Direct And Indirect Connections Between Primary Visceral Sensory Center In The Medulla And The Central Nucleus Of Amygdala And Related Neurotransmitters In These Pathways

The nucleus of the solitary tract has been thought to be a very importment primary center for visceral sensation. It receives the sensory information from the peripheral nerves, then relays it to forebrain via the direct or indirect connections. The central nucleus of the amygdala is one of forebrain structures, which is involved in visceral activities. However, the morphological details of these connections have not been delineated yet and the synaptic organization about these pathways has not been reported at electron microscoptic level. In present studies, a series of experiments (Chapter I-IV) have been performed to reveal details of the direct and indirect connections between the nucleus of the solitary tract and the central nucleus of the amygdala and related neurotransmitters in these pathways. The ultrastructural studies are stressed in the present papers.I. Evidence of GABAergic Control Over the CatecholaminergicProjection from the Medulla Oblongata to the Central Nucleus of the AmygdalaOne of the major pathway of visceral information to the forebrain is from the nucleus of the solitary tract(NTS) and ventrolateral medulla (VLM) to the central nucleus of the amygdala (Ce). Reversely, the Ce innervates the NTS and VLM. In order to better understand the significance of the reciprocal connection between the medulla and Ce in the rat, experiments have been performed to study the topography, synaptic organization and contained neurotransmitters of the medulla-amygdala projection and to determine whether gamma-aminobutyric acid (GABA) immunostaining neurons in the Ce project to the medulla and make direct synaptic contacts with catecholaminergic neurons of the NTS and VLM. This was achieved by combining the anterograde and retrograde transport of the lectin-conjugated horseradish peroxidase (WGA-HRP) with the pre-embedding immunocytochemistry for tyrosine hydroxylase (TH), and postembedding immunocytochemistry for GABA. Following injection of WGA-HRP in the Ce, retrograde labelling occurred primarily in the caudal NTS and VLM, and anterograde labelling was mainly located in the rostral NTS and VLM. The study of retrograde tracing with WGA-HRP combined with the immunocytochemistry for TH revealed that approximately 80% of projection neurons in the NTS and 90% in the VLM were TH-immunoreactive (TH-IR). Most of them were confined in the area around the level of the obex, predominantly ipsilateral to the WGA-HRP injection site. Some anterogradely labeled fibers from the Ce were present in close proximity to the TH-IR processes in the rostral NTS at the level of light microscope. Electron microscopic study showed that the GABAergic and non-GABAergic terminals formed synaptic contacts with the TH-IR or Ce-projecting or TH-IR/Ce-projecting double-labeled neurons of the NTS and VLM.