The origin and fate of histamine in the rabbit 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.

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.

The effects of serotonin drugs on horizontal and ganglion cells in the rabbit retina

We have investigated the effects of a serotonin 5-HT2 antagonist and a 5-HT1A agonist on horizontal and ganglion cells in the rabbit retina. Simultaneous intracellular horizontal cell and extracellular ganglion cell recordings were obtained from a superfused in vitro rabbit eyecup preparation and the effects of bath applied drugs on these cells' light responses observed. Sinusoidally modulated current was also injected into horizontal cells while the extracellular spike activity of nearby, single-unit ganglion cells was monitored. Although the ON components of the light-evoked responses of ganglion cells were reduced by the 5-HT2 antagonist or the 5-HT1A agonist, the membrane potential and the light responses of horizontal cells and the b-wave of the ERG were simultaneously unaffected. However, the drugs blocked current-driven ganglion cell spike activity induced by current injections into nearby horizontal cells. These results are discussed with respect to the site of action of these serotonin drugs and with respect to the circuitry of serotonergic neurons.

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.

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.

Circadian rhythm of histamine metabolism in the rabbit central nervous system (CNS): analysis of brain and ocular structures

The circadian rhythm of the level of histamine (HI) and histidine decarboxylase (HD) and histamine-methyltransferase (HMT) activity in 6 brain and 5 ocular structures of the rabbit was studied. Clear circadian variations of the histaminergic parameters in two brain (hypothalamus and lateral geniculate body) and two ocular (retina and iris-ciliary body) tissues were found. It is suggested that HI in some brain and ocular structures may be functionally involved in activity that depends on circadian rhythm.

Histamine H1-receptor in the retina: species differences

Histamine H1-receptors in membranes of the various mammalian retinas were studied by [3H]mepyramine binding assay. Specific [3H]mepyramine bindings to bovine, pig, dog and human retinas were observed with the dissociation constants (KD), 3.8 +/- 1.2 nM, 1.8 +/- 0.6 nM, 2.6 +/- 0.6 nM and 3.0 +/- 0.9 nM, respectively, which were similar to those found in brains. But there was no detectable specific binding in the guinea-pig and rabbit retinas. The number of binding sites (Bmax) ranged from negligible value to 290.7 +/- 51.7 fmole/mg protein(human retina). Some H1-antagonists acted as potent agents in competing with [3H]mepyramine binding to bovine and pig retinas. These results indicated that histamine H1-receptors exist in some mammalian retina and have similar characteristics to those in brain membranes, but they distributes in the wide difference of the binding capacities among the species, while in brain variations were smaller.

Histamine in the retina

Retina and choroid were rapidly and gently extracted from chickens, guinea-pigs, rats, rabbits and cows. They were frozen, weighted and pooled in 3% trichloroacetic acid in a refrigerator. Using a radioenzymic assay, histamine was found to be present in the retina of guinea-pig (4.26 +/- 0.677 nmol g-1), rabbit (1.08 +/- 0.422 nmol g-1) and cow (0.189 +/- 0.036 nmol g-1), but not detected in chicken and rat. In the choroid, histamine was found in all species except rat (chicken: 4.92 +/- 0.542 nmol g-1; guinea-pig: 121.5 +/- 4.46 nmol g-1; rabbit: 18.7 +/- 3.93 nmol g-1; cow; 0.36 +/- 0.095 nmol g-1). The study verifies the presence of histamine in the retina of some species, and thereby supports previous fluorometry assays.

Effect of electroconvulsive shock (ECS) treatment on the histaminergic system in rat brain: biochemical and behavioural studies

The effects of electroconvulsive shock (ECS) on brain histamine (HI) levels, 1-histidine decarboxylase (HD) and HI-methyltransferase (HMT) activities, and H2-receptor--mediated hypothermia were studied in rats. Single (x1) and repeated (x10, once daily) ECS had practically no effect on both HI levels and HMT activities in the rat hypothalamus, cerebral cortex and rest of the brain. ECS x10 significantly increased HI accumulation. ECS x1 (only after 24 h) and ECS x10 resulted in marked elevation of the brain HD activity; the most pronounced effect was observed in the cerebral cortex. Repeated, but not single, ECS reduced the hypothermic action of various centrally given histamine H2-receptor agonists (HI, 4-methylHI, dimaprit and impromidine). It is suggested that prolonged treatment with ECS activates the central histaminergic system in the rat. The histaminergic neurons in the cerebral cortex seem to be especially affected by repeated ECS.