The role of the lateral septum in anxiolysis

Physiology&Behavior.Vol. 53, pp. 1077-1083, 1993

0031-9384/93 $6.00 + .00 Copyright© 1993 PergamonPressLtd.

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The Role of the Lateral Septum in Anxiolysis E L N A Y A D I N , *~ E A R L T H O M A S , ] " H O L L Y L. G R I S H K A T ] " A N D C R A I G E. S T R I C K L A N D ] "

*Department of Psychiatry, The Medical College of Pennsylvania, 3200 Henry Avenue, Philadelphia, PA 19129 and ~'Department of Psychology, Bryn Mawr College, Bryn Mawr, PA 19010 Received 12 J u n e 1992 YADIN, E., E. THOMAS, H. L. GRISHKAT AND C. E. STRICKLAND. The roleofthe lateralseptum in anxiolysis. PHYSIOL BEHAV 53(6) 1077-1083, 1993.--Behavior of rats in the water-lick conflict test was examined during stimulation, and after lesions of the lateral septal nucleus. Continuous low-current stimulation resulted in an anxiolytic effect, an increase in the number of licks, and hence in the number of shocks, during a signaled, punished period. This effect is similar to the one seen with peripheral administration of benzodiazepine and other anxiolytic agents. Rewarding medial forebrain stimulation did not have this effect. Conversely, septal lesions resulted in an anxiogenic effect, a significant decrease in the number of licks during the punished period, Lesions had this effect when the conflict behavior was already well established. Septal lesions performed before acquisition of the conflict behavior resulted in initial retardation of acquisition. The results suggest an important role for the lateral septum in the inhibition of anxiety and in the acquisition of behaviors reinforced by alleviation of anxiety. Lateral septum

Anxiety

Conflict

Stimulation

Lesions

A recent review (12) has brought together evidence from various experimental approaches supporting a proposed function for the lateral nucleus of the septal area in emotion. The cumulative evidence from studies using lesions, stimulation, and unit activity recording suggests an important role for this nucleus in the inhibition of fear. Acquisition of behaviors that depend on fear reduction are impaired in animals with septal lesions. An example of such a behavior is avoidance behavior. Septally lesioned animals showed deficits in acquisition of both passive and one-way active avoidance behavior (3). Moreover, the ability of a conditioned inhibitor of fear to retard ongoing avoidance behavior (19) was abolished by electrolytic damage to the septal region (18). Stimulation of the septal area produced a general parasympathetic reaction (6) and inhibited responses elicited by stimulation of other brain structures, such as the hypothalamus (13). Cats that normally do not self-stimulate in the septum learned to perform an instrumental response to introduce septal stimulation only when aversive stimulation in the hypothalamus was present (13). If the aversive hypothalamic stimulation was discontinued, the instrumental response underwent extinction. Similarly, if the hypothalamic stimulation was nonaversive, the animals did not acquire the response. In a conditioned emotional response paradigm in rats, septal self-stimulation was impervious to the disruptive effects of the conditioned emotional stimulus, while MFB self-stimulation showed strong suppression (2). Multiple- and single-unit activity recorded from the lateral septum during conditioning seemed to reflect tonically the emotional state of the animal (14,21). Thus, the rate of firing of cells in the lateral nucleus of the septal area consistently increased during periods of relief from fear, such as during the presentations

J Requests for reprints should be addressed to Elna Yadin.

