Presence and distribution of histaminergic components in rat and bovine retina

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.

Marked increase in [3H](R) alpha-methylhistamine binding in the superior colliculus of visually deprived rats after unilateral enucleation

The binding of [3H](R)alpha-methylhistamine to histamine H3-receptors in visual structures of unilaterally enucleated rats was examined by quantitative autoradiography to clarify the involvement of histamine neurons in the visual system. [3H](R)alpha-Methylhistamine binding in the visually deprived superior colliculus, contralateral to the enucleated eye, was significantly increased 5, 15, 30 and 45 days after unilateral enucleation. Slight time-dependent increases in ligand binding were observed in the visual cortex, but the change was significant only 45 days after unilateral enucleation. Unilateral enucleation had no significant effect in the dorsal lateral geniculate nucleus at any time after enucleation. Continuous injection of (S)-alpha-fluoromethylhistidine, a specific inhibitor of L-histidine decarboxylase, attenuated the effect of unilateral enucleation in the superior colliculus. These results suggest that retinal deafferentation induced an increase in histamine H3-receptor binding sites, probably by selective adjustment of histamine neurons in response to unilateral enucleation.

Immunocytochemical and biochemical studies of histamine in the retina of the turtle Pseudemys scripta

A combination of immunocytochemical and biochemical methods was used to study histamine in the turtle retina. Histamine-like immunoreactivity was localized within paraboloids of certain cone photoreceptors by use of two different antisera directed against histamine. Preincubation of eyecups in Ringer's containing 10 microM histamine selectively increased the immunoreactivity of these photoreceptor paraboloids. The present localization of histamine in paraboloids indicated that, although histamine is in photoreceptors of the turtle retina, it may play some metabolic or neuromodulatory role, and not function as a neurotransmitter.

Phosphorylase alpha and labile metabolites during anoxia: correlation to membrane fluxes of K+ and Ca2+

The objective of the present study was to explore mechanisms responsible for activation of ion conductances in the initial phases of brain ischemia, particularly for the early release of K+ that precedes massive cell depolarization, and rapid downhill fluxes of K+, Na+, Cl-, and Ca2+. As it has been speculated that a K+ conductance can be activated either by an increase in the free cytosolic calcium concentration (Ca2+i) or by a fall in ATP concentration, the question arises whether the early increase in extracellular K+ concentration (K+e) is preceded by a rise in Ca2+i and/or a fall in ATP content. In the present experiments, ischemia was induced in rats by cardiac arrest, the time courses of the rise in K+e and cellular depolarization were determined by microelectrodes, and the tissue was frozen in situ through the exposed dura for measurements of levels of labile metabolites. including adenine nucleotides and cyclic AMP (cAMP), after ischemic periods of 15, 30, 60, and 120 s. Conversion of phosphorylase b to a was assessed, because it depends, among other things, on changes in Ca2+i. The K+e value rose within a few seconds following induction of ischemia, but massive depolarization (which is accompanied by influx of calcium) did not occur until after approximately 65 s. Activation of phosphorylase was observed already after 15 s and before glycogenolysis had begun. At that time, 3',5'-cAMP concentrations were unchanged, and total 5'-AMP concentrations were only moderately increased. The results demonstrate that a K+ conductance is activated at a time when the overall ATP concentration remains at 95% of control values.(ABSTRACT TRUNCATED AT 250 WORDS)

Histamine in the retina: recent progress and perspectives

This article presents some new findings, and shortly surveys recently published data, on histamine (HA) in the retina of several vertebrates (carp, hen, rabbit, rat, guinea pig, cow, man). Analysis of various parameters (HA level, activity of histidine decarboxylase and HA-methyltransferase, release and uptake of HA, 3H-mepyramine binding, effect of HA on cAMP accumulation, diurnal variations) in vertebrate retina and brain shows that histaminergic systems in these sites are comparable, although some species-dependent differences have been observed. Interestingly, exposure of dark-adapted rabbits to light affects several histaminergic parameters selectively, i.e. in the retina and not in the brain. The reviewed data suggest that in retinas of at least some vertebrates HA may be a physiologically important amine.

Histamine-mediated regulation of cAMP levels and inositol phosphate metabolism in isolated rabbit retina

In pieces of the rabbit retina, histamine (HI) had no significant effect on cAMP accumulation; however, HI inhibited forskolin-evoked accumulation of the nucleotide. Mepyramine (H1-antagonist) was without effect, while cimetidine (H2-antagonist) antagonized the HI action. Dimaprit (H2-agonist) mimicked the HI action, however its action was not significantly affected by H2-antagonists. Since phosphodiesterase (PDE) inhibitors prevented the HI action it is suggested that HI-induced inhibition of the forskolin effect results from HI-induced activation of cAMP breakdown. The muscarinic agonist carbachol, and to a lesser extent HI, both increased [3H]inositol incorporation and intensified accumulation of [3H]inositol phosphates in the rabbit retina.