Activation of muscarinic receptors during blockade of GABA(A)-mediated inhibition induces synchronous epileptiform activity in immature rat hippocampus

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.

Activation of muscarinic cholinergic receptors inhibits giant neurones in the caudal pontine reticular nucleus

Giant neurones in the caudal pontine reticular nucleus (PnC) play a crucial role in mediating the mammalian startle response. They receive input from cochlear, trigeminal and vestibular nuclei and project directly to motoneurones. Furthermore, they integrate modulatory input from different brain regions either enhancing or inhibiting startle responses. One prominent startle modulation is prepulse inhibition where a non-startling stimulus presented prior to the startle stimulus inhibits a subsequent startle response. Several behavioural studies have indicated that this inhibition is mediated by muscarinic receptors at the level of the PnC. Here, we performed whole-cell patch-clamp recordings from PnC giant neurones in acute rat brain slices in order to examine muscarinic inhibition. We stimulated afferent trigeminal and auditory fibres and applied muscarinic agonists and antagonists in order to investigate their effect on excitatory postsynaptic current amplitudes, paired-pulse ratio and passive membrane properties of PnC giant neurones. The cholinergic agonist carbachol and the muscarinic agonist oxotremorine significantly reduced excitatory postsynaptic current amplitudes and increased the paired-pulse ratio. Carbachol additionally reduced the membrane resistance of postsynaptic PnC giant neurones. The subtype-specific antagonists AF-DX116 (M2 preferring) and tropicamide (M4 preferring) antagonized the oxotremorine effect indicating that M4 and possibly M2 receptor subtypes are involved in this inhibition. The G-protein-activated inward rectifying potassium channel blocker tertiapin-Q had no effect on oxotremorine-induced inhibition of giant neurones. Our results show a mainly presynaptically mediated strong inhibition of PnC giant neurones by activation of M4 and possibly M2 receptors that presumably contribute to prepulse inhibition.

Calcium-activated afterhyperpolarizations regulate synchronization and timing of epileptiform bursts in hippocampal CA3 pyramidal neurons

Calcium-activated potassium conductances regulate neuronal excitability, but their role in epileptogenesis remains elusive. We investigated in rat CA3 pyramidal neurons the contribution of the Ca(2+)-activated K(+)-mediated afterhyperpolarizations (AHPs) in the genesis and regulation of epileptiform activity induced in vitro by 4-aminopyridine (4-AP) in Mg(2+)-free Ringer. Recurring spike bursts terminated by prolonged AHPs were generated. Burst synchronization between CA3 pyramidal neurons in paired recordings typified this interictal-like activity. A downregulation of the medium afterhyperpolarization (mAHP) paralleled the emergence of the interictal-like activity. When the mAHP was reduced or enhanced by apamin and EBIO bursts induced by 4-AP were increased or blocked, respectively. Inhibition of the slow afterhyperpolarization (sAHP) with carbachol, t-ACPD, or isoproterenol increased bursting frequency and disrupted burst regularity and synchronization between pyramidal neuron pairs. In contrast, enhancing the sAHP by intracellular dialysis with KMeSO(4) reduced burst frequency. Block of GABA(A-B) inhibitions did not modify the abnormal activity. We describe novel cellular mechanisms where 1) the inhibition of the mAHP plays an essential role in the genesis and regulation of the bursting activity by reducing negative feedback, 2) the sAHP sets the interburst interval by decreasing excitability, and 3) bursting was synchronized by excitatory synaptic interactions that increased in advance and during bursts and decreased throughout the subsequent sAHP. These cellular mechanisms are active in the CA3 region, where epileptiform activity is initiated, and cooperatively regulate the timing of the synchronized rhythmic interictal-like network activity.

