Reticular elicitation of hippocampal slow waves: common effects of some anxiolytic drugs

Model of theta frequency perturbations and contextual fear memory

Theta oscillations in the hippocampal local field potential (LFP) appear during translational movement and arousal, modulate the activity of principal cells, and are associated with spatial cognition and episodic memory function. All known anxiolytics slightly but consistently reduce hippocampal theta frequency. However, whether this electrophysiological effect is mechanistically related to the decreased behavioral expression of anxiety is currently unclear. Here, we propose that a reduction in theta frequency affects synaptic plasticity and mnemonic function and that this can explain the reduction in anxiety behavior. We test this hypothesis in a biophysical model of contextual fear conditioning. First, we confirm that our model reproduces previous empirical results regarding the dependence of synaptic plasticity on presynaptic firing rate. Next, we investigate how theta frequency during contextual conditioning impacts learning. These simulations demonstrate that learned associations between threat and context are attenuated when learning takes place under reduced theta frequency. Additionally, our simulations demonstrate that learned associations result in increased theta activity in the amygdala, consistent with empirical data. In summary, we propose a mechanism that can account for the behavioral effect of anxiolytics by impairing the integration of threat attributes of an environment into the cognitive map due to reduced synaptic potentiation.

Signalling pathways contributing to learning and memory deficits in the Ts65Dn mouse model of Down syndrome

Down syndrome (DS) is a genetic trisomic disorder that produces life-long changes in physiology and cognition. Many of the changes in learning and memory seen in DS are reminiscent of disorders involving the hippocampal/entorhinal circuit. Mouse models of DS typically involve trisomy of murine chromosome 16 is homologous for many of the genes triplicated in human trisomy 21, and provide us with good models of changes in, and potential pharmacotherapy for, human DS. Recent careful dissection of the Ts65Dn mouse model of DS has revealed differences in key signalling pathways from the basal forebrain to the hippocampus and associated rhinal cortices, as well as changes in the microstructure of the hippocampus itself. In vivo behavioural and electrophysiological studies have shown that Ts65Dn animals have difficulties in spatial memory that mirror hippocampal deficits, and have changes in hippocampal electrophysiological phenomenology that may explain these differences, and align with expectations generated from in vitro exploration of this model. Finally, given the existing data, we will examine the possibility for pharmacotherapy for DS, and outline the work that remains to be done to fully understand this system.

HippoBellum: Acute Cerebellar Modulation Alters Hippocampal Dynamics and Function

Here we examine what effects acute manipulation of the cerebellum, a canonically motor structure, can have on the hippocampus, a canonically cognitive structure. In male and female mice, acute perturbation of the cerebellar vermis (lobule 4/5) or simplex produced reliable and specific effects in hippocampal function at cellular, population, and behavioral levels, including evoked local field potentials, increased hippocampal cFos expression, and altered CA1 calcium event rate, amplitudes, and correlated activity. We additionally noted a selective deficit on an object location memory task, which requires objection-location pairing. We therefore combined cerebellar optogenetic stimulation and CA1 calcium imaging with an object-exploration task, and found that cerebellar stimulation reduced the representation of place fields near objects, and prevented a shift in representation to the novel location when an object was moved. Together, these results clearly demonstrate that acute modulation of the cerebellum alters hippocampal function, and further illustrates that the cerebellum can influence cognitive domains.SIGNIFICANCE STATEMENT The cerebellum, a canonically motor-related structure, is being increasingly recognized for its influence on nonmotor functions and structures. The hippocampus is a brain region critical for cognitive functions, such as episodic memory and spatial navigation. To investigate how modulation of the cerebellum may impact the hippocampus, we stimulated two sites of the cerebellar cortex and examined hippocampal function at multiple levels. We found that cerebellar stimulation strongly modulates hippocampal activity, disrupts spatial memory, and alters object-location processing. Therefore, a canonically cognitive brain area, the hippocampus, is sensitive to cerebellar modulation.

Dexamethasone impairs encoding and expression of aversive conditioning promoted by pentylenetetrazole

Behavioral and neuroendocrine responses following threatening situations promote the release of corticosterone, which is known to modulate trauma-related learning and memory process. However, it remains unknown whether the aversive learning generated by interoceptive fear conditioning is affected by glucocorticoid modulation. Therefore, the present study aimed to investigate the role of dexamethasone suppression in encoding and expression of pentylenetetrazole-induced olfactory fear conditioning (OFC) and in contextual second-order conditioning promoted by the conditioned odor. Adult male Long-Evans rats were treated with dexamethasone 60 min before the encoding or the expression in both OFC and contextual second-order conditioning. Dexamethasone treatment impaired encoding and expression of the OFC, but failed to impair encoding and expression of the contextual second-order conditioning. Altogether, our results show that although OFC and thereafter contextual second-order conditioning may allow the study of traumatic memories, each order of conditioning seems to present specific features related to their pharmacological modulation. These findings highlight the importance of addressing the role of neuromodulatory systems in first-order and second-order conditioning to gain a better understanding of these phenomena and support future therapies related to traumatic memories.

Theta Rhythm of the Hippocampus: Subcortical Control and Functional Significance

The theta rhythm is the largest extracellular synchronous signal that can be recorded from the mammalian brain and has been strongly implicated in mnemonic processes of the hippocampus. We describe (a) ascending brain stem–forebrain systems involved in controlling theta and nontheta (desynchronization) states of the hippocampal electroencephalogram; (b) theta rhythmically discharging cells in several structures of Papez's circuit and their possible functional significance, specifically with respect to head direction cells in this same circuit; and (c) the role of nucleus reuniens of the thalamus as a major interface between the medial prefrontal cortex and hippocampus and as a prominent source of afferent limbic information to the hippocampus. We suggest that the hippocampus receives two main types of input: theta rhythm from ascending brain stem– diencephaloseptal systems and information bearing mainly from thalamocortical/cortical systems. The temporal convergence of activity of these two systems results in the encoding of information in the hippocampus, primarily reaching it from the entorhinal cortex and nucleus reuniens.

Modulation of anxiety and fear via distinct intrahippocampal circuits

Recent findings indicate a high level of specialization at the level of microcircuits and cell populations within brain structures with regards to the control of fear and anxiety. The hippocampus, however, has been treated as a unitary structure in anxiety and fear research despite mounting evidence that different hippocampal subregions have specialized roles in other cognitive domains. Using novel cell-type- and region-specific conditional knockouts of the GABA_A receptor α2 subunit, we demonstrate that inhibition of the principal neurons of the dentate gyrus and CA3 via α2-containing GABA_A receptors (α2GABA_ARs) is required to suppress anxiety, while the inhibition of CA1 pyramidal neurons is required to suppress fear responses. We further show that the diazepam-modulation of hippocampal theta activity shows certain parallels with our behavioral findings, suggesting a possible mechanism for the observed behavioral effects. Thus, our findings demonstrate a double dissociation in the regulation of anxiety versus fear by hippocampal microcircuitry.

