The serine protease subtilisin suppresses epileptiform activity in rat hippocampal slices and neocortex in vivo.

Serine proteases of the S8A family and those belonging to the subtilase group generate a long-lasting inhibition of hippocampal evoked potentials, which shows little recovery and resembles long-term depression. The present work investigates the effects of subtilisin A on epileptiform activity induced in hippocampal slices. Interictal bursts were generated by perfusion with 4-aminopyridine in magnesium-free medium, whereas ictal bursts were produced by the addition of baclofen. Subtilisin A superfused for 10 min at concentrations of 50 nM and above reduced the duration of ictal bursts, whereas higher concentrations reduced the frequency of interictal activity with little or no recovery, indicating similarity with the long-term depression reported previously. The anti-epileptiform activity was not prevented by inhibitors of phosphatases or several kinases, but the inhibition of ictal activity was selectively reduced by the tyrosine kinase inhibitor genistein. The rho-activated coiled-coil kinase (ROCK) inhibitor Y-27632 had no effect on the suppression of ictal or interictal bursts. Subtilisin applied at nanomolar concentrations to the surface of the cerebral cortex in vivo also suppressed epileptiform spikes induced by bicuculline. It is concluded that serine proteases of the subtilase group are highly potent inhibitors of epileptiform activity, especially ictal bursts, and that tyrosine kinases may be involved in that inhibition. The mechanism of inhibition is different from the long-lasting depression of evoked potentials, which is partly mediated via ROCK.

Learning may provide neuroprotection against dementia.

A number of studies attempting to identify specific risk factors for dementia have noted an inverse relationship between educational background and the likelihood of developing dementia. This idea has been somewhat controversial as educational background can introduce a number of confounding factors that generally affect health and lifestyle. Despite these reservations, there is mounting evidence to support the concept of education (or increased mental activity) producing a functional reserve in the brain, a process that provides some protection against the clinical manifestation of dementia. Long-term potentiation (LTP) is a recognized neural correlate of learning and memory. We have shown recently that LTP reduces the sensitivity of hippocampal neurons to agonists of the neurotransmitter glutamate; additionally, we have reported that LTP protects the neurons from the effects of acute hypoxia. Given that the effect of hypoxia on neurons involves over-stimulation by glutamate, and hypoxia has been implicated in the aetio-pathology of some types of dementia, our observations suggest that LTP has a protective effect on neuronal tissue. Such an interaction offers a physiological basis for the epidemiological evidence that lifelong learning can protect a person from some types of dementia.

Learning may provide neuroprotection against dementia

A number of studies attempting to identify specific risk factors for dementia have noted an inverse relationship between educational background and the likelihood of developing dementia. This idea has been somewhat controversial as educational background can introduce a number of confounding factors that generally affect health and lifestyle. Despite these reservations, there is mounting evidence to support the concept of education (or increased mental activity) producing a functional reserve in the brain, a process that provides some protection against the clinical manifestation of dementia. Long-term potentiation (LTP) is a recognized neural correlate of learning and memory. We have shown recently that LTP reduces the sensitivity of hippocampal neurons to agonists of the neurotransmitter glutamate; additionally, we have reported that LTP protects the neurons from the effects of acute hypoxia. Given that the effect of hypoxia on neurons involves over-stimulation by glutamate, and hypoxia has been implicated in the aetio-pathology of some types of dementia, our observations suggest that LTP has a protective effect on neuronal tissue. Such an interaction offers a physiological basis for the epidemiological evidence that lifelong learning can protect a person from some types of dementia.

Long-term potentiation protects rat hippocampal slices from the effects of acute hypoxia.

We have previously shown that long-term potentiation (LTP) decreases the sensitivity of glutamate receptors in the rat hippocampal CA1 region to exogenously applied glutamate agonists. Since the pathophysiology of hypoxia/ischemia involves increased concentration of endogenous glutamate, we tested the hypothesis that LTP could reduce the effects of hypoxia in the hippocampal slice. The effects of LTP on hypoxia were measured by the changes in population spike potentials (PS) or field excitatory post-synaptic potentials (fepsps). Hypoxia was induced by perfusing the slice with (i) artificial CSF which had been pre-gassed with 95%N2/5% CO2; (ii) artificial CSF which had not been pre-gassed with 95% O2/5% CO2; or (iii) an oxygen-glucose deprived (OGD) medium which was similar to (ii) and in which the glucose had been replaced with sucrose. Exposure of a slice to a hypoxic medium for 1.5-3.0 min led to a decrease in the PS or fepsps; the potentials recovered to control levels within 3-5 min. Repeat exposure, 45 min later, of the same slice to the same hypoxic medium for the same duration as the first exposure caused a reduction in the potentials again; there were no significant differences between the degree of reduction caused by the first or second exposure for all three types of hypoxic media (P>0.05; paired t-test). In some of the slices, two episodes of LTP were induced 25 and 35 min after the first hypoxic exposure; this caused inhibition of reduction in potentials caused by the second hypoxic insult which was given at 45 min after the first; the differences in reduction in potentials were highly significant for all the hypoxic media used (P<0.01; paired t-test). The neuroprotective effects of LTP were not prevented by cyclothiazide or inhibitors of NO synthetase compounds that have been shown to be effective in blocking the effects of LTP on the actions of exogenously applied AMPA and NMDA, respectively. The neuroprotective effects of LTP were similar to those of propentofylline, a known neuroprotective compound. We conclude that LTP causes an appreciable protection of hippocampal slices to various models of acute hypoxia. This phenomenon does not appear to involve desensitisation of AMPA receptors or mediation by NO, but may account for the recognised inverse relationship between educational attainment and the development of dementia.

