Marra Vincenzo, Kemenes Ildikó, Vavoulis Dimitris, Feng Jianfeng, O'Shea Michael, Benjamin Paul R
Sussex Centre for Neuroscience, School of Life Sciences, University of Sussex Brighton, East Sussex, UK.
Front Behav Neurosci. 2010 Sep 15;4. doi: 10.3389/fnbeh.2010.00161. eCollection 2010.
Changes in the strength of excitatory synaptic connections are known to underlie associative memory formation in the molluscan nervous system but less is known about the role of synaptic inhibition. Tonic or maintained synaptic inhibition has an important function in controlling the Lymnaea feeding system and is known to suppress feeding in the absence of food or in satiated animals. Tonic inhibition to the feeding network is provided by the N3t interneuron that has inhibitory monosynaptic connection with the central pattern generator interneuron, the N1M. Here we asked whether a reduction in the level of tonic inhibition provided by the N3t cell could play a role in reward conditioning? Semi-intact preparations made from hungry snails were conditioned using a previously developed one-trial chemical conditioning paradigm. We recorded electrical activity in a feeding motoneuron, the B3, at various time-points after conditioning. This allowed us to measure the frequency of spike activity in the N3t interneuron and monitor fictive feeding patterns that generate the rhythmic movements involved in food ingestion. We show that there is a reduction in N3t spiking at 1, 2, 3, and 4 h after conditioning but not at 10 and 30 min and the reduction in N3t firing inversely correlates with an increase in the conditioned fictive feeding response. Computer simulation of N3t-N1M interactions suggests that changes in N3t firing are sufficient to explain the increase in the fictive feeding activity produced by conditioning. A network model is presented that summarizes evidence suggesting that reward conditioning in Lymnaea is due to the combined effects of reduced tonic inhibition and enhanced excitatory synaptic connections between the CS pathway and feeding command neurons.
已知兴奋性突触连接强度的变化是软体动物神经系统中联想记忆形成的基础，但关于突触抑制的作用却知之甚少。持续性或维持性突触抑制在控制椎实螺进食系统中具有重要作用，并且已知在没有食物或动物饱腹时会抑制进食。对进食网络的持续性抑制由N3t中间神经元提供，该神经元与中央模式发生器中间神经元N1M具有抑制性单突触连接。在这里，我们探讨了由N3t细胞提供的持续性抑制水平的降低是否在奖赏条件作用中发挥作用？使用先前开发的单次化学条件作用范式对饥饿蜗牛制成的半完整标本进行条件作用。我们在条件作用后的不同时间点记录了进食运动神经元B3中的电活动。这使我们能够测量N3t中间神经元的动作电位发放频率，并监测产生食物摄取中涉及的节律性运动的虚拟进食模式。我们发现，在条件作用后1、2、3和4小时，N3t的发放减少，但在10和30分钟时没有减少，并且N3t放电的减少与条件性虚拟进食反应的增加呈负相关。对N3t - N1M相互作用的计算机模拟表明，N3t放电的变化足以解释条件作用产生的虚拟进食活动的增加。本文提出了一个网络模型，该模型总结了相关证据，表明椎实螺中的奖赏条件作用是由于持续性抑制的降低以及条件刺激通路与进食指令神经元之间兴奋性突触连接增强的综合作用。
