Changes in adenosine A1 receptor-mediated regulation of hippocampal area CA2 following chronic high-dose caffeine treatment during adolescence in rats

记忆是一种高度复杂的认知功能，涉及众多大脑结构、蛋白质和信号分子之间的相互作用。事实上，神经元之间突触连接强度的经验依赖性变化是大脑信息存储的基础。因此，了解突触的高度弹性性质或“可塑性”，对于揭示大脑如何将我们的个人经历转化为持久的记忆至关重要。为此，调节性神经递质系统在形成突触过程中发挥核心作用，突触过程涉及神经回路中信息的编码和保留。然而，一个根本的问题仍然是理解外部因素如何招募、模仿甚至劫持这些内源性调节信号来影响对认知和记忆功能至关重要的大脑系统的发育。哺乳动物神经系统在出生后早期发育过程中经历了几个关键期，其中认知和感觉功能对一系列环境因素的持久干扰很敏感。事实上，越来越多的证据表明，啮齿动物的海马嘌呤能系统也经历了一段与腺苷A1受体(A1R)表达模式相关的显著成熟期。腺苷在大脑中扮演着神经调节剂的角色，海马体中腺苷受体上的信号已被证明通过调节突触的可塑性来影响记忆的形成和行为。在大鼠发育早期，A1R的表达模式并不是一成不变的。特别是，A1R的表面表达在青春期显著增加，在海马体的特定部分，被称为山角2区(或CA2)。然而，在发育过程中干扰腺苷能信号传递后，CA2区突触功能和神经元生理学的变化尚未在动物模型中进行探索。与CA2区A1Rs高表达一致的是，观察到咖啡因和其他A1R阻滞剂优先增强CA2区的兴奋性突触传递，其浓度对海马体其他部分的反应几乎没有影响。这表明CA2中的A1Rs可能在咖啡因的认知增强效应中起中心作用。咖啡因是一种天然的认知增强剂，被广泛用于改善注意力和增强记忆力。其主要作用机制被认为是通过阻断A1Rs，然而，关于A1Rs高表达的海马区CA2区以及A1R在青春期发育过程中表面表达变化剧烈的海马区，人们知之甚少。在过去的15年里，高咖啡因所谓的能量饮料的受欢迎程度急剧上升，最近的趋势突显出青少年经常饮用能量饮料，饮用量超过了推荐的每日限量。然而，目前尚不清楚青春期习惯性饮用能量饮料是否与大脑功能的持久变化有关。鉴于A1Rs在青春期急剧增加，以及咖啡因在改变CA2突触功能方面的特异性，本研究的目的是评估长期服用咖啡因对CA2生理和CA2依赖的认知形式的影响。为了实现这一目标，将在大鼠身上实验模拟人类青少年能量饮料的消费模式，并在给药方案结束时评估咖啡因诱导的突触功能、神经元形态和CA2区蛋白表达的变化。支持青少年可能因习惯性摄入咖啡因而易患持久神经功能障碍的发育窗口的证据，可能为修订政策和规范向弱势群体销售高咖啡因产品提供理由。

Memory is a highly complex cognitive function that involves interactions between numerous brain structures, proteins and signalling molecules. Indeed, experience-dependent changes in the strength of synaptic connections between neurons are though to underlie information storage in the brain. Thus, understanding the highly elastic nature, or the 'plasticity', of synapses is critical to uncovering how the brain converts our individual experiences into lasting memories. To this end, modulatory neurotransmitter systems play a central role in shaping synaptic processes involved in the encoding and retention of information within neural circuits. A fundamental problem remains, though, in understanding how extrinsic factors recruit, mimic or even hijack these endogenous regulatory signals to affect the development of brain systems critical to cognitive and mnemonic function.The mammalian nervous system undergoes several critical periods during early postnatal development in which cognitive and sensory functions are sensitive to lasting disruption by an array of environmental factors. Indeed, there is growing evidence to suggest that the hippocampal purinergic system in rodents also undergoes a period of significant maturation related to the expression pattern of the adenosine A1 receptor (A1R). Adenosine acts as a neuromodulator in the brain, and signalling at adenosine receptors in the hippocampus has been shown to influence memory formation and behaviour by regulating synaptic plasticity. In rats, the expression pattern of the A1R is not static during early development. In particular, the surface expression of the A1R increases significantly during adolescence in a specific part of the hippocampus, known as area Cornu Ammonis 2 (or CA2). However, changes in synaptic function and neuronal physiology in area CA2 following manipulations to disrupt adenosinergic signalling during development have yet to be explored in animal models.Consistent with the high expression of A1Rs in CA2 is the observation that caffeine and other A1R blockers preferentially enhance excitatory synaptic transmission in area CA2 at concentrations that have little effect on responses in other parts of the hippocampus. This suggests that A1Rs in CA2 may play a central role in mediating the cognitive enhancing effects of caffeine. Caffeine is a naturally-occurring cognitive enhancer that is widely-consumed to improve attention and augment memory. Its primary mechanism of action is thought to be through blockade of A1Rs, however, little is known about hippocampal area CA2 where A1Rs are highly expressed and where the surface expression of the A1R changes dramatically during adolescent development. The popularity of highly-caffeinated so-called 'energy' drinks has increased dramatically in the past 15 years, and recent trends highlight how teenagers consume energy drinks regularly and in quantities that exceed recommended daily limits. It is currently unknown, though, whether habitual consumption of energy drinks during adolescence is associated with lasting changes in brain function. Given that A1Rs increase dramatically during adolescence, together with the specificity that caffeine has in modifying synaptic function in CA2, the aim of this proposal is to evaluate the long-term effects of chronic caffeine administration on CA2 physiology and CA2-dependent forms of cognition. To achieve this, human adolescent energy drink consumption patterns will be modelled experimentally in rats and caffeine-induced changes in synaptic function, neuronal morphology and protein expression in CA2 will be assessed at the end of the dosing regimen. Evidence in support of a developmental window during which adolescents may be susceptible to lasting neurological dysfunction resulting from habitual caffeine consumption may provide justification to amend policy and regulate the sale of highly caffeinated products to vulnerable populations.