Acetylcholine, theta-rhythm and activity of hippocampal neurons in the rabbit--I. Spontaneous activity

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.

Intracranial EEG correlates of expectancy and memory formation in the human hippocampus and nucleus accumbens

The human brain is adept at anticipating upcoming events, but in a rapidly changing world, it is essential to detect and encode events that violate these expectancies. Unexpected events are more likely to be remembered than predictable events, but the underlying neural mechanisms for these effects remain unclear. We report intracranial EEG recordings from the hippocampus of epilepsy patients, and from the nucleus accumbens of depression patients. We found that unexpected stimuli enhance an early (187 ms) and a late (482 ms) hippocampal potential, and that the late potential is associated with successful memory encoding for these stimuli. Recordings from the nucleus accumbens revealed a late potential (peak at 475 ms), which increases in magnitude during unexpected items, but no subsequent memory effect and no early component. These results are consistent with the hypothesis that activity in a loop involving the hippocampus and the nucleus accumbens promotes encoding of unexpected events.

Dopamine D1/5 Receptor Modulation of Firing Rate and Bidirectional Theta Burst Firing in Medial Septal/Vertical Limb of Diagonal Band Neurons In Vivo

The medial septum/vertical limb of diagonal band complex (MS/vDB) consists of cholinergic, GABAergic, and glutamatergic neurons that project to the hippocampus and functionally regulate attention, memory, and cognitive processes. Using tyrosine hydroxlase (TH) immunocytochemistry and dark-field light microscopy, we found that the MS/vDB is innervated by a sparse network of TH-immunoreactive (putative catecholaminergic) terminals. MS/vDB neurons are known to fire in rhythmic theta burst frequency of 3–7 Hz to pace hippocampal theta rhythm. Extracellular single-unit recording in theta and non-theta firing MS/vDB neurons and antidromically identified MS/vDB-hippocampal neurons were made in urethan-anesthetized rats. Tail-pinch noxious stimuli and ventral tegmental area (VTA) stimulation (20 Hz) evoked spontaneous theta burst firing in MS/vDB neurons. Systemic D1/5 antagonists SCH23390 or SCH39166 (0.1 mg/kg iv) alone suppressed the spontaneous theta bursts, suggesting a tonic facilitatory endogenous dopamine D1 “tone” that modulates theta bursts in vivo. Activation of D1/5 receptor by dihydrexidine (10 mg/kg iv) led to an increase in mean firing rate in 60% of all theta and non-theta MS/vDB neurons with an increase in the number of theta bursts and spikes/burst in theta cells. In strong theta firing MS/vDB neurons, D1/5 receptor stimulation suppressed the occurrence of theta burst firing, whereas the overall increase in spontaneous mean firing rate remained. In low baseline theta MS/vDB neurons D1/5 receptor stimulation increases the occurrence of theta bursts along with a net increase in mean firing rate. Atropine injection consistently disrupts theta burst pattern and reduced the time spent in theta firing. Collectively, these data suggest that dopamine D1/5 stimulation enhances the mean firing rate of most MS/vDB neurons and also provides a state-dependent bidirectional modulation of theta burst occurrence. Some of these MS/vDB neurons may be cholinergic or GABAergic that may indirectly regulate theta rhythm in the hippocampus.

Ibotenic acid lesions of the medial septum retard delay eyeblink conditioning in rabbits (Oryctolagus cuniculus)

S. Berry and R. Thompson (1979) reported that electrolytic lesions of the medial septum significantly retard eyeblink conditioning. However, these electrolytic lesions were nonselective and may have also damaged the subcortical inputs to the hippocampus via the fimbria-fornix. In the present study, the medial septum was selectively lesioned with ibotenic acid in rabbits (Oryctolagus cuniculus), whose performance in a delay eyeblink conditioning paradigm was compared with that of intact controls. sham-operated controls, and intact controls given a systemic injection of scopolamine. Rabbits with selective medial septal lesions and rabbits receiving systemic scopolamine were significantly slower to condition than were intact and sham-lesioned rabbits. This finding demonstrates that the selective removal of the medial septum retards delay eyeblink conditioning in a manner similar to the disruption seen after systemic administration of scopolamine.

