Effect of anticholinergic drugs alone and in combination with nembutal on burst theta-neurons of the rabbit septum

Activity of neurones with rhythmic theta-bursts was recorded in the medial septum--diagonal band complex of the waking rabbits with intact and deafferented septum. Effects of anticholinergic (scopolamine, atropine) and cholinomimetic (physostigmine) drugs were investigated after i.v. injection. Cholinoblocking drugs in doses, suppressing the theta-rhythm in the hippocampal EEG, eliminated rhythmic activity in some cells with weak theta-modulation. Theta-bursts persisted in cells with stable continuous rhythmicity, though its regularity decreased in some of them. Strong reticular or sensory stimulation evoked an increase of burst frequency, involvement of additional septal cells into rhythmic activity and appearance of the theta-rhythm in the hippocampal EEG. Neither anticholinergic, nor cholinomimetic drugs influenced the frequency and basic characteristics of theta-bursts in any condition tested. The anticholinergic drugs have no selective effect upon low-frequency theta-bursts. The septohippocampal connections contain a significant non-cholinergic component. The theoretical concept of the septum as a sole source of the whole frequency band of the theta-rhythm is proposed.     

Theta-bursts of hippocampal and septal neurons

Investigation of neuronal activity on non-anaesthetized rabbits in non-stress chronic conditions revealed in the hippocampus a small proportion of neurones (10-12 percent) with a weak rhythmic (theta) modulation of their activity. In the medial and lateral septal nuclei this activity was more organized and was present in about one third of neuronal population. Disruption of septo-hippocampal connections completely abolished theta-rhythm in EEG and in neuronal activity of the hippocampus. Transection of the perforant path was followed by appearance of pronounced theta-bursts in a large group of neurones (38 percent in CA1 and 43 percent in CA3). Electrical stimulation of the hippocampus suppressed theta-bursts and decreased their frequency in the lateral spetal nucleus. Disruption of hippocampo-septal connections was followed by an increase of the population of septal neurones with theta-bursts (48 percent), which became more prounced. Some theoretical implications from these data concerning the nature of hippocampal theta-activity are discussed.     

The role of GABA-ergic regulation in organizing spontaneous and evoked activity of the septal neurons

Effects of GABA, pentobarbital and picrotoxin upon spontaneous and evoked activity of neurones of the medial septal nucleus and the nucleus of the diagonal band (MS-DB) were investigated in the guinea pig septal slices. GABA and pentobarbital have similar effect upon all neurones, but the cells with a regular single spike and rhythmic burst activity of pacemaker type were less sensitive to their inhibitory influence. Picrotoxin affects neither frequency, nor pattern of activity. Electrical stimulation of the medial forebrain bundle evoked initial suppression of activity in majority of the neurones (74%); the remaining cells reacted mainly with an initial burst. GABA and pentobarbital increased the duration of the initial inhibition and revealed it in all cells with initial excitation in the control state. Picrotoxin did not influence this type of response, but revealed initial short-latency bursts in the cells with inhibitory effect in control state. The experiments show double nature of the effect of afferent stimulation controlling the activity of the MS-DB neurones. The mechanism of synchronization of the rhythmic activity in MS-DB, resulting in generation of the hippocampal theta-rhythm, is discussed.     

The effect of disconnecting the hippocampus from the septum on the activity of septal neurons]

Extracellular recording of neuronal activity was performed in the medial and lateral septal nuclei (MS and LS) in unanaesthetized rabbits after coagulation of septo-hippocampal connections. The MS neuronal activity had many pathological features. The LS activity was normal in every respect. Spontaneous activity, reactivity to sensory stimuli and main characteristics of responses to sensory stimuli were preserved in LS (and in a part of MS neurones). Sensory effects were augmented in intensity and duration, the number of neurones in LS with theta-bursts increased twofold, theta-bursts were more regular, than in control animals. These effects may be explained by an increase of ascending RF influences, which is supported by the fact of outstanding similarity between sensory and reticular effects in septal neurones after hippocampal disconnection. The number of units with inhibition of activity in response to sensory stimuli decreased, habituation of responses was absent. That means that hippocampal influences are necessary for the organization of inhibitory phenomena in the septum, and, above all, for processes of gradual habituation.     

