Memory of an operant response and of depressed mood retained in activation states of 5-HT(1A) receptors: evidence from rodent models

A systematic mapping review of the evolution of the rat Forced Swim Test: Protocols and outcome parameters

As depression is projected to become the leading mental disease burden globally by 2030, understanding the underlying pathology, as well as screening potential anti-depressants with a higher efficacy, faster onset of action, and/or fewer side-effects is essential. A commonly used test for screening novel antidepressants and studying depression-linked aspects in rodents is the Porsolt Forced Swim Test. The present systematic mappping review gives a comprehensive overview of the evolution and of the most prevalently used set-ups of this test in rats, including the choice of animals (strain, sex, and age), technical aspects of protocol and environment, as well as reported outcome measures. Additionally, we provide an accessible list of all existing publications, to support informed decision-making for procedural and technical aspects of the test, to thereby enhance reproducibility and comparability. This should further contribute to reducing the number of unnecessarily replicated experiments, and consequently, reduce the number of animals used in future.

Role of state-dependency in memory impairment induced by acute administration of midazolam in mice

Although the memory deficits produced by pre-training benzodiazepines administration have been extensively demonstrated both in humans and in animal studies, there is considerable controversy about the involvement of the state-dependency phenomenon on benzodiazepines-induced anterograde amnesia. The present study aimed to characterize the role of state-dependency on memory deficits induced by the benzodiazepine midazolam (MID) in mice submitted to the plus-maze discriminative avoidance task (PM-DAT). This animal model concomitantly evaluates learning and retention of discriminative avoidance task, exploratory habituation as well as anxiety-like behavior and motor activity. Mice received 2mg/kg MID before training and/or before testing in the PM-DAT. Pre-training (but not pre-test) MID administration impaired the retention of the discriminative avoidance task, which was not counteracted by a subsequent pre-test administration of this drug, thus refuting the role of state-dependency. Conversely, the pre-training administration of MID also led to an impairment of the habituation of exploration in the PM-DAT (an animal model of non-associative memory). This habituation deficit was state-dependent since it was absent in pre-training plus pre-test MID treated mice. Concomitantly, MID pre-training administration induced anxiolytic effects and diminished the aversive effectiveness of the aversive stimuli of the task, leading to an impairment of the acquisition of the discriminative avoidance task. Our findings suggest that pre-training benzodiazepine administration can impair the retention of different types of memory by producing specific deleterious effects on learning or by inducing state-dependent memory deficits.

Inactivation of the dorsal raphé nucleus reduces the anxiogenic response of rats running an alley for intravenous cocaine

Rats traversing a straight alley once a day for delivery of a single i.v. injection of cocaine develop over trials an ambivalence about entering the goal box. This ambivalence is characterized by the increasing occurrence of "retreat behaviors" where animals leave the start box and run quickly to the goal box, but then stop at the entry point and "retreat" back toward the start box. This unique pattern of retreat behavior has been shown to reflect a form of "approach-avoidance conflict" that stems from the animals' concurrent positive (cocaine reward) and negative (cocaine-induced anxiety) associations with the goal box. Cocaine blocks reuptake of the serotonergic (5-HT) transporter and serotonin has been implicated in the modulation of anxiety. It was therefore of interest to determine whether inactivation of the serotonergic cell bodies residing in the dorsal raphé nucleus (DRN) and projecting to brain areas critical for the modulation of anxiety, would alter the anxiogenic state exhibited by rats running an alley for single daily i.v. injections of 1.0mg/kg cocaine. Reversible inactivation of the DRN was accomplished by intracranial application of a mixed solution of the GABA agonists baclofen and muscimol. While DRN inactivation had no impact on the subjects' motivation to initiate responding (i.e., latencies to leave the start box were unaffected) it reliably reduced the frequency of approach-avoidance retreat behaviors (conflict behavior). These data suggest that inactivation of the dorsal raphé reduces the conflict/anxiety otherwise present in experienced cocaine-seeking animals.

Effects of the delta-opioid receptor agonist SNC80 on learning relative to its antidepressant-like effects in rats

Delta-opioid receptor agonists produce decreases in immobility in the forced swim test, suggesting that these compounds have antidepressant-like activity. There is also the possibility that these compounds decrease immobility in the forced swim test by disrupting learning processes that occur during the swim, or with successive swim exposures, thus falsely identifying them as having "antidepressant" potential. This study investigated the effects of the delta-opioid receptor agonist, SNC80, on responding in a repeated-acquisition procedure and in the forced swim test in rats, and the effects were compared directly to those of scopolamine, a compound known to disrupt memory and learning. SNC80 disrupted acquisition of a response sequence (learning) and produced a significant antidepressant-like effect in the forced swim test. Scopolamine, however, produced larger decrements in learning without producing behavioral changes consistent with an antidepressant-like profile of action. These results suggest that SNC80 produces antidepressant-like activity through a mechanism independent of its disruptive effects on learning.

Memory Impairment Means Less Pain for Mice

BACKGROUND

Clinical observations have reported that individuals with memory deterioration, like in Alzheimer's disease, display a lesser pain sensibility than patients with no cognitive impairment.

OBJECTIVE

To clarify the link between pain and loss of memory, we studied how memory-impaired mice behave when submitted to hotplate nociceptive tests.