Computational model of carbachol-induced delta, theta, and gamma oscillations in the hippocampus

Field potential recordings from the rat hippocampus in vivo contain distinct frequency bands of activity, including delta (0.5-2 Hz), theta (4-12 Hz), and gamma (30-80 Hz), that are correlated with the behavioral state of the animal. The cholinergic agonist carbachol (CCH) induces oscillations in the delta (CCH-delta), theta (CCH-theta), and gamma (CCH-gamma) frequency ranges in the hippocampal slice preparation, eliciting asynchronous CCH-theta, synchronous CCH-delta, and synchronous CCH-theta with increasing CCH concentration (Fellous and Seinowski, Hippocampus 2000;1 0:187-197). In a network model of area CA3, the time scale for CCH-delta corresponded to the decay constant of the gating variable of the calcium-dependent potassium (K-AHP) current, that of CCH-theta to an intrinsic subthreshold membrane potential oscillation of the pyramidal cells, and that of CCH-gamma to the decay constant of GABAergic inhibitory synaptic potentials onto the pyramidal cells. In model simulations, the known physiological effects of carbachol on the muscarinic and K-AHP currents, and on the strengths of excitatory postsynaptic potentials, reproduced transitions from asynchronous CCH-theta to CCH-delta and from CCH-delta to synchronous CCH-theta. The simulations also exhibited the interspersed CCH-gamma/CCH-delta and CCH-gamma/CCH-theta that were observed in experiments. The model, in addition, predicted an oscillatory state with all three frequency bands present, which has not yet been observed experimentally.

Cholinergic induction of oscillations in the hippocampal slice in the slow (0.5-2 Hz), theta (5-12 Hz), and gamma (35-70 Hz) bands

Carbachol, a muscarinic receptor agonist, produced three distinct spontaneous oscillations in the CA3 region of rat hippocampal slices. Carbachol concentrations in the 4-13 microM range produced regular synchronized CA3 discharges at 0.5-2 Hz (carbachol-delta). Higher concentrations (13-60 microM) produced short episodes of 5-10 Hz (carbachol-theta) oscillations separated by nonsynchronous activity. Concentrations of carbachol ranging from 8-25 microM also produced irregular episodes of high-frequency discharges (carbachol-gamma, 35-70 Hz), in isolation or mixed with carbachol-theta and carbachol-delta. At carbachol concentrations sufficient to induce carbachol-theta, low concentrations of APV reversibly transformed carbachol-theta into carbachol-delta. Higher concentrations of D,L-2-amino-5-phosphonopentanoic acid (APV) reversibly and completely blocked carbachol-theta. A systematic study of the effects of carbachol shows that the frequency of spontaneous oscillations depended nonlinearly on the level of muscarinic activation. Field and intracellular recordings from CA1 and CA3 pyramidal cells and interneurons during carbachol-induced rhythms revealed that the hippocampal circuitry preserved in the slice was capable of spontaneous activity over the range of frequencies observed in vivo and suggests that the presence of these rhythms could be under neuromodulatory control.

