Biomedical subjects

W M Burnham

Publications and source records attributed to W M Burnham.

At least 55 records · Page 3Linked to original sources

PubMed1993

Long-term changes in regional brain cytochrome oxidase activity induced by electroconvulsive treatment in rats.

Quantitative cytochrome oxidase (CO) histochemistry was used to examine brain regional metabolic effects of electroconvulsive shock-induced seizures (ECS). Rats receive a course of either eight ECS or control treatments and were sacrificed either 24 h or 28 days after the last session. Regional CO activity (mumol/gT/min) was quantitated throughout the brain using internally calibrated standards. Twenty-four hours after the last ECS session there was no significant difference between ECS- and sham-treated brains in any of the 99 brain regions examined. In contrast, 28 days after the last session, ECS brains showed significant increases in CO activity in the interpeduncular nucleus (+20%), bed nucleus of the stria terminalis (+25%), dorsomedial hypothalamus (+20%), ventromedial hypothalamus (+12%), mammillary nucleus (+14%), pontine nucleus (+16%), basolateral amygdala (+14%), medial amygdala (+12%), piriform cortex (+12%) and ventromedial thalamus (+9%). These results suggest that ECS induces localized increases in brain CO activity which are long-lasting and may develop independently of additional stimulation. The fact that CO changes were predominantly in limbic areas suggests that they may be relevant to the antidepressant effects of ECS.

Animals↗

PubMed1992

Autoradiographic analysis of [35S]TBPS binding in entorhinal cortex-kindled rat brains.

A quantitative autoradiographic analysis of [35S]t-butylbicyclophosphorothionate (TBPS) binding to the gamma-aminobutyric acid (GABA)-mediated chloride ionophore was carried out in 104 brain areas of entorhinal cortex-kindled and control rats. Subjects were sacrificed either 24 h or 28 days after the last kindled seizure. Kindled subjects in the 24 h group showed reductions in mean [35S]TBPS binding in the lateral nucleus of the amygdala (-31%), the infralimbic cortex (-14%), and the paracentral nucleus of the thalamus (-22%). At 28 days, reductions in binding were observed in the infralimbic cortex (-15%) and the paracentral nucleus of the thalamus (-18%). These data suggest that repeated seizures (kindling) modify the GABA-mediated chloride ionophore, and that in some brain areas related to seizure generalization the modifications are very long lasting.

Animals↗

PubMed1992

[3H]Phenytoin identifies a novel anticonvulsant-binding domain on voltage-dependent sodium channels.

The voltage-dependent sodium channel has been proposed as a specific target for the actions of the anticonvulsant drug phenytoin. Working at 0-4 degrees, we previously reported the existence of specific [3H]phenytoin binding sites in rat brain membranes. In the present study, the binding of [3H]phenytoin was assessed at 22 degrees, a temperature favorable to the binding of sodium channel ligands. At 22 degrees, the site had a Kd of 1.5 microM, which is in the relevant therapeutic concentration range for anticonvulsant activity (1-10 microM), and a calculated Bmax of 4.5 pmol/mg of protein, which is similar to previous estimates of sodium channel concentration in brain membranes. In competition experiments, specific [3H]phenytoin binding was found to be inhibited by drugs that interact with the sodium channel, including antiarrhythmics, local anesthetics, anticonvulsants, and site-specific neurotoxins (the steroidal alkaloid activators, beta-scorpion venoms, and brevetoxin-3). Diazepam, used clinically in the management of tonic-clonic status epilepticus, and flunarizine, a calcium channel blocker with anticonvulsant activity, potentiated [3H]phenytoin binding at micromolar concentrations. Other drugs and ligands, including neurotransmitters, neuromodulators, and ligands for other ion channels, had no effect. Depolarization with KCl showed [3H]phenytoin binding to be voltage sensitive. Experiments with batrachotoxin (a specific site 2 toxin) and anticonvulsants demonstrated that the interactions between these compounds and the [3H]phenytoin binding site are allosteric in nature. These results provide direct evidence that phenytoin interacts with the voltage-dependent sodium channel and indicate that such interactions take place at therapeutic concentrations. They support previous proposals, based on toxin-binding and electrophysiological studies, that the therapeutic effects of phenytoin result from a selective inhibition of voltage-dependent sodium flux.

Animals↗

PubMed1992

The anticonvulsant effects of vitamin E: a further evaluation.