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Chlordiazepoxide

of a conditioned inhibitor of fear, and after termination of a predicted shock. Those same cells' firing rates were suppressed during periods associated with the elicitation of fear, such as during the presentations of a conditioned excitor of fear. Pharmacological manipulations have yielded evidence further supporting the notion of a fear-relieving role for the lateral septurn. In awake, freely moving rats, anxiolytic drugs increased firing rates of lateral septal units, enhanced the ability of a conditioned inhibitor of fear to increase firing rates, and blocked the activity-suppressing effect of a conditioned excitor of fear (22). If stimulation of the lateral septum has fear-relieving properties, it should mimic the behavioral effects of anxiolytic drugs. These properties should be detectable in a commonly used screening procedure for anxiolytic agents, the water-lick suppression conflict paradigm (17), Conversely, if lesioning the septal area results in an increased state of fear, anxiogenic changes would be expected in the conflict procedure. EXPERIMENT 1: STIMULATION EFFECTS This experiment was designed to determine whether lateral septal stimulation would increase punished responding in a water-lick suppression conflict paradigm in a similar manner to that seen after administration of benzodiazepines (19). To rule out the possibility that such effects would be due to the rewarding properties of the stimulation, a control group was included in which animals received stimulation of the medial forebrain bundle (MFB). The MFB at the level of the lateral hypothalamus has been well established as a site where stimulation is highly rewarding.

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5AI)IN t i "~I METHOD

Subjects The subjects in all experiments were male albino SpragueDawley rats, 90 days old at the start of the experiment. Their weights ranged from 350 to 500 g. They were individually housed in a temperature-controled colony with a 12-h dark (2000 to 0800 h), 12-h light (0800 to 2000 h) cycle. Food was continuously available while water was available only for 0.5 h after each daily session.

Apparatus The conflict chamber was a clear Plexiglas chamber measuring 34 × 26 × 22 cm with a grid floor. A water bottle was mounted on the outside of the front panel with a metal tube protruding into the chamber in the center of the panel, 5 cm above the floor. The apparatus was housed in a sound-attenuating cubicle, equipped with a houselight and speaker. White noise (70 dB SPL) was fed into the chamber throughout the experiment, except during presentations of the tone. Drinkometers were used to detect licking. Shocks were delivered through the grid floor, using constant current shock generators. Activation of the experimental sequences and recording of the results were controlled by a computer (IBM PC) and solid state equipment. The self-stimulation apparatus was a standard Plexiglas Skinner box with a lever. Stimulation in this apparatus, as well as in the conflict apparatus during testing, was delivered by a constant current square-wave stimulator.

Procedure Animals were trained (between 1300 to t 500 h) in a modified water-lick suppression conflict procedure in which they were exposed to four 2-min time periods, the second and fourth of which were signaled by an 80 dB/1000 Hz tone. The total number of licks within each 2-rain component was counted. The first few sessions were habituation sessions in which animals were exposed to the experimental apparatus and the sequence of stimuli, without their behavior being punished. Animals were then exposed to conflict training sessions during which every 20th lick in the second and fourth signaled periods was punished by a 0.5-s shock delivered through the grids. Shocks were 60 Hz sine waves and were adjusted individually (0.2-0.5 mA) for each animal to produce suppression of responding during the first punished period to between 10% and 40% of the animal's responding during the first unpunished period. This training phase lasted for approximately 2 weeks. Animals were then implanted under anesthesia (Nembutal, 40 mg/kg, IP) with unilateral bipolar stimulation electrodes (Plastics One), either in the lateral septum (LS; n = 6) or in the medial forebrain bundle (MFB; n = 5). The coordinates for LS implants were: 1.5 mm anterior to bregma, 0.7 mm lateral to midline, and 5.0 mm from the surface of the skull. The coordinates for MFB implants were: 3.0 mm anterior to bregma, 1.5 mm lateral to the midline, and 9.0 mm from the surface of the skull. After a l-week recovery period, all animals were tested for self-stimulation behavior in the Skinner box. Reinforcement for each lever press consisted of a 0.5-s train of square-wave pulses, 0.1 ms duration, delivered at a rate of 100 pulses per second. They were then rerun in the conflict apparatus for reminder sessions. Testing began with a conflict session in which animals were run with the connectors attached to the electrodes but with

