Marzagalli R, Leggio G M, Bucolo C, Pricoco E, Keay K A, Cardile V, Castorina S, Salomone S, Drago F, Castorina A
Department of Biomedical and Biotechnological Sciences, Section of Human Anatomy and Histology, University of Catania, Catania 95123, Italy.
Department of Biomedical and Biotechnological Sciences, Section of Pharmacology, University of Catania, Catania 95123, Italy.
Neuroscience. 2016 Mar 1;316:279-95. doi: 10.1016/j.neuroscience.2015.12.034. Epub 2015 Dec 21.
Dopamine D3 receptors (D3Rs) are implicated in several aspects of cognition, but their role in aversive conditioning has only been marginally uncovered. Investigations have reported that blockade of D3Rs enhances the acquisition of fear memories, a phenomenon tightly linked to the neuropeptide pituitary adenylate cyclase-activating peptide (PACAP). However, the impact of D3R ablation on the PACAPergic system in regions critical for the formation of new memories remains unexplored. To address this issue, levels of PACAP and its receptors were compared in the hippocampus and cerebral cortex (CX) of mice devoid of functional D3Rs (D3R(-/-)) and wild-types (WTs) using a series of comparative immunohistochemical and biochemical analyses. Morphometric and stereological data revealed increased hippocampal area and volume in D3R(-/-) mice, and augmented neuronal density in CA1 and CA2/3 subfields. PACAP levels were increased in the hippocampus of D3R(-/-) mice. Expression of PACAP receptors was also heightened in mutant mice. In the CX, PACAP immunoreactivity (IR), was restricted to cortical layer V in WTs, but was distributed throughout layers IV-VI in D3R(-/-) mice, along with increased mRNAs, protein concentration and staining scores. Consistently, PAC1, VPAC1 and VPAC2 IRs were variably redistributed in CX, with a general upregulation in cortical layers II-IV in knockout animals. Our interpretation of these findings is that disturbed dopamine neurotransmission due to genetic D3R blockade may enhance the PACAP/PAC1-VPAC axis, a key endogenous system for the processing of fear memories. This could explain, at least in part, the facilitated acquisition and consolidation of aversive memories in D3R(-/-) mice.
多巴胺D3受体（D3Rs）与认知的多个方面有关，但其在厌恶条件反射中的作用仅得到了少量揭示。研究报告称，阻断D3Rs可增强恐惧记忆的形成，这一现象与神经肽垂体腺苷酸环化酶激活肽（PACAP）密切相关。然而，D3R基因敲除对新记忆形成关键区域中PACAP能系统的影响仍未得到探索。为了解决这个问题，我们使用一系列比较免疫组织化学和生化分析方法，比较了缺乏功能性D3Rs的小鼠（D3R(-/-)）和野生型小鼠（WTs）海马体和大脑皮层（CX）中PACAP及其受体的水平。形态学和体视学数据显示，D3R(-/-)小鼠的海马体面积和体积增加，CA1和CA2/3亚区的神经元密度增大。D3R(-/-)小鼠海马体中的PACAP水平升高。突变小鼠中PACAP受体的表达也增强。在CX中，PACAP免疫反应性（IR）在WTs中仅限于皮层V层，但在D3R(-/-)小鼠中分布于IV-VI层，同时mRNA、蛋白质浓度和染色评分增加。一致地，PAC1、VPAC1和VPAC2 IRs在CX中发生了不同程度的重新分布，在基因敲除动物的皮层II-IV层普遍上调。我们对这些发现的解释是，由于基因阻断D3R导致的多巴胺神经传递紊乱可能会增强PACAP/PAC1-VPAC轴，这是处理恐惧记忆的关键内源性系统。这至少可以部分解释D3R(-/-)小鼠中厌恶记忆的形成和巩固得到促进的原因。
