Excitatory neurotransmitters in kindling: Excitatory amino acid, cholinergic, and opiate mechanisms

The role of excitatory neurotransmitters in kindling is selectively reviewed. Evidence from a variety of approaches strongly indicates that muscarinic cholinergic and excitatory amino acid neurotransmission contributes to electrical kindling in the rat. Among the latter group of neurotransmitters, N-m-d-aspartate receptors are known to play a role, but quisqualate and kainate receptors also may play a role. Delta and epsilon opiate receptors appear to be capable of mediating the development of kindled seizures, but their contribution to electrical kindling may be less than that of excitatory amino acid and cholinergic mechanisms. However, none of these systems is critical for kindling since antagonism of any one of them only retards kindling at best, but does not completely block it. Antagonism of both N-m-d-aspartate and muscarinic receptors results in greater retardation of electrical kindling than does antagonism of either receptor type alone, which implicates a mechanism involving the summation of excitatory neurotransmission in kindling. The use of the N-m-d-aspartate receptor antagonist aminophosphonovaleric acid for the pharmacological dissociation of kindling and long-term potentiation indicates that long-term potentiation in the perforant path-dentate circuit is critically dependent upon N-m-d-aspartate neurotransmission, but kindling is not. This suggests that long-term potentiation is not a critical element in the mechanism of kindling in this circuit.     

Excitatory neurotransmitters in the lateral habenula and pedunculopontine nucleus of rat modulate limbic seizures induced by pilocarpine.

The involvement of the excitatory neurotransmitter system in the lateral habenula and pedunculopontine nucleus in the initiation and propagation of limbic seizures induced by pilocarpine has been investigated in the rat. Limbic seizures occur in animals following bilateral microinjection into the lateral habenula of N-methyl-D-aspartate (NMDA) (5 and 12.5 nmol) or kainate (100 and 200 pmol), 15 min prior to a subconvulsant dose of pilocarpine (150 mg/kg, i.p.). In the absence of pilocarpine NMDA (5 and 12.5 nmol) or kainate (100 and 200 pmol), injected focally into the lateral habenula or pedunculopontine nucleus, produced sniffing, grooming and tremor but no electrographic or behavioural seizures. Limbic seizures also occur after a subconvulsant dose of pilocarpine when it is preceded by injection of NMDA (5 and 12.5 nmol) or kainate (50, 100 and 200 pmol) into the pedunculopontine nucleus. Behavioural and electrographic signs of limbic seizures following pilocarpine (380 mg/kg, i.p.) were attenuated or completely antagonized by focal injection into the lateral habenula of the NMDA antagonist, 2-amino-7-phosphonoheptanoate (AP7) (10 and 50 pmol) or kainate antagonist, gamma-D-glutamylaminomethylsulphonate (GAMS) (20 nmol). In addition, AP7 (0.05, 0.1 and 1.0 nmol) or GAMS (40 nmol) injected into the pedunculopontine nucleus suppressed limbic seizures induced by i.p. administration of pilocarpine (380 mg/kg). The relative efficacy of NMDA and non-NMDA receptor antagonists revealed that the selective NMDA antagonist, AP7, was more potent in its anticonvulsant activity in comparison to GAMS, a non-NMDA receptor antagonist.     

Antipsychotic drugs, neurotransmitters, and schizophrenia.

Inhibition of central dopamine functions appears to be a common basic property of antipsychotic drugs. The mesolimbic and nigrostriatal portions of the dopaminergic system are probably the main targets for the mental and the extrapyramidal actions, respectively, of these drugs. The fact that dopaminergic hyperfunction induced by amphetamines or dopa may lead to a disturbance mimicking paranoid schizophrenia lends further support for a key role of dopamine in mental functions. Although a primary disturbance in dopamine function in schizophrenia cannot be ruled out, the intimate relationship between dopaminergic and other neuronal systems must be emphasized. The possible involvement of other amine, amino acid, or peptide transmitters in schizophrenia cannot be disregarded.     

Corelease of two functionally opposite neurotransmitters by retinal amacrine cells: experimental evidence and functional significance.

