Short- and long-term effects of cannabinoids on the extinction of contextual fear memory in rats

Facilitation of memory extinction by manipulation of the endocannabinoid (eCB) system has been recently studied in several paradigms. Our previous results pointed to facilitation of contextual fear memory extinction by a low dose of a cannabinoid agonist, with a suggestion of short-term effects. The aim of the present study was to further investigate the effects of cannabinoid drugs in the short- and long-term extinction of conditioned fear using an extended extinction protocol. Male Wistar rats were placed in a conditioning chamber and after 3 min received a footshock (1.5 mA, 1 s). On the next day, they received i.p. drug treatment (WIN55212-2 0.25 mg/kg, AM404 10 mg/kg, SR141716 A 1 mg/kg) and were re-exposed to the conditioning chamber for 30 min (extinction training). No-Extinction groups received the same drug treatment, but were exposed for 3 min to the conditioning chamber. A drug-free test of contextual memory (3 min) was performed 7 days later. The cannabinoid agonist WI55212-2 and the inhibitor of eCB metabolism/uptake AM404 facilitated short-term extinction. In addition, long-term effects induced by treatments with WIN55212 and AM404 were completely divergent to those of SR141716A treatment. The present results confirm and extend previous findings showing that the eCB system modulates short-term fear memory extinction with long-lasting consequences.     

Pentylenetetrazole as an unconditioned stimulus for olfactory and contextual fear conditioning in rats

The association of five footshocks with a neutral odor is able to establish an olfactory fear conditioning in rats. The present study sought to investigate whether the systemic administration of pentylenetetrazole (PTZ; 3.75–15 mg/kg) would turn the coffee odor in a conditioned stimulus in the fear conditioning paradigm. The results showed that rats started to display risk assessment and avoidance after PTZ (15 mg/kg)–coffee odor pairing. When three mild footshocks (0.4 mA for 2 s) were delivered during this pairing, the conditioned response exhibited was greater than before. In both cases, however, pretreatment with the benzodiazepine midazolam (MDZ. 0.5 mg/kg i.p.) fully counteracted the expression of these defensive behaviors. Moreover, after being paired with 15 mg/kg of PTZ alone or combined with footshocks, the coffee odor was able to promote a new fear conditioning related to the context where it was re-exposed. The present findings point out the usefulness of PTZ as an unconditioned stimulus to promote fear conditioning to olfactory and contextual cues in rats.     

Lack of phenotype for LTP and fear conditioning learning in calpain 1 knock-out mice

We previously proposed the hypothesis that calpain activation played an important role in long-term potentiation (LTP) of synaptic transmission in hippocampus. Two forms of calpain are predominant in brain tissues, calpain 1 (mu-calpain), activated by micromolar calcium concentration and calpain 2 (m-calpain), activated by millimolar calcium concentration in vitro. In the present study, we tested the role of calpain 1 in LTP and in learning and memory using calpain 1 knock-out mice. Changes in learning and memory were assessed using both context and tone fear conditioning. No differences in freezing responses were observed between the knock-out and the wild-type animals during the acquisition phase of the training, eliminating the possibility that the knock-out animals could be differentially affected by the foot shock. Likewise, no differences in freezing responses elicited by either the context or the tone were observed during the retention phase. No differences in short-term potentiation (STP) or LTP were observed in hippocampal slices from the knock-out and matched wild-type mice. Several interpretations might explain these negative results. First, it is conceivable that calpain 2 plays a more dominant role in neurons, and that calpain 1 makes a minor contribution as opposed to its suspected predominant role in the hematopoietic system. Alternatively, it is conceivable that some as yet unknown compensatory mechanisms take effect, and that calpain 2 or another calpain isoform substitutes for the missing calpain 1.     

Effects of neonatal handling on play behavior and fear towards a predator odor in juvenile rats (Rattus norvegicus)

The effects of brief daily separation, also known as "handling," during the first 2 weeks of life on play behavior and fearfulness toward a predatory odor were assessed in juvenile rats. Handled rats were more playful than nonhandled control rats, and while handling had no effect on the direct response of these rats toward a predatory odor, handled rats did not exhibit a conditioned suppression of play when tested later in the same context where they had been exposed to the predatory odor. Handled rats were still wary of the environment in that they continued to show a heightened level of risk assessment behavior. These data suggest that early postnatal experiences may play a significant role in determining how an animal deals with predatory threats later in life.     

Inhibition of PKA anchoring to A-kinase anchoring proteins impairs consolidation and facilitates extinction of contextual fear memories

Both genetic and pharmacological studies demonstrated that contextual fear conditioning is critically regulated by cyclic AMP-dependent protein kinase (PKA). Since PKA is a broad range protein kinase, a mechanism for confining its activity is required. It has been shown that intracellular spatial compartmentalization of PKA signaling is mediated by A-kinase anchoring proteins (AKAPs). Here, we investigated the role of PKA anchoring to AKAPs in different stages of the memory process (acquisition, consolidation, retrieval and extinction) using contextual fear conditioning, a hippocampus-dependent learning task. Mice were injected intracerebroventricularly or intrahippocampally with the membrane permeable PKA anchoring disrupting peptides St-Ht31 or St-superAKAP-IS at different time points during the memory process. Blocking PKA anchoring to AKAPs resulted in an impairment of fear memory consolidation. Moreover, disrupted PKA anchoring promoted contextual fear extinction in the mouse hippocampus. We conclude that the temporal and spatial compartmentalization of hippocampal PKA signaling pathways, as achieved by anchoring of PKA to AKAPs, is specifically instrumental in long-term contextual fear memory consolidation and extinction, but not in acquisition and retrieval.     

Assessing the interplay between fear and learning in mice exposed to a live rat in a spatial memory task (MWM)

In this study we tested the hypothesis that fear might facilitate learning when experienced contextually to the task. To this purpose, learning and memory performance of CD-1 mice in a Morris Water Maze (MWM) was assessed in the presence of a live predator (rat). Results indicate that a live predator induced specific predatory-avoidance responses, such as diving behavior and thigmotaxis. The rat-exposed group showed the most adaptive strategy, balancing anti-predator behavior and escape responses, while the rat pre-exposed group showed impairment in the initial phases of the acquisition. The probe trial revealed distinct swimming patterns but equal memory abilities in the different groups. Overall, this procedure represents a novel and easy test to assess the effects of stressful stimuli, contextually to spatial learning and memory performance, in mice.