no stimulation being delivered, l'he lbllowing session was the test session. In this session animals were run in the contlict chamber with continuous stimulation on. The stinlulation ,aas comprised of biphasic square wave trains of 100 pulses per second, with 0.1 ms pulse duration. Current levels ranged between 25 and 100 #A. Several baseline sessions were administered following the stimulation test, and then conflict behavior was tested with peripherally administered chlordiazepoxide (CDP, 10 mg/kg). Animals were then sacrificed with an overdose of Nembutal. and perfused cardially with normal saline and t0% lbrmalin. The brains were removed, and frozen sections were taken at 120 #m, photographed using the method described by Guzman-Fqores et al. (4), and stained with cresyl violet. All statistical analyses were calculated on the first unpunished/punished part of the cycle. The second part was used to ensure that the animals" licking behavior came under stimulus control. Factors such as fatigue and satiation that may have affected the animals' behavior during this part rendered it inappropriate for statistical analysis. RESUI TS

Figure 1 is a photograph of fresh brain sections from representative animals with electrode tips in the lateral septum (A) and in the MFB (B). Self-stimulation behavior was seen in three out of six animals with electrodes in the lateral septum and in four out of five animals implanted with MFB electrodes. Figure 2 depicts the conflict behavior in animals that, on the test day, received continuous septal or MFB stimulation. The raw data for each animal's daily performance were converted to log (number of licks + 1). One was added to each score to allow the conversion when occasionally an animal's performance in the punished component was zero. It was found that the log transformation consistently normalizes the variances, especially when comparing punished and unpunished performance. In addition, it allowed the portrayal of both the punished and unpunished responding conveniently on the same axis. Unless otherwise specified, the statistical analyses were performed on the transformed data. In a dependent t-test, comparing performance under stimulation with an unstimulated baseline session in the same animals, the presence of continuous low-level stimulation during the test session resulted in a significant increase in punished behavior in the septally stimulated group only, tLS(5) = 3.51, p < 0,05. There were no changes in the performance of the MFB-stimulated animals, nor in the unpunished component of either group. The effects of stimulation of the lateral septum upon punished licking behavior were similar to the effect of a l0 mg/kg dose of CDP injected intraperitoneally. This latter manipulation resulted in a significant increase of the animals' licking rates during the punished component of the conflict test compared with the previous vehicle session, t(l 1) = 2.69, p < 0.05. DISCUSSION

Stimulation of the anterior dorsolateral portion of the septal area appears to possess anxiolytic properties as measured by an increased punished response in this water-lick suppression test. The change in punished behavior in the group receiving septal stimulation does not appear to be a consequence of its rewarding properties, as the group receiving MFB rewarding stimulation does not show the same kind of change in punished behavior. The effects of lateral septal stimulation closely resemble the effect of peripherally administered known anxiolytic agents, such as the benzodiazepines.

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FIG. 1. A photograph, using fresh brain sectionsas negatives,depictingthe electrode tip locations (arrows) from representative animals with an electrode implanted in the anterior dorsolateral septum (A) or in the medial forebrain bundle (B).

METHOD

EXPERIMENT 2: LESION EFFECTS The lateral septal nucleus appears to play a role in anxiolysis. If this nucleus is indeed involved in antianxiety activity, then removal of the structure would result in anxiogenesis. In the conflict procedure, this should be manifest by an added suppression of punished responding. The present experiment was conducted to test this possibility.

Procedure Naive animals were trained in the conflict procedure described in Experiment 1. After being trained they were subjected, under anesthesia (Nembutal, 40 mg/kg, IP), to bilateral electrolytic septal lesions (n = 9). The coordinates were: 1.0 mm anterior

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FIG. 2. Performance on a water-lick conflict test in animals receiving either lateral septal stimulation, medial forebrain bundle (MFB) stimulation, or intraperitoneal (IP) injections of chlordiazepoxide (10 mg/ kg). *Marks a significant difference from the previous session. Error bars represent the SEM. BAS, baseline: STIM, stimulation: SAL. saline: CDP, chlordiazepoxide.

