reßexes in choroid [353]. Our work in chicks suggests that the sensory input to the choroid may be involved in temperature-dependent regulation of ChBF (i.e., retinal thermoregulation) [318], which may be important for retinal homeostasis [268]. Reduced ocular blood ßow caused by elevated IOP has been shown to result in reduced scleral and retinal temperature [32]. Heating an eye in which blood ßow has been reduced by increased IOP (thereby reducing ocular perfusion pressure and ChBF) results in an exaggerated rise in ocular temperature, showing that choroidal blood ßow can be involved in ocular cooling as well [32]. Regulation of ocular temperature has been shown to be important for the normal physiological functioning of the retina [146, 241].

12.5.7 Intrinsic Choroidal Neurons

Studies over the past 25 years have proven that the choroid in many mammalian and avian species contains neuronal cells that possess various neuroactive substances typical of parasympathetic PPG neurons. Although intrachoroidal neurons were initially reported by silver staining methods in the nineteenth century [308], Terenghi et al. [359] Þrst noted them for their neuroactive substance content (VIP) in guinea pig choroid after colchicine treatment (which prevented transport of the neuroactive substances out of the perikarya). Shortly thereafter, Miller et al. [237] and Stone et al. [345] saw VIP+ neurons in human choroid. This phenomenon was subsequently more extensively studied by several groups. Using NADPHd histochemistry, which detects neurons using NOS to synthesize NO [25] reported the presence of many isolated or grouped NADPHd+ neurons in human choroid, with individual neurons being about 30 mm in perikaryal size. FlŸgel et al. [94] made similar observations of human choroid and reported that VIP was typically present in the NOS+ intrachoroidal neurons. The ganglion cells tended to be most numerous in the central retina around the human fovea. No such neurons were, however, observed in afoveate species such as rats, rabbits, tree shrews, cats, pigs, or owl mon-

keys [94, 95]. FlŸgel-Koch et al. [95] reported that NOS+ and VIP+ intrachoroidal ganglion cells are also present in cynomolgus monkeys, which have a fovea centralis. Since these neurons were absent from primate and nonprimate species lacking a fovea centralis, these authors suggested that the plexus was associated with choroidal blood ßow control in the region of the fovea. The intrachoroidal ganglion cells in humans give rise to processes that can be observed to join the perivascular network of NADPHd+ and VIP+ Þbers in the choroid, and in some cases observed to end directly on arteries. The intrachoroidal ganglion cells in humans have been noted to receive terminals, some of which contain NOS or VIP [94, 223]. Schršdl et al. [308] reported on the connectivity of the intrinsic choroidal neurons in human eye. Using immunolabeling for nNOS or VIP, as well as single-cell Þlling, they reported that intrachoroidal neurons send processes to other ganglion cells, as well as to vascular and nonvascular smooth muscle of the choroid. CGRP+ sensory boutons were also observed ending on intrachoroidal neurons. The evidence of sensory input to intrachoroidal ganglion cells suggests they have a role in reßexive modulation of ChBF in response to sensory stimuli such as heat or cold detected by the sensory Þbers.

Intrinsic choroidal neurons are also present in birds and of a similar size (30 mm) and neurochemistry (nNOS+ and VIP+) to those in mammals [26, 65, 309]. These neurons give rise to processes that contact choroidal blood vessels, and they receive CGRP+ contacts from sensory axons [306]. The evidence of sensory input to intrachoroidal ganglion cells again suggests they have a role in reßexive modulation of ChBF in response to sensory stimuli such as heat or cold. Intrachoroidal ganglion neurons in birds also receive sympathetic nerve terminals, indicating their sympathetic modulation as well [307]. The intrachoroidal ganglion neurons vary among avian groups in their abundance, with the highest abundance among studied groups being in goose, the next highest in turkeys and chickens, and the lowest in ducks [309]. Their spatial distribution also varied among studied avian groups, presumably in relation to retinal high acuity specializations.