Glutamatergic neurons of rat medial prefrontal cortex innervating the ventral tegmental area are positive for serotonin 5-HT1A receptor protein

The present study was designed to investigate whether serotonin 5-HT1A receptor protein (5-HT1A receptor-immunoreactivity), is present on cortical pyramidal neurons of the rat medial prefrontal cortex (MPC) innervating the ventral tegmental area (VTA). Recent data stress the role of serotonin 5-HT1A receptors in the pathology of schizophrenia, and in the mechanism of action of novel antipsychotic drugs. It was found that approximately 52% of cells in layers II/III of the MPC whose axons initial segments were immunoreactive for serotonin 5HT1A receptor were also labeled with Fluoro-Gold (FG), a retrograde tracer injected into the VTA, indicating that certain portion of neurons forming glutamatergic innervations of the VTA may be controlled by serotonin 5-HT1A receptors. In deep cortical layers (V/VI) retrogradely labeled neurons never colocalized with serotonin 5-HT1A receptormmunoreactivity. These anatomical data indicate that serotonin 5-HT1A receptors might potentially control the excitability and propagation of information transmitted by the pyramidal cells to the VTA. Moreover, our results indicate that the drugs operating via serotonin 5-HT1A receptors in the MPC, might control from this level the release of glutamate in the VTA and restore function of glutamate neurotransmission, whose dysfunction is observed for example in schizophrenia.

The long-term effects of the neurotoxin 1-trichloromethyl-1,2,3,4-tetrahydro-beta-carboline (TaClo) on cognitive performance in rats

The neurotoxin 1-trichloromethyl-1,2,3,4-tetrahydro-beta-carboline (TaClo) has been reported, both in vitro and in vivo models, to produce neurodegeneration and parkinsonian symptoms after prolonged exposure in rats. The aim of the present study was to investigate the effects of TaClo on the cognitive performance of rats. We used the COGITAT hole board system where rats can find hidden pellets by exploring the board. TaClo-treated rats found as many pellets as control rats treated with saline. Furthermore, their search was as efficient as that of control animals since there were no differences between the groups regarding explorative activity, visits to non-baited holes and time needed to find the pellets. These results suggest that there is no deficit in spatial memory following the chronic administration of TaClo to rats.

Cerebral oligemia and iron influence in cerebral structures--element of morbus parkinson models?

The consequences of short phases of restricted cerebral blood flow and iron enrichment of striatal tissues resulted in an animal model that could correspond to the basic features of a model for Parkinson's disease. An automatic and computerized hole-board offers simultaneous data on learning and cognitive memory capabilities, learning of distinct patterns of distributed food pellets found and eaten in a given time, switches between different locations of food in the holes and in different layout patterns. Wistar rats after 60 min of bilateral clamping of the carotid arteries (BCCA) under pentobarbital anesthesia received 1.5 microg FeCl3 injected one week after BCCA unilaterally into the ventrolateral striatum. The experiments showed that reduced cerebral blood flow and increased iron within the striatal tissue had the effect of retarding reactions. Rats after BCCA and iron need 180 s to find pellets deep inside holes that are distributed in a distinct pattern. During only 60 or 30s BCCA plus iron rats are no longer able to find the same number of pellets as over 180s. Rats after BCCA plus NaCl do not show such reduced success. These results point to the idea that cerebral oligemia and increased iron in the striatum stimulate the pathological symptoms of Parkinson's disease which need also more time to have reaction and success (see Fig. 5). The data covering abbreviated time-spans show how heavily the BCCA + Fe animals are dependent on longer times.

Consequences of a single short lasting cerebral oligemia and the influence of iron injected into the substantia nigra or in the ventrolateral striatum of the rat. Trigger of Parkinson?s disease pathogenesis?

One BCCA-phase (bilateral clamping of carotid arteria) leads to an extensive release of striatal dopamine with a subsequent formation of free radicals (Heim et al., 200b). Early investigations did not show histological damage to cerebral structures after 24 and 60 min duration of a BCCA phase (Melzacka et al., 1994). The study here turned out that oligemic damage and an increase in iron (FeCl3) concentration in the ventral striatum was responsible for most of the defective performance of the animals investigated. Striatal damaged animals were unable to correct their deficient performance to the same extent as was possible for animals which had been damaged through BCCA and FeCl3 in the substantia nigra. Furthermore it turn out that with the use of a comprehensive behaviour profile which was able to gather 22 parameters simultaneously, 15 of these parameters did not correspond in the performance of the controls already after BCCA alone. Since during the ageing process, pathological effects may occur in vulnerable structures not only from disturbances to cerebral blood-perfusion but also from enrichment of iron in vulnerable structures (Connor, 1992) the question arose whether this situation did not reveal pathological mechanisms that might triggered the early symptoms of Parkinson's disease.

