Palate morphogenesis. V. Effects of cholinergic agonists and antagonists on rotation in embryo culture

Morphological studies have shown that the pterygopalatine ganglion in the day-14.5 mouse palatal shelf lies adjacent to the putative contractile system of region-2 cells in the posterior palate. It is of interest to learn whether the ganglion could influence rotation of the palate. Results of embryo culture experiments showed that acetylcholine appeared to play a role in posterior shelf rotation since cholinergic agents (pyridostigmine, bethanechol and carbachol) stimulated elevation of that end of the palate. To characterize the putative receptors mediating the effects of the cholinergic agonists on palate shelf elevation, muscarinic or nicotinic antagonists were introduced into the emrbyo culture system. Atropine, a muscarinic blocking agent, did not produce any significant effect on palate shelf rotation when tested in overnight and 2 h embryo cultures at 3 X 10(-5) M and 10(-4) M, respectively. Neither did atropine inhibit significantly the stimulation produced by 10(-8) M bethanechol, Thus, the cholinergic effect was probably not on muscarinic receptors of the contractile system. However, hexamethonium, a nicotinic antagonist, at 10(-6) M and 10(-4) M profoundly inhibited posterior shelf rotation to about 35% of the control value in a 2 h incubation. In addition, 10(-4) M hexamethonium inhibited posterior palate rotation to 11% of the control value after overnight culture. Furthermore, hexamethonium was able to reverse the stimulation of posterior rotation produced by carbachol. Partial inhibition of palate rotation by hexamethonium was also demonstrated when pregnant dams were injected with drug at doses approximately corresponding to 10(-6) and 10(-4) M. Hexamethonium treatment resulted in approximately 30% of the palates not completely rotated at day 15.5 compared to only 9.3% in the control. Hexamethoniuim also produced a significant increase in the palate gap and a comparable decrease in palate fusion. These effects were slightly greater at the posterior end of the palate. Thus, the cholinergic ganglion in the posterior palate may play a role in regulating shelf rotation at that end through a nicotinic pathway.