Spontaneous activity and sensory responses of hippocampal neurons during persistent theta-rhythm evoked by median raphe nucleus blockade in rabbit

Spontaneous activity and responses to sensory stimuli were analysed in the hippocampal CA1 neurons of chronic unanesthetized rabbits before and after reversible functional blockade of the median raphe nucleus and medial septal area by local microinjections of anesthetic lidocaine. This evoked, correspondingly, persistent theta rhythm and its complete blockade for about 30 min. The results were compared to the neuronal data obtained earlier in the experiments with cholinergic drugs modulating expression of theta rhythm. Intra-median raphe nucleus injection of lidocaine evoked uniform increase of discharge rate in the hippocampal neurons with low and high spontaneous activity. Theta modulation of neuronal activity had increased regularity and frequency (by 0.5-2.0 Hz) and appeared in additional group of the neurons simultaneously with expression of persistent theta in the hippocampal electroencephalogram. Sensory responsiveness of the hippocampal neurons was drastically decreased (45% of the responses preserved). Reactions of all types were blocked, diminished, or inverted, but inhibitory responses were the most severely affected. Injection of lidocaine into medial septal area also blocked all brain stem afferents ascending to the hippocampus via medial septal area, thus, totally depriving hippocampus of brainstem-septal input. However, besides the total absence of theta modulation, spontaneous activity in majority of neurons was not significantly changed. Responsiveness to sensory stimuli also remained relatively high (77% of the responses preserved); on-effects were especially resistant to medial septal area blockade. Comparison of spontaneous and evoked activity in two theta states (physostigmine and median raphe nucleus blockade) revealed striking similarity of all characteristics, which suggested that theta-suppressing influences of median raphe nucleus (presumably serotonergic) are realized primarily through the control of cholinergic septo-hippocampal theta-generating mechanism. However, as the frequency of theta rhythm does not depend on it, an additional effect of disinhibition of activating reticular formation by the median raphe nucleus suppression is suggested. The data confirm that theta rhythm may be regarded as active filter in the information processing by the hippocampal neurons.

Perception of species-specific vocalizations in rats: role of the cholinergic septo-hippocampal pathway and aging

The effect of a chemical lesion of the cholinergic septo-hippocampal pathway induced by ethylcholine aziridinium (AF64A) on brain potentials evoked by species-specific vocalization containing informations of high biological relevance was studied in young adult (10 months) and aged (24 months) rats by means of neocortical electroencephalographic recordings. In control rats, the perception of a rat's vocalization in a life endangering situation (lasting 0.8 s) initiated an evoked potential followed by a late positive slow wave (LPSW)-complex and a direct current shift with a duration of up to 16 s. Four months after treatment with AF64A (2 nmol into each of the lateral ventricles), the mean negative component of the initial acoustic evoked potential (peak latency of about 60 ms after stimulus onset) was reduced (P = 0.04) both in young adult and aged rats. Further changes included a decrease of the late positive wave amplitude in young adult rats (P = 0.001) and a shorter duration of the LPSW-complex in aged rats (P = 0.03). AF64A induced also changes in specific components revealed by Principle Component Analysis, but only in the group of young rats. A decrease in the slow wave component (factor 1, 3000-4000 ms after stimulus onset; P = 0.02) was observed. Age per se affected the late positive potential shifts as indicated by a shorter latency of the late positive wave (P = 0.03). A detailed analysis of the major neurotransmitter markers proved an almost exclusive reduction of the activity of choline acetyltransferase in the ventral and dorsal hippocampus (up to 60%), which was equal in young adult and aged rats. The irreversible loss in choline acetyltransferase activity, which was restricted to the hippocampus, was associated by a slight reduction in serotonergic function. The present data suggest that the complex cognitive and emotional processes initiated by species-specific vocalization appear to be affected by aging. Furthermore, as a consequence of a cholinergic deficit in the hippocampus, the integration processes essential for the recognition of the biological meaning of a species-specific vocalization are considerably disturbed. These findings provide an experimental basis for studying disturbances in the perceptual response to stimuli of high emotional value in patients with hypocholinergic function as in Alzheimer's disease.

