George K. Kostopoulos

Adenosine as a putative transmitter in the cerebral cortex. Studies with potentiators and antagonists

Abstract Adenosine has a potent depressant action on cerebral cortical neurons, including identified corticospinal cells. Adenosine 2′-, 3′- and 5′-phosphates, including adenosine 5′-imidodiphosphate, had comparable depressant actions and 2-chloroadenosine was an even more potent depressant. Inhibitors of adenosine uptake, hexobendine and papaverine, potentiated the actions of adenosine and adenosine 5′-monophosphate. Theophylline and caffeine antagonized the depressant actions of adenosine and adenosine 5′-monophosphate. The results are compatible with the hypothesis that adenosine depresses neurons by activating an extracellular receptor and that this effect can be blocked by theophylline and caffeine.

Decreased ability of rat temporal hippocampal CA1 region to produce long-term potentiation

Tetanic stimulation of Schaffer collaterals in the CA1 region of transverse slices, taken from the septal (dorsal) part of young rat hippocampus, produced N-Methyl-D-aspartate-dependent long-term potentiation (LTP) of the rising slope of excitatory postsynaptic potential (mean 38%). Under identical conditions of stimulation (100 Hz, 1 s) slices taken from the temporal (ventral) third of hippocampus presented a substantially reduced ability for LTP (mean 5%). The defect appeared to lie with the induction rather than the maintenance phase of LTP. These results suggest that a significant functional differentiation at the local synaptic plasticity level occurs between the two poles of hippocampus, which together with the substantial differences in their extrinsic connections, may help explain the reported differential participation of neurons in these parts of hippocampus during animal memory tests.

Pentylenetetrazol-induced seizures decrease γ-aminobutyric acid-mediated recurrent inhibition and enhance adenosine-mediated depression

Summary: To elucidate the consequences of convulsions, we examined biochemically and electrophysiologically the brains of mice that had sustained two complete tonicclonic convulsions after administration of pentylenetetrazol (PTZ 50 mg/kg intraperitoneally, i.p.), 48 and 24 h before decapitation. Control mice were injected with saline. Input/output curves of the extracellular synaptic responses in the CAI area of hippocampal slices showed that PTZ‐induced seizures do not establish the persistent change in hippocampal excitability itself that can be detected in vitro. However, use of the paired‐pulse stimulation paradigm showed that γ‐aminobutyric acid, (GABA)‐mediated recurrent inhibition was significantly weaker (by 19–25%) in the CA1 area of slices from PTZtreated mice (PTZ slices) as compared with slices from control mice (control slices). The density of GABA, receptors (high‐affinity component) was also lower in hippocampus (by 19%) and cortex (by 14%) of PTZ‐treated mice. A GABA‐related disinhibitory mechanism underlying PTZ seizures may thus persist for 1 day after the seizure, predisposing the brain to subsequent seizures. On the other hand, the depressant effect of a single dose of adenosine 10 μM on the CA1 synaptic response was stronger (by 35% on population spikes) and longer lasting in PTZ slices as compared with controls. This could be attributed to significantly higher adenosine A1 receptor density in hippocampus (Bmax of [3H]CHA was higher by 34%) as well as cortex and cerebellum of these animals. The phenomenon may reflect an adenosine A1‐mediated adaptive mechanism that offers protection from subsequent seizures.

