The histamine degradative uptake pathway in human vascular endothelial cells and skin fibroblasts is dependent on extracellular Na+ and Cl-

The dynamics of signaling at the histaminergic photoreceptor synapse of arthropods

Histamine, a ubiquitous aminergic messenger throughout the body, also serves as a neurotransmitter in both vertebrates and invertebrates. In particular, the photoreceptors of adult arthropods use histamine, modulating its release to signal increases and decreases in light intensity. Strong evidence from various arthropod species indicates that histamine is synthesized and stored in photoreceptors, undergoes Ca-dependent release, inhibits postsynaptic interneurons by gating Cl channels, and is then recycled. In Drosophila, the synthetic enzyme, histidine decarboxylase, and the subunits of the histamine-gated chloride channel have been cloned. Possible histamine transporters at synaptic vesicles and for reuptake remain elusive. Indeed, the mechanisms that remove histamine from the synaptic cleft, and that help terminate histamine's action, are unexpectedly complex, their details remaining unresolved. A major pathway in Drosophila, and possibly other arthropod species, is by conjugation of histamine to beta-alanine to form carcinine in adjacent glia. This conjugate then returns to the photoreceptors where it is hydrolysed to liberate histamine, which is then loaded into synaptic vesicles. Evidence from other species suggests that direct reuptake of histamine into the photoreceptors may also occur. Light depolarizes the photoreceptors, causing histamine release and postsynaptic inhibition; dimming hyperpolarizes the photoreceptors, causing a decrease in histamine release and an "off" response in the postsynaptic cell. Further pursuit of histamine's action at these highly specialized synapses should lead to an understanding of how they signal minute changes in presynaptic membrane potential, how they reliably extract signals from noise, and how they adapt to a wide range of presynaptic membrane potentials.

The links between maternal histamine levels and complications of human pregnancy

Previous literature published since 1910 on maternal blood histamine levels and complications of pregnancy have been reviewed, showing links between hyper-histaminemia occurring in specific gestational complications including preeclampsia, spontaneous abortion, preterm labour and hyperemesis gravidarum. These complications may present with symptoms similar to those of experimentally induced high blood histamine or hyper-histaminemia in non-pregnant humans. Maternal levels of histamine in normal pregnancy decrease below values found in healthy non-pregnant women. However, in some complications of pregnancy, maternal blood histamine levels rise above those associated with normal pregnancy and may exceed normal non-pregnant circulating levels. These links between circulating maternal histamine levels and specific complications of human pregnancy suggest that further investigations to evaluate the outcome of managing maternal blood histamine levels during such complications may be warranted.