Epileptiform activity generated by endogenous acetylcholine during blockade of GABAergic inhibition in immature and adult rat hippocampus

Effect of morphine administration after status epilepticus on epileptogenesis in rats

INTRODUCTION

Morphine is widely used in patients and has been reported to alter seizure threshold, but its role in the development of epilepsy is unknown. In this study, role of morphine administration in the development of epilepsy using the status epilepticus (SE) model was determined in rats.

METHODS

Rats experiencing SE with lithium-pilocarpine (LiP) were randomized into four groups- saline, morphine low dose (5 mg/kg, s.c.), morphine high dose (5-20 mg/kg, s.c.), and naloxone (1 mg/kg, s.c.). Treatments were started 90 min after termination of SE and repeated twice daily for next three days. Rats were video monitored daily for 21 days to determine onset and frequency of spontaneous convulsive seizures (SS).

RESULTS

Morphine in low doses increased frequency of SS (1.51 ± 0.15 vs LiP 0.60 ± 0.12 seizures/rat/day, p-value = 0.0026) and seizures occurred during handling (SDH) (0.08 ± 0.02 vs LiP control 0.01 ± 0.01) (p-value = 0.0018). In high doses, no significant change in SS and SDH was found as compared to LiP. No effect of morphine on the onset of SS and percentage of rats experienced SS was found. No effect of naloxone per se was found on SS.

CONCLUSION

Morphine administration after SE does not affect epileptogenesis as no change in the onset of SS and percentage of rats experiencing SS was found. However, it might alter the susceptibility and frequency of SS. As no other study is available with a similar finding, it needs further evaluation.

Unexpected Effects of Acetylcholine Precursors on Pilocarpine Seizure- Induced Neuronal Death

BACKGROUND

Choline alfoscerate (α-GPC) and Cytidine 5'-diphosphocholine (CDPCholine) are both acetylcholine precursors and are considered to act as pro-cholinergic nootropic agents. Acetylcholine precursors have also recently found frequent use in the neurology clinic. Stroke and many types of dementia have been shown to respond favorably after treatment with these agents, not only in terms of cognitive dysfunction but also behavioral and psychological symptoms. The primary mechanisms of Acetylcholine precursors are the following: 1) Acetylcholine precursors themselves are used in the biosynthesis of acetylcholine and 2) byproducts like glycerophosphate have protective functions for neuronal phospholipids. However, whether acetylcholine precursors have a similar effect in treating cognitive impairment in patients with epilepsy remains controversial.

METHODS

Our previous studies investigating acetylcholine precursors in seizure-experienced animals have produced variable results that were dependent on the timing of administration.

RESULTS

Early administration of CDP-choline immediately after seizure increased neuronal death, blood-brain barrier (BBB) disruption and microglial activation in the hippocampus. However, administration of α-GPC starting 3 weeks after seizure (late administration) improved cognitive function through reduced neuronal death and BBB disruption, and increased neurogenesis in the hippocampus.

CONCLUSION

These seemingly contradictory results may be attributed to both epileptogenic features and neuroprotective functions of several acetylcholine precursors.

The change of picrotoxin-induced epileptiform discharges to the beta oscillation by carbachol in rat hippocampal slices

The study aimed to determine whether and how the activation of the acetylcholine receptor affects epileptiform discharges in the CA3 region in a rat hippocampus. Picrotoxin (100 μM), a GABA_A receptor antagonist, was applied to a hippocampal slice to induce epileptiform discharges. The effects of the cholinergic agonist, carbachol, on the discharges were examined at the several concentrations (1–30 μM). Carbachol had different impacts on epileptiform discharges at the different concentrations. Relatively low concentrations of carbachol (<10 μM) increased the frequency but decreased the amplitude of the discharges. At 10 μM, carbachol induced the discharges, including bursts of theta frequency oscillations. At 30 μM, carbachol could induce bursts of beta frequency oscillations instead of epileptiform discharges. The amplitudes of the oscillations were smaller than those of the discharges. Carbachol suppressed the evoked population EPSPs (pEPSPs) in a dose-dependent manner. These effects were blocked by the muscarinic cholinergic receptor antagonist atropine sulfate. The high level of muscarinic receptor activation can replace epileptiform discharges with theta or beta oscillation. These results suggest that the dose-dependent alternation of the acetylcholine receptor activation may provide the three different stages the epileptiform discharges, the bursts of theta oscillation, and the bursts of the beta oscillation.