Melanin-concentrating hormone and neuropeptide EI projections from the lateral hypothalamic area and zona incerta to the medial septal nucleus and spinal cord: a study using multiple neuronal tracers

The projection pathways of neurons containing melanin-concentrating hormone (MCH) and neuropeptide EI (NEI), two peptides colocalized in the lateral hypothalamic area (LHA) of the rat, were mapped using the retrogradely transported fluorescent dyes, true blue (TB) and diamidino yellow (DY). TB and DY were injected into the medial septum/diagonal band complex (MS/DBC) and the thoracic level of the spinal cord (SpCd), respectively. Brains from rats receiving only one or both tracer injections were immunohistochemically stained for MCH in the spinal cord and NEI in the forebrain. In the MS/DBC, NEI-immunoreactive (-ir) fibers are concentrated in the MS and in the vertical and horizontal limbs of the DBC. In the SpCd, MCH-ir fibers are concentrated primarily in lamina X. Of the diencephalic NEI-ir neurons, 37.15% project to the MS/DBC and reside in the rostromedial zona incerta (ZIm), in the LHAt and LHAp, and in the perifornical region. Of the diencephalic MCH-ir neurons, 20.2% project to the SpCd and reside in the LHAt and LHAp. In addition, 2. 2% of the MCH-ir cells and 8.7% of the NEI-ir cells in the hypothalamus were labeled with both retrograde tracers and thus project to both the MS/DBC and SpCd. These dual projection neurons are located mainly in the LHAt and LHAp. Anterograde injections of the tracer Phaseolus vulgaris leucoagglutinin into the LHAt and ZIm corroborated our findings in the retrograde studies. Potential autonomic and behavioral roles of the NEI and MCH systems in the MS/DBC and the SpCd are discussed.

Metrifonate treatment enhances acquisition of eyeblink conditioning in aging rabbits

The cholinergic system is known to show deterioration during aging and Alzheimer's disease. In response, a therapeutic approach to Alzheimer's disease has been to attempt to compensate for the decrease in central cholinergic function by potentiating the activity of the remaining intact cholinergic cells with cholinesterase inhibitors. In this study treatment with the long-lasting cholinesterase inhibitor metrifonate enhanced acquisition of eyeblink conditioning in aging rabbits without producing interfering side effects. The effects of metrifonate on central and peripheral cholinesterase activity were evaluated, as was the involvement of plasma atropine esterase activity on the central and peripheral response to metrifonate. Results demonstrate that metrifonate can produce predictable, dose-dependent ChE inhibition. Associative learning in the aging rabbit was improved by metrifonate-induced steady state ChE inhibition within a range of 30-80%. Metrifonate was behaviorally effective in the absence of the severe side effects which typically plague cholinesterase inhibitors, suggesting that metrifonate is a possible treatment for the cognitive deficits resulting from normal aging and Alzheimer's disease.

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)

Hippocampus, theta, and spatial memory

The past 18 months have witnessed interesting developments in several areas of hippocampal research. First, the mechanisms of hippocampal theta are becoming clear, as is its role in spatial coding; each theta cycle appears to act as a clock mechanism against which the firing of the place cells can be timed. Second, there has been a continued strengthening in the support for the spatial theory of hippocampal function from single unit and lesion experiments; particularly important is the finding that the deficit in (non-spatial) delayed non-match to sample memory experiments in the monkey following medial temporal lobe damage stems from the part of the cortex which surrounds the hippocampus, and not from the hippocampus itself. Third, in contrast, it is proving more difficult than originally thought to show a causal relationship between long-term potentiation at the synaptic level and place learning-induced changes in hippocampal synapses.

Acetylcholine, theta-rhythm and activity of hippocampal neurons in the rabbit--II. Septal input

