Medial Septal Modulation of the Ascending Brainstem HippocampalSynchronizing Pathways in the Anesthetized Rat

Jesse Jackson and Brian H. Bland*

ABSTRACT: Independent and combined electrical stimulation pairingsof the medial septum (MS), posterior hypothalamus (PH), and reticularpontine oralis (RPO) of the brainstem were performed in the acute ure-thane anesthetized rat, while recording field activity from electrodes ineither the stratum oriens or stratum moleculare of the hippocampal for-mation. Theta frequency and power were measured during independentstimulation of each nuclei and during combined stimulation using threepairings: (1) MS–PH (2) MS–RPO and (3) PH–RPO. Each pairing con-sisted of parameters known to elicit theta of a high frequency for onenucleus, and parameters known to elicit a low frequency for the secondnucleus. This methodology allowed us to observe whether one nucleuspreferentially modulated theta activity in the hippocampus in terms offrequency and power. The MS was observed to reset theta frequency inboth the upward and downward direction when stimulated in combina-tion with either the PH (Experiment 1) or the RPO (Experiment 2). InExperiment 3 (PH–RPO), the structure receiving the higher intensitystimulation had the predominate effect on theta frequency. With MSstimulation combinations, the power of the elicited theta activity wasfound to increase over the independent stimulation in some cases duringExperiment 1. Likewise, in Experiment 2, the combined stimulation pro-duced a power that in most cases was significantly greater than thatmeasured during the independent stimulations. This effect was notobserved with PH and RPO stimulation combinations. The combinedstimulation of the PH and RPO yielded a power similar to the independ-ent PH stimulations. The findings support the following conclusions: (1)the major theta generating activity of the ascending brainstem synchro-nizing pathways involves projections from the RPO to the PH, relayedthrough the MS, to the hippocampal formation; and (2) that the MSdirectly controls theta amplitude and secondarily translates the level ofascending brainstem activity into the appropriate frequency of hippo-campal theta. VVC 2005 Wiley-Liss, Inc.

KEY WORDS: medial septum; brainstem; hippocampal; synchronizing;pathways

INTRODUCTION

The ascending brainstem hippocampal synchronizing pathways provide amajor source of extrinsic theta generating inputs to the hippocampal forma-tion. These pathways originate in the rostral pontine region (nucleus pontis

oralis and the pedunculopontine tegmental nucleus),ascend, and synapse with midline caudal diencephalicnuclei (posterior hypothalamic nucleus and the supra-mammillary nucleus), which in turn send projections tothe medial septal region (medial septal nucleus and thevertical limb of the diagonal band of Broca) (Vertes andKocsis, 1997; Bland and Oddie, 1998; Vertes et al.,2004). The medial septal region functions as the node inthe ascending pathways, sending both cholinergic andGABA-ergic projections to the HPC (Bland, 2000).Oddie et al. (1994) carried out experiments designed toprovide evidence that the midline caudal diencephalicnuclei form a part of the ascending synchronizing path-ways and to determine the nature of the contributionthey make to ascending HPC synchronization. Theirstudy supported the conclusion that the PH and SUMhypothalamic nuclei were a critical part of the ascendingbrainstem HPC synchronizing pathways. Inputs ascend-ing from the brainstem caudal to the PH and SUMhypothalamic nuclei (originating in the reticular pontineoralis (RPO) and PPT nuclei of the rostral pontineregion) contributed primarily to the frequency of HPCtheta and secondarily to the amplitude of HPC theta.