Postnatal development of the cholinergic innervation in the dorsal hippocampus of rat: Quantitative light and electron microscopic immunocytochemical study

Choline acetyltransferase (ChAT) immunocytochemistry was used to examine the distribution and ultrastructural features of the acetylcholine (ACh) innervation in the dorsal hippocampus of postnatal rat. The length of ChAT-immunostained axons was measured and the number of ChAT-immunostained varicosities counted, in each layer of CA1, CA3, and dentate gyrus, at postnatal ages P8, P16, and P32. At P8, an elaborate network of varicose ChAT-immunostained axons was already visible. At P16, the laminar distribution of this network resembled that in the adult, but adult densities were reached only by P32. Between P8 and P32, the mean densities for the three regions increased from 8.4 to 14 meters of axons and 2.3 to 5.7 million varicosities per cubic millimeter of tissue. At the three postnatal ages, the ultrastructural features of ChAT-immunostained axon varicosities from the strata pyramidale and radiatum of CA1 were similar between layers and comparable to those in adult, except for an increasing frequency of mitochondria (up to 41% at P32). The proportion of these profiles displaying a synaptic junction was equally low at all ages, indicating an average synaptic incidence of 7% for whole varicosities, as previously found in adult. The observed junctions were small, usually symmetrical, and made mostly with dendritic branches. These results demonstrate the precocious and rapid maturation of the hippocampal cholinergic innervation and reveal its largely asynaptic nature as soon as it is formed. They emphasize the remarkable growth capacities of individual ACh neurons and substantiate a role for diffuse transmission by ACh during hippocampal development.

Iron-induced experimental cortical seizures: Electroencephalographic mapping of seizure spread in the subcortical brain areas

The iron-induced model of post-traumatic chronic focal epilepsy in rats was studied by depth-electrode mapping to investigate the spread of epileptiform activity into subcortical brain structures after its onset in the cortical epileptic focus. Electrical seizure activity was recorded in the hippocampal CA1 and CA3 areas, amygdala and caudate-putamen, in rats with iron-induced chronic cortical focal epilepsy. These experiments showed that the epileptiform activity with its onset in the cortical focus synchronously propagated into the studied subcortical brain areas. Seizure behaviours seemed to increase in correspondence with the spread of the epileptic electrographic activity in subcortical areas. Comparison of the cortical focus electroencephalographic and associated multiple-unit action potential recordings with those from the subcortical structures showed that the occurrence and evolution of the epileptiform activity in the subcortical structures were in parallel with that in the cortical focus. The intracerebral anatomic progression and delineation of seizure spread (mapped by field potential (EEG) and multiple-unit action potentials (MUA) recordings) indicated participation of these regions in the generalization of seizure activity in this model of epilepsy. The seizure-induced activation of the hippocampus appeared to evolve into an epileptic focus independent of the cortical focus. The present study demonstrates the propagation of epileptic activity from the cortical focus into the limbic and basal ganglia regions. Treatment of iron-induced epileptic rats with ethosuximide, an anti-absence drug, resulted in suppression of the epileptiform activity in the cortical focus as well as in the subcortical brain areas.

Developmental changes in presynaptic muscarinic modulation of excitatory and inhibitory neurotransmission in rat piriform cortex in vitro: relevance to epileptiform bursting susceptibility