METHODS

For 5 days (D1-D5), male CD1 mice were injected daily intraperitonealy with saline or scopolamine (s, an anticholinergic drug, 0.2 mg/kg) or ketamine (k, an N-methyl-D-aspartate receptor antagonist (NMDAr), 2.5 mg/kg), at doses leading to memory impairment with no analgesic effect. From D6 to D9, all received saline only. They were placed on the hotplate and removed at the first sign of discomfort, response time being recorded.

RESULTS

From D1 to D5, reaction time decreased significantly in controls only and did not change in mice with scopolamine or ketamine. From D6 to D9, response times decreased (p < 0.05 (s) and p < 0.0001 (k)) to reach the steady state of control animals. At D5, response time was significantly prolonged for scopolamine (p < 0.01) and ketamine (p < 0.05), compared to controls.

CONCLUSION

These results show that pain sensibility needs the integrity of the central cholinergic and of the NMDA systems, and that mice with memory impairment display a lesser pain sensibility than normal mice. Further research on the complex interactions of receptors and neurotransmitters involved in pain and cognition could assist in gaining a better understanding of pain and analgesia in patients with memory impairment and in demented individuals.

A persistent opioid-addiction state of memory

We determined whether tolerance develops to a morphine-induced state of memory. Rats were injected with 5 mg/kg of morphine and trained to complete a FR-10 schedule of lever presses in daily sessions. The dose-response curve of morphine (1.25-40 mg/kg) in enabling retrieval was tested in one group immediately after criterion had been reached and, in another group, after an additional 40 training sessions. The additional training enhanced, rather than attenuated, the dependence of retrieval on morphine; this was because the further gain in response latency that developed during additional training also became state-dependent. Thus, because tolerance did not develop to the morphine state, an increasingly large body of engrams became encoded in that state, rendering retrieval increasingly dependent.

Tianeptine: 5-HT uptake sites and 5-HT(1-7) receptors modulate memory formation in an autoshaping Pavlovian/instrumental task

Recent studies using invertebrate and mammal species have revealed that, endogenous serotonin (5-hydroxytryptamine, 5-HT) modulates cognitive processes, particularly learning and memory, though, at present, it is unclear the manner, where, and how long 5-HT systems are involved. Hence in this work, an attempt was made to study the effects of 5-HT endogenous on memory formation, using a 5-HT uptake facilitator (tianeptine) and, selective 5-HT(1-7) receptor antagonists to determine whether 5-HT uptake sites and which 5-HT receptors are involved, respectively. Results showed that post-training tianeptine injection enhanced memory consolidation in an autoshaping Pavlovian/instrumental learning task, which has been useful to detect changes on memory formation elicited by drugs or aging. On interaction experiments, ketanserin (5-HT(1D/2A/2C) antagonist) slightly enhanced tianeptine effects, while WAY 100635 (5-HT(1A) antagonist), SB-224289 (5-HT(1B) inverse agonist), SB-200646 (5-HT(2B/2C) antagonist), ondansetron (5-HT(3) antagonist), GR 127487 (5-HT(4) antagonist), Ro 04-6790 (5-HT(6) antagonist), DR 4004 (5-HT(7) antagonist), or fluoxetine (an inhibitor of 5-HT reuptake) blocked the facilitatory tianeptine effect. Notably, together tianeptine and Ro 04-6790 impaired learning consolidation. Moreover, 5-HT depletion completely reversed the tianeptine effect. Tianeptine also normalized an impaired memory elicited by scopolamine (an antimuscarinic) or dizocilpine (non-competitive glutamatergic antagonist), while partially reversed that induced by TFMPP (5-HT(1B/1D/2A-2C/7) agonist/antagonist). Finally, tianeptine-fluoxetine coadministration had no effect on learning consolidation; nevertheless, administration of an acetylcholinesterase inhibitor, phenserine, potentiated subeffective tianeptine or fluoxetine doses. Collectively, these data confirmed that endogenously 5-HT modulates, via uptake sites and 5-HT(1-7) receptors, memory consolidation, and are consistent with the emerging notion that 5-HT plays a key role on memory formation.

Evidence that mnesic states govern normal and disordered memory

Although it is recognized that retrieval may be state-dependent, only recently has a paradigm been identified that allows state-dependence in rats to be demonstrated reliably and at relevant doses of CNS agents. In humans, the effects of scopolamine constitute a valuable model of disordered memory. We used this paradigm to analyze the effects of scopolamine on different memory processes. Rats treated with either saline or 0.01-10 mg/kg doses of scopolamine learned to lever press for milk reward, and were then tested for retrieval while given the same or a different treatment. Saline-to-scopolamine as well as scopolamine-to-saline state changes produced robust failures to retrieve the response. Remarkably, the state produced by 2.5 mg/kg scopolamine, like that produced by saline, produced little intrinsic effect on learning or any other memory process (i.e. when the prevailing state was left unchanged). However, changing the implemented state from one to the other between two different processes, disrupted not only retrieval, but also learning, encoding and retention. We also determined whether the graded state changes produced by 0.01-10 mg/kg doses of scopolamine could mimic the peculiar and poorly understood temporally graded retrograde amnesia that occurs in Alzheimer's disease. In rats that had acquired a complex drug-discrimination task over a 6-month period, scopolamine-induced state changes seemed to produce dose-dependent deficits in the retrieval of recent information while preserving those abilities that had been acquired in the increasingly remote past. Beyond its role in retrieval, the findings implicate state dependence in learning, encoding and retention, and suggest that physiologically defined mnesic states govern each of these. The changes of mnesic state that are likely associated with excessively labile cholinergic neurotransmission may conceivably cause the complex disabilities of Alzheimer's disease.