The Dale's law postulates that a neuron releases the same neurotransmitter from all its branches. In the case of multiple neurotransmitters it would require all transmitters to be released from all branches. The retinal cholinergic amacrine cells contain and release gamma-aminobutyric (GABA) and, therefore, if GABA and acetylcholine (ACh) are released at the same sites, this could mean that amacrine cells simultaneously excite and inhibit postsynaptic cells. Conversely, if the two neurotransmitters are released at different synapses, or if their release is regulated in a distinct manner, they may play different physiological roles. Recent studies carried out in cultured cholinergic amacrine-like neurons showed that Ca(2+)-dependent release of ACh and GABA have a different sensitivity to membrane depolarization, to the effect of blockers of voltage gated Ca(2+) channels (VGCC) and to the effect of presynaptic A(1) adenosine receptors. Therefore, it is proposed that in retinal amacrine cells the Ca(2+)-dependent release of ACh and GABA occurs at distinct cellular locations. The possible nature of these release sites and the physiological significance of this model are discussed in this review.     

Pharmacology of a potent, new antithrombotic agent, 1,3-dihydro-7,8-dimethyl-2H-imidazo[4,5-b]quinolin-2-one (BMY-20844)

The effects of 1,3-dihydro-7,8-dimethyl-2H-imidazo[4,5-b]quinolin-2-one (BMY-20844) on platelet function and experimental thrombosis were evaluated in a series of in vitro, ex vivo and in vivo experiments. The compound inhibited platelet aggregation in vitro in platelet rich plasma obtained from humans, rats and rabbits with EC50s of less than 1 microgram/ml when aggregation was induced by ADP, collagen or thrombin. Supra-additive interaction against ADP aggregation was also observed when BMY-20844 was combined with prostacyclin. BMY-20844 was orally active with an ex vivo ED50 in the rat of 3.2 mg/kg vs ADP. Significant antithrombotic activity was observed in two animal models (laser induced thrombosis in the microcirculation of the rabbit ear and coronary artery thrombosis in the dog). Inhibitions of 52% at 3 mg/kg p.o. in the laser model and 100% at 1 mg/kg i.d. in the coronary artery thrombosis model were obtained. Modest inotropic and hemodynamic effects were observed in ferrets and dogs. BMY-20844 was found to be a potent, specific inhibitor of platelet low Km cyclic AMP phosphodiesterase.     

Effects of a new postnatal stress model on monoaminergic neurotransmitters in rat brains

Introduction: Stress and environmental perturbations influence postnatal brain development and may account for the high disability rates of preterm survivors following intensive care treatment. This study aims to investigate the impact of early environmental factors on the monoaminergic neurotransmitter system in the developing rat brain by using an innovative neonatal stress model. Materials and methods: After birth, in the experimental groups newborn rats were separated from their mothers and exposed to different stressful stimuli four times a day on day P0 to P6 for 10 min each. To mimic intensive care treatment, the stress protocol applied environmental factors like bright light, noise, and low temperature alternating with pain and handling stress at day- and night-time in a varying sequence. The non-stressed control mothers and litters were left completely undisturbed until sacrificing on day P7 or P20. Results: Brains of stressed animals revealed significantly higher levels of norepinephrine (NE) and dopamine (DA) as determined by HPLC-ED and electrochemical detection at day P7 as compared to controls. When returned to their mothers’ undisturbed care, juvenile rats at day P20 still showed higher (yet statistically not significant) concentrations of NE and DA in brain. The stressed animals gained less weight with significantly lower body weights at day P7 compared to controls. Their mothers developed various forms of stressed behaviour. Conclusions: A novel animal model for postnatal intensive care stress was established leading to changes in brain monoamine levels of newborn rats, while undisturbed maternal care seems to moderate the stress effects subsequently.     

Effects of subchronic exposure to benzo[a]pyrene (B[a]P) on learning and memory, and neurotransmitters in male Sprague-Dawley rat

