Acetylcholine induced modulation of hippocampal pyramidal neurons

Pharmacokinetic and Lipidomic Assessment of the In Vivo Effects of Parishin A-Isorhynchophylline in Rat Migraine Models

Migraine is a chronic brain disease that leads to periodic neurological attacks. Parishin A and isorhynchophylline (PI) is the active monomer component extracted from the traditional antimigraine Chinese medicinal combination of Gastrodia and Uncaria, respectively. In this study, using high-performance liquid chromatography coupled with tandem mass spectrometry (HPLC-MS/MS) technology, we performed pharmacokinetic and lipidomic study on migraine model rats after administration of PI. For the detection of the compounds in plasma, AB Sciex Triple Quad™ 4500 was applied for quantitative analysis, and the COSMOSIL C₁₈ column (2.1 × 100 mm, 2.6 μm) was used for separation. Isorhynchophylline (ISO: m/z 384.8-241.2) and its main metabolite rhynchophylline (RHY: m/z 384.8-160.2) were simultaneously detected under positive ion modes. Besides, parishin A (PA: m/z 995.1-726.9) and its main metabolite gastrodin (GAS: m/z 331.1-123.0) were simultaneously detected with negative ion modes. For the analysis of endogenous lipid components, Dionex Ultimate 3000 (UHPLC) Thermo Orbitrap Elite was applied for the detection, and the Waters UPLCRBEH C₁₈ column (1.7 μm 100 ∗ 2.1 mm) was used for separation. Chloroform/methanol (2 : 1, v : v) was used for extraction. The results demonstrated that PI exists significant difference in metabolism between single- and coadministration and can regulate lipid levels associated with migraine.

Modeling and simulation of organophosphate-induced neurotoxicity: Prediction and validation by experimental studies

Exposure to organophosphorus (OP) compounds, either pesticides or chemical warfare agents, represents a major health problem. As potent irreversible inhibitors of cholinesterase, OP may induce seizures, as in status epilepticus, and occasionally brain lesions. Although these compounds are extremely toxic agents, the search for novel antidotes remains extremely limited. In silico modeling constitutes a useful tool to identify pharmacological targets and to develop efficient therapeutic strategies. In the present work, we developed a new in silico simulator in order to predict the neurotoxicity of irreversible inhibitors of acetyl- and/or butyrylcholinesterase (ChE) as well as the potential neuroprotection provided by antagonists of cholinergic muscarinic and glutamate N-methyl-d-aspartate (NMDA) receptors. The simulator reproduced firing of CA1 hippocampal neurons triggered by exposure to paraoxon (POX), as found in patch-clamp recordings in in vitro mouse hippocampal slices. In the case of POX intoxication, it predicted a preventing action of the muscarinic receptor antagonist atropine sulfate, as well as a synergistic action with the non-competitive NMDA receptor antagonist memantine. These in silico predictions relative to beneficial effects of atropine sulfate combined with memantine were recapitulated experimentally in an in vivo model of POX in adult male Swiss mice using electroencephalic (EEG) recordings. Thus, our simulator is a new powerful tool to identify protective therapeutic strategies against OP central effects, by screening various combinations of muscarinic and NMDA receptor antagonists.

Temporally unstructured electrical stimulation to the amygdala suppresses behavioral chronic seizures of the pilocarpine animal model

Electrical stimulation applied to the basolateral amygdala in the pentylenetetrazole animal model of seizures may result in either a proconvulsant or an anticonvulsant effect depending on the interpulse intervals used: periodic or nonperiodic, respectively. We tested the effect of this electrical stimulation temporal coding on the spontaneous and recurrent behavioral seizures produced in the chronic phase of the pilocarpine animal model of temporal lobe epilepsy, an experimental protocol that better mimics the human condition. After 45 days of the pilocarpine-induced status epilepticus, male Wistar rats were submitted to a surgical procedure for the implantation of a bipolar electrical stimulation electrode in the right basolateral amygdala and were allowed to recover for seven days. The animals were then placed in a glass box, and their behaviors were recorded daily on DVD for 6h for 4 consecutive days (control period). Spontaneous recurrent behavioral seizures when showed in animals were further recorded for an extra 4-day period (treatment period), under periodic or nonperiodic electrical stimulation. The number, duration, and severity of seizures (according to the modified Racine's scale) during treatment were compared with those during the control period. The nonperiodically stimulated group displayed a significantly reduced total number and duration of seizures. There was no difference between control and treatment periods for the periodically stimulated group. Results corroborate previous findings from our group showing that nonperiodic electrical stimulation has a robust anticonvulsant property. In addition, results from the pilocarpine animal model further strengthen nonperiodic electrical stimulation as a valid therapeutic approach in current medical practice. Our working hypothesis is that temporally unstructured electrical stimulation may wield its effect by desynchronizing neural networks involved in the ictogenic process.

M1 and M4 receptors modulate hippocampal pyramidal neurons

Acetylcholine (ACh), acting at muscarinic ACh receptors (mAChRs), modulates the excitability and synaptic connectivity of hippocampal pyramidal neurons. CA1 pyramidal neurons respond to transient ("phasic") mAChR activation with biphasic responses in which inhibition is followed by excitation, whereas prolonged ("tonic") mAChR activation increases CA1 neuron excitability. Both phasic and tonic mAChR activation excites pyramidal neurons in the CA3 region, yet ACh suppresses glutamate release at the CA3-to-CA1 synapse (the Schaffer-collateral pathway). Using mice genetically lacking specific mAChRs (mAChR knockout mice), we identified the mAChR subtypes responsible for cholinergic modulation of hippocampal pyramidal neuron excitability and synaptic transmission. Knockout of M1 receptors significantly reduced, or eliminated, most phasic and tonic cholinergic responses in CA1 and CA3 pyramidal neurons. On the other hand, in the absence of other G(q)-linked mAChRs (M3 and M5), M1 receptors proved sufficient for all postsynaptic cholinergic effects on CA1 and CA3 pyramidal neuron excitability. M3 receptors were able to participate in tonic depolarization of CA1 neurons, but otherwise contributed little to cholinergic responses. At the Schaffer-collateral synapse, bath application of the cholinergic agonist carbachol suppressed stratum radiatum-evoked excitatory postsynaptic potentials (EPSPs) in wild-type CA1 neurons and in CA1 neurons from mice lacking M1 or M2 receptors. However, Schaffer-collateral EPSPs were not significantly suppressed by carbachol in neurons lacking M4 receptors. We therefore conclude that M1 and M4 receptors are the major mAChR subtypes responsible for direct cholinergic modulation of the excitatory hippocampal circuit.

Phasic cholinergic signaling in the hippocampus: functional homology with the neocortex?

Acetylcholine (ACh) acts as a neurotransmitter in both the hippocampus and neocortex to facilitate learning, memory, and cognitive function. Here we show that transient muscarinic ACh receptor (mAChR) activation inhibits action potential generation in CA1, but not in CA3, pyramidal neurons via activation of an SK-type calcium-activated potassium conductance. Hyperpolarizing responses generated by focal ACh application near the somata of CA1 pyramidal neurons were blocked by atropine or the M1-like mAChR antagonist pirenzepine, but not by the M2-like mAChR antagonist methoctramine. Inhibitory cholinergic responses required intracellular calcium signaling, as evidenced by their sensitivity to depletion of internal calcium stores or internal calcium chelation. Cholinergic inhibition did not require GABAergic synaptic transmission, but was blocked by apamin, an SK channel antagonist. In contrast to inhibitory effects in CA1 neurons, ACh was primarily depolarizing, and enhanced action potential firing in CA3 pyramidal neurons. These results, when combined with recent data in neocortical neurons, suggest a functional homology in phasic cholinergic signaling in the hippocampus and neocortex whereby ACh preferentially inhibits those neurons in the lower cortical layers (CA1 and layer 5 neurons) that provide the majority of extracortical efferent projections.

