Selective cholinergic depletion of the hippocampus spares both behaviorally induced Arc transcription and spatial learning and memory

Intrahippocampal Adeno-Associated Virus-Mediated Overexpression of Nerve Growth Factor Reverses 192IgG-Saporin-Induced Impairments of Hippocampal Plasticity and Behavior

One of the aspects of Alzheimer disease is loss of cholinergic neurons in the basal forebrain, which leads to development of cognitive impairment. Here, we used a model of cholinergic deficit caused by immunotoxin 192IgG-saporin to study possible beneficial effects of adeno-associated virus (AAV)-mediated overexpression of nerve growth factor (NGF) in the hippocampus of rats with cholinergic deficit. Suspension of recombinant AAV carrying control cassette or cassette with NGF was injected into both hippocampi of control rats or rats with cholinergic deficit induced by intraseptal injection of 192IgG-saporin. Analysis of choline acetyltransferase (ChAT) immunostaining showed that NGF overexpression in the hippocampus did not prevent strong loss of ChAT-positive neurons in the septal area caused by the immunotoxin. Induction of cholinergic deficit in the hippocampus led to impairments in Y-maze and beam-walking test but did not affect behavioral indices in the T-maze, open field test, and inhibitory avoidance training. NGF overexpression in the rats with cholinergic deficit restored normal animal behavior in Y-maze and beam-walking test. Recording of field excitatory postsynaptic potentials in vivo in the hippocampal CA1 area showed that induction of cholinergic deficit decreased magnitude of long-term potentiation (LTP) and prevented a decrease in paired-pulse ratio after LTP induction, and NGF overexpression reversed these negative changes in hippocampal synaptic characteristics. The beneficial effect of NGF was not associated with compensatory changes in the number of cells that express NGF receptors TrkA and NGFR in the hippocampus and medial septal area. NGF overexpression also did not prevent a 192IgG-saporin-induced decrease in the activity of acetylcholine esterase in the hippocampus. We conclude that NGF overexpression in the hippocampus under conditions of cholinergic deficit induces beneficial effects which are not related to maintenance of cholinergic function.

The effect of nerve growth factor on supporting spatial memory depends upon hippocampal cholinergic innervation

Nerve growth factor (NGF) gene therapy has been used in clinical trials of Alzheimer's disease. Understanding the underlying mechanisms of how NGF influences memory may help develop new strategies for treatment. Both NGF and the cholinergic system play important roles in learning and memory. NGF is essential for maintaining cholinergic innervation of the hippocampus, but it is unclear whether the supportive effect of NGF on learning and memory is specifically dependent upon intact hippocampal cholinergic innervation. Here we characterize the behavior and hippocampal measurements of volume, neurogenesis, long-term potentiation, and cholinergic innervation, in brain-specific Ngf-deficient mice. Our results show that knockout mice exhibit increased anxiety, impaired spatial learning and memory, decreased adult hippocampal volume, neurogenesis, short-term potentiation, and cholinergic innervation. Overexpression of Ngf in the hippocampus of Ngf gene knockout mice rescued spatial memory and partially restored cholinergic innervations, but not anxiety. Selective depletion of hippocampal cholinergic innervation resulted in impaired spatial memory. However, Ngf overexpression in the hippocampus failed to rescue spatial memory in mice with hippocampal-selective cholinergic fiber depletion. In conclusion, we demonstrate the impact of Ngf deficiency in the brain and provide evidence that the effect of NGF on spatial memory is reliant on intact cholinergic innervations in the hippocampus. These results suggest that adequate cholinergic targeting may be a critical requirement for successful use of NGF gene therapy of Alzheimer's disease.

Hippocampal acetylcholine depletion has no effect on anxiety, spatial novelty preference, or differential reward for low rates of responding (DRL) performance in rats

We investigated the role of the septo-hippocampal cholinergic projection in anxiety, spatial novelty preference, and differential reward for low rates of responding (DRL) performance. Cholinergic neurons of the rat medial septum (MS) and the vertical limb of the diagonal band of Broca (VDB) were lesioned using the selective immunotoxin, 192 IgG-saporin. Rats were then tested on several behavioral tests previously shown to be sensitive to either (a) hippocampal lesions or (b) nonselective MS/VDB lesions which target both cholinergic and γ-aminobutyric acid (GABA)-ergic projections, or both. Saporin lesions substantially reduced hippocampal cholinergic innervation, resulting in an absence of acetyl cholinesterase staining and markedly reduced choline acetyltransferase activity (mean reduction: 80 ± 5%; range: 50-97%). However, the saporin-lesioned rats did not differ from control rats in any of the behavioral tests. Thus we found no evidence from these lesion studies that the septo-hippocampal cholinergic projection plays an essential role in anxiety, spatial novelty preference, or DRL.

