Pharmacological differences between memory consolidation of habituation to an open field and inhibitory avoidance learning.

Rats implanted bilaterally with cannulae in the CA1 region of the dorsal hippocampus or the entorhinal cortex were submitted to either a one-trial inhibitory avoidance task, or to 5 min of habituation to an open field. Immediately after training, they received intrahippocampal or intraentorhinal 0.5-microl infusions of saline, of a vehicle (2% dimethylsulfoxide in saline), of the glutamatergic N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphono pentanoic acid (AP5), of the protein kinase A inhibitor Rp-cAMPs (0.5 microg/side), of the calcium-calmodulin protein kinase II inhibitor KN-62, of the dopaminergic D1 antagonist SCH23390, or of the mitogen-activated protein kinase kinase inhibitor PD098059. Animals were tested in each task 24 h after training. Intrahippocampal KN-62 was amnestic for habituation; none of the other treatments had any effect on the retention of this task. In contrast, all of them strongly affected memory of the avoidance task. Intrahippocampal Rp-cAMPs, KN-62 and AP5, and intraentorhinal Rp-cAMPs, KN-62, PD098059 and SCH23390 caused retrograde amnesia. In view of the known actions of the treatments used, the present findings point to important biochemical differences in memory consolidation processes of the two tasks.

Pharmacological differences between memory consolidation of habituation to an open field and inhibitory avoidance learning

Rats implanted bilaterally with cannulae in the CA1 region of the dorsal hippocampus or the entorhinal cortex were submitted to either a one-trial inhibitory avoidance task, or to 5 min of habituation to an open field. Immediately after training, they received intrahippocampal or intraentorhinal 0.5-æl infusions of saline, of a vehicle (2 percent dimethylsulfoxide in saline), of the glutamatergic N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphono pentanoic acid (AP5), of the protein kinase A inhibitor Rp-cAMPs (0.5 æg/side), of the calcium-calmodulin protein kinase II inhibitor KN-62, of the dopaminergic D1 antagonist SCH23390, or of the mitogen-activated protein kinase kinase inhibitor PD098059. Animals were tested in each task 24 h after training. Intrahippocampal KN-62 was amnestic for habituation; none of the other treatments had any effect on the retention of this task. In contrast, all of them strongly affected memory of the avoidance task. Intrahippocampal Rp-cAMPs, KN-62 and AP5, and intraentorhinal Rp-cAMPs, KN-62, PD098059 and SCH23390 caused retrograde amnesia. In view of the known actions of the treatments used, the present findings point to important biochemical differences in memory consolidation processes of the two tasks

NMDA receptor antagonism in the basolateral amygdala blocks enhancement of inhibitory avoidance learning in previously trained rats.

Extensive evidence suggests that N-methyl-D-aspartate (NMDA) glutamate receptor channels in the amygdala are involved in fear-motivated learning, and infusion of NMDA receptor antagonists into the amygdala blocks memory of fear-motivated tasks. Recent studies have shown that previous training can prevent the amnestic effects of NMDA receptor antagonists on spatial learning. In the present study, we evaluated whether infusion of the NMDA antagonist D,L-2-amino-5-phosphonopentanoic acid (AP5) into the basolateral nucleus of the amygdala (BLA) impairs reinforcement of inhibitory avoidance learning in rats given previous training. Adult male Wistar rats (220-310 g) were bilaterally implanted under thionembutal anesthesia (30 mg/kg, i.p.) with 9.0-mm guide cannulae aimed 1.0 mm above the BLA. Infusion of AP5 (5.0 microg) 10 min prior to training in a step-down inhibitory avoidance task (0.4 mA footshock) blocked retention measured 24 h after training. When infused 10 min prior to a second training session in animals given previous training (0.2 mA footshock), AP5 blocked the enhancement of retention induced by the second training. Control experiments showed that the effects were not due to alterations in motor activity or footshock sensitivity. The results suggest that NMDA receptors in the basolateral amygdala are involved in both formation of memory for inhibitory avoidance and enhancement of retention in rats given previous training.

