High-frequency stimulation of the hippocampus blocks fear learning sensitization and return of extinguished fear.

Patients with post-traumatic stress disorder (PTSD) present hippocampal (HPC) dysfunction, which may facilitate fear-related phenomena such as fear learning sensitization (i.e. potentiation of fear acquisition by initial fear conditioning (FC1)) and fear return (i.e. reactivation of extinguished fear). Fear return is sensitive to HPC high-frequency stimulation (HFS) in rats. The goal of the present study was to examine whether fear learning sensitization is also sensitive to HPC HFS in rats. We found in control conditions that, after FC1 (with 15 shock administrations) and extinction, conditioning in a different context with one shock administration was potentiated (proactive effect) and associated with fear return in the initial context (retroactive effect). Both phenomena were prevented by HPC HFS applied before the second conditioning. We also found that the effect of HPC HFS on fear learning sensitization required initial extinction. These findings suggest a pivotal role of the HPC in preventing proactive and retroactive effects of successive fear conditionings. These data also support the concept that HPC deactivation may be involved in fear learning sensitization and fear return in PTSD patients.

Low-frequency stimulation of the hippocampus following fear extinction impairs both restoration of rapid eye movement sleep and retrieval of extinction memory.

Post-learning rapid eye movement (REM) sleep deprivation has often been shown to impair hippocampal functioning, which results in deficit in retrieval of some types of memory. However, it remains to be determined whether post-learning alteration of hippocampal functioning affects, in turn, REM sleep. Recent studies have shown that both post-extinction REM sleep deprivation and post-extinction application of hippocampal low-frequency stimulation (LFS) impair memory of fear extinction, indicating possible bidirectional interactions between hippocampal functioning and REM sleep. To analyze the potential effect of post-extinction alteration of hippocampal functioning on REM sleep, rats were implanted with stimulating electrodes in the dorsal hippocampus for post-extinction LFS. Sleep was recorded before (two sessions, 1 day apart) and after conditioning (five tone and eyelid-shock pairings), and following extinction training (25 tone-alone presentations) for 6 h per session. Fear conditioning reduced time spent in REM sleep, which was restored with fear extinction. Hippocampal LFS, applied immediately following extinction training, abolished the restorative effect of fear extinction on REM sleep and impaired extinction retrieval. These data extend previous findings and suggest bidirectional interactions between hippocampal functioning and REM sleep for successful extinction retrieval.

Habenula lesions alter synaptic plasticity within the fimbria-accumbens pathway in the rat.

Both the habenula and the nucleus accumbens, and especially the glutamatergic innervation of the latter from the hippocampus, have been hypothesized to be involved, in different ways, in the pathophysiology of cognitive disturbances in schizophrenia. Lesions of the habenula produce disturbances of memory and attention in experimental animals. As the habenular nuclei have been shown to influence the release of many neurotransmitters, both in the hippocampus and the nucleus accumbens, we examined in this study the effects of bilateral habenula lesions on the plasticity of the fimbria-nucleus accumbens pathway, by means of the long-term depression phenomenon in freely moving rats. Long-term depression, induced within the shell region of the nucleus accumbens by low-frequency stimulation of the fimbria, was exaggerated and showed greater persistence in habenula-lesioned rats compared with sham-operated animals. These results indicate that plasticity in the fimbria-nucleus accumbens pathway is altered by habenula lesions in a way similar to previously-reported effects of stress and the psychosis-provoking agent ketamine. Moreover, they strengthen the views that the habenula belongs to systems, mediating higher cognitive functions, which involve the hippocampus and the nucleus accumbens. Finally, this study suggests that dysfunction of the habenula could contribute to cognitive alterations in diseases such as schizophrenia, where the habenula is reported to exhibit exaggerated calcification.

Variations in extracellular levels of dopamine, noradrenaline, glutamate, and aspartate across the sleep--wake cycle in the medial prefrontal cortex and nucleus accumbens of freely moving rats.

