The extrinsic modulation of hippocampal theta depends on the coactivation of cholinergic and GABA-ergic medial septal inputs.

The long trains of theta field activity recorded from the hippocampal formation of urethane-anesthetized rats are thought to be primarily dependent on cholinergic afferents originating in the medial septum/vertical limb of the diagonal band of Broca (MS/vDBB). Recent anatomical studies have revealed the existence of a septal GABA-ergic input to the hippocampal formation which synapses mainly on intrinsic GABA-ergic interneurons. The present work investigated the possibility that some form of interaction between cholinergic and GABA-ergic MS/vDBB inputs might be required for the generation of hippocampal theta field and cellular activities in urethane-anesthetized rats. Reversible inactivation of the MS/vDBB completely abolished theta field and theta-on cell activities, but "released" theta-off cells. The theta field and theta-on cell activities induced by direct intrahippocampal microinfusions of carbachol were also abolished by MS/vDBB inactivation. We speculated that septal suppression was producing two effects: 1) removing excitatory, cholinergic input; and 2) removing inhibitory control of hippocampal GABA-ergic interneurons, thereby increasing the overall level of hippocampal inhibition. Sequential administration of both carbachol and the GABA-A antagonist, bicuculline, resulted in theta-like oscillations similar to those seen in hippocampal slices bath perfused with carbachol alone. Thus, following MS/vDBB inactivation hippocampal GABA-ergic systems are overactive; this enhances intrinsic inhibition and blocks carbachol theta. By reducing the overall level of inhibition in the hippocampus with bicuculline, it is possible to reinstate its oscillatory properties. Conversely, increasing the level of inhibition in the hippocampus (with muscimol) results in the abolishment of theta field activity and the discharges of both theta-on and theta-off cells. Based on these findings we are proposing that cholinergic and GABA-ergic systems originating in the MS/vDBB act synergistically to modulate hippocampal theta. Cholinergic projections provide the afferent excitatory drive for hippocampal theta-on cells and septal GABA-ergic projections act to reduce the overall level of inhibition by inhibiting hippocampal GABA-ergic interneurons (hippocampal theta-off cells). Both activities must be present for the generation of hippocampal theta field and cellular activities. The balance between the cholinergic and GABA-ergic systems may determine whether hippocampal synchrony (theta) or asynchrony (LIA) occurs.     

Comparison of spontaneous and septally driven hippocampal theta field and theta-related cellular activity

Experiments were carried out for the purpose of comparing the electrophysiological properties of spontaneously occurring hippocampal theta field activity with those of theta-like field activity elicited by 5-Hz and 7-Hz electrical stimulation of the medial septum in urethane-anesthetized rats. Experiment 1 compared the amplitude and phase depth profiles for the three conditions of spontaneously occurring theta, theta elicited by 5-Hz medial septal stimulation, and theta elicited by 7-Hz medial septal stimulation. The results supported the conclusion that septally elicited theta field activity exhibited characteristics similar to those of spontaneously occurring theta field activity. Experiment 2 compared the discharge properties of hippocampal theta-related cellular discharges during spontaneous and septally elicited theta field activity. In contrast to the results of Experiment 1, the findings of Experiment 2 supported the conclusion that electrical stimulation of medial septal nuclei did not produce typical responses of hippocampal theta-related cellular activity. During spontaneously occurring field conditions, HPC theta-ON cells increased their discharge rates during spontaneous theta field activity, relative to LIA, and theta-OFF cells decreased (often to zero) their discharge rates during theta field activity relative to LIA. During septally elicited theta-like activity, phasic and tonic theta-ON cells decreased their discharge rates (some were totally inhibited), and most tonic theta-OFF cells increased their discharge rates (although two were totally inhibited). In addition, the discharges (albeit reduced) of the majority of both phasic and tonic theta-ON cells during septal driving became entrained to the stimulation pulses and thus exhibited rhythmicity and strong phase relations with the field activity. Furthermore, both cell types discharged near the positive peak of the septally elicited theta field activity during 5-Hz stimulation and near the negative peak during 7-Hz stimulation. The discharges of most tonic theta-OFF cells also became entrained to the stimulation pulses and exhibited similar phase relations to theta-ON cells during the 5-Hz and 7-Hz driving frequencies. Thus, based on cellular evidence, electrical stimulation of the medial septum activates the hippocampal neural circuitry involved in the generation of theta field activity in a nonphysiological manner. The findings of the present paper provide an explanation for why electrical stimulation of the medial septum in freely moving rats elicits a theta-like field activity that is dissociated from the normal behavioral correlates, in contrast to those elicited by stimulation of the posterior nucleus of the hypothalamus (Bland and Oddie. 2001. Behav Brain Res 127:119-136).     