Vitamin E (d-alpha-tocopherol) has proven to be a useful adjunct to anticonvulsant drugs in clinical studies. Improvement has occurred even in patients with complex partial seizures, which are often resistant to drug therapy. In animals, vitamin E is effective against ferrous chloride seizures, hyperbaric oxygen seizures and penicillin-induced seizures. It has failed, however, to show anticonvulsant effects in the standard animal models used for drug screening--the maximal electroshock and threshold pentylenetetrazol tests. The present experiments were designed to further explore the anti-epileptic actions of vitamin E in animals. Three models related to complex partial epilepsy were used: 1) the development of amygdala-kindled seizures; 2) the development of electrically-induced status in kindled animals; and 3) the development of kainic-acid seizures. Vitamin E failed to produce significant effects in any of the models.

Animals↗

PubMed1991

The effect of tricyclic antidepressants on cortex- and amygdala-kindled seizures in the rat.

The anticonvulsant effects of amitriptyline, imipramine, nortriptyline and desipramine were tested against focal and generalized seizures, triggered from either the amygdala or the cortex in fully kindled rats. Tests were administered on a 72- or a 24-hour schedule. Significant seizure suppression was achieved with only one drug, amitriptyline, and it occurred only at toxic or near-toxic doses. The differential, low-dose suppression of amygdala-kindled seizures, reported in earlier studies, was not seen in the present experiments. It may occur only in the short-interval test paradigms used by previous experimenters.

Amygdala↗

PubMed1990

Secondary generalization in non-kindled rats following acute administration of GABA-complex and adenosine antagonists.

In order to test the GABA hypothesis of kindling, GABA-complex antagonists were administered in a dose-response paradigm to rats that had been implanted with indwelling forebrain electrodes, but not kindled. Focal seizures were then elicited from either the cortex or the amygdala to see whether kindling-like secondary generalization would occur. Norharmane, a benzodiazepine inverse agonist, failed to promote secondary generalization from either the cortex or the amygdala. Bicuculline, a GABAA receptor antagonist, and picrotoxin, a chloride ionophore antagonist, enhanced generalization from both sites and, in amygdala-implanted subjects, appeared to produce a significant acceleration of kindling as well. Aminophylline, an adenosine antagonist tested for purposes of comparison, also enhanced secondary generalization from both sites, and in amygdala-implanted subjects produced long electrographic discharges which sometimes developed into status epilepticus.

Adenosine↗

PubMed1990

Regional brain [3H]muscimol binding in kindled rat brain: a quantitative autoradiographic examination.

Quantitative [3H]muscimol autoradiography was used to perform a comprehensive examination of the GABAA receptor site in entorhinal cortex kindled and handled control rats. Ninety-eight brain areas were analyzed 24 h or 28 days after the last kindled seizure. A significant post-kindling change was observed in only one area, the fascia dentata of the hippocampus, which showed an increase of 55% at 24 h, but not at 28 days. This acute change appeared in both the dorsal and ventral divisions of the hippocampus and in both the infrapyramidal and suprapyramidal blades of the fascia dentata. These data closely parallel the pattern previously obtained with [3H]flunitrazepam in the same animals. The comprehensive nature of this examination further suggests that the fascia dentata is the only site in the brain which shows altered GABAA binding after kindling. Although our data indicate that elevated [3H]muscimol binding is not a permanent feature of the kindled state, the possibility exists that the fascia dentata change observed at 24 h reflects a permanent alteration in the acute reactivity of hippocampal GABAA receptors to electrical stimulation.

Animals↗

PubMed1989

Autoradiographic analysis of benzodiazepine binding in entorhinal-kindled rat brains.

A quantitative autoradiographic analysis of binding to 'central' benzodiazepine ([3H]flunitrazepam) receptors was carried out in 95 brain areas from entorhinal-kindled and handled control rats. Animals were sacrificed either 24 h or 28 days after the last kindled seizure. A significant post-kindling change was observed in only one area, the fascia dentata, which showed an elevation of 31% at 24 h, but not at 28 days. This change, which appeared in both the dorsal and ventral divisions of the hippocampus, was largest in the suprapyramidal layer. These data confirm previous reports of transient post-kindling binding changes in the fascia dentata, and extend them by suggesting that the fascia dentata is the only site in the brain which shows altered benzodiazepine binding after kindling. These transient and circumscribed changes are unlikely to contribute significantly to the 'kindled state', although they may relate to the enhancement of inhibitory effects observed in fascia dentata after kindling.

Animals↗

PubMed1989

Effects of phenytoin, carbamazepine, and clonazepam on cortex- and amygdala-evoked potentials in partially kindled rats.

