Acetylcholinesterase (AChE) activity has been demonstrated ultrastructurally in neurons of the arcuate nucleus and associated with fibers in the arcuate nucleus neuropil and the median eminence (ME) of the rat. In addition, the effects of neonatal monosodium glutamate (MSG) treatment and Halasz deafferentation on the AChE staining and localization have been studied. Neonatal MSG-treatment resulted in loss of the majority of AChE positive neurons in the arcuate nucleus while leaving neuropil staining intact. Halasz deafferentation caused a loss of arcuate neuropil activity while leaving the neuronal staining unaltered. These observations are consistent with previous biochemical results suggesting the existence of a cholinergic tuberoinfundibular system with nerve cells in the arcuate nucleus and terminals in the median eminence. In addition, the deafferentation experiments indicated that extra hypothalamic cholinergic fibers may innervate the arcuate nucleus. Supporting evidence from other biochemical studies and the curious paucity of histochemical and biochemical AChE activity in the ME are also discussed.
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