Suppression of depolarizing postsynaptic potentials and isolated GABA-A receptor-mediated fast inhibitory postsynaptic potentials by the muscarinic acetylcholine receptor agonist, oxotremorine-M (10 microM), was investigated in adult and immature (P14-P30) rat piriform cortical (PC) slices using intracellular recording. Depolarizing postsynaptic potentials evoked by layers II-III stimulation underwent concentration-dependent inhibition in oxotremorine-M that was most likely presynaptic and M2 muscarinic acetylcholine receptor-mediated in immature, but M1-mediated in adult (P40-P80) slices; percentage inhibition was smaller in immature than in adult piriform cortex. In contrast, compared with adults, layer Ia-evoked depolarizing postsynaptic potentials in immature piriform cortex slices in oxotremorine-M, showed a prolonged multiphasic depolarization with superimposed fast transients and spikes, and an increased 'all-or-nothing' character. Isolated N-methyl-d-aspartate receptor-mediated layer Ia depolarizing postsynaptic potentials (although significantly larger in immature slices) were however, unaffected by oxotremorine-M, but blocked by dl-2-amino-5-phosphonovaleric acid. Fast inhibitory postsynaptic potentials evoked by layer Ib or layers II-III-fiber stimulation in immature slices were significantly smaller than in adults, despite similar estimated mean reversal potentials ( approximately -69 and -70 mV respectively). In oxotremorine-M, only layer Ib-fast inhibitory postsynaptic potentials were suppressed; suppression was again most likely presynaptic M2-mediated in immature slices, but M1-mediated in adults. The degree of fast inhibitory postsynaptic potential suppression was however, greater in immature than in adult piriform cortex. Our results demonstrate some important physiological and pharmacological differences between excitatory and inhibitory synaptic systems in adult and immature piriform cortex that could contribute toward the increased susceptibility of this region to muscarinic agonist-induced epileptiform activity in immature brain slices.

Histamine H(1) and H(3) receptors in the rat thalamus and their modulation after systemic kainic acid administration

In rat thalamus, histamine H(1) receptor and isoforms of H(3) receptor were expressed predominantly in the midline and intralaminar areas. Correspondingly, higher H(1) and H(3) receptor binding was also detected in these areas. All isoforms of H(3) receptor were expressed in several thalamic nuclei, but there were minor differences between their expression patterns. H(1) mRNA expression was high in the ventral thalamus, but the H(1) binding level was low in these areas. Since increased brain histamine appears to have an antiepileptic effect through the H(1) receptor activity, kainic acid (KA)-induced status epilepticus in rat was used to study modulation of H(1) and H(3) receptors in the thalamus following seizures. After systemic KA administration, transient decreases in mRNA expression of H(1) receptor and H(3) receptor isoforms with full-length third intracellular loops were seen in the midline areas and the H(1) receptor mRNA expression also decreased in the ventral thalamus. After 1 week, a robust increase in mRNA expression of H(3) receptor isoforms with a full-length third intracellular loop was found in the ventral posterior, posterior, and geniculate nuclei. The changes indicate a modulatory role of H(3) receptor in the sensory and motor relays, and might be involved in possible neuroprotective and compensatory mechanisms after KA administration. However, short-term increases in the H(3) receptor binding appeared earlier (72 h) than the increases of H(3) mRNA expression (1-4 w). The elevations in H(3) binding were evident in the intralaminar area, laterodorsal, lateral posterior, posterior and geniculate nuclei, and were likely to be related to the cortical and subcortical inputs to thalamus.

CA2: the most vulnerable sector to bicuculline exposure in rat hippocampal slice cultures

The vulnerability of the CA2 sector to chronic exposure to bicuculline was investigated in rat hippocampal slice cultures. Selective neuronal cell death was observed only in the CA2 sector after exposure to 6 microM bicuculline for 12 h, but the effect of the cell toxicity extended to the CA3 sector after 24 h. The effect was increased by adding 20 microM roscovitine but was reduced by adding 200 nM omega-agatoxin IVA. Bicuculline also induced a calcium influx into neuronal cells mainly in the CA2 sector. These results suggest that CA2 is the most vulnerable sector to bicuculline exposure in hippocampal slice cultures, and that neuronal cell death in the CA2 sector involves the P/Q-type voltage-dependent calcium channel.

Exaggeration of epileptic-like patterns by nicotine receptor activation during the GABA withdrawal syndrome