We used intracerebral microdialysis coupled with electrophysiologic recordings to determine relative changes in the concentrations of several neurotransmitters in the medial prefrontal cortex and nucleus accumbens of freely moving rats during waking, slow-wave sleep, and rapid eye movement (REM) sleep. The concentrations of noradrenaline, dopamine, glutamate, and aspartate in 2-min dialysate samples were analyzed by capillary electrophoresis combined with laser-induced fluorescence detection. Changes in glutamate and aspartate concentrations were found only in the nucleus accumbens, in which a decrease was obtained during both slow-wave sleep and REM sleep compared to waking. A progressive reduction in the release of noradrenaline was observed from waking to REM sleep in both structures. In contrast, dopamine concentrations were higher during waking and REM sleep compared to that during slow-wave sleep. The latter results demonstrate that contrary to the findings of earlier electrophysiologic studies carried out on ventral tegmental area dopaminergic neurons, changes in the release of dopamine in projection areas occur across the sleep-wake cycle. The elevated levels of dopamine during waking and REM sleep in the medial prefrontal cortex and the nucleus accumbens could result from changes during these two states in afferent modulation at the level of cell bodies or at the level of dopaminergic terminals.

Apamin produces selective improvements of learning in rats.

The effect of apamin on learning was examined using two behavioral tasks where the animals were subjected to two trials separated by a 24h interval. In the Y maze task, apamin administered before the acquisition session did not enhance performance on both the acquisition session and the restitution session. In the second behavioral task, animals were trained to press a lever to obtain a food pellet (fixed ratio 1). Then, to study the effect of apamin on extinction, animals were submitted to two sessions where a press on the lever was not reinforced. Apamin administered before the acquisition session reduced the number of lever presses during the first 3-min period of the restitution session. These results suggest that the blockade of SK channels could improve the acquisition but not when the task requires the processing of spatial information.

Further SAR studies of piperidine-based analogues of cocaine. 2. Potent dopamine and serotonin reuptake inhibitors.

The synthesis and monoamine transporter activity of additional members of a series of 3,4-disubstituted piperidines (truncated analogues of the WIN series) are described. All members of this series were prepared from arecoline hydrobromide in optically pure form and were evaluated for their ability to inhibit high affinity uptake of dopamine (DA), serotonin (5-HT) and norepinephrine (NE) into rat brain nerve endings (synaptosomes). Most of the compounds prepared in this series are reasonably potent DAT inhibitors (K(i) values of 4-400 nM) and have selectivity for the 5-HT transporter relative to both the NE transporter (3-9-fold) and to the DAT ( approximately 25-fold). In the present series, (-)-methyl 1-methyl-4beta-(2-naphthyl)piperidine-3beta-carboxylate (6) was found to be the most potent piperidine-based ligand, exhibiting K(i)'s of 21 nM and 7.6 nM at the DAT and 5-HTT, respectively. While the 5-HTT activity of compound 6 is comparable to that of the antidepressant medication fluoxetine, it is less selective. As is apparent from the data presented, the naphthyl substituted piperidines 6-9, which differ in their stereochemistry, show different degrees of selectivity for the three transporters. Consistent with results reported in the literature for the tropane analogues, removal of the methyl group from the nitrogen atom of 9 leads to a further enhancement in 5-HTT activity. To examine the in vivo effects of these piperidines, preliminary behavioral screening was carried out on piperidine 14. Despite its 2.5-fold greater DAT activity compared to cocaine, piperidine 14 was found to be about 2. 5-fold less potent in increasing distance traveled in mice. However, consistent with its DAT activity, piperidine 14 was found to be about 2.5-fold more potent than cocaine in enhancing stereotypic movements. Further studies of these piperidine-based ligands may provide valuable insights into the pharmacological mechanisms underlying the enhancement in distance traveled versus stereotypic movements. The present results have important implications for better understanding the structural motifs required in the design of agents with specific potency and selectivity at monoamine transporters.

Discovery of a novel dopamine transporter inhibitor, 4-hydroxy-1-methyl-4-(4-methylphenyl)-3-piperidyl 4-methylphenyl ketone, as a potential cocaine antagonist through 3D-database pharmacophore searching. Molecular modeling, structure-activity relationships, and behavioral pharmacological studies.

