Theta-activity-dependent and -independent muscarinic facilitation of long-term potentiation in guinea pig hippocampal slices

Adaptive control of synaptic plasticity integrates micro- and macroscopic network function

Synaptic plasticity configures interactions between neurons and is therefore likely to be a primary driver of behavioral learning and development. How this microscopic-macroscopic interaction occurs is poorly understood, as researchers frequently examine models within particular ranges of abstraction and scale. Computational neuroscience and machine learning models offer theoretically powerful analyses of plasticity in neural networks, but results are often siloed and only coarsely linked to biology. In this review, we examine connections between these areas, asking how network computations change as a function of diverse features of plasticity and vice versa. We review how plasticity can be controlled at synapses by calcium dynamics and neuromodulatory signals, the manifestation of these changes in networks, and their impacts in specialized circuits. We conclude that metaplasticity-defined broadly as the adaptive control of plasticity-forges connections across scales by governing what groups of synapses can and can't learn about, when, and to what ends. The metaplasticity we discuss acts by co-opting Hebbian mechanisms, shifting network properties, and routing activity within and across brain systems. Asking how these operations can go awry should also be useful for understanding pathology, which we address in the context of autism, schizophrenia and Parkinson's disease.

Presynaptic Inhibitory Effects of Acetylcholine in the Hippocampus: A 40-Year Evolution of a Serendipitous Finding

Cholinergic regulation of hippocampal circuit activity has been an active area of neurophysiological research for decades. The prominent cholinergic innervation of intrinsic hippocampal circuitry, potent effects of cholinomimetic drugs, and behavioral responses to cholinergic modulation of hippocampal circuitry have driven investigators to discover diverse cellular actions of acetylcholine in distinct sites within hippocampal circuitry. Further research has illuminated how these actions organize circuit activity to optimize encoding of new information, promote consolidation, and coordinate this with recall of prior memories. The development of the hippocampal slice preparation was a major advance that accelerated knowledge of how hippocampal circuits functioned and how acetylcholine modulated these circuits. Using this preparation in the early 1980s, we made a serendipitous finding of a novel presynaptic inhibitory effect of acetylcholine on Schaffer collaterals, the projections from CA3 pyramidal neurons to dendrites of CA1 pyramidal cells. We characterized this effect at cellular and pharmacological levels, published the findings in the first volume of the Journal of Neuroscience, and proceeded to pursue other scientific directions. We were surprised and thrilled to see that, nearly 40 years later, the paper is still being cited and downloaded because the data became an integral piece of the foundation of the science of cholinergic regulation of hippocampal function in learning and memory. This Progressions article is a story of how single laboratory findings evolve through time to be confirmed, challenged, and reinterpreted by other laboratories to eventually become part of the basis of fundamental concepts related to important brain functions.

Chemogenetic Activation of Excitatory Neurons Alters Hippocampal Neurotransmission in a Dose-Dependent Manner

Designer receptors exclusively activated by designer drugs (DREADD)-based chemogenetic tools are extensively used to manipulate neuronal activity in a cell type-specific manner. Whole-cell patch-clamp recordings indicate membrane depolarization, coupled with increased neuronal firing rate, following administration of the DREADD ligand, clozapine-N-oxide (CNO) to activate the Gq-coupled DREADD, hM3Dq. Although hM3Dq has been used to enhance neuronal firing in order to manipulate diverse behaviors, often within 30 min to 1 h after CNO administration, the physiological effects on excitatory neurotransmission remain poorly understood. We investigated the influence of CNO-mediated hM3Dq DREADD activation on distinct aspects of hippocampal excitatory neurotransmission at the Schaffer collateral-CA1 synapse in hippocampal slices derived from mice expressing hM3Dq in Ca²⁺/calmodulin-dependent protein kinase α (CamKIIα)-positive excitatory neurons. Our results indicate a clear dose-dependent effect on field EPSP (fEPSP) slope, with no change noted at the lower dose of CNO (1 µM) and a significant, long-term decline in fEPSP slope observed at higher doses (5-20 µM). Further, we noted a robust θ burst stimulus (TBS) induced long-term potentiation (LTP) in the presence of the lower CNO (1 µM) dose, which was significantly attenuated at the higher CNO (20 µM) dose. Whole-cell patch-clamp recording revealed both complex dose-dependent regulation of excitability, and spontaneous and evoked activity of CA1 pyramidal neurons in response to hM3Dq activation across CNO concentrations. Our data indicate that CNO-mediated activation of the hM3Dq DREADD results in dose-dependent regulation of excitatory hippocampal neurotransmission and highlight the importance of careful interpretation of behavioral experiments involving chemogenetic manipulation.