Involvement of the Thalamocortical System in Epileptic Loss of Consciousness

Summary: Experiments on putative neuronal mechanisms underlying absence seizures as well as clinical observations are critically reviewed for their ability to explain apparent “loss of consciousness.” It is argued that the initial defect in absences lies with corticothalamic (CT) neuronal mechanisms responsible for selective attention and/or planning for action, rather than with those establishing either the states or the contents of consciousness. Normally, rich thalamocortical (TC)–CT feedback loops regulate the flow of information to the cortex and help its neurons to organize themselves in discrete assemblies, which through high‐frequency (>30 Hz) oscillations bind those distributed processes of the brain that are considered important, so that we are able to focus on what is needed from moment to moment and be aware of this fact. This ability is transiently lost in absence seizures, because large numbers of CT loops are recruited for seconds in much stronger, low‐frequency (∼3 Hz) oscillations of EPSP/IPSP sequences, which underlie electroencephalographic (EEG) spike‐and‐wave discharges (SWDs). These oscillations probably result from a transformation of the normal EEG rhythm of sleep spindles on an abnormal increase of cortical excitability that results in strong activation of inhibitory neurons in the cortex and in nucleus reticularis thalami. The strong general enhancement of CT feedback during SWDs may disallow the discrete feedback, which normally selects specific TC circuits for conscious perception and/or motor reaction. Such a mechanism of SWD generation allows variability in the extent to which different TC sectors are engaged in the SWD activity and thus explains the variable ability of some patients to respond during an absence, depending on the sensory modality examined.

Dorsal-ventral differentiation of short-term synaptic plasticity in rat CA1 hippocampal region

Two forms of short-term synaptic plasticity (STP), paired-pulse facilitation (PPF) and frequency potentiation (FP) of CA1 field excitatory postsynaptic potentials (EPSP) to afferent stimulation were compared in slices taken from the dorsal and ventral parts of rat hippocampus. While dorsal slices showed significant PPF at all interpulse intervals (20-1400 ms, 80% at 40 ms), PPF in ventral slices was substantially weaker at intervals shorter than 100 ms (19% at 40 ms) and nil at longer intervals. While dorsal slices showed substantial FP at frequencies 1-40 Hz and frequency depression at 50-100 Hz, ventral slices showed only a much smaller potentiation at 1 Hz and substantial depression at 20-100 Hz. Decreasing [Ca(2+)](o) from 2 to 1 and 0.5 mM substantially reduced the baseline EPSPs in both groups of slices but its effect on PPF was greater in ventral slices. On the contrary when [Ca(2+)](o) was increased to 5 mM only dorsal slices showed an enhancement of baseline EPSP. It is concluded that ventral hippocampus CA1 area has a specific deficit in STP, which is related to the important presynaptic role of calcium and is consistent with a relatively higher transmitter release probability.

Upregulation of A1 adenosine receptors in human temporal lobe epilepsy: A quantitative autoradiographic study

A significant increase of A1 adenosine receptor binding (48% increase of mean) was detected in human neocortex obtained from patients suffering from temporal lobe epilepsy as compared to control neocortex from non-epileptic patients. Such increase was equally distributed in the six cortical layers and reached similar levels in each of the five specimens tested independently of age, sex and pharmacological treatment of the patient. Since adenosine exerts a depressant effect on neocortical neurons in slices obtained from epileptic patients, this upregulation of A1 receptor binding may constitute a protective mechanism against subsequent seizures, which is exerted by elevating the depressant response of the brain to endogenous adenosine.

Participation of cortical recurrent inhibition in the genesis of spike and wave discharges in feline generalized penicillin epilepsy.

Cortical recurrent inhibition (RI) evoked in pericruciate cortex by antidromic stimulation of the cerebral peduncle (CP) was studied in normal cats and in cats exhibiting the signs of feline generalized penicillin epilepsy (FGPE) following the i.m. injection of penicillin. Two measures of RI evoked by antidromic CP stimulation were used: (i) the averaged focal potential in the pericruciate gyrus; and (ii) the duration of the suppression or diminution of extracellularly recorded action potential (ap) discharge of antidromically activated pericruciate neurons measured in peristimulus time histograms (PSTHs). After i.m. injection of 350,000 IU/kg of penicillin RI remained preserved as long as only generalized spike and wave (SW) discharges appeared in the EEG, although in 5/17 neurons a modest to moderate reduction in the duration of RI occurred once SW discharges had appeared in the EEG. This inconstant reduction was probably not caused by a direct anti-inhibitory action of penicillin, but is a consequence of the increased number of ap discharges curtailing RI. At the small concentrations of penicillin existing in brain in FGPE its anti-inhibitory action evident with larger concentrations cannot be demonstrated. When focal or generalized tonic-clonic (T-C) seizures occurred, RI was reduced in slightly more than half of the instances for a few minutes before the onset of these seizures. This suggests that the transition from SW discharge to T-C seizure may be caused by a breakdown of RI.

