In vitro Effect of Thyroxine on Cholinergic Neurotransmission in Rat Sympathetic Superior Cervical Ganglion

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Abstract
This study aimed at examining the effect of thyroid hormones on cholinergic transmission in isolated rat superior cervical ganglia (SCG). In SCG explants incubated with 3H-choline, thyroxine (T4) and 3,3’,5-triiodothyronine (T3) added to the medium before a second depolarization stimulus of 60 mM K+ resulted in a dose-dependent increase of S2/S1 ratio for 3H release. The concentration of hormone that produced 50% of maximal increase in K+-induced radioactivity release was 8 × 10–9 M for T4 and 1.6 × 10–8M for T3 while 3,3’,5,5’-tetraiodothyroacetic acid was almost ineffective. Preincubation of SCG with 10–7M iopanoic acid for 30 min before S2, although not affecting by itself S2/S1 ratio, effectively prevented the increase given by T4 or T3. 3H-acetylcholine release by SCG was augmented in a high K+ and the effect was amplified by T4 to a similar extent as that for total 3H release. When added to the incubation medium together with 60 mM K+ for 30 min, T4 (10–7M) increased significantly the activity of choline acetyltransferase (ChAT). T4 did not affect ChAT activity in SCG exposed to 4.7 mM K+, nor in SCG homogenates. 3H-choline uptake measured immediately after exposure of SCG to 60 mM K+ decreased by 25%, whereas it increased by 71% after a subsequent 30-min incubation with 4.7 mM K+. Addition of 10–7M T4 prevented the changes in choline uptake observed in a high K+ medium. These results indicate that T4 increases SCG cholinergic transmission.

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