Adolescent Intermittent Alcohol Exposure: Deficits in Object Recognition Memory and Forebrain Cholinergic Markers

The long-term effects of intermittent ethanol exposure during adolescence (AIE) are of intensive interest and investigation. The effects of AIE on learning and memory and the neural functions that drive them are of particular interest as clinical findings suggest enduring deficits in those cognitive domains in humans after ethanol abuse during adolescence. Although studies of such deficits after AIE hold much promise for identifying mechanisms and therapeutic interventions, the findings are sparse and inconclusive. The present results identify a specific deficit in memory function after AIE and establish a possible neural mechanism of that deficit that may be of translational significance. Male rats (starting at PND-30) received exposure to AIE (5g/kg, i.g.) or vehicle and were allowed to mature into adulthood. At PND-71, one group of animals was assessed using the spatial-temporal object recognition (stOR) test to evaluate memory function. A separate group of animals was used to assess the density of cholinergic neurons in forebrain areas Ch1-4 using immunohistochemistry. AIE exposed animals manifested deficits in the temporal component of the stOR task relative to controls, and a significant decrease in the number of ChAT labeled neurons in forebrain areas Ch1-4. These findings add to the growing literature indicating long-lasting neural and behavioral effects of AIE that persist into adulthood and indicate that memory-related deficits after AIE depend upon the tasks employed, and possibly their degree of complexity. Finally, the parallel finding of diminished cholinergic neuron density suggests a possible mechanism underlying the effects of AIE on memory and hippocampal function as well as possible therapeutic or preventive strategies for AIE.

N-cadherin regulates molecular organization of excitatory and inhibitory synaptic circuits in adult hippocampus in vivo

N-Cadherin and β-catenin form a transsynaptic adhesion complex required for spine and synapse development. In adulthood, N-cadherin mediates persistent synaptic plasticity, but whether the role of N-cadherin at mature synapses is similar to that at developing synapses is unclear. To address this, we conditionally ablated N-cadherin from excitatory forebrain synapses in mice starting in late postnatal life and examined hippocampal structure and function in adulthood. In the absence of N-cadherin, β-catenin levels were reduced, but numbers of excitatory synapses were unchanged, and there was no impact on number or shape of dendrites or spines. However, the composition of synaptic molecules was altered. Levels of GluA1 and its scaffolding protein PSD95 were diminished and the density of immunolabeled puncta was decreased, without effects on other glutamate receptors and their scaffolding proteins. Additionally, loss of N-cadherin at excitatory synapses triggered increases in the density of markers for inhibitory synapses and decreased severity of hippocampal seizures. Finally, adult mutant mice were profoundly impaired in hippocampal-dependent memory for spatial episodes. These results demonstrate a novel function for the N-cadherin/β-catenin complex in regulating ionotropic receptor composition of excitatory synapses, an appropriate balance of excitatory and inhibitory synaptic proteins and the maintenance of neural circuitry necessary to generate flexible yet persistent cognitive and synaptic function.

Sustained transcription of the immediate early gene Arc in the dentate gyrus after spatial exploration

After spatial exploration in rats, Arc mRNA is expressed in ∼2% of dentate gyrus (DG) granule cells, and this proportion of Arc-positive neurons remains stable for ∼8 h. This long-term presence of Arc mRNA following behavior is not observed in hippocampal CA1 pyramidal cells. We report here that in rats ∼50% of granule cells with cytoplasmic Arc mRNA, induced some hours previously during exploration, also show Arc expression in the nucleus. This suggests that recent transcription can occur long after the exploration behavior that elicited it. To confirm that the delayed nuclear Arc expression was indeed recent transcription, Actinomycin D was administered immediately after exploration. This treatment resulted in inhibition of recent Arc expression both when evaluated shortly after exploratory behavior as well as after longer time intervals. Together, these data demonstrate a unique kinetic profile for Arc transcription in hippocampal granule neurons following behavior that is not observed in other cell types. Among a number of possibilities, this sustained transcription may provide a mechanism that ensures that the synaptic connection weights in the sparse population of granule cells recruited during a given behavioral event are able to be modified.