In vitro activation of the medial septum-diagonal band complex generates atropine-sensitive and atropine-resistant hippocampal theta rhythm: an investigation using a complete septohippocampal preparation

The medial septum and diagonal band complex (MS-DB) is believed to play a key role in generating theta oscillations in the hippocampus, a phenomenon critical for learning and memory. Although the importance of the MS-DB in hippocampal theta rhythm generation is generally accepted, it remains to be determined whether the MS-DB alone can generate hippocampal oscillations or is only a transducer of rhythmic activity from other brain areas. Secondly, it is known that hippocampal theta rhythm can be separated into an atropine-sensitive and insensitive component. However, it remains to be established if the MS-DB can generate both types of rhythm. To answer these questions, we used a new in vitro rat septohippocampal preparation placed in a hermetically separated two side recording chamber. We showed that carbachol activation of the MS-DB generated large theta oscillations in the CA1 and CA3 regions of the hippocampus. These oscillations were blocked by applying either the GABA(A) receptor antagonist bicuculline or the AMPA/kainate antagonist DNQX to the hippocampus. Interestingly, the application of the muscarinic receptor antagonist atropine produced only a partial decrease in the amplitude, without modification of the frequency, of theta. These results show for the first time, that upon optimal excitation, the MS-DB alone is able to generate hippocampal oscillations in the theta frequency band. Moreover, these MS-DB generated theta oscillations are mediated by muscarinic and nonmuscarinic receptors and have a pharmacological profile similar to theta rhythm observed in awake animals.     

The classification of medial septum-diagonal band cells as theta-on or theta-off in relation to hippocampal EEG states

The discharge patterns of cells located in the medial septum (MS)/vertical limb of the Diagonal Band of Broca (vDBB) were recorded simultaneously with hippocampal formation large amplitude irregular activity (LIA), or theta, in urethane-anesthetized rats. The main conclusion was that the majority of cells in the MS/vDBB were theta-related and could be classified according to the same scheme developed for theta-related cells in the hippocampal formation. That is, cells were classified as theta-on or theta-off, with subtypes defined as tonic or phasic, linear or non-linear. The discharge properties of hippocampal and MS/vDBB cell populations were compared. It was argued that this classification scheme encompassed all the crucial properties of theta-related cells in the hippocampal formation and the MS/vDBB. An alternative model to the septal pacemaker hypothesis, concerning the role of the medial septum in the generation of hippocampal formation theta activity, was presented and discussed.     

Extrinsic modulation of medial septal cell discharges by the ascending brainstem hippocampal synchronizing pathway

Single cells or simultaneously recorded cell pairs in the medial septum (MS) vertical limb of the diagonal band of Broca (vDBB) in the urethane-anesthetized rat were initially classified as either theta-on or non-related according to the system of Colom and Bland (1987, Brain Res 422:277–286). Subgroups of these cells were then studied under various test conditions that included electrical stimulation of the nucleus pontis oralis (PO) or posterior hypothalamus (PH), microinfusion of carbachol into the PO or PH, and the microinfusion of atropine sulfate or procaine hydrochloride into the PH. Electrical stimulation of either the PO or PH induced theta (θ) activity in the hippocampal formation (HPC), and electrical stimulation of the PO resulted in a simultaneous increase in the discharge rate of all MS/vDBB θ-on cells tested, compared to the rates recorded during HPC large-amplitude irregular activity (LIA). Five of the MS/vDBB θ-on cells were tested consecutively with electrical stimulation of the PO and PH, and were shown to be activated in a similar manner in either condition. Microinfusion of carbachol into either the PO or PH resulted in the induction of HPC θ field activity and the simultaneous intense activation of all MS/vDBB θ-on cells tested. Following the microinfusion of either atropine sulfate or procaine into the PH, electrical stimulation of the PO failed to induce HPC θ field activity or the concomitant rhythmic discharges of all MS/vDBB phasic θ-on cells tested. Microinfusing procaine into the PH also abolished the coupling between all MS/vDBB cell pairs during HPC θ field activity including that between two cell pairs that were coupled during HPC LIA. The data support the following conclusions: (1) The brainstem HPC synchronizing pathway originating in the pons region ascends to the medial septum via the midline posterior hypothalamic region; (2) the present results taken together with previous work suggest that a major component of the ascending synchronizing pathway, up to and including the hippocampal formation, is cholinergic, cholinoceptive, or both, and the receptors involved are primarily muscarinic; (3) the midline posterior hypothalamic region is an important source of inputs to the medial septum and their major contribution is to provide frequency-coded inputs to the MS/vDBB for relay into the hippocampal formation. © 1994 Wiley-Liss, Inc.     

