Influences of hypothalamic stimulation upon septal and hippocampal electrical activity in the cat

Brain Research, 107 (1976) 55-68

55

© ElsevierScientificPublishing Company,Amsterdam- Printed in The Netherlands

INFLUENCES OF HYPOTHALAMIC STIMULATION UPON SEPTAL AND HIPPOCAMPAL ELECTRICAL ACTIVITY IN THE CAT

CHARLES L. WILSON, BRAD C. MOTTER AND DONALD B. LINDSLEY

Departments of Psychology, Physiology, Psychiatry and Brain Research Institute, University of California, Los Angeles, Calif. 90024 (U.S.A.) (Accepted September 23rd, 1975)

SUMMARY

Spontaneous patterns of hippocampal EEG and septal cell activity were studied in immobilized cats, and the influences of high frequency stimulation of medial hypothalamus (MH) and lateral hypothalamus (LH) were determined. Septal cells were divided into 3 classes on the basis of their discharge patterns: (1), rhythmic bursting (2), non-rhythmic bursting and (3), non-bursting, and the relationship of these discharge patterns to hippocampal theta rhythm was analyzed. Rhythmic bursting cells displayed close frequency and phase relations to hippocampal theta rhythm and were located chiefly in the diagonal band of Broca. Cells of the other two categories were found both within and outside of the diagonal band region. High frequency stimulation of medial hypothalamus evoked theta rhythm in hippocampus and bursts of repetitive firing at theta frequency from rhythmic bursting septal cells, whereas lateral hypothalamic stimulation desynchronized hippocampal EEG activity and disrupted rhythmic bursting patterns in the septum. The majority (59 ~o) of rhythmic bursting cells ceased bursting and fired continuously at an increased rate during high frequency stimulation of lateral hypothalamus, while other (33 %) rhythmic bursting cells ceased bursting and were either completely inhibited or fired at a markedly reduced rate. Of the rhythmic bursting cells tested with high frequency stimulation of lateral hypothalamus 73 % responded with short latencies (1.7-4.5 msec). No rhythmic bursting cells were found which responded to medial hypothalamic stimulation at fixed latencies. Brain stem-hypothalamic pathways which mediate these differential effects upon septum and hippocampus are discussed.

INTRODUCTION

Anchel and Lindsley1 have described two systems or pathways in the hypothalamus where high frequency electrical stimulation produces contrasting effects

56 upon hippocampal electrical activity. Stimulation of the medial system caused hippocampal theta rhythm, while stimulation of the more laterally placed system in the region of the medial forebrain bundle led to desynchronization of hippocampal activity. Macadar e t al. i t found that these contrasting hippocampal patterns of electrical activity could also be produced by stimulation of several specific sites in the mesencephalic and pontine regions of the brain stem. Anchel and Lindsley1 suggested that the medial, or theta, system exerted its effect upon the hippocampus via the septum, since Petsche e t al. 19 had previously presented evidence that cells in the medial septum may act as 'pacemakers' for hippocampal theta rhythm in the rabbit. Further support for this view derives from the fact that lesions of the septum or dorsal fornix abolish hippocampal theta activity1,31. The purpose of the present study was to investigate how stimulation of the two hypothalamic systems which influence the hippocampus in distinctive and contrasting ways affect the firing patterns of septal cells in relation to hippocampal electrical activity. These investigations have been carried out in acute, immobilized cats during non-stimulated states when hippocampal electrical activity manifested either spontaneous theta rhythms or mixtures of irregular fast and slow activity, and during high frequency stimulation of the medial hypothalamic system which induces theta rhythm, or during stimulation of the lateral hypothalamic system which causes desynchronization of hippocampal electrical activity.

