Behavioural Brain Research, 58 (1993) 91-98 © 1993 Elsevier Science Publishers B.V. All rights reserved. 0166-4328/93/$06.00 91 BBR 1509 Memory processing by the limbic system: role of specific neurotransmitter systems Ivan Izquierdo ~'*, Jorge H. Medina b, Marino Bianchin a, Roger Walz a, Marilene S. Zanatta a, Ricardo C. Da Silva a, Marcia Bueno E Silva a, Anelise C. Ruschel a and Natalia Paczko a a Centro de Memoria, Departamento de Bioquimica, lnstituto de Biociencias, Universidade Federal do Rio Grande do Sul, 90046-900 Porto Alegre, RS (Brazil), b Laboratorio de Neurorreceptores, Instituto de Biologia Celular, Facultad de Medicina, Universidad de Buenos Aires, Paraguay 2155, 1121 Buenos Aires (Argentina) (Received 15 January 1993) (Accepted 11 October 1993) Key words: Long-term potentiation and memory; Glutamatergic mechanisms and memory; GABAergic mechanisms and memory; Amygdala and memory; Hippocampus and memory; Medial septum and memory; Entorhinal cortex and memory Experiments using localized infusions into selected brain structures of agonists and antagonists of various synaptic receptors, given before or after behavioral training, have led to the following conclusions: (1) Memory is processed shortly after training in the amygdala, medial sep- tum and hippocampus by glutamatergic NMDA and AMPA receptors activated in that sequence. Cholinergic muscarinic receptors are acti- vated concurrently with the former. GABAA receptors modulated by brain benzodiazepines and by beta-noradrenergic receptors inhibit the process. (2) The sequential involvement of NMDA and AMPA receptors suggests that long-term potentiation (LTP) of the synapses activated by the learning experiences in the hippocampus and/or amygdala and medial septum is the crucial event. Expression of this LTP at the time of testing is necessary for retrieval: AMPA receptor blockade in the hippocampus and amygdala at the time of testing hinders retrieval. This suggests that the LTP underlies the memory process itself. (3) The amygdala, medial septum and hippocampus mediate different types of me- mory and/or different components of memories. The entorhinal cortex, through mechanisms that require intact NMDA receptors and are in- hibited by GABAA receptors, intervenes in post-training memory processing 90-180 rain after the other limbic regions. The entorhinal cortex integrates consecutively acquired memories; this role could be maintained by the LTP that is generated after training in the amygdala, hippoc- ampus and medial septum. Post-training intervention of the entorhinal cortex does not occur if this region is inhibited at the time of training. INTRODUCTION is the case with long-term memories, the maintenance phase of LTP can persist for very long periods even if Like long-term potentiation (LTP) 8'44'54'56,long-term its expression is withheld or prevented 56. The early, memories are initially labile and after a few minutes labile phase of long-term memories is often called the become stable for very long periods 25'33'46. The initial consolidation phase, because it is believed that it in- labile phase of LTP is called the induction phase. It is volves a process of strengthening of the underlying neu- mediated by N-methyl-D-aspartate (NMDA) gluta- ral processes 25"46. LTP, of course, consists of the matergic receptors 8, can be modulated by cholinergic strengthening of synaptic transmission 8'27'56. In LTP 56, muscarinic and beta-noradrenergic receptors (see refs. as in long-term memory 25'61'67, re-iteration of the stimuli 30, 42) and is inhibited by gamma-amino-butyrate type that brought them into being will trigger their expres- A (GABAA) receptors (see ref. 8). The late, stable phase sion. The expression of memories is called re- of LTP is expressed through 7-amino-3-hydroxy-5- trieval25,21,67. methyl-4-isoxazole propionate (AMPA) glutamatergic Indeed, the two most widely accepted current hy- receptors and is called the maintenance phase 8'56. As potheses of memory storage are that it involves LTP 1"54"55 and/or the operation of neural networks 39'43. * Corresponding author. Centro de Memoria, Departamento de Bio- These hypotheses are not mutually exclusive. The ex- quimica, Instituto de Biociencias, UFRGS (centro), 90050 Porto pression of LTP in the particular synapses that had Alegre, RS, Brazil. Fax: (55)(51)227-2343. been specifically activated during training could sub- 92 serve important functions in amplifying the signals that The agonist, glutamate has opposite effects to those are to enter the neural networks 27'37. In addition, LTP of AP5 and the effects of both are not restricted to the can underlie the function of components of the net- amygdala. In fact, the two drugs affect memory retro- works 1. Network theory explains the continued func- actively when infused into the hippocampus and the tioning of neural circuits through alternative pathways medial septum as well, depending on the task. Post- in spite of lesions 43, which may in turn explain the training infusion of AP5 into the amygdala, medial sep- persistence