In keeping with the importance of the MS as the nodeof the ascending synchronizing pathways, electrical stim-ulation of the MS nuclei does produce a theta-like fieldactivity in the HPC (Ball and Gray, 1971), but there is anotable difference in stimulus parameters comparedwith those applied to nuclei in the pontine and dience-phalic regions. Theta field frequencies recorded, in theHPC mirror, the frequency of electrical stimulation ofthe MS, such that, for example, 5 Hz stimulation produ-ces a theta frequency of 5 Hz. A study by Scarlett andBland (1997) investigated the differences between elec-trical stimulation parameters of the PH and the MS inan attempt to clarify the role played by these two regionsin the modulation of HPC theta frequency and ampli-tude. The PH and the MS were stimulated electricallywith the appropriate parameters to elicit HPC theta fieldactivity, both independently and combined. The studydemonstrated that independent stimulation of the MSor the PH resulted in HPC theta field activity that corre-sponded to either the frequency of stimulation (MS) orthe intensity (PH), respectively. However, when the MSand PH were stimulated simultaneously, the frequencyof the stimulus-induced HPC theta field activity wasfound to match the frequency of the theta elicited by theMS, regardless of the frequency of the PH-induced

Department of Psychology, Behavioral Neuroscience Research Group,The University of Calgary, Calgary, Alberta T2N 1N4, CanadaGrant sponsor: Natural Sciences and Engineering Research Council ofCanada; Grant number: A9935.*Correspondence to: Brian H. Bland, Department of Psychology, Behav-ioral Neuroscience Research Group, University of Calgary, 2500 Univer-sity Dr. NW, Calgary, Alberta, T2N 1N4, Canada.E-mail: bhbland@ucalgary.caAccepted for publication 9 September 2005DOI 10.1002/hipo.20135Published online 3 November 2005 in Wiley InterScience (www.interscience.wiley.com).

HIPPOCAMPUS 16:1–10 (2006)

VVC 2005 WILEY-LISS, INC.

HPC theta field activity. That is, MS stimulation was capable ofresetting PH-induced frequencies in both directions, upwards ordownwards. Furthermore, the addition of MS stimulation duringPH stimulation was found to increase the amplitude of the stimu-lation-induced HPC theta field activity.

The objective of the present work was to replicate andextend the findings of Scarlett and Bland (1997) by includingindependent electrical stimulation of the reticular pontis oralis(RPO) along with combined RPO–MS stimulation and RPO–PH stimulation. Such a design would allow evaluation of sev-eral hypotheses: (1) the theta generating pathways originatingin the RPO projected to the PH and then on to the hippocam-pus via the MS; and (2) the MS directly controls theta ampli-tude and secondarily translates the level of ascending brainstemactivity into the appropriate frequency of hippocampal theta.

METHODS AND MATERIALS

Subjects and Surgical Procedures

Data were obtained from fifteen male Long–Evans rats(0.300–0.400 kg) supplied by the Animal Care Facility atthe University of Calgary. Five animals were used in each ofthree experiments. Experimental protocols were approved bythe Life and Environmental Sciences Animal Research Council(LESARC) at the University of Calgary. Animals were initiallyanesthetized using a mixture of halothane and oxygen (mini-mum 1.0% alveolar concentration) to allow for the insertion oftracheal and a jugular vein cannulae, the latter used for thesubsequent administration of the urethane anesthetic (1 g/kg).Halothane and oxygen was then discontinued and urethaneanesthetic was used for the remaining experiment. The animalwas titrated to an appropriate depth of anesthesia before beingplaced in the stereotaxic apparatus. Core temperature was

maintained at 378C (Harvard Instruments heating pad) andheart rate was monitored throughout the experiment.

Surgery

A standard sterotaxic apparatus was used to secure the animal,and the skull was leveled between bregma and lambda. An unin-sulated tungsten wire was placed anterior to bregma. In allexperiments, tungsten microelectrodes (0.2–0.8 MO) were low-ered bilaterally at coordinates (with reference to bregma), ante-rior/posterior (A/P): �4.5 mm, medial/lateral (M/L): 3.0 mm,and to a depth of 2.0–2.6 mm ventral to dural surface. Signalsfrom the two hippocampal recording electrodes were amplifiedby Grass model P511 preamplifiers, led into a Grass model 7Dpolygraph, displayed on a digital oscilloscope (Tektronix TDS420), and then stored on an FM tape recorder (Teac XR – 30)for subsequent off-line data analysis. All experiments employedthe use of two bipolar stimulating electrodes consisting of stain-less steel wire (250 um in diameter) connected to two miniaturemale Winchester pins. Medial septum (MS) coordinates were A/P: þ0.5 mm, M/L: 0 mm, and 4.5–5.5 mm ventral to dural sur-face. Posterior hypothalamus (PH) coordinates were A/P: �3.4mm, M/L: 0.0 mm, and 7.2–8.0 mm ventral to dural surface.Rostral pontine oralis (RPO) coordinates used were A/P: �8.6mm, M/L: 1.6 mm, and D/V: �7.0 to 8.0 mm (see Fig. 1).