Effects of pre- and postnatal exposure to 5-methoxytryptamine and early handling on an object-place association learning task in adolescent rat offspring

A reduction in 5-HT1A receptor response enhances learning and memory performance in rats. Pre- and postnatal treatment with 5-methoxytryptamine (5MT), a non-selective serotonergic agonist, and early handling, reduce the number of 5-HT1A receptors in neonatal and pre-pubertal rat progeny. The aim of this study was to investigate in adolescent male rats the consequences of pre- and postnatal treatment with 5MT and its interaction with early handling on an object-place association learning task, the "Can test", a motivated, non-aversive, spatial/object discrimination task. Results show that a single daily injection of 5MT from gestational days 12 to 21 (1 mg/kg s.c.) and from postnatal days 2 to 18 to pups (0.5 mg/kg s.c.), increases the level of activity and the number of correct responses, and decreases the number of reference memory errors in the progeny as adolescent, compared to vehicle-treated rats. Similar effects are observed following a daily, brief, maternal separation of the pups from postnatal days 2 until 21. Furthermore, when 5MT-treated rats underwent to early handling procedure, the effects induced by 5MT increased handling-induced facilitation of the object-place association. These results suggest that pre- and postnatal treatment with 5MT enhances learning in the "Can test", probably due to a reduction in 5-HT1A receptors in the hippocampus. Whether the potentiation exerted by pre- and postnatal 5MT on early handling effects may be related to a further damping of 5-HT1A receptor response is not yet assessed; however, our data demonstrate that this association is able to induce long-term facilitative effects on spatial learning performance in a non-aversive spatial/object discrimination task in the adolescent rat offspring.

Development of Cholinergic Modulation and Graded Persistent Activity in Layer V of Medial Entorhinal Cortex

During muscarinic modulation, principal neurons from layer V of rat medial entorhinal cortex (mEC) respond to repeated applications of a brief stimulus with a graded change in persistent firing frequency. This pattern of discharge has been proposed to represent an intrinsic mechanism for short-term memory operations. To investigate the implementation of persistent activity in mEC during development, we characterized the electrophysiological properties of layer V principal neurons in the mEC over a range of postnatal stages. We observed significant differences in both passive (resistance, time constant, and resting membrane potential) and active properties (threshold, action potential, and adaptation) of principal neurons from rats aged 5–7, 10–13, 16–19, and 21–23 days. We also examined the properties of muscarinic-dependent persistent activity in EC slices from different age groups. Recordings were conducted using the perforated-patch whole cell technique because persistent activity runs down in the ruptured-patch configuration. Although no neuron in the youngest group exhibited graded persistent activity in response to muscarinic receptor activation, this activity was recorded in the 10- to 13-day-old group and its occurrence increased from 69% in the 16- to 19-day-old group to 76% in the 21- to 23-day-old group. This postnatal increase in neurons endowed with persistent firing properties in mEC was found to parallel the increase in density of ChAT-positive immunostaining of fibers and the developmental changes in M1 muscarinic receptor mRNA levels. All these data suggest that the implementation of mnemonic properties in mEC principal neurons matches the ontogenic development of afferent cholinergic circuits and their signaling components.

Differential nicotinic acetylcholine receptor subunit expression in the human hippocampus

Neuronal nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels composed of alpha and beta subunits with specific structural, functional and pharmacological properties. In this study the distribution of alpha3, alpha4, alpha7, beta2 and beta4 nAChR subunits in the human hippocampus was investigated using immunohistochemistry. Most pyramidal neurons, pre-alpha cells of the entorhinal cortex and dentate granule cells were immunoreactive for all subunits. Small islands of alpha7 immunoreactive cells were present in the outer presubiculum. alpha4 and beta2, and alpha3, alpha4 and beta2 immunoreactive fibre tracts were present in the stratum radiatum and subiculum, respectively, suggesting nAChRs may play a role in modulating inputs to the hippocampus via Schaffer collaterals and along the perforant pathway. Some astrocytes were immunoreactive for alpha3, alpha7 and beta4 subunits. Immunoreactivity to all subunits was noted in association with blood vessels. These results indicate the involvement of multiple nAChR subtypes in the modulation of both neuronal and non-neuronal functions in the human hippocampus.

Interaction of Noradrenergic and Cholinergic Agonists with Ligands Increasing K-conductance of Guinea Pig Hippocampal Neurons, in vitro

Single electrode current clamp and voltage clamp recordings were employed to study the effects of noradrenergic agonists and a cholinergic agonist (carbachol, Cch) on the resting membrane potential of CA3 neurons in guinea pig hippocampal slices. Stimulation of muscarinic and beta-adrenergic receptors depolarized, and stimulation of alpha1-adrenergic receptor hyperpolarized, CA3 neurons but the membrane potential changes were small. Hyperpolarizations or outward currents induced by baclofen, adenosine or serotonin (5-HT) were strongly potentiated by alpha-noradrenergic agonists and suppressed by Cch at concentrations ten times lower than those having any direct effects on membrane potential. Both the enhancement of the baclofen-induced hyperpolarization by phenylephrine and its suppression by Cch were pronounced at low concentrations of baclofen, but diminished at higher concentrations. The modulatory effects persisted after blockade of sodium spikes by tetrodotoxin and after blockade of fast inhibitory and excitatory synaptic transmission by picrotoxin and 6-cyano-7-nitroquinoxaline-2,3-dione. Our data suggest that, through the postsynaptic interaction with ligands activating potassium conductance, noradrenergic and muscarinic receptor stimulation can exert a stronger inhibitory and excitatory effect on CA3 pyramidal neurons at their resting membrane potential than would be expected from the changes in membrane potential induced by these neuromodulators on their own.

The M1 muscarinic agonist CI-1017 facilitates trace eyeblink conditioning in aging rabbits and increases the excitability of CA1 pyramidal neurons

The M1 muscarinic agonist CI-1017 was administered intravenously to aging rabbits on a daily basis before and during hippocampally dependent trace eyeblink conditioning sessions. Circulating levels of CI-1017 were significantly related to the drug dose. The drug was found to significantly increase the rate and amount of learning in a dose-dependent manner with no significant effects on the amplitude, area, or latency of conditioned responses. There was no evidence of pseudoconditioning at the highest drug concentration, and the minimally effective dose produced only mild and temporary hypersalivation as a side effect. CI-1017 (10 microM) was also found to increase the excitability of CA1 pyramidal neurons recorded from hippocampal slices from young and aging naive rabbits as measured by changes in spike-frequency adaptation and the postburst afterhyperpolarization. These biophysical changes were reversed with either atropine (1 microM) or pirenzepine (1 microM). These results suggest that M1 agonists ameliorate age-related learning and memory impairments at least in part by reducing the afterhyperpolarization and spike-frequency adaptation of hippocampal pyramidal neurons and that M1 agonists may be an effective therapy for reducing the cognitive deficits that accompany normal aging and/or Alzheimer's disease.

S 15535, a benzodioxopiperazine acting as presynaptic agonist and postsynaptic 5-HT1A receptor antagonist, prevents the impairment of spatial learning caused by intrahippocampal scopolamine

1 The effect of S 15535 (4-benzodioxan-5-yl)1-(indan-2-yl)piperazine), an agonist at presynaptic and antagonist at postsynaptic 5-HT1A receptors, on the impairment of spatial learning caused by intrahippocampal scopolamine in a two-platform spatial discrimination task was studied. 2 Scopolamine (4.0 microg microl-1), injected bilaterally into the CA1 region of the dorsal hippocampus 10 min before each training session, impaired choice accuracy with no effect on choice latency and errors of omission. 3 Administered subcutaneously 30 min before each training session, S 15535 1.0 (but not 0.3) mg kg-1 did not modify choice accuracy but prevented its impairment by intrahippocampal scopolamine. 4 WAY 100635, a 5-HT1A receptor antagonist, injected into the dorsal raphe at 1.0 microg 0.5 microl-1 5 min before scopolamine, had no effect on choice accuracy and latency or errors of omission and did not modify the effect of scopolamine but completely antagonized the effect of S 15535 (1.0 mg kg-1) on scopolamine-induced impairment of choice accuracy. 5 The results confirm a previous report (Carli et al., 1998) that stimulation of presynaptic 5-HT1A receptors in the dorsal raphe counteracts the deficit caused by intrahippocampal scopolamine, probably by facilitating the transfer of facilitatory information from the entorhinal cortex to the hippocampus. 6 Drugs that stimulate action on presynaptic 5-HT1A receptors, such as S 15535 and other partial 5-HT1A receptors agonists, may be useful in the symptomatic treatment of human memory disturbances associated with loss of cholinergic innervation to the hippocampus.