Convulsant bicuculline modifies CNS muscarinic receptor affinity

Background

Previous work from this laboratory has shown that the administration of the convulsant drug 3-mercaptopropionic acid (MP), a GAD inhibitor, modifies not only GABA synthesis but also binding of the antagonist [³H]-quinuclidinyl benzilate ([³H]-QNB) to central muscarinic receptors, an effect due to an increase in affinity without modifications in binding site number. The cholinergic system has been implicated in several experimental epilepsy models and the ability of acetylcholine to regulate neuronal excitability in the neocortex is well known. To study the potential relationship between GABAergic and cholinergic systems with seizure activity, we analyzed the muscarinic receptor after inducing seizure by bicuculline (BIC), known to antagonize the GABA-A postsynaptic receptor subtype.

Results

We analyzed binding of muscarinic antagonist [³H]-QNB to rat CNS membranes after i.p. administration of BIC at subconvulsant (1.0 mg/kg) and convulsant (7.5 mg/kg) doses. Subconvulsant BIC dose failed to develop seizures but produced binding alteration in the cerebellum and hippocampus with roughly 40% increase and 10% decrease, respectively. After convulsant BIC dose, which invariably led to generalized tonic-clonic seizures, binding increased 36% and 15% to cerebellar and striatal membranes respectively, but decreased 12% to hippocampal membranes. Kd value was accordingly modified: with the subconvulsant dose it decreased 27% in cerebellum whereas it increased 61% in hippocampus; with the convulsant dose, Kd value decreased 33% in cerebellum but increased 85% in hippocampus. No change in receptor number site was found, and Hill number was invariably close to unity.

Conclusion

Results indicate dissimilar central nervous system area susceptibility of muscarinic receptor to BIC. Ligand binding was modified not only by a convulsant BIC dose but also by a subconvulsant dose, indicating that changes are not attributable to the seizure process itself. Findings support the notion that the muscarinic receptors play a major role in experimental epilepsy and provide a new example of differential neuronal plasticity.

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.

The A3 adenosine receptor agonist 2-Cl-IB-MECA facilitates epileptiform discharges in the CA3 area of immature rat hippocampal slices

The effects of the A(3) adenosine receptor agonist 2-Cl-IB-MECA were tested on epileptiform field potentials recorded in the CA3 area of postnatal days 10-20 immature hippocampal slices, during perfusion with the GABA(A) receptor antagonist bicuculline (10 microM). Evoked potentials: 2-Cl-IB-MECA (1-50 microM, n = 17) had consistently excitatory effects, blocked by the A(3) receptor antagonist MRS 1220 (1 microM, n = 7), but not occluded in the presence of the A(1) antagonist DPCPX (1 microM, n = 12) or the A(2A) antagonist ZM-241385 (0.1 microM, n = 12). 2-Cl-IB-MECA reversed the inhibitory effects (n = 5) of the adenosine uptake blocker nitrobenzylthioinosine (NBTI, 50 microM), but did not increase its excitatory effects (n = 19). Spontaneous discharges: 2-Cl-IB-MECA (1 microM) induced them or increased their frequency in 14/30 slices, an effect reversed by MRS 1220 (n = 3), and observed also following pre-perfusion with DPCPX (n = 11), ZM-241385 (n = 11) or both (n = 10). In the presence of the A(1) antagonist DPCPX, NBTI increased the frequency of spontaneous discharges, an effect partially reversed by MRS 1220 (n = 8), thus suggesting that a rise in endogenous adenosine during disinhibition may activate A(3) receptors. In conclusion, these findings suggest strongly that activation of A(3) receptors, following a rise in endogenous adenosine (i.e. during seizures, hypoxia), facilitates excitation, thus limiting the known inhibitory and/or neuroprotective effects of adenosine in immature brain.

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.