DOI:http://dx.doi.org/10.7554/eLife.14120.001

Fear and anxiety can be thought of as different but related emotional states. Fear is triggered by specific harmful situations, such as the immediate presence of a predator. Anxiety instead results from the possibility of an obscure threat, such as being in an exposed environment, which increases the chance of being detected by a predator. Evidence suggests that slightly different areas of the brain control fear and anxiety, but much remains unknown about the specific brain regions that help to regulate these two emotional states.

One brain region that has been implicated in both anxiety and fear – as well as in learning and memory – is the hippocampus. Named after the Greek word for seahorse because of its shape, the hippocampus is made up of three subregions: CA1, CA3 and the dentate gyrus. Each of these subregions has a distinct role in learning and memory. However, their individual contributions to the control of fear and anxiety were not known.

An inhibitory receptor protein found in the surface of some hippocampal neurons had previously been shown to be involved in controlling fear and anxiety. Now, Engin et al. have studied three different groups of genetically modified mice, each of which lacks the receptor protein in a different subregion of the hippocampus. The mice completed tests that stimulated anxiety or fear, some while under the influence of the anxiety and fear-reducing drug diazepam. Notably, diazepam failed to reduce fear in animals that lacked the inhibitory receptor protein in the CA1 subregion of the hippocampus, suggesting that this subregion participates in the fear response. However, mice that lacked the receptor in the dentate gyrus or CA3 responded normally to the drug (they showed reduced fear when given diazepam).

In tests of anxiety, the picture was exactly the opposite. Diazepam failed to reduce anxiety in animals lacking the inhibitory receptor in the dentate gyrus or CA3, indicating that these subregions are involved in the regulation of anxiety. However, the drug still reduced anxiety in mice that lacked the receptor protein in the CA1 subregion.

Further studies are now needed to clarify how manipulating specific subregions of the hippocampus alters how it communicates with other brain structures to generate changes in anxiety or fear-related behaviors.

DOI:http://dx.doi.org/10.7554/eLife.14120.002

The frequency of hippocampal theta rhythm is modulated on a circadian period and is entrained by food availability

The hippocampal formation plays a critical role in the generation of episodic memory. While the encoding of the spatial and contextual components of memory have been extensively studied, how the hippocampus encodes temporal information, especially at long time intervals, is less well understood. The activity of place cells in hippocampus has previously been shown to be modulated at a circadian time-scale, entrained by a behavioral stimulus, but not entrained by light. The experimental procedures used in the previous study of this phenomenon, however, necessarily conflated two alternative entraining stimuli, the exposure to the recording environment and the availability of food, making it impossible to distinguish between these possibilities. Here we demonstrate that the frequency of theta-band hippocampal EEG varies with a circadian period in freely moving animals and that this periodicity mirrors changes in the firing rate of hippocampal neurons. Theta activity serves, therefore, as a proxy of circadian-modulated hippocampal neuronal activity. We then demonstrate that the frequency of hippocampal theta driven by stimulation of the reticular formation also varies with a circadian period. Because this effect can be observed without having to feed the animal to encourage movement we were able to identify what stimulus entrains the circadian oscillation. We show that with reticular-activated recordings started at various times of the day the frequency of theta varies quasi-sinusoidally with a 25 h period and phase-aligned when referenced to the animal’s regular feeding time, but not the recording start time. Furthermore, we show that theta frequency consistently varied with a circadian period when the data obtained from repeated recordings started at various times of the day were referenced to the start of food availability in the recording chamber. This pattern did not occur when data were referenced to the start of the recording session or to the actual time of day when this was not also related to feeding time. This double dissociation demonstrates that hippocampal theta is modulated with a circadian timescale, and that this modulation is strongly entrained by food. One interpretation of this finding is that the hippocampus is responsive to a food entrainable oscillator (FEO) that might modulate foraging behavior over circadian periods.

The lateral septum as a regulator of hippocampal theta oscillations and defensive behavior in rats

Hippocampal theta oscillations are linked to various processes, including locomotion, learning and memory, and defense and affect. The lateral septum (LS) has been implicated in the generation of the hippocampal theta rhythm, but its precise role in this process is not well understood. Here, we investigated the effects of direct pharmacological inhibition or disinhibition of the dorsal LS (dLS) on the frequency of hippocampal theta activity elicited by stimulation of the reticular formation in urethane-anesthetized rats. We found that bilateral infusions of the GABAA receptor agonist muscimol into the dLS significantly increased theta frequency. Strikingly, intra-dLS infusions of the GABAA receptor antagonist GABAzine largely abolished reticularly elicited theta activity. We also locally injected these same compounds into the medial septum (MS) to test for neuroanatomical specificity. In contrast to the effects seen in the dLS, intra-MS infusions of muscimol had no effect on theta frequency, whereas intra-MS infusions of GABAzine increased theta frequency. Given the hypothesized role of hippocampal theta in behavioral defense, we also examined the effects of intra-dLS application of muscimol in two models of anxiety, the elevated plus maze and the novelty-induced suppression of feeding paradigm; both tests revealed clear, anxiolytic-like effects following muscimol infusions. The fact that dLS-muscimol increased theta frequency while also reducing anxiety-like behaviors challenges the influential theta suppression model of anxiolysis, which predicts a slowing of theta with anxiolytic compounds. More importantly, the experiments reveal a novel role of the LS, especially its dorsal aspects, as an important gating mechanism for the expression of theta oscillations in the rodent hippocampus.

Activation of 5-HT6 receptors modulates sleep-wake activity and hippocampal theta oscillation

The modulatory role of 5-HT neurons and a number of different 5-HT receptor subtypes has been well documented in the regulation of sleep-wake cycles and hippocampal activity. A high level of 5-HT(6) receptor expression is present in the rat hippocampus. Further, hippocampal function has been shown to be modulated by both 5-HT(6) agonists and antagonists. In the current study, the potential involvement of 5-HT(6) receptors in the control of hippocampal theta rhythms and sleep-wake cycles has been investigated. Hippocampal activity was recorded by intracranial hippocampal electrodes both in anesthetized (n = 22) and in freely moving rats (n = 9). Theta rhythm was monitored in different sleep-wake states in freely moving rats and was elicited by stimulation of the brainstem reticular formation under anesthesia. Changes in theta frequency and power were analyzed before and after injection of the 5-HT(6) antagonist (SAM-531) and the 5-HT(6) agonist (EMD386088). In freely moving rats, EMD386088 suppressed sleep for several hours and significantly decreased theta peak frequency, while, in anesthetized rats, EMD386088 had no effect on theta power but significantly decreased theta frequency, which could be blocked by coadministration of SAM-531. SAM-531 alone did not change sleep-wake patterns and had no effect on theta parameters in both unanesthetized and anesthetized rats. Decreases in theta frequency induced by the 5-HT(6) receptor agonist correspond to previously described electrophysiological patterns shared by all anxiolytic drugs, and it is in line with its behavioral anxiolytic profile. The 5-HT(6) antagonist, however, failed to potentiate theta power, which is characteristic of many pro-cognitive substances, indicating that 5-HT(6) receptors might not tonically modulate hippocampal oscillations and sleep-wake patterns.

Elicited hippocampal theta rhythm: a screen for anxiolytic and procognitive drugs through changes in hippocampal function?

