Galanin immunoreactivity within the primate basal forebrain: Evolutionary change between monkeys and apes

Abstract

Galanin immunoreactivity (GAL‐ir) is differentially expressed within the basal forebrain of monkeys and humans. Most monkey magnocellular basal forebrain neurons colocalize GAL‐ir. In contrast, virtually no human magnocellular basal forebrain neurons express GAL‐ir. Rather, an extrinsic galaninergic fiber plexus innervates these neurons in humans. The present study examined the expression of GAL‐ir within the basal forebrain of apes to establish the phylogenetic level at which this transformation occurs. The staining patterns of GAL‐ir within the basal forebrain of both lesser (gibbons) and great (chimpanzee and gorilla) apes were compared to that previously observed within monkeys and humans. All apes displayed a pattern of basal forebrain GAL‐ir indistinguishable from humans. GAL‐ir was not expressed within ape basal forebrain magnocellular neurons as seen in monkeys. Rather like humans, a dense collection of GAL‐ir fibers was seen in close apposition to magnocellular perikarya. In addition, a few GAL‐ir parvicellular neurons were scattered within the ape basal forebrain. These data indicate that the evolutionary change in the expression of GAL‐ir within the primate basal forebrain occurs at the branch point of monkeys and apes.