Genetic control of hippocampal cholinergic and dynorphinergic mechanisms regulating novelty-induced exploratory behavior in house mice

Cholinergic modulation of rearing in rats performing a spatial memory task

Spatial memory encoding depends in part on cholinergic modulation. How acetylcholine supports spatial memory encoding is not well understood. Prior studies indicate that acetylcholine release is correlated with exploration, including epochs of rearing onto hind legs. Here, to test whether elevated cholinergic tone increases the probability of rearing, we tracked rearing frequency and duration while optogenetically modulating the activity of choline acetyltransferase containing (i.e., acetylcholine producing) neurons of the medial septum in rats performing a spatial working memory task (n = 17 rats). The cholinergic neurons were optogenetically inhibited using halorhodopsin for the duration that rats occupied two of the four open arms during the study phase of an 8-arm radial arm maze win-shift task. Comparing rats' behaviour in the two arm types showed that rearing frequency was not changed, but the average duration of rearing epochs became significantly longer. This effect on rearing was observed during optogenetic inhibition but not during sham inhibition or in rats that received infusions of a fluorescent reporter virus (i.e., without halorhodopsin; n = 6 rats). Optogenetic inhibition of cholinergic neurons during the pretrial waiting phase had no significant effect on rearing, indicating a context-specificity of the observed effects. These results are significant in that they indicate that cholinergic neuron activity in the medial septum is correlated with rearing not because it motivates an exploratory state but because it contributes to the processing of information acquired while rearing.

Cholinergic modulation of rearing in rats performing a spatial memory task

Spatial memory encoding depends in part on cholinergic modulation. How acetylcholine supports spatial memory encoding is not well understood. Prior studies indicate that acetylcholine release is correlated with exploration, including epochs of rearing onto hind legs. Here, to test whether elevated cholinergic tone increases the probability of rearing, we tracked rearing frequency and duration while optogenetically modulating the activity of choline acetyltransferase containing (i.e., acetylcholine producing) neurons of the medial septum in rats performing a spatial working memory task (n = 17 rats). The cholinergic neurons were optogenetically inhibited using halorhodopsin for the duration that rats occupied two of the four open arms during the study phase of an 8-arm radial arm maze win-shift task. Comparing rats' behavior in the two arm types showed that rearing frequency was not changed but the average duration of rearing epochs became significantly longer. This effect on rearing was observed during optogenetic inhibition but not during sham inhibition or in rats that received infusions of a fluorescent reporter virus (i.e., without halorhodopsin; n = 6 rats). Optogenetic inhibition of cholinergic neurons during the pre-trial waiting phase had no significant effect on rearing, indicating a context-specificity of the observed effects. These results are significant in that they indicate that cholinergic neuron activity in the medial septum is correlated with rearing not because it motivates an exploratory state but because it contributes to the processing of information acquired while rearing.

Something new and something blue: Responses to novelty in a rodent model of depression and epilepsy comorbidity

A bidirectional comorbidity exists between depression and epilepsy such that patients with epilepsy are at higher risk for developing depression, and vice versa. Each of these conditions individually can be complicated by behavioral effects that worsen quality of life, but less is known about these interactions within the comorbidity of depression and epilepsy. The SwLo rat has been selectively bred for depression-relevant behaviors and exhibits enhanced limbic seizure susceptibility. This study sought to characterize the effects of novelty and stress on the SwLo rodent model of this comorbidity. It was hypothesized that SwLo rats would exhibit altered responses to novelty, reflected in hyperactivity-, anxiety-, sensation seeking-, and/or compulsive behaviors, and that this would be exacerbated with stress. Compared to the SwHi rat (their depression- and epilepsy-resistant counterparts), SwLo rats showed increased entries in all areas of the Open Field Test and spent significantly more time in the light compartment of the Light-Dark Box. SwLo rats also had a significantly higher number of rearing behaviors in the inner squares of the Open Field Test, the closed arms of the Elevated Plus Maze, and both areas of the Light-Dark Box. They demonstrated increased Nestlet shredding but showed no difference in a marble burying task or in latency to consume food in a novelty suppressed feeding task. Interestingly, restraint stress showed little effect on these behaviors, despite increasing corticosterone levels. Combined, these results suggest an increase in exploratory sensation seeking and hypervigilant information-gathering behaviors in the SwLo rat that are not dependent on corticosterone levels. This shows the utility of this model for studying behavioral effects of comorbid depression and epilepsy and allows for their use in identifying underlying mechanisms or screening treatment strategies for this complex comorbidity.