The spontaneous neuronal activity of the hippocampus during the modulation of the theta rhythm by cholinergic substances]

Background activity of the hippocampal neurons, extracellularly recorded in waking chronic rabbits, was analysed in control state and after systemic injection of physostigmine and scopolamine. Similar analysis was done in the hippocampus chronically deprived of ascending brain stem afferents. Cholinergic drugs controlled the number of hippocampal neurons with theta-modulation and the degree of its stability but not the frequency. Activation of cholinergic theta-rhythm resulted also in regularization of activity with suppression of delta-modulation and complex spike discharges; its blockade was accompanied by the opposite changes. Both drugs shifted the level of background activity in the majority of neurons, but the overall mean frequency did not vary between the states. Regression analysis demonstrated significant negative correlations with dominating decrease in the level of activity in high-frequency neurons ( > 25 sp/s) and its increase in low-frequency ones ( < 25 sp/s) after injection of both drugs. Stability of the overall mean frequency and uniformity of its shifts presumably indicate that the frequency, unlike the pattern of the background activity, is not directly controlled by the cholinergic septal input.     

Frequency modulation of theta bursts of the septal neurons in the removal of ascending afferent effects in rabbits

Medial septal-diagonal band (MS-DB) units were examined extracellularly in chronic rabbits under two experimental conditions: 1) in an intact septum, under anaesthetic doses of pentobarbital (40 mg/kg, i.v.); 2) in a basally undercut septum of unanaesthetized rabbits. The background rhythmic burst activity was undistinguishable in both states. Low-frequency electric stimulation of afferent inputs (MFB, CA1, LS) led to entrainment of the theta-cycles. The upper limit of following was almost normal in the undercut septum, but was strongly reduced under pentobarbital. In units with the driving "by pause", the duration of the initial silent period under pentobarbital was increased almost twofold but in the basally undercut septum it was the same as in the normal state. Some MS-DB units with weak or absent theta-modulation reacted to stimulation by stimulus-locked single spike discharges which followed up to high frequencies in both conditions.     

Antidromic and synaptic potentials evoked in neurons of the lateral vestibular nucleus by stimulation of the interstitial nucleus of Cajal and Darkshevich's nucleus in cats

Effects of stimulation of the interstitial nucleus of Cajal and Darkshevich's nucleus on unit activity in the lateral vestibular nucleus of Dieters were investigated in cats anesthetized with pentobarbital. Stimulation of the above-mentioned structures was shown to lead to antidromic and orthodromic activation of Dieters neurons. Axon collateral of vestibular neurons, ascending to the above-mentioned brain-stem structures were discovered electrophysiologically. Stimulation of the nuclei of Cajal and Darkshevich was shown to evoke mono- and polysynaptic EPSPs and IPSPs in neurons of Deiters nucleus. Convergence of influences from both nuclei on the neurons studied was demonstrated. The particular features and functional role of the influences observed are discussed.L. A. Orbeli Institute of Physiology, Academy of Sciences of the Armenian SSR, Erevan. Translated from Neirofiziologiya, Vol. 16, No. 6, pp. 822–829, November–December, 1984.     

Sensory reactions of hippocampal neurons in rabbit during functional suppression of theta rythm- controlling structures

Responses to sensory stimuli were analyzed in hippocampal CA1 neurons of unanesthetized rabbits in chronic experiments before and after reversible functional blockade of the median raphe nucleus (MR) and medial septal area (MS-DB) by local microinjections of anesthetic lidocaine. The MR blockade, which resulted in an enhancement of theta-modulation of the background activity of the hippocampal neurons, was followed by a depression of sensory responsiveness (only 46.7% of the neurons reactive before the blockade retained their responses). Reactions of all types were blocked, diminished, or inverted, but inhibitory responses were affected most severely. Lidocaine injection into the MS-DB, which blocked also all brain stem afferents ascending to the hippocampus via the MS-DB input, resulted in the total absence of theta-modulation; however, responsiveness to sensory stimuli remained relatively high (76.7% of the responses were preserved); on-effects were especially resistant to the MS-DB blockade. Comparison of evoked activity in two conditions of continuous theta rhythm generation (physostigmine injection and MR blockade) revealed striking similarities. This suggests that the theta-suppressing influence of the MR (presumably serotonergic) is primarily actualized through the control of cholinergic septo-hippocampal theta-generating mechanism. The results provide support for the view that the theta-rhythm acts like an active filter in information processing performed by hippocampal neurons.     