to bregma, 0.6 m m lateral to the midline, and 5.5 m m below the surface of the skull. The lesions were produced with 2 mA anodal direct current presented for 20 s. A second group of animals was given sham lesions (n = 9), in which an electrode was lowered into the designated target site but no electrical current was applied. After at least a l-week recovery period, animals were again tested in the conflict apparatus during several consecutive sessions. Means were calculated of performance over the first three sessions after resumption of testing (POSTOPI) and of three sessions 4 days later (POSTOP2). Sacrifice, perfusions, and histological procedures were identical to those described for Experiment 1.

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Figure 3 is a photograph of a flesh brain section at the level of the septum from a representative animal with a bilateral septal lesion. In most of the lesioned animals the lateral septum was destroyed. Figure 4 depicts the conflict behavior in animals with septal or sham lesions. The lesion had a significant suppressing effect on the punished component of the test, as seen in a one-way A N O V A with repeated measures, F(2, 16) - 5.77, p < 0.05. This additional suppression of punished licking behavior was seen both in the early sessions after the lesion and during later sessions after destruction of the septal area, teosmpj(8) = 2.92, p < 0.05: teosvop2(8) = 2.73, p < 0.05. The unpunished component of the test was unaffected by the lesion. Sham lesions resulted in no significant changes in the animals" water-lick suppression conflict behavior. DISCUSSION

Lesioning the septal area, including the anterior dorsolateral portion, in animals that have been well trained in the conflict procedure produces a proconflict effect. Septally lesioned animals consistently lick less during the signaled punished period, both in comparison with their own preoperative performance and when compared with sham-lesioned controls. These results support the notion that the lateral septum has a role in the reduction of fear or anxiety. E X P E R I M E N T 3: LESION EFFECTS ON A C Q U I S I T I O N One of the most consistently reported effects ofseptal lesions is an impairment in passive avoidance (3), usually attributed to a deficit in the ability to suppress or withhold somatomotor responses. The conflict paradigm employed here is, of course, a form of the punishment or passive avoidance procedure in which drinking is suppressed by response-contingent shocks. Thus, the results of the first two experiments may be seen as at variance

FIG. 3. A photograph, using a fresh brain section as a negative, depicting a bilateral lateral septal lesion in a representative animal.

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with the bulk of the data on the effects of septal lesions on passive avoidance and with the attendant theories of septal function. An important difference exists between Experiment 2 and those experiments that report deficits in passive avoidance. In most of the experiments that show such deficits, the animals were tested in acquisition. In the present experiment, the animals were highly trained, even overtrained, prior to the lesion. In order to determine if this is an important variable, Experiment 3 was designed to examine the effects of septal lesions upon the acquisition of lick suppression in the conflict paradigm. METHOD Procedure

Animals were habituated to the experimental apparatus where they were allowed to lick water during four consecutive 2-min periods, the second and fourth of which were signaled by tone. No shocks were presented at this time. They were then given bilateral septal lesions under anesthesia, using the same methods and coordinates as in Experiment 2 (n = 8). A second group of animals was given sham lesions (n = 15). After a 1-week recovery period, animals were trained on the conflict task, introducing a 0.3 mA shock, every 20th lick, during the signaled periods. This training routine was maintained for several sessions before euthenizing and perfusing the animals for verification of the extent of damage sustained by the lesion.