METHODS

These experiments employed 22 adult cats of both sexes, weighing between 2.5 and 3.5 kg. Acute preparations were anesthetized with 1.5 ~o halothane in a I :l mixture of nitrous oxide and oxygen during tracheotomy, alignment in a stereotaxic frame, and introduction of stimulating and recording electrodes through burr holes drilled in the skull. After wound margins and pressure points were infiltrated with l ~ lidocaine hydrochloride (Xylocaine), halothane was discontinued, gallamine triethiodide (Flaxedil) was administered to the point of respiratory paralysis, and ventilation with the 1 : 1 mixture of nitrous oxide and oxygen was continued for the remainder of the experiment. Continuous venous perfusion of Flaxedil at a rate of 5 mg/kg/h maintained immobility. Local administration of Xylocaine was continued at regular intervals. Rectal temperature was maintained at 37-38 °C with a heating pad. Cortical EEG activity was recorded from stainless steel screws placed over posterior sigmoid (A 26, L 4) and marginal gyri (P 2, L 2). Stainless steel, concentric, bipolar recording electrodes with tip separation of 0.5 mm were lowered at coordinates A 4.0, L 5.5, until the inversion of hippocampal theta waves indicated that the lower tip of the electrode had entered the stratum radiatum and the upper tip was just above the stratum pyramidale of the dorsal CA 1 hippocampal fiel&. Concentric bipolar stimulating electrodes with tip exposure and separation of 0.2 mm were lowered into the ipsilateral posteromedial hypothalamus at coordinates A 10.0, L 0.5, H --1.5 to --3.0, and anterolateral hypothalamus at A 12.0, L 3.0, H--2.0 to --4.0.

57 Stimulation consisted of trains of biphasic pulses (100 Hz, 0.15 msec) delivered by a Grass stimulator (S-88) and stimulus isolation units (SIU-5). Tungsten microelectrodes with tips of 1 #m and impedances ranging from 15 to 30 Mf~ were lowered to the ipsilateral septum at an angle of 4° from the vertical in order to avoid the midsagittal sinus, through a 5 mm diameter trephine hole covered with agar. Coordinates of the areas sampled ranged from A 14.0 to 18.0, L 0.0-2.0, and H ÷ 6.0 to --3.0. Extracellular recording was carried out with a high impedance probe (Grass P-17) at a bandpass width of 30 Hz-3 kHz and EEG recording with a polygraph (Grass Model 78) at 1-15 Hz and 1-100 Hz bandwidths. Cortical and hippocampal electrical activity and septal unit discriminated output were recorded on EEG paper; hippocampal and septal activity was also monitored on a CRO and recorded with an FM magnetic tape recorder (Tandberg 100) for later analysis. Isolated septal cells were tested for response to sensory stimulation, to physostigmine salicylate, administered i.v., and to 0.15 msec single pulse stimulation of MH and LH systems. Response patterns of isolated septal cells and of hippocampal EEG activity were determined during 2-7 sec trains of 100 Hz MH and LH stimulation at current intensities ranging from 100 to 400/~A. At the conclusion of experiments, animals were deeply anesthetized and perfused with isotonic saline and 10 ~o formalin. Microelectrode and EEG recording tracks, and stimulating electrode tracks were marked by electrolytic lesions produced by passage of 100/~A of anodal DC current for 15 sec, and at least one microelectrode track in each animal was marked at two points in order to control for tissue shrinkage during histology. Subsequent data analyses were based only on cells whose loci were histologically verified on 40/~m frontal plane sections using the method of Guzman-Flores

et aL9. RESULTS

Characteristics of hippocampal and septal activity Spontaneous hippocampal activity. There are two general patterns of hippocampal activity characteristically observed in cats immobilized with Flaxedil and maintained under a nitrous oxide-oxygen mixture. These are (1), an irregular pattern of activity consisting of a mixture of slow and fast waves and (2), rhythmic slow waves (theta rhythm) ranging in frequency from 2 to 6 Hz and in amplitude from 0.5 to 1.5 mV. The irregular pattern is usually predominant, although theta rhythm frequently appears spontaneously and can easily be elicited by peripheral sensory stimulation. In addition to these two general patterns of activity brief (1-2 sec) periods of low voltage fast activity sometimes occur spontaneously. Spontaneous activity ofseptal cells. Septal cells have been classified on the basis of discharge patterns as rhythmic bursting (RB), non-rhythmic bursting (NRB) and non-bursting (NB) cells. RB cells displayed a firing pattern similar to that described by Petsche et aL 19 in the rabbit, consisting of rhythmic bursts which occurred at the frequency of the hippocampal theta rhythm. NRB cells exhibited patterns of burst activity which were irregular and bore no constant frequency or phase relation to