Partial loss of dopaminergic neurons in the substantia nigra, ventrotegmental area and the retrorubral area -- model of the early beginning of Parkinson's symptomatology?

To test substances which might have protective effects on the dopaminergic system it is necessary to use models with a pathological symptomatology of the early beginning, i.e. models in which the chance exists to arrest the otherwise progressive pathological processes (see Heim et al., 2001). 6-hydroxydopamine (6-OHDA) injected unilaterally into the ventrolateral striatum of rats (6 microg dissolved in 2 microl 0.2% ascorbic acid) leads to specific stereotyped movements after subcutaneous injection of apomorphine both 3 and 13 weeks after surgery. Ten weeks after surgery decreased spontaneous motor activity could be observed. Twelve weeks after 6-hydroxydopamine injection, the animals had difficulties in performing a spatial navigation task when the submerged escape platform was moved to another position. The switching of motor programs was less pronounced. The application of tyrosine-hydroxylase-staining showed a loss of ipsilateral neurones of the substantia nigra compacta as well as of dendrites in the pars reticulata, neurones in the ventral tegmental area and in the retrorubral area ipsilaterally as well as a loss of dopaminergic fibres both ipsilaterally and contralaterally in the striatum which should belong to the contralateral acting substantia nigra afferents. The loss of the neurones and the afferents was induced by the retrograde denervation following the 6-OHDA injection within the ventrolateral striatum. The question arises whether the model used here with the partially loss of dopaminergic neurons and fibres reflects some of pathological symptoms of Parkinson's disease in the early states.

Effect of repeated treatment with reboxetine on the central alpha1-adrenergic system

The obtained results indicate that repeated administration of reboxetine (selective noradrenaline reuptake inhibitor, without any affinity for neurotransmitter receptors) increased the responsiveness of alpha1-adrenergic system (potentiating the methoxamine-induced exploratory hyperactivity in rats and clonidine-induced aggressiveness in mice), as tricyclic antidepressants did. However, the question whether the increased functional responsiveness found in the present study is important for the clinical antidepressant efficacy, remains open.

Locomotor hypoactivity and motor disturbances--behavioral effects induced by intracerebellar microinjections of dopaminergic DA-D2/D3 receptor agonists

In the light of recent findings, DA-D3 dopamine receptors with an unclear physiological function are present in the cerebellar cortex. Our preliminary results seem to indicate that bilateral injection of 7-OH-DPAT, a DA-D2/D3 receptor agonist (1 and 10 microg/0.5 microl), to lobule 9/10 of rat cerebellar cortex reduces spontaneous locomotor activity (hypolocomotor effects) and induces balance and motor coordination disturbances, respectively. Similar effects can be observed in the case of analogous microinjection of the DA-D3/D2 agonist pramipexole. In earlier studies, peripheral (ip) injection of nafadotride (0.6 mg/kg), a D3 receptor antagonist, neither affected per se spontaneous motor activity, nor modified the above described effects of 7-OH-DPAT. Participation of cerebellar DA-D3 and DA-D2 receptors in hypolocomotor effects, as well as putative participation of other receptors in the generation of motor disturbances, has been discussed.

Behavioral alterations after unilateral 6-hydroxydopamine lesions of the striatum. Effect of alpha-tocopherol

6-Hydroxydopamine (6-OHDA) injected unilaterally into the striatum of rats induced contralateral circling, and increased the duration of stereotyped movements after subcutaneous (sc) injection of apomorphine both 3 and 13 weeks after surgery. Ten weeks after surgery, the spontaneous locomotor activity during 24 h of observation was decreased. Twelve weeks after 6-OHDA injection, the animals had difficulties in carrying out a spatial navigation task in the water maze when the submerged escape platform was moved to another position on each of four consecutive days. When learning to find a new platform position required switching behavior-strategies, latency and swim paths were increased because of significantly more perseverative crossings of the previous platform positions. Intraperitoneal (ip) injection of alpha-tocopherol for 8 days increased the ability of naive control animals to find the hidden platform positions in the water maze one week later. In intrastriatal sham-operated rats, 8 daily pre-injections of alpha-tocopherol significantly increased the duration and number of bursts of stereotyped movements during 30 min following a sc injection of apomorphine if measured 13 weeks after surgery. In 6-OHDA-lesioned rats, alpha-tocopherol prevented the increased response to apomorphine, reduced the apomorphine-induced circling at 3 and 13 weeks, and prevented the decrease in spontaneous locomotion at 10 weeks, as well as the perseverative platform crossings which are caused by an impairment in switching behavior-strategies in the navigation task 12 weeks after surgery. Alpha-Tocopherol has, however, no influence on 6-OHDA-induced changes in problem solving strategies. The used model reflects some of the pathological symptoms of Parkinson's disease, and it seems that alpha-tocopherol may be an effective drug in the early initial stages of the disease.