Hippocampal damage produces cognitive deficits similar to dementia and changes in emotional and motivated reactions similar to anxiolytic drugs. The gross electrical activity of the hippocampus contains a marked 'theta rhythm'. This is a relatively high voltage sinusoidal waveform, resulting from synchronous phasic firing of cells, variation in which correlates with behavioural state. Like the hippocampus, theta has been linked to both cognitive and emotional functions. Critically, it has recently been shown that restoration of theta-like rhythmicity can restore lost cognitive function. We review the effects of systemic administration of drugs on hippocampal theta elicited by stimulation of the reticular formation. We conclude that reductions in the frequency of reticular-elicited theta provide what is currently the best in-vivo means of detecting antianxiety drugs. We also suggest that increases in the power of reticular-elicited theta could detect drugs useful in the treatment of disorders, such as dementia, that involve memory loss. We argue that these functionally distinct effects should be seen as indirect and that each results from a change in a single form of cognitive-emotional processing that particularly involves the hippocampus.

Hippocampal theta rhythm: a tag for short-term memory

The theta rhythm is the largest extracellular synchronous signal that can be recorded from the mammalian brain, and has been strongly implicated in mnemonic functions of the hippocampus. We advance the proposal that the theta rhythm represents a "tag" for short-term memory processing in the hippocampus. We propose that the hippocampus receives two main types of input, theta from ascending brainstem-diencephalo-septal systems and "information bearing" mainly from thalamocortical and cortical systems. The temporal convergence of activity of these two systems results in the encoding of information in the hippocampus, primarily reaching it via cortical routes. By analogy to processes associated with long-term potentiation (LTP), we suggest that theta represents a strong depolarizing influence on NMDA receptor-containing cells of the hippocampus. The temporal coupling of a theta-induced depolarization and the release of glutamate to these cells from intra- and extrahippocampal sources activates them. This, in turn, initiates processes leading to a (short-term) strengthening of connections between presynaptic ("information bearing") and postsynaptic neurons of the hippocampus. Theta is selectively present in the rat during active exploratory movements. During exploration, a rat continually gathers and updates information about its environment. If this information is temporally coupled to theta (as with the case of locomotion), it becomes temporarily stored in the hippocampus by mechanisms similar to the early phase of LTP (E-LTP). If the exploratory behavior of the rat goes unreinforced, these relatively short-lasting traces (1-3 h) gradually weaken and eventually fade-to be reupdated. On the other hand, if the explorations of the rat lead to rewards (or punishments), additional modulatory inputs to the hippocampus become activated and convert the short-term, theta-dependent memory, into long-term stores.

Different systems in the posterior hypothalamic nucleus of rats control theta frequency and trigger movement

Reduced frequency of theta activity is thought to compromise hippocampal function and so behavioural inhibition. The anxiolytic benzodiazepine chlordiazepoxide (CDP) reduces theta frequency when injected into the medial supramammillary nucleus (mSuM), posterior hypothalamic nucleus (PH) and dorsomedial hypothalamic nucleus (DMH). These hypothalamic effects on theta could underlie at least some behavioural effects of benzodiazepines. We have previously shown that in a fixed interval 60-s schedule (FI60), CDP injected into mSuM reduced both theta frequency and behavioural inhibition. The present experiments test the effect of injections into PH and DMH on theta and hippocampal-sensitive behaviour (FI60 and open field ambulation). Systemic CDP (5mg/kg i.p.) released, but PH/CDP (20microg in 0.5microl vehicle) suppressed FI responding, though they both reduced FI theta frequency. In the open field, both CDP i.p. and PH/CDP reduced ambulation, but only the systemic injection reduced ambulation theta frequency. Taken together with previous research, these results support a role for PH in the control of voluntary behaviour. They imply that this function may be suppressed, independently of theta, by benzodiazepines. An anxiolytic effect of PH/CDP in FI60 may, therefore, have been masked by a concurrent action of CDP on the PH motor system. DMH/CDP did not affect behaviour or theta in either experiment, despite the fact that this nucleus is involved in benzodiazepine mediation of risk assessment and the flight response. This suggests that, like the control of theta frequency by the hypothalamus, the neural mechanisms underlying anxiety are distributed in complex networks.

Multiple hypothalamic sites control the frequency of hippocampal theta rhythm

Stimulation of a neural pathway originating in the brainstem reticular formation, with synapses in the medial hypothalamus, activates the hippocampal theta rhythm. The frequency of reticular-elicited theta is determined in the medial supramammillary nucleus (mSuM) completely in anaesthetised rats, but only partially when the animal is awake. We tested other medial hypothalamic sites for their capacity to control theta frequency in awake rats. Blockade of sodium channels (1 microl fast infusion of the local anaesthetic procaine, experiment 1) or increased inhibition by GABA (Chlordiazepoxide [CDP], experiment 2) was found to reduce or increase the frequency of reticular-elicited theta, depending on the precise site of injection, in the region of the dorsomedial hypothalamic nucleus (DMH) and the posterior hypothalamic nucleus (PH). A band of null sites for CDP was located in the region of the ventral border of PH and dorsal border of mSuM. Using 0.5 and 1 microl CDP, and slow infusions (experiment 3), it was found that effective PH sites were also separate from mSuM in the rostrocaudal direction. In experiment 4, the DMH/PH region was mapped with unilateral and bilateral slow infusions of 0.5 microl CDP. CDP significantly reduced frequency in medial (periventricular) and dorsal PH, but not DMH. Bilateral injections appeared to generally sum the usual effects of unilateral injection, producing effects of intermediate size. However, the absolute frequency change in any given site, or with any pair of sites, did not exceed 1 Hz, which is similar to what is seen with single injections in mSuM. Overall, it appears that at, any one time, theta frequency may be determined by a complex interplay between distinct but interacting modulatory regions in the medial hypothalamus.

Similar effects of medial supramammillary or systemic injection of chlordiazepoxide on both theta frequency and fixed-interval responding

The frequency of theta activity may be important for hippocampal function. Anxiolytic drugs reduce theta frequency and have behavioral effects that are similar to those of hippocampal lesions. The effect of the anxiolytic benzodiazepine chlordiazepoxide (CDP) on theta frequency is partially mediated by the medial supramammillary nucleus (mSuM), part of an ascending theta-activating system. Rats were trained on the hippocampal-sensitive fixed-interval 60-sec schedule (FI60). CDP (5 mg/kg i.p.) released responding suppressed by nonreward, seen as increased leverpressing, and reduced theta frequency concurrently. Microinfusion of CDP (20 microg in 0.5 microl saline) into mSuM had as large effects on both frequency and behavior. Other nuclei mediate the benzodiazepine reduction of theta frequency in the open field and the water maze. But the mSuM appears to be the major, if not sole, nucleus controlling theta frequency and, so, hippocampal-mediated behavioral inhibition in the FI60 lever task.