The kindling technique has been reported to produce a long-lasting enhancement in both the early and late phases of evoked potentials triggered from the kindled focus. It also alters paired-pulse facilitation and depression in the pathways which support these phenomena. The present experiment was designed to determine whether the drugs which antagonize secondary generalization in the kindling model also antagonize kindling-enhanced excitation in the pathways leading out of the focus. Multiple doses of phenytoin, carbamazepine, and clonazepam were therefore tested against single- and double-pulse evoked potentials triggered from the focus in rats that had been subjected to parital kindling from either the amygdala or the cortex. Responses were recorded in monosynaptic sites and in the mesencephalic reticular formation--a polysynaptic site thought to play an important role in secondary generalization. No drug-related effects were found on early evoked potential components, either in the single-pulse or the double-pulse paradigm. Kindling-enhanced late components ("late waves"), however, were clearly and selectively antagonized by clonazepam.

Amygdala↗

PubMed1989

The GABA hypothesis of kindling: recent assay studies.

The GABA (gamma-aminobutyric acid) hypothesis of kindling suggests that the permanent changes caused by the kindling procedure result from a loss of GABA-mediated inhibition. Pharmacological studies have generally supported this hypothesis: GABA-complex antagonists accelerate (or stimulate) kindling, whereas GABA-complex agonists retard (or reverse) it. Assay studies, however, have presented an inconsistent picture. Earlier studies found no GABAergic brain changes after kindling, whereas recent studies have reported postkindling changes in a number of GABA-related parameters. The crucial difference seems to be that earlier studies assayed GABA parameters in "whole tissue," whereas recent studies have concentrated on "synaptic" GABA. As indicated by recent studies, when the "metabolic pool" is excluded, kindled subjects show a variety of persistent abnormalities in the GABA system. These data are generally consistent with the GABA hypothesis of kindling.

Animals↗

PubMed1988

Brain neurotransmitters in glycine encephalopathy.

We measured neurotransmitter markers in autopsied brain of infants with glycine encephalopathy (GE). Because patients with GE develop intractable seizures, special attention was devoted to those neurotransmitter systems implicated in human epilepsy. Mean levels of glycine in the frontal cortex of GE patients were three times higher than control values. No abnormalities were observed for concentrations of gamma-aminobutyric acid (and related receptors), other major neurotransmitter amino compounds, or activities of cholineacetyltransferase and aspartate aminotransferase. Mean acetylcholinesterase activity was significantly elevated by 46%. As experimental data suggest, glycine markedly potentiates the action of the excitatory neurotransmitter glutamic acid. To the extent that the brain seizures in patients with GE can be explained by this mechanism, pharmacotherapy with excitatory amino acid antagonists may represent a new approach to the treatment of GE.

Amino Acid Metabolism, Inborn Errors↗

PubMed1987

The effect of repeated seizures on anticonvulsant drug response in the kindling model.

Clinical reports suggest that the occurrence of numerous seizures prior to the onset of anticonvulsant therapy is associated with a poor prognosis for drug control. This seems to imply that seizures become harder to suppress as they recur--a possibility which argues for the early initiation of anticonvulsant therapy. The present study attempted to determine whether this effect could be demonstrated under controlled laboratory conditions. The kindling preparation, an animal model involving the elicitation of repeated focal seizures, was used. Drug response (ED50) was measured in Royal Victoria hooded rats after either a small or a large number of pretreatment seizures, administered in the kindling paradigm. A variety of experiments were performed, involving: different drugs (phenobarbital, phenytoin, and carbamazepine), different seizure types (amygdala focal seizure, cortical focal seizure, generalized convulsion), and different stimulation parameters (0, 20, 40 or 100 pretreatment seizures: 'near-threshold' vs. 'standard' stimulation). In no case were seizures found to be harder to suppress following repeated pretreatment seizures. After large numbers of pretreatment seizures (40, 100), drug response was actually enhanced. These data indicate that the mere repetition of seizures does not automatically lead to a decrease in anticonvulsant effectiveness. They offer no particular rationale for the early initiation of anticonvulsant therapy in clinical situations.

Animals↗

PubMed1986

Unchanged norepinephrine turnover and concentrations in amygdala-kindled rat brain regions 2 months postseizure.

Norepinephrine turnover and concentrations were examined in eight brain regions of amygdala-kindled and yoked control rats. The subjects were killed 2 months after the kindled subjects had exhibited their sixth "stage 4-5" generalized seizure. No significant differences were found in norepinephrine turnover or concentrations between kindled and yoked control rats in any of the brain regions examined. Dopamine turnover and concentrations in brain regions were also unchanged in kindled rats compared with their yoked controls. The data suggest that amygdala kindling does not produce any long-lasting alterations in brain regional catecholamine turnover or concentrations.

