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 Ca ²⁺/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.