Brain of the tree pangolin (Manis tricuspis). III. The unusual locus coeruleus complex

Here, we used a range of immunohistochemical stains, focussing on tyrosine hydroxylase and dopamine-β-hydroxylase, to show that within the pons of tree pangolins clusters of noradrenergic neurons are present. No noradrenergic neurons were observed in the pontine periventricular gray matter (A6 and A4 groups missing), with all noradrenergic neurons being found within the pontine tegmentum (A7 and A5 groups). The tree pangolin is unique in lacking the locus coeruleus (A6) cell group observed in all vertebrates previously studied; however, noradrenergic axons and terminal networks were found throughout the cerebral cortex. We propose this is achieved through a unique structural reorganization of this system. First, the number of noradrenergic neurons in the compact portion of the subcoeruleus (A7sc) of the tree pangolin is increased, providing a total number of noradrenergic neurons in the pontine tegmentum (A7diffuse, A7sc, A5) that is equivalent to the entire locus coeruleus complex in related species of similar brain mass. Second, the most medially located noradrenergic neurons of the A7sc have dendrites that extend into the ventrolateral periventricular gray matter, in the location where the A6 neurons should have been located, forming a "pseudo A6" region. Third, the topological relationships of this "pseudo A6" region to other neurochemical systems that interact with the A6 neurons, such as the orexinergic, cholinergic, and serotonergic systems, appear to be maintained. Thus, a unique structural plasticity of this region appears to maintain the standard functions of the locus coeruleus complex in this unusual mammalian species.