The uveal blood vessels, especially those of the choroid, have a rich innervation from the autonomic nervous system. The superior cervical ganglion is the source of the sympathetic nerve fibres, and the facial nerve, with a synapse in the pterygopalatine ganglion, is the major parasympathetic supply in mammals. Neuropeptide YNPY) is the most abundant peptide in the sympathetic nerve fibres, whereas (vasoactive intestinal polypeptide (VIP) is the main peptide in the parasympathetic nerve fibres. Pituitary adenylate cyclase activating polypeptide (PACAP-27 and PACAP-38) and peptide histidine isoleucine (PHI/PHM) are also present in the parasympathetic fibres.
NPY is a potent vasoconstrictor in the rabbit uvea, but has only minor effect on uveal blood flow in the cat. These observations agree well with the effects of sympathetic nerve stimulation, which in cats causes an uveal vasoconstriction that is completely abolished by α-adrenoreceptor blockade. In rabbits, the uveal vasoconstriction is resistant to α-adrenoreceptor blockade at high frequencies. The contribution of different neurotransmitters to the uveal vasodilatation caused by facial nerve stimulation seems to differ as well. In cats, the effect is almost abolished by muscarinic blockade combined with inhibition of nitric oxide synthase (NOS), indicating that nitric oxide and acetylcholine are the major transmitters. NO-inhibition and muscarinic blockade have only minor effects on the parasympathetic vasodilatation in the rabbit, however, suggesting that a peptide(s) can be the main mediator(s). In agreement with this, VIP, PHI and PACAP-27 and PACAP-38 are potent vasodilators in the rabbit choroid, but have no effect on uveal blood flows in the cat. The potency of PACAP-27 and PACAP-38 is about 100 times higher than that of VIP and PHI, indicating that selective PAC1 receptors may be involved. In humans, PACAP has been shown to increase blood flow velocity in the ophthalmic artery, indicating choroidal vasodilatation.
Receptors for VIP/PACAP are present on the ciliary epithelium from several species, and VIP and PACAP-27 stimulates aqueous humour flow in monkeys.
The significance of the various neuropeptides for the regulation ocular blood flow seems to vary considerably between species and the effects of neuropeptides on ocular circulation in humans is largely unknown.
Kerala, India: Research Signpost , 2009. 147-167 p.