From adagio to allegretto: The changing tempo of theta frequencies in epilepsy and its relation to interneuron function

Despite decades of research, our understanding of epilepsy, including how seizures are generated and propagate, is incomplete. However, there is growing recognition that epilepsy is more than just the occurrence of seizures, with patients often experiencing comorbid deficits in cognition that are poorly understood. In addition, the available therapies for treatment of epilepsy, from pharmaceutical treatment to surgical resection and seizure prevention devices, often exacerbate deficits in cognitive function. In this review, we discuss the hypothesis that seizure generation and cognitive deficits have a similar pathological source characterized by, but not limited to, deficits in theta oscillations and their influence on interneurons. We present a new framework that describes oscillatory states in epilepsy as alternating between hyper- and hypo-synchrony rather than solely the spontaneous transition to hyper-excitability characterized by the seizures. This framework suggests that as neural oscillations, specifically in the theta range, vary their tempo from a slowed almost adagio tempo during interictal periods to faster, more rhythmic allegretto tempo preictally, they impact the function of interneurons, modulating their ability to control seizures and their role in cognitive processing. This slow wave oscillatory framework may help explain why current therapies that work to reduce hyper-excitability do not completely eliminate seizures and often lead to exacerbated cognitive deficits.

Carbachol-Induced theta-like oscillations in the rodent brain limbic system: Underlying mechanisms and significance

Theta oscillations (4-12 Hz) represent one of the most prominent physiological oscillatory activity in the mammalian EEG. They are observed in several areas of the hippocampus and in parahippocampal structures. Theta oscillations play important roles in modulating synaptic plasticity during memory and learning; moreover, they are dependent on septal cholinergic inputs. Theta oscillations can be reproduced in vitro in several regions of the temporal lobe in the absence of the septum by employing the cholinergic agonist carbachol (CCh). Here, we review the mechanisms underlying CCh-induced theta oscillations. We address: (i) the ability of temporal lobe neuronal networks to oscillate independently at theta frequency during CCh treatment; (ii) the contribution of intrinsic ionic currents; (iii) the participation of principal cells and interneurons; and (iv) their pharmacological profiles. We also discuss the similarities between CCh-induced theta oscillations and physiological type II theta activity, as well as their roles in synaptic plasticity. Finally, we consider experimental evidence pointing to the contribution of spontaneous and CCh-induced theta activity to epileptiform synchronization.

Balancing Arc synthesis, mRNA decay, and proteasomal degradation: maximal protein expression triggered by rapid eye movement sleep-like bursts of muscarinic cholinergic receptor stimulation

Cholinergic signaling induces Arc/Arg3.1, an immediate early gene crucial for synaptic plasticity. However, the molecular mechanisms that dictate Arc mRNA and protein dynamics during and after cholinergic epochs are little understood. Using human SH-SY5Y neuroblastoma cells, we show that muscarinic cholinergic receptor (mAchR) stimulation triggers Arc synthesis, whereas translation-dependent RNA decay and proteasomal degradation strictly limit the amount and duration of Arc expression. Chronic application of the mAchR agonist, carbachol (Cch), induces Arc transcription via ERK signaling and release of calcium from IP(3)-sensitive stores. Arc translation requires ERK activation, but not changes in intracellular calcium. Proteasomal degradation of Arc (half-life ∼37 min) was enhanced by thapsigargin, an inhibitor of the endoplasmic calcium-ATPase pump. Similar mechanisms of Arc protein regulation were observed in cultured rat hippocampal slices. Functionally, we studied the impact of cholinergic epoch duration and temporal pattern on Arc protein expression. Acute Cch treatment (as short as 2 min) induces transient, moderate Arc expression, whereas continuous treatment of more than 30 min induces maximal expression, followed by rapid decline. Cholinergic activity associated with rapid eye movement sleep may function to facilitate long term synaptic plasticity and memory. Employing a paradigm designed to mimic intermittent rapid eye movement sleep epochs, we show that application of Cch in a series of short bursts generates persistent and maximal Arc protein expression. The results demonstrate dynamic, multifaceted control of Arc synthesis during mAchR signaling, and implicate cholinergic epoch duration and repetition as critical determinants of Arc expression and function in synaptic plasticity and behavior.