Stimulation Protocol

Experiment 1

Following the implantation of the recording electrodes, thetwo stimulating electrodes were lowered. The final depth forthe MS stimulating electrode was determined by stimulating atprogressively deeper locations until a maximal response wasobserved from the recording hippocampal electrodes (in terms

FIGURE 1. Saggital view of the brain showing recording and stimulating electrode place-ments for Experiments 1, 2, and 3.

2 JACKSON AND BLAND

of amplitude and frequency of theta). This ensured that loca-tion for the stimulating electrode was precise. For the PH, briefstimulations were administered as the electrode was loweredand final depth was determined by the location where thetaactivity could be elicited at the lowest intensity (thresholdintensity), which was usually between 200 and 300 lA. Inaddition, electrode location was determined by the ability ofthe stimulation to induce behavioral effects such as rapid vibris-sae movement, increased heart rate, and increased respiration.Once both electrodes were in position, in this and all subse-quent experiments, the animal was titrated with urethane to alevel where theta field activity no longer occurred spontane-ously, and could only be elicited by a very strong tail pinch.Stimulation trials were then initiated. In all the followingexperimental protocols, the intertrial stimulation period wasdetermined by the time it took the field activity to return toLIA. Independent stimulations of the MS consisted of a bipha-sic pulse with 0.1 ms duration, at an intensity of 300–600 lA,and at two frequencies, 5 and 10 Hz, which were referred to asthe MS5 and MS10 conditions, respectively. The independentstimulations of the PH consisted of a 100 Hz biphasic pulsewith a 0.1 ls duration at threshold intensity (PHL, L ¼ low),and a high intensity (900 lA) stimulation (PHH, H ¼ high).For the combined stimulations, 5 Hz MS stimulation wasadministered with a high intensity (800–900 lA) PH stimula-tion termed the MS5–PHH condition. The second combinedstimulation consisted of the 10 Hz MS stimulation with athreshold PH stimulation. This combination was referred to asthe MS10–PHL condition.

Experiment 2

The RPO stimulating electrode was lowered until thetaactivity could be elicited at threshold intensity (100–300 lA).Concomitant behavioral signs such as vibrissae movement,increased respiration, and increased heart rate also indicatedideal location in the RPO. The MS stimulating electrode wasthen lowered at the location and stimulation parameters speci-fied in Experiment 1. Stimulation periods began after bothelectrodes had been lowered. Independent stimulations of theRPO consisted of a 100 Hz biphasic pulse with a 0.1 ms dura-tion delivered at threshold intensity (RPOL) and at a highintensity of 900 lA (RPOH). Combined stimulations consistedof a 5 Hz MS stimulation paired with 900 lA RPO stimula-tion (MS5–RPOH) and a 10 Hz MS stimulation paired withthe threshold RPO stimulation (MS10–RPOL).

Experiment 3

For the third experiment, the same coordinates and stimu-lation parameters for the PH and RPO were used as describedin Experiments 1 and 2. Independent stimulations were carriedout for both the PH and RPO at threshold and high intensities(900 lA). Combined stimulations consisted of PH at thresh-old paired with RPO at 900 lA (PHL–RPOH), and PH at900 lA paired with an RPO stimulation at threshold intensity(PHH–RPOL).

All independent and combined stimulations for each experi-ment were five seconds in duration and applied in randomorder. Interstimulus intervals varied from 10 s to 2 min, thustotal time taken to complete the stimulation trials was usuallybetween 45 and 60 min per animal.