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.

Selective cholinergic depletion in medial septum leads to impaired long term potentiation and glutamatergic synaptic currents in the hippocampus

Cholinergic depletion in the medial septum (MS) is associated with impaired hippocampal-dependent learning and memory. Here we investigated whether long term potentiation (LTP) and synaptic currents, mediated by alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors in the CA1 hippocampal region, are affected following cholinergic lesions of the MS. Stereotaxic intra-medioseptal infusions of a selective immunotoxin, 192-saporin, against cholinergic neurons or sterile saline were made in adult rats. Four days after infusions, hippocampal slices were made and LTP, whole cell, and single channel (AMPA or NMDA receptor) currents were recorded. Results demonstrated impairment in the induction and expression of LTP in lesioned rats. Lesioned rats also showed decreases in synaptic currents from CA1 pyramidal cells and synaptosomal single channels of AMPA and NMDA receptors. Our results suggest that MS cholinergic afferents modulate LTP and glutamatergic currents in the CA1 region of the hippocampus, providing a potential synaptic mechanism for the learning and memory deficits observed in the rodent model of selective MS cholinergic lesioning.

Recognition of novel objects and their location in rats with selective cholinergic lesion of the medial septum

The importance of cholinergic neurons projecting from the medial septum (MS) of the basal forebrain to the hippocampus in memory function has been controversial. The aim of this study was to determine whether loss of cholinergic neurons in the MS disrupts object and/or object location recognition in male Sprague-Dawley rats. Animals received intraseptal injections of either vehicle, or the selective cholinergic immunotoxin 192 IgG-saporin (SAP). 14 days later, rats were tested for novel object recognition (NOR). Twenty-four hours later, these same rats were tested for object location recognition (OLR) (recognition of a familiar object moved to a novel location). Intraseptal injections of SAP produced an 86% decrease in choline acetyltransferase (ChAT) activity in the hippocampus, and a 31% decrease in ChAT activity in the frontal cortex. SAP lesion had no significant effect on NOR, but produced a significant impairment in OLR in these same rats. The results support a role for septo-hippocampal cholinergic projections in memory for the location of objects, but not for novel object recognition.

Localization of pre- and postsynaptic cholinergic markers in rodent forebrain: a brief history and comparison of rat and mouse

Rat and mouse models are widely used for studies in cognition and pathophysiology, among others. Here, we sought to determine to what extent these two model species differ for cholinergic and cholinoceptive features. For this purpose, we focused on cholinergic innervation patterns based on choline acetyltransferase (ChAT) immunostaining, and the expression of muscarinic acetylcholine receptors (mAChRs) detected immunocytochemically. In this brief review we first place cholinergic and cholinoceptive markers in a historic perspective, and then provide an overview of recent publications on cholinergic studies and techniques to provide a literature survey of current research. Next, we compare mouse (C57Bl/J6) and rat (Wistar) cholinergic and cholinoceptive systems simultaneously stained, respectively, for ChAT (analyzed qualitatively) and mAChRs (analyzed qualitatively and quantitatively). In general, the topographic cholinergic innervation patterns of both rodent species are highly comparable, with only considerable (but region specific) differences in number of detectable cholinergic interneurons, which are more numerous in rat. In contrast, immunolabeling for mAChRs, detected by the monoclonal antibody M35, differs markedly in the forebrain between the two species. In mouse brain, basal levels of activated and/or internalized mAChRs (as a consequence of cholinergic neurotransmission) are significantly higher. This suggests a higher cholinergic tone in mouse than rat, and hence the animal model of choice may have consequences for cholinergic drug testing experiments.

Acetylcholine and attention

Historically, ACh has been implicated in learning and short-term memory functions. However, more recent studies have provided support for a role of cortical ACh in attentional effort, orienting and the detection of behavioral significant stimuli. The current review article summarizes studies in animals and humans which have investigated the role of ACh in attention and cognition. An attempt has been made to differentiate between brain regions involved in attentional processes versus those important for other cognitive functions. To this purpose, various experimental methods and interventions were used. Animal behavioral studies have injected the selective immunotoxin IgG-saporin to induce specific cholinergic lesions, employed electrochemical techniques such as microdialysis, or have administered cholinergic compounds into discrete parts of the brain. Human studies that give some indication on the link between central cholinergic signaling and cognition are obviously confined to less invasive, imaging methods such as fMRI. The brain areas that are deemed most important for intact attentional processing in both animals and humans appear to be the (pre)frontal, parietal and somatosensory (especially visual) regions, where ACh plays a vital role in the top-down control of attentional orienting and stimulus discrimination. In contrast, cholinergic signaling in the septohippocampal system is suggested to be involved in memory processes. Thus, it appears that the role of ACh in cognition is different per brain region and between nicotinic versus muscarinic receptor subtypes.

