Limbic modulation of the autonomic nervous system

Abstract

Nuclei in the hypothalamus and brainstem are responsible for maintaining homoeostasis by controlling rhythmic motor and autonomic activities. However, during emotional responses to real or potential environmental challenges, significant changes in motor and autonomic rhythm also occur.Forebrain structures that form the limbic system are responsible for perceiving environmental challenges and orchestrating appropriate emotional responses. Thus, it is only through descending synaptic interactions of the forebrain areas with the hypothalamus and brainstem, that modulation of motor and autonomic activities is possible during the expression of emotions. However, there is little knowledge of the forebrain areas that modulate autonomic activities. Also, in coordinating physiologic responses, forebrain structures operate through functional networks but the neural pathways and mechanisms involved particularly during emotional behaviours are not clear.The hippocampus is an important component of the limbic system involved in learning and memory, but evolving evidence implicates it in the expression of emotions such as defensive fear and anxiety reactions. Studies also suggest possible hippocampal connections with brainstem autonomic centres.Thus, the central hypothesis tested in this thesis is that discrete neurone populations in the ventral hippocampus, via neuronal projections to the amygdala, can modulate bulbar motor and autonomic output. Specific aims were to 1) Investigate and describe the respiratory and cardiovascular changes induced by chemically mapping the dorsal and ventral hippocampus with microinjections of the excitatory amino acid, D, L-Homocysteic acid, 2) Investigate the distribution of descending projections of the hippocampus using classical neuroanatomical tract tracing techniques, with a particular focus on the connectivity of the ventral hippocampus and the amygdala, 3) To extend the chemical mapping to specific areas in the amygdala that receive projections from the hippocampus and determine how such areas differ in their influence over motor and autonomic functions and 4) To investigate the effects of inhibiting of the amygdala while stimulating the ventral hippocampus.Experiments were conducted using urethane-anaesthetised (1.5 g/kg IP) adult Sprague-Dawley rats (n = 136; weight = 300 -600 g; either sex). The hippocampus was stereotaxically mapped to locate discrete neurone populations that can modulate motor and autonomic activities. The specific parameters monitored include blood pressure, heart rate, respiratory frequency, inspiratory and expiratory durations, tracheal pressure and the motor expression of augmented breaths.Microinjections of the excitatory amino acid (EAA), D, L, Homocysteic acid (DLH; 50 mM, pH 7.4, iii n = 95), were used for chemical stimulation. Using this stimulation technique, discrete neurone populations in the ventral hippocampus were identified that can generate significant changes in both respiratory and cardiovascular parameters. Augmented breaths, which have...