Fast and slow-twitching muscles are differentially affected by reduced cholinergic transmission in mice deficient for VAChT: A mouse model for congenital myasthenia

Congenital myasthenic syndromes (CMS) result from reduced cholinergic transmission at neuromuscular junctions (NMJs). While the etiology of CMS varies, the disease is characterized by muscle weakness. To date, it remains unknown if CMS causes long-term and irreversible changes to skeletal muscles. In this study, we examined skeletal muscles in a mouse line with reduced expression of Vesicular Acetylcholine Transporter (VAChT, mouse line herein called VAChT-KDHOM). We examined this mouse line for several reasons. First, VAChT plays a central function in loading acetylcholine (ACh) into synaptic vesicles and releasing it at NMJs, in addition to other cholinergic nerve endings. Second, loss of function mutations in VAChT causes myasthenia in humans. Importantly, VAChT-KDHOM present with reduced ACh and muscle weakness, resembling CMS. We evaluated the morphology, fiber type (myosin heavy chain isoforms), and expression of muscle-related genes in the extensor digitorum longus (EDL) and soleus muscles. This analysis revealed that while muscle fibers atrophy in the EDL, they hypertrophy in the soleus muscle of VAChT-KDHOM mice. Along with these cellular changes, skeletal muscles exhibit altered levels of markers for myogenesis (Pax-7, Myogenin, and MyoD), oxidative metabolism (PGC1-α and MTND1), and protein degradation (Atrogin1 and MuRF1) in VAChT-KDHOM mice. Importantly, we demonstrate that deleterious changes in skeletal muscles and motor deficits can be partially reversed following the administration of the cholinesterase inhibitor, pyridostigmine in VAChT-KDHOM mice. These findings reveal that fast and slow type muscles differentially respond to cholinergic deficits. Additionally, this study shows that the adverse effects of cholinergic transmission, as in the case of CMS, on fast and slow type skeletal muscles are reversible.

Strain and sex based characterization of behavioral expressions in non-induced compulsive-like mice

There is currently a lack of understanding how genetic background and sex differences attribute to the heterogeneity of obsessive-compulsive disorder (OCD). An animal model of compulsive-like behaviors has been developed through bidirectional selection of house mice (Mus musculus) for high (big cotton nests; BIG mice) and low levels (small nests; SMALL mice) of nest-building behavior. The BIG male strains have predictive and face validity as a spontaneous animal model of OCD. Here, we evaluated compulsive-, anxiety-, cognitive-, and depression-like behaviors among male and proestrus female replicate strains each of BIG (BIG1, BIG2) and SMALL (SML1, SML2) nest-builders, and randomly-bred Controls (C1, C2). BIG1 and BIG2 males and females had higher nesting scores when compared to SMALL and Control strains. Male BIG1 and BIG2 strains showed more compulsive-like nesting than BIG1 and BIG2 proestrus females, which was not observed among the other strains. Nesting scores were also different between BIG replicate male strains. A similar pattern was observed in the compulsive-like marble burying behavior with BIG strains burying more marbles than SMALL and Control strains. Significant replicate and sex differences were also observed in marble burying among the BIG strains. The open field test revealed replicate effects while the BIG strains showed less anxiety-like behavior in the elevated plus maze test compared to the SMALL strains. For novel object recognition only the Control strains showed replicate and sex differences. In the depression-like forced swim test proestrus females demonstrated less depression-like behavior than males. BIG and SMALL nest-building strains had a higher corticosterone stress response than the Control strains. Together these results indicate a strong interplay of genetic background and sex in influencing expression of behaviors in our compulsive-like mouse model. These results are in congruence with the clinical heterogeneity of OCD.

Ovarian Sex Hormones Modulate Compulsive, Affective and Cognitive Functions in A Non-Induced Mouse Model of Obsessive-Compulsive Disorder

There is currently a lack of understanding of how surgical menopause can influence obsessions, compulsions and associated affective and cognitive functions in female obsessive-compulsive disorder (OCD) patients. Early menopause in women due to surgical removal of ovaries not only causes dramatic hormonal changes, but also may induce affective and cognitive disorders. Here, we tested if surgical removal of ovaries (ovariectomy, OVX), which mimics surgical menopause in humans, would result in exacerbation of compulsive, affective and cognitive behaviors in mice strains that exhibit a spontaneous compulsive-like phenotype. Female mice from compulsive-like BIG, non-compulsive SMALL and randomly-bred Control strains were subjected to OVX or sham-surgery. After 7 days animals were tested for nest building and marble burying to measure compulsive-like behavior. The elevated plus maze and open field tests measured anxiety-like behaviors, while memory was assessed by the novel object recognition. Acute OVX resulted in exacerbation of compulsive-like and anxiety-like behaviors in compulsive-like BIG mice. No significant effects of OVX were observed for the non-compulsive SMALL and Control strains. Object recognition memory was impaired in compulsive-like BIG female mice compared to the Control mice, without an effect of OVX on the BIG mice. We also tested whether 17 β-estradiol (E2) or progesterone (P4) could reverse the effects of OVX. E2, but not P4, attenuated the compulsive-like behaviors in compulsive-like BIG OVX female mice. The actions of the sex steroids on anxiety-like behaviors in OVX females were strain and behavioral test dependent. Altogether, our results indicate that already existing compulsions can be worsened during acute ovarian deprivation concomitant with exacerbation of affective behaviors and responses to hormonal intervention in OVX female mice can be influenced by genetic background.