Metrifonate increases neuronal excitability in CA1 pyramidal neurons from both young and aging rabbit hippocampus

The effects of metrifonate, a second generation cholinesterase inhibitor, were examined on CA1 pyramidal neurons from hippocampal slices of young and aging rabbits using current-clamp, intracellular recording techniques. Bath perfusion of metrifonate (10-200 microM) dose-dependently decreased both postburst afterhyperpolarization (AHP) and spike frequency adaptation (accommodation) in neurons from young and aging rabbits (AHP: p < 0.002, young; p < 0.050, aging; accommodation: p < 0.024, young; p < 0.001, aging). These reductions were mediated by muscarinic cholinergic transmission, because they were blocked by addition of atropine (1 microM) to the perfusate. The effects of chronic metrifonate treatment (12 mg/kg for 3 weeks) on CA1 neurons of aging rabbits were also examined ex vivo. Neurons from aging rabbits chronically treated with metrifonate had significantly reduced spike frequency accommodation, compared with vehicle-treated rabbits. Chronic metrifonate treatment did not result in a desensitization to metrifonate ex vivo, because bath perfusion of metrifonate (50 microM) significantly decreased the AHP and accommodation in neurons from both chronically metrifonate- and vehicle-treated aging rabbits. We propose that the facilitating effect of chronic metrifonate treatment on acquisition of hippocampus-dependent tasks such as trace eyeblink conditioning by aging subjects may be caused by this increased excitability of CA1 pyramidal neurons.

Cholinergic facilitation of trace eyeblink conditioning in aging rabbits

The hippocampus is importantly involved in learning and memory, and is severely impacted by aging. In in vitro hippocampal slices, both the post-burst afterhyperpolarization (AHP) and spike-frequency accommodation are reduced in hippocampal pyramidal neurons after hippocampally-dependent trace eyeblink conditioning, indications of increased cellular excitability. The AHP results from the activation of outward potassium currents, including sI(AHP) and muscarine-sensitive I(M). The AHP is significantly increased in aging hippocampal neurons, potentially contributing to age-associated learning deficits. Compounds which reduce the AHP and spike-frequency accommodation could facilitate learning in normal aging or in age-associated dementias such as Alzheimer's disease. The cholinesterase inhibitor metrifonate enhances trace eyeblink conditioning by aging rabbits and reduces the AHP and accommodation in hippocampal CA1 neurons in a dose-dependent manner. These reductions are mediated by muscarinic cholinergic transmission as they are blocked by atropine. Hippocampal neurons from metrifonate treated but behaviorally naive rabbits were more excitable and not desensitized to the effects of metrifonate since the AHP and accommodation were further reduced when metrifonate was bath applied to the neurons. These observations suggest that the facilitating effect of chronic metrifonate on acquisition of hippocampally dependent tasks is mediated at least partially by increasing the baseline excitability of CA1 pyramidal neurons. The issue of whether learning can be facilitated with muscarinic cholinergic agonists, in addition to cholinesterase inhibitors, was addressed by training aging rabbits during intravenous treatment with the M1 agonist CI1017. A dose-dependent enhancement of acquisition was observed, with rabbits receiving 1.0 or 5.0 mg/ml CI1017 showing comparably improved learning rates as those receiving 0.5 mg/ml or vehicle. Sympathetic side effects, mainly excess salivation, were seen with the 5.0 mg/ml dose. Post-training evaluations suggested that the effective doses of CI1017 were enhancing responsivity to the tone conditioned stimulus. These studies suggest that muscarinic cholinergic neurotransmission is importantly involved in associative learning; that learning in aging animals may be facilitated by enhancing cholinergic transmission; and that the facilitation may be mediated through actions on hippocampal neurons.

Cholinergic excitation of dendrites in neocortical neurons

Discharge patterns were studied in response to iontophoretic application of acetylcholine to the soma and dendrites of 128 neocortical pyramidal neurons of layer V. Extracellular recordings were obtained from slices of the guinea-pig parietal cortex. All responses found were excitatory and were better expressed in spontaneously firing cells than in silent ones. Sensitivity to acetylcholine was approximately the same at somatic and dendritic sites in all the cells. Activation of muscarinic receptors gave rise to firing patterns with equal latencies and intensities when applied to both soma and dendrites. The latter suggests that membrane excitation elicited in dendrites by binding of acetylcholine to muscarinic cholinoreceptors is likely to propagate towards the soma through intracellular biochemical processes. Modulating effect of acetylcholine on output firing patterns, elicited by dendritic application of excitatory amino acids, included shortening of the somatic response latency and increase of response intensity and duration. We propose that, in contrast to glutamatergic excitation, the spread of cholinergic excitation along dendrites involves intra-cellular chemical signalling and results in changing the electrical properties of dendrites all over their length.

alpha-bungarotoxin- and methyllycaconitine-sensitive nicotinic receptors mediate fast synaptic transmission in interneurons of rat hippocampal slices

This study demonstrates for the first time that alpha7 nicotinic receptors (nAChRs) mediate fast synaptic transmission in conventional hippocampal slices. In the presence of antagonists of muscarinic, AMPA, NMDA, GABAA, ATP, and 5-HT3 receptors, spontaneous and evoked postsynaptic currents (PSCs) recorded from CA1 interneurons were blocked by the alpha7 nAChR antagonists methyllycaconitine and alpha-bungarotoxin and by a desensitizing concentration of the alpha7 nAChR agonist choline. Spontaneous nicotinic PSCs were also accompanied by Na+ transients, indicating that alpha7 nAChR-mediated transmission serves as an excitatory signal to the CA1 interneurons in the hippocampus.

Neuromodulatory control of hippocampal function: towards a model of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder of cognitive function whose cellular pathology and molecular etiology have been increasingly and dramatically unraveled over the last several years. Despite this substantial knowledge base, the disease remains poorly understood due to a basic lack of understanding of how memories are stored and recalled in the brain. We describe a preliminary attempt at constructing a detailed model of these basic neural mechanisms; in particular, the natural dynamics of neuronal activity in hippocampal region CA3 and the modulation and control of these dynamics by subcortical cholinergic and GABAergic input to the hippocampus. We view the construction of such a model, with sufficient detail at the cellular and subcellular level, to be a necessary first step in understanding the effect of AD pathology on the functional behavior of the underlying neural circuitry. The network is based on the 66-compartment hippocampal pyramidal cell model of Traub and colleagues and their 51-compartment interneuron interconnected with realistic AMPA-, NMDA-, and GABA(A)-mediated synapses. Traub and others have shown that a network composed of these modeled cells is capable of synchronization in the gamma frequency range. We demonstrate here that this synchronization mechanism can implement an attractor-based autoassociative memory. A new input pattern arrives at the beginning of each theta cycle (comprised of 5-10 gamma cycles), and the pattern of activity across the network converges, over several gamma cycles, to a stable attractor that represents the stored memory. In this model, cholinergic deprivation, one of the hallmarks of AD, leads to a slowing of the gamma frequency which reduces the number of "cycles" available to reach an attractor state. We suggest that this may be one mechanism underlying the memory loss and cognitive slowing seen in AD. Our results also support the idea that acetylcholine acts on individual neurons to induce and maintain a transition from intrinsic bursting to spiking in pyramidal cells. These results are consistent with the hypothesis that spiking and bursting in CA3 pyramidal cells mediate separate behavioral functions, and that cholinergic input is required for the transition to and support of behavioral states associated with the online processing and recall of information.

Muscarinic-receptor antagonist scopolamine rescues hippocampal neurons from death induced by glutamate

Cultured hippocampal neurons were used to test the hypothesis that modulation of muscarine receptors can modify glutamate-induced neurodegeneration. Treatment of hippocampal cultures with scopolamine (1 nM to 1 mM) under glutamate incubation had beneficial effect on neuronal viability. Thus, blockade of muscarinic-receptor sites increased the threshold for glutamate neurotoxicity. These data show that interactions between the NMDA, muscarinic receptors and their corresponding neurotransmitter inputs to hippocampal neurons may play a crucial role in neurodegeneration.

