Glucocorticoid effects on working memory impairment require l-type calcium channel activity within prefrontal cortex

Previous findings have indicated that glucocorticoid hormones impair working memory via an interaction with the β-adrenoceptor-cAMP signaling cascade to rapidly increase cAMP-dependent protein kinase (PKA) activity within the prefrontal cortex (PFC). However, it remains elusive how such activation of PKA can affect downstream cellular mechanisms in regulating PFC cognitive function. PKA is known to activate l-type voltage-gated Ca²⁺ channels (LTCCs) which regulate a broad range of cellular processes, including neuronal excitability and neurotransmitter release. The present experiments examined whether LTCC activity within the PFC is required in mediating glucocorticoid and PKA effects on spatial working memory. Male Sprague Dawley rats received bilateral administration of the LTCC inhibitor diltiazem together with either the glucocorticoid receptor agonist RU 28362 or PKA activator Sp-cAMPS into the PFC before testing on a delayed alternation task in a T-maze. Both RU 28362 and Sp-cAMPS impaired working memory, whereas the LTCC inhibitor diltiazem fully blocked the working memory impairment induced by either RU 28362 or Sp-cAMPS. Conversely, bilateral administration of the LTCC agonist Bay K8644 into the PFC was sufficient to impair working memory. Thus, these findings provide support for the view that glucocorticoids, via an interaction with the β-adrenergic signaling cascade and enhanced PKA activity levels, impair working memory by increasing LTCC activity in the PFC.

Cardiac-specific mutation of Clock alters the quantitative measurements of physical activities without changing behavioral circadian rhythms

Even though peripheral circadian oscillators in the cardiovascular system are known to exist, the daily rhythms of the cardiovascular system are mainly attributed to autonomic or hormonal inputs under the control of the central oscillator, the suprachiasmatic nucleus (SCN). In order to examine the role of peripheral oscillators in the cardiovascular system, we used a transgenic mouse where the Clock gene is specifically disrupted in cardiomyocytes. In this cardiomyocyte-specific CLOCK mutant (CCM) mouse model, the circadian input from the SCN remains intact. Both CCM and wild-type (WT) littermates displayed circadian rhythms in wheel-running behavior. However, the overall wheel-running activities were significantly lower in CCM mice compared to WT over the course of 5 weeks, indicating that CCM mice either have lower baseline physical activities or they have lower physical adaptation abilities because daily wheel running, like routine exercise, induces physical adaptation over a period of time. Upon further biochemical analysis, it was revealed that the diurnal oscillations of phosphorylation states of several kinases and protein expression of the L-type voltage-gated calcium channel (L-VGCC) α1D subunit found in WT hearts were abolished in CCM hearts, indicating that in mammalian hearts, the daily oscillations of the activities of these kinases and L-VGCCs were downstream elements of the cardiac core oscillators. However, the phosphorylation of p38 MAPK exhibited robust diurnal rhythms in both WT and CCM hearts, indicating that cardiac p38 could be under the influence of the central clock through neurohormonal signals or be part of the circadian input pathway in cardiomyocytes. Taken together, these results indicate that the cardiac core oscillators have an impact in regulating circadian rhythmicities and cardiac function.