Histological Verification

Upon completion of each experiment, animals were euthan-ized with an overdose of urethane, and perfused transcardiallywith 0.9% saline, followed by a 10% formalin solution. Thebrains were removed and stored in formalin, then transferredto a 30% sucrose solution before sectioning into 40 lm slicesusing a cryostat and mounting on glass slides for confirmationof electrode placement.

Data Analysis

Data was analyzed with a PC microcomputer, utilizing thesoftware program DataWave (Longmount, CO). Hippocampalfield activity was sampled at 133 Hz and processed through a12-bit A/D converter. For every animal, three data segments ofat least 4,000 ms were analyzed for all independent and com-bined stimulations. Fast-Fourier Transform analysis was per-formed, providing the peak frequency and power (voltagesquared) for each segment of elicited theta activity. The fre-quency and power of combined stimulations were comparedwith the independent stimulations of which it was comprised,to evaluate any significant differences. Frequency and powerdata were analyzed using one-way ANOVA analyses and com-

FIGURE 2. Coronal sections showing representative histologi-cal verifications of (a) bilateral hippocampal recording electrodeplacements, (b) MS stimulating electrode placement in Experi-ments 1 and 2, (c) posterior hypothalamic stimulating electrodeplacements in Experiments 1 and 3, and (d) rostal pontine oralisstimulating electrode placements in Experiments 2 and 3.

SEPTAL MODULATION OF HIPPOCAMPAL PATHWAYS 3

parisons were tested for significance using the independent t-tests, at the 0.05 level of probability.

RESULTS

Histological analysis revealed the accurate placement of bilat-eral hippocampal recording electrodes in all fifteen experiments.Electrodes were all located in the stratum oriens of CA1 orstratum molecular layer of the dentate gyrus (DG). Coronalsections of representative recording and stimulating electrodesare shown in Figure 2.

Experiment 1: Medial Septum and PosteriorHypothalamus

The results for all stimulations were averaged across eachanimal (n ¼ 5) and a grand mean obtained for all independentand combined stimulation conditions. Group data of the meantheta frequency for each stimulation condition is shown (lowerpanel, Figs. 3 and 4), in addition to representative sample

waveform segments and their respective fast-Fourier transformspectrograms (upper panel, Figs. 3 and 4). Single factor analysisof variance (ANOVA) revealed a significant difference in thetafrequency between groups (F(5,24) ¼ 160.2, P < 0. 001). Dif-ferences in theta frequency elicited by the MS5 (4.89 6 0.003Hz) and MS10 (9.68 Hz) stimulations were significant at P <0.05, and differences between theta frequency elicited by PHL(4.93 6 0.05 Hz) and PHH (6.95 6 0.15 Hz) were also sig-nificant at P < 0.05. The combined stimulation pairing, MS5–PHH, provided a mean theta frequency (4.91 6 0.004 Hz)not statistically significant than the MS5 stimulation, whereasthere was a significant difference between this combined stimu-lation and the independent PHH stimulation, t(4) ¼ 3.695,P < 0.05 (lower panel, Fig. 3). The combined MS10–PHLstimulation produced a 9.68 Hz theta frequency for every trial,which was unchanged from the MS10 independent stimulation,although it was significantly larger than the independent PHLstimulation, t(4) ¼ 18.7, P < 0.05 (lower panel, Fig. 4).

Group data of means and standard error for theta power eli-cited in each stimulation condition for Experiment 1 are shown

FIGURE 3. Upper panel: Representative EEG segments and FFT’s for the PHH, MS5, andMS5–PHH stimulation conditions. Lower panel: Grand means for theta frequency elicited during thestimulation conditions listed above. The asterisk indicates significant differences between frequencies.

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in Figure 5. Theta power for all combined stimulation conditionswas compared with the independent stimulations of which it wascomprised. Combined stimulation conditions, MS5–PHH andMS10–PHL, yielded an increase in theta power over independentPHL and PHH stimulations. The MS5–PHH stimulation wasaccompanied by a higher power than the PHH independentstimulation, t(39) ¼ 1.68, P < 0.05. Likewise, the MS10–PHLstimulation showed a mean power of 0.150 V2, whereas the PHLstimulation had a significantly smaller power measure of 0.05V2, t(30) ¼ 5.05, P < 0.05. There were no significant differencesbetween the power of the MS5–PHH and the MS5 stimulations,or between theta power measured during the MS10–PHL andthe MS10 independent stimulation periods.