Short- and long-term memory are differentially affected by metabolic inhibitors given into hippocampus and entorhinal cortex.

Rats were implanted with cannulae in the CA1 area of the dorsal hippocampus or in the entorhinal cortex and trained in one-trial step-down inhibitory avoidance. Two retention tests were carried out in each animal, one at 1.5 h to measure short-term memory (STM) and another at 24 h to measure long-term memory (LTM). The purpose of the present study was to screen the effect on STM of various drugs previously shown to affect LTM of this task when given posttraining at the same doses that were used here. The drugs and doses were the guanylyl cyclase inhibitor LY83583 (LY, 2.5 microMg), the inhibitor of Tyr-protein kinase at low concentrations and of protein kinase G (PKG) at higher concentrations lavendustin A (LAV, 0.1 and 0.5 microMg), the PKG inhibitor KT5823 (2.0 microMg), the protein kinase C (PKC) inhibitor staurosporin (STAU, 2.5 microMg), the inhibitor of calcium/ calmodulin protein kinase II (CaMKII) KN62 (3.6 microMg), the protein kinase A (PKA) inhibitor KT5720 (0.5 microMg), and the mitogen-activated protein kinase kinase (MAPKK) inhibitor PD098059 (PD, 0.05 microMg). PD was dissolved in saline; all the other drugs were dissolved in 20% dimethyl sulfoxide. In all cases the drugs affected LTM as had been described in previous papers. The drugs affected STM and LTM differentially depending on the brain structure into which they were infused. STM was inhibited by KT5720, LY, and PD given into CA1 and by STAU and KT5720 given into the entorhinal cortex. PD given into the entorhinal cortex enhanced STM. LTM was inhibited by STAU, KN62, KT5720, KT5823, and LAV (0.5 microMg) given into CA1 and by STAU, KT5720, and PD given into the entorhinal cortex. The results suggest that STM and LTM involve different physiological mechanisms but are to an extent linked. STM appears to require PKA, guanylyl cyclase, and MAPKK activity in CA1 and PKA and PKC activity in the entorhinal cortex; MAPKK seems to play an inhibitory role in STM in the entorhinal cortex. In contrast, LTM appears to require PKA and PKC activity in both structures, guanylyl cyclase, PKG, and CaMKII activity in CA1, and MAPKK activity in the entorhinal cortex.

Time-dependent impairment of inhibitory avoidance retention in rats by posttraining infusion of a mitogen-activated protein kinase kinase inhibitor into cortical and limbic structures.

Mitogen-activated protein kinase (MAPK) is abundantly expressed in postmitotic neurons of the developed nervous system. MAPK is activated and required for induction of long-term potentiation (LTP) in the CA1 area of the hippocampus, which is blocked by the specific inhibitor of the MAPK kinase, PD 098059. Recently it was demonstrated that MAPK is activated in the hippocampus after training and is necessary for contextual fear conditioning learning. The present work tests the role of the MAPK cascade in step-down inhibitory avoidance (IA) retention. PD 098059 (50 microM) was bilaterally injected (0.5 microl/side) into the CA1 region of the dorsal hippocampus or entorhinal cortex at 0, 90, 180, or 360 min, or into the amygdala or parietal cortex at 0, 180, or 360 min after IA training in rats using a 0.4-mA foot shock. Retention testing was carried out 24 h after training. PD 098059 impaired retention when injected into the dorsal hippocampus at 180 min, but not 0, 90, and 360 min after training. When infused into the entorhinal cortex, PD 098059 was amnestic at 0 and 180 min, but not at 90 and 360 min after training. The MAPKK inhibitor also impairs IA retention when infused into the parietal cortex immediately after training, but not at 180 or 360 min. Infusions performed into amygdala were amnestic at 180 min, but not at 0 and 360 min after training. Our results suggest a time-dependent involvement of the MAPK cascade in the posttraining memory processing of IA; the time dependency is different in the hippocampus, amygdala, entorhinal cortex, or parietal cortex of rats.