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Fig. 1. Discharge patterns of 4 rhythmic bursting septal (S) cellsduring periods of spontaneous hippocampal (H) theta rhythm. A and B: cells displaying long periods of constant rhythmic activity; note lack of burst pattern in small background cell in B. C and D: septal rhythmic bursting cells with less consistent relationship to spontaneous hippocampal theta rhythm. Calibrations: 0.5 mV, 1.0 sec. Polarity, positive up in this and all septal unit records. Bipolar hippocampal EEG activity, bandpass 1-15 Hz in A and D; 1-100 Hz in B and C. hippocampal theta rhythm. NB cells were characterized by more or less continuous irregular firing without burst-like grouping of unit discharges.

Characteristics of spontaneous rhythmic bursting cell activity All RB cells manifest the characteristic of rhythmic bursts, some in continuous sequences corresponding in frequency and phase to ongoing hippocampal theta rhythms, others showing only short periods of rhythmic bursting interspersed with periods of continuous or irregular unit discharges. Fig. 1 illustrates these different modes of firing in 4 different RB cells during periods of continuous hippocampal theta rhythm. Records A and B show highly rhythmic bursting patterns of discharge correlated in frequency and phase with the theta rhythm simultaneously recorded in dorsal hippocampus. The correspondence between the rhythmic cellular discharges in the septum and the rhythmic theta waves of the hippocampus indicate a close functional relationship between these two structures. It should be noted, however, that not all RB cells manifest this close and continuous correspondence with hippocampal theta rhythm; some, like that shown in record C, display short periods of rhythmic bursting synchronized with theta activity but with intervals of continuous or irregular

59

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Fig. 2. Location of septal units classified by discharge pattern plotted on a diagram of a frontal section of the cat septum at A 16.6 taken from the atlas of Andy and Stephan2. Rhythmic bursting (RB) cells are plotted on the left side of the section; non-rhythmic bursting (NRB) cells and non-bursting (NB) cells are plotted together on the right side. Abbreviations: CC, corpus callosum; NC, nucleus caudatus; LS, lateral septum; VDB, ventral nucleus of the diagonal band of Broca. Inverted 'V' shaped border demarcates the dorsal and ventral extent of the diagonal band region.

firing, and others, such as that illustrated in record D, show only occasional periods of bursting discharges synchronized with hippocampal theta waves. During changes in the state of hippocampal electrical activity 3 types of RB cell discharges were noted. A majority of cells discharged rhythmically only during periods of theta rhythm in hippocampus and fired in arrhythmic bursts or singly in the absence of hippocampal theta rhythm. A lesser number of cells displayed only brief periods of rhythmic burst activity during prominent epochs of hippocampal theta rhythm or when theta waves were enhanced by stimulation. Finally, a few cells fired with relatively continuous rhythmic bursting, regardless of whether theta waves were present in the hippocampus. Physostigrnine salicylate (100/zg/kg, i.v.) was effective in producing long periods (20-30 min) of continuous theta waves in the hippocampus which were synchronized with burst discharges of septal RB cells as previously shown in the rabbit a2. Repetitive bursts recorded from RB cells ranged in frequency from 2 to 6 Hz and each burst ranged in duration from 20 to 250 msec, with a mean burst duration of 110 msec. The number of spikes per burst ranged from 2 to 36, but usually were in a range of 3-12/burst. Interspike intervals within bursts were in the 4--40 msec range. Spontaneous discharge frequency of RB cells ranged from 4 to 63 spikes/see with a median rate of 18 spikes/see.

