Karl-Heinz Sontag

Spatial learning is affected by transient occlusion of common carotid arteries (2VO) : comparison of behavioural and histopathological changes after 2VO and four-vessel-occlusion in rats

Place learning in the Morris water maze following transient (24 min) occlusion of the common carotid arteries (2VO), and following permanent occlusion of both vertebral arteries plus transient (20 min) clamping of the carotids (4VO) was investigated in rats 6-9 days after occlusion. Both 2VO and 4VO treatment increased the latency to find the hidden platform during escape trials; spatial bias, tested in a probe trial, was decreased. Histological analysis revealed neural necrosis in the CA1 sector of the hippocampus in 4VO rats but not in 2VO animals. The data suggest that an experimentally induced reduction in cerebral blood flow of 50% (after 2VO) or 95% (after 4VO) produces persistent functional changes even in absence of actual neural damage.

Morphine-induced muscular rigidity in rats.

Abstract Morphine in doses of 7.5 to 17.5 mg/kg, i.p. induced dose-dependent, electromyographically recorded muscular activity (rigidity), which could be inhibited by the morphine antagonist, naloxone and by drugs increasing central dopaminergic activity, L-DOPA and apomorphine. The results suggest that the rigidity was morphine specific and was due to a functional dopamine deficiency in the striata. There was a good parallelism between catalepsy and rigidity. Since both effects were easily reversed by L-DOPA, it is suggested that in our system studied the primary action of morphine was a presynaptic effect in the nigrostriatal neurons and not blockade of striatal dopamine receptors. The results could be explained by ‘diversion’ of newly synthetized dopamine from storage sites to sites of catabolism resulting in a lack of dopamine at striatal receptor sites which might lead to a subsequent compensatory increase of dopamine utilization.

Muscle Relaxant Action of Excitatory Amino Acid Antagonists

Antagonists of neuronal excitation induced by dicarboxylic amino acids were tested in genetically spastic rats of the Han-Wistar strain. These animals exhibit an increased muscle tone which can be measured as a spontaneous tonic activity in the electromyogram of the gastrocnemius-soleus muscle. Compounds that block excitation due to N-methyl-D-aspartic acid reduced the spontaneous activity measured in the electromyogram in a dose-related manner. The most potent compounds, 2-amino-7-phosphonoheptanoic and kynurenic acids were effective muscle relaxants when given either intraperitoneally or intracerebroventricularly. 2-Amino-5-phosphonopentanoic acid possessed much weaker muscle relaxant activity, while L-glutamic acid diethylester was inactive by either route. The results suggest that blockade of N-methyl-D-aspartic acid receptors results in a myorelaxant effect. Specific antagonists of excitation at N-methyl-D-aspartic acid receptors may provide a new class of muscle relaxants.

Susceptibility to seizures produced by pilocarpine in rats after microinjection of isonnazid or γ-vinyl-GABA into the substantia nigra

Pilocarpine, given intraperitoneally to rats, reproduces the neuropathological sequelae of temporal lobe epilepsy and provides a relevant animal model for studying mechanisms of buildup of convulsive activity and pathways operative in the generalization and propagation of seizures within the forebrain. In the present study, the effects of manipulating the activity of the gamma-aminobutyric acid (GABA)-mediated synaptic inhibition within the substantia nigra on seizures produced by pilocarpine in rats, were investigated. In animals pretreated with microinjections of isoniazid, 150 micrograms, an inhibitor of activity of the GABA-synthesizing enzyme, L-glutamic acid decarboxylase, into the substantia nigra pars reticulata (SNR), bilaterally, non-convulsant doses of pilocarpine, 100 and 200 mg/kg, resulted in severe motor limbic seizures and status epilepticus. Electroencephalographic and behavioral monitoring revealed a profound reduction of the threshold for pilocarpine-induced convulsions. Morphological analysis of frontal forebrain sections with light microscopy revealed seizure-related damage to the hippocampal formation, thalamus, amygdala, olfactory cortex, substantia nigra and neocortex, which is typically observed with pilocarpine in doses exceeding 350 mg/kg. Bilateral intrastriatal injections of isoniazid did not augment seizures produced by pilocarpine, 200 mg/kg. Application of an irreversible inhibitor of GABA transaminase, gamma-vinyl-GABA (D,L-4-amino-hex-5-enoic acid), 5 micrograms, into the SNR, bilaterally, suppressed the appearance of electrographic and behavioral seizures produced by pilocarpine, 380 mg/kg. This treatment was also sufficient to protect animals from the occurrence of brain damage. Microinjections of gamma-vinyl-GABA, 5 micrograms, into the dorsal striatum, bilaterally, failed to prevent the development of convulsions produced by pilocarpine, 380 mg/kg. The results demonstrate that the threshold for pilocarpine-induced seizures in rats is subjected to the regulation of the GABA-mediated synaptic inhibition within the substantia nigra.

