Cholinergic and VIPergic vasodilator actions of parasympathetic nerves on the thyroid blood flow in rats.

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Abstract
The present experiment was performed to investigate the effect of stimulating the parasympathetic superior laryngeal nerve (SLN) on thyroid blood flow and its mediator substances in urethane-chloralose anesthetized rats. Thyroid blood flow was measured by counting number of blood drops from a thin catheter inserted into the thyroid vein. SLNs were cut bilaterally and their peripheral portions were electrically stimulated. Electrical stimulations (intensity, 10V; pulse duration, 0.5ms) of SLNs increased thyroid blood flow in a frequency-dependent manner as stimulus frequencies increased from 2 to 40Hz. The intravenous administration of atropine (0.5mg/kg) reduced these responses, but did not abolish them. The basal secretion rate of vasoactive intestinal peptide (VIP). from thyroid glands in the resting state was nearly zero (0.25±0.14pg/(kg•min)). SLN stimulation increased markedly the VIP secretion rate to 3.40±0.64pg/(kg•min). The VIP secretion responses evoked by SLN stimulation remained in atropinized rats. Furthermore, exogenously applied VIP increased the thyroid blood flow dose-dependently. These results suggest that SLN stimulation increases the thyroid blood flow by dilating thyroid blood vessels via activation of cholinergic and non-cholinergic (probably VIP-containing) nerve fibers. Thus, these parasympathetic vasodilation systems may play a supplementary role in regulating the secretion of thyroid hormone by changing the thyroid blood flow in addition to the role of hormonal regulation by the thyroid stimulating hormone (TSH).