The aim of this paper was to evaluate the cholinergic component of the septohippocampal input signals in neuronal activity of the hippocampal fields CA1 and CA3 recorded extracellularly in chronic alert rabbits. Effects of electrical stimulation of the medial septal area were analysed in the control state, on the background of an increased level of endogenous acetylcholine (by physostigmine injection) and during its blockade by antimuscarinic drugs (scopolamine, atropine). Two groups of animals were used in the experiments: intact rabbits and rabbits with complete chronic undercutting of the septum, depriving the septohippocampal system of ascending medial forebrain bundle afferents. Primary inhibitory effects of standard duration (40-140 ms) evoked by medial septal area stimulation dominated in the hippocampus of intact rabbits (54%), though some neurons responded by initial diffuse excitation (37.5%); responses by single-spike on-effects were observed in a minority of neurons (8.5%). The primary suppression of activity prevailed (90%) in animals with basal undercutting of the septum. In intact rabbits under physostigmine action, the effects of medial septal area stimulation were depressed or completely blocked in 78% of hippocampal neurons on the background of increased theta modulation of activity. Neuronal responses to medial septal area stimulation recovered at the background of muscarinic antagonists. These effects of cholinergic drugs were reproduced in animals without medial forebrain bundle. It is concluded that the initial effect of the septal input upon the hippocampal neurons consists of a general suppression of their activity (reset), depending upon a non-cholinergic (presumably GABAergic) component of the septohippocampal connections.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholine, theta-rhythm and activity of hippocampal neurons in the rabbit--IV. Sensory stimulation

Modifications of responses of hippocampal neurons to sensory stimuli at the background of increased endogenous acetylcholine level (injection of physostigmine) and during blocking by scopolamine were analysed in the chronic alert rabbit. A significant decrease of reactivity (about 40%) of hippocampal neurons to sensory stimuli occurred after physostigmine injection, inducing stable theta modulation. Suppression and decrease of inhibitory responses (including initial reset phase) and of some excitatory reactions (including on-effects) were observed. However, a limited group of excitatory responses was augmented and prolonged under physostigmine action. Scopolamine, which blocked electroencephalogram theta-rhythm, did not change the responsiveness of hippocampal neurons. Some of the inhibitory and excitatory effects of sensory stimuli, especially on-responses, were strongly facilitated. Tonic responses were shorter, but they were stably reproduced without typical gradual habituation. All these effects were also present in the hippocampus after basal undercutting of the septum, which eliminates ascending brainstem input. It is suggested that under normal conditions a new or significant sensory stimulus evokes, in the hippocampus, an initial inhibitory reset of neuronal activity with subsequent coordinated triggering of rhythmic theta modulation by the septal input and arrival of the cortical input signal phase-locked to it. During the period of theta triggered by the stimulus, its processing and fixation in memory occurs, while the other, interfering stimuli, which are not phase-locked to the ongoing theta activity, are actively filtered out. Thus, septohippocampal interactions may participate in the organization of selective attention as a necessary condition for memory trace formation.

Acetylcholine, theta-rhythm and activity of hippocampal neurons in the rabbit--III. Cortical input

Cholinergic modulation of single cell responses and field potentials evoked in the hippocampus by electrical stimulation of the perforant path and mossy fibres was investigated in two groups of chronic unanesthetized rabbits--with intact hippocampus and with basally undercut septum (without ascending medial forebrain bundle afferents). In both groups of animals responses to stimulation were blocked or significantly depressed by i.v. physostigmine injection in many neurons (50% in the intact hippocampus and 69% in the hippocampus without medial forebrain bundle). In minor groups of neurons (10 and 8%, respectively), facilitation of responses was observed. Scopolamine restored initial responsiveness of hippocampal neurons and augmented effects of stimulation in some of them. The effect of physostigmine was reproduced by stimulation of the medial septum. Depressive influence of medial septal area stimulation was increased by physostigmine and blocked by scopolamine. Population spikes evoked by stimulation of the perforant path of the intact group were equally suppressed (by 43%) during sensory stimulation evoking natural theta, after physostigmine and after medial septal area stimulation. In the group of animals without medial forebrain bundle these influences resulted in a complete suppression of field potentials; scopolamine restored them. It is concluded that the main function of the septohippocampal cholinergic input consists of filtering out the signals appearing at the background of theta-rhythm triggered by a previous signal, thus preventing their interference with its processing and registration.