Observation of the ultrastructure and synaptic relationships of angiotensin II-like immunoreactive neurons in the rat area postrema

The ultrastructure and synaptic relationships of the angiotensin II-contain ing neurons in the area postrema of the rat were studied by immunocytochemi stry using the avidin-biotin-complex-DAB method, and also using silver-gold intensification following the DAB reaction. At the light microscopic level , the angiotensin II-like immunoreactive neurons were observed within the a rea postrema, especially in the upper region. At the electron microscopic l evel, the angiotensin II-like immunoreactive cell bodies were observed as h aving a round, unindented nucleus. The nuclei of these neurons were not imm unostained. The angiotensin II-like immunoreactive axon terminals often con tained a few dense core vesicles in addition to many small clear synaptic v esicles. Numerous axon terminals were found to make synapses on immunonegat ive dendrites; they were also found to make synapses on angiotensin II-like immunoreactive dendrites. Many angiotensin II-like immunoreactive dendrite s received synapses from immunonegative axon terminals. Although angiotensi n II-like immunoreactive cell bodies were sometimes postsynaptic to immunor eactive axon terminals, they did not receive synapses from immunonegative a xon terminals. These results provide solid morphological evidence of AP end ogenous angiotensin II and confirm that in spite of circulating angiotensin II, the local neurons in the AP may also play an important role in angiote nsin II-induced cardiovascular regulation. (C) 2000 Wiley-Liss, Inc.