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

We investigated the effects of the cholinergic agonist carbachol (25 microM) on the synaptic potentials recorded extracellularly and intracellularly from the CA3 area of immature hippocampal slices of the rat (postnatal days 10-20). In control conditions, carbachol reduced the amplitude of evoked synaptic responses (n=8) and did not induce any spontaneous synchronous activity (n=12); the depressant effect of carbachol was mimicked by acetylcholine (100 microM, in eserine 10 microM, n=5) and was reversed by the muscarinic antagonist atropine (1 microM, n=2). The GABA(A)-receptor antagonist bicuculline (10 microM) enhanced the amplitude and duration of the evoked synaptic responses and induced infrequent (0.016-0.045 Hz) spontaneous synchronous discharges in 23/37 of the slices. Application of carbachol in the presence of bicuculline reduced the amplitude of the evoked synaptic responses (n=21) and in addition induced synchronous discharges with rates of occurrence 0.075-0.225 Hz, in 64/68 slices. Both effects were mimicked by acetylcholine and eserine, and antagonized by atropine. The specific muscarinic antagonists pirenzepine (M1-type), tripitramine (M2-type), 4-diphenylacetoxy-N-methylpiperidine methiodide (M3-type) and tropicamide (M4-type) (all tested at 0.1-1 microM) reversibly reduced the frequency of synchronous carbachol-induced discharges. In addition, these discharges were reversibly blocked by high Ca2+ perfusion medium (7 mM CaCl2, n=4) and by the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM, n=7). Synchronous epileptiform discharges were recorded from both CA1 and CA3 areas in intact slices (n=3), but only from CA3 following disruption of the CA1-CA3 synaptic connections (n=3). These experiments suggest that activation of muscarinic receptors during blockade of GABA(A)-mediated potentials, may enhance synchronous epileptiform activity in immature (postnatal days 10-20) hippocampus, through activation of local excitatory circuits and that endogenous acetylcholine may be sufficient to play this role.

WAY 100635, a 5-HT1A receptor antagonist, prevents the impairment of spatial learning caused by intrahippocampal administration of scopolamine or 7-chloro-kynurenic acid

The effect of WAY 100635, a 5-HT1A receptor antagonist, on the impairment of spatial learning caused by intrahippocampal administration of scopolamine, a cholinergic muscarinic receptor antagonist, or 7-chloro-kynurenic acid, an antagonist at the glycine site associated with the NMDA receptor complex, was studied in a two-platform spatial discrimination task. Scopolamine (4 microg/microl) or 7-chloro-kynurenic acid (3 microg/microl), administered bilaterally into the CA1 region of the dorsal hippocampus 10 min before each training session, impaired choice accuracy with no effect on choice latency and errors of omission. Administered subcutaneously at 1 (but not at 0.3) mg/kg 30 min before each training session, WAY 100635 did not modify the acquisition of spatial learning, but prevented the impairment of choice accuracy caused by intrahippocampal scopolamine or 7-chloro-kynurenic acid. These findings suggest that blockade of 5-HT1A receptors can compensate the loss of cholinergic or NMDA-mediated excitatory input to pyramidal cells in the hippocampus. The mechanisms involved and the importance of these findings for the symptomatic treatment of memory disorders in man are discussed.

Differential effects of scopolamine on neuronal survival in ischemia and glutamate neurotoxicity: relationships to the excessive vulnerability of the dorsoseptal hippocampus

The neurodegeneration in the CA1 subfield of hippocampus exhibited a dorsal-ventral gradient of susceptibility in global ischemia (82% dorsoseptally and only 16% ventrotemporally). Scopolamine (SCOP) did not improve the neuronal damage caused by the global ischemic challenge in rats and did not reduce the infarct area after the focal MCA-occlusion in mice. No differences were observed between saline and SCOP-treated animals in the physiologic parameters, except for a slight increase in rectal temperature. In contrast, treatment of hippocampal cultures with increasing concentrations of SCOP (1 nM to 1 mM) under glutamate incubation had a beneficial effect on neuronal viability. These data show that (1) there is substantial gradient of vulnerability of the hippocampus from dorsal to ventral in global ischemia and (2) that interactions between the NMDA, muscarinic receptors and their corresponding neurotransmitter inputs to hippocampal neurons are evident in vitro and may play a crucial role in neuronal neurodegeneration. However, the mechanisms underlying the high vulnerability of dorsal hippocampus still remain enigmatic.

Muscarinic depolarization of SH-SY5Y human neuroblastoma cells as determined using oxonol V

Membrane potential was measured in the suspension of SH-SY5Y cells using the anionic potentiometric probe, oxonol V. The relation of fluorescence to membrane potential was assessed by increasing the external [K+] in the presence of the K+ ionophore valinomycin. The response was linear in the range of 5 to 30 mM K+ (membrane potential change of approximately 40 mV). Muscarine increased the fluorescence indicating a depolarization. The competitive inhibitory constant (112 nM) of the muscarinic antagonist pirenzepine (5,11-dihydro-11-([4-methyl-1-piperazinyl]acetyl)-6H-pyrido[2,3-b] (1,4)benzodiazepin-6-one-dihydrochloride) suggests that Hm1 receptors are not involved. The protein kinase C inhibitor, GF 109203X (3-[1-(3-demethylaminopropyl)-indol-3-yl]-3-(indol-3-yl)-maleimide ), and a reduction of extracellular Na+ both produced an additive partial inhibition. The results suggest that muscarinic receptors depolarize these cells by separate Na(+)-dependent and -independent mechanisms, the Na(+)-independent mechanism being protein kinase C-dependent.

(S)-WAY 100135, a 5-HT1A receptor antagonist, prevents the impairment of spatial learning caused by intrahippocampal scopolamine

Scopolamine, 3.75 micrograms/microliters infused bilaterally into the CA1 region of the dorsal hippocampus 10 min before each training session, impaired choice accuracy but had no effect on choice latency or errors of omission in rats trained in a two-platform spatial discrimination task. Administered subcutaneously at 3 and 10 mg/kg 30 min before each training session, N-tert-butyl-3-4-(2-methoxyphenyl)piperazin-1-yl-2-phenylpropanami de dihydrochloride ((S)-WAY 100135), a 5-HT1A receptor antagonist, prevented the impairment of choice accuracy induced by intrahippocampal scopolamine. No subcutaneous dose of (S)-WAY 100135 by itself modified the acquisition of spatial learning. Administered into the dorsal hippocampus 15 min before each training session, (S)-WAY 100135 at doses of 0.2, 1 and 5 micrograms/microliters did not affect the acquisition of spatial learning but dose dependently prevented the impairment of choice accuracy caused by scopolamine, 3.75 micrograms/microliters infused into the same area. These findings suggest that blockade of 5-HT1A receptors can compensate the loss of cholinergic excitatory input on pyramidal cells, probably by favouring the action of other excitatory transmitters.

Expression, control, and probable functional significance of the neuronal theta-rhythm

The data on theta-modulation of neuronal activity in the hippocampus and related structures, obtained by the author and her colleagues have been reviewed. Analysis of extracellularly recorded neuronal activity in alert rabbits, intact and after various brain lesions, in slices and transplants of the hippocampus and septum allow one to make the following conclusions. Integrity of the medial septal area (MS-DB) and its efferent connections are indispensable for theta-modulation of neuronal activity and EEG of the hippocampus. The expression of hippocampal theta depends on the proportion of the MS-DB cells involved in the rhythmic process, and its frequency in the whole theta-range, is determined by the corresponding frequencies of theta-burst in the MS-DB. The neurons of the MS-DB have the properties of endogenous rhythmic burst and regular single spike oscillators. Input signals ascending to the MS-DB from the pontomesencephalic reticular formation increase both the frequency of the MS-DB theta-bursts and the proportion of neurons involved in theta-activity; serotonergic midbrain raphe nuclei have the opposite effect on the MS-DB rhythmic activity and hippocampal EEG theta. Increase of endogenous acetylcholine (by physostigmine) also increases the proportion of the MS-DB neurons discharging in theta-bursts (both in intact and basally-undercut septum), but does not influence the theta-frequency. The primary effect of the MS-DB on hippocampal neurons (pyramidal and non-pyramidal) consists in GABAergic reset inhibition. Reset inhibition, after which theta-modulation follows in constant phase relation, is triggered also by sensory stimuli. About two-thirds of the hippocampal pyramidal neurons are tonically inhibited by sensory stimuli which evoke EEG theta, while others are excited, or do not change their activity. Anticholinergic drugs restrict the population of rhythmic neurons but do not completely suppress theta-bursts in the MS-DB and hippocampus. Under their action, EEG theta can be evoked (presumably through GABAergic MS-DB influences) by strong reticular or sensory stimuli with corresponding high frequency. However information processing in this condition is defective: expression of reset is increased, responses to electrical stimulation of the perforant path and to sensory stimuli are often augmented, habituation to sensory stimuli is absent and tonic responses are curtailed. On a background of continuous theta induced by increase of endogenous acetylcholine, reset is absent or reduced, responsiveness of the hippocampal neurons to electrical and sensory stimulation is strongly reduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitative evaluation of neuronal loss in the dorsal hippocampus in rats with long-term pilocarpine seizures