Amygdala↗

PubMed1985

Core mechanisms in generalized convulsions.

A hypothetical model is proposed to account for the generalized convulsions observed in rats. Two versions of the hypothesis are discussed: an earlier, more specific formulation that accounts for kindled convulsions, and a later, general version that applies to generalized convulsions as a whole. Observations leading to the specific formulation of the model included: 1) kindling data that suggested a single downstream center responsible for kindled convulsions; 2) brain-stem stimulation data that indicated that the reticular core of that structure could initiate and maintain generalized convulsive behavior; and 3) spinal hemisection data that indicated that nonspecific systems in the cord could also maintain convulsions, even in the absence of direct input from the brain. The more specific version of the model suggests that kindled convulsions are nonspecific core seizures that occur when self-sustained epileptic activation spreads from forebrain foci to involve descending polysynaptic pathways in the nonspecific core of the brain stem and cord. Observations leading to a more general formulation of the model include the facts that: 1) maximal and submaximal convulsions in a variety of models resemble each other; and 2) they also resemble the maximal and submaximal seizures produced by direct stimulation of the brain stem and cord. The more general formulation of the nonspecific core hypothesis suggests that a wide variety of convulsions in rats may be nonspecific core seizures, differences in tonic-clonic and rostrocaudal configuration being related primarily to differences in the intensity of core activation.

Animals↗

PubMed1984

Benzodiazepine receptor binding following amygdala-kindled convulsions: differing results in washed and unwashed brain membranes.

The binding of [3H]flunitrazepam and [3H]RO5-4864 was measured in unwashed brain homogenates and in extensively washed brain membranes from amygdala-kindled and "yoked" control rats sacrificed 2 weeks following the sixth stage 5 convulsion. In unwashed homogenates, [3H]flunitrazepam binding was reduced in both the hypothalamus and ipsilateral right cortex of kindled rats (unchanged in other areas). In washed brain membranes, [3H]flunitrazepam binding was unaltered in these regions; it was bilaterally elevated, however, in both the amygdala and hippocampus (unchanged in other areas). In washed membranes, the in vitro addition of gamma-aminobutyric acid enhanced [3H]flunitrazepam binding to a similar extent in kindled and control membranes. These data indicate that the type of benzodiazepine binding abnormality observed after kindling depends on the type of tissue preparation employed in the assay procedure.

Amygdala↗

PubMed1983

Binding of [3H]flunitrazepam and [3H]Ro5-4864 to crude homogenates of amygdala-kindled rat brain: two months post-seizure.

The binding of [3H]flunitrazepam and [3H]Ro5-4864 to crude homogenates of amygdala-kindled and 'yoked' control rat brains was evaluated in subjects sacrificed 60 days after the sixth generalized convulsion. Decreases in the Bmax for [3H]flunitrazepam binding to 'central-type' benzodiazepine receptors were observed in the hypothalamus and ipsilateral cortex of kindled animals. The binding of [3H]Ro5-4864 to 'peripheral-type' benzodiazepine receptors was unchanged.

Amygdala↗

PubMed1983

Enhancement of 3H-phenytoin binding by diazepam and (+)bicuculline.

3H-Phenytoin binding to the particulate fraction of rat wholebrain homogenate was studied using the filter assay technique. It was found that diazepam and (+)bicuculline methobromide caused a concentration-dependent enhancement of the total binding of 3H-phenytoin, whereas GABA and (-)bicuculline methobromide (the inactive bicuculline isomer) had no effect. In subsequent competition experiments (labelled versus unlabelled phenytoin), it was found that the presence of a potentiating concentration of diazepam transformed the biphasic phenytoin competition isotherm into a simple curve with a Hill coefficient of approximately one, and a Ki of 110 nM.

Animals↗

PubMed1983

Decreased benzodiazepine receptor binding in amygdala-kindled rat brains.

3H-Flunitrazepam (3H-FLU) binding was measured in multiple brain regions of amygdala-kindled rats two weeks following the sixth Stage 5 convulsion. As compared to 'yoked' controls, the kindled animals displayed significant reductions in 3H-FLU binding in the ipsilateral cortex (20%) and in the hypothalamus (20%). Scatchard plots revealed that these reductions were due to changes in the maximal number of available binding sites (Bmax) rather than to alterations in receptor affinity (KD). No significant changes were found in the contralateral cortex, or in either the contralateral or ipsilateral amygdala, hippocampus or striatum. These data suggest that kindling is associated with long-lasting changes in the benzodiazepine receptor system and possibly with related changes in GABA-mediated neural inhibition.

Amygdala↗