Priming stimulation in the basolateral amygdala modulates synaptic plasticity in the rat dentate gyrus

Neuroscience Letters 270 (1999) 83±86

Priming stimulation in the basolateral amygdala modulates synaptic plasticity in the rat dentate gyrus Irit Akirav, Gal Richter-Levin* Laboratory of Behavioral Neuroscience, Department of Psychology, University of Haifa, Haifa 31905, Israel Received 22 March 1999; received in revised form 26 May 1999; accepted 26 May 1999

Abstract We investigated the effects of basolateral amygdala (BLA) priming on long-term potentiation (LTP) in the dentate gyrus (DG). In the control animals, the induction of high-frequency stimulation (HFS) to the perforant path (PP) resulted in hippocampal LTP at all the time intervals tested. A priming stimulation to the BLA prior to the application of HFS to the PP resulted in the enhancement of the excitatory post-synaptic potential (EPSP)-LTP and population spike (PS)-LTP in the DG from 90-min post-HFS onwards. These ®ndings suggest that the amygdala has a potential role in the modulation of some aspects of memory that are mediated by the hippocampus. q 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Memory; Basolateral amygdala; Hippocampus; Dentate gyrus; Long-term potentiation; Amygdalo-hippocampal interaction; synaptic plasticity; rat

It is widely recognized that there are different kinds of learning and memory capacities in the brain [15,19]. The hippocampal formation is an essential component of the brain system underlying the explicit recollection of past events and the processing of spatial, con®gural, contextual and/or relational information [11]. Long-term potentiation (LTP) of evoked potentials in the hippocampus has become widely regarded as a possible physiological substrate for some aspects of learning and memory in the mammalian brain [2]. The amygdala is involved in certain types of learning and memory as well as emotional and motivational aspects of behavior [5,6,9,13]. The activation of the amygdala in¯uences neural activity and possibly memory storage in the hippocampus and the caudate [16]. Neural projections from the amygdala to the hippocampus have been con®rmed by several physiological and anatomical studies [1,8,20]. For example, single-pulse stimulation of the amygdala evoked synaptic potentials in the dentate gyrus (DG), and the DG response to perforant path (PP) stimulation was enhanced by the preceding stimulation of the amygdala [20]. Evidence also shows monosynaptic and excitatory projections from the amygdala to the entorhinal cortex [4]. In addition to these functional amygdalo-hippocampal * Corresponding author. Tel.: 1972-4-824-0962; fax: 1972-4824-0966. E-mail address: [email protected] (G. Richter-Levin)

interactions, there have been reports regarding the role of the amygdaloid inputs in the generation of hippocampal LTP. A series of studies [6±8] has suggested that the basolateral neurons of the amygdala (BLA) participate in the formation of the population spike (PS)-LTP in the DG. Hippocampal PS-LTP was attenuated by lesions of the BLA [6] and was facilitated by electrical stimulation of the BLA [7]. However, an increase in the PS size may result from the enhanced ef®ciency of synaptic transmission or from alterations of the excitability of postsynaptic cells. The increase in the excitatory postsynaptic potentials (EPSP's) slope of the recorded ®eld potential is considered a more direct re¯ection of the ef®ciency of synaptic transmission. Thus, the effects of BLA priming on hippocampal EPSP-LTP should be studied to understand BLA modulation of synaptic ef®cacy in the hippocampus. The BLA appears to be the nucleus most crucially involved in the modulation of memory [17,18]. The BLA has a role in modulating hippocampal-dependent in¯uences on memory storage and is important for the enhanced longterm declarative memory associated with emotional events [3]. Therefore, in the present study we examined the effects of priming of the BLA on synaptic plasticity in the DG by measuring both EPSP-LTP and PS-LTP. In addition, we characterized the temporal pro®le of these effects by recording ®eld potential responses at different time points following a high-frequency stimulation (HFS) to the PP (at 30, 90,

0304-3940/99/$ - see front matter q 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S03 04 - 394 0( 9 9) 00 48 8- 7

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Fig. 1. A schematic indication of the location of the BLA and cortex stimulating electrodes. After the completion of the experiment, the rats were given marking lesions of the BLA and the cortex. The ®gure shows a coronal view at a position 2.8-mm posterior to bregma. The locations of the BLA (A) and cortex (B) are indicated by solid black circles.

150 and 180 min post-HFS). To test for the speci®city of stimulating the amygdala, a second control group was added in which we stimulated the area adjacent to the rhinal ®ssure (the cortex group).

