Neuropeptides are widely used as neurotransmitters in vertebrates and invertebrates. In vertebrates, a detailed understanding of their functions as transmitters has been hampered by the complexity of the nervous system. The marine mollusk Aplysia, with a simpler nervous system and many large, identified neurons, presents several advantages for addressing this question and has been used to examine the roles of tens of peptides in behavior. To screen for other peptides that might also play roles in behavior, we observed immunoreactivity in individual neurons in the central nervous system of adult Aplysia with antisera raised against the Aplysia peptide FMRFamide and two mammalian peptides that are also found in Aplysia, cholecystokinin (CCK) and neuropeptide Y (NPY), as well as serotonin (5HT). In addition, we observed staining of individual neurons with antisera raised against mammalian somatostatin (SOM) and peptide histidine isoleucine (PHI). However, genomic analysis has shown that these two peptides are not expressed in the Aplysia nervous system, and we have therefore labeled the unknown peptides stained by these two antibodies as X_SOM and X_PHI. There was an area at the anterior end of the cerebral ganglion that had staining by antisera raised against many different transmitters, suggesting that this may be a modulatory region of the nervous system. There was also staining for X_SOM and, in some cases, FMRFamide in the bag cell cluster of the abdominal ganglion. In addition, these and other studies have revealed a fairly high degree of colocalization of different neuropeptides in individual neurons, suggesting that the peptides do not just act independently but can also interact in different combinations to produce complex functions. The simple nervous system of Aplysia is advantageous for further testing these ideas.

中文翻译：

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海兔中枢神经系统中几种肽神经递质的分布、细胞定位和共定位

神经肽广泛用作脊椎动物和无脊椎动物的神经递质。在脊椎动物中，神经系统的复杂性阻碍了对其作为递质的功能的详细了解。海洋软体动物海兔具有更简单的神经系统和许多大型的、已识别的神经元，在解决这个问题方面具有多种优势，并已被用来研究数十种肽在行为中的作用。为了筛选可能也在行为中发挥作用的其他肽，我们观察了成年海兔中枢神经系统中单个神经元的免疫反应性，并使用针对海兔肽 FMRFamide 和海兔中也发现的两种哺乳动物肽胆囊收缩素 (CCK)产生的抗血清。和神经肽 Y (NPY)，以及血清素 (5HT)。此外，我们观察了针对哺乳动物生长抑素（SOM）和肽组氨酸异亮氨酸（PHI）的抗血清对单个神经元的染色。然而，基因组分析表明，这两种肽在海兔神经系统中并不表达，因此我们将这两种抗体染色的未知肽标记为X _SOM和X _PHI。大脑神经节前端有一个区域被针对许多不同递质的抗血清染色，表明这可能是神经系统的调节区域。腹部神经节的袋细胞簇中也有 X _{SOM染色，在某些情况下还有 FMRFamide 染色。}此外，这些和其他研究揭示了不同神经肽在单个神经元中具有相当高程度的共定位，这表明这些肽不仅独立发挥作用，而且还可以以不同的组合相互作用以产生复杂的功能。海兔的简单神经系统有利于进一步检验这些想法。