Repeated treatment with SA4503, a selective sigma1 receptor agonist, up-regulates alpha-adrenergic system. a behavioral study

The obtained results indicate that SA4503, a selective sigma1 receptor agonist, given repeatedly (but not acutely) enhanced the effects of phenylephrine, alpha1-adrenoceptor agonist, and clonidine (stimulating the postsynaptic alpha1-adrenoceptors at high dose) in behavioral models (hyperexploratory activity in rats and aggressiveness in mice, respectively).

The role of dopamine D2 receptor in the behavioral effects of imipramine--study with the use of antisense oligonucleotides

Antisense strategies have a potential to specifically block the production of a given protein, e.g. receptor subtype, thus may help to uncover its behavioral and/or biochemical function. In the present study we demonstrated the utility of this approach for studying the role of dopamine D2 receptors in the anti-immobility effect of imipramine in the forced swimming test. Following intracerebroventricular (i.c.v.) administration of phosphorothioate oligonucleotide complementary to mRNA encoding for dopamine D2 receptors (D2 antisense ODN; 1 nmol/1 microl H2O, twice a day for 5 days) to the rats, the decrease in the locomotor activity (shortened total distance travelled and decrease in vertical activity, without differences in the stereotypic movements of animals), as well as the decrease of specific binding of [3H]raclopride in the striatum and limbic forebrain were observed. At the same time, i.c.v. administration of D2 antisense ODN reversed the effect of imipramine in the forced swimming test, what may indicate that the dopamine D2 receptors play a significant role in the behavioral anti-immobility effects of imipramine.

The effects of the 21-aminosteroid U-74389G on spatial orientation in rats after a cerebral oligemic episode and iron-induced oxidative stress

Oligemic episodes and increased iron concentration have both been proposed as being involved in neurodegenerative diseases. In animal models, a combination of both of these might therefore mimic the clinical pathology in humans. In rats, intrastriatal injections of ferric chloride, FeCl3, one week after a 60-minute oligemic episode, produced by bilateral clamping of the carotid arteries under pentobarbital anaesthesia (BCCA) impaired the animals' learning ability in a water maze task. Median adult rats, after intrastriatal 0.3 microg FeCl3, are impaired when challenged during the first three trial blocks, while after 0.06 microg FeCl3, an impairment is seen during the process of habituation to the challenge. Two-year-old animals do not show any learning effect at all after the combination of BCCA and intrastriatal FeCl3. Lazaroid U-74389G, a potent inhibitor of iron-induced lipid peroxidation, totally prevents the learning impairments in both median adult and aged animals, suggesting that iron-induced lipid peroxidation may be responsible for the late learning deficiencies. However, when U-74389G is applied one week after the oligemic episode but without the additional injection of iron, U-74389G on its own also impairs the animals' learning ability. The present animal model, when applied to clinical studies of lazaroids in humans, does seem able to give reliable information concerning the neuroprotective properties of such drugs.

The analysis system COGITAT for the study of cognitive deficiencies in rodents

COGITAT is an automated hole board system that, following minimal experimental interventions, makes it possible to measure a variety of parameters associated with learning, memory, relearning, cognition, and cognitive shifts, but also changes in exploratory and sensorimotor performance in rodent models. The individual parameters--that is, overall exploratory activity, number of visits (deep in the hole) into or inspections of (at the upper surface) holes, number of baited, unbaited, or previously baited holes visited or inspected, reinspections of or revisits into any holes, number of pellets eaten, time to find pellets, serial order collection (without intermediate inspections or visits), and reference and working memory errors (visits, inspections, or total)--are obtained simultaneously, and the results are immediately available after the end of each experiment. The system appears to be well suited to neurophysiological, neuropharmacological, and gene-technological investigations in rodent models.