Systemic administration of the cholinergic agonist pilocarpine (350-400 mg/kg, i.p.) to rats induces acute behavioral and EEG status epilepticus followed by apparent complete neurological recovery. In rats receiving higher doses of pilocarpine (i.e., 380-400 mg/kg), recurrent seizures reappear 2-2.5 weeks later and continue to occur as long as the rats are kept alive. Stereological estimates of neurons in regions CA1, CA3 and the dentate granule cell layer in the dorsal hippocampus show a dose-dependent neuronal loss in the CA3 and CA1 subregions. The granule cell layer of the dentate gyrus is not affected. No progressive neuronal loss was observed in the regions studied after 3, 6 and 12 weeks during which the animals displayed spontaneous recurrent seizures. The temporal profile of the epileptic condition induced by pilocarpine and the resulting pattern of neuronal loss in the rat hippocampus are similar to those seen in many cases of human temporal lobe epilepsy. The neuronal loss is dose-dependent and primarily results from the acute pilocarpine-induced seizures as chronic seizures do not produce any measurable additional cell loss in the regions examined in the experimental model used in this study.

Cholinoceptive cells in rat cerebral cortex: somatodendritic immunoreactivity for muscarinic receptor and cytoskeletal proteins

Adult rat telecephalon was surveyed for cells demonstrating immunopositivity for muscarinic receptor (M35 antibody), microtubule-associated proteins, neurofilaments, and brain-spectrin. Neurons immunostained for muscarinic receptor were found in frontal, parietal, temporal, and occipital isocortex where they accounted for approximately 15-16% of all neurons. This labeling involved a large proportion of layer V pyramidal cells, some layer III pyramidal cells and a small proportion of non-pyramidal cells in layers II-VI. In the hippocampus, pyramidal cells, non-pyramidal cells and granular cells were immunoreactive, as were many pyramidal cells in subicular and entorhinal cortices. In every cortical region examined, cells demonstrating muscarinic receptor were morphologically identical to cells stained lightly to moderately for acetylcholinesterase following pretreatment with diisopropylfluorophosphate, and they were found in similar numbers and in a similar laminar distribution. These characteristics further corresponded to those of cells whose somatodendritic compartments were intensely immunostained by antibodies to microtubule-associated proteins (MAP): MAP-1, MAP-2, MAP-5; neurofilament proteins (NF): NF-68kD, NF-160kD, NF-200kD; and brain-spectrin. Double immunostaining using a fluorescence method followed by an avidin-biotin staining procedure revealed that cortical cells which possessed immunoreactivity for muscarinic receptor demonstrated an 80-85% overlap with cells that were immunoreactive for MAP-2 (and tau) or NF-200kD. Following unilateral ibotenic acid lesions of the nucleus basalis, MAP-2 immunostaining was reduced in the ipsilateral isocortex. This significant reduction was most evident in the parietal cortex, exactly where maximal loss of acetylcholinesterase-containing fibers occurred. The same lesion produced no significant difference in immunodensity of muscarinic receptor, MAP-1, MAP-5 NF-68kD, NF-160kD and NF-200kD. Thus, cortical cholinoceptive cells are enriched with cytoskeletal components and cholinergic afferents modulate cortical MAP-2.

Postnatal development of cholinergic presynaptic inhibition in rat hippocampus

Muscarinic depression of field potential excitatory postsynaptic potentials (fEPSPs) in striatum radiatum of area CA1 was compared in hippocampal slices from rats of different ages. Bath application of 4 microM muscarine reversibly depressed the fEPSP slope by 68.4% in slices from adult animals (P43-P60), but caused only a 32.2% depression in slices from P5-P7 animals. The magnitude of the depression increased with age during the first postnatal month. Reduced sensitivity of excitatory synaptic transmission to cholinergic depression during postnatal development could be one factor contributing to the hyperexcitability of immature hippocampus.

Characterization of a calcium-dependent current generating a slow afterdepolarization of CA3 pyramidal cells in rat hippocampal slice cultures

A depolarization-induced, slowly decaying inward current was examined in slice-cultured CA3 pyramidal cells by voltage-clamp techniques and microfluorometric measurements of cytosolic free Ca2+ concentration ([Ca2+]i). Action potentials elicited by intracellular injection of short-lasting (50-100 ms) depolarizing current pulses were followed by a slowly decaying afterhyperpolarization (AHP). After switching to voltage-clamp mode, short-lasting (50-100 ms) depolarizing voltage jumps from -60 mV to between -30 and 0 mV induced a slowly decaying outward aftercurrent (IAHP) which was depressed by bath application of muscarine (0.5 microM). In the presence of muscarine, the same depolarizations induced a slowly decaying afterdepolarization (ADP) or inward aftercurrent (IADP) in voltage-clamp mode. This current was also induced in the presence of trans(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD, 5 microM), an agonist of metabotropic glutamate receptors, but not in the presence of noradrenalin (5 microM), while both of these agonists depressed IAHP. IADP was depressed by reducing the external Ca2+ concentration from 3.8 to 0.5 mM, by external Co2+ (1 mM) and by external Cd2+ (10-100 microM). Combined voltage-clamp recordings and microfluorometric measurements of [Ca2+]i using the Ca2+ indicator fura-2 revealed that the amplitude of IADP was correlated with the amplitude of depolarization-induced Ca2+ influx. IADP was absent at membrane potentials < -90 mV, and reached maximal amplitudes at approximately -55 mV. Raising the extracellular K+ concentration from 2.7 to 13.5 mM increased the amplitude of IADP and resulted in a positively directed shift of the apparent reversal potential of IADP. When the external Na+ concentration was reduced from 157 to 33 or 18 mM the current reversed at more negative potentials and was reduced to 40 and 21%, respectively, of control amplitude. Lowering the external CI- concentration from 159 to 20 mM did not affect IADP. We conclude that IADP most likely represents a Ca(2+)-activated cation current, rather than a Ca2+ tail current, or an electrogenic Ca2+ extrusion current.

Characterization of cholinergic and noradrenergic slow excitatory postsynaptic potentials from rat cerebral cortical neurons

Intracellular recordings from layer V pyramidal neurons in rat somatosensory neocortical slices were used to investigate the effects of electrically stimulating slices known to contain cholinergic and noradrenergic fibers. Repetitive electrical stimulation ventral to the recording site elicited a series of fast excitatory postsynaptic potentials followed by an inhibitory postsynaptic potential. These potentials were followed by a slow excitatory postsynaptic potential that lasted up to tens of seconds. The slow excitatory postsynaptic potential was more prominent when neurons were depolarized to 5-10 mV below firing threshold and was associated with increased input resistance and generated action potentials. The slow excitatory postsynaptic potential increased the amplitude of membrane potential oscillations and blocked the slow afterhyperpolarization which followed trains of action potentials. The amplitude of the slow excitatory postsynaptic potential was sensitive to extracellular potassium concentration. Blockade of postsynaptic action potentials by QX-314 did not block slow excitatory postsynaptic potentials. Exposure of slices to tetrodotoxin did block slow excitatory postsynaptic potentials, indicating they were dependent on propagated action potentials. Application of antagonists of glutamate and fast GABA responses failed to block slow excitatory postsynaptic potentials. Exposure to atropine or either propranolol or atenolol partially antagonized slow excitatory postsynaptic potentials, but only when atropine was added in combination with one of the other agents was the slow excitatory postsynaptic potential completely blocked. Exposure of slices to eserine, imipramine, or cocaine enhanced slow excitatory postsynaptic potentials. It is concluded that the slow excitatory postsynaptic potential triggered in neocortical slices is a composite of a cholinergic and a noradrenergic slow excitatory postsynaptic potential, and these potentials are capable of altering the firing properties of neurons for tens of seconds.