A novel, fairly potent dopamine transporter (DAT) inhibitor, 4-hydroxy-1-methyl-4-(4-methylphenyl)-3-piperidyl 4-methylphenyl ketone (3, K(i) values of 492 and 360 nM in binding affinity and inhibition of dopamine reuptake, respectively), with significant functional antagonism against cocaine and a different in vitro pharmacological profile from cocaine at the three transporter sites (dopamine, serotonin, and norepinephrine) was discovered through 3D-database pharmacophore searching. Through structure-activity relationships and molecular modeling studies, we found that hydrophobicity and conformational preference are two additional important parameters that determine affinity at the DAT site. Chemical modifications of the lead compound (3) led to a high affinity analogue (6, K(i) values of 11 and 55 nM in binding affinity and inhibition of dopamine reuptake, respectively). In behavioral pharmacological testing, 6 mimics partially the effect of cocaine in increasing locomotor activity in mice but lacks cocaine-like discriminative stimulus effect in rats. Taken together, these data suggest that 6 represents a promising lead for further evaluations as potential therapy for the treatment of cocaine abuse.

Nicotine improves memory in an object recognition task in rats.

Nicotine was investigated for its mnemonic effect in a two trials object recognition task. In the first trial, two copies of the same object were presented. In the second trial (24 h after), one of the familiar object and a new object were presented. The time spent exploring the new object by control rats was not significantly different from the exploration time of the familiar object, indicating that they did not remember the familiar object. Rats injected with nicotine before the first trial, after the first trial or before the second trial spent more time in exploring the new object than the familiar one at the second trial. These results suggest that, in normal rats, acute nicotine enhances acquisition, consolidation and restitution of the information in an object recognition task.

Effect of apamin, a selective blocker of Ca2+-activated K+-channel, on habituation and passive avoidance responses in rats.

The effects of apamin, a selective blocker of the small conductance K(Ca) channels were examined in both passive avoidance and habituation of exploratory activity in rats. In the two experiments, animals were subjected to two trials separated by a 24 h interval. Apamin was administered either before or after the first session (acquisition) or before the second session (restitution). In the passive avoidance test, apamin did not alter performance whenever the time of administration. In the habituation task, apamin (0.4 mg/kg) decreased activity (distance travelled, rearing) on the two sessions only when it was injected before acquisition but not when injection took place just after the acquisition session or before the restitution session. Taken together, these findings support the view that the blockade of apamin sensitive K(Ca) channels improved the acquisition in non-stressful task, but not in a stressful situation.

Apamin improves learning in an object recognition task in rats.

Object recognition was investigated in rats in a two trial unrewarded task. In the first trial, two copies of the same object were presented. In the second trial, one of the familiar object and a new object were presented. Rats explored the new object longer than the familiar object when the intertrial time was 1 h, indicating that they remembered the familiar object, but not when the intertrial time was 24 h. Rats injected with apamin (a toxin which blocks specifically Ca(2+)-activated K(+)-channels) before the first trial spent more time in exploring the new object than the familiar object at the second trial, when it took place 24 h after the first trial. Injection of apamin just after the first trial or before the second trial did not modify the difference in exploration time between the new and the familiar object. These results suggest that the blockade of Ca(2+)-activated K(+)-channels could improve learning, but not consolidation nor restitution of the information, in an object recognition task.

Effects of penitrem A on rat's performances in passive avoidance and Morris water maze tests.

Intraperitoneal administration of the mycotoxin penitrem A 30 min before a training session in passive avoidance task, impaired performance of rats subjected to a test-session 24 h after. This effect was not antagonised by pretraining administration of physostigmine or bicuculline. Administration of penitrem A 20 min before a training session or 30 min before a test-session did not impair performance. In the Morris water maze, doses of penitrem A that induces slight to moderate tremors, but not a lower dose, disrupted place learning. These results suggest that penitrem A disrupts the processes that take place at the time of acquisition, but not those just after acquisition, and does not alter the restitution of information. This effect would not be related to a decrease of cholinergic neurotransmission nor to a stimulation of GABA A receptors. Nevertheless, it could not be totally excluded that the performance impairments induced by penitrem A would be secondary to a motor disruption.