Olfactory Bulb Glomerular NMDA Receptors Mediate Olfactory Nerve Potentiation and Odor Preference Learning in the Neonate Rat

Rat pup odor preference learning follows pairing of bulbar beta-adrenoceptor activation with olfactory input. We hypothesize that NMDA receptor (NMDAR)-mediated olfactory input to mitral cells is enhanced during training, such that increased calcium facilitates and shapes the critical cAMP pattern. Here, we demonstrate, in vitro, that olfactory nerve stimulation, at sniffing frequencies, paired with beta-adrenoceptor activation, potentiates olfactory nerve-evoked mitral cell firing. This potentiation is blocked by a NMDAR antagonist and by increased inhibition. Glomerular dishinhibtion also induces NMDAR-sensitive potentiation. In vivo, in parallel, behavioral learning is prevented by glomerular infusion of an NMDAR antagonist or a GABAA receptor agonist. A glomerular GABAA receptor antagonist paired with odor can induce NMDAR-dependent learning. The NMDA GluN1 subunit is phosphorylated in odor-specific glomeruli within 5 min of training suggesting early activation, and enhanced calcium entry, during acquisition. The GluN1 subunit is down-regulated 3 h after learning; and at 24 h post-training the GluN2B subunit is down-regulated. These events may assist memory stability. Ex vivo experiments using bulbs from trained rat pups reveal an increase in the AMPA/NMDA EPSC ratio post-training, consistent with an increase in AMPA receptor insertion and/or the decrease in NMDAR subunits. These results support a model of a cAMP/NMDA interaction in generating rat pup odor preference learning.大鼠幼崽的气味偏好学习，依托于嗅球β肾上腺素能受体激活与嗅觉输入的配对范式。我们提出假说：训练过程中，介导嗅觉信号输入至僧帽细胞的N-甲基-D-天冬氨酸受体（NMDA receptor, NMDAR）活性会被增强，进而通过钙离子浓度升高促进并塑造关键的环磷酸腺苷（cAMP）信号模式。本研究通过体外实验证实：以嗅探频率给予嗅神经刺激，并与β肾上腺素能受体激活相配对，可增强嗅神经诱发的僧帽细胞放电活动。该突触增强效应可被NMDAR拮抗剂以及抑制性信号增强所阻断。嗅球肾小球去抑制同样可诱导依赖于NMDAR的突触增强效应。在体平行实验中，向嗅球肾小球灌注NMDAR拮抗剂或γ-氨基丁酸A型（GABAA）受体激动剂，可阻断行为学学习过程；若将GABAA受体拮抗剂与气味配对施用于嗅球肾小球，则可诱导依赖于NMDAR的学习行为。训练后5分钟内，气味特异性嗅球肾小球中的NMDA GluN1亚基发生磷酸化，这提示在学习习得阶段，NMDAR已被早期激活且钙离子内流增强。学习完成3小时后，GluN1亚基表达下调；训练后24小时，GluN2B亚基同样出现表达下调，上述分子事件或有助于维持记忆稳定性。使用经训练幼崽的嗅球进行的离体实验显示，训练后AMPA/NMDA兴奋性突触后电流（EPSC）比值升高，该结果与AMPA受体膜插入量增加或NMDAR亚基表达下调的结论相一致。上述研究结果支持了一个模型：环磷酸腺苷（cAMP）与NMDAR的相互作用参与介导大鼠幼崽的气味偏好学习过程。