Characterization of acute neurotoxic effects of trimethylolpropane phosphate via neuronal network biosensors

We have utilized cultured neuronal networks grown on microelectrode arrays to demonstrate rapid, reliable detection of a toxic compound, trimethylolpropane phosphate (TMPP). Initial experiments, which were performed blind, demonstrated rapid classification of the compound as a convulsant, a finding consistent with previous whole animal neurobehavioral studies. TMPP (2-200 microM) reorganized network spike activity into synchronous, quasi-periodic burst episodes. Integrated burst amplitudes invariably increased, reflecting higher spike frequencies within each burst. The variability of network burst parameters, quantified as coefficients of variation (CVs), was decreased. Mean CVs for burst duration, interburst interval, and burst rate were lowered by 42+/-13, 58+/-5.5, and 62+/-1.8%, respectively (mean+/-SEM, n=8 cultures, 197 channels). These changes in network activity paralleled the effects induced by bicuculline, a known disinhibitory and seizure-inducing drug, and confirmed classification of TMPP as a potential epileptogenic compound. Simple pharmacological tests permit exploration of mechanisms underlying observed activity shifts. The EC(50) for GABA inhibition of network activity was increased from 2.8 to 7.0 microM by 20 microM TMPP and to 20.5 microM by 200 microM TMPP. Parallel dose-response curves suggest that TMPP acts by a competitive antagonism of GABA inhibition, and are consistent with reported patch-clamp analysis of TMPP-induced reduction of inhibitory postsynaptic current amplitudes. The potency of TMPP in generating epileptiform activity in vitro was comparable to concentrations reported for in vivo studies. TMPP and bicuculline produced both increases and decreases in burst rate depending on native spontaneous bursting levels. These results demonstrate a need for multivariate analysis of network activity changes to yield accurate predictions of compound effects.

Muscarinic Depression of Synaptic Transmission in the Epileptogenic GABA Withdrawal Syndrome Focus

The GABA withdrawal syndrome (GWS) is a model of local status epilepticus consecutive to the interruption of a prolonged GABA infusion into the rat somatomotor cortex. Bursting patterns in slices from GWS rats include intrinsic bursts of action potentials (APs) induced by intracellular depolarizing current injection and/or paroxysmal depolarization shifts (PDSs) induced by white matter stimulation. Possible changes in the effects of cholinergic drugs after in vivo induction of GWS were investigated on bursting cells ( n = 30) intracellularly recorded in neocortical slices. In GWS slices, acetylcholine (Ach, 200-1000 μM) or carbachol (Cch, 50 μM) applications increased the number of bursts induced by depolarizing current injection while synaptically induced PDSs were significantly diminished (by 50–60%) or even blocked independently of the cholinergic-induced depolarization. The intrinsic burst facilitation and PDS depression provoked by Ach or Cch were mimicked by methyl-acetylcholine (mAch, 100–400 μM, n = 11), were reversed by atropine application (1–50 μM, n = 3), and were not mimicked by nicotine (50–100 μM, n = 4), indicating the involvement of muscarinic receptors. In contrast, in nonbursting cells from the same epileptic area ( n = 42) or from equivalent area in control rats ( n = 24), a nonsignificant muscarinic depression of EPSPs was induced by Cch and Ach. The mAch depression of excitatory postsynaptic potential (EPSPs) was significantly lower than that seen for PDSs in GWS rats. None of the cholinergic agonists caused bursting appearance in these cells. Therefore the present study demonstrates a unique implication of muscarinic receptors in exerting opposite effects on intrinsic membrane properties and on synaptic transmission in epileptiform GWS. Muscarinic receptor mechanisms may therefore have a protective role against the development and spread of epileptiform activity from the otherwise-activated epileptic focus.

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.

Novel role for the NMDA receptor redox modulatory site in the pathophysiology of seizures

