Role of hippocampal NO in the acquisition and consolidation of inhibitory avoidance learning.

Nitric oxide (NO), an unconventional neurotransmitter in the brain, has been postulated as a retrograde intercellular messenger necessary for the induction, but not the maintenance phase, of activity-dependent forms of synaptic plasticity in the hippocampus. Here we report on the effects of an inhibitory avoidance learning task on hippocampal NO synthase (NOS) activity and on the effects of intrahippocampal infusion of a NOS inhibitor in the acquisition and consolidation of this task in rats. NOS activity increases by 45% in the hippocampus immediately after training (0 min) but not at 60 min after training. No changes were observed in cerebellar NOS activity. The bilateral intrahippocampal microinjection of nitro-arginine (NO-arg), an NOS inhibitor, provoked retrograde amnesia for the inhibitory avoidance when given 10 min before or immediately after training, but not 60 min after training. These results suggest that NO-regulated processes in the hippocampus play an important role at the time of training or very shortly thereafter of an inhibitory avoidance learning.

Memory enhancement by intrahippocampal, intraamygdala, or intraentorhinal infusion of platelet-activating factor measured in an inhibitory avoidance task.

Platelet-activating factor (PAF; 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine), which is thought to be a retrograde messenger in long-term potentiation (LTP), enhances glutamate release and LTP through an action on presynaptic nerve endings. The PAF antagonist BN 52021 blocks CA1 LTP in hippocampal slices, and, when infused into rat dorsal hippocampus pre- or posttraining, blocks retention of inhibitory avoidance. Here we report that memory is affected by pre- or posttraining infusion of the PAF analog 1-O-hexadecyl-2-N-methylcarbamoyl-sn-glycerol-3-phosphocholine (mc-PAF) into either rat dorsal hippocampus, amygdala, or entorhinal cortex. Male Wistar rats were implanted bilaterally with cannulae in these brain regions. After recovery from surgery, the animals were trained in step-down inhibitory avoidance or in a spatial habituation task and tested for retention 24 h later. mc-PAF (1.0 microgram per side) enhanced retention test performance of the two tasks when infused into the hippocampus before training without altering training session performance. In addition, mc-PAF enhanced retention test performance of the avoidance task when infused into (i) the hippocampus 0 but not 60 min after training; (ii) the amygdala immediately after training; and (iii) the entorhinal cortex 100 but not 0 or 300 min after training. In confirmation of previous findings, BN 52021 (0.5 microgram per side) was found to be amnestic for the avoidance task when infused into the hippocampus or the amygdala immediately but not 30 or more minutes after training or into the entorhinal cortex 100 but not 0 or 300 min after training. These findings support the hypothesis that memory involves PAF-regulated events, possibly LTP, generated at the time of training in hippocampus and amygdala and 100 min later in the entorhinal cortex.

Experiments suggesting a role for nitric oxide in the hippocampus in memory processes.

Nitric oxide (NO) has been proposed to be involved in the induction of long-term potentiation (LTP) and in other processes. When coupled with weak tetanic stimulation, NO produces a long-term synaptic enhancement on its own. N-Nitroarginine (NO-Arg) inhibits NO-synthase, the enzyme that produces NO, and blocks LTP in hippocampal slices. We investigated the effect on memory of the pre- or post-training infusion of NO-Arg and of the post-training infusion of the No donor, S-nitroso-N-acetylpenicillamine (SNAP) into the hippocampus. Male Wistar rats were implanted bilaterally with cannulae in the dorsal hippocampus. After recovery from surgery, the animals were trained in step-down inhibitory avoidance using a 0.4-mA footshock and tested for retention 24 h later. NO-Arg (2.0 microgram) hindered retention test performance when infused either before or immediately after training, but not 30 or 60 min later. SNAP (5.0 microgram) enhanced retention test performance when given 0, 60, or 150 min, but not 300 min, after training. The results suggest that memory storage depends on NO-sensitive processes in the hippocampus, perhaps, as suggested in previous papers, LTP generated at the time of training.

