Galanin, a 29-amino acid peptide, is uniquely distributed in human basal forebrain and may play a role in cholinergic cell dysfunction in Alzheimer's disease. We report a detailed evaluation of galanin receptors in human basal forebrain (67 +/- 12 years) and hypothalamus (67 +/- 15 years) with radioligand binding techniques. The binding of [125I]galanin (porcine) (agonist) or [125I]galantide [GAL (1-3)-substance P (5-11)-NH2] (putative antagonist) saturated in 2 hr, and only 15% to 30% of either radioligand was removed in the presence of unlabeled peptide. [125I]Galanin or [125I]galantide binding in basal forebrain revealed similar Bmax values, with [125I]galanin having a higher affinity for the galanin receptor. In contrast, [125I]galanin showed a lower affinity and labeled 42% more receptors than [125I]galantide in the hypothalamus. Differences were noted in competition studies of galanin and galanin chimeric peptides (M15, M35, M40 and C7) between [125I]galanin and [125I]galantide binding and in both regions. M35, M40 and C7 showed high affinity for galanin receptors in the hypothalamus with Hill coefficients close to unity, whereas in the basal forebrain these peptides competed differently. 5'-Guanylylimidodiphosphate reduced the specific binding of either radioligand in both regions. Based on the derived data, both radioligands irreversibly bind with high affinity and act as agonists at galanin receptors in human basal forebrain and hypothalamus. Galanin and galanin chimeric peptides compete differently for galanin receptors depending on the radioligand and region tested, suggesting subtype differences.