Early-life stress induces EAAC1 expression reduction and attention-deficit and depressive behaviors in adolescent rats

Neonatal maternal separation (NMS), as an early-life stress (ELS), is a risk factor to develop emotional disorders. However, the exact mechanisms remain to be defined. In the present study, we investigated the mechanisms involved in developing emotional disorders caused by NMS. First, we confirmed that NMS provoked impulsive behavior, orienting and nonselective attention-deficit, abnormal grooming, and depressive-like behaviors in adolescence. Excitatory amino acid carrier 1 (EAAC1) is an excitatory amino acid transporter expressed specifically by neurons and is the route for the neuronal uptake of glutamate/aspartate/cysteine. Compared with that in the normal control group, EAAC1 expression was remarkably reduced in the ventral hippocampus and cerebral cortex in the NMS group. Additionally, EAAC1 expression was reduced in parvalbumin-positive hippocampal GABAergic neurons in the NMS group. We also found that EAAC1-knockout (EAAC1-/-) mice exhibited impulsive-like, nonselective attention-deficit, and depressive-like behaviors compared with WT mice in adolescence, characteristics similar to those of the NMS behavior phenotype. Taken together, our results revealed that ELS induced a reduction in EAAC1 expression, suggesting that reduced EAAC1 expression is involved in the pathophysiology of attention-deficit and depressive behaviors in adolescence caused by NMS.

The neuropsychology of schizophrenia

A model is proposed for integrating the neural and cognitive aspects of the positive symptoms of acute schizophrenia, using evidence from postmortem neuropathology and neurochemistry, clinical and preclinical studies of dopaminergic neurotransmission, anatomical connections between the limbic system and basal ganglia, attentional and other cognitive abnormalities underlying the positive symptoms of schizophrenia, specific animal models of some of these abnormalities, and previous attempts to model the cognitive functions of the septohippocampal system and the motor functions of the basal ganglia. Anatomically, the model emphasises the projections from the septohippocampal system, via the subiculum, and the amygdala to nucleus accumbens, and their interaction with the ascending dopaminergic projection to the accumbens. Psychologically, the model emphasises a failure in acute schizophrenia to integrate stored memories of past regularities of perceptual input with ongoing motor programs in the control of current perception. A number of recent experiments that offer support for the model are briefly described, including anatomical studies of limbic-striatal connections, studies in the rat of the effects of damage to these connections, and of the effects of amphetamine and neuroleptics, on the partial reinforcement extinction effect, latent inhibition and the Kamin blocking effect; and studies of the latter two phenomena in acute and chronic schizophrenics.

Morphological correlates of emotional and cognitive behaviour: insights from studies on inbred and outbred rodent strains and their crosses

Every study in rodents is also a behavioural genetic study even if only a single strain is used. Outbred strains are genetically heterogeneous populations with a high intrastrain variation, whereas inbred strains are based on the multiplication of a unique individual. The aim of the present review is to summarize findings on brain regions involved in three major components of rodent behaviour, locomotion, anxiety-related behaviour and cognition, by paying particular attention to the genetic context, genetic models used and interstrain comparisons. Recent trends correlating gene expression in inbred strains with behavioural data in databases, morpho-behavioural-haplotype analyses and problems arising from large-scale multivariate analyses are discussed. Morpho-behavioural correlations in multiple strains are presented, including correlations with projection neurons, interneurons and fibre systems in the striatum, midbrain, amygdala, medial septum and hippocampus, by relating them to relevant transmitter systems. In addition, brain areas differentially activated in different strains are described (hippocampus, prefrontal cortex, nucleus accumbens, locus ceruleus). Direct interstrain comparisons indicate that strain differences in behavioural variables and neuronal markers are much more common than usually thought. The choice of the appropriate genetic model can therefore contribute to an interpretation of positive results in a wider context, and help to avoid misleading interpretations of negative results.