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Anatomical distribution of recorded cells The loci of 107 cells were histologically verified and are plotted in Fig. 2 on a diagram of a frontal plane section of the cat septum taken from the atlas of Andy and StephanL The cell loci illustrated extend from 0.8 mm anterior and 1.0 mm posterior to this frontal section at A 16.6. RB cells, which comprise 39 % of the 107 cells located, are represented by solid circles plotted on the left side of the figure, and were confined chiefly to the dorsal and ventral regions of tbe diagonal band o f Broca within the septum. Thirty of the 42 RB cell loci were concentrated within a region ~ 0.5 mm anterior or posterior to this frontal plane, although an area extending from A 17.9 to A 14.7 was sampled. Of the 3 RB cells located outside of the diagonal band, two were in medial septum and one in lateral septum. The 26 N R B cell loci (triangles) a p d 39 NB cell loci (squares) are plotted together on the right side of the frontal section. These recording sites were distributed about equally within and outside the diagonal band region, as well as in both anterior-posterior directions from frontal plane A 16.6.

61

Effects of stimulation upon septal rhythmic bursting, cells Fig. 3 illustrates how a single, septal, rhythmic bursting cell changes its firing pattern from (A) one of irregularity during a non-stimulated state when no theta rhythm was present in the hippocampus to (B) one of regularity and rhythmicity of bursting discharge synchronized in frequency and phase relations with hippocampal theta waves elicited by peripheral sensory stimulation (fur stroking). In record (C) stimulation of the medial hypothalamic system elicited theta rhythm in the hippocampus and caused the septal cell to discharge with a rhythmic burst pattern synchronized with the theta waves. In contrast to the effects of stimulating the medial hypothalamic system, stimulation of the lateral hypothalamic system (D) resulted in desynchronization of hippocampal electrical activity and eliminated the rhythmic bursting pattern of the same septal cell causing it to fire continuously and regularly at a high rate. Records E and F show the effects of repeated single shocks to medial and lateral hypothalamus, respectively, upon the same septal cell. The specific effects of each of the foregoing types of stimulation will now be described. Peripheral sensory stimulation. Stimulation of different sense modalities was effective in eliciting hippocampal theta rhythm and concurrent rhythmic bursting of septal cells. Brief light flashes or a sudden loud noise, such as a handclap, elicited a short period of rhythmic activity in both the hippocampus and septum; repeated stimulation led to rapid habituation. Somatosensory stimulation, such as stroking the fur of the cat, was more effective in eliciting trains of rhythmic activity in hippocampus and septum, which ranged from 5 to 30 see in duration (see Fig. 3B). Medial hypothalamic stimulation. Stimulation of the medial hypothalamic system at 100 Hz induced hippocampal theta rhythm and synchronized rhythmic burst discharges in the septal cell (Fig. 3C). Whereas somatosensory stimulation evoked a rhythmic pattern at 3.5 Hz in hippocampus and septum, MH stimulation elicited rhythmic activity at 4.8 Hz. The cell also responded to MH stimulation with an increase in the number of spikes/burst, an effect commonly seen in RB cells during this type of stimulation as compared to periods of spontaneous rhythmicity. Whereas the phase relation between septal bursts and hippocampal theta waves induced by MH stimulation (Fig. 3C) was the same as that during periods of spontaneous hippocampal theta rhythm (not illustrated), there was a shift in phase relations during somatosensory stimulation. The change in phase relation during the response of this cell to somatosensory stimulation, as compared to MH stimulation, shows that phase relations between hippocampal and septal rhythmic acitivities are not always invariant, and phase shifts were occasionally noted during recordings from other cells. In general, however, phase relations of the discharge of any one septal cell to hippocampal theta waves remained constant whether the rhythmicity occurred spontaneously or was elicited by sensory or MH stimulation. In contrast, phase relations between septal bursts and hippocampal theta waves often varied from cell to cell. Of the 27 RB cells tested with 100 Hz MH stimulation, 21 responded with rhythmic bursts synchronized with hippocampal theta rhythm, 3 were unaffected, and 3 cells were partially inhibited. Lateral hypothalamie stimulation. Hippocampal and septal responses to LH