Phase dependency of long-term potentiation induction during the intermittent bursts of carbachol-induced β oscillation in rat hippocampal slices

The rodent hippocampus possesses theta (θ) and beta (β) rhythms, which occur intermittently as bursts. Both rhythms are related to spatial memory processing in a novel environment. θ rhythm is related to spatial memory encoding process. β rhythm is related to the match/mismatch process. In the match/mismatch process, rodent hippocampus detects a representation matching sensory inputs of the current place among the retrieved internal representations of places. Long-term synaptic potentiation (LTP) is induced in both processes. The cholinergic agent carbachol induces intermittent θ and β oscillations in in vitro slices similar to in vivo bursts. LTP is facilitated during the generation of θ oscillation, suggesting that the facilitation of LTP is dependent upon the phases of intermittent burst (burst phases) of the oscillation. However, whether this is the case for β oscillation has not yet been studied. In the present study, LTP-inducing θ-burst stimulation was administered at the different burst phases of carbachol-induced β oscillations (CIBO), and the synaptic changes were measured at CA3-CA3 pyramidal cell synapses (CA3 synapse) and at CA3-CA1 pyramidal cell synapses (CA1 synapse). At the CA3 synapse, the largest magnitude of LTP was induced at the late burst phases of CIBO. At the CA1 synapse, LTP was induced only at the late burst phases. Modulation of LTP was suppressed when CIBO was blocked by the application of atropine at both synapses. The results suggest that the bursts of hippocampal β rhythm can determine the optimal temporal period for completing with the match/mismatch process.

The cholinergic system and hippocampal plasticity

Acetylcholine is an essential excitatory neurotransmitter in the central nervous system and undertakes a vital role in cognitive function. Consequently, there is ample evidence to suggest the involvement of both nicotinic and muscarinic acetylcholine receptors in the modulation of synaptic plasticity, which is believed to be the molecular correlate of learning and memory. In the hippocampus in particular, multiple subtypes of both nicotinic and muscarinic receptors are present at presynaptic and postsynaptic loci of both principal neurons and inhibitory interneurons, where they exert profound bi-directional influences on synaptic transmission. Further evidence points to a role for cholinergic activation in the induction and maintenance of synaptic plasticity, and key influences on hippocampal network oscillations. The present review examines these multiple roles of acetylcholine in hippocampal plasticity.

Treadmill exercise improves cognitive function and facilitates nerve growth factor signaling by activating mitogen-activated protein kinase/extracellular signal-regulated kinase1/2 in the streptozotocin-induced diabetic rat hippocampus

This study aimed to investigate the effects of regular treadmill exercise on nerve growth factor (NGF) expression, the improvement of cognitive function in the hippocampus of diabetic rats, and to understand the molecular mechanisms through which the relevant signaling factors act. We investigated the effects of regular treadmill exercise for 6 weeks on NGF, tyrosine kinase receptor A (TrkA), p75 receptor, phosphatidylinositol 3-kinase (PI3-K), mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase 1/2 (Erk1/2), cyclic AMP response element-binding protein (CREB), and caspase-3 protein levels; we also assessed cell survival and cognitive function. Forty male Sprague-Dawley rats were divided into four groups: (1) normal control group (NCG: n=10); (2) normal exercise group (NEG: n=10); (3) diabetes control group (DCG: n=10), and (4) diabetes exercise group (DEG: n=10). Diabetes was induced by injecting streptozotocin (STZ; 55 mg/kg dissolved in 0.05 M citrate buffer, pH 4.5, i.p.) into rats. Rats were subjected to treadmill exercise for 5 days a week over 6 weeks, and the speed of the treadmill was gradually increased. In a passive avoidance test, the retention latency in the DCG was significantly shorter than that in the DEG (P<0.05). Increased 5-bromo-2'-deoxyuridine-5'-mono-phosphate (BrdU)-labeled cells (P<0.001) and significant increases in NGF and TrkA protein levels were observed in the hippocampal dentate gyrus in the NEG and DEG (P<0.001 and P<0.01, respectively). The p75 receptor protein level significantly increased in the NEG but decreased in the DCG (P<0.001). The p-PI3-K and t-CREB protein levels significantly increased in the NEG (P<0.001 and P<0.05, respectively), whereas t-Erk1/2 significantly decreased in the DCG (P<0.01, P<0.01, respectively). p-Erk1/2 and p-CREB protein levels significantly increased in the NEG and DEG (P<0.001, P<0.001, and P<0.01, respectively). Caspase-3 protein levels significantly increased in the DCG (P<0.001). These results show that treadmill exercise improves cognitive function, increases the number of BrdU-labeled cells, and increases NGF levels, by the activation of the MAPK/Erk1/2 signaling pathway in the hippocampus of diabetic rats.