Adult, male Sprague±Dawley rats were anaesthetized (40% urethane, 5% chloral hydrate in saline, 0.5 ml/100 g, i.p.), and mounted in a Steolting stereotaxic frame. The scalp was incised and retracted, and the head position was adjusted to place bregma and lambda in the same horizontal plane. Small burr holes (2 mm diameter) were drilled unilaterally in the skull for the placement of recording and stimulating electrodes. A recording microelectrode (glass, tip diameter 2±5 mm, ®lled with 2 M NaCl, resistance 1±4 MV) was placed in the DG (coordinates: 4 mm posterior, 2.5 mm lateral to bregma, depth adjusted to yield largest EPSP response to stimulation of the PP). A bipolar 125 mm stimulating electrode was implanted in the PP (coordinates: 8 mm posterior, 4 mm lateral to bregma, depth adjusted to yield maximal response of the DG). In the BLA group, a second stimulating electrode was implanted in the BLA (coordinates: 3 mm posterior, 5.3 mm lateral to bregma, depth 7.4 mm). In the cortex group, the second stimulating electrode was implanted near the rhinal ®ssure (coordinates: 5 mm posterior, 6.5 mm lateral to bregma, depth 6.2 mm). Test stimuli (monopolar pulses, 100 ms duration, intensity adjusted to yield a population spike of 30±50% of the maximal pre-tetanus value) were delivered at 0.1 Hz. After positioning the electrodes, the rat was left for 20 min before commencing the experiment. Evoked responses were digitized (10 kHz) and analyzed using the Cambridge Electronic Design 14011 and its

Fig. 2. The potentials recorded from the DG immediately before HFS to the PP (left) and 180 min post-HFS (right). (A) Evoked potentials in the control group before HFS to the PP; (B) evoked potentials in the control group 180 min post-HFS to the PP. (C) Evoked potentials in the BLA group before HFS to the PP. (D) Evoked potentials in the BLA group 180 min post-HFS to the PP.

I. Akirav, G. Richter-Levin / Neuroscience Letters 270 (1999) 83±86

Fig. 3. Changes in the EPSP slope and the PS size following BLA priming. The increase in EPSP slope was measured as a percentage of baseline values immediately before the tetanus. The PS amplitude increase was measured as the difference in milliVolts between pre-tetanic and post-tetanic values. Field potential recordings were taken 30, 90, 150 and 180 min after the induction of HFS to the PP. In the control group (n ˆ 7) HFS (three sets of 10 trains, each one consists of 10 pulses at 100 Hz) was applied to the PP axons. In the BLA group (n ˆ 7) a stimulation to the BLA (10 trains of ®ve pulses at 100 Hz) was applied 30 s before HFS to the PP. (A) Stimulation of the BLA did not signi®cantly increase the EPSP-LTP relative to the control group 30 min post-HFS to the PP. However, at 90, 150 and 180 min post-HFS the EPSP-LTP of the BLA group was signi®cantly enhanced relative to the control group. Stimulation of the BLA signi®cantly increased the EPSP-LTP relative to the cortex group at all the time points tested. Moreover, there was no signi®cant difference between the control and the cortex group at all the time points tested. (B) Stimulation of the BLA did not signi®cantly increase the PS-LTP relative to the control or the cortex groups 30 min post-HFS to the PP. However, at 90, 150 and 180 min post-HFS, the PS-LTP of the BLA group was signi®cantly enhanced relative to the control and the cortex groups. Moreover, there was no signi®cant difference between the control and the cortex group at all the time points tested.

Spike2 software. Off-line measurements were made of the amplitude of the PS and the slope of the EPSP, using averages of ®ve successive responses to a given stimulation intensity applied at 0.1 Hz. The increase in EPSP slope was measured as a percentage of baseline values immediately before the tetanus. The PS amplitude increase was measured