Hippocampal theta field activity and theta-on/theta-off cell discharges are controlled by an ascending hypothalamo-septal pathway

The nature of the control of hippocampal formation field activity [theta (theta) and large-amplitude irregular activity (LIA)] and theta-on/theta-off cell discharges by an ascending hypothalamo-septal pathway was investigated in urethane-anesthetized rats. Electrical stimulation of the dorsomedial-posterior hypothalamus in the range of 0.1-1.0 mA in 0.1-mA steps produced theta in the hippocampal formation, with a linear positive relation between stimulus intensity, theta frequency, and theta amplitude. Reversible blockade of the medial septal (MS)/vertical limb of the diagonal band of Broca (vDBB) region by microinjection of procaine hydrochloride abolished spontaneous and hypothalamically elicited theta, resulting in a field activity at 1-min post-procaine that had a lower power than pre-procaine LIA levels. The efficacy and recovery of the MS/vDBB suppression was tested at 1, 10, 20, 30, 40, 50, and 60 min post-procaine using 0.5, 0.8, and 1.0 mA of hypothalamic stimulation. All three of the dependent measures of hippocampal field activity (frequency, amplitude, and power) showed a progressive recovery during the 60-min post-procaine period. Frequency exhibited a rapid recovery with a shallow slope between 20 and 60 min post-procaine. In contrast, the amplitude and power of theta exhibited a gradual recovery with a steeper slope between 20 and 60 min post-procaine. During the time of maximal MS/vDBB suppression, theta-on cell discharges were reduced to 0 in most cases. The initial recovery of theta-on cells in the post-procaine condition was correlated with the first appearance of theta and was characterized by low discharge rates.(ABSTRACT TRUNCATED AT 250 WORDS)     

Intraseptal Microinfusion of Muscimol: Effects on Hippocampal Formation Theta Field Activity and Phasic Theta-ON Cell Discharges

The effect of intraseptal microinfusions of the GABA-A agonist muscimol on spontaneously occurring or hypothalamically induced hippocampal formation (HPC) theta field activity and the simultaneously occurring discharge properties of CA1 pyramidal and dentate granule layer phasic theta-ON cells, was investigated in urethane-anesthetized rats. The microinfusion of 5.0–12.5 nmol of muscimol into the medial septum/vertical limb of the diagonal band of Broca (MS/vDBB) resulted in a progressive reduction (beginning 5 min postinfusion) in the power (amplitude) and finally the total loss of theta field activity. In contrast, theta field frequency remained unaffected during the entire postinfusion period that theta field activity was present. In the time immediately following the first 1-min intraseptal microinfusion of 5 nmol muscimol, (before changes in theta amplitude occurred) a brief period of increased phasic theta-ON cell excitability was noted. This was manifested as an increase in the number of discharges per rhythmic burst. Associated with the progressive reduction of the amplitude of theta field activity, phasic theta-ON cell discharge rates progressively decreased for a period beginning 5 min postinfusion of 5 nmol muscimol. Despite the progressive decrease in the number of discharges and a noticeable reduction in the degree of rhythmicity, phasic theta-ON cells maintained their preferred timing of discharges in relation to the phase of theta field activity, while the latter was present. Just prior to the complete abolishment of theta field activity, phasic theta-ON cells ceased discharging. During the period when theta field activity was replaced by low amplitude asynchronous activity, phasic theta-ON cells discharged in bursts correlated with every occurence of sharp wave field activity. The results support the following conclusions: (1) the brief excitatory effect on HPC theta-ON cell discharges may be correlated pharmacologically with an initial brief increase in HPC ACh turnover. The reduction of phasic theta-ON cell discharges and theta field activity may be correlated with the longer lasting reduction of HPC ACh turnover, controlled by MS/vDBB GABA-A inputs to MS/vDBB cholinergic septohippocampal neurons, possibly along with a direct inhibition of the GABAergic septohippocampal projection; (2) the primary contribution of the MS/vDBB nuclei, as a nodal point in the ascending brainstem HPC synchronizing system, is the modulation of the amplitude of HPC formation theta field activity and secondarily to relay frequency-coded inputs from the posterior hypothalamic region (posterior and supramammillary nuclei); (3) HPC theta and sharp wave field activity represent functionally distinct neural inputs to the same population of phasic theta-ON cells located in both the CA1 pyramidal and dentate granule cell layers.     

