脾后皮质(RSC)在处理上下文恐惧条件中起着核心作用。除了皮质皮质和丘脑皮质投射,RSC接收皮层下输入,包括脑桥被盖层中核的大量突起。这个GABA能投射包含神经肽,松弛素-3(RLN3),通过其Gi/o蛋白偶联受体抑制靶神经元,RXFP3。为了评估这种肽能系统在上下文恐惧条件下的作用,我们向成年大鼠RSC注射了腺相关病毒(AAV),表达嵌合RXFP3激动剂R3/I5或对照AAV,并使它们受到上下文恐惧条件的影响。注射R3/I5的大鼠与对照注射和未处理的大鼠相比没有显示出任何差异,但显示出显著延迟的灭绝。随后,我们采用特异性RXFP3激动剂肽的急性双侧注射,RXFP3-类似物2(A2),进入RSC。虽然在每次灭绝试验之前施用A2对灭绝过程没有影响,在每次采集试验前用A2处理导致延迟灭绝。在相关的解剖学研究中,我们在RSC的深层中检测到RLN3免疫反应性神经纤维的富集,RXFP3mRNA与囊泡GABA转运蛋白(vGAT)mRNA的共定位水平高于整个RSC的囊泡谷氨酸转运蛋白1(vGLUT1)mRNA,与RLN3/RXFP3信号传导对内在的影响一致,RSC内的抑制电路。这些发现表明,RSC中的上下文条件过程涉及,在某种程度上,RLN3传入激活局部抑制性神经元,提供更强的记忆获取,反过来,延缓灭绝进程。
The retrosplenial cortex (RSC) plays a central role in processing contextual fear conditioning. In addition to corticocortical and thalamocortical projections, the RSC receives subcortical inputs, including a substantial projection from the nucleus incertus in the pontine tegmentum. This GABAergic projection contains the neuropeptide, relaxin-3 (RLN3), which inhibits target neurons via its Gi/o-protein-coupled receptor, RXFP3. To assess this peptidergic system role in contextual fear conditioning, we bilaterally injected the RSC of adult rats with an adeno-associated-virus (AAV), expressing the chimeric RXFP3 agonist R3/I5 or a control AAV, and subjected them to contextual fear conditioning. The R3/I5 injected rats did not display any major differences to control-injected and naïve rats but displayed a significantly delayed extinction. Subsequently, we employed acute bilateral injections of the specific RXFP3 agonist peptide, RXFP3-Analogue 2 (A2), into RSC. While the administration of A2 before each extinction trial had no impact on the extinction process, treatment with A2 before each acquisition trial resulted in delayed extinction. In related anatomical studies, we detected an enrichment of RLN3-immunoreactive nerve fibers in deep layers of the RSC, and a higher level of co-localization of RXFP3 mRNA with vesicular GABA transporter (vGAT) mRNA than with vesicular glutamate transporter-1 (vGLUT1) mRNA across the RSC, consistent with an effect of RLN3/RXFP3 signalling on the intrinsic, inhibitory circuits within the RSC. These findings suggest that contextual conditioning processes in the RSC involve, in part, RLN3 afferent modulation of local inhibitory neurons that provides a stronger memory acquisition which, in turn, retards the extinction process.