Investigations into the organisation and morphology of vagal preganglionic neurones and the anatomical identification of the chemistry and origin of some of their synaptic inputs
The investigations in this thesis involve the use of neuroanatomical techniques in the study of vagal preganglionic neurones (VPNs) in the medulla oblongata of the rat and cat. Light microscopic examination of VPNs identified by the injection of horseradish peroxidase (HRP) into the cervical vagus nerve of the rat revealed that they lay mainly in two areas of the medulla - the dorsal vagal nucleus (DVN) and the nucleus ambiguus (NA). Labelled cells in the DVN (median diameter 18jim) were arranged in a tightly packed group. In contrast, labelled neurones in the NA were arranged in two identifiable groups. The most dorsal of these groups consisted of tightly packed cells with median diameter of 30μm (compact group - cNA) whereas the other group were smaller (median D 25μm) and were scattered in ventrolateral regions of the medulla (vINA). Neurones in each group were identified as being statistically different in diameter and soma area from those in other groups (Student's t-test). Cardiac vagal preganglionic neurones (CVPNs), retrogradely labelled by the injection of cholera toxin-HRP into the right atrium, were located mainly in the vINA with median soma diameter 25μm. Ultrastructural examination of the groups of VPNs revealed that they shared some morphological characteristics. VPNs in the NA, particularly those in the vINA, were observed to be intermingled with neurones retrogradely labelled from the phrenic motor nucleus in the spinal cord and were often morphologically indistinguishable. VPNs in the NA were also intermingled with neuropeptide Y immunoreactive neurones. The chemistry of inputs to VPNs was examined using a combination of retrograde tracing to identify VPNs, immunocytochemistry to detect various neurotransmitter chemicals, and electron microscopy. Serotonin, substance P and neuropeptide Y were identified in boutons forming asymmetric type synaptic contacts with VPNs in the nucleus ambiguus of the rat which were retrogradely labelled from the heart or the cervical vagus nerve. The origin of inputs to VPNs was investigated by the ionophoretic injection of HRP into regions of the cat ventral medulla where antidromic potentials were recorded to stimulation of the cervical vagus nerve. Retrogradely labelled cells were localised both contralaterally and ipsilateral ly in the nucleus tractus solitarius, raphe nuclei, parabrachial nucleus, periaqueductal gray matter and the NA. To determine if the NTS was a source of synaptic input to VPNs in the NA, VPNs were identified by retrograde tracing followed by the ionophoretic injection of the anterograde tracer biocytin into regions of the NTS where evoked potentials were recorded to stimulation of the carotid sinus or cervical vagus nerve. Light microscopic examination revealed anterogradely labelled boutons and fibres in the NA, some of which were in close association with retrogradely labelled VPNs. Subsequent electron microscopic examination revealed some of these boutons form synaptic specialisations with retrogradely labelled VPNs
