Endocannabinoid system and the aging locus coeruleus

Abstract

The locus coeruleus (LC) is a small nucleus located in the pons of the brainstem, representing the major source of noradrenaline (NE) in the central nervous system. Although the LC consists of a relatively low number of noradrenergic neurons, they widely innervate cortex, hippocampus, amygdala, hypothalamus and the brainstem itself. The LC plays a crucial role in the modulation of numerous cognitive functions including attention, memory, the regulation of sleep–wake states and the stress response. The peculiarity of noradrenergic neurons, in fact, is to act as a real attention filter switching from a tonic firing mode, related to a “low attention” stimulus, to a phasic firing mode, in response to relevant “focus-demanding” stimuli. It has been widely demonstrated that the LC undergoes structural changes in neurodegenerative diseases, however very little is known about changes in this region during physiological aging. A recent human in vivo study showed the presence of an age-related neurodegeneration of the LC in healthy adults, confirming this brain region as particularly susceptible to aging. Thus, LC seems to be expressly dependent on the activity of neuroprotective mechanisms to compensate for the factors contributing to its vulnerability: increased oxidative stress, pacemaking activity, high arborization, high load on mitochondria due to the big energetic need. Interestingly, all these factors are influenced by the endocannabinoid system, whose activity is thought to be neuroprotective and to influence aging. Yet, whether the endocannabinoid system has a protective effect on the survival of these neurons during aging is still not known. Thus, the aim of this thesis was to investigate the aging process of the LC and to comprehend the role that the endocannabinoid system plays in it. <br /> We discovered that the LC of C57BL/6J mice goes through an age-related neurodegenerative process, as the number of noradrenergic neurons starts decreasing from 8 months of age, for a total 20 % reduction throughout life. We also found that the rate of neuronal loss was higher in animals lacking of the CB1r (Cnr1^-/-), indicating a crucial role of this receptor in the survival of noradrenergic neurons. Moreover, a lower number of noradrenergic neurons was associated with a reduced density of noradrenergic axon terminals in the main projecting areas of LC such as cortex and all the hippocampal subregions. Also in this case, the reduction was more severe in Cnr1^-/- mice compared to wild-types, suggesting a poorer (if any) compensatory rate. This marked neuronal death in Cnr1^-/- animals was also associated with alterations in microglia morphology and mitophagic activity. <br /> In further support to the hypothesis that the endocannabinoid system plays a key role in the aging process of this neuronal type, we found that DAGL enzyme increased with age selectively in noradrenergic neurons, in a timepoint subsequent to the onset of the neurodegenerative process, suggesting a protective role of it in the outlived neurons. Furthermore, we investigated whether the age-related reduction in LC neurons that we identified could be directly responsible for the cognitive alterations in working memory, attention and memory extinction characteristic of old age. Our data clearly show that even this mild decrease in noradrenergic signalling can lead to an impairment in working memory and to a greater distractibility in the execution of attention-required tasks. <br /> Overall, our data provide the basics for a better comprehension of the role of the noradrenergic system in brain aging and for a future exploration of the endocannabinoid system's potential as a new valuable therapeutic target.