Mesolimbic NMDA receptors are implicated in the expression of conditioned morphine reward

Systemic administration of a variety of N-methyl-D-aspartate (NMDA) receptor antagonists inhibits morphine's rewarding properties in the conditioned place preference test. In this study, we investigated the anatomical loci implicated in the inhibition of expression of morphine's reward by bilateral microinjections of a selective NMDA antagonist into the mesolimbic areas, including ventral tegmental area and nucleus accumbens. During conditioning, injections of 1 mg/kg morphine were associated with placing rats in one chamber of the place preference box; the exposures to the other chamber were associated with placebo administration. On the test day, drug-free control subjects demonstrated a marked preference for the morphine-associated chamber. Systemic administration of 5 mg/kg and 10 mg/kg of the competitive NMDA antagonist, NPC 17742 (2R,4R,5S-2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoic acid), significantly reduced the expression of morphine-induced conditioned place preference; the dose of 10 mg/kg produced also an inhibition of locomotor activity. Similar attenuation of the expression of morphine-induced conditioned place preference was observed in rats receiving 15.6 and 62.5 ng/0.5 ml side of NPC 17742 injected bilaterally into the nucleus accumbens and ventral tegmental area. While the higher intra-accumbal dose of NPC 17742 produced behavioral stimulation, intra-tegmental injection did not affect locomotor activity. These findings suggest that activation of NMDA receptors in the nucleus accumbens and ventral tegmental area is necessary for the elicitation of approach by environments previously associated with morphine's rewarding action.

Effect of local intracerebral administration of EMD 57445, a selective sigma receptor ligand, on the locomotor activity of the rat

EMD 57445 is a new compound which has been characterized by high affinity for sigma receptor sites. It has not been shown to bind to any other receptors, including dopamine ones. However, hitherto existing data (behavioral as well as biochemical) have suggested that this drug exhibits functional antidopaminergic activity. Therefore, in the present study, the local cerebral administration of EMD 57445 has been used in order to better elucidate the actual site of action of this compound in the central nervous system. EMD 57445 given unilaterally into the nucleus accumbens, striatum and lateral ventricle decreases the locomotor activity of the rats, the effect being the most pronounced in the case of administration into the nucleus accumbens. Moreover, unilateral intraaccumbal injection of EMD 57445 significantly diminishes apomorphine (given peripherally)-induced hyperactivity. Local administration of EMD 57445 into the prefrontal cortex of the rats also attenuated the locomotor activity but the effect was statistically significant only after bilateral administration of the compound. Additionally, the lack of influence of EMD 57445, administered po, on the dopamine D1 and D2 receptor binding in the striatal membranes was observed in these studies. However, in limbic forebrain the density of D2 receptors decreased after the higher dose of EMD 57445. In conclusion, the results obtained in this paper support the hypothesis that EMD 57445 exerts antidopaminergic activity through indirect inhibition of dopamine system.

Quantification of rigidity and tremor activity in rats by using a new device and its validation by different classes of drugs

Subjective techniques employed for measuring skeletal muscle tone and tremor, leading symptoms of several diseases, have certain limitations. Objective methods are usually more sensitive and accurate. The equipment developed by the authors allows the objective and rapid measurement of experimentally induced rigidity and tremor in the same small laboratory animal (rat). The present method considerably reduces the number of animals needed to investigate the activity of drugs, especially when compounds should be screened. Due to the greater sensitivity of the equipment, doses of reserpine and oxotremorine which do not cause any postural, autonomic or parasympathetic symptoms can be used to induce muscular rigidity and tremor. Therefore, not only the number of animals but also their stress can be reduced. It was possible to differentiate the oxotremorine-induced tremor from the spontaneous motor activity and to determine qualitative differences in tremor caused by antitremor agents. A number of clinically effective muscle relaxants and antiparkinsonian drugs were examined in this model in order to determine its utility.