Redox-active compounds modulate NMDA receptors (NMDARs) such that reduction of NMDAR redox sites increases, and oxidation decreases, NMDAR-mediated activity. Because NMDARs contribute to the pathophysiology of seizures, redox-active compounds also may modulate seizure activity. We report that the oxidant 5, 5'-dithio-bis(2-nitrobenzoic acid) (DTNB) and the redox cofactor pyrroloquinoline quinone (PQQ) suppressed low Mg(2+)-induced hippocampal epileptiform activity in vitro. Additionally, in slices exposed to 4-7 microM bicuculline, DTNB and PQQ reversed the potentiation of evoked epileptiform responses by the reductants dithiothreitol and Tris(2-carboxyethyl)phosphine (TCEP). NMDA-evoked whole-cell currents in CA1 neurons in slices were increased by TCEP and subsequently decreased by DTNB or PQQ at the same concentrations that modulated epileptiform activity. However, DTNB and PQQ had little effect on baseline NMDA-evoked currents in control medium, and PQQ did not alter NMDAR-dependent long-term potentiation. In contrast, in slices returned to control medium after low Mg(2+)-induced ictal activity, DTNB significantly inhibited NMDAR-mediated currents, indicating endogenous reduction of NMDAR redox sites under this epileptogenic condition. These data suggested that PQQ and DTNB suppressed spontaneous ictal activity by reversing pathological NMDAR redox potentiation without inhibiting physiological NMDAR function. In vivo, PQQ decreased the duration of chemoconvulsant-induced seizures in rat pups with no effect on baseline behavior. Our results reveal endogenous potentiation of NMDAR function via mass reduction of redox sites as a novel mechanism that may enhance epileptogenesis and facilitate the transition to status epilepticus. The results further suggest that redox-active compounds may have therapeutic use by reversing NMDAR-mediated pathophysiology without blocking physiological NMDAR function.

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

We tested the effects of the acetylcholinesterase inhibitor eserine (10 microM), an indicator of the activity of endogenous ACh, on the generation of epileptiform discharges during blockade of inhibitory GABA(A)-mediated potentials by bicuculline (10 microM), in the CA3 area of hippocampal slices from postnatal days 4-20 (P4-P20) immature and adult rats. Eserine provoked or significantly increased the frequency of spontaneous synchronous epileptiform discharges, in 6/22 (27%) P4-P10 slices, 34/35 P11-P20 slices and 18/18 adult slices, an epileptogenic effect. In immature slices, spontaneous discharges showed a stable frequency throughout perfusion with eserine, while in 5/11 adult slices an initial fast frequency was followed by a slower steady-state one. The cholinergic agonist carbachol (CCh, 25 microM) provoked only transient or no spontaneous synchronous discharges in adult slices (n=8), thus suggesting that massive activation of cholinergic receptors may lead to suppression of epileptiform activity in adult brain. Stimulus-induced excitatory CA3 responses, were depressed by eserine in approximately half of 20 P4-P10, 45 P11-P20 and 11 adult slices. The depression consisted of a decrease in the amplitude, duration, and number of population spikes of the field potentials by about 30%, a minor neuroprotective effect, which did not change with maturation. The different developmental profiles of the epileptogenic and neuroprotective effects of endogenous ACh suggest that they are mediated by different mechanisms. These experiments demonstrate that, endogenous ACh is sufficient to induce epileptogenesis during a decrease or failure of GABAergic inhibition, in both >/=P10 immature and in adult hippocampus. We therefore suggest that clinical or behavioral conditions which raise the concentration of endogenous ACh may lower the threshold to seizures.

Muscarinic receptor modulation of GABA-mediated giant depolarizing potentials in the neonatal rat hippocampus

1. The whole-cell patch clamp technique was used to study the role of muscarinic receptors in regulating the frequency of giant depolarizing potentials (GDPs) in CA3 hippocampal neurones in slices from postnatal (P) P1-P8 rats. 2. Atropine (1 microM) reduced the frequency of GDPs by 64.2 +/- 2.9 %. The acetylcholinesterase inhibitor edrophonium (20 microM) increased the frequency of GDPs in a developmentally regulated way. This effect was antagonized by the M1 muscarinic receptor antagonist pirenzepine. 3. In the presence of edrophonium, tetanic stimulation of cholinergic fibres induced either an enhancement of GDP frequency (179 +/- 79 %) or a membrane depolarization (27 +/- 16 mV) associated with an increase in synaptic noise. These effects were prevented by atropine. 4. Application of carbachol (3 microM) produced an increase in GDP frequency that at P5-P6 was associated with a membrane depolarization and an increase in synaptic noise. These effects were prevented by atropine, pirenzepine (3 microM) and bicuculline (10 microM). 5. In the presence of pirenzepine, carbachol reduced GDP frequency by 50 +/- 4 %. Conversely, in the presence of methoctramine (3 microM), carbachol enhanced GDP frequency by 117 +/- 4 %. 6. It is concluded that endogenous acetylcholine, through the activation of M1 receptors, enhances the release of gamma-aminobutyric acid (GABA), in a developmentally regulated way. On the other hand, carbachol exerts both an up- and downregulation of GABA release through the activation of M1 and M2 receptors, respectively.