Chlordiazepoxide specifically impairs nonspatial reference memory in the cued radial arm maze in rats

Anxiolytic benzodiazepines, at low doses, reportedly impair the radial arm maze with nonspatial visual/tactile but not spatial cues. We replicated the former result controlling for changes in drug state and cue effectiveness. Rats learned an eight-arm radial maze with reward in only four arms. The reward varied in spatial position from trial to trial but was always cued by a piece of sandpaper at the entry to the arm. Chlordiazepoxide (5 mg/kg, ip) impaired acquisition. Rats that switched from saline during acquisition to chlordiazepoxide showed an impairment of performance that only lasted for 1 day. Removal of the cues reduced the performance of controls and switched rats to the level of the rats that received chlordiazepoxide during acquisition but did not affect the latter. These data suggest that chlordiazepoxide does indeed impair nonspatial reference memory in the radial arm maze while leaving working memory, and, possibly, spatial reference memory, intact but that the previous report of this effect was the result of a change in drug state rather than of the drug itself.

Neuronal activity of the septal pacemaker of theta rhythm under the influence of stimulation and blockade of the median raphe nucleus in the awake rabbit

The control of theta rhythm in neuronal activity of the medial septal area and hippocampal electroencephalogram by the brainstem structures was investigated in waking rabbits. In the first series of experiments stimulating electrodes were implanted into the midbrain reticular formation and median raphe nucleus. The standard frequency of theta-bursts in medial septal area neurons and in the electroencephalogram was uniformly and chronically decreased in all rabbits with electrodes implanted into the median raphe nucleus (4.7 +/- 0.5 Hz versus 5.2 +/- 0.19 Hz in animals without electrodes in median raphe nucleus). Weak electrical stimulation of the median raphe nucleus resulted in additional decrease of theta expression in the medial septal area neurons and its disappearance from the hippocampal electroencephalogram, where it was substituted by delta-waves and spindles. Stimulation of the reticular formation had the opposite effect, with an increase in theta frequency, regularity and expression in medial septal area neuronal activity and hippocampal electroencephalogram. In the second series of experiments reversible functional blockade of the median raphe nucleus by local microinjection of lidocaine was performed. This resulted in expression of theta-bursts in an additional group of medial septal area neurons, an increase in theta-burst frequency (by 0.5-2 Hz) and regularity with concomitant changes in the electroencephalogram. The effects of sensory stimuli on the background of increased theta activity were suppressed or significantly decreased. It is concluded that, in accordance with the data of other authors, the median raphe nucleus can be regarded as a functional antagonist of the reticular formation, powerfully suppressing theta-bursts of the medial septal area neurons and hippocampal theta rhythm. It is suggested that, in combination with the theta-enhancing influences of reticular formation, the median raphe nucleus may participate in termination of attention, its switching to other stimuli and stabilization of the effects of learning.

The medial supramammillary nucleus, spatial learning and the frequency of hippocampal theta activity

Previous studies have shown that the presence of hippocampal theta activity (theta) is important for learning and that the medial supramammillary nucleus (SuM) is involved in the control of the frequency of theta. In the present experiments, a single-day version (20 trials) of the Morris water maze was used to investigate the effects of drug injections into SuM on hippocampal theta frequency and spatial learning. Two groups of rats received an injection of chlordiazepoxide (CDP, 0.5 microl, 40 microg/microl) or saline (0.5 microl) into SuM 10 min before training in the Morris water maze. Two other groups of rats received an i.p. injection of 5 mg/kg CDP or saline, and two further groups received short (10 min) or long (15 min) immersion in cool water (22 degrees C) before training. The results showed: (1) in all groups theta frequency was an inverse logarithmic function of training time; (2) systemic CDP or long cool water exposure decreased theta frequency to a greater extent (by 1 Hz), and also impaired learning to a greater extent, than the other treatments; (3) that SuM-CDP produced a modest decrease in theta frequency (0.35-0.5 Hz) and a modest impairment of spatial learning. These data suggest that theta frequency per se may be important for spatial learning and that total abolition of theta is not necessary for dysfunction; and that while a lesser part of the effect of i.p. CDP on spatial learning appears to be mediated by SuM the greater part appears to involve other nuclei as well.

Cognitive dysfunction resulting from hippocampal hyperactivity--a possible cause of anxiety disorder?

Pure cognition and hence pure cognitive dysfunction might be expected to have no direct relation to any specific emotion. Changes in cognitive processing will change the assessment of stimuli and thus could change emotional responses nonspecifically. However, neurology suggests a more direct relation between at least some aspects of cognition and emotion. The limbic system in general and the hippocampus in particular have been suggested at different times to be crucial for both memory and emotion. Even recently, O'Keefe and Nadel (The hippocampus as a cognitive map, Oxford University Press, 1978) proposed that the hippocampus is a spatial, or cognitive, map, while Gray (The neuropsychology of anxiety: An enquiry into the functions of the septo-hippocampal system. Oxford University Press, 1982) proposed that it is central to anxiety. This apparent incongruity can be resolved by combining recent developments in the psychology of anxiety (which emphasise changed processing biases), recent extensions of Gray's theory (which bring it closer to cognitive views), and recent theories of the role of the hippocampus in memory (which see it as controlling rather than storing information). This paper proposes that at least some instances of clinical anxiety could result from hyperactivity of the septo-hippocampal system, which would produce cognitive dysfunction in the form of increased negative association of stimuli with a consequential increase in anxiety when the stimuli are subsequently presented.

Regulation of the septal pacemaker theta rhythm by the cervical nuclei of the midbrain

Neuronal activity in the medial septal region (the medial nucleus and the diagonal band nucleus, MN-DBN) was recorded along with hippocampal EEG traces in conscious rabbits with stimulatory electrodes implanted in the median cervical nucleus (MCN) and the reticular formation (RF) of the midbrain and pons. In all animals with electrodes in the MCN, the background theta activity frequency was low (4.6 +/- 0.15 Hz) as compared with intact rabbits or those with electrodes implanted only in the RF (5.2 +/- 0.19 Hz, p < 0.5). Stimulation of the MCN with weak low-frequency impulses reduced theta volleys from MN-DBN cells, reducing their frequency and regularity and inducing the appearance or strengthening of low-frequency delta modulation. The number of spikes in a volley decreased, and the duration of inter-volley intervals increased. Stimulation of the MCN led to a gradual decrease in the frequency and amplitude of theta waves, induced irregular delta waves and spindles of 12 Hz in the hippocampal EEG. Stimulation of the RF produced the opposite changes in volley activity in the MN-DBN and hippocampal EEG, with increases in theta and decreases in delta components. These results support a role for the midbrain cervical nuclei as structures limiting the generation of theta activity by the reticular-septal system, but do not support the existence of an MN-DBN-independent high-frequency serotoninergic theta rhythm. It is proposed that the effect of the MCN may be important for suppression and switching of attention.

