Identification of C-type Natriuretic Peptide as a novel target in age-related cognitive decline.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
C-Type Natriuretic Peptide (CNP) is an important neuroendocrine regulator, suggested to be involved with cognitive decline and pathological aging. From the literature, and based on pilot work contributing to a Master’s Thesis, three questions were identified for investigation regarding a putative involvement of CNP in age-related cognitive decline. 1) Do CNP and its related signalling system vary in cerebral tissue during normal brain aging? 2) Could an involvement of CNP activity be identified in relation to a rodent model of spatial/episodic-like learning and memory? 3) What effects would administration of CNP have on rodent models of working, object and location recognition memory? During investigation of this final question, rodents were identified as cognitively impaired.
Question one was addressed by housing aged rodents (23 months old) in an enriched environment, previously indicated as useful for modifying endogenous CNP expression and degradative regulation in young (8 to 9-month-old) rats. Response of CNP and related markers to this manipulation was markedly different in aged rats compared with young rodents. Data acquired indicated CNP synthesis and regulation to be dysfunctional within hippocampus and retrosplenial cortex during aging, identifying CNP as a novel target in age-related cognitive decline.
A study using a radial arm maze task of spatial reference memory was designed to address question two. For rodents learning a spatial reference task, CNP synthesis was increased in retrosplenial cortex and mammillary bodies during memory consolidation. In a second group, interference was introduced to the reference memory component by use of a forced-arm choice, yoked to rodents learning the spatial reference memory task. Increases in CNP synthesis were restricted to mammillary bodies in this group. Rodents in both behaviourally trained groups and a third control group (accounting for effects of environmental conditions) had reduced CNP concentrations in hippocampus compared with previous reports, suggesting an effect of food-deprivation on central CNP activity. Findings provided the first indication that endogenous CNP activity is modified during spatial/episodic-like memory formation, commonly impaired in cognitive disorders.
As working memory, object recognition and memory ‘pattern separation’ are impaired during cognitive decline, the final question was addressed by investigating effects of infused CNP in rodent models of each. A working memory task (radial arm maze) was negatively affected by behavioural outcomes of CNP infusion, resulting in inclusion of a rodent model testing anxiety (elevated plus maze). Object recognition was examined using a standard novel-object recognition paradigm, with pattern separation tested using a recent adaptation of spontaneous location recognition (Bekinschtein et al., 2014). Hypothesising that CNP would increase anxiety, decrease locomotion and interfere with recognition memory in a dose-dependent manner, doses ranging from 200 to 1000ng were administered (icv) prior to each task in a repeated-measures design. Contrary to the hypothesis, locomotion increased across increasing doses of CNP, an effect that was attenuated across multiple tasks, as were anxiogenic effects of the peptide. In line with the hypothesis, CNP interfered with recognition memory dose dependently in a spontaneous location recognition task with low pattern separation load. However, recognition memory was facilitated when pattern separation load was high. Dose-dependent effects on recognition were no longer apparent during novel object recognition, but recognition (measured by discrimination ratio) was overall improved compared with previous tasks. Immunohistochemistry for cyclic Guanosine Monophosphate (cGMP; directly stimulated by CNP) provided supporting evidence for attenuation of behavioural effects across chronic administrations, and indicated bidirectional effects of CNP on memory may relate to variable cGMP stimulation in frontal cortical regions. Taken together, results of this study provide the first experimental evidence that CNP can variably affect memory acquisition, but may overall improve memory deficits within a rodent model of cognitive decline.
Several novel findings of this thesis present interesting implications regarding the role of CNP and its signalling system in relation to aging, memory and anxiety. Additionally, identification of dysregulation of this system during aging, partly accounted for by modifications to the Natriuretic Peptide C receptor, introduces theoretical considerations for Natriuretic Peptide research at large. Combined, investigations presented here point to a special relationship between CNP and retrosplenial cortex. This suggests CNP not only as a novel target in cognitive decline, but also for ongoing research within intersecting neural systems for incorporating sensation and emotion into memory.