cGMP, but not cAMP, in rat hippocampus is involved in early stages of object memory consolidation

Impaired memory following predatory stress in mice is improved by fluoxetine

The first purpose of the present study was to investigate possible effects of predatory stress (i.e., 5-min cat exposure) on short-term learning abilities in Swiss mice using the object recognition test (ORT). The second aim was to evaluate the effects of anxiolytics (i.e., diazepam and fluoxetine) on learning/memory abilities in the ORT following predatory stress. Results showed that predatory exposure impaired learning and produced amnesia of acquired information or impairment to retrieve learned information (48 and 96 h poststressor). The learning impairment in the ORT in stressed mice was restored by acute fluoxetine treatment, but not by diazepam that instead affected learning in nonstressed animals. Taken together, these findings indicate that this animal model of exposure of mice to unavoidable predatory stimuli produces early cognitive changes analogous to those seen in patients with acute stress disorder (ASD).

Neurotoxic methamphetamine regimen severely impairs recognition memory in rats

Methamphetamine (mAMPH), when administered repeatedly to rodents or primates, is neurotoxic to some cortical neurons and to forebrain dopaminergic and serotonergic axon terminals. The aim of the present study was to investigate the effects of a neurotoxic regimen of mAMPH on two hippocampus-dependent memory tasks: object recognition, a nonspatial memory task, and the Morris water maze, a spatial memory task. Male rats were treated with mAMPH (4 x 4.0 mg/kg, s.c.) or saline and trained in the object recognition task 1 week and 3 weeks later. During training, animals explored two identical copies of the same object. In retention test sessions one of the objects was replaced by a novel object. mAMPH-treated rats showed no recognition memory during the short-term memory (STM) test, given 90 min after the training session, and showed marked impairments in the long-term memory (LTM) test, given 24 h after training. Even 3 weeks after drug injections, the mAMPH-treated animals were unable to discriminate between the novel and familiar objects during both STM and LTM tests. Despite the severe deficits observed in the recognition memory, no effects of prior mAMPH treatment were seen in the water maze task. Damage to monoamine terminals was confirmed by significant 30-40% losses of [(125)I]RTI-55 binding to striatal dopamine transporter and hippocampal serotonin transporter sites at both 1 and 3 weeks after mAMPH treatments. Thus, administration of mAMPH restricted to a single day can produce a profound, persistent, and selective deficit in a nonspatial hippocampus-dependent memory.

On the role of nitric oxide in hippocampal long-term potentiation

Nitric oxide (NO) functions in several types of synaptic plasticity, including hippocampal long-term potentiation (LTP), in which it may serve as a retrograde messenger after postsynaptic NMDA receptor activation. In accordance with a prediction of this hypothesis, and with previous findings using guinea pig tissue, exogenous NO, when paired with a short tetanus (ST) to afferent fibers, generated a stable NMDA receptor-independent potentiation of rat CA1 hippocampal synaptic transmission that occluded LTP. Contrary to predictions, however, the pairing-induced potentiation was abolished in the presence of NO synthase inhibitors, indicating that endogenous NO is required for exogenous NO to facilitate LTP. Periodic application of NO while endogenous NO synthesis was blocked indicated that a tonic low level is necessary on both sides of the NO-ST pairing for the plasticity to occur. A similar dependence on tonic NO seems to extend to LTP, because application of an NO synthase inhibitor 5 min after tetanic stimulation blocked LTP as effectively as adding it beforehand. The posttetanus time window during which NO operated was restricted to <15 min. Inhibition of the guanylyl cyclase-coupled NO receptor indicated that the potentiation resulting from NO-ST pairing and the NO signal transduction pathway during early LTP are both through cGMP. We conclude that NO does not function simply as an acute signaling molecule in LTP induction but has an equally important role outside this phase. The results resonate with observations concerning the role of the hippocampal NO-cGMP pathway in certain types of learning behavior.

Effects of two selective phosphodiesterase type 5 inhibitors, sildenafil and vardenafil, on object recognition memory and hippocampal cyclic GMP levels in the rat

The present study investigated the effects of two cyclic GMP-specific phosphodiesterase enzyme type 5 inhibitors, sildenafil and vardenafil, on the memory performance in the object recognition task. Both compounds were given per orally (1, 3 and 10 mg/kg sildenafil; 0.1, 0.3, 1 and 3 mg/kg vardenafil) immediately after the exposure to two identical objects. The memory for the objects was tested 24 h later. Vehicle-treated rats spent equal times exploring a new and the familiar object demonstrating that they did not remember the familiar one. However, sildenafil improved the object discrimination performance of the rats with a high discrimination performance at a dose of 3 mg/kg. Rats treated with vardenafil also showed an improved object discrimination performance. Compared with sildenafil, vardenafil appeared to be even more potent in this respect since it already produced a high discrimination performance at a dose of 0.3 mg/kg. The effects of both compounds on cyclic GMP and cyclic AMP accumulation were studied in rat hippocampal slices incubated in vitro. Cyclic GMP levels were increased after incubation with the highest concentration of 100 microM vardenafil (together with 0.1 mM sodium nitroprusside), although no changes in cyclic GMP levels were detected after incubation with different concentrations of sildenafil. Both compounds had no effect on cyclic AMP levels. Additional cyclic GMP immunocytochemistry showed that incubation with vardenafil (in the presence of sodium nitroprusside) resulted in a concentration-dependent staining of cyclic GMP. Staining was predominantly found in neuronal fibres in the hippocampal CA2/CA3 region. It was already detected at a concentration of 0.1 microM vardenafil. Also positive fibres were detected after incubation with sildenafil but at a higher concentration of 10 microM. Taken together, these results suggest that inhibition of phosphodiesterase enzyme type 5 improves object recognition memory. This effect might be explained by increased levels of central cyclic GMP.