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

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

Effects of long-term administration of phenelzine on reticular-elicited hippocampal rhythmical slow activity

All anxiolytics so far tested show a common reduction in the frequency of reticular-elicited hippocampal rhythmical slow activity (RSA). Acute administration of the tricyclic antidepressant imipramine shares this effect with anxiolytics. The present experiment tested whether the MAO inhibitor antidepressant phenelzine shares this common effect of anxiolytics and imipramine on hippocampal RSA. Rats, implanted with reticular stimulating electrodes and subicular recording electrodes, received four acute doses (0.2, 2.0, 6.0 and 18 mg/kg) or one chronic dose (2 mg/kg/day for 35 days) of phenelzine. Acute administration of phenelzine failed to systematically affect RSA frequency. Chronic injections of phenelzine eventually produced a reduction in RSA frequency combined with a gradual increase in baseline RSA frequency. The absence of immediate action and the production of a chronic reduction in RSA frequency are distinct from the documented effects of anxiolytics and imipramine, whereas the increase in baseline RSA frequency is similar to imipramine. The different influence of phenelzine on RSA frequency compared with anxiolytics (including imipramine) is consistent with the different clinical profiles of these drugs.

Contribution of synapses in the medial supramammillary nucleus to the frequency of hippocampal theta rhythm in freely moving rats

We have previously shown that in urethane-anesthetized rats the frequency of rhythmical slow activity in the hippocampus ("theta") is controlled by the medial supramammillary nucleus (SuM). In particular, injections of procaine into SuM in urethane-anesthetized animals reduce the frequency of theta. However, it has been reported that, in freely moving animals, lesions of SuM do not affect theta. The present experiments were designed to resolve this anomaly. Injections of procaine or chlordiazepoxide into SuM in urethane-anesthetized animals reduced the frequency of theta elicited by reticular stimulation. Mapping showed that procaine injections in freely moving animals were effective in the same locations as under urethane anesthesia. Injections of chlordiazepoxide were effective in a more restricted area than procaine, consistent with an action on synapses in SuM and sparing fibers afferent to SuM. Analysis of the functional spread indicated an effective radius of diffusion of the drugs of 500 microns. With optimal placements, this implied an action on at least 80% of SuM. However, in contrast to the results under urethane, the maximal frequency reductions obtained were less than 50% of the theoretical maximum. In a number of animals receiving repeated injections into SuM, lesions developed which encompassed the whole of SuM. As previously reported, theta was largely intact in SuM-lesioned animals. However, the frequency of theta produced by reticular stimulation was reduced after lesion by approximately the same amount as by procaine injections before lesion. These results suggest that in freely moving animals SuM is only one of two or more nuclei which jointly control the frequency of reticular-elicited theta.

Food carrying in rats is blocked by the putative anxiolytic agent buspirone

The effects of the putative anxiolytic agent buspirone on food-handling behavior of laboratory rats were investigated. Rats trained to travel from a covered shelter to a food source were provided with food pellets of six sizes. Smaller pellets were eaten at the exposed food source, whereas larger pellets were carried back to the shelter for consumption. Subcutaneous administration of buspirone hydrochloride (0.2-2.0 mg/kg) reduced carrying of larger food pellets in a dose-dependent manner. Instead, these pellets were also eaten at the exposed food source. Carrying was maximally suppressed 1 h after drug administration. Handling of smaller pellets, travel times, and eating times were not affected by buspirone. Similar results have previously been obtained with diazepam. Buspirone appears to exert its effects through 5-HT1A and/or dopamine receptors, whereas diazepam interacts with benzodiazepine receptors. Thus, manipulations of distinct transmitter systems may have similar behavioral consequences on the food carrying responses of rats.

Fine structure of the supramammillary nucleus of the rat: analysis of the ultrastructural character of dopaminergic neurons

The supramammillary nucleus projecting to widespread regions contains dopaminergic and non-dopaminergic neurons. The present study provided a comprehensive electron microscopic analysis of these dopaminergic and non-dopaminergic neurons in the supramammillary nucleus of the rat. The normal supramammillary nucleus was composed of round or spindle-shaped, small and medium-sized neurons (12.7 x 8.0 microns, 78.0 microns 2) containing a light oval nucleus with invaginated envelope, mitochondria, Golgi apparatus, lysosomes, less-developed rough endoplasmic reticulum, and no Nissl bodies. The majority of terminals (more than 70%) in the normal neuropil were small (diameter less than 1.0 microns) and contained round vesicles forming asymmetric synaptic contacts. The terminals often contained dense-cored vesicles. To determine the morphological features of dopaminergic neurons, we examined the ultrastructural localization of tyrosine hydroxylase (TH) immunoreactivity, which is the synthetic enzyme of dopamine, and compared TH-immunoreactive neurons to non-TH-immunoreactive neurons. Their shape and size were similar. The average number of axosomatic terminals in a sectional plane was 5.0 in TH-neurons and 2.4 in non-TH-neurons; the bouton covering ratio was 16.5% in the former and 8.6% in the latter. Both numbers were significantly larger in TH-neurons than in non-TH-neurons. Serial ultrathin sections of these neurons revealed that the average total number of axosomatic terminals was 55.7 in the TH-neuron and 28.4 in the non-TH-neuron. Characteristic lamellar bodies and sub-surface cisternae were often present in TH neurons. There were no TH-labeled terminals. These results indicate that dopaminergic neurons receive more inputs than neurons containing other neurotransmitters.

A comparison of the acute effects of a tricyclic and a MAOI antidepressant on septal driving of hippocampal rhythmical slow activity

In free-moving male rats, the function relating frequency to the threshold current required to drive hippocampal rhythmical slow activity (RSA) with septal stimulation has a minimum at 130 ms. Both classical anxiolytics (e.g. benzodiazepines) and the novel anxiolytic buspirone show similar effects on septal driving of RSA. The tricyclic antidepressant imipramine may be as effective as anxiolytic drugs in treatment of generalized anxiety disorder. The antidepressant monoamine oxidase inhibitor phenelzine has also been reported to be effective in treating anxiety, but this may reflect an action on "atypical depression" rather than "anxiety". The present study therefore compared the effects of acute administration of imipramine and phenelzine on septal driving of RSA to determine whether either would mimic anxiolytics in this test. Rats were chronically implanted with septal stimulating electrodes and subicular recording electrodes. Three groups of rats received IP injection of either imipramine (5.9-13.3 mg/kg or 13.3-30 mg/kg) or phenelzine (0.2-5.4 mg/kg). The effects produced by imipramine were very similar to the effects produced by anxiolytic drugs. In contrast, the effects produced by phenelzine were essentially opposite to those of both anxiolytic drugs and imipramine. The present experiment suggests that imipramine may act as a true anxiolytic, in addition to its conventional antidepressant properties. In contrast, phenelzine may be effective in cases where the etiology is essentially that of depression even when the symptomatology appears to be that of anxiety.

