Vogt M B, Mistretta C M
Department of Biologic and Materials Sciences, School of Dentistry, University of Michigan, Ann Arbor 48109-1078.
J Neurosci. 1990 Sep;10(9):3148-57. doi: 10.1523/JNEUROSCI.10-09-03148.1990.
To determine the type and extent of neural rearrangements that are made during functional differentiation of circuits for salt taste processing, we determined receptive field size and salt response characteristics of second-order taste cells in 3 age groups of sheep. Neurophysiological recordings were made from single cells in the nucleus of the solitary tract (NST) in fetal, perinatal, and postnatal sheep. Responses to NH4Cl, NaCl, and KCl were measured, and location and number of fungiform papillae in the receptive field were determined by stimulating individual papillae with anodal electrical current. The data are compared with previous, parallel measures from chorda tympani nerve afferent taste fibers to permit conclusions about convergence or divergence onto second-order cells. Receptive field size of second-order taste neurons increases during development, in contrast to the decrease in field size observed previously for chorda tympani nerve fibers during the same period. Furthermore, receptive fields of second-order cells are significantly larger than those of first-order fibers at perinatal and lamb ages, but not fetal. Thus, there is convergence of first-order taste afferents onto brain-stem neurons, and the convergence increases remarkably between fetal and perinatal periods. Associated with the increase in convergence are increased salt response frequencies relative to afferent fibers for NaCl in perinatal animals and lambs, and for KCl in lambs. The increase in frequencies occurs before NST neurons are functionally mature, as indicated by the rapid response adaptation of many cells in young animals. Convergence in NST during development apparently functions to maximize gain for processing neural responses to NaCl. In the periphery, response frequencies to NaCl are very low in fetuses, and increase progressively during development. In the NST, NaCl response frequencies are high even in fetuses, and remain high. The process of convergence onto second-order cells is accomplished with maintenance of order in afferent projections because receptive fields of NST neurons are composed of fungiform papillae that are clustered together, not dispersed over the tongue. Our quantification of taste receptive field size at 2 neural levels provides strong evidence for increasing convergence in the NST during development. Altering patterns of afferent neural input and geometry of second-order neurons may have a role in establishing convergence. The convergence has an apparently special function: to increase gain for NaCl taste sensation. Therefore, neural rearrangements during differentiation of salt taste pathways result in specific functional outcomes.
为了确定在盐味处理回路的功能分化过程中发生的神经重排的类型和程度，我们测定了3个年龄组绵羊中二阶味觉细胞的感受野大小和盐反应特性。对胎儿期、围产期和出生后绵羊的孤束核（NST）中的单个细胞进行了神经生理学记录。测量了对氯化铵、氯化钠和氯化钾的反应，并通过用阳极电流刺激单个乳头来确定感受野中菌状乳头的位置和数量。将这些数据与先前从鼓索神经传入味觉纤维获得的平行测量结果进行比较，以便得出关于向二阶细胞的汇聚或发散的结论。与之前观察到的同一时期鼓索神经纤维感受野大小减小相反，二阶味觉神经元的感受野大小在发育过程中增加。此外，在围产期和羔羊期，二阶细胞的感受野明显大于一阶纤维的感受野，但在胎儿期并非如此。因此，一阶味觉传入纤维汇聚到脑干神经元上，并且这种汇聚在胎儿期和围产期之间显著增加。与汇聚增加相关的是，围产期动物和羔羊中相对于氯化钠传入纤维以及羔羊中相对于氯化钾传入纤维的盐反应频率增加。频率增加发生在NST神经元功能成熟之前，这表现为许多幼小动物细胞的快速反应适应。发育过程中NST中的汇聚显然起到了最大化处理对氯化钠神经反应增益的作用。在周围，胎儿对氯化钠的反应频率非常低，并在发育过程中逐渐增加。在NST中，即使在胎儿期，氯化钠反应频率也很高，并保持高位。由于NST神经元的感受野由聚集在一起而不是分散在舌头上的菌状乳头组成，所以向二阶细胞的汇聚过程是在传入投射保持有序的情况下完成的。我们在两个神经水平对味觉感受野大小的量化为发育过程中NST中汇聚增加提供了有力证据。改变传入神经输入模式和二阶神经元的几何结构可能在建立汇聚中起作用。这种汇聚具有明显的特殊功能：增加对氯化钠味觉的增益。因此，盐味通路分化过程中的神经重排导致了特定的功能结果。
