Histamine modulates the activity of baboon retinal ganglion cells in vitro

Abstract

Abstract: : Purpose: In primates, neurons projecting from the posterior hypothalamus to the retina use histamine as a neurotransmitter, and these are most active during the day. To determine the function of this pathway, we studied the effects of histamine on ganglion cells in the baboon retina. Methods: Extracellular recordings were made in vitro using superfused pieces of dark adapted retina with RPE, choroid and sclera attached. Histamine (5 – 50 µM) and methylhistamine (5 µM), an H3 receptor agonist, were added to the superfusate. Results: Histamine produced both increases (17/29) and decreases (10/29) the maintained firing rates; 2 cells did not respond to histamine. Methylhistamine increased the maintained activity in 4 cells, decreased it in 3 and had no effect in 4. When histamine and methylhistamine were administered in succession, they gave similar responses in 3 cases. However, in 3 cases histamine increased the firing rate, while methylhistamine had no effect. Histamine had no effect on maintained activity in low Ca⁺²/high Mg⁺² Ames medium (n = 9). The maintained activity in the dark ranged from 3 – 65 spikes/sec; however, there was no correlation between the responses to drugs and the maintained rates. The responses of ganglion cells to full field light stimulation were either decreased or unaffected by histamine or methylhistamine. Cells that did not respond to histamine application (8/14) were more common than those with decreased light responses (6/14). ON and OFF types were affected in roughly equal proportions. In a rare type of ON–OFF cell with a protracted OFF response, histamine decreased both the transient ON and the protracted OFF response; the conventional OFF response was unaffected. Conclusions: Exogenous histamine or methylhistamine modulated the maintained activity of most (29/32) of the ganglion cells recorded in the baboon retina. Because histamine decreases the light responses of a subset of retinal ganglion cells, centrifugal axons are also expected to play a role in light adaptation. These effects are mediated by H3 and at least one other type of histamine receptor. In these respects, the actions of histamine in the retina are similar to those elsewhere in the CNS.

Keywords: ganglion cells • electrophysiology: non–clinical • neurotransmitters/neurotransmitter systems