Effects of intracranial infusions of chlordiazepoxide on spatial learning in the Morris water maze. II. Neuropharmacological specificity

In the preceding paper it was found that infusions of chlordiazepoxide (CDP) into the medial septal region, but not several other regions possessing a high density of benzodiazepine receptors, impaired spatial learning, but not cue learning or swim speed, in the Morris water maze. The present investigation sought to further characterize the neuropharmacological profile of this effect. Initially, it was reconfirmed that systemically administered CDP impaired spatial learning, but not cue learning or swim speed, in the water maze. Additionally, it was found that systemically administered scopolamine, a muscarinic antagonist, impaired both spatial and cue learning, but not swim speed, confirming the detrimental effects of cholinergic hypofunction on maze learning. In new rats, a dose-response assessment revealed that 60 and 30 nmol, but not 10 nmol, CDP infused into the medial septum impaired spatial learning, but not cue learning or swim speed. On the following day, rats from each dose group, now undrugged, acquired a reversed platform location at control levels, suggesting that the previously observed impairment was not due to a neurotoxic effect. Additionally, it was found that systemically administered flumazenil (10 mg/kg) blocked the spatial learning deficit produced by the 60 nmol dose of CDP infused into the medial septum. However, intraseptal infusions of flumazenil (10, 20, or 30 nmol) failed to attenuate the spatial learning deficit produced by systemically administered CDP. Finally, systemically administered tetrahydroaminoacridine (1 or 3 mg/kg), an acetylcholinesterase inhibitor, failed to attenuate the spatial learning deficit produced by intraseptal CDP (60 nmol). Together these results implicate benzodiazepine receptors in the medial septum in the amnesic actions of CDP but suggest that additional sites also mediate this action. The present results fail to support the idea that the spatial learning deficit produced by intraseptal infusions of CDP is due to a suppression of septo-hippocampal cholinergic activity and it is proposed that CDP impairs spatial learning by exacerbating hippocampal inhibition by inhibiting septo-hippocampal GABAergic projection neurons.

Differential effects of benzodiazepine receptor agonists on hippocampal long-term potentiation and spatial learning in the Morris water maze

The amnesic effect of benzodiazepine drugs has been well documented, though the mechanisms mediating this effect are unknown. Long-term potentiation (LTP) has been proposed as a mechanism by which information is stored in the mammalian central nervous system. This experiment sought to determine if benzodiazepines impair mnemonic processes by blocking LTP. Rats implanted with a stimulating electrode in the perforant path and a recording electrode in the dentate gyrus were given high-frequency stimulation after the administration of either chlordiazepoxide (5 mg/kg), diazepam (5 mg/kg) or CL 218,872 (10 mg/kg). None of these drugs completely blocked the induction of LTP as measured by changes in the magnitude of the population spike amplitude, though CL 218,872 significantly suppressed potentiation over the duration of recording (24 h). Moreover, the potentiation observed in diazepam-treated rats returned to baseline after 24 h. Two weeks after the last recording, the same implanted rats were given their previous drug and dose and then tested for spatial learning ability in the Morris water maze. Each drug resulted in a severe impairment of spatial learning, but had no effect on cue learning. Two days later, in the absence of drugs, the same rats readily acquired a reversed platform location. Together these results suggest that CL 218,872 may impair spatial learning by suppressing LTP in the perforant path but that chlordiazepoxide and diazepam can impair spatial learning in the absence of LTP suppression in this pathway.

Neuroanatomical study of afferent projections to the supramammillary nucleus of the rat

We examined the regions projecting to the supramammillary nucleus of the rat with retrograde transport of WGA-HRP and WGA, and anterograde transport of Phaseolus vulgaris leucoagglutinin. The supramammillary nucleus receives major descending afferents from the infralimbic cortex, the dorsal peduncular cortex, the nucleus of the diagonal band of Broca, the medial and lateral preoptic nuclei, bilaterally. The major ascending afferents come from the pars compacta of the nucleus centralis superior, the ventral tegmental nucleus, and the laterodorsal tegmental nucleus. The supramammillary nucleus also receives a few (but distinct) fibers from the anterior and lateral hypothalamic nuclei, the ventral premammillary nucleus, the interpeduncular nucleus, the cuneiform nucleus, the dorsal raphe nucleus, the incertus nucleus, and the C3 region including the prepositus hypoglossi nucleus. All descending fibers run through the medial forebrain bundle. Almost all ascending fibers from the pars compacta of the nucleus centralis superior and the laterodorsal tegmental nucleus run through the mammillary peduncle, and terminate throughout the supramammillary nucleus. A few fibers from the laterodorsal tegmental nucleus and the C3 region run through the fasciculus longitudinalis dorsalis and terminate in the dorsal part of the supramammillary nucleus including the supramammillary decussation.

Buspirone produces a dose-related impairment in spatial navigation

Classical anxiolytic drugs and hippocampal lesions have common behavioural effects that include loss of place navigation in the water maze. The novel anxiolytic drug buspirone, unlike classical anxiolytic drugs, does not interact with GABA and is not muscle relaxant, sedative, hypnotic, anticonvulsant, or addictive. Buspirone affects hippocampal electrophysiology in a similar fashion to classical anxiolytics and so we predicted it would have similar effects on spatial navigation. Rats injected with buspirone (0.1-10.0 mg/kg, IP) showed a loss of acquisition of spatial navigation in the water maze that has a similar dose dependence to that reported for the effects of buspirone on the hippocampus. This finding demonstrates that the effects of anxiolytics on spatial navigation are not due to their side effects and supports the view that changes in hippocampal function may underlie some components of clinical anxiolytic action.

Effect of a new anxiolytic, DN-2327, on learning and memory in rats

The effects of a new anxiolytic, (2-(7-chloro-1,8-naphthyridin-2-yl)-3- [(1,4-dioxa-8-azaspiro[4.5]dec-8-yl)-carbonylmethyl] isoindolin-1-one (DN-2327), on the execution of step-through passive avoidance and delayed spontaneous alternation tasks were assessed and compared with those of diazepam (DZP) and buspirone. DN-2327 and buspirone (both 10 and 20 mg/kg, PO) impaired performance in the 48-h passive avoidance recall test when given prior to the test session, but not when given before the training trial. DZP impaired the performance at doses of more than 5 and more than 10 mg/kg PO when given prior to the test session and when given before the training trial, respectively. The action of DZP (10 mg/kg PO) when given before the training trial was antagonized by flumazenil (20 mg/kg, IP) and tended to be antagonized by DN-2327 (10 and 30 mg/kg, PO), but was not affected by buspirone. No evidence for possible amnesic effects of DN-2327 or buspirone on working memory was found in the delayed spontaneous alternation task, but DZP (3 and 10 mg/kg, PO) caused significant impairment of working memory. Electroshock sensitivities detected by flinch, jump, and vocalization thresholds were not influenced significantly by DN-2327 (30 and 100 mg/kg, PO), DZP (10 and 30 mg/kg, PO) or buspirone (30 and 100 mg/kg, PO).(ABSTRACT TRUNCATED AT 250 WORDS)

Pindolol antagonizes the effects on hippocampal rhythmical slow activity of clonidine, baclofen and 8-OH-DPAT, but not chlordiazepoxide and sodium amylobarbitone