Differential effects of post-training muscimol and AP5 infusions into different regions of the cingulate cortex on retention for inhibitory avoidance in rats.

Adult male Wistar rats were bilaterally implanted with indwelling cannulae in four different coordinates of the cingulate cortex: (1) the anterior cingulate (AC), (2) the rostral region of the posterior cingulate (RC), (3) the upper portion of the caudal region of the posterior cingulate (UC), and (4) the lower portion of the caudal region of the posterior cingulate (LC). After recovery, animals were trained in a step-down inhibitory avoidance task (3.0-s, 0.4-mA foot shock). Either immediately, or 90 or 180 min after training, animals received a 0.5-microl infusion of vehicle (phosphate buffer, pH 7.4), of muscimol (0.5 microg), or of AP5 (5.0 microg). Retention testing was carried out 24 h after training. Muscimol was amnestic when given into any of the three coordinates of the posterior cingulate cortex 90 min after training, and when given into LC immediately post-training. In addition, AP5 was amnestic when given into UC 90 min post-training, but not when given into any other region and/or at any other time. None of the treatments had any effect when given into AC. The results suggest that memory processing of the inhibitory avoidance task is regulated by the posterior but not by the anterior cingulate cortex, through muscimol-sensitive synapses, relatively late after training. AP5-sensitive synapses appear to play a very limited role in these processes, restricted to UC.

Stimulators of the cAMP cascade reverse amnesia induced by intra-amygdala but not intrahippocampal KN-62 administration.

Infusion of the calcium-calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-62 (3.5 ng/side) 0 h after training into rat hippocampus CA1 or amygdala has been known for years to cause retrograde amnesia for step-down inhibitory avoidance. On the other hand, drugs that indirectly stimulate protein kinase A (PKA) (8-Br-cAMP, 1.25 microg/side; norepinephrine, 0.3 microg/side; the dopamine D1 receptor agonist, SKF38393, 7.5 microg/side) infused 3 h posttraining into CA1 but not amygdala markedly facilitate retention of this task. Here we find that 8-Br-cAMP, norepinephrine, or SKF38393 given 3 h posttraining into rat CA1 reverses the amnestic effect of KN-62 given into the amygdala 0 h after training, but not that of KN-62 given into CA1 0 h posttraining. The findings bear on the participation of CaMKII and of the cAMP/PKA cascade in memory processes in the hippocampus and the amygdala. Both cascades have been proposed to play a role in memory: CaMKII in the early phase and PKA in the transition between the early phase and long-term memory. Clearly, in CA1, both cascades are involved and are crucial, and the CaMKII cascade must precede the PKA cascade. In contrast, in the amygdala, only the CaMKII cascade is active, and it does not play a central role in memory, inasmuch as its deleterious effect may be fully recovered by stimulation of the PKA cascade in the hippocampus. This further supports the contention that the hippocampus is essential for memory formation of this task, as it is for many others, whereas the amygdala appears to play instead an early modulatory role.

Effects of post-training infusions of a mitogen-activated protein kinase kinase inhibitor into the hippocampus or entorhinal cortex on short- and long-term retention of inhibitory avoidance.