Endogenous alkaloids in man. XXVI. Determination of the dopaminergic neurotoxin 1-trichloromethyl-1,2,3,4-tetrahydro-beta-carboline (TaClo) in biological samples using gas chromatography with selected ion monitoring

Highly chlorinated beta-carbolines have a potential in vivo relevance to Parkinson's disease. In this paper, a gas chromatographic method for the determination of the neurotoxic 1-trichloromethyl-1,2,3,4-tetrahydro-beta-carboline (TaClo), the condensation product of tryptamine and chloral hydrate, is described. The specific and sensitive assay involves purification of the biological samples by solid-phase extraction with C18 cartridges, derivatization with heptafluorobutyric anhydride, and chromatography on a non-polar fused-silica capillary column. Detection of TaClo was achieved by the registration of characteristic mass fragments of the TaClo heptafluorobutyric amide derivative using selected ion monitoring. The method was utilized to detect and quantify TaClo in blood, urine, bile, faeces, and brain tissue of rats treated with this alkaloid-type heterocycle. Four-fold deuterium-labelled TaClo was used as an internal standard.

Locomotor activity and motor disturbances induced by the competitive NMDA receptor antagonist CGP 37849

The study compares the effects of the competitive NMDA receptor antagonist, CGP 37849, on the locomotor activity and motor disturbances after local, unilateral microinjection into nucleus accumbens and caudateputamen or peripheral ip administration. Independently on the route of administration, the compound induced dose-dependent increase in the locomotor activity as well as the symptoms of motor disturbances. In all cases the symptoms of motor disturbances appeared earlier and lasted longer than increase in locomotor activity induced by the same dose of the drug. The long-lasting observation indicated, that CGP 37849 administered in a dose of 20 mg/kg ip increased the locomotor activity of the rats for about 2 h while motor disturbances persisted for about 6 h. Administration of the lowest dose of CGP 37849, i.e. 0.05 microgram into the nucleus accumbens, or 10 mg/kg ip-while without any effect on the locomotor activity-induced the symptoms of motor disturbances. Pretreatment with SKF-525-A (proadifen, an agent inhibiting the metabolism of the drugs), fully antagonized the locomotor hyperactivity induced by CGP 37849 given in a dose of 20 mg/kg, and-in the same time-diminished the degree of motor disturbance symptoms. On the other hand, in the case of the dose of 10 mg/kg of CGP 37849, SKF-525-A did not affect the locomotor activity, but enhanced the motor disturbance symptoms induced by this dose of CGP 37849. In conclusion, the obtained results indicate that CGP 37849 administered peripherally or locally to the brain regions cited above induces motor disturbances at doses lower (or at least similar) than these which stimulate the locomotor activity.

Injection of a minuscule dose of FeCl3 within the ventrolateral striatum causes a chronic disturbance of the integrative function within the limbic part of the ventral striatum

The present study shows that low amounts of applied iron have a potent effect on the ventrolateral striatum. This is reflected by an influence on spontaneous night activity, cognitive behaviour during the water maze navigation task, exploratory activity and in response to postsynaptic apomorphine stimulation. Such functional disturbances could be observed up to months after a single application of either 0.3 microgram or 1.5 micrograms FeCl3. The low dose of iron stimulates while 1.5 micrograms inhibits the spontaneous dopaminedependent locomotor night and explorative activity. The low concentration of ionic iron injected intrastriatally also increases lipid peroxidation in striatal and hippocampal tissues. These results suggest that the functional integrity of the ventral striatum and the regulation of the iron metabolism are critical for the sensorimotor performance.

Attenuation of the fentanyl-induced muscle rigidity by the selective 5HT1A agonist 8-OH-DPAT

Fentanyl given in a low dose of 25 microgram/kg (ip) evoked a marked muscle rigidity measured directly by a mechanographic method in non-anesthetized rats. The selective 5HT1A receptor agonist 8-OH-DPAT (0.1, 0.3 and 1.0 mg/kg ip) showed only a tendency to attenuate the natural muscle tone. However, when that compound was given 40 min before (0.3 and 1.0 mg/kg ip) or 20 min after (1.0 mg/kg ip) fentanyl, it abolished the muscle rigidity. It is concluded that the serotonergic transmission, possibly via 5HT1A receptors, may participate in elucidation of the mechanism(s) of the fentanyl-induced muscle rigidity. These results seem to be clinically important in case other 5HT1A agonists, buspirone or gepirone (potent anxiolytics), also prevented fentanyl-induced muscle rigidity in humans.