Medial septal cell interactions in relation to hippocampal field activity and the effects of atropine.

Simultaneously recorded pairs (from a single electrode) of cells in the medial septum (MS) vertical limb of the diagonal band of Broca (vDBB) were studied during the simultaneous occurrence of field activity in the hippocampal formation of urethane-anesthetized rats and following the systemic administration of atropine sulphate (ATSO4). The 2 types of field activities recorded were cholinergically mediated type 2 theta (theta) and large-amplitude irregular activity (LIA). The study had 3 objectives: (1) to determine if cells in close proximity in the MS/vDBB had a high probability of having similar discharge properties; (2) to determine the possible functional relationships occurring between pairs of MS/vDBB cells using standard cross correlational analyses; and (3) to determine the effects of ATSO4 on both the discharge properties of MS/vDBB cells and the interactions between these cells. Of the 143 cells recorded, 133 (93%) were theta-related and classified as theta-on or theta-off cells. theta-on cells had a significant increase in mean discharge rate during theta compared to their mean discharge rate during LIA and/or had a linear increase in discharge rate in relation to increases in theta frequency. theta-off cells had a significant increase in mean discharge rate during LIA compared to their mean discharge rate during theta and/or had a linear decrease in discharge rate in relation to increases in theta frequency. The remaining 10 (7%) cells were classified as nonrelated to theta. A probability analysis carried out on each of the separate criteria for classification revealed that several of these could predict whether or not cells were in close proximity of one another. Cells occurred in close proximity according to whether they had higher discharge rates during theta or LIA and according to whether or not they varied their discharge rates as theta frequency varied (linear-nonlinear). Cell discharge patterns of rhythmicity (phasic), or nonrhythmicity (tonic) on the other hand, were not good predictors of whether cells were in close proximity since there was an equal probability of a phasic cell being paired with either a phasic or a tonic cell. The presence of a phasic discharge pattern was, however, the main determinant of whether a correlation occurred between cell pairs. Cross correlations of the cell pair spike trains revealed that 50% of the 36 phasic-phasic cell pairs were correlated during both theta and LIA field activities. The characteristics of the cross correlation functions of these cell pairs (strong symmetrical correlations with sharp primary peaks or troughs occurring around the origin) suggested that they received shared excitatory and inhibitory inputs. The remaining 50% of the phasic-phasic pairs were cross correlated during theta only.(ABSTRACT TRUNCATED AT 400 WORDS)     

In vivo contribution of h-channels in the septal pacemaker to theta rhythm generation

One of the most intriguing network-level inferences made on the basis of in vitro and modelling data regarding the role of Ih current was that they participate in rhythmogenesis in different parts of the brain. The nature of Ih contribution to various neuronal oscillations is far from uniform however, and the proper evaluation of the role of Ih in each particular structure requires in situ investigations in the intact brain. In this study we tested the effect of Ih blockade in the medial septum on hippocampal theta rhythm in anaesthetized and freely behaving rats. We could not confirm the recent report of elimination of theta by septal injection of ZD7288 [C. Xu et al. (2004) Eur. J. Neurosci., 19, 2299-2309]; the observed effects were more subtle and more specific. We found that Ih blockade in the medial septum substantially decreased the frequency of hippocampal oscillations without changing the context in which theta occurred, i.e. specific behaviours in freely moving rats and spontaneous switching and brainstem stimulation under anaesthesia. Septal injection of ZD7288 eliminated atropine-resistant theta elicited by high intensity electrical stimulation of the reticular formation in anaesthetized rats but was ineffective in combination with the muscarinic agonist, carbachol. Thus, functional Ih was necessary for the septum to generate or transmit high frequency theta rhythm elicited by strong ascending activation, whereas low frequency theta persisted after Ih blockade. These results suggest that Ih plays a specific role in septal theta generation by promoting fast oscillations during exploratory behaviour and rapid eye movement sleep.     