Interactions of glutamate receptor agonists with long-term potentiation in the rat hippocampal slice.

Previous work has described the apparent desensitisation of neuronal networks in the rat neocortex to amino acid agonists, following prior exposure several minutes earlier. Since long-term potentiation is believed to involve activation of amino acid receptors, we have now sought to determine whether long-term potentiation can modify the sensitivity of neurones to glutamate receptor agonists in rat hippocampal slices. Responses were measured as the change in population spike or postsynaptic potential (e.p.s.p.) size. Two applications of N-methyl-D-aspartate (NMDA), quinolinic acid, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or kainate, 45 min apart, did not exhibit any apparent desensitisation. However, the induction of long-term potentiation produced a marked loss of sensitivity to quinolinic acid, with smaller effects on NMDA, AMPA and kainate responses. No marked changes were obtained of e.p. s.p. size. In order to localise the cellular sites of these changes, agonists were also applied by microiontophoresis to the cell bodies or dendritic regions of CA1 neurones. Responses to quinolinic acid showed apparent desensitisation at both sites, whereas no decrease was observed in responses to NMDA or AMPA application. The induction of long-term potentiation again produced a decrease in the size of responses to NMDA and AMPA. Inhibition of nitric oxide (NO) synthase prevented the long-term potentiation-induced loss of responsiveness to NMDA, but not AMPA, implying a role for NO in the loss of NMDA sensitivity. Recordings of single cell activity during the iontophoretic application of agonists and induction of long-term potentiation showed that responses to NMDA were often suppressed to a greater extent than to quinolinic acid. The results indicate that long-term potentiation can modify the sensitivity of hippocampal neurones to glutamate receptor agonists, and that differences exist in the pharmacology of NMDA and quinolinic acid.

NMDA-induced changes in a cortical network in vivo are prevented by AMPA.

Analogues of glutamic acid including N-methyl-D-aspartic acid (NMDA) depolarise neurones of the cerebral cortex in vivo and thus change the size of the somatosensory evoked potentials (SEPs). The potentials recover rapidly despite maintained superfusion with NMDA, suggesting a form of neuronal desensitisation or network adaptation. In this study potentials were evoked at the cortical surface by electrical stimulation of the contralateral forepaw and compounds applied topically to the cortical surface by a cortical cup. NMDA at 50-250 microM caused a concentration-dependent decrease in the amplitude of the SEPs, with the highest concentration always abolishing them. AMPA at 50 microM did not affect evoked potentials when applied alone, but prevented the NMDA. Such AMPA-NMDA interactions were inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and enhanced by cyclothiazide (which prevents AMPA desensitisation). Superfusion with potassium did not change sensitivity to NMDA. These results suggest that, in the rat cerebral cortex in vivo, activation of AMPA receptors can induce a loss of the network response to activation of NMDA receptors. Such a phenomenon may have physiological and therapeutic implications.

Interactions of glutamate receptor agonists with long-term potentiation in the rat hippocampal slice