Evidence for the involvement of hippocampal CO production in the acquisition and consolidation of inhibitory avoidance learning.

Carbon monoxide (CO), produced through the action of haem oxygenase (HO) isoenzymes, has been recently postulated as a retrograde messenger in the early stages of long-term potentiation (LTP). In the present study, rats submitted to an inhibitory avoidance task there is a significant increase (+76%) in hippocampal HO activity immediately after training (0 min), but not at 60 min post-training. No changes were observed in cerebral cortical and cerebellar HO activity. Bilateral intrahippocampal infusion of the HO inhibitor zinc-protoporphyrin-IX (ZnPP) (2 micrograms side-1) caused full amnesia for inhibitory avoidance when given 10 min before training or immediately after training, but not 60 min after training. These findings provide evidence that CO production in the hippocampus is important for the early stages of memory processing of an inhibitory avoidance training.

Intrahippocampal, but not intra-amygdala, infusion of an inhibitor of heme oxygenase causes retrograde amnesia in the rat.

Zinc protoporhyrin-9 (ZnPP) is an inhibitor of heme oxygenase, the enzyme involved in the biosynthesis of carbon monoxide (CO). CO regulates the activity of glutamatergic synapses and has been proposed to play a role in the early phases of long-term potentiation. The present paper reports on the effect of ZnPP on memory of inhibitory avoidance and of habituation to a novel environment. The bilateral infusion of ZnPP (2 micrograms/side) into the dorsal hippocampus caused amnesia for the inhibitory avoidance task when given before training or 0 or 30 min, but not 60 or 100 min, after training. The immediate post-training intrahippocampal infusion of ZnPP also caused amnesia for the habituation task. The immediate post-training intra-amygdala infusion of ZnPP had no effect on retention of the avoidance task. The data are consistent with the hypotheses that memory involves long-term potentiation initiated at the time of training in the hippocampus, and that hippocampal but not amygdala long-term potentiation may be regulated by CO.

Effect of antagonists of platelet-activating factor receptors on memory of inhibitory avoidance in rats.

Platelet-activating factor (PAF) is present in the brain. It enhances glutamate release and long-term potentiation (LTP) through an action on synaptic membrane receptors sensitive to the antagonist, BN 52021, and has been proposed as a retrograde messenger in the genesis of LTP. In addition, PAF has other, metabolic actions mediated by microsomal receptors sensitive to the antagonist, BN 50730. We investigated the effect on memory of the pre- or post-training infusion of BN 52021 or BN 50730 into the hippocampus and that of BN 52021 in the amygdala and the entorhinal cortex. Male Wistar rats were implanted bilaterally with cannulae aimed at these brain regions. After recovery from surgery, the animals were trained in step-down inhibitory avoidance using a 0.5-mA foot shock and tested for retention 24 h later. BN 52021 (0.5 microgram/side) was amnestic when given into the hippocampus or the amygdala either before or immediately after training but not 30 or 100 min later. BN 52021 was also amnestic when given into the entorhinal cortex 100 but not 0 or 300 min after training. Intrahippocampally administered BN 50730 had no effect on memory. The findings are compatible with the suggestion from previous findings that memory of this task depends on the generation of LTP at the time of training in hippocampus and amygdala and, 90-180 min later, in the entorhinal cortex.

Intrahippocampal or intraamygdala infusion of KN62, a specific inhibitor of calcium/calmodulin-dependent protein kinase II, causes retrograde amnesia in the rat.

We investigated the effect of a bilateral post-training intracerebral infusion of KN62, a specific inhibitor of calcium/calmodulin-dependent protein kinase II (CaM-II), on memory. This enzyme plays a crucial role in the early phases of long-term potentiation. Male Wistar rats were implanted bilaterally with cannulae aimed at the CA1 region of the dorsal hippocampus or at the junction between the central and the basolateral nuclei of the amygdala. After recovery, rats were trained in step-down inhibitory avoidance using a 0.5-mA footshock and tested for retention 24 h later. At various times after training (0, 30, 120, or 240 min for the animals implanted into the hippocampus; 0 or 240 min for the animals implanted in the amygdala) they received, through the cannulae, an infusion of vehicle (0.1% dimethylsulfoxide in water) or KN62 (100 mumol/side). KN62 caused full retrograde amnesia when given 0 min after training into either the amygdala or the hippocampus. When given into the hippocampus 30 min post-training it had a partial amnestic effect. When given 120 min after training into the hippocampus, or 240 min after training into either structure, KN62 had no effect. The data suggest that the early phase of memory requires intact CaM-II activity in the amygdala and hippocampus and support the hypothesis that memory involves long-term potentiation initiated at the time of training in both structures.