Effect of pentylentetrazol-induced seizures on A1 adenosine receptor regional density in the mouse brain: a quantitative autoradiographic study.

Adenosine has been shown to be a major regulator of neuronal activity in convulsive disorders, exerting its anticonvulsant effect through central A1 adenosine receptors. The aim of the present study was to investigate the effect of generalized tonic-clonic seizures induced by pentylentetrazol on regional changes in A1 adenosine receptor density and distribution in the mouse brain by in vitro quantitative autoradiography. As radioligand the specific agonist of A1 receptors [3H]cyclohexyladenosine was used. After two consecutive (once daily) pentylentetrazol-induced convulsions a widespread upregulation of A1 receptor density was detected with a marked enhancement in structures that mediate seizure activity like hippocampus, mamillary bodies, septum, substantia nigra, thalamic nuclei and cerebral cortices. On the contrary, in basal ganglia a significant downregulation of A1 receptors was observed. These results indicate that: (i) the observed increases or decreases in A1 receptor density are organized in selective anatomical structures related to seizure development rather than uniform in the brain; and (ii) since the upregulation of A1 receptors is sufficient to enhance the physiological depressive response of adenosine, the overall evoked increases seen here may lead to a stronger inhibitory tone and accordingly to a more efficient anticonvulsant effect of endogenous adenosine.

Reduction of A1 adenosine receptors in rat hippocampus after kainic acid-induced limbic seizures

In a temporal lobe epilepsy (TLE) model induced by kainic acid (KA), we examined the effect of limbic seizures on A1 adenosine receptor distribution in hippocampus and cortex. By using quantitative autoradiography, we determined a progressive decrease in A1 receptor density in CA1 and CA3 regions of hippocampus, which coincided in time with the degenerating process of hippocampal pyramidal cells. This result indicates that a great amount of A1 receptors are located postsynaptically on pyramidal cell dendrites. No difference in A1 receptor density was observed in the inner compared to the outer molecular layer of dentate gyrus, or in the infrapyramidal band compared to the outer layer of stratum oriens of CA3. This could indicate that the newly sprouted mossy fiber glutamatergic terminals do not contain A1 receptors, thus lacking a restrain in the release of glutamate.

Spontaneous GABAA-dependent synchronous periodic activity in adult rat ventral hippocampal slices

The present study shows that adult rat transverse slices from the ventral hippocampus perfused with standard medium persistently generate spontaneous synchronous field potentials. In CA1 st. pyramidale this regular ventral hippocampus spontaneous synchronous activity (VHSSA) was positive with mean amplitude 0.18 +/- 0.02 mV (n=80 slices) and occurred every 0.48 +/- 0.02 s. Simultaneous intracellular recordings from CA1 pyramidal neurons demonstrated that concomitant hyperpolarizations invariably occurred in association to this field activity and could thus constitute its electrical generators. These hyperpolarizations, had mean amplitude 2.7 +/- 0.6 mV, duration at half amplitude 44.8 +/- 6.6 ms, they reversed at -72.6 +/- 1.5 mV (n=10 cells), they effectively suspended the depolarization-induced tonic neuronal firing of all ten pyramidal neurons and they were reversibly abolished, together with field potentials, by the GABA(A) receptor antagonist bicuculline (5 microM, n=4). VHSSA was also dependent on fast glutamatergic transmission, since it was blocked by the antagonist of AMPA/kainate receptors 6-Cyano-7-nitroquinoxaline-2,3-dione disodium (10 microM, n=3). We propose that, under standard in vitro conditions, synchronous GABA(A)-mediated hyperpolarizing potentials are spontaneously generated in pyramidal neurons presumably resulting from the phasic quasi-rhythmic discharge of a local interneuronal network of ventral hippocampus.