Behavioural effects after cholinergic stimulation of the reticular thalamic nucleus in rats

This study investigated the functional relationship between the experimentally induced changes in the activity of the cholinergic, muscarinergic system of the rostral area of the nucleus reticularis thalami (TRN) and the motor behaviour. The effect of direct stimulation of the rostral TRN by the cholinergic agonist carbachol on the behaviour of freely moving rats was observed. Unilateral injection of carbachol (0.2-3.2 micrograms/0.5 microliters) into the rostral TRN caused catalepsy which appeared rapidly and was short-lasting. Furthermore, it induced impairment of the performance on the rota rod. Both effects were dose-dependent. The cholinergic antagonist scopolamine (6.66 micrograms) coadministered with the equimolar dose of carbachol (3.2 micrograms) antagonized the effects of carbachol on both behavioural tests. The described effects seem to be cholinergic- and site-specific within the rostral TRN. The present results suggest that activation of the cholinergic, muscarinergic receptors in the rostral TRN modulate the motor function of rats.

Direct mechanomyographic measurement of the myorelaxant action of baclofen and diazepam in normal and reserpinized rats

A pathologically increased muscle tone is frequently observed. Objective methods for measuring directly the muscle-relaxant action of drugs in laboratory animals are scarce. This study shows that a computerized mechanomyographic method allows the assessment of myorelaxant drugs. The method consists of the successive bending and straightening of the rat's foot in the ankle joint and the separate measurement of resistance of extensors and flexors of the hind foot to passive movements. The two well-known antispastic drugs diazepam and baclofen reduced the normal muscle tone in both groups of muscles in non-treated animals as well as the muscle tone increased by reserpine (10 mg/kg i.p.). The results obtained show that the mechanomyographic method directly and reliably reveals the myorelaxant action of drugs.

The role of the ventromedial thalamic nucleus in the catalepsy evoked from the substantia nigra pars reticulata in rats

Picrotoxin (25, 50 and 100 ng), injected unilaterally into the posterior part of the substantia nigra pars reticulata (SNR) of rats, evoked a dose-dependent catalepsy. The catalepsy evoked by 100 ng of picrotoxin injected into the SNR was abolished by a subsequent bilateral injection of the same drug (200 ng) into the ventromedial thalamic nuclei. It is suggested that impulses pertinent to the catalepsy evoked from the SNR are transmitted via a GABAergic pathway to the ventromedial thalamic nucleus, wherefrom they reach the striatum, as had been shown previously.

The neostriatal inhibition of catalepsy, but not of muscle rigidity, evoked from the substantia nigra pars reticulata

The effects of a bilateral blockade of neo- and palleostriatal GABAergic mechanisms on catalepsy and muscle rigidity resulting from picrotoxin injection into the substantia nigra pars reticulata (SNR) were studied. The catalepsy and rigidity were induced by a unilateral injection of 100 ng/0.5 microliter of picrotoxin. Bilateral injections of 250 ng/l microliter of picrotoxin into the intermediate-ventral parts of the caudato-putamen (CP) abolished the catalepsy but had no effect on the muscle rigidity induced by an intranigral injection of the drug. Bilateral injections of 250 ng/l microliter of picrotoxin into the globus pallidus (GP) did not influence the catalepsy and rigidity induced by the intranigral injection of the drug. The results indicate that the impulses, connected with the catalepsy evoked from the SNR seem to be transmitted back to the CP and blocked therein by inhibition of GABAergic synapses in its intermediate-ventral part. The impulses, connected with the muscle rigidity evoked from the SNR, presumably do not return to the striatum.

Mechanographic analysis of muscle rigidity after morphine and haloperidol: a new methodological approach

The new method described in this study was based on consecutive repeated measurements of the resistance of flexor and extensor muscles of the hind foot of the rat to forced flexions and extensions of the foot. Locomotor movements of the rat were restrained with a metaplex box which had a slot for the hind limb. The control muscle tone measured by this method was constant for more than 2 h, and amounted to approx. 25 g for flexor muscles, and approx. 45 g for extensors. Morphine (2.5, 5, 10, 20 mg/kg) enhanced dose-dependently the resistance of flexor muscles up to approx. 45 g, 70 g, 100 g and 140 g, respectively, and the resistance of extensors of the paw up to approx. 100 g, 140 g, 180 g and 240 g, respectively. Haloperidol (5 and 10 mg/kg) enhanced dose-dependently the resistance of flexor muscles up to approx. 45 g and 70 g, respectively, and that of extensors of the foot up to approx. 75 g and 120 g, respectively. Morphine rigidity, measured as resistance of respective muscles to forced movements, was almost completely inhibited by a consecutive injection of 0.2 mg/kg of naloxone. The new method seems to have considerable advantages in comparison with electromyographical (EMG) or other kinds of mechanographical measurements of the muscle tone.