Negative occurrence between hippocampal rhythmic slow activity and epileptiform spikes: influence of the striatum

The effects of caudate and septal stimulation on hippocampal activity were studied. Sodium penicillin was intravenously injected in order to induce a steady rate of interictal epileptic activity. Penicillin injection caused significant reduction of the rate of occurrence and duration of hippocampal rhythmic slow activity (RSA). Spontaneous RSA as well as RSA-eliciting caudate and septal stimulation induced a marked inhibition on frequency of epileptiform complexes. Lesions of the medial septal nucleus made it impossible to record RSA or to observe any sort of influence on hippocampal epileptiform activity by caudate stimulation. This result suggests that the septum is important for RSA genesis in the striato-hippocampal pathway or in the reciprocal septo-hippocampal connections.     

An investigation of phase relations between theta-rhythm potentials in the neocortex and hippocampus of rabbits

Phase relations between rhythmic activity recorded from the hippocampus and from various levels of the neocortex were calculated on a two-dimensional model on the assumption of the passive spread of hippocampal activity. The calculation showed that if such a spread of activity takes place an appreciable phase shift, caused by the spatio-temporal structure of the hippocampal source of the theta-rhythm, may exist between the rhythmic components of potentials recorded from the neocortex and hippocampus. In some series of electrophysiological experiments on rabbits various consequences of the model calculations were tested. Phase relations of the theta-rhythm recorded in the neocortex and in the hippocampus in the presence of maximal correlation between cortical potentials cannot be explained, in the overwhelming majority of cases, purely by the passive spread of hippocampal activity to the surface of the neocortex.     

Formation of "caudate spindles" in the rabbit sensomotor cortex during ontogeny

Electrical activity of the sensomotor and visual areas of the neocortex during stimulation of the caudate nucleus was recorded in young rabbits aged 3–60 days and in adults. Single stimulation of the caudate nucleus was found to cause the appearance of characteristic bursts of spindle-like rhythmic activity ("caudate spindles"), described previously in cats and monkeys, in the adult rabbit cortex. The latent period of the caudate spindle was about 200 msec, its duration 1–3 sec, and the frequency of its oscillations of the order of 12 Hz. Caudate spindles were most marked in the sensomotor cortex of the ipsilateral hemisphere. In rabbits under 10 days old caudate spindles were not found even if the intensity of stimulation was increased many times. Starting from the age of 15 days bursts of rhythmic activity resembling caudate spindles, but with lower frequency (about 8 Hz), longer latent period (up to 350 msec), and also with a higher threshold, appeared in the sensomotor cortex. The definitive type of caudate spindles was established toward the end of the first month of postnatal life, corresponding to the time of formation and complication of conditioned-reflex activity in developing animals.Brain Institute, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 17, No. 1, pp. 11–15, March, 1985.     

Frequency modulation of theta volleys of septal neurons during rhythmic oligosynaptic stimulation in the rabbit

Activity of the neurones with stable theta-bursts was recorded extracellularly in intact and hippocampectomized septum of unanaesthetized chronic rabbits during low-frequency (3-17 Hz) stimulation of horizontal limb of the diagonal band or the lateral septal nucleus. Gradual entrainment and phase-locking of the spontaneous theta-cycles occurred. Two types of entrainment were observed: "entrainment by pause", where interburst interval was reset by the stimuli; and "entrainment by burst", where bursts were time-locked to the stimuli. Such reorganization of the spontaneous bursts occurred in a narrow frequency range of stimulation (from 4 Hz up to 9-12 Hz), with the best resonance following in the range of "basic" theta frequencies of the awake rabbit (5-6 Hz). With stimulation beyond the theta-range three phenomena occurred: shift of the burst frequencies to higher or lower harmonics of stimulation frequencies; complex interactions of basic background frequency with the rhythm of stimulation ("beating"); escape from the influence of the stimuli with return to background theta-burst frequency.     

Characteristics of the neuronal activity of structures of the hippocampal formation (subiculum)]

Spontaneous and evoked neuronal activity of subiculum was investigated in chronic unanaesthetized rabbits. Rhythmic components of two different types were observed in the spontaneous activity of 58% of units: 4.5-6.0 cps bursts (13%) and 2.0-3.8 cps (45%). Besides their basic frequency, these types of rhythmic activity were differentiated by several additional criteria. 63% of the cells responded to applied sensory stimuli, among which auditory stimuli were the most effective. Multimodal units were slightly more numerous (59%) than unimodal ones. In 50% of the recorded units responses were of tonic type, in 31%--of phasic type, and in 19%--specific on-off responses were observed. In majority of units with repeated stimulations relatively rapid process of habituation developed after a short incremental phase. Interruption of septal afferents resulted in disappearence of 4.5-6.0 cps (theta) bursts, in reduction in the number of tonic responses and in absence of habituation. Similarity of neuronal characteristics in the field CA1 and subiculum is discussed.     