DISCUSSION Acquisition of conflict behavior is retarded by lesions of the septal area. Similar to other passive avoidance tasks, the deficit is seen mainly in the first session after the lesion. The effect on punished responding found here cannot be attributed to the hyperreactivity produced by the septal lesion, because when the lesion is given after conflict behavior has been acquired the effect on punished behavior is in the opposite direction (Experiment 2). The finding from Experiment 3 lends support to the contention that the septum is important in the acquisition of behaviors motivated by the reduction of fear. The fact that the animals do learn to suppress their punished responding in the following sessions suggests that the septum may only be one part within a larger fear-relieving network. GENERAL DISCUSSION The composite of results from these experiments appears to be consistent with the proposed anxiolytic function for the lateral septal nucleus (12). Continuous low-level stimulation of the lateral septum produced higher licking rates during the punished component of the conflict test when compared to the performance on a previous baseline session, during which no stimulation was present. The behavior of MFB-stimulated animals did not differ from their performance during the baseline session. This suggests that it is not the rewarding effect of the stimulation that is crucial in raising punished behavior, but rather a more specific fear-reducing property of lateral septal stimulation that is responsible for the change in punished licking. This anticonflict change is similar to the change seen after administration of chlordiazepoxide and of other anxiolytic agents reported in the literature (9). The animals in the current experiments, when injected peripherally with chlordiazepoxide, displayed characteristic anticonflict effects: their punished behavior was released under the drug condition while their unpunished behavior remained unchanged. Conversely, destruction of the septai area yielded a proconflict effect. When animals, well trained in the conflict procedure, underwent septal lesions, their punished licking behavior consistently became further suppressed. Interestingly, when the septal lesion was made before acquisition of the conflict task, that acquisition was impaired in the first session during which shocks

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Figure 5 displays the mean number of licks during the punished component of the conflict test in septal- and sham-lesioned animals, before acquisition began (PRE), and in five consecutive sessions after introduction of the shocks. While there was no difference in the animals' licking behavior to the signal alone, upon presentation of the shocks in the first session the mean number of licks (and hence the mean number of shocks taken) in the lesioned group was significantly higher than that in the sham control group, t(21) = 2.20, p < 0.05. As may be seen, in subsequent sessions conflict behavior of the animals with septal lesions is not different from behavior of the sham-lesioned group.

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HG. 5. Performance on acquisition of a water-lick conflict test in animals given sham (SHAM) or lateral septum (SEPT) lesions. *Marks a significant difference between the two groups. Error bars represent the SEM. U, unpunished; P, punished; PRE, Licking behavior before lesioning.

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were introduced. In subsequent sessions, the animals learned to suppress their punished licking response. As training proceeded, the septal-lesioned animals' punished responding tended to dip below the control animals' responding, resembling the proconflict effect seen in Experiment 2, where animals had acquired the task prior to the septal lesions. The pattern of deficits early in training is quite typical of the effects of septal lesions seen in a number of passive avoidance paradigms (3). The reasons for the discrepancy between the effects of septal lesions upon acquisition of water-lick suppression and its maintenance are a matter of speculation. The data would suggest that there may be different mechanisms responsible for the acquisition of aversively controlled behavior and for its maintenance. One possibility is that the acquisition of avoidance, but not its maintenance, requires fear reduction for its reinforcement. It has been proposed that the lateral septum plays an important role in mediation of relief from fear (12). If the underlying presumption is that acquisition of all avoidance behavior requires fear reduction for its reinforcement, it would follow that in the absence of fear reduction provided by the lateral septum the animal would be in a high state of fear, during which its behavior is unable to provide relief. However, when the task has been well learned, the increased fear resulting from the lesion contributes to the proconflict effect. The findings in this report support the contention that an intact lateral septal area is necessary (but probably not sufficient) for the reinforcement of behaviors that are motivated by relief from fear. In this case, as in other passive avoidance situations, withholding the licking response during the punished period should be reinforced by relief from fear. This seems to be happening in the sham-lesioned control animals but not in the septallesioned animals. The similarity between the change in conflict behavior as a consequence of stimulation of the lateral septal nucleus and the behavioral change obtained with anxiolytic agents raises the possibility that comparable mechanisms of action may be underlying these behavioral changes. Since the discovery of specific benzodiazepine receptors in the brain (11), evidence has accumulated suggesting the involvement of the benzodiazepine/ GABA chloride ionophore complex in mediating the action of anxiolytic agents (5). The lateral septum seems to be one important site tbr such involvement. Thus, BZD receptors are present in relatively high concentrations in this area (11), as is the concentration of GABAergic neurons (7). Plastic changes in BZD-like molecules in rats trained in a punishment paradigm have been shown to exist in the septum (20). Furthermore, the GABA agonist muscimol produced an anxiolytic effect when injected directly into the lateral septum (1). Is it possible that the effects of lateral septal stimulation detected here with one type of anxiolytic screening procedure are mediated by the BZD/ GABA chloride ionophore complex as well? It would be ira-

