked three waves of activity (I-waves) in the spinal cord, and a small EMG response in the hand. A prior conditioning stimulus to the other hemisphere suppressed the size of both the descending spinal cord volleys and the EMG responses evoked by the test stimulus when the interstimulus interval was greater than 6 ms. In the spinal recordings, the effect was most marked for the last I-wave (I3), whereas the second I2-wave was only slightly inhibited, and the first I-wave (I1) was not inhibited at all. We conclude that transcranial stimulation over the lateral part of the motor cortex of one hemisphere can suppress the excitability of the contralateral motor cortex.

Adult↗
PubMed1999

Effects of voluntary contraction on descending volleys evoked by transcranial electrical stimulation over the motor cortex hand area in conscious humans.

The spinal volleys evoked by electric anodal and cathodal stimulation over the cerebral motor cortex hand area were recorded from a bipolar electrode inserted into the cervical epidural space of two conscious human subjects. We measured the size of volleys elicited by electric stimulation at active motor threshold and at 3% of maximum stimulator output above this value with subjects at rest and during maximum voluntary contraction of the contralateral first dorsal interosseous muscle. Surface EMG activity was recorded at the same time. Electrical anodal stimulation evoked a single negative wave that we termed D-wave in analogy with data in experimental animals. Cathodal stimulation evoked a single negative wave with a latency of 0.2 ms longer than the D-wave recruited by anodal stimulation. At both intensities tested, voluntary contraction did not modify the amplitude of the descending waves. We conclude that changes in cortical excitability induced by voluntary activity do not modify the corticospinal volley evoked by electric stimulation and that the D-waves evoked by both anodal and cathodal electric stimulation are probably initiated several nodes distant to the cell body.

Adult↗
PubMed1999

Intracortical origin of the short latency facilitation produced by pairs of threshold magnetic stimuli applied to human motor cortex.

Under certain conditions, EMG responses evoked by pairs of transcranial magnetic stimuli over the motor cortex are larger than the sum of the responses to each stimulus given alone. This occurs with interstimulus intervals of around 1.3, 2.5 and 4.3 ms and could be due to interaction between the responses to each stimulus at either the cortex or spinal cord. We recorded the descending volleys set up by such pairs of stimuli from the cervical epidural space of five patients implanted with chronic stimulators for pain control. Interstimulus intervals of 1, 1.2, 1.4 and 2 ms were used to investigate the first peak of facilitation. Enhanced EMG responses occurred after pairs of stimuli at 1, 1.2 and 1.4 ms, and these were accompanied by larger and more numerous descending volleys than expected from the sum of each stimulus alone. We conclude that facilitatory interaction between the stimuli can occur within the cerebral cortex. This may involve elements that produce repetitive I-wave activity in response to a single stimulus.

Adult↗
PubMed1999

Parallel processing of sensory inputs: an evoked potentials study in Parkinsonian patients implanted with thalamic stimulators.

In two drug-resistant Parkinsonian subjects, who underwent thalamic chronic stimulation for extrapyramidal symptoms relief, median nerve somatosensory evoked potentials (SEPs) were recorded before and at different times following the thalamic lead implant. In both subjects, a transient obliteration of post-rolandic SEPs components was detected; pre-rolandic waves' amplitude was preserved or showed a tendency to increase after the beginning of chronic stimulation. Parietal waves' amplitude totally recovered pre-surgical values after 1 month. Latency of both pre- and post-central components remained stable. The 'dissociate behaviour' of the examined waves following the thalamic implant reinforces the hypothesis that short-latency sensory inputs are processed by separate and independent routes which are functionally segregated at subcortical level.

Aged↗
PubMed1999

The effect of deep brain stimulation on the frontal N30 component of somatosensory evoked potentials in advanced Parkinson's disease patients.

OBJECTIVES: In the present study we investigated whether in advanced Parkinson's disease (PD) patients the frontal component of short somatosensory evoked potentials (SEPs) to median nerve stimulation may be modified by basal ganglia deep brain stimulation (DBS). METHODS: We recorded the SEPs in 6 PD patients undergoing bilateral functional neurosurgery in the internal globus pallidus (GPi) (4 patients) and in the nucleus subthalamicus (STN) (two patients) during ineffective and effective bilateral BDS. Pre-operatively, the SEPs were also recorded in off therapy and during apomorphine infusion. RESULTS: From the evaluation of the latency and the amplitude characteristics of the major parietal (N20 and P25) and frontal (N30) components, we observed that whereas the parietal waves did not vary in any condition, the N30 potential showed a remarkable amplitude increase during apomorphine as well as during effective bilateral GPi or STN DBS. Furthermore, after the stimulators were turned off we noticed that the N30 amplitude potential progressively faded almost in parallel with the attenuation of DBS clinical effects. CONCLUSIONS: Our results lead to the conclusion that the bilateral DBS of both GPi and STN is really effective in producing a selective increase of frontal N30 amplitude probably improving the supplementary motor area functional activity, but these results do not clarify whether this amelioration is due to a central or a 'long loop' mechanism.