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as the difference in milliVolts between pre-tetanic and posttetanic values. During the course of the experiment, body temperature was monitored and maintained at 37 ^ 0:58C by a feedback regulated heating pad. LTP was induced by a `theta'-like stimulation (HFS) to the PP (three sets of 10 trains, each train consisting of 10 pulses at 100 Hz; inter-train interval: 200 ms; inter-set interval: 1 min). The BLA and the cortex groups received a tetanic stimulation (10 trains of ®ve pulses at 100 Hz, inter-train interval: 200 ms) 30 s before HFS was applied. Potentials were recorded from the DG 30, 90, 150 and 180 min after the tetanus. Histological veri®cation of the stimulating electrode location was performed on all the rats that had been implanted with a stimulating electrode in the BLA or the cortex. After electrophysiological testing, marking lesions were made by passing anodal currents (10 mA for 3 s) to the metal bipolar stimulating electrode. Brains were removed, post-®xed over 3 nights in formaldehyde (10%), and sectioned (120 mm) on a sledge microtome. The sections were mounted on gelatin-coated slides, stained in cresyl violet, dehydrated, and cover-slipped. The electrode tract and lesion locations were then identi®able under a light microscope examination (Fig. 1). For statistical analysis, we used 3 £ 4 (treatment (time post-HFS) mixed ANOVA with LSD multiple comparison post-hoc tests. In control animals (n ˆ 7), applying HFS induced a signi®cant potentiation of the EPSP slope (one sample ttest at 30 min post-HFS: t…6† ˆ 3:05, P ˆ 0:0225, signi®cantly different from zero). The potentiation lasted for at least 3 h. A comparison between the EPSP-LTP in the BLA and control and cortex groups revealed a signi®cant group effect (Figs. 2 and 3A; F…2; 16† ˆ 5:81, P ˆ 0:013). Post-hoc tests indicated that, except for 30 min post-HFS to the PP, the stimulation of the BLA (n ˆ 7) signi®cantly increased the EPSP-LTP relative to the control group at 90 (P ˆ 0:046), 150 (P ˆ 0:032), and 180 min (P ˆ 0:028) post-HFS. There was no signi®cant difference between the control and the cortex group (n ˆ 5). Moreover, EPSP-LTP in the BLA group was signi®cantly increased also relative to the cortex group at all the time points tested (at 30 min: P ˆ 0:034; at 90 min: P ˆ 0:042; at 150 min; P ˆ 0:029; at 180 min: P ˆ 0:003). A comparison between the PS-LTP in the BLA, control and cortex groups revealed a signi®cant group effect (Figs. 2 and 3B; F…2; 16† ˆ 5:97, P ˆ 0:012). Post-hoc tests indicated that, except for 30 min post-HFS to the PP, the stimulation of the BLA signi®cantly increased the PS-LTP relative to the control and the cortex groups at 90 min post-HFS (control: P ˆ 0:017; cortex: P ˆ 0:022), at 150 min (control: P ˆ 0:024; cortex: P ˆ 0:006), and at 180 min (control: P ˆ 0:018; cortex: P ˆ 0:003) post HFS. The BLA priming stimulation enhanced both the EPSP-

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LTP and the PS-LTP in the DG relative to the control group, an effect that was statistically signi®cant from 90-min postHFS and onwards. The current results provide additional evidence of BLA-hippocampus interactions in¯uencing neuroplasticity [6±8], by demonstrating BLA priming effects not only on PS-LTP but also on EPSP-LTP, which is considered a more direct re¯ection of synaptic plasticity. Furthermore, the results indicate that the enhancing effect is long lasting (.3 h). Interestingly, posttraining activation of the amygdala can produce memory enhancement or impairment [16±18]. Assuming that LTP represents some aspects of learning and memory in the hippocampus [2], these observations may support the notion that the BLA is involved in the modulation of hippocampal-dependent in¯uences on memory storage [17]. Since the amygdala plays a critical role in emotional and motivational aspects of behavior [10], activity-dependent facilitation of hippocampal synaptic plasticity may serve as a useful model for studying the neural mechanisms underlying memory enhancement associated with emotion [12]. Recent studies [14] point to intracellular mechanisms that enable transient synaptic changes to be stabilized if they occur in close temporal proximity to important events. The emotionally activated amygdala may serve as a marker for important events, processed by the hippocampus, to be stabilized and thus remembered. The work was supported by a BSF grant No. 96±291 to G.R.-L. [1] Aggelton, J.P. and Mishkin, M., The amygdala: sensory gateway to the emotions. In R. Plutchik and H. Kellerman (Eds.), Emotion: Theory, Research and Experience, Vol. 3, Academic Press, Orlando, FL, 1986, pp. 281±299. [2] Bliss, T.V.P. and Collingridge, G.L., A synaptic model of memory: long-term potentiation in the hippocampus. Nature, 361 (1993) 31±39. [3] Cahill, L. and McGaugh, J.L., Mechanisms of emotional arousal and lasting declarative memory. Trends Neurosci., 21 (1998) 294±299. [4] Finch, D.M., Wong, E.E., Derian, E.L., Chen, X.H., NowlinFinch, N.L. and Brothers, L.A., Neurophysiology of limbic system pathways in the rat: projections from the amygdala to the entorhinal cortex. Brain Res., 370 (1986) 273±284. [5] Gallagher, M., Kapp, B.S., Musty, R.E. and Driscoll, P.A.,

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