Carbachol-induced EEG 'theta' activity in hippocampal brain slices

Application of the cholinergic agonist carbachol (50 microM) produced theta-like rhythmical waveforms, recorded in the stratum moleculare of the dentate gyrus. Atropine sulfate (50 microM) antagonized the carbachol-induced theta-like activity, consistent with this action of atropine in vivo. These results provide the first direct evidence that hippocampal neurons are capable of producing synchronized slow-wave activity when isolated from pulsed rhythmic inputs of the medial septum and other brain regions.     

The role of inhibition in oscillatory wave dynamics in the cortex

Cortical oscillations arise during behavioral and mental tasks, and all temporal oscillations have particular spatial patterns. Studying the mechanisms that generate and modulate the spatiotemporal characteristics of oscillations is important for understanding neural information processing and the signs and symptoms of dynamical diseases of the brain. Nevertheless, it remains unclear how GABAergic inhibition modulates these oscillation dynamics. Using voltage-sensitive dye imaging, pharmacological methods, and tangentially cut occipital neocortical brain slices (including layers 3-5) of Sprague-Dawley rat, we found that GABAa disinhibition with bicuculline can progressively simplify oscillation dynamics in the presence of carbachol in a concentration-dependent manner. Additionally, GABAb disinhibition can further simplify oscillation dynamics after GABAa receptors are blocked. Both GABAa and GABAb disinhibition increase the synchronization of the neural network. Theta frequency (5-15-Hz) oscillations are reliably generated by using a combination of GABAa and GABAb antagonists alone. These theta oscillations have basic spatiotemporal patterns similar to those generated by carbachol/bicuculline. These results are illustrative of how GABAergic inhibition increases the complexity of patterns of activity and contributes to the regulation of the cortex.     

Respiratory cycle entrainment of septal neurons mediates the fast coupling of sniffing rate and hippocampal theta rhythm

Memory for odour information may result from temporal coupling between the olfactory and hippocampal systems. Respiration defines the frequency of olfactory perception, but how the respiratory rate affects hippocampal oscillations remains poorly understood. The afferent connectivity of the medial septum/diagonal band of Broca complex (MS/DB) proposes this region as a crossroads between respiratory and limbic pathways. Here we investigate if the firing rates of septal neurons integrate respiratory rate signals. We demonstrate that approximately 50% of MS/DB neurons are temporally correlated with sniffing frequency. Moreover, a group of slow‐spiking septal neurons are phase‐locked to the sniffing cycle. We show that inter‐burst intervals of MS/DB theta cells relate to the sniff rate. Intranasal odour infusion evokes sniff phase preference for the activity of fast‐spiking MS/DB neurons. Concurrently, the infusion augments the correlation between sniffing and limbic theta oscillations. During periods of sniffing–theta correlation, CA1 place cells fired preferentially during the inhalation phase, suggesting the theta cycle as a coherent time frame for central olfactory processing. Furthermore, injection of the GABAergic agonist muscimol into medial septum induces a parallel decrease of sniffing and theta frequencies. Our findings provide experimental evidence that MS/DB does not merely generate theta rhythm, but actively integrates sensorimotor stimuli that reflect sniffing rate. Such integration may provide temporal oscillatory synchronisation of MS/DB‐innervated limbic structures with the sniffing cycle.     

Dissociable pathways facilitate theta and non-theta states in the median raphe--septohippocampal circuit