of memories after extensive brain damage turn or hippocampus blocks the consolidation of step- as is seen in the early stages of Alzheimer's disease or down inhibitory avoidance 3235. The infusion of AP5 of other organic brain syndromes 21. into the hippocampus but not into the amygdala or Lesion and drug infusion studies have shown that the medial septum causes retrograde amnesia for habitua- main areas involved in consolidation are the amygdala, tion to a novel environment32'35. The agonist, glutamate, the hippocampus, the medial septum and the entorhinal has effects exactly opposite to those of AP5. Its imme- cortex, which are parts of the so-called limbic diate post-training infusion into the amygdala, medial system 16'17"24'25"28"32-35'46. Evidence suggests that other septum or hippocampus causes retrograde facilitation regions of the brain are involved in parallel 25"64 or al- of inhibitory avoidance; its intrahippocampal but not ternatively62 to the limbic areas of the temporal lobe. its intraseptal or intra-amygdala infusion causes retro- These parallel or alternative areas include the cerebel- grade facilitation of habituation 3~. lum 64, the caudate nucleus 2"68 and various regions of Recent evidence suggests a role for cholinergic mus- the cerebral cortex 1'62. It is possible that these extra- carinic receptors in consolidation processes in addition limbic circuits specialize in types of memory not to glutamatergic receptors. A role for cholinergic handled by the limbic structures referred to above 2'62. mechanisms in memory had long been suspected mainly The present article reviews recent data from our because of the fact that systemic administration of the laboratories on the neurotransmitter mechanisms in- cholinergic muscarinic receptor antagonist, scopola- volved in memory processing by the amygdala, hippoc- mine, causes amnesia in humans and animals (see refs. ampus and medial septum, and on the interaction of 26, 38). these structures with the entorhinal cortex in the for- The comparative effect on memory consolidation of mation of long-term memories. As will be seen, the data cholinergic muscarinic antagonists or agonists given indicate that LTP in the amygdala, hippocampus and into specific brain structures was studies for the :first medial septum is at the core of the memory process. time by our group. In a step-down inhibitory avoidance task, post-training intra-amygdala, intraseptal or intra- hippocampal scopolamine administration causes retro- NEUROTRANSMITTERS INVOLVED IN MEMORY CON- SOLIDATION grade amnesia, whereas that of the agonist, oxotremo- rine, causes instead retrograde facilitation 32"35. In the Previous work in several laboratories had shown that habituation task, the intrahippocampal infusion of sco- glutamatergic synapses play a key role in memory. This polamine causes retrograde amnesia and that of ox- is not surprising since glutamatergic synapses are by far otremorine causes retrograde facilitation; intra- the most abundant excitatory synapses of the brain, amygdala or intraseptal infusions of these substances and can generate LTP. The systemic administration of was ineffective32"3s. indirect (i.e. Ca 2+ channel) blockers of NMDA (N- It is possible that cholinergic muscarinic transmis- methyl-D-aspartate) receptors to glutamic acid causes sion may act by facilitating the induction of LTP and amnesia for a variety of tasks in rats 59'65. Intracere- neighboring glutamatergic synapses 42. The amnesia broventricular infusion of the indirect NMDA receptor caused by scopolamine in humans can be alleviated by antagonist, dizolcipine, or of the direct antagonist, D-2- D-cycloserine, a modulator of the glycine site of NMDA amino-5-phosphonopentanoic acid (AP5), hinders re- receptors 38. tention of spatial learning54'55. AP5 and its heptanoic acid analogue, AP7, disrupt retention of a conditioned startle response when infused into the amygdala prior THE ROLE OF GABA A RECEPTORS to training 51. D-cycloserine, a partial agonist at the gly- cine modulatory site of NMDA receptors 57, enhances The immediate post-training infusion of the indirect memory in rats 53 and has been proposed as a useful (C1- channel) blocker of GABAA receptors, picro- drug in Alzheimer's disease TM. Sensitivity of the NMDA toxin, into the amygdala, medial septum or hippocam- receptor complex to glycine is reduced in Alzheimer's pus causes retrograde memory facilitation of inhibitory disease s7. avoidance behaviour and counteracts the amnesic ac- 93 tion of AP5 and/or scopolamine32'35. In the habitation amygdala25'28 and, at least in the amygdala, are possi- task, similar effects are found but only in the hippoc- bly mediated by beta-noradrenergic synapses 46. Recent ampus 32'35. The effect of systemic or intra-amygdala data suggest an involvement of dopaminergic synapses picrotoxin on inhibitory avoidance behavior is shared in the caudate nucleus in post-training memory pro- by another C1- channel blocker. R05-4864 (4'- cesses 68. It is not known whether dopaminergic syn- chlordiazepam) 11.