Galanin mRNA in the nucleus basalis of Meynert complex of baboons and humans

Abstract

Galanin, a 29-amino acid peptide, has been shown by immunocytochemistry to occur in most large acetylcholinergic neurons of the complex that includes the nucleus basalis of Meynert and the nucleus of the diagonal band of Broca in nonhuman primates. In contrast, several studies have reported that most large neurons of the human nucleus basalis of Meynert complex appear to lack galanin immunoreactivity. We investigated this apparent species-difference by hybridization histochemistry for galanin messenger ribonucleic acid (mRNA) in humans and baboons. The results confirm previous immunocytochemical data; very few large neurons of the nucleus basalis of Meynert complex in humans contained detectable galanin messenger RNA, whereas most such cells in baboons were labeled by the oligodeoxynucleotide probe. The few labeled neurons in humans were primarily medial or ventral to the main body of the nucleus basalis of Meynert and corresponded in location to a minor population of relatively intensely labeled cells in baboons. These findings indicate that the indetectability of immunoreactive galanin in most cells of the nucleus basalis of Meynert complex in humans is due to a paucity or an absence of galanin messenger RNA and not to differences in posttranslational processing or transport of the peptide. Inasmuch as the probe labeled neurons in several other nuclei of both species, it is unlikely that differences in galanin messenger RNA sequences underlie the species-related disparity in hybridization in the nucleus basalis of Meynert complex. The indetectability of galanin messenger RNA in most cells of the human nucleus basalis of Meynert complex indicates that the expression of the galanin gene is regulated by as yet unidentified influences that differ in human and nonhuman primates. The varying phenotypes of galanin in primates suggest potentially important species-differences in the function of galanin in neurons of the nucleus basalis of Meynert complex.

Figure 1.

Semischematic, computer-assisted diagram of the nbM complex of a 52-year-old human showing 1,372 unlabeled magnocellular neurons (small dots) and 11 neurons labeled for galanin mRNA (large dots). The dorsomedialmost neurons may not be associated with the nbM complex proper. Other galaninergic neurons that are clearly outside the nbM complex are not indicated. Compare with the baboon nbM complex in Figure 4, in which all indicated cells contain galanin mRNA.

Figure 2.

Hybridization histochemistry for galanin mRNA in nbM complex neurons of a 61-year-old human (bar = 25 μm). A. Large, lightly labeled galaninergic neuron (arrow) among unlabeled magnocellular neurons of the anteromedial nbM. B. Galaninergic smaller neuron (arrow) in the anteromedial nbM. C. Galaninergic neuron in the horizontal limb of the ndbB.

Figure 3.

Hybridization histochemistry for galanin mRNA (A) or TH mRNA (B,C) in a 61-year-old human (bar = 25 μm). A. Positive control: galanin mRNA-labeled neurons in the anterior hypothalamus. B. Known TH-producing neurons of the hypothalamic paraventricular nucleus are labeled by the TH probe. Compare with nbM complex neurons in Figure 3C. C. Negative control: the TH probe did not label most magnocellular neurons of the nbM. These neurons are in the same section as the labeled cells of the paraventricular nucleus shown in Figure 3B.

Figure 4.

Semischematic, computer-assisted diagram of neurons containing galanin mRNA in the nbM complex of a 2.5-year-old baboon. Each small dot represents one of 392 relatively lightly labeled neurons (see Fig. 5A); each large dot represents one of 11 more heavily labeled neurons (see Fig. 6A,B). Greater than 90% of all magnocellular neurons contained galanin mRNA in baboons, which was not the case in humans (see Fig. 1). Note that the heavily labeled galaninergic cells are generally medial and/or ventral to the typical dense clusters of magnocellular neurons. Galanin-hybridoreactive neurons outside the nbM complex are not indicated.

Figure 5.

Hybridization histochemistry for galanin mRNA in nbM complex neurons of baboons (bar = 25 μm). A. Cluster of galaninergic neurons in the anteromedial nbM of a 2.5-year-old baboon. B. Galaninergic neuron in the vertical limb of the ndbB of a 5-year-old baboon. C. Galaninergic neuron in the horizontal limb of the ndbB of a 2.5-year-old baboon. D. Small galaninergic neuron (arrow) next to a large, labeled neuron in the anteromedial nbM of a 2.5-year-old baboon.

Figure 6.

Hybridization histochemistry for galanin mRNA in the nbM complex and hypothalamus of baboons (bar = 25 μm). A. Relatively heavily labeled galaninergic neuron in the horizontal limb of the ndbB of a 5-year-old baboon. B. Relatively heavily labeled galaninergic neuron in the ventral nbM of a 2.5-year-old baboon. C. Positive control: labeled galaninergic neuron (arrow) next to an unlabeled cell in the anterior hypothalamus of a 2.5-year-old baboon.