To understand how nicotinic cholinergic receptors may participate in epileptic seizures, we tested the effects of nicotine and of the competitive nicotinic antagonists dihydro-beta-erythroidine and alpha-bungarotoxin on synaptic paroxysmal depolarization shifts (PDSs) and intrinsic bursts of action potentials recorded in slices from rats presenting a cortical status epilepticus. This model named GABA-withdrawal syndrome (GWS) appears consecutive to the interruption of a prolonged intracortical GABA infusion. Effects of both nicotinic antagonists suggest a distinct involvement of alpha4-beta2 and alpha7 subunits in shaping individual PDSs and patterning repetitive bursts. On one hand, in GWS rats, an increase of PDS latency and prolongation of PDS and bursts were induced by nicotine and reduced by dihydro-beta-erythroidine, but not by alpha-bungarotoxin. The K+ blocker tetraethylammonium also increased duration without changing latency. Thus, dihydro-beta-erythroidine-sensitive receptors exert distinct controls on the presynaptic generation of PDS and on the process which terminates PDSs and bursts. On the other hand, alpha-bungarotoxin depolarized neurons and generated rhythmic discharges of clustered bursts. Clustered bursts were also observed in slices obtained from GWS rats treated with the acetylcholinesterase inhibitor eserine. We suggest that both dihydro-beta-erythroidine and alpha-bungarotoxin-sensitive sites control paroxysmic activities in GWS and could be involved in some human and animal epilepsies presenting mutations of nicotinic cholinergic receptors.

Further characterization of muscarinic agonist-induced epileptiform bursting activity in immature rat piriform cortex, in vitro

The characteristics of muscarinic acetylcholine receptor agonist-induced epileptiform bursting seen in immature rat piriform cortex slices in vitro were further investigated using intracellular recording, with particular focus on its postnatal age-dependence (P+14-P+30), pharmacology, site(s) of origin and the likely contribution of the muscarinic acetylcholine receptor agonist-induced post-stimulus slow afterdepolarization and gap junction functionality toward its generation. The muscarinic agonist, oxotremorine-M (10 microM), induced rhythmic bursting only in immature piriform cortex slices; however, paroxysmal depolarizing shift amplitude, burst duration and burst incidence were inversely related to postnatal age. No significant age-dependent changes in neuronal membrane properties or postsynaptic muscarinic responsiveness accounted for this decline. Burst incidence was higher when recorded in anterior and posterior regions of the immature piriform cortex. In adult and immature neurones, oxotremorine-M effects were abolished by M1-, but not M2-muscarinic acetylcholine receptor-selective antagonists. Rostrocaudal lesions, between piriform cortex layers I and II, or layer III and endopiriform nucleus in adult or immature slices did not influence oxotremorine-M effects; however, the slow afterdepolarization in adult (but not immature) lesioned slices was abolished. Gap junction blockers (carbenoxolone or octanol) disrupted muscarinic bursting and diminished the slow afterdepolarization in immature slices, suggesting that gap junction connectivity was important for bursting. Our data show that neural networks within layers II-III function as primary oscillatory circuits for burst initiation in immature rat piriform cortex during persistent muscarinic receptor activation. Furthermore, we propose that muscarinic slow afterdepolarization induction and gap junction communication could contribute towards the increased epileptiform susceptibility of this brain area.

Modulation of muscarinic facilitation of epileptiform discharges in immature rat neocortex

We examined the cholinergic effects on epileptiform discharge generation in immature (postnatal days 10-20) rat neocortex. Evoked and spontaneous field potentials were recorded from the deep layers of neocortical slices during GABA(A) receptor blockade by bicuculline methiodide (BMI, 50 microM). The anticholinesterase eserine (10 microM) as well as the ACh-analog carbamylcholine chloride (CCh, 25 microM) decreased the amplitude and duration of evoked field potentials and in parallel, increased significantly the rate of occurrence of spontaneous discharges. These effects were reversed by the muscarinic antagonist atropine (2.5 microM, n = 20), but not by the nicotinic receptor antagonist hexamethonium (50 microM, n = 3). The M1 subtype-selective muscarinic antagonist pirenzepine (1 microM, n = 12) blocked spontaneous discharges in 8/12 slices, while muscarinic antagonists of the M2 (AFDX 116 n = 4), M3 (4-DAMP n = 4) and M4 (gallamine n = 5, tropicamide n = 6) type, all at 1 microM, only reduced their frequency. CCh-induced spontaneous discharges were blocked by the combination of the glutamate receptor antagonists AP5 and CNQX (both at 10 microM; n = 11). Gap junction blockers abolished them (halothane, n = 7) or reduced their frequency by 65% (carbenoxolone, n = 8). Inhibiting Ca2+ release from intracellular stores by dantrolene (100 microM, n = 5) or thapsigargin (1 microM, n = 5) also depressed their frequencies by 55-65%. By contrast, their rates were not altered by perfusion with high Ca2+ (7 mM; n = 6) medium, a manipulation suppressing polysynaptic connections. These findings demonstrate that activation of muscarinic receptors, notably of the M1 type, in immature rat neocortex facilitates the generation of glutamatergic epileptiform discharges. These discharges are strongly inhibited by gap junction blockers, and are also partly mediated by the, presumably muscarinic receptor-dependent, mobilization of intracellular calcium.