The harmful effects of the environmental carcinogen, benzo[a]pyrene (B[a]P), on mammalian neurodevelopment and behavior as yet remain unclear. Several studies have suggested that B[a]P impairs learning and memory. In the present investigation, we investigated the effects of subchronic exposure to B[a]P on rats. Male rats received daily injection of B[a]P (0, 1.0, 2.5, and 6.25 mg/kg, i.p.) or vehicle for 13 weeks. Employing the Morris water maze (MWM) test, we observed that rats exposed to either 2.5 mg/kg or 6.25 mg/kg B[a]P had modified behavior compared to controls as indicated by the increased mean latencies, the decreased number of crossing platform and the decreased swimming time in the target area. B[a]P treatment decreased the levels of malondialdehyde (MDA), nitric oxide (NO), nitric oxide synthase (NOS), superoxide dismutase (SOD), acetylcholine (ACh), choline acetyltransferase (ChAT), and increased the activity of acetylcholinesterase (AChE). Endogenous monoamine levels, norepinephrine (NE), adrenaline (A), dopamine (DA) and 5-hydroxytryptamine (5-HT) and their selected metabolites dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5-HIAA) in hippocampus were measured using high performance liquid chromatography (HPLC). B[a]P at both doses, 2.5 and 6.25 mg/kg, increased NE, DA, DOPAC and 5-HT content in the hippocampus. Our results suggested a close link between the modified levels of neurotransmitters in the hippocampus and the impaired behavioral performance, indicating that B[a]P is a potential neurotoxic pollutant.     

The modulation of the functional properties of the skin thermoreceptors]

The characteristic property of skin cold receptors is the presence of static and dynamic activity with their maximum at any skin temperature. Parallel with the common properties there are some differences in the temperature range of receptors' functioning, frequency of static and dynamic activity, concentration of sensitive receptors on the skin. Mechanisms of these differences are unknown. It is found that the character of skin cold receptors functioning can be changed in individual life of the organism. Some parameters of static and dynamic activity as well as concentration of cold receptors functioning change both with adaptation of the organism to cold and with a rise of the noradrenaline concentration in blood. The reason of adaptive modifications of skin thermoreceptors may be an increase of their sensitivity to noradrenaline observed with adaptation to cold. It can be supposed that the modulating influence of biological active substances is very important in forming activity and sensitivity of skin thermoreceptors.     

The actions of amphetamine on neurotransmitters: a brief review.

The central stimulant actions of d-amphetamine are not altered in animals in which brain stores of catecholamines have been depleted with reserpine, but they are blocked by alpha-methyltyrosine, which inhibits catecholamine synthesis. The results of a variety of experiments suggest that the central actions of amphetamine result primarily from the ability of the drug to facilitate the release of newly synthesized dopamine from nerve terminals in the forebrain. The results of experiments in animals in which dopaminergic nerve terminals in various brain regions have been selectively destroyed by intracranial microinjection of 6-hydroxydopamine reveal that the locomotor stimulant actions of relatively low doses of amphetamine are dependent upon mesolimbic dopaminergic neurons, whereas the stereotyped behaviors induced by relatively larger doses of amphetamine are dependent upon nigrostriatal dopaminergic neurons. The central actions of amphetamine appear to be the primary result of interactions with dopamine neurons, but secondarily the drug also alters the dynamics of other putative neurotransmitters (e.g. acetylcholine, 5-hydroxytryptamine) in the brain.     

Development and Characterization of NEX- Pten, a Novel Forebrain Excitatory Neuron-Specific Knockout Mouse

The phosphatase and tensin homolog located on chromosome 10 (PTEN) suppresses the activity of the phosphoinositide-3-kinase/Akt/mammalian target of rapamycin (mTOR) pathway, a signaling cascade critically involved in the regulation of cell proliferation and growth. Human patients carrying germ line PTEN mutations have an increased predisposition to tumors, and also display a variety of neurological symptoms and increased risk of epilepsy and autism, implicating PTEN in neuronal development and function. Consistently, loss of Pten in mouse neural cells results in ataxia, seizures, cognitive abnormalities, increased soma size and synaptic abnormalities. To better understand how Pten regulates the excitability of principal forebrain neurons, a factor that is likely to be altered in cognitive disorders, epilepsy and autism, we generated a novel conditional knockout mouse line (NEX-Pten) in which Cre, under the control of the NEX promoter, drives the deletion of Pten specifically in early postmitotic, excitatory neurons of the developing forebrain. Homozygous mutant mice exhibited a massive enlargement of the forebrain, and died shortly after birth due to excessive mTOR activation. Analysis of the neonatal cerebral cortex further identified molecular defects resulting from Pten deletion that likely affect several aspects of neuronal development and excitability.     

Evaluation of functional response of cerebral arteries by a new morphometric technique.