Unit activity of rabbit medial septum in early ontogenesis]

In experiments on 7-20 and 75-day-old rabbits, the activity of units of the septal nuclei, the nucleus of the diagonal band of Broca (NBD), nucleus medialis (NSM) and the EEG of dorsal hippocamp (the CA2-CA3 fields) have been recorded. It was shown that the specific discharge pattern of septal neurons (the bursts) appears on 8-11th days under action of eserine. The distinct bimodal pattern of the interval histograms of the septal units appeared from the 14th day. The burst discharge pattern of NBD and NSM neurones reached a definitive level by the 20th day. The parallel evolution of discharge pattern of the septal units and the EEG of the hippocamp is observed in ontogenesis, the formation of hippocampal theta activity correlating with enhancement of the average frequency of the septal (NBD, NSM) neuronal activity, with shortening to the interval between the discharges within the bursts and with enhancement of the regularity of the bursts.     

Immunfluorescence study of neuropeptides in identified neurons of the rat auditory superior olivary complex

 The present study was conducted to investigate the distribution and immunohistochemical characteristics of ascending and descending projection neurons of the rat superior olivary complex (SOC), a group of interrelated brainstem nuclei. Ascending neurons were identified by injection of cholera toxin B subunit (CTB) into the central nucleus of the inferior colliculus (IC), descending neurons were labeled by application of Fluoro-Gold (FG) into the scala tympani of the cochlea, ipsilaterally to the IC injection. In accordance with the literature, we observed neurons innervating the IC located in the lateral superior olivary nucleus (LSO) and dorsal periolivary groups (DPO) on both sides, in the superior paraolivary nucleus (SPO) predominantly ipsilateral, as well as in the ipsilateral medial superior olivary nucleus (MSO) and the medial nucleus of the trapezoid body (MNTB). Cochlear projection neurons were found predominantly in the ipsilateral LSO as well as in the bilateral SPO, DPO, MSO and MNTB. In addition, a considerable population of neurons in the ipsilateral LSO and SPO were identified as being both ascending and descending. To further characterize these double-projecting neurons, brainstem sections were incubated in antisera directed against different neuroactive substances. The majority of ascending/descending cells in the LSO contained calcitonin gene-related peptide, but not substance P (SP), met-enkephalin (ENK) or tyrosine hydroxylase (TH). Some of these neurons apparently were contacted by ENK- or SP-immunoreactive fibers and terminals. In addition, we found TH-immunoreactive neurons in the lateral MNTB region. These neurons, which were labeled upon tracer injection into the cochlea (but not upon IC injection), probably belong to the C1 catecholaminergic cell group and may represent a division of the uncrossed olivocochlear bundle. The present results reveal the existence of a previously unknown subpopulation of SOC neurons that project to both the cochlea and the inferior colliculus. Their CGRP immunoreactivity and their uncrossed projection pattern provide evidence that they may belong to the cholinergic, putatively excitatory cell group. Received: 4 January 1999 / Accepted: 17 February 1999     

Spectral correlation parameters of neocortical potentials in the rabbit undergoing paired isorhythmic stimulation

At pairing of isorhythmic stimuli beyond the theta-rhythm frequency limits (3 and 8 Hz), in power spectra of EEGs of the sensorimotor and visual neocortical areas of rabbits, the frequencies are present both of the theta-range and of the stimulation frequency, in the background activity as well as during the stimulation. Both rhythms are in reciprocal relations. The frequency of the theta-rhythm approaches the frequency divisible by that of the stimulation. Under the action of the conditioned stimulus, crosscorrelation coefficients (CC) between the potentials of the areas under study decrease in most cases in comparison to their background values. Combination of the conditioned stimulus with the unconditioned one, leads approximately in equal number of cases to an increase or decrease of CC. After elimination of the stimuli, in most cases CC increases. CC of the background activity does not increase in the course of paired stimuli presentation though a conditioned response is being formed. At presence of stimuli frequency fluctuations simultaneously in the potentials of both areas, the rise of coherence function at this frequency does not occurs always. Thus, the above spectral-correlation parameters of rabbit's cortical potentials differ from those which arise at pairing of continuous nonrhythmic stimuli. This difference is probably due to different characteristics of the stimuli presented.     

