Retinopetal axons in mammals: emphasis on histamine and serotonin

Abstract

Since 1892, anatomical studies have demonstrated that the retinas of mammals, including humans, receive input from the brain via axons emerging from the optic nerve. There are only a small number of these retinopetal axons, but their branches in the inner retina are very extensive. More recently, the neurons in the brain stem that give rise to these axons have been localized, and their neurotransmitters have been identified. One set of retinopetal axons arises from perikarya in the posterior hypothalamus and uses histamine, and the other arises from perikarya in the dorsal raphe and uses serotonin. These serotonergic and histaminergic neurons are not specialized to supply the retina; rather, they are a subset of the neurons that project via collaterals to many other targets in the central nervous system, as well. They are components of the ascending arousal system, firing most rapidly when the animal is awake and active. The contributions of these retinopetal axons to vision may be predicted from the known effects of serotonin and histamine on retinal neurons. There is also evidence suggesting that retinopetal axons play a role in the etiology of retinal diseases.

FIGURE 1

Two of the three types of retinopetal axons in mammalian retinas. (A, B) Two types of retinopetal axon terminals were redrawn from plates 74 and 93 of Polyak, respectively. One type from a chimpanzee (A), terminates near the border of the inner plexiform (IPL) and inner nuclear layer (INL). A second type from a macaque has more broadly stratified branches in the center of the IPL (B). (C) In the dog retina, two retinopetal axons were redrawn from plate AI-36 from Ramon y Cajal. They have large diameters, terminate in the outer strata of the IPL, and end in large swellings. The orientation of the drawing is photoreceptors are up; subdivisions 1–5 represent the sublamina of the IPL.

FIGURE 2

A THY1-YFP positive retinopetal axon in the mouse retina labeled from a transgenic mouse (line H, Jackson Laboratories) that expresses THY1-YFP in a subset of neurons. The labeling in the retinas was enhanced with a rabbit polyclonal antibody to GFP conjugated to Alexa Fluor 488 (Molecular Probes). (A) A low-power photomontage of a whole-mounted retina. (B) A Neurolucida (Microbrightfield, Williston, VT, USA) drawing of a retinopetal axon that emerges from the optic disk (arrow) and branches in the retina. Axon branches reach to the ora serrata, indicated by the thick line. Scale bar = 0.5 mm.

FIGURE 3

A histamine-immunoreactive retinopetal axon in the macaque retina. This axon was labeled with a histamine antiserum in whole-mounted retina, as previously described in Gastinger et al. It emerged from the optic disk (OD) and ran to the temporal retina where it made an orthogonally projecting branch (*) to the inner plexiform layer. The axon branches were studded with varicosities and stratified in the center of the inner plexiform layer. Scale bar = 1.0 mm.

FIGURE 4

Localization of histamine-immunoreactive neurons and retinopetal neurons in the macaque hypothalamus. Two coronal sections redrawn from Labandiera-Garcia et al. show the regions where retrogradely labeled neurons from the cut optic nerve (black circles) are found. The sites of histamine-immunoreactive neurons were superimposed on these sections (shaded areas), based on descriptions by Manning et al. TM, tuberomamillary nucleus; PH, posterior hypothalamus; LH, lateral hypothalamus; OT, optic tract; PM, premamillary; CI, capsula interna; Cd, caudate nucleus; GP, globus pallidus.

FIGURE 5

Histaminergic neurons in the posterior hypothalamus are most active during waking in the cat. A single histaminergic neuron, recorded in vivo, fires action potentials at a steady rate of 4–6 Hz during waking, more slowly early in slow wave sleep, and is silent during paradoxical sleep. Modified and reprinted from Vanni-Mercier et al.

FIGURE 6

HR3 localization in the outer plexiform layer of a macaque retina. This vertical section was double-labeled with a rabbit anti-HR3 (red, Chemicon) and mouse anti-mGluR6 (a gift from Noga Vardi) antibodies. Both HR3 and mGluR6 were found on the tips of ON-bipolar cell dendrites. Reprinted from Gastinger et al. Scale bar = 5 μm.

FIGURE 7

HR1 localization in rat retina. This rat retina was double-labeled with a rabbit anti-HR1 (red, Chemicon) and mouse anti-tyrosine hydroxylase (green, Sigma). All retinal layers were scanned using a confocal microscope, and HR1 receptors were found in all parts of the TH-IR amacrine cells in stratum 4 of the IPL (A), in S1 of the IPL (B), and in the INL (C). Reprinted from Gastinger et al. Scale bar = 50 μm.

FIGURE 8

Darkfield photomicrographs of retrogradely labeled neurons in the dorsal raphe of a green monkey. St. Kitts vervet monkeys (n =2) were given a 60-μl intravitreal injection of cholera toxin subunit B (CTB; 1 mg/ml) with 10% dimethyl sulfoxide. After a 2-week survival, the animals were perfusion fixed with 4% paraformaldehyde. Coronal sections of the brain were cut on a Vibratome, incubated in a goat anti-CTB antibody (1:10000, List Biological Laboratories), and visualized using an immunoperoxidase technique. (A–D) CTB-labeled neurons were found in the dorsal raphe. Each panel represents an increasing level of magnification. Scale bar = 200 μm (A, B), scale bar = 100 μm (C). (D) Retrogradely labeled neurons like this one (arrow) also contained immunoreactive serotonin (not illustrated). Scale bar = 20 μm. DR, dorsal raphe; Aq, cerebral aqueduct; SC, superior colliculus.

FIGURE 9

Serotonin receptors in the rabbit retina. (A-B) Serotonin receptors of the 5-HT_2A subtype are localized to rod bipolar cell terminals in the rabbit retina. These vertical sections were labeled with antibodies to 5-HT_2A (red) and protein kinase C (PKC, green). 5-HT_2A-IR puncta were found in the inner plexiform layer (IPL, arrows) (A) and outer plexiform layer (OPL) (B). All of the PKC-IR rod bipolar cells expressed 5-HT_2A in both the IPL and OPL. (C-D) 5-HT_3A receptors in the rabbit retina are localized to rod spherules in the outer plexiform layer (OPL). Using an antibody to 5-HT_3A (red), large puncta were labeled in the OPL. The yellow regions in panel (C) demonstrate that the 5-HT_3A-IR were colocalized with a marker for rod spherules, B16 (green). Alternatively in panel (D), a marker of cone pedicles, peanut agglutin (green), was not associated with 5-HT_3A-IR puncta. (A-B) Modified and reprinted from Pootanakit and Brunken. (C-D) Modified and reprinted from Pootanakit et al.