portant to determine whether benzodiazepine and/or GABA antagonists block the original effects found with lateral septal stimulation. Since the data for this experiment were gathered, a report has appeared in the literature suggesting that septal lesions may have an antianxiety effect (16). These authors found that septal lesions decreased defensive burying in rats and increased the number of entries onto the open arm of an elevated plus-maze. Both of these effects are indices of anxiolytic action. It should be pointed out that there are a number of differences between the present experiments and those of Treit and Pesold (16). The most obvious one is the difference in the behaviors studied. Although conflict, defensive burying, and elevated plus-maze performance are all considered to be tests of anxiolytic drug action, and all respond to the classic benzodiazepine anxiolytics, there are instances where these tasks respond differently to the same drugs. While the choice of tasks might account tot the seemingly conflicting results of the two studies, they are more likely a consequence of the regional differences in the precise locus of the lesions. The effective lesions in the Treit and Pesold (16) study were posterior, and included both lateral septum and the medial septum/diagonal band area. The lesions in the present study were considerably more anterior and did not extend greatly into the medial septum/diagonal band region. Preliminary. studies (unpublished data) using large lesions that extend into the medial septum/diagonal band region have shown an anxiolytic effect in the plus-maze similar to that reported by Treit and Pesold (16). Small lesions restricted to the dorsolateral septum produced neophobic effects in rats tested for entry into a novel chamber (8). These lesions had an anxiogenic action in rats when tested in the elevated plus-maze (Schneider, personal communication). Single-unit recording experiments have shown that cells in the anterior dorsal septum increase their firing in the presence of stimuli that inhibit anxiety, whereas cells in the medial septum tend to increase their firing in the presence of stimuli that signal anxiety (15), These data suggest that the medial and lateral septurn may have somewhat different and perhaps opposite functions in relation to anxiety. Thus, a lesion that extends into both may have unpredictable effects. In sum, the evidence would seem to support an anxiolytic function for the lateral septum. Continuous stimulation (at subconvulsive threshold currents) of the anterior dorsolateral region of the septal area in the rat appears to have anxiety-relieving properties. The anxiogenic effects of septal lesions provide complementary evidence in support of an important role for the lateral septum in relief of fear or anxiety. ACKNOWLEDGEMENTS We would like to acknowledge the contributions of the following

undergraduate students: Shaft Burke, Tamara Holt, and Rita Hunt.

REFERENCES 1. Drugan, R. C.; Skolnick, P.; Paul, S. M.; Crawley, J. M. Low doses of muscimol produce anticonflict actions in the lateral septum of the rat. Neuropharmacology 25:203-205; 1986. 2. Grauer, E.; Thomas, E. Conditioned suppressionof medial forebrain bundle and septal intracranial self-stimulationin the rat: Evidence for a fear-reliefmechanismof the septum. J. Comp. Physiol. Psychol. 96:61-70; 1982. 3. Gray, J. A.; McNaughton, N. Comparison between the behavioral effects of septal and hippocampal lesions: A review. Neurosci. Biobehav. Rev. 7:117-188; 1983.