Buspirone, benzodiazepines, barbiturates and ethanol all reliably reduce the frequency of reticular-elicited hippocampal rhythmical slow activity. In the present experiments we tested a number of drugs which are not usually used for treating generalized anxiety disorders but which have been reported to have some anxiolytic properties. Clonidine (0.3 mg/kg, i.p.), baclofen (6 mg/kg, i.p.) and 8-hydroxy-di-n-propylamino tetralin (8-OH-DPAT) (2.5 mg/kg, i.p.) all reduced the frequency of rhythmical slow activity. The effect of all three drugs was reduced by the 5-hydroxytryptamine 1a antagonist pindolol (2 mg/kg, i.p.). Pindolol had no effect on the reduction in rhythmical slow activity produced by sodium amylobarbitone, as has been previously reported for the benzodiazepine chlordiazepoxide. Flumazenil (10 mg/kg, i.p.), a benzodiazepine receptor antagonist, reduced the effects of chlordiazepoxide (5 mg/kg, i.p.), but not buspirone (10 mg/kg, i.p.). A combination of the selective beta 1 adrenergic receptor antagonist metoprolol (20 mg/kg, i.p.) and the beta 2 adrenergic receptor antagonist ICI 118,551 (4 mg/kg, i.p.) did not reduce the effects of either buspirone (10 mg/kg, i.p.) or diazepam (1 mg/kg, i.p.). These data show that there are at least two separate routes through which anxiolytic agents reduce the frequency of hippocampal rhythmical slow activity. Buspirone, clonidine, baclofen and 8-OH-DPAT act via a system dependent on 5-hydroxytryptamine 1a receptor activation. Benzodiazepines act via activation of the benzodiazepine receptor and probably share with barbiturates action at the GABA-benzodiazepine-chloride ionophore complex but do not produce their effects, directly or indirectly, by 5-hydroxytryptamine 1a receptor activation.

Effects of long-term administration of anxiolytics on reticular-elicited hippocampal rhythmical slow activity

Buspirone is effective in treating clinical anxiety but, unlike classical anxiolytics, does not have anti-convulsant, sedative or muscle relaxant side-effects and does not interact with GABA. Buspirone may also differ from classical anxiolytics in requiring a period of 2 weeks or more to achieve its full therapeutic action. It has previously been shown that all anxiolytic drugs, including buspirone, reduce the frequency of reticular-elicited hippocampal rhythmical slow activity (RSA). The present experiments tested whether the time course of the effect of buspirone on rhythmical slow activity differed from that of the anxiolytic benzodiazepine chlordiazepoxide. Rats, implanted with reticular stimulation electrodes and subicular recording electrodes, received three intraperitoneal injections per day of buspirone (2.5 mg/kg), chlordizepoxide (5 mg/kg) or saline for 45 days. Both buspirone and chlordiazepoxide reduced the frequency of rhythmical slow activity on the first day of testing and Ro15-1788 (10 mg/kg) blocked the effects of chlordiazepoxide but not buspirone. There was no increase in the effect of buspirone with time. These results showed that, if the effect of anxiolytic drugs on rhythmical slow activity provides any basis for their clinical action, then some additional factors are required to explain both the delayed action of buspirone and the immediate action of classical anxiolytic drugs.

Neurochemically dissimilar anxiolytic drugs have common effects on hippocampal rhythmic slow activity

Previous experiments have shown that anxiolytic drugs reduce the frequency of hippocampal rhythmic slow activity, induced by high frequency stimulation of the reticular formation and flatten the function relating threshold septal stimulation to the frequency of driven rhythmic slow activity. All of the drugs involved are known to augment GABAergic transmission. The present experiments investigated the effects of the novel anxiolytic compound buspirone which, unlike conventional anxiolytics, does not interact with GABA, yet is a clinically effective anxiolytic. Buspirone (0.156-40 mg/kg, i.p.) was found to reduce the frequency of reticular-elicited rhythmic slow activity, in a similar manner to chlordiazepoxide (0.019-20 mg/kg, i.p.). Buspirone did not change the linearity of the voltage-frequency function. Buspirone (10 mg/kg, i.p.) also altered the threshold for septal driving of rhythmic slow activity, in a similar manner to classical anxiolytics. The combination of chlordiazepoxide (5 mg/kg, i.p.) with corticosterone (0.2 mg, s.c.) removed the minor differences between buspirone and chlordiazepoxide in both the septal and reticular tests. These results show that buspirone altered the control of rhythmic slow activity in the hippocampus, in a manner which appeared functionally equivalent to other anxiolytics but which depends on mechanisms which are likely to be neurally and pharmacologically distinct from those of other anxiolytic drugs.

Effects of GABAA and GABAB receptor agonists on reticular-elicited hippocampal rhythmical slow activity

Hippocampal rhythmical slow activity (RSA) can be elicited by stimulation of the midbrain reticular formation. All classes of anxiolytic drug so far tested reduce the frequency of this RSA. Anxiolytic barbiturates and benzodiazepines, as opposed to compounds such as buspirone, are thought to act as receptor agonists of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). In the present study muscimol (a GABAA receptor agonist) and baclofen (a GABAB receptor agonist) were injected into freely moving rats. Baclofen produced a dose-related decrease in frequency of RSA in the range 1 to 9 mg/kg i.p. and abolished RSA at 27 mg/kg. Muscimol produced an increase in RSA frequency with an inverted U-shaped dose response curve in the range 0.0001 to 1.0 mg/kg with maximal effect at 0.001 mg/kg. The effects of classical anxiolytic drugs in the present test resemble those of the GABAB receptor agonist baclofen more than they resemble those of the GABAA receptor agonist muscimol but it is possible that they are acting via GABA systems which employ low rather than high affinity GABAA receptors or through some transmitter system other than GABA.

Effects of long-term administration of imipramine on reticular-elicited hippocampal rhythmical slow activity

All anxiolytics so far tested show a common reduction in the frequency of reticular-elicited hippocampal rhythmical slow activity (RSA). The present experiments tested whether imipramine, an antidepressant drug which has also been used to treat generalized anxiety disorders, shares the common characteristics of anxiolytics on hippocampal RSA. Rats implanted with reticular stimulating electrodes and subicular recording electrodes received both acute and chronic injection of different doses of imipramine. Only relatively high doses (20 and 30 mg/kg, IP) of imipramine produced a reduction in RSA frequency after a single administration. Long-term administration of 20 mg/kg (but not 10 mg/kg, IP) imipramine induced an increase in baseline RSA frequency but there was no change in the acute frequency-reducing effect of the drug. These results suggest that changes in hippocampal RSA reflect different mechanisms of action for chronic versus acute treatment with antidepressant. It is possible that, at high doses, apparently anxiolytic effects of imipramine may be mediated by similar mechanisms to conventional anxiolytic drugs.

Buspirone affects hippocampal rhythmical slow activity through serotonin1A rather than dopamine D2 receptors

Buspirone reduces anxiety clinically but, unlike classical anxiolytics, is not muscle relaxant, sedative, anticonvulsant or effective in increasing GABA function. The basis for its clinical action is not known, but action at both dopamine D2 and serotonin1A receptors has been suggested. Buspirone, like classical anxiolytics, produces a general reduction in the frequency of hippocampal rhythmical slow activity elicited by stimulation of the midbrain in the rat. Methysergide (3 mg/kg i.p.), GR38032F (0.3 mg/kg i.p.) and haloperidol (0.2 mg/kg and 2.0 mg/kg i.p.) failed to block this effect of buspirone (10 mg/kg i.p.). Apomorphine (0.3 mg/kg i.p.) had minor effects, but did not produce a general reduction in frequency. Pindolol (2 mg/kg i.p.) produced a small reduction in frequency itself. In the presence of pindolol, buspirone was without effect, while the effect of chlordiazepoxide (5 mg/kg i.p.) was potentiated. These results show that: (a) the similar effects of buspirone and classical anxiolytics such as chlordiazepoxide on reticular-elicited hippocampal rhythmical slow activity are achieved through different mechanisms; (b) the effects of buspirone in this particular test are more likely to depend on its interaction with serotonin1A receptors than its interaction with D2 receptors; and (c) that, as in other tests, buspirone does not act via serotonin2 or serotonin3 receptors.