The contractile response of the rabbit basilar artery under four conditions was determined: (1) response in a resting condition without exclusion of the sympathetic nervous system (control group I); (2) response in a resting condition with alpha-adrenoceptor blockade (group II); (3) response to subarachnoid haemorrhage (SAH) (group III); and (4) response to SAH with alpha-adrenoceptor blockade (group IV). It was also ascertained whether it was possible to measure contractile response using a new morphometric method. Vessels were prepared by intracardial perfusion fixation, stained by haematoxylin and eosin, and the length of the intimal corrugations were measured by computer image analysis. Two procedures were followed in order to express the intensity of intimal corrugation, indicating the contractibility of the basilar arteries: (1) the corrugation coefficient (CC) of the basilar artery intima was estimated by dividing the precisely measured length of the intimal corrugations by the length of the measured vessel wall section of the vessel cross-sections (obtained histologically); (2) the lumen reduction coefficient (LRC) of the basilar artery was determined by dividing the "ideal" luminal area (calculated from the total length of the intimal circumference) by the real luminal cross-section area. The results of CC measurements revealed the smoothest intima (mean CC = 1.146, P = 0.00) and the least reduction of lumen (mean LRC = 0.26, P = 0.000) in group II (rabbits without SAH but with alpha-blocker phenoxybenzamine), and in group IV (SAH group of rabbits with alpha-blocker phenoxybenzamine) where the mean CC was 1.141 (P = 0.001) and the mean LRC was 0.33 (P = 0.002) in comparison with the SAH-only group III, pointing out the effectiveness of alpha-blockade even against SAH vasospastic stimuli. Control group I (without SAH and without treatment) showed a greater degree of corrugation in the intima and an increased reduction in the lumina than in groups II and IV, but still significantly less than in group III (mean CC = 1.197, P = 0.001, and mean LRC = 0.40, P = 0.028), thus demonstrating a certain resting tone of the basilar arteries (in an ideal situation, without any tone at all, the CC and LRC would be equal one). The highest degree of intimal corrugation and the greatest lumina reduction were discovered in the SAH-only group III (mean CC = 1.374 and mean LRC = 0.60). The differences among groups I, II and IV were insignificant. The results of this study suggest four conclusions: (i) the possibility of evaluating the functional response of rabbit cerebral arteries using this new morphometric technique; (ii) the adrenergic influence on resting tone of these arteries; (iii) the likely preventive role of an alpha-blockade on post-SAH vasospasm of basilar arteries in rabbits; and (iv) good comparability of the results of CC and LRC measurements with the angiographically estimated vessel diameters of other similar studies.     

Centromuclear myopathy with late clinical manifestations. Clinical, histological and ultrastructural study of a new case]

A new case of late-onset centronuclear myopathy is reported. Clinical manifestations only occurred after 27 years age and consisted in diffuse muscle weakness without any ocular involvement. No other case was found in the family. In muscle biopsies the frequence of central nuclei was close to 100 p. 100. With the myofibrillar A.T.Pase reaction, almost all the fibers were type I. Furthermore, the electron microscopic study showed frequent vacuoles and splitting aspects of the muscle fibres, with "en passant" myotendinous junctions. Unusual granular bodies were found near the basement membrane, in particular within the subneural folds of the end-plates.     

Pharmacology of LY201409, a Potent Benzamide Anticonvulsant

LY201116 [4-amino-N-(2,6-dimethylphenyl)benzamide], an effective anticonvulsant in several animal models, is rapidly metabolized by N-acetylation in rats, mice and monkeys. In an attempt to preclude metabolic N-acetylation sterically, we investigated LY201409, an analogue possessing two methyl groups ortho to the 4-amino substituent. This structural modification successfully altered the metabolic pathway, and LY201409 displayed potent anticonvulsant activity. LY201409 antagonized maximal electroshock (MES)-induced seizures with ED50 values of 16.2 and 4.2 mg/kg after oral administration to mice and rats, respectively. The compound did not effectively antagonize seizures induced by a variety of chemical convulsants in rats, but did block pentylenetetrazol-induced seizures in mice. Thus, among the classic anticonvulsants, the profile of phenytoin most closely resembles that of LY201409. Studies conducted with the rotorod and horizontal screen assays in mice and behavioral studies in rats suggested that doses of LY201409 that produced CNS side-effects such as sedation or ataxia were well separated from the anti-MES doses. LY201409 was a potent, dose-dependent potentiator of hexobarbital-induced sleeping time in mice. Oral administration of 6.0 mg/kg led to a 372% increase in sleep time relative to control values. Although LY201409 is a potent and effective anticonvulsant, it is also one of the most potent potentiators of hexobarbital-induced sleep time yet described.