Synapse formation between dissociated basal forebrain neurones and hippocampal cells in culture

Dissociated neurones from rat basal forebrain and hippocampus were co-cultured in vitro for 8-15 days. Patch-clamp recordings from individual presumed hippocampal pyramidal cells revealed synaptic currents following focal extracellular stimulation of single presumed basal forebrain neurones. Of 18 neurone pairs, 13 showed inward (excitatory) synaptic currents, 4 showed outward (inhibitory) synaptic currents, and 1 showed a mixed current. Latencies varied from 4 to 15 ms, suggesting both mono- and polysynaptic currents. These experiments indicate that synaptic connections can be established between basal forebrain and hippocampal cells in dissociated cell culture.

Cardiovascular responses to combined microinjection of substance P and acetylcholine in the intermediolateral nucleus of the rat

As microinjection of either substance P (SP) or acetylcholine (ACh) into the right intermediolateral cell nucleus (IML) at the T2 level elicits increases in heart rate (HR) in the anesthetized rat, we investigated the possibility of a synergistic effect on HR and arterial pressure (AP) of ACh and SP microinjected in this nucleus. Moreover, we studied the effect on HR and AP of microinjection of either ACh or SP into the IML combined with activation of cardiovascular neurons in the ipsilateral rostral ventrolateral medulla (RVLM) by microinjection of glutamate (Glu). Male Wistar rats (n = 16) were anesthetized with urethane (1.4 g/kg i.p.), artificially ventilated, and the dorsal medulla and spinal cord (T1-T3) were exposed. Micropipettes containing SP and ACh were positioned in the right IML at the T2 level. Microinjection of threshold amounts of ACh (5 x 10(-2) M, 2-10 nl) and SP (3 x 10(-6) M, 2-10 nl) that caused small or no changes in HR or AP (less than 10 bpm or mmHg) elicited statistically significant synergistic increases in HR (22.9 +/- 3.3 bpm) but no changes in AP. Threshold microinjections of Glu (0.18 M, 2-10 nl) into the right RVLM combined with microinjections of threshold amounts of SP or ACh into the ipsilateral IML elicited significant synergistic increases in HR of 13.1 +/- 1.9 bpm and 10.6 +/- 1.9 bpm and in AP of 9.7 +/- 1.9 mmHg and 10.8 +/- 1.7 mmHg, respectively. These results indicate that SP and ACh interact to influence cardioacceleratory spinal preganglionic neurons (SPN) and interact with the transmitter released in the IML by RVLM stimulation to elicit increases in HR.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholinergic and adrenergic agents modify the initiation and termination of epileptic discharges in the dentate gyrus

A unique type of limbic seizures, maximal dentate activation, was used to examine the effects of cholinergic and adrenergic agents on the processes involved in epileptogenesis. The time to onset of maximal dentate activation was used to monitor the initiation of seizures while the duration of maximal dentate activation monitored termination of seizures. The cholinergic agonist pilocarpine shortened maximal dentate activation at 20 mg/kg and lengthened maximal dentate activation at 50 mg/kg, while both doses delayed the onset of maximal dentate activation. Atropine, a cholinergic antagonist, at 50 mg/kg, slowed the rate of lengthening of maximal dentate activation that occurred with repeated stimulation. The beta-adrenergic antagonist propranolol also slowed the rate of lengthening of maximal dentate activation at 3 mg/kg and shortened maximal dentate activation at 10 mg/kg. The alpha 2-agonist clonidine, at 0.5 mg/kg, shortened maximal dentate activation and increased the time to onset; at 0.1 mg/kg, clonidine did not affect maximal dentate activation. Pretreatment with reserpine had no effect on either the time to onset or duration of maximal dentate activation. These results indicate that both cholinergic and adrenergic mechanisms play important roles in the initiation and termination of limbic seizures.

Long-lasting facilitation of excitatory postsynaptic potentials in the rat hippocampus by acetylcholine

1. The effects of acetylcholine (ACh) on excitatory postsynaptic potentials (EPSPs) evoked by stimulating Schaffer-commissural afferents and on ionophoretically applied L-glutamate ligands, were investigated in CA1 neurones of hippocampal slices using current- and voltage-clamp techniques. 2. ACh produced a transient suppression followed by a long-lasting facilitation of EPSPs. The facilitation was also seen in Cs(+)-filled cells under voltage-clamp conditions. Both suppressing and facilitating effects were blocked by atropine. 3. All components of the EPSP were reduced in the initial phase of ACh action, while only the slow component was enhanced during the later phase. The facilitation was blocked by an N-methyl-D-aspartate (NMDA) receptor antagonist, d-2-amino-5-phosphonovalerate (2-APV) and by hyperpolarization. 4. ACh also facilitated responses to ionophoretically applied NMDA in voltage-clamped, Cs(+)-filled cells in Ba2(+)-treated slices. ACh facilitated responses to L-glutamate which was blocked by 2-APV. ACh failed to affect responses to kainate or quisqualate. 5. We conclude that ACh, acting on muscarinic receptors, exerts a primary effect in the hippocampus to specifically amplify NMDA receptor-mediated synaptic responses and thereby facilitate EPSPs.

Effects of diisopropyl phosphorofluoridate (DFP) on CA3 and CA1 responses in rat hippocampus

Diisopropyl phosphorofluoridate (DFP), an insecticide, is a potent anticholinesterase that binds essentially irreversibly to acetylcholinesterase, resulting in severe, acute neurologic pathology, and less severe, but longer-lasting, delayed neuropathy. We report here on the short-term effects of bath-applied DFP on extracellularly recorded responses from CA3 and CA1 of rat hippocampus. Exposure to 10 microM DFP evokes low amplitude, spontaneous bursts in CA3 generally within 10 minutes, and the bursting does not reverse with washing. The CA1 neuronal population usually bursts synchronously with CA3, but the population events are of low amplitude and sometimes not detectable, implying a differential sensitivity to DFP. These effects were partially blocked by the muscarinic antagonist atropine, while the cholinergic antagonist gallamine had little effect. Also, the reversible anticholinesterase physostigmine could, within temporal limits, protect slices from DFP's effects, implicating the cholinergic system as the probable mediator in the first stages of DFP-induced epileptogenesis.

Ethanol diminishes a voltage-dependent K+ current, the M-current, in CA1 hippocampal pyramidal neurons in vitro

Previous in vivo studies showed that systemic ethanol enhanced hippocampal neuronal responses to iontophoretically applied acetylcholine and somatostatin while having little or no effect on responses to other transmitters. We previously reported that these two agonists reciprocally regulate the non-inactivating, voltage-dependent K+ current called the M-current. Therefore, we tested ethanol superfusion on this current in rat hippocampal pyramidal neurons in vitro, using intracellular recording and single electrode voltage-clamp methods. Tetrodotoxin (TTX) was used to block Na+ spikes and synaptic transmitter release. Ethanol in low concentrations (22-44 mM), like muscarinic agonists, greatly reduced the M-current amplitude at depolarized membrane potentials and at 44 mM antagonized its augmentation by somatostatin. These changes were often accompanied by an inward baseline current with a conductance decrease. Other than a small inward current in some cells there was little or no consistent ethanol effect at resting membrane potentials. Atropine 1 microM (and TTX) did not alter the ethanol effect on the M-current. Therefore, the site of ethanol action is most likely distal to the muscarinic receptor. Ethanol reduction of the M-current, by summation of like effects, may account for the potentiation of acetylcholine responses seen in vivo and in vitro, and provides a mechanism for the excitatory effects of ethanol on some central neurons.

Acetylcholine potentiates responses to N-methyl-D-aspartate in the rat hippocampus

The effect of acetylcholine (ACh) on intracellular responses to ionophoretic application of N-methyl-D-aspartate (NMDA) was examined in rat hippocampal slice. Recordings were obtained from CA1 neurons under current- and voltage-clamp conditions. Drugs were applied topically by ionophoretic and microdrop techniques. ACh produced an atropine-sensitive potentiation of responses to NMDA. The effect of ACh on NMDA receptor-mediated responses was independent of changes in voltage or potassium conductances caused by ACh. ACh also potentiated responses to L-glutamate but not to kainate or quisqualate. This effect was blocked by DL-2-amino-5-phosphonovalerate (2-APV), an NMDA receptor antagonist. We conclude that ACh, acting on muscarinic receptors, potentiates selectively, the NMDA subclass of L-glutamate receptor.

