Handling and novel object recognition modulate fear response and endocannabinoid signaling in nucleus basalis magnocellularis

Abstract

Storage of aversive memories is of utmost importance for survival, allowing animals to avoid upcoming similar stimuli. However, without reinforcement, the learned avoidance response gradually decreases over time. Although the molecular mechanisms controlling this extinction process are not well known, there is evidence that the endocannabinoid system plays a key role through CB1 receptor‐mediated modulation of cholinergic signaling. In this study, we measured fear extinction throughout 7 months using naïve rats, assessed in passive avoidance (PA) test in a non‐reinforced manner. Then, we evaluated the effect of gentle handling and non‐aversive novel object recognition test (NORT) on the extinction and expression of fear memories by measuring passive avoidance responses. Neurochemical correlates were analyzed by functional autoradiography for cannabinoid, cholinergic, and dopaminergic receptors. Despite results showing a gradual decrease of passive avoidance response, it did not fully disappear even after 7 months, indicating the robustness of this process. Meanwhile, in rats that received gentle handling or performed NORT after receiving the PA aversive stimulus, extinction occurred within a week. In contrast, gentle handling performed before receiving the aversive stimulus exacerbated fear expression and triggered escape response in PA. The neurochemical analysis showed increased cannabinoid and cholinergic activity in the nucleus basalis magnocellularis (NBM) in rats that had performed only PA, as opposed to rats that received gentle handling before PA. Additionally, a correlation between CB1 mediated‐signaling in the NBM and freezing in PA was found, suggesting that the endocannabinoid system might be responsible for modulating fear response induced by aversive memories.

Cannabinoid Receptors and Glial Response Following a Basal Forebrain Cholinergic Lesion

“…Together, these results suggest that, in areas where we have determined that microglia immunoreactivity increased following the lesion, the expected increase in the coupling to G i/0 -proteins evoked by HU308 related to microglial CB 2 was not observed. At the minimum concentration required for these experiments (10 μM), 20 , 54 − 56 the coupling to G i/0 -proteins evoked by HU308 seems to correspond mainly to the CB 1 receptor subtype. This is in line with a previous study also using SR141716A and HU308, which reports that WIN55,212-2, a CB 1 /CB 2 receptor agonist, ameliorated disease progression in a mouse model of MS, exerting CB 1 -mediated anti-inflammatory effects 57 and another study indicating that, in the 5xFAD mice model of AD, CB 1 blockade exacerbated inflammation.…”

“…Briefly, tissue sections mounted on slices were first immersed in copling jars for preincubation and later incubated in the presence of [ 35 S]­GTPγS. CB 1 /CB 2 receptor-mediated coupling to G i/0 -proteins was determined with CP55,940 (10 μM) or HU308 (10 μM) agonists, which was determined in previous studies as the optimal concentration for these type of experiments. ,− Although many neurotransmitter receptors bind agonists with high affinity ( K d ) in the nanomolar range, micromolar concentrations of the same agonists are required to elicit a functional effect and such is the case of the [ 35 S]­GTPγS binding assays. Basal coupling to G i/0 -proteins for each brain area was determined in the absence of agonists.…”

The effect of late-life environmental enrichment on stress and anxiety: The role of sex and age-related differences in coping with aversive stimuli

“…After 24 h (test phase), the rats were again placed in the illuminated compartment, and the time it took for the animal to enter the dark chamber (latency) was recorded. If the rat remained in the illuminated compartment for the duration of 300 s, the test was considered complete [ 22 ]. Rats with latencies exceeding 200 s or falling below 100 s were categorized as high latency or low latency, respectively.…”