Nonlinear dynamical analysis of carbachol induced hippocampal oscillations in mice

AIM

Hippocampal neuronal network and synaptic impairment underlie learning and memory deficit in Alzheimer's disease (AD) patients and animal models. In this paper, we analyzed the dynamics and complexity of hippocampal neuronal network synchronization induced by acute exposure to carbachol, a nicotinic and muscarinic receptor co-agonist, using the nonlinear dynamical model based on the Lempel-Ziv estimator. We compared the dynamics of hippocampal oscillations between wild-type (WT) and triple-transgenic (3xTg) mice, as an AD animal model. We also compared these dynamic alterations between different age groups (5 and 10 months). We hypothesize that there is an impairment of complexity of CCh-induced hippocampal oscillations in 3xTg AD mice compared to WT mice, and that this impairment is age-dependent.

METHODS

To test this hypothesis, we used electrophysiological recordings (field potential) in hippocampal slices.

RESULTS

Acute exposure to 100 micromol/L CCh induced field potential oscillations in hippocampal CA1 region, which exhibited three distinct patterns: (1) continuous neural firing, (2) repeated burst neural firing and (3) the mixed (continuous and burst) pattern in both WT and 3xTg AD mice. Based on Lempel-Ziv estimator, pattern (2) was significantly lower than patterns (1) and (3) in 3xTg AD mice compared to WT mice (P<0.001), and also in 10-month old WT mice compared to those in 5-month old WT mice (P<0.01).

CONCLUSION

These results suggest that the burst pattern (theta oscillation) of hippocampal network is selectively impaired in 3xTg AD mouse model, which may reflect a learning and memory deficit in the AD patients.

Modulation of normal and altered hippocampal excitability states by septal networks

The septal region of the basal forebrain plays a dual role: 1) It modulates hippocampal excitability, facilitating synaptic plasticity within hippocampal circuits. Through this mechanism, the septum facilitates diverse cognitive processes that involve hippocampal circuits. 2) Additionally, the septum maintains the hippocampal networks working within normal ranges, decreasing the probability of abnormal excitability states. Through this second mechanism, the septum prevents the occurrence of epileptic discharges. Thus, septal alterations may lead to both decreased cognitive functions and epilepsy, as observed in elderly patients affected with Alzheimer's disease.

Cholinergic suppression of glutamatergic synaptic transmission in hippocampal region CA3 exhibits laminar selectivity: Implication for hippocampal network dynamics

Acetylcholine may help set the dynamics within neural systems to facilitate the learning of new information. Neural models have shown that if new information is encoded at the same time as retrieval of existing information that is already stored, the memories will interfere with each other. Structures such as the hippocampus have a distinct laminar organization of inputs that allows this hypothesis to be tested. In region CA1 of the rat (Sprague Dawley) hippocampus, the cholinergic agonist carbachol (CCh) suppresses transmission in stratum radiatum (SR), at synapses of the Schaffer collateral projection from CA3, while having lesser effects in stratum lacunosum-moleculare (SLM), the perforant path projection from entorhinal cortex (Hasselmo and Schnell, 1994). The current research extends support of this selectivity by demonstrating laminar effects in region CA3. CCh caused significantly greater suppression in SR than in SLM at low concentrations, while the difference in suppression was not significant at higher concentrations. Differences in paired-pulse facilitation suggest presynaptic inhibition substantially contributes to the suppression and is highly concentration and stratum dependent. This selective suppression of the recurrent excitation would be appropriate to set CA3 dynamics to prevent runaway modification of the synapses of excitatory recurrent collaterals by reducing the influence of previously stored associations and allowing incoming information from the perforant path to have a predominant influence on neural activity.

Age-dependent loss of NGF signaling in the rat basal forebrain is due to disrupted MAPK activation

The loss of nerve growth factor (NGF) and its high affinity receptor TrkA has been implicated in the loss of cholinergic tone and function in Alzheimer's disease (AD) and normal aging. We employed an animal model of aging, the aged rat, which also exhibits memory loss and NGF alterations. Basal forebrain TrkA levels increased after injection of NGF in the hippocampus within 1h in young rats, but this response was diminished in aged animals as determined by Western blot analysis. Further, NGF activated MAPK pathways without changing total ERK levels and the activation of these pathways was also diminished in aged animals. The exogenous NGF injection did not appear to activate the PI-3K pathway or alter total levels of Akt significantly. These data shed light on mechanisms of NGF signaling in the CNS, and alterations in this signaling cascade associated with age and memory loss. These findings might lead to development of novel treatment therapies for the memory loss associated with AD and other age-associated neurodegenerative diseases.