We recently demonstrated the time-dependent impairment of long-term retention of a step-down inhibitory avoidance task in rats induced by post-training infusion of the specific MAPKK (mitogen-activated protein kinase kinase) inhibitor PD 098059 into the hippocampus (HIP), amygdala (AMY), entorhinal cortex (EC) and posterior parietal cortex (PPC). Here we investigate the role of the MAPK cascade in the HIP and the EC on both short- and long-term retention of inhibitory avoidance in rats, using three different doses of the MAPKK inhibitor PD 098059. Adult male Wistar rats were trained and tested in inhibitory avoidance and given an infusion of PD 098059 (0.5, 5.0 or 50.0 microM) at 0, 30, 90, 120, 180, 270 or 360 min after training. A retention test session was carried out at 90, 180 or 270 min after training (short-term memory, STM) and/ or 24 h after training (long-term memory, LTM). When infused into the HIP at 0 min, but not at 30, 90, 120 or 180 min after training, PD 098059 impaired STM. Intrahippocampal PD 098059 impaired LTM when infused at 180 min, but not at 0, 30, 90, 120 or 270 min after training. When infused into the EC, PD 098059 enhanced STM when given at 0 min after training and had no effect when given at 30, 90, 120 or 180 min after training. In this structure, PD 098059 impaired LTM when given at 180 or 270 min, but not at 30, 90, 120 or 360 min after training. All effects were dose-dependent. These findings indicate that the MAPK cascade in the HIP and EC is differentially involved in short- and long-term retention of inhibitory avoidance in rats.

Differential involvement of cortical receptor mechanisms in working, short-term and long-term memory.

Rats received, through bilaterally implanted indwelling cannulae, 0.5 microliter infusions of 6-cyano-7-nitroquinoxaline2,3-dione (CNQX) (0.5 microgram), D-2-amino-5-phophono pentanoic acid (AP5) (5.0 micrograms), muscimol (0.5 microgram), scopolamine (2.0 micrograms), SCH23390 (2.5 micrograms), saline or a vehicle into the CA1 region of the hippocampus, or into the antero-lateral prefrontal (PRE), posterior parietal (PP) and entorhinal cortex (EC). The infusions were given 6 min prior to one-trial step-down inhibitory avoidance training in order to measure their effect on working memory (WM), or immediately post-training in order to measure their effect on short-term (STM) and long-term memory (LTM), 1.5 and 24 h later, respectively. WM was inhibited by CNQX or muscimol given into any of the cortical areas, by SCH23390 given into CA1, PRE or PP, and by scopolamine given into PRE or EC. STM was unaffected by any of the treatments given into PRE, and was inhibited by CNQX or muscimol given into CA1, PP and EC and by scopolamine given into PP, and enhanced by SCH given into CA1. LTM was inhibited by CNQX, muscimol, scopolamine or SCH23390 given into PRE, by scopolamine given into PP, by SCH23390 given into the entorhinal cortex, and by AP5, CNQX, muscimol or scopolamine given into CA1. The results indicate a differential involvement of the various neurotransmitter systems in the three types of memory in the various brain areas, and a separation of the mechanisms and of the regions involved in each. In addition, some of the findings suggested links between WM and LTM processing in PRE, between WM and STM processing in EC and PP, and between all three types of memory in CA1.

Intrahippocampal infusion of the NMDA receptor antagonist AP5 impairs retention of an inhibitory avoidance task: protection from impairment by pretraining or preexposure to the task apparatus.

The purpose of the present research was to further evaluate the role of hippocampal N-methyl-D-aspartate (NMDA) receptors in inhibitory avoidance task consolidation. Adult male Wistar rats were trained and tested in a step-down inhibitory avoidance task (0.4 mA footshock, 24 h training-test interval). Immediately after training, animals received a 0.5-microl intrahippocampal infusion of the NMDA receptor antagonist aminophosphonopentanoic acid (AP5) (5.0 microg) or vehicle (phosphate buffer in saline, pH 7.4). The infusion of AP5 impaired retention test performance. Both pretraining with a low footshock intensity (0.2 mA) or preexposure to the inhibitory avoidance box 24 h before training prevented the amnestic effect of AP5. The results suggest that hippocampal NMDA receptors are critical for neither the enhancement of retention induced by an additional training session nor the inhibitory avoidance retention in animals that have previously learned about the task environment by preexposure to the apparatus.