Experiment 2: Medial Septum and RostralPontine Oralis

Single factor ANOVA (F(5,24) ¼ 527.5, P < 0.001) indi-cated a significant difference between theta frequencies forExperiment 2. In a similar manner as Experiment 1, MS10and MS5 stimulations resulted in theta frequency of 9.62 6

0.003 Hz and 4.84 6 0.00 Hz, respectively. Independent PROstimulation at low and high intensities produced theta of 4.096 0.06 Hz and 5.94 6 0.03 Hz, respectively, and these values

FIGURE 5. Mean power of theta measured during stimulationconditions of Experiment 1. The corresponding asterisk indicatesthat the power measured during the combined stimulation was sig-nificantly greater than the independent stimulation indicated inthe x-axis.

FIGURE 4. Upper panel: Representative EEG segments and FFT’s for the PHL, MS10, andMS10–PHL stimulation conditions. Lower panel: Grand means for theta frequency elicited during thestimulation conditions listed above. The asterisk indicates significant differences between frequencies.

SEPTAL MODULATION OF HIPPOCAMPAL PATHWAYS 5

were significantly different at the P < 0.05 level. The MS5–RPOH stimulation pairing produced a theta frequency of4.84 Hz for every trial, which was significantly lower than theRPOH independent stimulation t(4) ¼ 6.290, P < 0.05 (lowerpanel, Fig. 6). In the second combined stimulation condition(MS10–RPOL), the mean theta frequency of 9.64 6 0.004 Hzmatched that observed in the MS10 stimulation, and was sig-nificantly higher than the frequency elicited by the RPOL con-dition (lower panel, Fig. 7), t(4) ¼ 14.43, P < 0.05.

Group data of the means and standard error for theta powerelicited in each stimulation condition for Experiment 2 areshown in Figure 8. Theta power for each of the combinedstimulation conditions was compared with the independentstimulations of which it was comprised. The MS5–RPOHstimulation elicited a mean theta power (0.132 V2), which wassignificantly higher than both the MS5 and RPOH stimula-tions, (t(47) ¼ 2.25, P < 0.05; t(34) ¼ 1.72, P < 0.05;Fig. 8). The MS10–RPOL combined stimulation yielded thetaactivity with a mean power of 0.19 V2, which was significantly

greater than the 0.07 V2 elicited with the RPOL pairing, t(39)¼ 3.56, P < 0.05. The power accompanying the MS10 stimu-lation was not significantly different in the MS10–RPOL con-dition, although a trend in that direction was observed (Fig. 8).

Experiment 3: Posterior Hypothalamusand Rostral Pontine Oralis

Single factor ANOVA proved significant for a difference intheta frequency between stimulation conditions, F(5,24) ¼76.3, P < 0.001. The independent PHL and PHH stimula-tions elicited different theta frequencies of 4.22 6 0.02 Hzand 6.38 6 0.04 Hz, respectively, significantly different at P <0.05. Independent stimulations of the RPO elicited a theta fre-quency of 4.10 6 0.08 Hz for RPOL, and 5.95 6 0.12 Hzfor the RPOH condition, which was significantly different atthe P < 0.05 level. Combined stimulation of these two nucleiresulted in a mean theta frequency of 5.65 6 0.05 Hz for thePHL–RPOH condition and 6.21 6 0.09 Hz for the PHH–

FIGURE 6. Upper panel: Representative EEG segments and FFT’s for the RPOH, MS5, andMS5–RPOH stimulation conditions. Lower panel: Grand means for theta frequency elicitedduring the stimulation conditions listed above. The asterisk indicates significant differencesbetween frequencies.