Cholinergic hypofunction impairs memory acquisition possibly through hippocampal Arc and BDNF downregulation

Recent evidence suggests that activity-regulated cytoskeleton associated protein (Arc) and brain-derived neurotrophic factor (BDNF) are key players in the cellular mechanisms that trigger synaptic changes and memory consolidation. Cholinergic deafferentiation of hippocampus has been largely shown to induce memory impairments in different behavioral tasks. However, the mechanisms underlying cholinergic-induced memory formation remain unclear. The role of hippocampal cholinergic denervation on synaptic consolidation and further acquisition of spatial memory was hereby examined by analyzing Arc and BDNF in standard environment and after behavioral training in Morris water maze (MWM). In standard environment, a cholinergic hypofunction induced by the toxin (192) IgG-saporin led to significant decreases in Arc protein and mRNA as well as in BDNF. Lesioned rats subjected to MWM showed a worse acquisition performance that was reversed after galantamine treatment. Recovery of memory acquisition was accompanied by normalization of Arc and BDNF levels in hippocampus. Stimulation of muscarinic, but not nicotinic receptors, in hippocampal primary neurons caused a rapid induction of Arc production. These data suggest that cholinergic denervation of hippocampus leads to deficits in muscarinic-dependent induction of Arc and a subsequent impairment of spatial memory acquisition.

Sympathetic sprouting in visual cortex stimulated by cholinergic denervation rescues expression of two forms of long-term depression at layer 2/3 synapses

Cholinergic innervation of hippocampus and cortex is required for some forms of learning and memory. Several reports have shown that activation of muscarinic m1 receptors induces a long-term depression (mLTD) at glutamate synapses in hippocampus and in several areas of cortex, including perirhinal and visual cortices. This plasticity likely contributes to cognitive function dependent upon the cholinergic system. In rodent models, degeneration of hippocampal cholinergic innervation following lesion of the medial septum stimulates sprouting of adrenergic sympathetic axons, originating from the superior cervical ganglia (SCG), into denervated hippocampal subfields. We previously reported that this adrenergic sympathetic sprouting occurs simultaneously with a reappearance of cholinergic fibers in hippocampus and rescue of mLTD at CA3-CA1 synapses. Because cholinergic neurons throughout basal forebrain degenerate in aging and Alzheimer's disease, it is critical to determine if this compensatory sprouting occurs in other regions impacted by cholinergic cell loss. To this end, we investigated whether lesion of the nucleus basalis magnocellularis (NbM) to cholinergically denervate cortex stimulates adrenergic sympathetic sprouting and the accompanying increase in cholinergic innervation. Further, we assessed whether the presence of sprouting positively correlates with the ability of glutamate synapses in acute visual cortex slices to express mLTD and low frequency stimulation induced LTD (LFS LTD), another cholinergic dependent form of plasticity in visual cortex. We found that both mLTD and LFS LTD are absent in animals when NbM lesion is combined with bilateral removal of the SCG to prevent possible compensatory sprouting. In contrast, when the SCG remain intact to permit sprouting in animals with NbM lesion, cholinergic fiber density is increased concurrently with adrenergic sympathetic sprouting, and mLTD and LFS LTD are preserved. Our findings suggest that autonomic compensation for central cholinergic degeneration is not specific to hippocampus, but is a general repair mechanism occurring in other brain regions important for normal cognitive function.

Bidirectional changes to hippocampal theta-gamma comodulation predict memory for recent spatial episodes

Episodic memory requires the hippocampus, which is thought to bind cortical inputs into conjunctive codes. Local field potentials (LFPs) reflect dendritic and synaptic oscillations whose temporal structure may coordinate cellular mechanisms of plasticity and memory. We now report that single-trial spatial memory performance in rats was predicted by the power comodulation of theta (4-10 Hz) and low gamma (30-50 Hz) rhythms in the hippocampus. Theta-gamma comodulation (TGC) was prominent during successful memory retrieval but was weak when memory failed or was unavailable during spatial exploration in sample trials. Muscimol infusion into medial septum reduced the probability of TGC and successful memory retrieval. In contrast, patterned electrical stimulation of the fimbria-fornix increased TGC in amnestic animals and partially rescued memory performance in the water maze. The results suggest that TGC accompanies memory retrieval in the hippocampus and that patterned brain stimulation may inform therapeutic strategies for cognitive disorders.