62 stimulation were in distinct contrast to those during MH stimulation. In Fig. 3D, LH stimulation at 100 Hz desynchronized hippocampal activity and the septal cell responded continuously and regularly rather than with a rhythmic bursting pattern. The spontaneous rate of firing of this cell before stimulation was 42 spikes/sec as compared to 104 spikes/sec during LH stimulation. The cell responded to nearly every pulse in the stimulus train. In Fig. 3F, 6 responses taken from a 100 Hz stimulus train are superimposed, showing that the cell responded to each pulse with a latency of 2-3 msec. The variability of response latency suggests that the cell was orthodromically activated. Similar testing of 26 RB cells with 100 Hz stimulation showed that 73 ~ responded with latencies ranging from 1.7 to 4.5 msec. All of these cells displayed small variability in response latencies ranging from 0.5 to 1.5 msec, but many were incapable of following stimulation at a frequency of 100 Hz. Of 18 cells from the same population tested with single pulse stimulation, 33 ~ responded with fixed latencies within the same 1.7-4.5 msec range. The greater number of cells responding with fixed latencies during high frequency stimulation suggests a potentiation effect. In Fig. 3E, MH stimulation of the same cell at 100 Hz produced no responses having a fixed latency. None of the 28 RB cells tested with MH stimulation at either high or low frequetacy responded with a fixed latency. Using a criterion of at least 30 ~ change in rate of firing in comparison with spontaneous discharge rate, 16 of 27 RB cells (59 ~), tested with LH stimulation responded with an increase in frequency of discharge. The average frequency increase in rate of firing to LH stimulation in this population of cells was 183 ~. Common to all of these cells with LH stimulation was the tendency to fire continuously rather than with grouped-spike burst discharges. Whereas stimulation of the medial hypothalamic system characteristically produces hippocampal theta rhythm and correlated rhythmic bursting of RB cells, as illustrated for two different cells in Fig. 4A and C, stimulation of the lateral hypothalamic system which desynchronizes hippocampal theta rhythm has a differential effect upon RB cell discharge patterns. For example, in Fig. 4B, LH stimulation inhibited RB cell discharge, but in the case of another RB cell (Fig. 4D) LH stimulation caused continuous discharge during the period of hippocampal desynchronization. This cell's rate of discharge during LH stimulation was 20.8/sec compared to 9.8/sec during spontaneous discharge. The inhibitory effect of LH stimulation was observed in 9 of 27 RB cells (33 ~o), while the increased firing rate effect occurred in 59 ~ of RB cells. RB cells of the inhibitory type showed an average decrease in firing rate of 75 ~ during LH stimulation as compared to spontaneous firing, and some RB cells were completely inhibited during LH stimulation. In general, septal RB cells which were inhibited by LH stimulation showed less prominent rhythmic bursting during periods of spontaneous hippocampal theta rhythm or during theta rhythm elicited by MH stimulation than did RB cells which showed continuous discharge and increased rate of firing during LH stimulation. Usually, septal cell response to LH stimulation was tested at stimulating sites and at current values which were effective in eliciting a clear desynchronization pattern in hippocampus. On some occasions, low levels of stimulation in LH were found to

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Characteristics of non-rhythmic bursting cells and non-bursting cells Approximately 80 ~o of 62 NRB and NB cells tested, responded to both M H and L H stimulation with substantial changes in firing rate as compared to spontaneous rates of discharge. There were no consistent differences between NRB and NB populations in number of cells showing increases or decreases in firing rates during

64 MH as compared to LH stimulation, nor in the locus within the septum of cells showing specific changes in firing rate. However, differences between NRB and NB categories did appear in terms of the number of cells which responded at a specific latency to hypothalamic stimulation. Of 21 NRB cells tested with LH stimulation 11 responded with fixed latencies ranging from 2 to 5 msec: only 3 of 24 NB cells tested responded with similar fixed latencies. None of the NRB cells responded to M H stimulation at a fixed latency; 3 N B cells responded to M H stimulation with fixed latencies of 10, 12 and 28 msec. DISCUSSION