The human immunodeficiency virus-1 protein Tat and its discrete fragments evoke selective release of acetylcholine from human and rat cerebrocortical terminals through species-specific mechanisms

The effect of the human immunodeficiency virus-1 protein Tat was investigated on neurotransmitter release from human and rat cortical nerve endings. Tat failed to affect the release of several neurotransmitters, such as glutamate, GABA, norepinephrine, and others, but it evoked the release of [3H]ACh via increase of cytosolic [Ca2+]. In human nerve terminals, the Tat effect partly depends on Ca2+ entry through voltage-sensitive Ca2+ channels, because Cd2+ halved the Tat-evoked release. Activation of group I metabotropic glutamate receptors (mGluR) and mobilization of Ca2+ from IP3-sensitive intraterminal stores are also involved, because the Tat effect was prevented by mGluR antagonists 2-methyl-6-(phenylethynyl)pyridine hydrochloride and 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester and by the IP3 receptor antagonists heparin and xestospongin C. Furthermore, the group I selective mGlu agonist (RS)-3,5-dihydroxyphenylglycine enhanced [3H]ACh release. In rat nerve terminals, the Tat-evoked release neither depends on external Ca2+ ions entry nor on IP3-mediated mechanisms. Tat seems to cause mobilization of Ca2+ from ryanodine-sensitive internal stores because its effect was prevented by both 8-bromo-cyclic adenosine diphosphate-ribose and dantrolene. The Tat-evoked release from human synaptosomes was mimicked by the peptide sequences Tat 32-62, Tat 49-86, and Tat 41-60. In contrast, the Tat 49-86 and Tat 61-80 fragments, but not the Tat 32-62 fragment, were active in rat synaptosomes. In conclusion, Tat elicits Ca2+-dependent [3H]ACh release by species-specific intraterminal mechanisms by binding via discrete amino acid sequences to different receptive sites on human and rat cholinergic terminals.

Endogenous acetylcholine facilitates epileptogenesis in immature rat neocortex

In the presence of the gamma-amino butyric acid-A (GABAA) antagonist bicuculline methiodide (50 microM), synchronous spontaneous and evoked potentials were recorded extracellularly from the deep layers of immature neocortex (postnatal days 10-31, P10-P31) in vitro. Addition of the anticholinesterase eserine (10 microM) depressed the amplitude (by 29.5+/-6.6%, n=13) and duration (by 26.3+/-4.7%, n=11) of the evoked field potentials in 13/19 slices (68%), and increased significantly the rates of occurrence of spontaneous epileptiform discharges or induced them in 9/19 slices (47%). All these effects were blocked by the muscarinic antagonist atropine (2.5 microM, n=3), suggesting that they were mediated by the activation of muscarinic receptors by endogenous acetylcholine. The cholinergic inhibitory effect is unlikely to terminate seizures, while the excitatory effect, could conceivably promote or aggravate their manifestation. In conclusion, these findings demonstrate that endogenous acetylcholine may contribute to epileptogenesis in immature neocortex.

Immature rat convulsions and long-term effects on hippocampal cholinergic neurotransmission

Generalized tonic-clonic convulsions were induced on 2 consecutive days by pentylenetetrazol (PTZ) in immature rats (postnatal days 10 and 20), and hippocampal slices were prepared at different intervals post-injection. The anticholinesterase eserine provoked interictal-like discharges in the CA3 area of PTZ-injected rats (19/33), but not in controls (0/15), an effect mimicked by carbachol and reversed by atropine. This enhanced response to eserine was recorded in slices from 25-100% of the PTZ-injected rats, the percentage varying with the age at injection and post-injection interval. These results suggest that seizures in immature brain may have long-term consequences in cholinergic neurotransmission, converting a rise in endogenous ACh into an epileptogenic stimulus, which in turn would presumably facilitate the recurrence of seizures.