The Arc of synaptic memory

The immediate early gene Arc is emerging as a versatile, finely tuned system capable of coupling changes in neuronal activity patterns to synaptic plasticity, thereby optimizing information storage in the nervous system. Here, we attempt to overview the Arc system spanning from transcriptional regulation of the Arc gene, to dendritic transport, metabolism, and translation of Arc mRNA, to post-translational modification, localization, and degradation of Arc protein. Within this framework we discuss the function of Arc in regulation of actin cytoskeletal dynamics underlying consolidation of long-term potentiation (LTP) and regulation of AMPA-type glutamate receptor endocytosis underlying long-term depression (LTD) and homeostatic plasticity. Behaviorally, Arc has a key role in consolidation of explicit and implicit forms of memory, with recent work implicating Arc in adaptation to stress as well as maladaptive plasticity connected to drug addiction. Arc holds considerable promise as a “master regulator” of protein synthesis-dependent forms of synaptic plasticity, but the mechanisms that modulate and switch Arc function are only beginning to be elucidated.

Segregated populations of hippocampal principal CA1 neurons mediating conditioning and extinction of contextual fear

Learning processes mediating conditioning and extinction of contextual fear require activation of several key signaling pathways in the hippocampus. Principal hippocampal CA1 neurons respond to fear conditioning by a coordinated activation of multiple protein kinases and immediate early genes, such as cFos, enabling rapid and lasting consolidation of contextual fear memory. The extracellular signal-regulated kinase (Erk) additionally acts as a central mediator of fear extinction. It is not known however, whether these molecular events take place in overlapping or nonoverlapping neuronal populations. By using mouse models of conditioning and extinction of fear, we set out to determine the time course of cFos and Erk activity, their cellular overlap, and regulation by afferent cholinergic input from the medial septum. Analyses of cFos(+) and pErk(+) cells by immunofluorescence revealed predominant nuclear activation of either protein during conditioning and extinction of fear, respectively. Transgenic cFos-LacZ mice were further used to label in vivo Fos(+) hippocampal cells during conditioning followed by pErk immunostaining after extinction. The results showed that these signaling molecules were activated in segregated populations of hippocampal principal neurons. Furthermore, immunotoxin-induced lesions of medial septal neurons, providing cholinergic input into the hippocampus, selectively abolished Erk activation and extinction of fear without affecting cFos responses and conditioning. These results demonstrate that extinction mechanisms based on Erk signaling involve a specific population of CA1 principal neurons distinctively regulated by afferent cholinergic input from the medial septum.

Rapid activation of plasticity-associated gene transcription in hippocampal neurons provides a mechanism for encoding of one-trial experience

The hippocampus is hypothesized to support rapid encoding of ongoing experience. A critical prerequisite for such function is the ability to readily recruit enduring synaptic plasticity in hippocampal neurons. Hippocampal long-term potentiation (LTP) and memory consolidation require expression of the immediate-early gene (IEG) Arc. To determine whether Arc transcription could be driven by limited and controlled behavioral experience, we used a rectangular track paradigm. In past electrophysiological studies, pyramidal neurons recorded from rats running in one direction on similar tracks typically exhibited a single firing field. Using fluorescence in situ hybridization, we show that the behavioral activity associated with a single lap around the track was sufficient to trigger Arc transcription in complete CA3 neuronal ensembles, as predicted given the role of CA3 in one-trial learning. In contrast, Arc transcription in CA1 ensembles was recruited incrementally, with maximal activation achieved after four laps a day for 4 consecutive days. To test whether Arc transcription is linked to learning and plasticity, or merely elicited by location-specific firing, we inactivated the medial septum, a treatment that compromises hippocampus-dependent learning and LTP but spares location-specific firing in CA1 neurons. Septal inactivation abolished track training-induced Arc transcription in CA1 and CA3 neurons, showing that Arc transcription requires plasticity-inducing stimuli. Accordingly, LTP induction activated Arc transcription in CA1 neurons in vivo. These findings demonstrate for the first time that a single brief experience, equivalent to a single crossing of a firing field, can trigger IEG expression required for long-term plasticity in the hippocampus.

