Abstract
Evolutionarily, what was the earliest engram? Biology has evolved to encode representations of past events, and in neuroscience, we are attempting to link experience-dependent changes in molecular signaling with cellular processes that ultimately lead to behavioral output. The theory of evolution has guided biological research for decades, and since phylogenetically conserved mechanisms drive circadian rhythms, these processes may serve as common predecessors underlying more complex behavioral phenotypes. For example, the cAMP/MAPK/CREB cascade is interwoven with the clock to trigger circadian output, and is also known to affect memory formation. Time-of-day dependent changes have been observed in long-term potentiation (LTP) within the suprachiasmatic nucleus and hippocampus, along with light-induced circadian phase resetting and fear conditioning behaviors. Together this suggests during evolution, similar processes underlying metaplasticity in more simple circuits may have been redeployed in higher-order brain regions. Therefore, this notion predicts a model that LTP and metaplasticity may exist in neural circuits of other species, through phylogenetically conserved pathways, leading to several testable hypotheses.
Highlights
What was the earliest engram? Biology has evolved to encode representations of past events, and in neuroscience, we are attempting to link experience-dependent changes in molecular signaling with cellular processes that lead to behavioral output
It has been shown that the number of photons of light correlated with the amount of the immediate-early gene c-fos mRNA expression in the suprachiasmatic nucleus (SCN), which in turn correlated with the amount of phase-shift behavioral response, at times when the circadian clock is susceptible to phase-shifts (Kornhauser et al, 1990), and these effects are tightly coupled with CREB phosphorylation (Ginty et al, 1993)
Future work examining the role of these molecules, and how they relate to SCN period-length-dependent phase shifting, should provide fundamental information on basic nervous system function, and could prove to be a very useful model for examining how its networks integrate properties of excitability with metaplasticity (Jedlicka, 2002; Abraham, 2008) or other forms of plasticity, such as homeostatic plasticity (Nelson and Turrigiano, 2008)
Summary
The repetitive nature of cycling environmental stimulus factors, such as light and temperature, and their overlap with the availability of essential nutrients would have led to early life exhibiting a “timed” and coordinated molecular signature, and in turn this could have led to an organization of molecular and cellular processes contributing to a behavioral response which coincided with regular, cyclical, and predictable stimuli (Figure 1) These stimulating events, while repetitive, would have retained some variance over time, such as annual periodic changes in day length. For life to survive optimally, an adaptive quality with this changing spectrum is absolutely essential, and rationale for why circadian clocks are thought to have evolved out of periodic changes in the environment (Paranjpe and Sharma, 2005; McIntosh et al, 2010) This manuscript is not meant to set a base for this argument, but instead propose that the metaplasticity that is observed in neuronal networks and complex behavior in higher-order organisms today could have evolved out of more simple adaptive molecular machinery, such as from clock-forming
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
