Reciprocal synaptic relationships between angiotensin II-containing neurons and enkephalinergic neurons in the rat area postrema

A preembedding double immunostaining technique was used to study synaptic relationships between angiotensin-II-like immunoreactive and enkephalin-like immunoreactive neurons in the rat area postrema. The angiotensin-II-like immunoreactive neurons were detected by silver-gold intensification of the DAB reaction results while the enkephalin-like immunoreactive neurons were detected by simple ABC-DAB reaction. The synaptic relationships were reciprocal between the two neurons. Most of the synapses found between these two neurons were the presynaptic enkephalin-like immunoreactive axon terminals that made synapses on the angiotensin-II-like immunoreactive perikarya and dendrites. Both the axo-somatic and axo-dendritic synapses were symmetrical. However, although angiotensin-II-like immunoreactive axon terminals also made synapses on enkephalin-like perikarya and dendrites, the axo-somatic synapses were symmetrical, while the axo-dendritic synapses were asymmetrical. The present results confirm the presence of angiotensin-II-like immunoreactive neurons in the area postrema and suggest that these angiotensinergic neurons in the area postrema may play a role in the regulation of blood pressure via coordinated synaptic interactions with enkephalinergic neurons.

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-containing neurons in the area postrema of the rat were studied by immunocytochemistry 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 area postrema, especially in the upper region. At the electron microscopic level, the angiotensin II-like immunoreactive cell bodies were observed as having a round, unindented nucleus. The nuclei of these neurons were not immunostained. The angiotensin II-like immunoreactive axon terminals often contained a few dense core vesicles in addition to many small clear synaptic vesicles. Numerous axon terminals were found to make synapses on immunonegative dendrites; they were also found to make synapses on angiotensin II-like immunoreactive dendrites. Many angiotensin II-like immunoreactive dendrites received synapses from immunonegative axon terminals. Although angiotensin II-like immunoreactive cell bodies were sometimes postsynaptic to immunoreactive axon terminals, they did not receive synapses from immunonegative axon terminals. These results provide solid morphological evidence of AP endogenous angiotensin II and confirm that in spite of circulating angiotensin II, the local neurons in the AP may also play an important role in angiotensin II-induced cardiovascular regulation.