GABA mechanisms of ventromedial thalamic nucleus in morphine-induced muscle rigidity

The aim of the study was to investigate the role of the ventromedial thalamic nucleus in the rigidity induced by morphine. Muscle rigidity was assessed using an electromyographic method (EMG) in non-anaesthesized rats with electrodes implanted unilaterally in the gastrocnemius soleus muscle. Subcutaneous (s.c.) injections of morphine in doses of 5, 10 or 20 mg/kg evoked tonic EMG activity in the gastrocnemius soleus muscle; this was estimated as muscle rigidity. Picrotoxin was injected bilaterally into the ventromedial nucleus in doses of 50-400 ng/0.5 microliter 30 min after morphine administration. Picrotoxin in doses of 200 and 400 ng attenuated the tonic EMG activity induced by morphine, 10 mg/kg s.c. Picrotoxin in a dose of 400 ng reduced the tonic activity induced by morphine, 20 mg/kg s.c. The results suggest that the thalamic ventromedial nucleus mediates the morphine-induced rigidity.

Thalamus as a relay station for catalepsy and rigidity

The aim of the study was to determine to what extent catalepsy and tonic rigidity of muscles induced by muscimol administration into the ventral thalamic nuclei disturb the motor activity of rats. This study also aimed to test whether the ventromedial thalamic nucleus (Vm) was involved in transmitting effects evoked by the systemic injection of neuroleptics or opioids. For this purpose muscimol and/or picrotoxin was injected into the ventral thalamic nuclei and the behaviour of the animals was assessed in a series of test situations. It was found that muscimol administration to the Vm disturbs not only the initiation and performance of voluntary movements but also the occurrence of avoidance when the animal's life is endangered. Postural reflexes remained, however, undisturbed. Those effects seemed to be GABA- and site-specific to Vm. The haloperidol catalepsy was strongly inhibited by administration of picrotoxin to the Vm while the morphine catalepsy remained unchanged after picrotoxin. The Vm plays a crucial role in the motor behaviour and transmission of cataleptogenic effects of haloperidol, whereas similar effects produced by morphine appear to by-pass the investigated thalamic region.

The effect of kainic acid administration into globus pallidus on EEG and behavior of rabbits

Injection of 0,5 microgram of kainic acid into the globus pallidus of rabbits carrying chronically implanted electrodes and cannulas, resulted in an increase of cerebral electrical activity up to appearance of epileptoidal discharges. These effects were prevented by diazepam pretreatment. Behavioral changes produced by kainic acid were biphasic; ipsilateral rotations or postural asymmetries, observed in the early phase, gave way later to contralateral asymmetries. Kainic acid facilitated sniffing episodes, which appeared simultaneously with epileptoidal discharges in the hippocampus. The stimulation of globus pallidus neurons with kainic acid resulted also in electrical activation of the ventral anterior thalamic nucleus. The results indicate that the majority of the neurons of globus pallidus display selective sensitivity to kainic acid and that the globus pallidus is functionally connected with the hippocampus and the ventral anterior thalamic nucleus.

Alterations in apomorphine concentration in spinal cord and brain follow the time course of catalepsies induced by different treatments

Because evidence for the neurotransmitter role of dopamine in the gray matter of the spinal cord is accumulating, a question arises of whether or not spinal dopamine receptors are also involved in the effects of dopaminomimetics which are believed to induce beneficial effects in Parkinson's disease through an action thought to be mediated mainly by striatal dopamine receptors. To test this hypothesis muscimol and picrotoxin were injected unilaterally into the posterior part of the substantia nigra of rabbits permanently implanted with stainless-steel cannulae. Muscimol (a GABA-mimetic) enhanced locomotor activity, evoked a stereotyped behavior and contralateral rotations, and increased apomorphine-induced gnawing. Picrotoxin, a substance which inhibits GABA transmission, induced ipsilateral rotations, evoked catalepsy and muscle rigidity, and inhibited locomotor activity. Picrotoxin abolished apomorphine-induced gnawing, and increased haloperidol-mediated catalepsy. The catalepsy induced by an intranigral injection of picrotoxin, and the picrotoxin-evoked blockade of the apomorphine-induced gnawing disappeared within 16 h after the intranigral injection. Alterations in the apomorphine concentration in brain structures (n. caudatus and cerebral cortex) and in spinal cord after picrotoxin injection followed the same time course as the behavioral changes, and returned to the control values 16 h after injection of picrotoxin. Apomorphine was always injected 30 min before the rabbits were killed. Moreover, the substantial increase (to 300%) in apomorphine concentration in the spinal cord probably reflects the antagonism between behavioral changes induced by picrotoxin and the haloperidol catalepsy, rather than the decreased apomorphine concentrations observed in the brain structures. We suggest, therefore, that there exists a correlation between the behavioral effects, which are generally accepted as laboratory models of Parkinson's disease, and the enhanced apomorphine concentration in the spinal cord.