Object recognition memory deficit and depressive-like behavior caused by chronic ovariectomy can be transitorialy recovered by the acute activation of hippocampal estrogen receptors

It is well known that estradiol (E2) replacement therapy is effective on restoring memory deficits and mood disorders that may occur during natural menopause or after surgical ovarian removal (ovariectomy, OVX). However, it is still unknown the effectiveness of acute and localized E2 administration on the effects of chronic OVX. Here we tested the hypothesis that the intra-hippocampal E2 infusion, as well as specific agonists of estrogen receptors (ERs) alpha (ERα) and beta (ERβ), are able to mend novel object recognition (NOR) memory deficit and depressive-like behavior caused by 12 weeks of OVX. We found that both ERα and ERβ activation, at earlier stages of consolidation, recovered the NOR memory deficit caused by 12 w of OVX. Conversely, only the ERβ activation was effective in decreasing the depressive-like behavior caused by 12 w of OVX. Furthermore, we investigated the effect of OVX on hippocampal volume and ERs expression. The structural MRI showed no alteration in the hippocampus volume of 12 w OVX animals. Interestingly, ERα expression in the hippocampus decreased after one week of OVX, but increased in 12 w OVX animals. Overall, we may conclude that the chronic estrogen deprivation, induced by 12 weeks of OVX, modulates the hippocampal ERα expression and induces NOR memory deficit and depressive-like behaviors. Nonetheless, it is noteworthy that the acute effects of E2 on NOR memory and depressive-like behavior are still apparent even after 12 weeks of OVX.

Estradiol enhances object recognition memory in Swiss female mice by activating hippocampal estrogen receptor α

In rodents, 17β-estradiol (E2) enhances hippocampal function and improves performance in several memory tasks. Regarding the object recognition paradigm, E2 commonly act as a cognitive enhancer. However, the types of estrogen receptor (ER) involved, as well as the underlying molecular mechanisms are still under investigation. In the present study, we asked whether E2 enhances object recognition memory by activating ERα and/or ERβ in the hippocampus of Swiss female mice. First, we showed that immediately post-training intraperitoneal (i.p.) injection of E2 (0.2 mg/kg) allowed object recognition memory to persist 48 h in ovariectomized (OVX) Swiss female mice. This result indicates that Swiss female mice are sensitive to the promnesic effects of E2 and is in accordance with other studies, which used C57/BL6 female mice. To verify if the activation of hippocampal ERα or ERβ would be sufficient to improve object memory, we used PPT and DPN, which are selective ERα and ERβ agonists, respectively. We found that PPT, but not DPN, improved object memory in Swiss female mice. However, DPN was able to improve memory in C57/BL6 female mice, which is in accordance with other studies. Next, we tested if the E2 effect on improving object memory depends on ER activation in the hippocampus. Thus, we tested if the infusion of intra-hippocampal TPBM and PHTPP, selective antagonists of ERα and ERβ, respectively, would block the memory enhancement effect of E2. Our results showed that TPBM, but not PHTPP, blunted the promnesic effect of E2, strongly suggesting that in Swiss female mice, the ERα and not the ERβ is the receptor involved in the promnesic effect of E2. It was already demonstrated that E2, as well as PPT and DPN, increase the phospho-ERK2 level in the dorsal hippocampus of C57/BL6 mice. Here we observed that PPT increased phospho-ERK1, while DPN decreased phospho-ERK2 in the dorsal hippocampus of Swiss female mice subjected to the object recognition sample phase. Taken together, our results suggest that the type of receptor as well as the molecular mechanism used by E2 to improve object memory may differ in Swiss female mice.

A role for the endocannabinoid system in exercise-induced spatial memory enhancement in mice

It is well known that physical exercise has positive effects on cognitive functions and hippocampal plasticity. However, the underlying mechanisms have remained to be further investigated. Here we investigated the hypothesis that the memory-enhancement promoted by physical exercise relies on facilitation of the endocannabinoid system. We observed that the spatial memory tested in the object location paradigm did not persist in sedentary mice, but could be improved by 1 week of treadmill running. In addition, exercise up-regulated CB1 receptor and BDNF expression in the hippocampus. To verify if these changes required CB1 activation, we treated the mice with the selective antagonist, AM251, before each period of physical activity. In line with our hypothesis, this drug prevented the exercise-induced memory enhancement and BDNF expression. Furthermore, AM251 reduced CB1 expression. To test if facilitating the endocannabinoid system signaling would mimic the alterations observed after exercise, we treated sedentary animals during 1 week with the anandamide-hydrolysis inhibitor, URB597. Mice treated with this drug recognized the object in a new location and have increased levels of CB1 and BDNF expression in the hippocampus, showing that potentiating the endocanabinoid system equally benefits memory. In conclusion, the favorable effects of exercise upon spatial memory and BDNF expression depend on facilitation of CB1 receptor signaling, which can be mimic by inhibition of anandamide hydrolysis in sedentary animals. Our results suggest that, at least in part, the promnesic effect of the exercise is dependent of CB1 receptor activation and is mediated by BDNF.