Cholinergic modulation of responses to single tones produces tone-specific receptive field alterations in cat auditory cortex

Acetylcholine (ACh), acting via muscarinic receptors, is known to modulate neuronal responsiveness in primary sensory neocortex. The administration of ACh to cortical neurons facilitates or suppresses responses to sensory stimuli, and these effects can endure well beyond the period of ACh application. In the present study, we sought to determine whether ACh produces a general change in sensory information processing, or whether it can specifically alter the processing of sensory stimuli with which it was "paired". To answer this question, we restricted acoustic stimulation in the presence of ACh to a single frequency, and determined single neuron frequency receptive fields in primary auditory cortex before and after this pairing. During its administration, ACh produced mostly facilitatory effects on spontaneous activity and on responses to the single frequency tone. Examination of frequency receptive fields after ACh administration revealed receptive field modifications in 56% of the cells. In half of these cases, the receptive field alterations were highly specific to the frequency of the tone previously paired with ACh. Thus ACh can produce stimulus-specific modulation of auditory information processing. An additional and unexpected finding was that the type of modulation during ACh administration did not predict the type of receptive field modulation observed after ACh administration; this may be related to the physiological "context" of the same stimulus in two different conditions. The implications of these findings for learning-induced plasticity in the auditory cortex is discussed.

Cholinergic effects on spinal dorsal horn neurons in vitro: an intracellular study

The cholinoceptive properties of dorsal horn neurons (lamina III-V) were investigated by means of intracellular recordings from the rat isolated spinal cord slice preparation. In half of the neurons investigated, acetylcholine (ACh) evoked a dose-dependent slow depolarization and increase in excitability; hyperpolarization was observed in 10% of neurons. Acetyl-beta-methylcholine (MCh) similarly depolarized 39% and hyperpolarized 25% of neurons tested; depolarization was also observed following bethanechol. Responses to the muscarinic agonists were abolished by atropine (10(-5) M). Nicotine depolarized 84% of tested neurons; dihydro-beta-erythroidine (5 x 10(-5) M) and (+)-tubocurarine (10(-6) M) antagonized this depolarization. ACh-, MCh- and nicotine-induced depolarizations, associated with changes in input resistance, were maintained in the presence of tetrodotoxin (10(-6) M). Substance P, as well as repetitive electrical stimulation of the dorsal root, also evoked depolarization in ACh-sensitive neurons. Atropine, but not (+)-tubocurarine, diminished responses to both substance P and dorsal root stimulation. These results indicate that dorsal horn neurons are ACh-sensitive and possess both muscarinic and nicotinic receptors. In addition, the parallel sensitivity of neurons to muscarinic agonists, substance P and dorsal root stimulation, as well as the parallel antagonistic effect of atropine, are supportive of a common ionic mechanism underlying the activation of muscarinic and substance P receptors.

Effect of anticholinergic drug on long-term potentiation in rat hippocampal slices

Long-term potentiation (LTP) was studied in CA1 neurons by tetanization of the Schaffer-commissural pathway in rat hippocampal slices. A brief tetanus (200 Hz for 1 s) caused an increase in amplitude of the population spike recorded from the CA1 area (typically about 200% of control), which lasted for more than 2 h. LTP was suppressed by perfusion of the muscarinic antagonist scopolamine (10(-5) M) from 5 min before to 15 min after the tetanus. If perfusion of the drug was begun after the tetanus, there was no affect on LTP. Scopolamine perfused without tetanus did not change the amplitude of the population spike. These results suggest that cholinergic system may affect the generation mechanisms of LTP.

Potentiation and suppression by eserine of muscarinic synaptic transmission in the guinea-pig hippocampal slice

1. The effects of the anticholinesterase eserine on CA3 pyramidal cells and dentate gyrus granule cells in guinea-pig hippocampal slices were investigated with single-electrode current-clamp and voltage-clamp recording. 2. In the majority of cells superfused with eserine (0.5-10 microM) for 3-10 min, tetanic stimulation near the cell layers elicited a delayed depolarization (slow EPSP; duration up to 60 s) at a pre-stimulation membrane potential of -60 mV. The slow EPSP was blocked by atropine (1 microM). 3. Under voltage clamp at -60 mV holding potential, an apparent inward current (slow EPSC) with a similar time course to the slow EPSP was observed. 4. The amplitude of the delayed inward current was about 50 pA. The amplitude increased at holding potentials more positive than -60 mV. At holding potentials negative to -60 mV, the delayed inward current was too small to allow reliable analysis. In the absence of eserine, there was a delayed inward current, which was rather small, however, due to a superimposed outward current. 5. Eserine reduced the after-hyperpolarization following a train of action potentials. This effect was antagonized by atropine, but not to pirenzepine. In voltage-clamp recording, eserine reduced a current termed IAHP. 6. CA3 neurones treated with eserine exhibited a region of negative slope conductance (in tetrodotoxin). The slow inward current which developed at clamp potentials between -50 and -40 mV was reduced by Ni2+ (50 microM). The effect of eserine on slope conductance increased with time of exposure. In all neurones superfused with eserine for more than 60 min, burst discharges were observed. Burst discharges were blocked by atropine and Ni2+, but not by pirenzepine. 7. In cells superfused with eserine for more than 1 h, tetanic stimulation failed to elicit a slow EPSP or EPSC. Currents induced by focal acetylcholine (ACh) application were first enhanced by eserine, but blocked after exposure to eserine for more than 1 h. Blockade of ACh-induced currents was also observed after bath application of carbachol (CCh) in a concentration (0.2 microM) in which it did not induce an inward current at -60 mV holding potential. Further, the slow EPSP faded when elicited by repeated tetanic stimulation. 8. While the observed effects of eserine on hippocampal neurones can be explained by eserine's well-known ability to block acetylcholinesterase activity, our data indicate that the effects of eserine involve more than one muscarinic receptor site, i.e. desensitizing and non-desensitizing postsynaptic receptor sites.

Cholinergic modulation of frequency receptive fields in auditory cortex: II. Frequency-specific effects of anticholinesterases provide evidence for a modulatory action of endogenous ACh

Exogenously applied muscarinic agonists--for example, acetylcholine (ACh) and acetyl-beta-methacholine (MCh)--modify frequency receptive fields in auditory cortex of unanesthetized animals in a frequency-specific rather than global manner. The present study sought to relate these findings to endogenous actions of ACh by using the anticholinesterase agents eserine sulphate and soman (0-1,2,2-trimethylpropylmethylphosphonofluoridate) to facilitate the effects of endogenous ACh. Frequency receptive fields (FRF) were determined by presenting sequences of different isointensity tones before, during, and after application of ACh, MCh, eserine, or soman; also the cholinesterase blockers were applied between applications of ACh or MCh. The major effects produced by the inhibitors were similar to those of the agonists. Predominant effects were frequency-specific changes in FRF. Further, eserine and soman, similar to ACh and MCh, produced shifts in the best frequency (BF) of FRF due mainly to coordinated depression of responses to the BF and increased responses to adjacent, non-BF. The results indicate that exogenous and endogenous ACh, acting via muscarinic receptors, can significantly influence the physiological functioning of cortical neurons and consequently their processing of sensory information.

Synaptic and non-synaptic mechanisms underlying low calcium bursts in the in vitro hippocampal slice

1. The epileptiform activity generated by lowering extracellular [Ca++] was studied in the CA1 subfield of rat hippocampal slices maintained "in vitro" at 32 degrees C. Extracellular and intracellular recordings were performed with NaCl and KCl filled microelectrodes. 2. Synaptic potentials evoked by stimulation of the stratum radiatum and alveus were blocked upon perfusion with artificial cerebrospinal fluid (ACSF) containing 0.2 mM Ca++, 4 mM Mg++. Blockade of synaptic potentials was accompanied by the appearance of synchronous field bursts which either occurred spontaneously or could be induced by stimulation of the alveus. 3. Both spontaneous and stimulus-induced low Ca++ bursts recorded extracellularly in stratum pyramidale consisted of a negative potential shift with superimposed population spikes. This extracellular event was closely associated with intracellularly recorded action potentials rising from a prolonged depolarization shift. Steady hyperpolarization of the cell membrane potential decreased the amplitude of the depolarizing shift suggesting that synaptic conductance were not involved in the genesis of the low Ca++ burst. 4. Spontaneous depolarizing inhibitory potentials recorded in normal ACSF with KCl filled microelectrodes were reduced in size in low Ca++ ACSF. However, small amplitude potentials could still be observed at a time when low CA++ bursts were generated by hippocampal CA1 pyramidal neurons. 5. Bicuculline methiodide, an antagonist of gamma-aminobutyric acid (GABA), was capable of modifying the frequency of occurrence and the shape of synchronous field bursts. The effects evoked by bicuculline methiodide were, however, not observed when 81-100% of NaCl was replaced with Na-Methylsulphate.(ABSTRACT TRUNCATED AT 250 WORDS)