Thalamic mediation of impulses arriving from the substantia nigra

Unilateral injection of carbachol (CCh), 3 mu, to the pars reticularis (SNr) of the substantia nigra evoked electroencephalographic epileptiform discharges in the ventro-lateral thalamic nucleus (VLTh), nucleus caudatus (NC( and substantia nigra (SN) in rabbits. Administration of atropine, 2.5 micrograms, to the VLTh, prior to the CCh injections, completely blocked the epileptiform discharges in all the examined structure. Injections of picrotoxin, 1 microgram, on the border-line between the SNr and pedunculus cerebri (PC) evoked also epileptiform discharges, which were blocked by an intrathalamic injection of GABA, 40 micrograms. It is concluded that both acetylcholine and GABA receptors are present both in the SN and VLTh, that between SN and VLTh there exists functional connection represented most probably by the nigro-thalamic pathway, and that GABA receptors of the VLTh are linked with the endings of the nigro-thalamic pathway while the cholinergic thalamic receptors are connected with another thalamic input.

Nigral output neurons are engaged in regulation of static fusimotor action onto flexors in cat

Picrotoxin and muscimol were unilaterally injected into the postero-lateral part of the reticular zone of substantia nigra (plSNR) through chronically implanted guide cannulae in ketamine-anesthetized cats. Afferent activity of pretibial flexor muscle spindle primary and secondary endings was recorded before and after drug administration, and spindle sensitivity monitored during both sinusoidal and ramp stretch of the receptor-bearing muscle. From changes in spindle sensitivity after drug injection it is deduced that unilateral block by picrotoxin of the action of GABA on postsynaptic receptors in plSNR removes tonic static fusimotor action from flexor muscle spindle primary endings. Secondary endings seemed largely unaffected. The effect on primary endings is reversed by a subsequent injection of muscimol. It is concluded that the central nervous system, through GABA-modulated nigral output neurons, can control static fusimotor action onto flexor muscle spindle primary and secondary endings separately to some extent.

The effects of muscimol and picrotoxin injections into the cat substantia nigra

The behaviour of cats after unilateral injections of muscimol, picrotoxin and bicuculline into the posterior and lateral parts of the substantia nigra was observed. The antagonism between muscimol and picrotoxin was limited to some effects of the drugs: (1) muscimol-induced sniffing, licking and enhanced locomotor activity was attenuated by picrotoxin in contrast to the muscimol-induced contralateral turning which was even enhanced by picrotoxin; (2) picrotoxin-induced balance disorders as well as characteristic motor disturbances in the hind legs remained unaffected following an additional treatment with muscimol. Furthermore, subcutaneously given apomorphine did not affect the picrotoxin-induced balance and hind leg disorders, although the apomorphine-induced stereotyped behaviour itself was enhanced and suppressed by intranigral administration of muscimol and picrotoxin respectively. The conclusion is reached that there are at least two distinct populations of GABA receptors within the substantia nigra: one group controlling or being controlled by dopaminergic neurons and one group operating fully independently of the former neurons. The similarities to experiments on rats are stressed. The mechanism underlying the muscimol-resistant picrotoxin-induced effects (balance disorders and motor disturbance of the hind legs) is discussed.

Effects of intrastriatal injections of atropine and methacholine on the apomorphine-induced gnawing in the rabbit

To find out the anatomical location of the target point of cholinergic-dopaminergic equilibrium, atropine (40 microng) or methacholine (10 microng) were injected through previously implanted cannulas into various places of caudate nucleus and putamen of the rabbit, and the effect of the injections on stereotype gnawing induced by subcutaneously or intravenously administered apomorphine (1--2 mg/kg) was assessed. The intensity of gnawing was measured using a special apparatus, counting each bite. Atropine inhibited the stereotype, while metacholine potentiated it. The effects were evident with the method used, but difficult to reveal with the classical method of assessing the intensity of stereotyped behavior, based on visual observation. The results suggest that the striatum is not a target point for the cholinergic component of the cholinergic-dopaminergic equilibrium in the central nervous system.