Histochemical and immunocytochemical investigations of the marginal nuclei in the spinal cord of pigeons (Columba livia)

Abstract

In birds there are segmentally organized marginal nuclei at the lateral or ventrolateral border of the spinal cord. In most regions of the spinal cord these nuclei are within the outline of the cord. However, in the lumbosacral region they form accessory lobes protruding into the vertebral canal. Histochemical and immunocytochemical investigations were performed to study the neurochemical features of the marginal nuclei of the pigeon. Despite histological differences (only accessory lobe neurons are embedded in glia-derived glycogen cells), there was no difference in the chemical neuroanatomy of the two types of marginal nuclei. These nuclei contained cholinergic neurons and there was also evidence for a cholinergic innervation. NADPH-diaphorase activity, which is considered to indicate nitric oxide synthesis, was faint in marginal neurons. No serotonin immunoreactivity was found. However, all neurons showed immunoreactivity to glutamate and glycine, and some were immunoreactive to γ-aminobutyric acid (GABA). A GABAergic innervation of non-GABAergic neurons could also be demonstrated. The lack of difference in the chemical neuroanatomical features between cervical marginal nuclei and lumbosacral accessory lobes suggests a similar origin of all marginal neurons. A comparison with the chemical neuroanatomy of marginal neurons in other vertebrates shows both similarities and differences.

Introduction

Neurons at the lateral margin of the spinal cord form segmentally organized nuclei in birds and reptiles [19]. Marginal neurons have also been found in other classes of vertebrates 1, 19, 31 and the best characterized marginal cells are the edge cells in lampreys 13, 14, 15. In birds there are additional specializations in the lumbosacral enlargement where the marginal neurons are embedded in glia-derived glycogen cells. These cells protrude into the vertebral canal forming accessory lobes [20], which are also named Hofmann’s major nuclei as compared to Hofmann’s minor nuclei in all other segments [19]. In the present investigation the lumbosacral marginal nuclei will be named accessory lobes, and those of all other segments will be defined as marginal nuclei. In the lumbosacral region the marginal neurons have to be distinguished from paragriseal cells 17, 19 located nearby in the white matter of the lateral and ventral funiculi. Paragriseal cells are very numerous in lumbosacral segments and have been shown to project to the cerebellum both in the pigeon [22] and in the chicken [38].

Recent investigations suggest that the accessory lobes may have a sensory function in a putative sense organ of equilibrium located in the avian synsacrum 24, 26. Marginal nuclei in the cervical region of the pigeon receive input from peripheral primary afferent fibers, which suggests that they function as relays in an afferent pathway [25]. Marginal cells have been characterized neurochemically in lampreys [36], elasmobranchs 1, 2, turtles [12], and chickens 3, 11.

To characterize further neurons of the marginal nuclei in the pigeon, we examined the distribution of neurotransmitters (γ-aminobutyric acid [GABA], glutamate, glycine, serotonin) and of the neurotransmitter-related enzymes acetylcholine esterase (AChE), NADPH-diaphorase (NADPHd), and choline acetyltransferase (ChAT). Because of the histological differences between the normal marginal nuclei and the accessory lobes, we compared both types of marginal nuclei and related the results to the chemical neuroanatomy of marginal nuclei of other vertebrates.