The properties of carbachol-induced beta oscillation in rat hippocampal slices

The rhythmical and pharmacological properties of carbachol-induced beta oscillation were studied using rat hippocampal slices. With the application of 30 microM carbachol, beta-range oscillations with frequencies of 13-20 Hz were recorded from the CA3 region. An AMPA receptor antagonist, CNQX, diminished the oscillations. An NMDA receptor antagonist, APV, significantly suppressed the pre-established beta oscillations. The pre-application of APV blocked the start of the carbachol-induced beta oscillations. When bicuculline (BIC), a GABAA receptor antagonist, was applied to the pre-established beta oscillations, the frequency decreased to the theta-range. When 5 microM BIC was applied with 30 microM carbachol, the beta oscillations did not start; instead, theta-like activities were induced. It has been reported that carbachol in hippocampal slices can induce theta-like activities, which are not modulated by BIC, while BIC's facilitating the start of the activities. The results of the present study suggest that the GABAA receptor-mediated inhibitory transmission modulates the beta oscillation and that the transmission is needed for the start process of the oscillations. Therefore, the start and generation mechanisms of carbachol-induced beta oscillation will be different from those of carbachol-induced theta-like activities.

Phase shift of subthreshold theta oscillation in hippocampal CA1 pyramidal cell membrane by excitatory synaptic inputs

Hippocampal CA1 neurons receive multiple rhythmical inputs with relatively independent phases during theta activity. It, however, remains to be determined how these multiple rhythmical inputs affect oscillation properties in membrane potential of the CA1 pyramidal cell. In order to investigate oscillation properties in the subthreshold membrane potential, we generated oscillations in the membrane potential of the CA1 pyramidal cells in rat hippocampal slices in vitro with a sinusoidal current injection into the pyramidal soma at theta band frequencies (4-7 Hz), and analyzed effect of rhythmically excitatory synaptic inputs. The Schaffer collaterals were stimulated with a cyclic Gaussian stimulation method, whose pulse intervals were distributed at 10 pulses/cycle (5 cycles/s). We found that the cyclic Gaussian stimulations induced membrane potential oscillations and their phase delays from the mean of the pulse distribution were dependent on membrane potential oscillation amplitude. We applied four pairs of cyclic Gaussian stimulations and somatic sinusoidal current stimulations at the same frequency (5 Hz) with varying phase differences (-pi/2, 0, pi/2, pi rad). The paired stimulations induced phase distributions of the oscillation in the membrane potential, which showed a dependency on an increasing membrane potential oscillation amplitude response to cyclic Gaussian stimulation. This membrane potential dynamic was exhibited by the mixture of the membrane potential oscillation-amplitude-dependent phase delay and the linear summation of the two sinusoidal waves. These suggest that phases of the membrane potential oscillation are modulated by excitatory synaptic inputs. This phase-modulation by excitatory synaptic inputs may play a crucial role for memory operation in the hippocampus.

Long-term depression in rat CA1-subicular synapses depends on the G-protein coupled mACh receptors

The subiculum, which is the primary target of CA1 pyramidal neurons and sending efferent fibres to many brain regions, serves as a hippocampal interface in the neural information processes between hippocampal formation and neocortex. Long-term depression (LTD) is extensively studied in the hippocampus, but not at the CA1-subicular synaptic transmission. Using whole-cell EPSC recordings in the brain slices of young rats, we demonstrated that the pairing protocols of low frequency stimulation (LFS) at 3 Hz and postsynaptic depolarization of -50 mV elicited a reliable LTD in the subiculum. The LTD did not cause the changes of the paired-pulse ratio of EPSC. Furthermore, it did not depend on either NMDA receptors or voltage-gated calcium channels (VGCCs). Bath application of the G-protein coupled muscarinic acetylcholine receptors (mAChRs) antagonists, atropine or scopolamine, blocked the LTD, suggesting that mAChRs are involved in the LTD. It was also completely blocked by either the Ca2+ chelator BAPTA or the G-protein inhibitor GDP-beta-S in the intracellular solution. This type of LTD in the subiculum may play a particular role in the neural information processing between the hippocampus and neocortex.

