Eisthen H L, Northcutt R G
Neurobiology Unit, Scripps Institution of Oceanography, San Diego, California 92093-0201, USA.
J Comp Neurol. 1996 Jun 24;370(2):159-72. doi: 10.1002/(SICI)1096-9861(19960624)370:2<159::AID-CNE3>3.0.CO;2-1.
The terminal nerve is a ganglionated cranial nerve with peripheral processes that enter the nasal cavity and centrally directed processes that enter the forebrain. Members of all classes of gnathostomes have been found to possess a terminal nerve, some components of which demonstrate immunoreactivity to the peptides Phe-Met-Arg-Phe-NH2 (FMRFamide) and gonadotropin-releasing hormone (GnRH). To explore the possibility that lampreys possess a terminal nerve, we examined the distribution of these peptides in the silver lamprey, Ichthyomyzon unicuspis, by using antisera to FMRFamide and to four forms of GnRH. We found cells with FMRFamide-like immunoreactivity in the preoptic area and the isthmal gray region of the mesencephalon, and found labeled fibers throughout the preoptic-infundibular region. Occasional labeled fibers were scattered through many regions of the brain, including the optic nerve and olfactory bulb; however, unlike species that possess a terminal nerve, lampreys have no immunoreactive cells or fibers in the olfactory nerve or nasal epithelia. In addition, we observed GnRH-immunoreactive cell bodies in the preoptic area of all animals and in the ventral hypothalamus of one individual. Most of the labeled fibers extended ventrally to the hypothalamus, with other fibers extending throughout the striatum and hypothalamic-neurohypophyseal region. A few fibers in other regions, including the optic nerve, were also labeled; we detected no immunoreactivity in the olfactory bulb, olfactory nerve, or nasal epithelia. The use of different GnRH antisera resulted in remarkably similar patterns of labeling of both cells and fibers. In summary, we did not observe either GnRH or FMRFamide-like immunoreactivity in the olfactory regions that represent the typical path of terminal nerve fibers, nor were we able to locate a terminal nerve ganglion. We conclude that lampreys may lack a terminal nerve, and that the previously described fiber bundle extending from the nasal sac to the ventral forebrain may constitute an extra-bulbar olfactory pathway.
终神经是一条有神经节的脑神经，其外周突起进入鼻腔，中枢突起进入前脑。已发现所有类别的颌口脊椎动物都有终神经，其一些成分对肽苯丙氨酸-甲硫氨酸-精氨酸-苯丙氨酸-酰胺（FMRF酰胺）和促性腺激素释放激素（GnRH）表现出免疫反应性。为了探究七鳃鳗是否有终神经，我们通过使用针对FMRF酰胺和四种形式GnRH的抗血清，研究了这些肽在银七鳃鳗（Ichthyomyzon unicuspis）中的分布。我们在视前区和中脑峡部灰色区域发现了具有FMRF酰胺样免疫反应性的细胞，并在整个视前-漏斗区域发现了标记纤维。偶尔有标记纤维散布在大脑的许多区域，包括视神经和嗅球；然而，与有终神经的物种不同，七鳃鳗在嗅神经或鼻上皮中没有免疫反应性细胞或纤维。此外，我们在所有动物的视前区和一只个体的腹侧下丘脑观察到了GnRH免疫反应性细胞体。大多数标记纤维向腹侧延伸至下丘脑，其他纤维则贯穿纹状体和下丘脑-神经垂体区域。其他区域（包括视神经）的一些纤维也有标记；我们在嗅球、嗅神经或鼻上皮中未检测到免疫反应性。使用不同的GnRH抗血清导致细胞和纤维的标记模式非常相似。总之，我们在代表终神经纤维典型路径的嗅觉区域未观察到GnRH或FMRF酰胺样免疫反应性，也未能找到终神经节。我们得出结论，七鳃鳗可能没有终神经，并且先前描述的从鼻囊延伸至腹侧前脑的纤维束可能构成一条球外嗅觉通路。
