Structural and neurochemical changes in the maturation of the carotid body.

Functional maturation of the carotid body in the postnatal period relies partly on structural and neurochemical changes, which are reviewed here. Structural changes include changes in cytological composition, and increases in glomic tissue volume, dense-cored granules of type I cells, synapses of type I cells with type II cells and afferent nerve fibres. Vascular volume also increases, but in the same proportion as extravascular volume. During maturation, the carotid body also shows higher density and hypoxic sensitivity of K(+)-channels and an increased hypoxic [Ca(2+)](i) response. Modulation of content and release of catecholamine occurs, together with decreased expression of tyrosine hydroxylase and dopamine β-hydroxylase and increased expression of choline acetyltransferase. Expression of dopamine 2 receptor and nicotinic α3 and α7 receptor subunits increases, and muscarinic M1 receptor protein, nicotinic α4 and β2 receptor subunits and adenosine receptor 1 decrease. Maturation of the carotid body may also be explained with reference to the developmentally regulated expression of trophic factors and their receptors.