Pacemaker concepts and septal cell discharge characteristics

The proposal that septal cells serve as 'pacemakers' for hippocampal theta rhythm originated with Petsche et al. 19 and subsequent studies by these5,6,1°,32 and other investigatofs3,4,12,16,z3,3a,z5 have since provided further evidence for this concept in both rabbit and rat. Our results in the cat, showing a correspondence in frequency and phase relations between the rhythmic bursting of septal cells and hippocampal theta waves provide additional support for the pacemaker role of such cells. Whereas Petsche and coUaborators ~s indicated that in the rabbit all cells within the dorsal part of the nucleus of the diagonal band have the potentiality of firing in bursts synchronous with hippocampal theta rhythm, our results in the cat show that a substantial number of septum diagonal band cells do not manifest this kind of rhythmic bursting activity. For example, of 71 cells sampled within the diagonal band region of the septum 39 or 55~o showed rhythmic bursting synchronized with hippocampal theta rhythm, whereas 32 or 45 ~ manifested non-rhythmic bursting or no bursting, either spontaneously or during medial hypothalamic stimulation which induced hippocampal theta rhythm. The concept of septal 'pacemaker' cells includes several possible mechanisms: (1) such cells may have intrinsic properties of rhythmicity, (2) rhythmicity may be engendered by synaptic drive from one or more sources, directly or indirectly, (3) local circuits of excitatory and recurrent inhibitory interaction within the septum may be responsible for phasic rhythmic bursting of medial septum cells, (4) medial septal cell rhythmicity may depend upon reciprocal relations2s,29 between septum and hippocampus. With respect to possible mechanisms by which septal cells become 'pacemakers' for hippocampal theta rhythm, as Petsche et al. 19 contend, or, alternatively, that medial septal rhythmic bursting cells require hippocampal input via fimbria and lateral septum to maintain their bursting activity as McLennan and Milled 3,1~ have proposed, the present study offers additional information with regard to the role of hypothalamic input and its influences upon septal cell rhythmicity. Our results emphasize the importance of the medial hypothalamic system input as a primary source responsible for rhythmic bursting of medial septal cells. It is as yet uncertain whether rhythmic bursting of medial septal cells is due to synaptic drive upon them directly, or indirectly via lateral septum or hippocampal feedback pathways. However, no rhyth-

65 mic bursting cell in medial septum responded to MH stimulation with a constant latency, and high frequency MH stimulation gave rise to rhythmic bursting, but without direct response to individual pulses. In contrast, lateral hypothalamic system input appears to be responsible for disrupting the rhythmic bursting discharges of medial septal cells by either desynchronizing or inhibiting their activity. The lateral hypothalamic influence must be relatively direct since medial septal cell responses to LH stimulation have latencies as short as 1.7 msec. Brain stem-hypothalamic systems influencing septal and hippocampal activity

Anatomical studies have provided information concerning the pathways which may be involved in mediating changes in septal and hippocampal activity. Mesencephalic and pontine structures are known to project via the hypothalamus to the septum and hippocampus. Nauta and Kuypers 17 described two main projections from the midbrain of the cat (1), ascending via the dorsal longitudinal fasciculus of Schtitz which distributes fibers to the periventricular and medial hypothalamic nuclei, and (2) via the mammillary peduncle and medial forebrain bundle (MFB) to septum and hippocampus. The periventricular and dorsal nuclei of the medial hypothalamus have cell bodies which project to the septum and hippocampus 24-26. In the present study stimulation of sites in the medial hypothalamus (MH) elicited hippocampal theta rhythm and rhythmic bursting discharges in septal cells. In the lateral hypothalamus (LH), fibers of the MFB provide important afferent projections to the septal nucleiS, 15, 21,22. The most pronounced desynchronizing effects of LH stimulation upon septal and hippocampal electrical activity were produced at stimulating sites in the region of the medial forebrain bundle. Specific projections from nucleus locus coeruleus and the raphe nuclei to septum and hippocampus have recently been identified by silver staining ~4,27, autoradiographic 2°, and horseradish peroxidase labeling 24-26 techniques. Electrophysiological investigations carried out in this laboratory have provided correlative information on the functional characteristics of some of these anatomically defined ascending brain stem-hypothalamic projections to septum and hippocampus. For example, Macadar et al. 11 mapped hippocampal response to high frequency stimulation of a large portion of the mesencephalic and pontine brain stem and found that stimulation in 5 different and relatively localized sites, including nucleus locus coeruleus, nucleus reticularis pontis oralis, nucleus reticularis gigantocellularis of the pontine tegmental field, midbrain tegmentum and the ventrolateral region of the periaqueductal gray substance, was effective in eliciting hippocampal theta rhythm, while stimulation of the nuclei of the raphe and nucleus reticularis pontis caudalis desynchronized hippocampal activity. Gogolak et al. 5 investigated the effect of midbrain reticular stimulation upon septal and hippocampal activity in the rabbit. They reported that etectrical stimulation of the reticular formation at low intensities elicited hippocampal theta rhythm and a rhythmic bursting pattern in septal cells. Higher stimulation intensities at the same site desynchronized hippocampal activity and produced continued rhythmic bursting in some septal cells and disruption of rhythmic bursting in others. On the basis of the results of Anchel and Lindsley I (see also refs. 30, 33 and 36), that both hippocampal

