An Ultrastructural Analysis of the Distribution of Angiotensin II in the Rat Brain

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Abstract
Immunopositive angiotensin II nerve fibres and terminals are widely distributed throughout the rat brain, including areas of the brain with and without a blood-brain barrier. Ultrastructural examination indicates that in the circumventricular organs (areas which lack a blood-brain barrier), many angiotensin ll-positive nerve terminals are closely aligned with fenestrated blood vessels and do not have synaptic specializations. This appearance is in contrast to that of angiotensin II terminals in regions with a blood-brain barrier where there exists a more typical synaptic configuration. In both cases, angiotensin II is contained within large (100 to 125 nm) vesicles which coexist with smaller, lucent, non-immunoreactive vesicles. These observations suggest a possible duality of function such that angiotensin II in circumventricular organs may be secreted into the circulation, whereas angiotensin II in the remainder of the brain is more likely to be acting as a neurotransmitter or neuromodulator.