Gamma-endorphin affects retrieval of an inhibitory avoidance task.

Rats were trained in a step-down inhibitory avoidance task and retrieval was measured during a test session conducted 24 h after training. The ip administration of a low dose of gamma-endorphin (0.2 micrograms/kg) immediately after training reduced retrieval time from 40.6 to 13.5 s (N = 15). Higher doses of gamma-endorphin given 5 min before testing (1.0 micrograms/kg) or immediately after training (5 micrograms/kg) enhanced retrieval time from 38.6 to 300 s (N = 15) and from 40.6 to 104.4 s (N = 15). All of these effects were blocked by pretreatment with naloxone (0.4 mg/kg). Alpha-endorphin (0.2, 1.0 or 5.0 micrograms/kg) administered either after training or before testing had no effect on retrieval. Since at least the amnestic effect of gamma-endorphin is similar to that of beta-endorphin, and gamma-endorphin is a possible metabolite of beta-endorphin by limited proteolysis of the carboxyl-terminal amino acid, it is suggested that at least some effects of beta-endorphin on memory may be mediated by its proteolysis product, gamma-endorphin.

Chronic ethanol ingestion selectively affects memory modulation in rats.

Post-training treatment alters memory by different mechanisms. Naloxone enhances memory by antagonism of endogenous beta-endorphin-induced state dependency. Epinephrine facilitates consolidation at low doses and generates state dependency at high doses. Exposure to a session of tones causes retroactive interference through a cognitive effect. The present data show that chronic ethanol ingestion, in rats, inhibited the post-training effect of naloxone and of a high dose of epinephrine on the retention of an inhibitory avoidance task but did not inhibit retrograde interference by a session of tones or retrograde facilitation by a low dose of epinephrine. Therefore, ethanol appears to selectively affect post-training influences related to state dependency.

Chronic ethanol ingestion selectively affects memory modulation in rats

Post-training treatment alters memory by different mechanisms. Naloxone enhances memory by antagonism of endogenous ß-endorphin-induced state dependency. Epinephrine facilitates consolidation at low doses and generates state dependency at high doses. Exposure to a session of tones causes retroactive interference through a cognitive effect. The present data show that chronic ethanol ingestion, in rats, inhibited the post-training effect of naloxone and of a high dose of epinephrine on the retention of an inhibitory avoidance task but did not inhibit retrograde interference by a session of tones or retrograde facilitation by a low dose of epinephrine. Therefore, ethanol appers to selectively affect post-training influences related to state dependency

Gamma-endorphin affects retrieval of an inhibitory avoidance task

Rats were trained in a step-down inhibitory avoidance task and retrieval was measured during a test session conducted 24 h after training. The ip administration of a low dose of gamma-endorphin (0.2 microng/kg) immediately after training reduced retrieval time from 40.6 to 13.5 s (N = 15). Higher doses of gamma-endorphin given 5 min before testing (1.0 microng/kg) or immediately after training (5 microng/kg) enhanced retrieval time from 38.6 to 300 s (N = 15) and from 40.6 to 104.4 s (N = 15). All of these effects were either after training or before testing had no effect on retrieval. Since at least the amnestic effect of gamma-endorphin is similar to that of beta-endorphin, and gamma-endorphin is a possible metabolite of beta-endorphin by limited proteolysis of the carboxyl-terminal amino acid, it is suggested that at least some effects of beta-endorphin on memory may be mediated by its proteolysis product, gamma-endorphin