L-type voltage-gated calcium channels in conditioned fear: a genetic and pharmacological analysis

Using pharmacological approaches, others have suggested that L-type voltage-gated calcium channels (L-VGCCs) mediate both consolidation and extinction of conditioned fear. In the absence of L-VGCC isoform-specific antagonists, we have begun to investigate the subtype-specific role of LVGCCs in consolidation and extinction of conditioned fear using a molecular genetics approach. Previously, we used this approach to demonstrate that the Ca(v)1.3 isoform mediates consolidation, but not extinction, of contextually conditioned fear. Here, we used mice in which the gene for the L-VGCC pore-forming subunit Ca(v)1.2 was conditionally deleted in forebrain excitatory neurons (Ca(v)1.2(cKO) mice) to address the role of Ca(v)1.2 in consolidation and extinction of conditioned fear. We demonstrate that Ca(v)1.2(cKO) mice consolidate and extinguish conditioned fear as well as control littermates. These data suggest that Ca(v)1.2 is not critical for these processes and together with our previous data argue against a role for either of the brain-expressed L-VGCCs (Ca(v)1.2 or Ca(v)1.3) in extinction of conditioned fear. Additionally, we present data demonstrating that the L-VGCC antagonist nifedipine, which has been used in previous conditioned fear extinction studies, impairs locomotion, and induces an aversive state. We further demonstrate that this aversive state can enter into associations with conditioned stimuli that are present at the time that it is experienced, suggesting that previous studies using nifedipine were likely confounded by drug toxicity. Taken together, our genetic and pharmacological data argue against a role for Ca(v)1.2 in consolidation of conditioned fear as well as a role for L-VGCCs in extinction of conditioned fear.

The perforant path: projections from the entorhinal cortex to the dentate gyrus

This paper provides a comprehensive description of the organization of projections from the entorhinal cortex to the dentate gyrus, which together with projections to other subfields of the hippocampal formation form the so-called perforant pathway. To this end, data that are primarily from anatomical studies in the rat will be summarized, complimented with comparative data from other species. The analysis of the organization of any of the connections of the hippocampus, including that of the entorhinal cortex to the dentate gyrus, is severely hampered because of the complex three-dimensional shape of the hippocampus. In particular in rodents, but to a lesser extent also in primates, all traditional planes of sectioning will result in sections that at some point or another do not cut through the hippocampus at an angle that is perpendicular to its long axis. To amend this, we will describe own unpublished tracing data obtained in the rat with the use of the so-called extended preparation. A number of issues will be addressed. First, data will be summarized which will clarify the laminar origin of the perforant pathway within the entorhinal cortex. Second, we will discuss whether or not a radial organization, along the proximo-distal dendritic axis of granule cells, characterizes the entorhinal-dentate projection. Third, we will discuss whether this projection is governed by any transverse organization, and fourth, we will focus on the organization along the longitudinal axis. Finally, the synaptic organization and the contralateral entorhinal-dentate projection will be described briefly. Taken together, the available data suggest that the projection from the entorhinal cortex to the dentate gyrus is a fairly well conserved connection, present in all species studied, exhibiting a grossly similar organization.

Flanking gene and genetic background problems in genetically manipulated mice

Mice carrying engineered genetic modifications have become an indispensable tool in the study of gene functioning. The interpretation of results obtained with targeted mutants is not completely straightforward, however, because of genetic complications due to linkage and epistasis. Effects of closely linked genes flanking the targeted locus might sometimes be responsible for phenotypic changes ascribed to the null mutation. The effects of the latter might also be modified by the general genetic background. This review presents some examples and discusses some simple strategies to deal with these complications.

Behavioural and physiological characterization of inbred mouse strains: prospects for elucidating the molecular mechanisms of mammalian learning and memory

With the advent of recombinant DNA methodology, it has become possible to dissect the molecular mechanisms of complex traits, including brain function and behaviour. The increasing amount of available information on the genomes of mammalian organisms, including our own, has facilitated this research. The present review focuses on a somewhat neglected area of genetics, one that involves the study of inbred mouse strains. It is argued that the use of inbred mice is complementary to transgenic approaches in the analysis of molecular mechanisms of complex traits. Whereas transgenic technology allows one to manipulate a single gene and investigate the in vivo effects of highly specific, artificially induced mutations, the study of inbred mouse strains should shed light on the roles of naturally occurring allelic variants in brain function and behaviour. Systematic characterization of the behavioural, electrophysiological, neurochemical, and neuroanatomical properties of a large number of inbred strains is required to elucidate mechanisms of mammalian brain function and behaviour. In essence, a 'mouse phenome' project is needed, entailing the construction of databases to investigate possible causal relationships amongst the phenotypical characteristics. This review focuses on electrophysiological and behavioural characterization of mouse strains. Nevertheless, it is emphasized that the full potential of the analysis of inbred mouse strains may be attained if techniques of numerous disciplines, including gene expression profiling, biochemical analysis, and quantitative trait loci (QTL) mapping, to name but a few, are also included.