66 theta rhythm and desynchronization patterns could be obtained by stimulation of a dorsolateral midbrain reticular formation site similar to that employed by Gogolak et al. 5, it is possible that closely adjacent or overlapping ascending pathways in this region were simultaneously activated, thus explaining the continued rhythmic bursting of some septal cells during hippocampal desynchronization. In the present study, activation of the ascending pathways of the medial hypothalamic system and the lateral hypothalamic system produced more clearly differentiated responses in both septum and hippocampus. MH stimulation at low or high current levels elicited theta rhythm in hippocampus and rhythmic bursting in the septum, while LH stimulation characteristically produced hippocampal desynchronization and disruption of septal rhythmic bursting patterns. On occasions in which LH stimulation produced a 'mixed' pattern of response however, it was possible to elicit a rhythmic bursting response from some septal cells during low current stimulation which produced small amplitude hippocampal theta rhythm, and to disrupt rhythmic septal bursting with higher intensity stimulation that desynchronized hippocampal activity. Anchel a~d Lindsley1 attributed the 'mixed' patterns of hippocampal response during LH stimulation chiefly to spread of current to the medial hypothalamic area. However, data obtained during observation of septal response in the present study, combined with other recent evidence, suggests that the organization of these ascending hypothalamic pathways may be more complex. First, during low current LH stimulation which produced septal rhythmic bursting, individual discharges durivg each burst were time-locked to LH stimulus pulses with a 2-4 msec latency, but no septal cell ever responded at a specific latency to pulses of a train of MH stimulation. Second, anatomical evidence20,24-27 indicates that nucleus locus coeruleus projects from the brain stem to septum and hippocampus primarily through the lateral hypothalamus as one of the fiber components of the MFB. Locus coeruleus was one of the brain stem structures shown by Macadar et al. 11 to be art effective site for eliciting hippocampal theta rhythm, and therefore, stimulation of fiber projections from this nucleus ascending via LH might be expected to elicit theta rhythm in hippocampus if the effect were not masked by simultaneous activation of a desynchronizing fiber group. Third, the excitatory response of some septal RB cells to LH stimulation and the inhibitory response of other septal RB cells to the same type of stimulation may also be indicative of multiple influences from MFB fiber groups ascending through the lateral hypothalamus. Analysis of septal response to stimulation of the brain stem areas mapped by Macadar et al. 11 may provide needed information to isolate anatomically distinct ascending brain stem-hypothalamic systems which, via their differential effects upon patterns of septal cellular discharge, have specific influences upon hippocampal electrical activity. REFERENCES

1 ANCHEL, H., AND LINDSLEY, D. B., Differentiationof two reticulo-hypothalamicsystemsregulating hippocarnpal activity.Eletroenceph. din. Neurophysiol., 32 (1972) 209-226. 2 ANDY,O. J., ANDSTEPHAN,H., The Septum of the Cat, Thomas, Springfield,Ill., 1964.

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