Antiparkinson Agents↗
PubMed1999

Thiazole derivatives as inhibitors of purified bovine liver mitochondrial monoamine oxidase-B: structure-activity relationships and theoretical study.

Structure-activity relationships were performed on a new series of thiazole derivatives which selectively inactivate monoamine oxidase-B (MAO-B), purified from mitochondrial beef liver. All of the synthesized and tested compounds showed non-competitive inhibition, suggesting the formation of a stable adduct between the tertiary amine function, linked to the thiazolyl derivatives and the active site of the enzyme. The mechanism of MAO-B inhibition is discussed in terms of the Ionization Potential of the amine nitrogen atom and the conformational flexibility of the inhibitors.

Animals↗
PubMed1998

Effects of voluntary contraction on descending volleys evoked by transcranial stimulation in conscious humans.

1. The spinal volleys evoked by single transcranial magnetic or electric stimulation over the cerebral motor cortex were recorded from a bipolar electrode inserted into the cervical epidural space of three conscious human subjects. These volleys were termed direct (D) and indirect (I) waves according to their latency. 2. We measured the size and number of volleys elicited by magnetic stimulation at various intensities with subjects at rest and during 20 or 100 % maximum contraction of the contralateral first dorsal interosseous muscle (FDI). Surface EMG activity was also recorded. 3. Electrical stimulation evoked a D-wave volley. Magnetic stimulation at intensities up to about 15 % of stimulator output above threshold evoked only I-waves. At higher intensities, a D-wave could be seen in two of the three subjects. 4. At all intensities tested, voluntary contraction increased the number and size of the I-waves, particularly during maximum contractions. However, there was only a small effect on the threshold for evoking descending activity. Voluntary contraction produced large changes in the size of EMG responses recorded from FDI. 5. Because the recorded epidural activity is destined for muscles other than the FDI, it is impossible to say to what extent increased activity contributes to voluntary facilitation of EMG responses. Indeed, our results suggest that the main factor responsible for enhancing EMG responses in the transition from rest to activity is likely to be increased excitability of spinal motoneurones, rather than increases in the corticospinal volley. The latter may be more important in producing EMG facilitation at different levels of voluntary contraction.

Aged↗
PubMed1998

Magnetic transcranial stimulation at intensities below active motor threshold activates intracortical inhibitory circuits.

A magnetic transcranial conditioning stimulus given over the motor cortex at intensities below threshold for obtaining electromyographical (EMG) responses in active hand muscles can suppress responses evoked in the same muscles at rest by a suprathreshold magnetic test stimulus given 1-5 ms later. In order to define the mechanism of this inhibitory effect, we recorded descending volleys produced by single and paired magnetic transcranial stimulation of motor cortex through high cervical, epidural electrodes implanted for pain relief in two conscious subjects with no abnormality of the central nervous system. The conditioning stimulus evoked no recognisable descending activity in the spinal cord, whilst the test stimulus evoked 3-4 waves of activity (I-waves). Conditioning stimulation suppressed the size of both the descending spinal cord volleys and the EMG responses evoked by the test stimulus. Inhibition of the descending spinal volleys was most pronounced at ISI 1 ms and had disappeared by ISI 5 ms. It was evident for all components following the I1-wave, while the I1-wave itself was not inhibited at all. We conclude that a small conditioning magnetic stimulus can suppress the excitability of human motor cortex, probably by activating local corticocortical inhibitory circuits.

Aged↗
PubMed1998

Comparison of descending volleys evoked by transcranial magnetic and electric stimulation in conscious humans.

OBJECTIVES: The present experiments were designed to compare the understanding of the transcranial electric and magnetic stimulation of the human motorcortex. METHODS: The spinal volleys evoked by single transcranial magnetic or electric stimulation over the cerebral motor cortex were recorded from a bipolar electrode inserted into the cervical epidural space of two conscious human subjects. These volleys were termed D- and I waves, according to their latency. Magnetic stimulation was performed with a figure-of-eight coil held over the right motor cortex at the optimum scalp position, in order to elicit motor responses in the contralateral FDI using two different orientations over the motor strip. The induced current flowed either in a postero-anterior or in a latero-medial direction. RESULTS: At active motor threshold intensity, the electric anodal stimulation evoked pure D activity. At this intensity, magnetic stimulation with the induced current flowing in a posterior-anterior direction evoked pure I1 activity. When a latero-medial induced current was used, magnetic stimulation evoked both D and I1 activity. Using electric anodal stimulation, at a stimulus intensity of 9% of the stimulator output above the active motor threshold (corresponding approximately to 1.5 active motor threshold), a small I1 wave appeared only in subject 1. Using magnetic stimulation with a posterior-anterior induced current, at a stimulus intensity of 21% of maximum stimulator output above the active motor threshold (corresponding approximately to 1.8 times threshold in subject 1 and to two times threshold in subject 2), a small D wave appeared in subject 1 but not in subject 2. CONCLUSIONS: Present results demonstrate that, in conscious humans at threshold intensities, electric stimulation evokes D waves and magnetic stimulation (with a posterior-anterior induced current) evokes I waves, while magnetic stimulation (with a latero-medial induced current) evokes both activities.

Differential Threshold↗