title: Biochemical and physiological evidence that histamine is the transmitter of barnacle photoreceptors.
abstract: We tested the hypothesis that histamine is the transmitter released by barnacle photoreceptors. Median and lateral ocelli were incubated with 3H-histidine and found to synthesize 3H-histamine, identified by high-voltage electrophoresis. Synthesis could be blocked by the histidine decarboxylase inhibitor (S)-alpha-fluoromethylhistidine. Histamine was applied to I-cells either by superfusion or by pressure ejection from a pipette (100 microM or 1 mM histamine) positioned close to the I-cell's soma. When bath-applied at concentrations ranging from 5-100 microM, histamine hyperpolarized the I-cell in a dose-dependent fashion and increased its conductance. At 100 microM, histamine abolished the I-cell's response to light. The response to a pulse of pressure-applied histamine was a hyperpolarization whose amplitude was graded with dose (determined by the duration of the pulse). This response persisted in concentrations of Co2+ and Cd2+ that blocked synaptic transmission from the photoreceptors. Cimetidine, an antagonist of mammalian H2 receptors, markedly decreased the cell's responses both to HA and to light at 100 microM and blocked both responses at 1 mM. Pyrilamine and triprolidine, H1 antagonists, had a complex effect on the I-cell's responses to histamine and to light. Neither H1 nor H2 antagonists, nor histamine itself, affected the voltage or light responses recorded in the presynaptic terminal region, or any phase of calcium-dependent action potentials induced in the terminal in the presence of tetraethylammonium ion. Thus, biochemical, immunocytochemical, and physiological evidence suggests that HA is the transmitter from these photoreceptors to the I-cells. Although gamma-aminobutyric acid (GABA) is also present in the photoreceptors, it did not affect the I-cell's responses to light or to histamine when bath-applied at 100 microM. Thus, GABA does not appear to modulate transmission from the photoreceptor to the I-cell.
authors.affiliation: Department of Zoology, University of Washington, Seattle.
authors.forename: J C
authors.initials: JC
authors.lastname: Callaway
authors.forename: A E
authors.initials: AE
authors.lastname: Stuart
mesh_terms: D000818:Animals; D002104:Cadmium; D003035:Cobalt; D006632:Histamine / Q000096:biosynthesis / Q000378:metabolism / Q000502:physiology*; D006634:Histamine H1 Antagonists; D006635:Histamine H2 Antagonists; D006640:Histidine Decarboxylase / Q000037:antagonists & inhibitors; D008027:Light; D008564:Membrane Potentials / Q000187:drug effects; D008762:Methylhistidines; D018377:Neurotransmitter Agents / Q000096:biosynthesis / Q000378:metabolism / Q000502:physiology*; D010786:Photoreceptor Cells / Q000166:cytology / Q000201:enzymology / Q000502:physiology* / Q000528:radiation effects; D013569:Synapses / Q000502:physiology; D001468:Thoracica
chemical_list: D006634:Histamine H1 Antagonists; D006635:Histamine H2 Antagonists; D008762:Methylhistidines; D018377:Neurotransmitter Agents; D002104:Cadmium; D003035:Cobalt; C028309:alpha-fluoromethylhistidine; D006632:Histamine; D006640:Histidine Decarboxylase