Gene targeting: technical confounds and potential solutions in behavioral brain research

Gene targeting allows one to create null mutations in mice and to analyze how the mutant organism responds to the lack of a single gene product. This has facilitated the molecular dissection of such complex characteristics as mammalian brain function and behavior, including learning, memory, aggression, and maternal behavior to mention a few. However, the interpretation of the phenotypical changes that arise in null mutant mice has been questioned. The possibility that genes other than the targeted one may contribute to phenotypical alterations has been raised and the importance of compensatory mechanisms has been brought to attention. This review focuses on recent advances in the literature that illustrate the caveats associated with gene targeting and also presents an overview of potential solutions for the discussed problems.

Genetic dissection of mouse exploratory behaviour

A large variety of apparatus and procedures are being employed to measure mouse exploratory behaviour. Definitions of what constitutes exploration also vary widely. The present article reviews two studies whose results permit a genetic dissection of behaviour displayed in an open-field situation. The results agree that factors representing exploration and stress/fear underlie this type of behaviour. Both factors appear to be linked to neuroanatomical variation in the sizes of the hippocampal intra- and infrapyramidal mossy fibre terminal fields. Multivariate analysis of genetic correlations may render important insights into the structure of behaviour and its relations with neuroanatomical and neurophysiological systems.

Non-selective attention in a rat model of hyperactivity and attention deficit: subchronic methylphenydate and nitric oxide synthesis inhibitor treatment

The involvement of dopamine (DA) and nitric oxide (NO) in the process of non-selective attention (NSA) to environmental stimuli has been investigated in the juvenile Spontaneously Hypertensive rat (SHR). To this aim the frequency and duration of rearing episodes in a novelty situation, which is thought to monitor NSA, have been measured in male SHR and Wistar-Kyoto (WKY) control rats following subchronic treatment with methylphenidate (MP; 3 mg/kg) or the nitric oxide synthase (NOS) inhibitor L-Nitro-arginine-methylester (L-NAME; 1 mg/kg) or vehicle daily for two weeks. Different groups were tested at 0.5 h or 24 h after the last injection in a Làt-maze. Tests were repeated twice at a 24 h interval and lasted 10 min each. Upon first exposure, there was a differential drug effect only in the SHR. In fact, MP and L-NAME yielded a shift to the left and to the right, i.e. towards episodes of lower or higher duration, respectively. This shift was more pronounced in the group tested 0.5 h after the last injection. In contrast, both drugs produced a significant lengthening of the rearing episodes in the SHR only in comparison with the vehicle-treated rats over days of testing. Therefore both MP and L-NAME appear to shear a similar effect on non-selective attention, although the effect of L-NAME is somewhat paradoxical. The latter is likely to be due to increased arginine selective uptake due to negative feedback with the NO production. The consequent increased arginine availability displaces the NOS inhibitor, thus leading to increased NO production. In conclusion, dopamine and nitric oxide play a role in non-selective attention by synaptic and extrasynaptic mechanisms, respectively, in a rat model of hyperactivity and attention-deficits.

Diet Restriction Increases Enkephalin- and Dynorphin-like Immunoreactivity in Rat Brain and Attenuates Long-term Retention of Passive Avoidance

The present study examines effects of diet restriction (DR) on behavior and on the opioid peptides enkephalin (Enk) and dynorphin (Dyn). Female rats were assigned to ad libitum food intake (AL), DR 60% (DR60) or 40% (DR40) of AL. After 4 weeks, DR reduced fearful behavior in the elevated plus maze. DR rats displayed good retention of passive avoidance at 24 h, but DR40 rats had reduced retention, at 5 and 11 days post training. Changes in Enk- and Dyn-like immunoreactivity (LI) in the hippocampal mossy fibers (MF), hypothalamus, septum, central nucleus of amygdala (CeAm) and thalamus depended on the severity of DR. In DR60, Enk-LI and Dyn-LI were not changed except for reduction in CeAm. In DR40, Dyn-LI increased significantly above AL levels in MF, CeAm and hypothalamus, whereas Enk-LI increased significantly above AL levels in the CA3 subregion of the MF system and in thalamus. Serum glucose was tightly correlated with Enk-LI reaching highest values in the MF (r= -0.82). Increased opioid-LI in CeAm and MF was associated with reduced fearfulness in the elevated plus maze. Thus, hippocampal and amygdala opioid subsystems are uniquely sensitive to DR and may be relevant to psychophysiological problems in human starvation including anorexia.