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RPOL condition. Theta frequency recorded during the PHL–RPOH stimulation was found to be significantly different thanPHL condition (t(4) ¼ 3.97, P < 0.05, lower panel, Fig. 9),and not significantly different than the RPOH condition. The6.21 6 0.09 Hz mean frequency recorded with the PHH–RPOL stimulation was significantly higher than the frequencyof the RPOL condition (t(4) ¼ 4.68, P < 0.05), but did notdiffer from the frequency recorded during the PPH stimulationperiods (lower panel, Fig. 10).

Group data of the means and standard error for theta powerelicited in each stimulation condition in Experiment 3 are shownin Figure 11. Theta power for all combined stimulation condi-tions was compared with the independent stimulations of whichit was comprised. Combined stimulation of the PH and RPOdid not result in any increases in power. Instead, the PHL–RPOH condition maintained the power observed during PHLindependent stimulation, and was significantly smaller than thetapower during the RPOH condition, (t(86) ¼ 2.69, P < 0.05).Likewise, in the PHH–RPOL stimulation condition, power

remained at a relatively constant value when compared with thePHH stimulation, but was reduced significantly from the 0.071V2 recorded during RPOL stimulations, (t(81) ¼ 4.52, P <0.05, Fig. 11).

DISCUSSION

The present findings demonstrated that the MS has the abil-ity to reset the frequency of theta field activity elicited by pos-terior hypothalamic and rostral pontine reticular stimulation.These results confirm and extend the findings of Scarlett andBland (1997). In Experiments 1 and 2, the frequency of hippo-campal theta activity depended on the frequency of the MSstimulation, regardless of the intensity of PH or RPO stimula-tion. The MS served to modulate the inputs from the PH(Experiment 1) and the RPO (Experiment 2) by resetting HPCtheta frequency in both upward and downward directions. TheMS5 stimulation condition resets the frequency of hippocampal

FIGURE 7. Upper panel: Representative EEG segments and FFT’s for the RPOL, MS10,and MS10–RPOL stimulation conditions. Lower panel: Grand means for theta frequency eli-cited during the stimulation conditions listed above. The asterisk indicates significant differen-ces between frequencies.

SEPTAL MODULATION OF HIPPOCAMPAL PATHWAYS 7

theta activity when paired with a high intensity PH stimulationand when paired with a high intensity RPO stimulation. Like-wise, the MS10 stimulation condition resets the frequency ofhippocampal theta activity when paired with the low intensityPH stimulation and when paired with the low intensity RPOstimulation. These findings agree with those of Scarlett andBland (1997), who showed identical effects for the MS–PHstimulation combinations. The present work added to theirstudy by showing a similar reset of hippocampal theta fre-quency when the MS stimulation is paired with the RPO stim-ulation, along with the modulation of theta amplitude. Thesemodulatory effects of the MS extend to a freely moving animal,as shown by Bland et al. (2005). These authors demonstrated areset of theta frequency equal to the values of the frequency ofthe MS stimulation (7 and 10 Hz) for the combined MS–PHstimulation conditions. In addition, the MS stimulation servedto reset running speed in a downward direction, when the MSstimulation was paired with PH stimulation. Running speedwas found to decrease over speed measured during independentPH stimulation. We also investigated whether a similar modu-

FIGURE 9. Upper panel: Representative EEG segments and FFT’s for the PHL, RPOH,and PHL–RPOH stimulation conditions. Lower panel: Grand means for theta frequencyelicited during the stimulation conditions listed above. The asterisk indicates significant differ-ences between frequencies.

FIGURE 8. Mean power measured during stimulation condi-tions of Experiment 2. The corresponding asterisk indicates that thepower measured during the combined stimulation was significantlygreater than the independent stimulation indicated in the x-axis.