Sensory modulation of hippocampal transmission. II. Evidence for a cholinergic locus of inhibition in the Schaffer-CA1 synapse

The present work studied the neurotransmitter mediating the depressive effect of sensory stimulation on the Schaffer-CA1 transmission. Field responses of the CA1 region evoked by ipsilateral CA3 stimuli were recorded in paralyzed, locally anesthetized rats following the same experimental paradigm as in the previous work. The tissue zone under recording was perfused in vivo by an implanted hollow fiber (brain dialysis device) with either Krebs-Ringer bicarbonate (KRB), or KRB with penicillin, atropine, acetylcholine or eserine. Results were the following: (1) atropine increased the field excitatory postsynaptic potential (EPSP) amplitude in a dose-dependent manner and totally abolished the modulatory action of sensory stimulation; (2) both the field EPSP and the modulatory action of sensory stimulation remained unaltered during the blockade of GABAergic activity by penicillin; (3) acetylcholine as well as eserine induced a great diminution of both field EPSP and population spike amplitudes, without altering the effect of sensory stimulation; (4) penicillin and atropine induced multiple population spikes, reversing the effect of sensory stimulation and increasing the cell excitability. These results demonstrate that the sensory modulation of information transfer through the Schaffer-CA1 synapse is mediated by a muscarinic cholinergic mechanism. The dose-dependent increase in the field EPSP by muscarinic blockade is evidence for the existence of a cholinergic presynaptic inhibition on the Schaffer collateral terminals.

Somatostatin augments the M-current in hippocampal neurons

Immunocytochemical and electrophysiological evidence suggests that somatostatin may be a transmitter in the hippocampus. To characterize the ionic mechanisms underlying somatostatin effects, voltage-clamp and current-clamp studies on single CA1 pyramidal neurons in the hippocampal slice preparation were performed. Both somatostatin-28 and somatostatin-14 elicited a steady outward current and selectively augmented the noninactivating, voltage-dependent outward potassium current known as the M-current. Since the muscarinic cholinergic agonists carbachol and muscarine antagonized this current, these results suggest a reciprocal regulation of the M-current by somatostatin and acetylcholine.

Muscarinic agonists modulate spontaneous and evoked unit discharge in auditory cortex of cat

The present experiments studied the effects of cholinergic agonists and antagonists on the spontaneous and acoustic-evoked discharge of auditory cortical neurons and examined whether these effects were mediated by muscarinic cholinergic receptors. A primary focus of this report is the analysis of specific effects of these agents on the spontaneous and tone-evoked discharge and on different temporal components of the evoked discharge. Single neurons were recorded in the auditory cortex of chronically prepared, awake cats with multibarrel micropipette electrodes. The responses to acoustic stimuli were obtained before, during, and following continuous ejection of cholinergic agonist or antagonists by micropressure. The mean rate of discharge of the neurons was analyzed quantitatively for spontaneous discharge and for different peaks of the tone-evoked PSTH corresponding to tone "on," "through," and "off" responses. Acetylcholine (ACh) and acetyl-beta-methacholine (MCh) produced significant effects on spontaneous activity in 72% and 68% of neurons tested, respectively. Tone-evoked responses were effected in 92% and 82% of cells tested, respectively. The ability of these agonists to modify spontaneous or evoked activity was dose-dependent. Agonist effects on spontaneous and evoked activity were often different in the same cell; however, effects on spontaneous activity did predict effects on "through" responses. The most common effect of ACh or MCh on evoked activity was facilitation of the tone "on" response. For neurons with multicomponent discharge patterns in response to tones, the agonists had nonuniform effects on different response components. However, the effects of ACh on the "on" and "off" responses covaried. Hence cholinergic agonists produce heterogeneous, selective effects on different components of the responses of auditory cortical neurons rather than simple increases or decreases in discharge level. The effects of cholinergic agonists were modified in the presence of atropine. The effects of MCh were blocked by atropine in a higher proportion of cases than those of ACh.

Differential effects of acetylcholine in the chicken auditory neostriatum and hyperstriatum ventrale--studies in vivo and in vitro

1. The effect of acetylcholine (ACh) on the response properties of single units in the caudal auditory telencephalon was studied both in awake chickens and in an in vitro slice preparation. 2. Two types of electrophysiological behavior in response to ACh were observed: an inhibition of cell firing typical for the majority of neurons in the auditory hyperstriatum ventrale and a facilitation of neuronal responses seen predominantly in neostriatal auditory units. 3. The facilitatory effect of ACh is also present in hyperstriatal cells, but is usually dominated by an indirect inhibition. 4. ACh-induced facilitation on single unit responses could be mimicked in awake birds by applying potentially arousing sensory stimuli. 5. The effects of ACh are antagonized by the muscarinic receptor blocker scopolamine. 6. Inhibitory responses can also be antagonized by the GABA-antagonist bicuculline and thus can be attributed to an ACh-induced activation of GABAergic inhibitory interneurons. Evidence is given that the facilitatory responses result from a closure of voltage-dependent potassium channels. 7. The results are discussed with respect to a possible role of cholinergic afferents in telencephalic processing of auditory information and in comparison with the cholinergic influences in the mammalian neocortex.

The effects of aging on hippocampal and cortical projections of the forebrain cholinergic system

It has been proposed that disruption of cholinergic input to the hippocampus and cortex contributes to the learning and memory deficits associated with aging. The data reviewed here, however, suggest that the oft-stated generalization that normal aging is characterized by disruption of cholinergic input to the hippocampus and cortex is not entirely correct. Instead it appears that age-related changes are not consistently found on measures such as the activity of ChAT or the content of ACh in these regions, basal levels of ACh release in cortex, and the number of cholinergic neurons in the basal forebrain (source of cholinergic input to the hippocampus and cortex). These observations suggest that unlike Alzheimer's disease, normal aging does not reliably produce a degeneration of the cholinergic innervation of the hippocampus and cortex. The responsivity of the cholinergic system, however, is altered during normal aging. ACh synthesis and stimulation-induced release of ACh are diminished in aged animals. Further, the electrophysiological response of postsynaptic neurons to ACh is reduced during aging. Although some regional differences in these age-related changes may be present, the generalization that the functioning of the cholinergic system is impaired during aging is probably accurate. Thus, investigation of these changes in the dynamic properties of cholinergic input to the hippocampus and cortex during aging may provide clarification of the relationship between cholinergic dysfunction and age-related decline in learning and memory and may also provide a more reasonable rationale for treatment approaches.

Cholinomimetics induce theta rhythm and reduce hippocampal pyramidal cell excitability

The actions of cholinomimetics and of physostigmine were tested on two parameters reflecting hippocampal activity, namely theta activity and pyramidal cell excitability. In rats pretreated with methylscopolamine and anaesthetized with urethane i.v. administration of the cholinomimetics oxotremorine and arecoline and the cholinesterase blocker physostigmine evoked theta wave activity in the hippocampus, which was blocked by scopolamine. Spectral analysis demonstrated that the frequency of the theta waves induced was dose-related, ranging from about 3 Hz to between 5 and 6 Hz. theta Activity could not be induced by arecoline in animals with large septal lesions. Pyramidal cell excitability is known to be increased by endogenous acetylcholine released from cholinergic fibres. In the present study, however, i.v. injections of oxotremorine, arecoline and physostigmine in doses that induce theta activity diminished the excitability of CA1 pyramidal cells in a dose-dependent manner, as judged by the reduction in the amplitude of the population spike and the dendritic epsp. These depressant effects were attenuated by scopolamine but not by methylscopolamine. The depressant effect of arecoline was attenuated in rats with extensive lesions in the medial septal area. The present findings demonstrate that exogenously administered cholinomimetics only partly mimic the action of endogenous acetylcholine in the hippocampus. The central sites of action of exogenously administered cholinomimetics for mediation of theta activity and alteration of pyramidal cell excitability remain to be elucidated.