Plasticity of Galaninergic Fibers Following Neurotoxic Damage within the Rat Basal Forebrain: Initial Observations

Galanin immunoreactive fibers hypertrophy and hyperinnervate remaining cholinergic basal forebrain neurons within the septum–diagonal band complex in Alzheimer's disease. The present investigation determined whether a similar hyperinnervation of galanin immunoreactive fibers occurs following intraparenchymal injections of ibotenic acid within the cholinergic medial septum or diagonal band nucleus in young adult rats. Sections through the medial septum and the diagonal band were either concurrently immunostained for galanin and the low-affinity p75 neurotrophin receptor (an excellent marker of cholinergic basal forebrain neurons) or single stained for choline acetyltransferase. Following chemical lesion, an increase in the density of galanin immunoreactivity was seen within the medial septum on the lesion, as opposed to the contralateral control side. In contrast, within diagonal band-lesioned animals, the increase in galanin immunoreactivity was low to moderate. In either lesion paradigm we did not observe hyperinnervation of remaining cholinergic basal forebrain neurons. In fact, there was no correlation between the galanin hypertrophy and the amount of cholinergic cell loss. We hypothesize that galanin hyperinnervation within the cholinergic basal forebrain may provide a protective effect by down-regulating acetylcholine release following brain insult.