Previous work has described the apparent desensitisation of neuronal networks in the rat neocortex to amino acid agonists, following prior exposure several minutes earlier. Since long-term potentiation is believed to involve activation of amino acid receptors, we have now sought to determine whether long-term potentiation can modify the sensitivity of neurones to glutamate receptor agonists in rat hippocampal slices. Responses were measured as the change in population spike or postsynaptic potential (e.p.s.p) size. Two applications of N-methyl-D-aspartate (NMDA), quinolinic acid, teta-amino-3-hydroxy-5methyl-4-isoxazolepropionic acid (AMPA) or kainate, 45 min apart, did not exhibit any apparent desensitisation. However, the induction of long-term potentiation produced a marked loss of sensitivity to quinolinic acid, with smaller effects on NMDA,AMPA and kainate responses. No marked changes were obtained of e.p.s.p. size. In order to localise the cellular sites of these changes, agonist were also applied by microiontophoresis to the cell bodies or dendritic regions of CA1 neurones. Responses to quinolinic acid showed apparent desensitisation at both sites, whereas no decrease was observed in the responses to NMDA and AMPA application. THe induction of long-term potentiation again produced a decrease in the size of responses to NMDA and AMPA. Inhibition of nitric oxide (NO) synthase prevented the long-term potentiation-induced loss of responsiveness to NMDA, but not AMPA, implying a role for NO in the loss of NMDA sensitivity. Recordings of single cell activity during the iontophoretic application of agonists and induction of long-term potentiation showed that responses to NMDA were often suppressed to a greater extent that to quinolinic acid. The results indicate that long-term potentiation showed that responses to NMDA were often suppressed to a greater extent than to quinolinic acid. The results indicate that long-term potentiation can modify the sensitivity of hippocampal neurones to glutamate receptor agonists, and that differences exist in the pharmacology of NMDA and quinolinic acid (AU)

Neuroprotection by caffeine and pentoxifylline during experimental cerebral ischaemia.

Cerebral ischaemia was induced in anaesthetized rats by occlusion of the ipsilateral common carotid and middle cerebral arteries. The response to ischaemia was assessed by the reduction of the amplitude of recorded somatosensory evoked potentials (SSEPs), and the rate of recovery of the SSEPs during reperfusion. Caffeine and pentoxifylline when applied at 70 mM to the cortex for 60 min prior to induction of ischaemia significantly reduced the ischaemia induced attenuation of the SSEPs and hastened recovery to control levels. In contrast, application of normal saline or of the drugs for 15 min did not reduce the effect of ischaemia on the SSEPs. These results suggest that caffeine and pentoxifylline have potential roles in the management of patients with cerebral ischaemia.

Neuroprotection by caffeine and pentoxifylline during experimental cerebral ischaemia

Cerebral ischaemia was induced in anaesthetized rats by occlusion of the ipsilateral common carotid and middle cerebral arteries. The response to ischaemia was assessed by the reduction of the amplitude of recorded somatosensory evoked potentials (SSEPs), and the rate of recovery of the SSEPs during reperfusion. Caffeine and pentoxifylline when applied at 70 mM to the cortex for 60 min prior to induction of ischaemia significantly reduced the ischaemia induced attenuation of the SSEPs and hastened recovery to control levels. In contrast, application of normal saline or of the drugs for 15 min did not reduce the effect of ischaemia on the SSEPs. These results suggest that caffeine and pentoxifylline have potential roles in the management of patients with cerebral ischaemia.

Entrance qualifications and student performance in a hybrid problem based and traditional medical programme

A hybrid problem based learning (PBL) and traditional medical programme was started at the Trinidad campus of the University of the West Indies in 1989. Analyses were carried out to determine the extent to which the entrance qualifications of the students were related to their performances at the examinations in the Phase I (preclinical and paraclinical) and Phase II (clinical) programmes. Students who were admitted on the basis of their results in the secondary school General Certificate of Examination (GCE), 'A' level scored higher at the Phase I, but not at the Phase II, level than those who already had university education. Among the 'A' level students, there was positive correlation between the total 'A' level scores and the examination marks in the medical programme, particularly at the Phase I level. Furthermore, multiple regression analyses indicated that the grades in 'A' level Chemistry and, to a lesser extent in Biology, had the most influence on performances at the Phase I examinations, with much less influence on performances at the Phase II examinations. These results suggest that good grades at 'A' level examinations are significant factors, but not the only important ones, that favour high achievement in the initial stages of this type of PBL/traditional medical programme.

Entrance qualifications and student performance in a hybrid problem based and traditional medical programme.

A hybrid problem based learning (PBL) and traditional medical programme was started at the Trinidad campus of the University of the West Indies in 1989. Analyses were carried out to determine the extent to which the entrance qualifications of the students were related to their performances at the examinations in the Phase I (preclinical and paraclinical) and Phase II (clinical) programmes. Students who were admitted on the basis of their results in the secondary school General Certificate of Examination (GCE), 'A' level scored higher at the Phase I, but not at the Phase II, level than those who already had university education. Among the 'A' level students, there was positive correlation between the total 'A' level scores and the examination marks in the medical programme, particularly at the Phase I level. Furthermore, multiple regression analyses indicated that the grades in 'A' level Chemistry and, to a lesser extent in Biology, had the most influence on performances at the Phase I examinations, with much less influence on performances at the Phase II examinations. These results suggest that good grades at 'A' level examinations are significant factors, but not the only important ones, that favour high achievement in the initial stages of this type of PBL/traditional medical programme.

The role of glutamic acid in the pathogenesis of stroke and the development of neuroprotective drugs.