Hippocampal theta rhythms in vivo are modulated by a synchronizing projection from the medial septum (MS) and a desynchronizing input from the median raphe nucleus (MRn) of the brainstem. Inactivation of the MS suppresses theta rhythms while inactivation of the median raphe produces persistent theta. However, different pathways arise from within the MS and the median raphe and therefore different brain states could be facilitated by different forms of median raphe or septohippocampal inputs. Here, we found in urethane anesthetized rats that suppression of outputs from the MRn with procaine leads to persistent hippocampal theta as previously reported. The discharge properties of hippocampal theta-related cells recorded during both spontaneously occurring theta and MRn 8-OH-DPAT-induced theta did not differ significantly. This persistent theta was abolished by inactivation of the MS with either procaine or atropine sulfate. Selective inactivation of serotonergic median raphe outputs with the 5-HT-1A agonist (8-OH-DPAT) induced theta that was also abolished by medial septal inactivation using procaine. Thus, persistent theta following complete median raphe inactivation or selective serotonergic inactivation arises from a median raphe to MS pathway. However, 8-OH-DPAT infusions into the median raphe together with atropine infusions in the MS did not abolish theta activity. These data suggest that the non-serotonergic (possibly glutamatergic) median raphe projections to the MS can facilitate the generation of hippocampal theta in the absence of medial septal cholinergic tone. These results demonstrate that dissociable neuronal pathways in the median raphe-MS-hippocampal circuit promote different brain states (theta or non-theta) and a median raphe non-serotonergic (likely glutamatergic) system may serve a separate function from the ascending serotonergic raphe projection in the regulation of hippocampal network activity.     

The generation of theta rhythm in hippocampal formation maintained in vitro

The most spectacular example of oscillations and synchrony which appear in the brain is the rhythmic slow activity (theta) of the limbic cortex. Theta rhythm is the best synchronized electroencephalographic activity that can be recorded from the mammalian brain. Hippocampal formation is considered to be the main structure involved in the generation of this activity. Although detailed studies of the physiology and pharmacology of theta‐band oscillations have been carried out since the early 1950s, the first demonstration of atropine‐sensitive theta rhythm, recorded in completely deafferented hippocampal slices of a rat, was performed in the second half of the 1980s. Since the discovery of cholinergically induced in vitro theta rhythm recorded from hippocampal formation slices, the central mechanisms underlying theta generation have been successfully studied in in vitro conditions. Most of these experiments were focused on the basic question regarding the similarities between the cholinergically induced theta activity and theta rhythm examined in vivo. The results of numerous in vitro experiments strongly suggest that cholinergically induced theta rhythm recorded in hippocampal slices is a useful analogue of theta observed in intact animals, and could be helpful in searching for the mechanisms of oscillations and synchrony in the central nervous system neuronal networks. The objective of the present review is to discuss the main results of experiments concerning theta oscillations recorded in in vitro conditions. It is our intent to provide, on the basis of these results, the characteristics of essential mechanisms underlying the generation of atropine‐sensitive in vitro theta.     

Carbachol-induced rhythmic slow activity (theta) in cat hippocampal formation slices.

Application of the cholinergic agonist, carbachol, produced theta-like rhythmical waveforms, recorded in the stratum moleculare of the dentate gyrus in the cat hippocampal formation slices. This effect of carbachol was antagonized by atropine but not D-tubocurarine. These results provide first direct evidence that the hippocampal formation neuronal network in the cat is capable of producing synchronized slow wave activity when isolated from pulsed rhythmic inputs of the medial septum.     

Microinjection of carbachol in the supramammillary region suppresses CA1 pyramidal cell synaptic excitability

Previous studies have established that the posterior hypothalamus-supramammillary (SUM) region is involved in the control of the hippocampal theta rhythm and also modulates the synaptic excitation of hippocampal neurons. Particularly, the medial but not lateral SUM region mediates reticular stimulation-induced suppression of CA1 pyramidal cell synaptic excitation to Schaffer collateral stimulation. In the present study using urethane anesthetized rats, we have investigated the effect of direct chemical stimulation of the posterior hypothalamus-SUM region on CA1 pyramidal cell excitability. It was observed that microinjection of the cholinergic muscarinic receptor agonist, carbachol (0.1 microl, concentration of either 0.0052, 0.156, or 0.625 microg/microl), evoked concentration-dependent suppression of CA1 pyramidal cell excitability that was dissociated from theta activation. Further, carbachol microinjection preferentially recruited the lateral SUM region when compared with the medial SUM and the posterior hypothalamic regions. In this context, the shortest latencies to suppression at the lowest concentration of carbachol and the strongest suppression at higher concentrations were observed with lateral microinjections. The carbachol-induced suppression was attenuated by inactivation of the medial septal region by microinjection of procaine (0.5 microl, 20% w/v). These results underscore a possible role for cholinergic mechanisms in the lateral SUM region in modulation of CA1 pyramidal cell synaptic excitation via the medial septal region. Furthermore, the present findings when juxtaposed with the medial SUM mediation of reticularly-elicited suppression suggest a medial-lateral topographic organization of the SUM region in modulation of CA1 excitability.     