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latory mechanism was present in the PH by utilizing combinedRPO and PH stimulation conditions. Our findings demon-strated that the PH did not induce a reset of theta frequency,as observed with the MS. When stimulated in combination,the nuclei receiving the higher intensity stimulation had thedominant effect on theta frequency. In the PHH–RPOL stimu-lation, theta frequency most closely mirrored theta frequencymeasured in the PHH independent stimulation, which showedan overriding of the RPOL stimulation. However, in the PHL–RPOH stimulation, theta frequency did not differ from thatobserved during the RPOH condition. In this case, the RPOhad the dominant effect in determining theta frequency.Although no modulation existed in the combined stimulationof these two nuclei, it is entirely possible that some other dien-cephalic structure has the capability to modulate ascendinginformation from the RPO. The supramammillary nucleus(SUM) lies ventral to the PH and contains phasic theta-on cellscrucial for the determination of theta frequency (Kirk andMcNaughton, 1993; Bland and Oddie, 2001; Pan andMcNaughton, 2004; Woodnorth and McNaughton, 2005).

Thus, the SUM, if stimulated in combination with the RPOmay be capable of resetting theta frequency.

As shown by Scarlett and Bland (1997), the MS modulatedthe power of theta and reset theta frequency when stimulatedsimultaneously with the PH. Although we did not see a signifi-cant increase in power with the MS5–PHH or MS10–PHL stim-ulations compared with independent MS5 and MS10 stimula-tions, we did observe a significant increase in power with theMS5–PHH and MS10–PHL combinations compared with thePHH and PHL independent stimulations. Again, in Experiment2, the MS modulated power, and in this case, the combined stim-ulation yielded an overall increase in power compared with inde-pendent stimulations. Together, these results provide further evi-dence that the MS played a role in translating the level of activa-tion of the ascending pathways to the appropriate frequency ofHPC theta field activity. Our results also support work fromMcNaughton’s lab demonstrating the importance of the PH andSUM in controlling HPC theta frequency (Kirk and McNaugh-ton, 1993; Pan and McNaughton, 2004; Woodnorth andMcNaughton, 2005). In Experiment 3, the power of theta eli-

FIGURE 10. Upper panel: Representative EEG segments and FFT’s for the PHH, RPOL,and PHH–RPOL stimulation conditions. Lower panel: Grand means for theta frequencyelicited during the stimulation conditions listed above. The asterisk indicates significant differ-ences between frequencies.

SEPTAL MODULATION OF HIPPOCAMPAL PATHWAYS 9

cited by the combined stimulation of PH and RPO resulted inpower that closely mirrored theta power during periods of inde-pendent PH stimulation. Thus, the cumulative increase in powerobserved when stimulating the MS in combination was notobserved in the PH–RPO stimulation conditions.

In summary, the data supported the following conclusions: (1)the major theta generating activity of the ascending brainstemsynchronizing pathways involves projections from the RPO tothe PH, relayed through the MS, to the hippocampal formation;and (2) theta field frequency is primarily determined by thebrainstem nuclei below the MS while the MS relays frequencyinformation and contributes primarily to theta amplitude.

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Pan WX, McNaughton N. 2004. The supramammillary area: itsorganization, functions and relationship to the hippocampus. ProgNeurobiol 74:127–166.

Scarlett D, Bland BH. 1997. Evidence that the medial septum controlsthe reset of hippocampal theta frequency. Soc Neurosci Abstr 23:486.

Scarlett D, Dypvik AT, Bland BH. 2004. Comparison of spontaneousand septally driven hippocampal theta field and theta related cellu-lar activity. Hippocampus 14:99–106.

Vertes RP, Kocsis B. 1997. Brainstem-diencephalo-septohippocampalsystems controlling theta rhythms in the hippocampus. Neuro-science 81:893–926.

Vertes RP, Hoover WB, Di Prisco GV. 2004. Theta rhythm of thehippocampus: subcortical control and functional significance.Behav Cog Neurosci Rev 3:173–200.

Woodnorth MA, McNaughton N. 2005. Different systems in the pos-terior hypothalamic nucleus of rats control theta frequency andtrigger movement. Behav Brain Res 163:107–114.

FIGURE 11. Mean power measured during stimulation condi-tions of Experiment 3. Note that the combined stimulation condi-tion (PHL–RPOH) produced a power insignificantly different tothat measured during the PHL condition. Likewise, the PHH–RPOL stimulation yielded theta power most similar in value tothe PHH stimulation condition. No increase in power wasobserved during the combined stimulation of the PH and thereticular pontine oralis.

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