Chapter 3 Histamine in the retina and some other components of the visual system

Retinopetal axons in mammals: emphasis on histamine and serotonin

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.

Histamine receptors in mammalian retinas

Mammalian retinas are innervated by histaminergic axons that originate from perikarya in the posterior hypothalamus. To identify the targets of these retinopetal axons, we localized histamine receptors (HR) in monkey and rat retinas by light and electron microscopy. In monkeys, puncta containing HR3 were found at the tips of ON-bipolar cell dendrites in cone pedicles and rod spherules, closer to the photoreceptors than the other neurotransmitter receptors. This is the first ultrastructural localization of any histamine receptor and the first direct evidence that HR3 is present on postsynaptic membranes in the central nervous system. In rat retinas, most HR1 were localized to dopaminergic amacrine cells. The differences in histamine receptor localization may reflect the differences in the activity patterns of the two species.

Histamine induces opposing vasoactive effects at different levels of the ocular vasculature

PURPOSE

To characterize the vasoactive properties and receptors of histamine in porcine retinal arterioles compared to that in the long posterior ciliary artery.

METHODS

Isolated perfused porcine retinal arterioles were used to study vessel diameter in response to histamine (10(-10) to 10(-4) M). Ring segments from the long posterior ciliary artery were used to study contractile force in response to histamine.

RESULTS

Histamine-induced vasodilatation in pig retinal arterioles but contraction in the long posterior ciliary artery. In retinal arterioles, the involvement of H(2) receptors was implied by the attenuation of the histamine response in the presence of cimetidine. Further attenuation of the histamine response by pyrilamine suggests the involvement of H1 receptors. In retinal arterioles denuded of endothelial cells, the histamine response was only slightly reduced.

CONCLUSION

Histamine induces opposing vasoactive effects at different levels of the porcine ocular vasculature.

The effects of histamine on rat and monkey retinal ganglion cells

Mammalian retinas receive input from the posterior hypothalamus, and the neurotransmitter in this pathway is histamine. To determine whether histamine influences ganglion cells, we analyzed the effects of histamine on their maintained and light-evoked activity in vitro. In monkeys, histamine increased the maintained firing rate in 42% of ganglion cells, decreased it in 38%, and had no effect in 20%. When histamine and the HR3 agonist, methylhistamine, were applied to the same cells in succession, their effects were sometimes different, a finding suggesting that at least one other histamine receptor is present. In addition, the responses of some ganglion cells to full-field light stimuli were decreased by histamine and methylhistamine. In rats, the effects of histamine were somewhat different. Histamine increased the maintained firing rate of 82% of ganglion cells. Methylhistamine and the HR2 agonist, dimaprit, had the same effects as histamine. In some cells, histamine increased the light responses, but in others it decreased them. Histamine had no effect on ganglion cells in either species when synaptic transmission was blocked by low Ca2(+)/high Mg2+ Ames medium. Thus, the major effects of histamine were on the maintained activity of retinal ganglion cells. In both rats and monkeys, 80% or more of the ganglion cells were affected by histamine, and these responses were mediated by at least two of the histamine receptor subtypes.

Histamine H(2) -like receptors in chick cerebral cortex: effects on cyclic AMP synthesis and characterization by [(3) H]tiotidine binding

In this study, histamine (HA) receptors in chick cerebral cortex were characterized using two approaches: (1) analysis of the effects of HA-ergic drugs on the cAMP-generating system, and (2) radioreceptor binding of [(3) H]tiotidine, a selective H(2) antagonist. HA was a weak activator of adenylyl cyclase in a crude membrane preparation of chick cerebrum. On the other hand, HA (0.1-1000 microm) potently and concentration dependently stimulated cAMP production in [(3) H]adenine pre-labelled slices of chick cerebral cortex, displaying an EC(50) value (concentration that produces 50% of maximum response) of 2.65 microm. The effect of HA was mimicked by agonists of HA receptors with the following rank order of potency: HA >or= 4-methylHA (H(2)) >or= N alpha,N alpha-dimethylHA (H(3) >> H(2) = H(1)) >> 2-methylHA (H(1)) >> 2-thiazolylethylamine (H(1)) >or= R alpha-methylHA (H(3)) >> amthamine, dimaprit (H(2)), immepip (H(3), H(4)). The HA-evoked increase in cAMP production in chick cerebral cortex was antagonized by selective H(2) receptor blockers (aminopotentidine >or= tiotidine > ranitidine >> zolantidine), and not significantly affected by mepyramine and thioperamide, selective H(1) and H(3) /H(4) receptor blockers, respectively. A detailed analysis of the antagonistic action of aminopotentidine (vs. HA) revealed a non-competitive mode of action. The binding of [(3) H]tiotidine to chick cortical membranes was rapid, stable and reversible. Saturation analysis resulted in a linear Scatchard plot, suggesting binding to a single class of receptor binding site with high affinity [equilibrium dissociation constant (K (d)) = 4.42 nm] and high capacity [maximum number of binding sites (B (max) ) = 362 fmol/mg protein]. The relative rank order of HA-ergic drugs to inhibit [(3) H]tiotidine binding to chick cerebrum was: antagonists - tiotidine >> aminopotentidine = ranitidine >or= zolantadine >> thioperamide - triprolidine; agonists - HA >or= 4-methylHA >> 2-methylHA >or=R alpha-methylHA - dimaprit. In conclusion, chick cerebral cortex contains H(2) -like HA receptors that are linked to the cAMP-generating system and are labelled with [(3) H]tiotidine. The pharmacological profile of these receptors is different from that described for their mammalian counterpart. It is suggested that the studied receptors represent either an avian-specific H(2) -like HA receptors or a novel subtype of HA receptors.

Histamine in the chick pineal gland: origin, metabolism, and effects on the pineal function

The chick pineal gland contains histamine and tele-methylhistamine. The levels of both substances are elevated after treatment of chicks with the amino acid precursor of histamine, L-histidine (1 g/kg, ip). In control and L-histidine-loaded animals the pineal levels of histamine and tele-methylhistamine are higher in light-exposed than in dark-adapted animals (measured at the end of the light phase and in the middle of the dark phase of 12 hr light, 12 hr dark illumination cycle, respectively). The chick pineal gland contains histamine-immunofluorescent cells displaying mast cell morphology; they are seen in the vicinity of the capsule and in the parenchyma. Enzymatic studies showed the presence of the activity of histamine synthesizing and inactivating enzyme, i.e., L-histidine decarboxylase (HDC) and histamine-methyltransferase (HMT). The detected enzyme activities were sensitive to specific inhibitors of HDC (alpha-fluoromethylhistidine and alpha-hydrazinohistidine) and HMT (quinacrine and metoprine); inhibitors of aromatic amino acid decarboxylase alpha-methyl-DOPA and NSD-1015 were inactive on HDC. Exogenous histamine added to organ-cultured chick pineals strongly stimulated endogenous cyclic AMP accumulation and moderately increased melatonin secretion. The data, considered collectively, suggest that in avians histamine, probably originating from the pineal mast cell compartment, may function as a regulator of pineal gland activity.