Effects of diazepam on extracellular brain neurotransmitters in pilocarpine-induced seizures in rats

The present study was undertaken to gain insights into the mechanism of action of diazepam in focally-evoked pilocarpine-induced seizures by concomitantly assessing the changes produced in the extracellular levels of glutamate, GABA (gamma-aminobutyric acid) and dopamine. In vivo microdialysis, coupled to continuous monitoring of electrocorticographic (ECoG) recordings, was performed in freely moving rats. Intrahippocampal perfusion with 10 mM pilocarpine (40 min, 2 microl/min) produced limbic seizures. A single dose of intraperitoneal diazepam (5 mg/kg) was administered 2 h after pilocarpine perfusion was started. Dialysates were sampled both from hippocampus and cerebellum and analysed by microbore liquid chromatography. Diazepam produced instant inhibition of behavioural and ECoG seizure activity. Pilocarpine-induced increases in the extracellular levels of glutamate and dopamine in hippocampus were promptly reduced by diazepam. No concurrent alterations in pilocarpine-induced increases in the extracellular levels of GABA in either hippocampus or cerebellum were seen. Pilocarpine enhanced cerebellar glutamate levels only transiently and levels returned to baseline before diazepam administration. No further changes in cerebellar glutamate levels were observed with diazepam. Our findings suggest that the anti-convulsant action of diazepam against pilocarpine-induced seizures is associated with a prompt attenuation of extracellular hippocampal glutamate overflow without concurrent alteration of pilocarpine-induced increases in endogenous GABA levels. Diazepam also significantly decreased pilocarpine-induced increases in extracellular dopamine levels within the hippocampus. No immediate alterations of the basal levels of the neurotransmitters monitored were observed with diazepam.

A comparison of the adenosine-mediated synaptic inhibition in the CA3 area of immature and adult rat hippocampus

We compared the effects of the adenosine A1 receptor activation on the postsynaptic potentials (psps) recorded from the CA3 area of immature (postnatal days 10-20) and adult rat hippocampal neurons in vitro. The adenosine A1 receptor agonist 2-phenyl-isopropyl-adenosine (PIA, 1 microM) depressed the stimulus-induced psps less in immature and more in adult neurons. In the presence of the GABAA receptor antagonist bicuculline methiodide (BMI, 10 microM), PIA reduced the duration and number of action potentials of the stimulus-induced paroxysmal depolarizations (PDs) in immature neurons, while it blocked PDs in adult neurons. Spontaneous BMI-induced PDs, were blocked by PIA in less than half (5/12) immature and all (6/6) adult neurons. The adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 1 microM) enhanced the stimulus-induced psps in immature and adult neurons alike; this effect did not lead to stimulus-induced bursting in immature neurons. DPCPX induced spontaneous bursts (proconvulsant effect) in only 2/16 immature but in all adult (12/12) neurons. In BMI, DPCPX increased the duration and number of action potentials of the stimulus-induced PDs in immature and adult neurons alike (by about 30%), but it increased the rates of occurrence of spontaneous PDs in significantly more adult neurons. In conclusion, our results suggest that adenosine, acting via A1 receptors, is a more effective endogenous anti-epileptic in adult than in immature hippocampus, a fact which may contribute to the susceptibility of the latter to epileptogenesis.