Reduction of ocular blood flow results in glial fibrillary acidic protein (GFAP) expression in rat retinal Müller cells

Reduction of blood flow to the rat retina was achieved by either clamping both carotid arteries briefly (24 min) or combining clamping of the carotid arteries with permanent occlusion of the vertebral arteries. Analysis of retinas 6 days after operations showed that GFAP immunoreactivity is expressed throughout the retinal Müller cells, although this was variable in retinas from animals where only the carotid arteries were clamped. GFAP immunoreactivity was not associated with retinal Müller cells from control animals and no obvious neuronal damage was observed in retinas from operated animals. These data suggest that Müller-cell GFAP expression may be used as an index to follow possible processes leading to an ischemic insult.

Neuromuscular responses to gait perturbations in freely moving cats

SummaryIf an obstacle impedes the forward swing of a cats foot, the animal responds by rapidly lifting the foot over the obstacle. In freely moving cats, the electrical activity of hindlimb flexors and extensors was recorded during such reactions elicited both mechanically and electrically. The sequencing of muscle activity was more complex and longer in duration in the mechanically elicited reactions. Anaesthesia of the foot dorsum abolished responses in ankle extensors and knee flexors, and converted the responses of ankle flexors to simple stretch reflexes. Although our findings closely resemble those reported for chronic spinal kittens, there are interesting points of difference, which should be taken into account if the notion of a purely spinal mediation of the placing reaction during stepping is to be accepted.

Muscular rigidity and delineation of a dopamine-specific neostriatal subregion: Tonic EMG activity in rats

In order to investigate the role of neostriatal dopamine receptors in muscular rigidity we have studied catalepsy and spontaneous muscle tone in rats receiving haloperidol injections into various parts of the neostriatum. It was found that low doses of haloperidol (250-750 ng/0.5 microliter) induced a tonic activity in the EMG of the gastrocnemius-soleus muscle when injected into the most rostral part of the neostriatum (A 8620-9650). This tonic EMG activity was found to be both dose-dependent and dopamine-specific: the haloperidol effect of 500 ng could be inhibited by 500 ng apomorphine. No muscular rigidity could be observed when haloperidol (500 ng/0.5 microliter) was injected further caudally into the neostriatum or into the medial part of the nucleus accumbens. With regard to the haloperidol-induced catalepsy as measured by means of the bar test, a similar distribution of effective and ineffective injection sites was observed; however, higher doses of haloperidol (1.5-2.5 micrograms/0.5 microliter) were required. Thus catalepsy and muscular rigidity were found to be closely related phenomena within the neostriatum. Finally, it is discussed how information relevant to tonic EMG activity is transmitted from the substantia nigra pars compacta to the superior colliculus and the ventromedial thalamus via the neostriatum and the substrate nigra pars reticulata.

Muscle relaxant and anticonvulsant activity of 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid, a novel N-methyl-D-aspartate antagonist, in rodents.

A novel 4-substituted derivative of piperazine-2-carboxylic acid, 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), with potent N-methyl-D-aspartate (NMDA) antagonist activity was evaluated as a muscle relaxant in genetically spastic rats. CPP, 0.02-0.1 mmol/kg, given intraperitoneally reduced the tonic activity in the electromyogram recorded from the gastrocnemius muscle of genetically spastic rats in a dose- and time-dependent manner. Muscle relaxation was also seen following intrathecal application of CPP, 0.0002-0.002 mumol, in genetically spastic rats. CPP, 0.1 mmol/kg, while not affecting Hoffman (H-) reflexes, depressed flexor reflexes in anesthetized rats following intravenous administration. In mice, CPP, 0.001 mumol, given intracerebroventricularly preferentially antagonized myoclonic seizures induced by NMDA and quinolinate, and had no effect on convulsions elicited by kainate, quisqualate and L-glutamate. These observations identify CPP as the most potent preferential NMDA antagonist so far tested with muscle relaxant and anticonvulsant activity resembling the profile of action of 2-amino-7-phosphonoheptanoate.

Motor reactions to perturbations of gait: proprioceptive and somesthetic involvement

If an obstacle impedes the forward swing of a cats foot during stepping, the animal responds by rapidly lifting the foot over the obstacle. Electrical activity of hindlimb muscles was recorded during such reactions in conscious cats. After anaesthesia of the dorsum of the foot, some muscles no longer took part in the reactions and simple stretch responses were observed in the ankle flexors, which had previously shown a more complex pattern of activation. This result emphasizes the importance of cutaneous afferents in contributing to the control of motor responses to unexpected perturbations.

Retinal ischemia induced by occlusion of both common carotid arteries in rats as demonstrated by electroretinography

In 2 models of reduced cerebral blood flow-permanent occlusion of the vertebral arteries plus transient occlusion of the common carotid arteries (4VO) and transient clamping of the common carotid arteries (BCCA)-the acute effects on the electrical function of the retina were monitored by recording the photopic electroretinogram. During both 4VO and BCCA the amplitude of the b-wave was reduced. Within 30 min of reperfusion after 4VO and after BCCA the b-wave had fully recovered. In contrast, the a-wave was not affected by either treatment. The data suggest that occlusion of common carotid arteries leads to retinal ischemia and might represent a useful model of amaurosis fugax.