Depth amplitude and phase profiles of carbachol-induced theta in hippocampal formation slices

Phase and amplitude depth profiles of EEG theta-like activity (TLA) induced by bath perfusion of 50 micro M of the cholinergic agonist carbachol were investigated using rat hippocampal formation slices. The data showed that the amplitude and phase of laminar profiles of TLA recorded in vitro were the same as the type III theta profiles recorded in in vivo rat preparations. These results supported the conclusion that the neural mechanisms involved in the generation of theta field activity in in vivo preparations and the generation of TLA in in vitro preparations were similar. The study provided further validation for the use of carbachol in vitro slice preparation as a model for studying the neural circuitry underlying theta generation in the hippocampal formation.     

Electrical coupling underlies theta oscillations recorded in hippocampal formation slices

The role of gap junction coupling in generation of carbachol-induced theta-like activity (TLA) in hippocampal formation (HPC) slices was investigated in this study. Two gap junction (GJ) blockers, carbenoxolone (100 microM) and quinine (100 microM), were tested. Both GJ blockers abolished cholinergically induced theta-like activity and related cell discharges. Abolishing effects were observed after 40-45 min of drug perfusion. These effects were found to be slowly and partially reversible. Our results provide evidence for the contribution of gap junction communication in mechanisms of neural synchrony, underlying the production of theta oscillations in limbic cortex maintained in vitro.     

Involvement of calmodulin-dependent protein kinase II in carbachol-induced rhythmic activity in the hippocampus of the rat

The role of calcium and protein kinases in rhythmic activity induced by muscarinic receptor activation in the CA1 area in rat hippocampal slices was investigated. Extracellular recording showed that carbachol (20 microM) induced synchronized field potential activity with a dominant frequency of 7.39+/-0.68 Hz. Pretreatment with the membrane permeable Ca(2+) chelator BAPTA-AM (50 microM) or with thapsigargin (1 microM), a compound which depletes intracellular calcium stores, reduced the dominant power of carbachol-induced theta-like activity by 83% and 78%, respectively. Inhibition of calmodulin-dependent protein kinase II (CaMKII) by the cell permeable inhibitor KN-93 (10 microM) reduced the power of carbachol-induced theta-like activity by 80%. In contrast the protein kinase C (PKC) inhibitor calphostin C did not significantly (P>0.05) affect the effect of carbachol. Whole-cell recording indicated that KN-93 also blocked carbachol-induced suppression of slow I(AHP) and strongly inhibited the carbachol-induced plateau potential. Our data suggest that activation of CaMKII by carbachol is crucial for local theta-like activity in the CA1 area of the rat hippocampus in vitro. Furthermore, involvement of CaMKII in carbachol-induced suppression of the slow I(AHP) and the induction of plateau potentials could play a role in the induction of theta-like rhythmic activity by carbachol.     

Medial septal unit firing characteristics following injections of 8-OH-DPAT into the median raphe nucleus

Extracellular single-unit recording techniques were used to examine the firing characteristics of neurons in the medial septum/diagonal band of Broca complex (MS/1313) following injections of the 5-HT_1A agonist, 8-OH-DPAT, into the median raphe nucleus (MRN) of urethane-anesthetized rats. It had previously been shown that MRN injections of 8-OH-DPAT produce hippocampal theta rhythm. Injections of 8-OH-DPAT into the MRN produced a change in firing characteristics of MS/1313 neurons from an irregular discharge to a pattern of rhythmical bursting in synchrony with hippocampal theta rhythm. Cross-correlational and coherence analyses demonstrated that the rhythmical firing pattern of MS/1313 neurons strongly correlated with rhythmical fluctuations in the hippocampal EEG during periods of hippocampal theta produced by 8-OH-DPAT injections, but not during baseline conditions (i.e. hippocampal desynchronization). The results suggest that MRN control of the hippocampal EEG is modulated by the MS/DB. Serotonergic projections from the MRN to the MS/DB may normally act to inhibit the rhythmical bursting of MS/1313 neurons, thereby producing hippocampal desynchronization. Suppression of MRN 5-HT neurons by MRN injections of 8-OH-DPAT would disinhibit MS/1313 neurons, allowing them to burst rhythmically and thereby produce hippocampal theta rhythm.