Variation in hippocampal dynorphin b-immunoreactive mossy fiber terminal fields of apomorphine-(un)susceptible rats

The size of distinct hippocampal sub-fields were measured in the apomorphine-susceptible and apomorphine-unsusceptible rat lines. Mossy fiber terminal fields were delineated using dynorphin B immunoreactivity and area measurements were taken from (1) the supra-pyramidal mossy fiber terminal field; (2) the intra- and infra-pyramidal mossy fiber terminal field; (3) the hilus of the fascia dentata (4) the non dynorphin B immunoreactive area of the regio inferior and fascia dentata and (5) the total area of regio inferior and fascia dentata. The data indicate that statistically significant differences in the morphometry of the hippocampal subfields of the apomorphine susceptible and unsusceptible rats are confined to the intra- and infra terminal field: the relative size of the left and right intra- and infra terminal field of apomorphine unsusceptible rats are significantly larger than those of the apomorphine susceptible rats. These data explain at least in part the differential response of these rats to novelty.

Strain-dependent involvement of D1 and D2 dopamine receptors in muscarinic cholinergic influences on memory storage

These experiments examined the interaction of muscarinic and dopaminergic systems in influencing memory for one-trial inhibitory avoidance training in mice of the C57BL/6 and DBA/2 strains. In both strains, immediate post-training systemic administration of the muscarinic cholinergic agonist oxotremorine enhanced retention and the cholinergic antagonist atropine impaired retention. No effects were seen with injections 2 h post-training. Furthermore, the drugs did not affect retention performance of animals that received no footshock on the training trial. These results confirm previous findings indicating that muscarinic cholinergic drugs affect memory by influencing memory consolidation. In C57 mice, pretreatment with selective D1 or D2 dopamine (DA) receptor agonists (SKF 38393 or LY 171555, respectively) in otherwise non-effective doses (5 and 0.25 mg/kg, respectively) potentiated the effects of oxotremorine (0.04 mg/kg). Furthermore, in C57 mice pretreatment with selective D1 or D2 receptor antagonists (SCH 23390 or (-)-sulpiride) in otherwise non-effective doses (0.025 and 6 mg/kg, respectively) blocked the memory enhancing effects of oxotremorine. The memory impairing effects of atropine (3 mg/kg) were blocked by the D1 and D2 selective agonists and potentiated by the selective D1 or D2 antagonists. In contrast, in DBA mice, the D1 and D2 selective agonists antagonised the memory enhancing effects of oxotremorine (0.02 mg/kg) and potentiated the effects of atropine (2 mg/kg). Furthermore, the D1 and D2 antagonists potentiated the effects of oxotremorine and antagonised those of atropine. These findings indicate that although muscarinic cholinergic influences on memory storage are comparable in mice of these two strains, the cholinergic-dopaminergic interactions are opposite in the two strains. These results have implications for hypotheses of cholinergic and dopaminergic regulation of memory storage.

Effects of neonatal naltrindole treatment on antinociceptive and behavioral responses to mu and kappa agonists in rats

The effects of a daily injection of the delta selective opioid antagonist naltrindole (1 mg/kg), from birth to postnatal day 19, on antinociceptive and behavioral responses to the mu selective agonist alfentanil (65 microg/kg) and the kappa selective agonist CI-977 (50 microg/kg) in 20-day-old male rats were investigated. Antinociception was assessed using the tail immersion test and behavioral testing was performed by employing an open field. The functional blockade of the delta receptor by naltrindole blocked the antinociceptive response to alfentanil but did not affect the antinociception induced by CI-977. The effects of alfentanil (increased exploration) and CI-977 (a marked hypoactivity) in the open field were not modified by neonatal naltrindole treatment. The results suggest a functional interaction between delta and mu receptors in the postnatal period but not between delta and kappa receptors. The data also suggest differences in the delta and mu receptors interacting in the modulation of antinociception and those involved in behavioral responses in the open field.

Non-selective attention and nitric oxide in putative animal models of Attention-Deficit Hyperactivity Disorder