The role of glutamic acid (glutamate) in the pathogenesis of stroke is now fairly well established. As a result, many drugs which act on glutamate receptors are currently under investigation for their ability to prevent the damage induced by glutamate under ischaemic conditions. The efficacy of these compounds in protecting central neurones from the effects of stroke may be indicative of the importance of the role that glutamate plays in this process.

The sigma ligand 1,3-di-o-tolylguanidine depresses amino acid-induced excitation non-selectively in rat brain.

The sigma ligand 1,3-di-o-tolylguanidine (DTG) has been applied by microiontophoresis to neurones in the rat hippocampal slice and to neurones in the neocortex and hippocampus of rats anaesthetised with urethane. DTG depressed the excitatory responses of cells to both N-methyl-D-aspartate (NMDA) and quisqualate on a majority of the units tested, in no case causing an enhancement. Haloperidol had no consistent effect of its own and did not prevent the depressant effects of DTG. It is concluded that in the preparations used, DTG did not selectively modify neuronal sensitivity to NMDA.

Depression of purine induced inhibition during NMDA receptor mediated activation of hippocampal pyramidal cells--an iontophoretic study.

Single pyramidal cells in the rat hippocampal slice preparation were stimulated by iontophoretic application of excitatory amino acids and acetylcholine. The purine adenosine 5'-monophosphate (AMP), applied iontophoretically, readily depressed acetylcholine stimulated cell firing, was less effective on quisqualic acid stimulated cells and virtually ineffective during stimulation by N-methyl-D,L-aspartate (NMA). Inhibition could be restored if the AMP ejection current was increased 3-fold. In contrast, the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) exerted a comparable level of inhibition under all 3 neuroexcitants. These data support previously published results which suggest that purine mediated inhibition may be reduced during NMDA receptor channel activation. This may have important implications for the action of adenosine during seizures and ischaemic events as well as neuronal phenomena such as long term potentiation.

Effects of ackee fruit extracts on bronchomotor tone in rats

The unripe ackee fruit, when eaten, is known to cause serious clincial manifestations, including vomitting, hypoglycaemia and acidosis. The effects, of various extracts from the arilli of the unripe ackee fruit (including hypoglycin-A) on the lungs from rats were examined in an in vitro preparation. All the extracts were found to induce moderately severe broncho-constriction, indicating a possible contribution of these effects to the observed toxicity of ackee

A re-examination of the mechanism of ackee-induced vomiting sckness

The hypoglycemia seen in ackee poisoning almost certainly results from the presence of hypoglycin A in the aril. However, the mechanisms underlying the vomiting and neurological disrders have not been properly established. We have, in thes review, re-examined the latter and proposed that the vomiting of glutamic and neurological feactures of ackee poisoning probably result from the excitotoxic properties of glutamic and aspartic acids derived directly and indirectly from ackee intake

Purine receptors and kynurenic acid modulate the somatosensory evoked potential in rat cerebral cortex.

Adenosine and analogues, and antagonists to adenosine and putative excitatory amino acid transmitters were topically applied to the cerebral cortex of urethane-anaesthetised rats and their effects on the somatosensory evoked potentials (SEPs) examined. 2-Chloro-adenosine decreased the amplitude of the SEPs whereas adenosine did not. Both L-(-)N6-phenyl-isopropyladenosine (L-PIA) and 5'-N-ethylcarboxamide adenosine (NECA) depressed the SEPs; the effect of L-PIA was more marked than that of NECA. 8-p-Sulphophenyl theophylline increased the amplitude of the SEPs and also inhibited the effects of 2-chloro-adenosine and L-PIA. Kynurenic acid decreased the amplitude of the SEPs. The results suggest that the initial component of the SEP is a post-synaptic event and that endogenous adenosine probably modulates thalamo-cortical synaptic transmission.

Effects of anticonvulsants on responses to excitatory amino acids applied topically to rat cerebral cortex.

1. Five clinically used anticonvulsants were examined to find out if they block the effects of excitatory amino acids in the cerebral cortex of urethane anaesthetised rats. 2. Compounds were tested by topical application to the cortical surface and following their intraperitoneal injection at anticonvulsant doses. 3. Pentobarbital and diphenylhydantoin blocked the effect of quisqualic acid but only at concentrations higher than the therapeutically relevant levels. 4. Pentobarbital and diphenylhydantoin did not alter the effects of N-methyl-D,L-aspartic acid (NMDLA) or kainic acid. 5. Diazepam prevented the kainic acid-induced development of distorted somatosensory evoked potentials (SEPs) at therapeutically relevant levels. 6. Diazepam had no effect on NMDLA or quisqualic acid. 7. Carbamazepine and chlormethiazole had no effect on NMDLA, kainic acid or quisqualic acid. 8. The anticonvulsive effects of these drugs, with the exception of diazepam, probably do not involve antagonism of endogenous EAAs.