Effect of serotonin on brown adipose tissue and on its sympathetic neurons

Previously we have shown that there are two types of sympathetic neurons in the rat interscapular brown fat-pad: long adrenergic neurons with cell bodies located in the paraspinal ganglia and short adrenergic neurons with cell bodies located within the pad itself. The present studies examine 1) the uptake of [3H]norepinephrine by these neurons, 2) the effect of serotonin on release of the catecholamine, and 3) the metabolic action of serotonin on brown fat. [3H]norepinephrine uptake from the medium in which brown fat slices are incubated is linear with time up to 90 min. This uptake is blocked by both cocaine and imipramine. As these drugs block uptake by sympathetic neurons, the [3H]norepinephrine is probably taken up by neurons and not by some other cell in the pad. Serotonin creatinine sulfate, but not creatinine sulfate, promotes the release of the catecholamine from these neurons. Serotonin also exerts a norepinephrine-like metabolic effect on slices of normal brown fat. It stimulates lipolysis and inhibits lipogenesis. Serotonin has no effect on isolated brown fat cells or on slices of reserpinized tissue. It also does not potentiate the action of norepinephrine on these last two preparations. Therefore serotonin's action on brown fat is mediated by norepinephrine-containing stromal cells. The data suggest that the rat brown fat-pad has sympathetic neurons that can take up norepinephrine and that serotonin exerts its metabolic effect by stimulating the release of norepinephrine from these neurons.