PINEAL SYNAPTIC RIBBON NUMBERS AND MELATONIN SYNTHESIS OF RAT ARE RESISTANT TO GUANETHIDINE SYMPATHECTOMY

Chemical sympathectomy is widely used to study the impact of the norad renergic system on neuronal and neuroendocrine circuits. We tested the effects of intraperitoneal injections of guanethidine, an adrenergic neuron blocking agent, on selected functional parameters of the rat pi neal gland which are known to be under sympathetic influence. The reli ability of the method was demonstrated by the clear enophthalmus devel oped by experimental animals. However, neither the numbers of 'synapti c' ribbons nor melatonin synthesis differed between treated and contro l rats, both parameters exhibiting the nocturnal increase seen in inta ct animals. These results are in striking contrast to those obtained u pon chemical sympathectomy with 6-hydroxydopamine or surgical superior cervical ganglionectomy. We conclude that guanethidine is not capable of sufficiently removing noradrenergic influence from the rat pineal gland, and that this substance is thus inferior to other experimental methods of sympathectomy.