Functional convergence of dopaminergic and cholinergic input is critical for hippocampus-dependent working memory

Although Parkinson's disease is a movement disorder, in many patients cognitive dysfunction is an important clinical sign. It is not yet clear whether this is attributable solely to a decrease in dopamine levels, or whether other neurotransmitter systems might be involved as well. In the present study, the importance of the mesocorticolimbic dopamine pathway and a possible convergence with forebrain cholinergic projections to neocortex and hippocampus in the regulation of learning and memory abilities were investigated by using specific lesion paradigms in one or both systems. Lesioning of dopaminergic neurons in the ventral tegmental area resulted in an impaired performance in the reference memory task, whereas the execution of the working memory tasks appeared to be unaffected in the Morris water maze. Analysis of the swim paths revealed that the dopamine-depleted animals were capable of adapting a search strategy on a given testing day but failed to transfer this information to the next day, suggesting a deficit in information storage and/or recall. In contrast, cholinergic lesions alone were without effect in all test paradigms. However, when both dopamine and acetylcholine were depleted, animals were also impaired in the working memory task, indicating that a functional convergence of the inputs from these systems was critical for acquisition of spatial memory. Interestingly, such an additional acquisition deficit appeared only after hippocampal cholinergic depletion regardless of a concurrent disruption of basalo cortical cholinergic afferents. Thus, further analyses of cholinergic alterations may prove useful in better understanding the cognitive symptoms in Parkinson's disease.

The memory-impairing effects of septal GABA receptor activation involve GABAergic septo-hippocampal projection neurons

Septal infusions of the gamma-aminobutyric acid (GABA)(A) agonist muscimol impair memory, and the effect likely involves the hippocampus. GABA(A) receptors are present on the perikarya of cholinergic and GABAergic septo-hippocampal (SH) projections. The current experiments determined whether GABAergic SH projections are involved in the memory-impairing effects of septal GABA(A) receptor activation. Experiment 1 tested whether combining septal co-infusions of subeffective doses of muscimol with scopolamine, a drug that selectively influences GABA SH projections, would produce memory deficits. Experiment 2 tested whether hippocampal infusions of a GABA(A) receptor antagonist would block the effects of septal muscimol infusions. Fifteen minutes prior to assessing spontaneous alternation (SA) or training in a multiple trial inhibitory avoidance (CMIA) task, male Sprague-Dawley rats were given septal infusions of vehicle, muscimol, scopolamine, or co-infusions of muscimol with scopolamine, or septal infusions of vehicle or muscimol combined with hippocampal infusions of vehicle or bicuculline. Septal co-infusions of muscimol with scopolamine significantly impaired SA and CMIA. Hippocampal bicuculline infusions blocked deficits produced by septal muscimol infusions in SA and attenuated deficits produced in CMIA. Combined, these findings suggest that GABAergic SH projections are involved in the memory-impairing effects of septal GABA receptor activation.

Networks of neurons, networks of genes: an integrated view of memory consolidation

Investigations into the mechanisms of memory formation have abided by the central tenet of the consolidation theory-that memory formation occurs in stages which differ in their requirement for protein synthesis. The current most widely accepted hypothesis posits that new memories are encoded as neural activity-induced changes in synaptic efficacy, and stabilization of these changes requires de novo protein synthesis. However, the basic assumptions of this view have been challenged by concerns regarding the specificity of the effects of the protein synthesis inhibitors used to support the claim. Studies on immediate-early genes (IEGs), in particular Arc, provide a distinct and independent perspective on the issue of the requirement of new protein synthesis in synaptic plasticity and memory consolidation. The IEG Arc and its protein are dynamically induced in response to neuronal activity, and are directly involved in synaptic plasticity and memory consolidation. Although we provide extensive data on Arc's properties to address the requirement of genomic and proteomic responses in memory formation, Arc is merely one element in a network of genes that interact in a coordinated fashion to serve memory consolidation. From gene expression and other studies, we propose the view that the stabilization of a memory trace is a continuous and ongoing process, which does not have a discrete endpoint and cannot be reduced to a single deterministic "molecular cascade". Rather, memory traces are maintained within metastable networks, which must integrate and update past traces with new ones. Such an updating process may well recruit and use many of the plasticity mechanisms necessary for the initial encoding of memory.

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.