4. Guzman-Flores, C. M.; Alcarez, M.; Fernandez--Guardiola,A. Rapid procedure to localize electrodes in experimental neurophysiology. Bol. Inst. Estud. Med. Biol. 16:26-31; 1958. 5. Hommer, D. W.; Skolnick, P.; Paul, S. M. The benzodiazepine/ GABA receptor complex and anxiety. In: Meltzer, H. Y., ed. Psychopharmacology: The third generation of progress. New York: Raven Press; 1987:977-983. 6. Kaada, B. R. Somatomotor, autonomic and electrocorticographic responses to electrical stimulation of "rhinencephalic" and other structures in primates, cats, and dogs. Acta Physiol. Scand. 24:1258; 1951.

LATERAL SEPTUM AND ANXIETY 7. Panula, P.; Revuelta, A. V.; Cheney, D. L.; Wu, J.-Y.; Costa, E. An immunohistochemical study on the location of GABAergic neurons in rat septum. J. Comp. Neurol. 222:69-80; 1984. 8. Schneider, A. M.; Thomas, E.; Martin, W.; Folwell, M. G.; Payne, A. Repeated injections of scopolamine hydrobromide ameliorate the behavioral effects of subsequent septal lesions. Soc. Neurosci. Abstr. 17:1401; 1991. 9. Sepinwall,J.; Cook, L. Behavioral pharmacology of antianxiety drugs. In: Iverson, L. L.; Iverson, S. D.; Snyder, S. H., eds. Handbook of psychopharmacology, vol. 13. New York: Plenum Press; 1978:345-393. 10. Siegel, A.; Skog, D. Effects of electrical stimulation of the septum upon attack behavior elicited from the hypothalamus of the cat. Brain Res. 23:371-380; 1971. 11. Squires, R. F.; Braestrup, C. Benzodiazepine receptors in rat brain. Nature 266:732-734; 1977. 12. Thomas, E. Forebrain mechanisms in the relief of fear: The role of the lateral septum. Psychobiology 16:36-44; 1988. 13. Thomas, E.; Evans, G. J. Septal inhibition of aversive emotional states. Physiol. Behav. 31:673-678; 1983. 14. Thomas, E.; Yadin, E. Neural correlates of conditioning assessed by extracellular unit recording: Implications for neuroplasticity. In: Milgram, N. W.; McLeod, C. M.; Petit, T. L., eds. Neuroplasticity, learning and memory. New York: Allan R. Liss; 1987:199-229.

1083 15. Thomas, E.; Yadin, E.; Strickland, C. E. Septal unit activity during classical conditioning: A regional comparison. Brain Res. 547:303308; 1991. 16. Treit, D.; Pesold, C. Septal lesions inhibit fear reactions in two models of anxiolytic drug action. Physiol. Behav. 47:365-371; 1990. 17. Vogel, J. R.; Beer, B.; Clody, D. E. A simple and reliable conflict procedure for testing antianxiety agents. Psychopharmacologia 21: 1-7; 1971. 18. Wagman, R. Septal area modulation of conditioned emotional states in cats. Bryn Mawr College; 1972. Master's thesis. 19. Weisman, R. G.; Litner, J. S. The role of Pavlovian events in avoidance learning. In: Boakes, R. A.; Halliday, M. S., eds, Inhibition and learning. London: Academic Press; 1972:253-270. 20. Wolfman, C,; Cunha, C. D.; Jerusalinsky, D.; de Stein, M. L.; Viola, H.; Izquierdo, I.; Medina, J. H. Habituation and inhibitory avoidance training alter brain regional levels of benzodiazepine-like molecules and are affected by intracerebral flumazenil microinjection. Brain Res. 548:74-80; 1991. 21. Yadin, E.; Thomas, E. Septal correlates of conditioned inhibition and excitation. J. Comp. Physiol. Psychol. 95:331-340; 1981. 22. Yadin, E.; Thomas, E.; Vaughan, M. P. Effects of anxiolytic agents on firing of lateral septal units in acute and chronic preparations. Soc. Neurosci. Abstr. 12:935; 1986.