Effect of pentobarbital on unit activity of reticular and ventrolateral thalamic nuclei in cats

Responses of single units in the reticular and ventrolateral thalamic nuclei were studied in acute experiments on curarized cats before and after intravenous injection of small doses (0.5–15 mg/kg) of pentobarbital, with simultaneous derivation of activity by two electrodes. After injection of pentobarbital, unit activity in the reticular nucleus consisted of high-frequency grouped (52.5% of 40 neurons) or continuous (30%) discharges as long as barbiturate spindles were present in the electrocorticogram. Activity of only four neurons (10%) of this nucleus was inhibited during the presence of spindles. In all other neurons of the reticular nucleus (7.5%) the character of discharges was unchanged after injection of pentobarbital. The appearance of grouped discharges, repeated several times (66.5% of 40 neurons), or blocking of activity (30%) throughout the period of spindle recording was observed in neurons of the ventrolateral nucleus. The remaining neurons of that nucleus (3.5%) did not respond to intravenous pentobarbital. The appearance of high-frequency discharges in neurons of the reticular nucleus while spindles were recorded coincided with a period of silence in neurons of the ventrolateral nucleus (58.5% of 34 pairs of neurons). High-frequency electrical stimulation of the mesencephalic reticular formation led to asynchronous activation of neurons of the ventrolateral nucleus (82%) and inhibition of unit activity in the reticular nucleus (88%).I. S. Beritashvili Institute of Physiology, Academy of Sciences of the Georgian SSR, Tbilisi. Translated from Neirofiziologiya, Vol. 14, No. 5, pp. 517–524, September–October, 1982.     

Dopaminergic regulation of theta activity of septohippocampal neuron in the awake rabbit

The effects of dopamine reuptake blocker nomifensine and nonselective antagonist of dopamine receptors haloperidol on the theta rhythmicity of the medial septal neurons and hippocampal EEG were investigated in the rabbit. Bilateral intracerebroventricular infusion of nomifensine (9 micrograms in each ventriculus) produced an increase in both the rate of firing and the theta modulation of medial septal neurons; the theta power of the hippocampal EEG also augmented. The degree of neuronal theta stability (time constant of damping, tao theta) significantly increased. The frequency of rhythmic bursts in the neuronal firing also substantially elevated. The amplitude, regularity and frequency of theta waves in the hippocampal EEG also increased. The antagonist haloperidol (12.5 mg) caused the opposite effect. The theta activity of medial septal neurons and the theta power of the hippocampal EEG decreased after haloperidol injection. Theta rhythmicity of septal neurons significantly diminished, the rate of rhythmic bursts in the neuronal firing also decreased, although not substantially. The theta amplitude and regularity in the hippocampal EEG also decreased. Effects of both drugs built up rapidly and then gradually attenuated. Nomifensine infusion against the background of exposure to haloperidol provoked neither increasing neuronal firing rate, nor elevating theta activity. These finding suggest that dopaminergic system produces activation of the septohippocampal system in situations that require selective attention to functionally important information.     

Effect of cortisol microiontophoresis on discharge time structure of dorsal hippocampal neurons in rabbits

The effect of microiontophoretic application of cortisol to single neurons of the dorsal hippocampus on the character of distribution of interspike intervals in their discharges was studied in chronic experiments on rabbits. Cortisol modified the time structure of regular and rhythmic discharges of hippocampal neurons. Regularization of discharges in the form of bursting activity appeared as the result of cortisol in cells with irregular spontaneous activity. Activity of more than half of the neurons, in which bursting discharges corresponded in frequency to the theta-rhythm, was intensified as a result of microapplication of cortisol. In neurons discharging complex spikes, in which under normal conditions a phenomenon of reduction of spike amplitude was observed within each burst, no definite rule as regards changes in the time structure of the discharges could be observed after administration of the hormone. It is suggested that cortisol plays a modulating role in mechanisms of generation of spike activity by hippocampal neurons.P. K. Anokhin Research Institute of Normal Physiology, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 13, No. 6, pp. 628–635, November–December, 1981.     

Dynamics of the spectral composition of the hippocampal theta-rhythm in the dog during switching-over of heterogeneous instrumental reflexes

In two dogs with electrodes chronically implanted in the hippocampus, hippocampal electrical activity and the heart rate were recorded in the process of elaboration of a conditioned switch-over of operant alimentary and defensive responses. Averaged computerized spectral characteristics of hippocampal theta-rhythm were essentially different between the alimentary and defensive situations in intersignal period as well as during the action of triggering conditioned stimulus. Individual differences in the dominant frequency of bioelectrical activity were observed. The animal with a higher theta-rhythm frequency elaborated conditioned switch-over of heterogeneous operant responses sooner and more successfully.