Non-selective attention (NSA) to environmental stimuli has been measured in putative animal models of Attention-Deficit Hyperactivity Disorder (ADHD), such as the Spontaneously Hypertensive (SHR) and the Naples High-Excitability (NHE) rat lines. A series of experiments has been carried out on male juvenile SHR and Wistar-Kyoto (WKY) controls (experiment 1) and on the NHE and two controls, i.e. the Naples Low-Excitability (NLE) and a random-bred (NRB) line (experiment 2). It was done under basal conditions or following a single injection of the nitric oxide synthase (NOS) inhibitor L-nitro-arginine-methylester (L-NAME: 0.1-10 mg/kg, i.p.), or vehicle, 30 min before testing on day 1 and vehicle alone before testing on days 2 and 3 in SHR/WKY (experiment 3) and the Naples lines (experiment 4). The behavior in a Lát maze during three consecutive 10-min exposures at 24-h intervals was monitored by a CCD video camera and analyzed off-line for frequency and duration of rearings on hindlimbs per 1-min blocks. The results demonstrated that both SHR and NHE rats showed a higher frequency of rearings of shorter duration than controls. With time of testing, the duration of rearings tended to increase in the WKY but not the SHR. In the Naples lines the duration tended to increase in all but mostly in the NHE rats. The acute inhibition of NOS by L-NAME significantly increased the duration of rearing episodes both in SHR and NHE rats only at 10 mg/kg in the second part of the testing period. Therefore, NSA, as indexed by the duration of rearings, is defective in both hyperactivity models against different genetic backgrounds. In addition, this impairment is dependent upon nitric oxide (NO), which appears to play a significant role in these processes.

Hippocampal cholinergic blockade enhances hypothalamic-pituitary-adrenal responses to stress

We examined the role of the hippocampal cholinergic system, which is known to mediate processes related to fear and anxiety, in the regulation of stress-induced hypothalamic-pituitary-adrenal (HPA) activity. Bilateral intra-hippocampal injections (30 microg per side) of the muscarinic antagonist Scopolamine augmented adrenocorticotropin and corticosterone responses to restraint without altering basal HPA activity compared to vehicle-treated animals. These results suggest that the hippocampal cholinergic system regulates stress-induced HPA activity and may serve to coordinate behavioral and neuroendocrine responses to stress.

No major differences in locomotor responses to dexamphetamine in high and low responders to novelty: a study in Wistar rats

The aim of the study was to compare locomotor responses to acute and sub-chronic dexamphetamine in two distinct types of Wistar rats, namely the Nijmegen high responders to novelty (HR) and Nijmegen low responders to novelty (LR). HR and LR were chosen because they differ in neurochemical processes relevant to the control of the locomotor effects of dexamphetamine, such as the dopaminergic and adrenergic activity in the nucleus accumbens. In experiment 1, a dexamphetamine dose-response curve (0.0-2.0 mg/kg/i.p.) was established using standard activity boxes. The dose-response curve slightly, but significantly, differed between HR and LR: especially the increase elicited by 1.5 mg/kg dexamphetamine was significantly greater in HR than in LR. In experiment 2, locomotor effects of sub-chronic administration of dexamphetamine (1.0 mg/kg/i.p.) were analyzed in HR and LR for 5 consecutive days. HR showed a higher locomotor response to dexamphetamine than LR; however, the two groups did not differ in their sensitization rate. It is concluded that there are neither major HR-LR differences in the locomotor response to acute administration of various doses of dexamphetamine nor HR-LR differences in the rate of sensitization of this locomotor response to sub-chronic administration of dexamphetamine. Type-specific differences in the mutual interaction between corticosteroids and dexamphetamine as well as the nature of the chosen dependent variable, namely locomotor activity, are hypothesized to underlie the results of the present study.

Modulation of the reaction of hippocampal neurons to sensory stimuli by cholinergic substances

The influences of increasing endogenous acetylcholine (eserine) and its blockade (scopolamine) on the effects of sensory stimuli were analyzed through the extracellular recording of the activity of individual hippocampal neurons of awake rabbits. An increase in the level of acetylcholine, accompanied by the appearance of stable theta rhythm, leads to a substantial decrease in the reactivity of neurons, the suppression, attenuation, and inversion of the majority of inhibitory reactions and of a substantial proportion of activational reactions including on-responses of a specific type. At the same time, a limited group of activational reactions is intensified and extended against the background of eserine. Scopolamine, which blocks theta rhythm, does not change or intensifies inhibitory and some activational reactions, including on-responses. Tonic reactions are shortened; however, their gradual extinction disappears. The effects described are preserved in the hippocampus in the presence of basal undercutting of the septum which eliminates ascending brainstem pathways. These data make it possible to draw the conclusion that, under normal conditions, a new (significant) sensory stimulus elicits in the hippocampus an initial stoppage (reset) of activity with the coordinated triggering of theta rhythm and the passage against this background of signals along the cortical input in a specific phase relationship to it. The period of theta modulation switched on by the signal fosters its recording and the limitation of the passage of subsequent, interfering signals. The septohippocampal influences may thus support the mechanism of selective attention, as a necessary precondition for memory.