Inhibition of hippocampal CA1 neurons by 5-hydroxytryptamine, derived from the dorsal raphe nucleus and the 5-hydroxytryptamine1A agonist SM-3997

Electrophysiological studies, using chloral hydrate-anesthetized rats, were undertaken to determine whether hippocampal pyramidal neurons, receiving input from the medial septal nucleus, were affected by 5-hydroxytryptamine (5-HT) derived from the dorsal raphe nucleus. The pyramidal neurons in the CA1 region of the hippocampus were classified into short- and long-latency neurons, based on their response to stimulation of the medial septal nucleus. Microiontophoretically applied atropine inhibited the generation of spikes upon stimulation of the medial septal nucleus in short-latency neurons, but had no effect on long-latency neurons. In the short-latency neurons, the stimulation-induced spikes of the medial septal nucleus were inhibited by conditioning stimuli applied to the dorsal raphe nucleus and iontophoretic application of 5-HT and the 5-HT1A agonists, SM-3997 (3 a alpha,4 beta,7 beta,7a alpha-hexahydro-2-(4-(4-(2-pyrimidinyl)-1- piperazinyl)-butyl)-4,7-methano-1H-isoindole-1,3(2H)-dione dihydrogen citrate) and 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin). The conditioning effect of the dorsal raphe nucleus was antagonized by methysergide. However, in the long-latency neurons, the spikes elicited by stimulation of the medial septal nucleus were not affected by the conditioning stimulation of the dorsal raphe nucleus, or iontophoretically applied 5-HT. These results indicate that 5-HT, originating in the dorsal raphe nucleus inhibited hippocampal pyramidal neurons receiving cholinergic input from the medial septal nucleus, but not those receiving non-cholinergic input from the medial septal nucleus. The drug SM-3997 inhibited the activity of hippocampal pyramidal neurons, that receive excitatory cholinergic input from the medial septal nucleus by acting on 5-HT1A receptors.

Chlordiazepoxide reduces discriminability but not rate of forgetting in delayed conditional discrimination

Benzodiazepine and anticholinergic drugs interfere with septo-hippocampal function in similar but not identical ways. They also share a number of common behavioural effects and, in particular, both classes of drug interfere with spatial memory in the Morris Water Maze--a test which is very sensitive to hippocampal dysfunction. We have previously shown that the anticholinergic drug scopolamine impairs discriminability, but not rate of forgetting, in delayed conditional discrimination. In the present study forgetting was quantified by fitting a negative exponential function to estimates of discriminability derived from a signal detection analysis of data from an auditory delayed conditional discrimination task. Chlordiazepoxide produced a highly significant decrease in discriminability which was monotonically related to the logarithm of dose in the range 0.67-18.0 mg/kg IP. The rate of forgetting was not increased. These data confirm the pharmacological independence of changes in discriminability and rate of forgetting; demonstrate that in this task chlordiazepoxide has similar effects to scopolamine; and suggest that the effects of chlordiazepoxide in other working memory tasks could be more a result of changed stimulus processing than impairment of memorial processes.

Effects of ethanol and Ro 15-4513 in an electrophysiological model of anxiolytic action

Previous research has implicated hippocampal rhythmical slow activity in the mechanisms of action of the anxiolytic drugs. In this study ethanol and a putative ethanol antagonist, Ro 15-4513, were investigated with reticular elicitation of rhythmical slow activity. Doses of ethanol between 0.6 and 3.1 g/kg were used. Ethanol reduced the frequency of reticular-elicited rhythmical slow activity in the same way as has been reported for anxiolytic barbiturates and benzodiazepines. This effect was linearly related to log dose of ethanol in the range of 1.7-3.1 g/kg. Ro 15-4513 at a dose of 2 mg/kg reduced the effect of ethanol (2.0 g/kg) but had no action itself. Ethanol also decreased the slope of the stimulation voltage-rhythmical slow activity frequency function but this effect was not reduced by Ro 15-4513. These results show that ethanol acts in a similar manner to conventional anxiolytic drugs but that only one component of this action can be reduced by Ro 15-4513.

Effects of the NMDA antagonists CPP and MK-801 on delayed conditional discrimination

N-methyl-D-aspartate (NMDA) receptor/channel antagonists have previously been shown to impair spatial working memory and hippocampal long-term potentiation. The present experiment investigated the effects of a variety of doses of NMDA antagonists on a working memory task in rats involving an auditory delayed conditional discrimination. Signal detection analysis and an exponential memory decay model were used to extract independent measures of stimulus discriminability and rate of forgetting. A competitive NMDA antagonist, (CPP, 0.33, 1.0, 10.0 mg/kg, IP) produced a reduction in discriminability which was linearly related to log dose, but which was only clear at the 10 mg/kg dose. Rate of forgetting was not increased by any dose. Similar results were obtained with a non-competitive antagonist (MK-801, 0.1, 0.33 mg/kg, IP). These data suggest that doses of NMDA receptor channel antagonists sufficient to disrupt hippocampal long-term potentiation and radial arm maze performance will also disrupt delayed conditional discrimination. The effect on delayed conditional discrimination is due to a disruption of stimulus discriminability and not to an increased rate of forgetting. The extent to which these effects relate to the reported changes in hippocampal long-term potentiation and radial arm maze performance remains to be determined.

Chlordiazepoxide, an anxiolytic benzodiazepine, impairs place navigation in rats

There are separate proposals that the hippocampus is involved in 'spatial memory' and in the control of 'anxiety'. Despite a larger number of common effects of anxiolytic drugs and hippocampal lesions, no effect of anxiolytic drugs has yet been reported in those spatial tasks which are particularly sensitive to the effects of hippocampal lesions. The present study addresses this issue. Separate groups of rats were treated, i.p., with 5 mg/kg chlordiazepoxide (an anxiolytic benzodiazepine), 1 mg/kg scopolamine (a muscarinic antagonist which has previously been shown to impair spatial learning) and a saline placebo. They were then trained to find a platform which was hidden in a constant location just under the surface of opaque water in a swimming pool. Separate groups of rats were trained with 4 trials per day and with 1 trial per day. Number of trials per day did not significantly influence the effects of the drugs. Chlordiazepoxide and scopolamine produced similar degrees of impairment in spatial learning to each other--but less impairment than has previously been reported with hippocampal lesions. The effectiveness of the anxiolytic drug chlordiazepoxide in the swimming pool, a specifically spatial task, suggests that the opposing concepts of 'spatial memory' and 'anxiety' which have been used previously to describe hippocampal function may represent different aspects of a unitary concept.