Comparison of the effects of M1 and M2 muscarinic receptor activation in the absence of GABAergic inhibition in immature rat hippocampal CA3 area

We have reported previously that addition of carbachol (CCh, 25 microM) in the presence of the GABAA antagonist bicuculline (BMI, 10 microM), depresses evoked synaptic responses and significantly increases the frequency of spontaneous synchronous discharges in immature (postnatal days 10-20, P10-P20) hippocampus in vitro. The present experiments were designed in order to compare the activity of CCh with that of the selective muscarinic agonists McN-A-343 (M1 agonist) and oxotremorine-M (M2 agonist), in inducing the above-mentioned effects. Cholinergic agonists were tested in concentrations ranging from 2.5 nM to 5 microM and synaptic field potentials (evoked, spontaneous) were recorded from the CA3 pyramidal layer of hippocampal slices in the presence of BMI (10 microM). Depression of the evoked responses was measured as decrease (i) in their duration, (ii) in the number of population spikes/response, and (iii) in the amplitude of the first population spike of each sequence. Sigmoid dose-response curves were plotted, and the IC50s were calculated. For all three indexes, the profile of effectiveness was Oxotremorine-M>carbachol=McN-A-343. CCh and oxotremorine-M induced or increased the frequency of spontaneous synchronous discharges in the presence of BMI in a concentration-dependent manner. At the range of concentrations tested, McN-A-343 did not mimic this effect. In conclusion, our results suggest that M2 muscarinic receptors play a major role in both the depression of synaptic responses and the increase in the frequency of spontaneous synchronous discharges observed in the absence of GABAergic inhibition.

Muscarinic agonist-induced burst firing in immature rat olfactory cortex neurons In vitro

Age-related changes in pre-/postsynaptic muscarinic (mAChR) and metabotropic-glutamate (mGluR) responsiveness were studied in slices of olfactory cortex from both immature [postnatal day 16-22 (P16-P22)] and adult (>/=P40) rats, using a conventional intracellular recording technique. In adult neurons, bath application of the mAChR agonist oxotremorine-M (OXO-M; 10 microM), or the selective mGluR agonist 1-aminocyclopentane-1S-3R-dicarboxylic acid (1S,3R-ACPD; 10 microM) evoked sustained membrane depolarizations, increases in input resistance, intense repetitive firing, and the appearance of a slow poststimulus afterdepolarizing potential (sADP). Excitatory postsynaptic potentials (EPSPs) evoked by local electrical stimulation of association fiber terminals were also depressed. In contrast, in neurons from immature slices, the 10 microM OXO-M-induced membrane depolarization was followed by the appearance of spontaneous rhythmic epileptiform activity, which was voltage independent and reversible on drug wash out. Epileptiform bursts were abolished or reduced by coapplication of tetrodotoxin (1 microM), atropine (1 microM), pirenzepine (100-200 nM), the N-methyl-D-aspartate (NMDA) receptor antagonist -amino-5-phosphonovaleric acid (-APV; 100 microM), the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 5-20 microM), the anesthetic-sedative barbiturate pentobarbitone (100 microM), or by raising the extracellular Mg2+ concentration, whereas a clear facilitatory effect was exhibited by the selective gamma-aminobutyric acid-A (GABAA) receptor blocker (-)-bicuculline methiodide (10 microM). The epileptogenic effects induced by OXO-M were indistinguishable from those produced by 4-aminopyridine (4-AP; 100-200 microM), although these latter actions were unaffected by atropine. In slices from immature animals, electrical stimulation of layer III association fibers in the presence of 10 microM OXO-M was accompanied by a dramatic prolongation of evoked depolarizing postsynaptic potentials (PSPs), with the appearance of recurrent superimposed spike discharges. This effect was readily reversed on wash out of OXO-M. No comparable age-dependent differences were observed in the nature or time course of 1S,3R-ACPD-evoked pre- (or post)synaptic responses, even in immature cells where muscarinic epileptiform activity had previously been demonstrated. We suggest that the overall susceptibility toward muscarinic-induced epileptiform discharge in immature olfactory cortical neurons may depend on the functional integrity of presynaptic inhibitory mAChRs; additional contributing mechanisms were also considered.