Effects of 17β-estradiol on chemically induced long-term depression

In this study, we have investigated the effects of 17beta-estradiol (E2) on chemically induced long-term depression (LTD). LTD was induced by a brief application of N-methyl-D-aspartate (NMDA) or (R,S)-3,5-dihydroxyphenylglycine (DHPG), a group I metabotropic glutamate receptor agonist. Bath application of E2 alone potentiated population excitatory postsynaptic potentials. This potentiation was readily reversed by washing with control saline. The effect of E2 on NMDA-induced LTD was a conversion of LTD to long-term potentiation (LTP). An application of NMDA in the presence of E2 induced LTP. The induction of LTP was inhibited by an inhibitor of calcium/calmodulin dependent protein kinase (CaMKII). The results suggest that E2 potentiates NMDA receptor function and induces an increase in postsynaptic Ca2+ concentration. An increase in postsynaptic Ca2+ concentration activates CaMKII, leading to LTP. In contrast to NMDA-induced LTD, an application of DHPG in the presence of E2 induced significantly larger LTD. The results suggest that E2 potentiates an as yet unidentified process(es) in inducing LTD by an application of DHPG. The effects of E2 both on NMDA-induced and DHPG-induced LTD were suppressed by an estrogen receptor antagonist.

Window effect of temperature on carbachol-induced theta-like activity recorded in hippocampal formation in vitro

The effect of different temperatures of ACSF (18-42 degrees C) on carbachol (CCH)-induced field potentials were examined in the present study. Two hundred and thirty one experiments were performed on hippocampal formation slices maintained in a gas-liquid interface chamber. All slices were perfused with 50 microM CCH. A recording electrode was positioned in the region of CA3c pyramidal cells. The experiments gave two main findings. First, in a presence of continuous cholinergic stimulation the temperature of the bathing medium per se determined the rate of synchronization of the field potentials and pattern of EEG activity recorded. Second, within the temperature range from 33 degrees to 37 degrees C a window effect of temperature on CCH-induced theta-like activity (TLA) was noted: in this temperature range all slices tested responded only with one pattern of EEG activity-TLA. The results are discussed in light of temperature effects on hippocampal neuronal networks.

Desynchronization of carbachol-induced theta-like activities by alpha-adrenergic agents in guinea pig hippocampal slices

The effect of alpha-adrenergic agents on theta-like activity (TLA) in guinea-pig hippocampal slices was studied. TLA was induced by a cholinergic agent, carbachol. TLA had a frequency of 4.66+/-0.08 Hz (mean+/-S.E.M.) and an amplitude of 96.3+/-8.3 microV in the dentate gyrus (DG). The alpha-adrenergic agents epinephrine and clonidine increased the frequency and decreased the amplitude of TLA in a concentration-dependent manner. The agents also increased antidromic and orthodromic population spike (PS) amplitudes of the granule cells, but did not have any effect on population EPSP. Another alpha-adrenergic agent, guanabentz, had the same effect as clonidine. The adenylate cyclase inhibitor SQ22536 suppressed the increase in PS amplitude by clonidine. The results suggest that these alpha2-adrenergic agents facilitate the activity of granule cells through the activation of the alpha2-adrenoreceptor and cAMP pathway, and the facilitation causes the desynchronization of TLA.

Suppression of pair-pulse depression of the population spike in the dentate gyrus during carbachol-induced theta-like activity in guinea pig hippocampal slices

In pair-pulse stimulation experiments, pair-pulse depression (PPD) of the population spike (PS) occurred at intervals shorter than 20 ms in the dentate gyrus in guinea pig hippocampal slices. Application of 50 microM carbachol resulted in an increase in the test PS amplitude and caused suppression of PPD. This suppression was antagonized by atropine sulfate, a muscarinic receptor antagonist. Carbachol at 50 microM induced intermittent bursts of theta-like activity (TLA). We compared the pair-pulse index (PPI) during TLA with that in a rest period between bursts of TLA. The PPI was defined as the ratio of the amplitude of the test PS to that of the conditioning PS. The PPI during TLA were significantly larger than that during the rest period, although there were no significant differences in the conditioning PS amplitude and the test pEPSP slope. When TLA was induced, the PPI during the rest period was increased by bicuculline. The PPI during TLA did not change significantly with the drug. The increase by bicuculline in the PPI during the rest period was caused by increase in the test PS amplitude. PPD can occur due to inhibition of granule cell activity by inhibitory neurons. Our findings suggest that the action of inhibitory neurons on granule cell activity is suppressed by activation of muscarinic receptors, with stronger suppression during TLA than during the rest period between bursts of TLA.