Senior Honors Projects, 2010-current

Date of Award

Fall 2017

Document Type

Thesis

Degree Name

Bachelor of Science (BS)

Department

Department of Biology

Advisor(s)

Dr. Justin Brown

Abstract

Sudden Infant Death Syndrome (SIDS) is a leading cause of infant mortality. Alterations in brainstem development of Serotonin (5HT) and γ-aminobutyric acid (GABA) are linked to its cause. The sympathetic premotor neurons located in the Nucleus of the Raphe Pallidus (NRP) in the brainstem have both 5-HT1A and GABA-A receptor subtypes that mediate protective cardiovascular responses to environmental stress. It is hypothesized that alteration in these receptors at the NRP will also modify protective thermoregulatory responses to hypoxic stress such as hypothermia. Using aseptic techniques, male and female Sprague-Dawley rats (230-385g) were instrumented with radiotelemetry probes to non-invasively measure core temperature (Tc). Using a stereotaxic device, a steel cannula was also inserted into the brainstem, which allowed microinjection at the NRP. After recovery (1 week), rats were housed in a thermal gradient that allowed them to select their ambient temperature (STa) and thereby facilitated behavioral thermoregulation. Once acclimated to the gradient, 30 mM of either a 5-HT1A agonist (8OH-DPAT or “DPAT”), antagonist (WAY100635), a GABA-A agonist (Muscimol), antagonist (Bicuculine) or ACSF (control vehicle) was then microinjected into the NRP immediately before exposure to either 6% O2 (hypoxia) or 21% O2 (normoxia) for 60 min. In the normoxic control group, the typical short-term hyperthermic response to handling was abolished in rats injected with Muscimol but persisted in all other groups. Rats exposed to 6% O2 stress (hypoxia) demonstrated a typical hypothermic response. When the 5-HT1A receptor was activated by DPAT in hypoxic rats, this protective hypothermic response was significantly exacerbated compared to the control (ACSF). Similar responses resulted from injection of the 5-HT1A antagonist (WAY). Modulation of the GABA-A receptor however had no effect versus ACSF injection. Also in the hypoxic stress group, there were mild decreases in STa of ACSF injected rats (4.3˚C) which was exacerbated in DPAT injected rats (8˚C). Rats injected with WAY seemed to reverse this trend initially with an increase in STa (3˚C) which quickly faded. Importantly, the STa responses to hypoxic stress helped facilitate Tc changes suggesting coordination between behavioral and autonomic thermoregulatory mechanisms that facilitated the protective hypothermic response. Previous data confirmed that GABA-A and 5-HT1A receptors in the NRP help mediate cardiovascular responses to stress. Although there was a sympatholytic effect from Muscimol injection in the normoxic group, the GABA-A receptors do not seem to facilitate thermoregulatory responses to hypoxic stress. This suggests that hypoxic stress is such a salient stimulus that it over rides the sympatholytic effect of GABA-A receptor activation or does so via alternative pathways. Hypoxic stress may instead mediate its hypothermic responses via the 5-HT1A receptor. When that receptor is activated by DPAT, there is an exacerbation of the hypothermic response to hypoxia. The behavioral thermoregulatory responses also facilitate the hypothermia in this group. The results from blockade of the 5-HT1A receptor with WAY are confounded by the short half-life of this receptor antagonist. Alterations in 5HT neuronal development in the brain stem may cause inadequate behavioral (STa decrease) and autonomic (Tc decrease) thermoregulatory responses to hypoxic stress. Understanding how dysfunction in brainstem 5HT leads to impairment in these protective responses to stress is essential in the diagnosis, and eventual prevention, of SIDS.

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