The cholinergic system in the basal forebrain of the Atlantic white‐sided dolphin (Lagenorhynchus acutus)

Abstract

The basal forebrain (BFB) cholinergic neurotransmitter system is important in a number of brain functions including attention, memory, and the sleep-wake cycle. The size of this region has been linked to the increase in encephalization of the brain in a number of species. Cetaceans, particularly those belonging to the family Delphinidae, have a relatively large brain compared to its body size and it is expected that the cholinergic BFB in the dolphin would be a prominent feature. However, this has not yet been explored in detail. This study examines and maps the neuroanatomy and cholinergic chemoarchitecture of the BFB in the Atlantic white-sided dolphin (Lagenorhynchus acutus). As in some other mammals, the BFB in this species is a prominent structure along the medioventral surface of the brain. The parcellation and distribution of cholinergic neural elements of the dolphin BFB was comparable to that observed in other mammals in that it has a medial septal nucleus, a nucleus of the vertical limb of the diagonal band of Broca, a nucleus of the horizontal limb of the diagonal band of Broca, and a nucleus basalis of Meynert. The observed BFB cholinergic system of this dolphin is consistent with evolutionarily conserved and important functions for survival.

Distribution of acetylcholinesterase in the hippocampal formation of the Atlantic white‐sided dolphin (Lagenorhynchus acutus)

“…The presence of this layer in the AWSD suggests the existence of the septo‐hippocampal pathway. However, the apparent reduction, in terms of width and staining intensity, implies a potential reduction of innervation from cholinergic neurons in the basal forebrain (Reid et al, 2018) compared to other species. Although it is difficult to come to conclusions regarding the importance of the of septal‐hippocampal pathway in the AWSD using just AChE staining, it would appear that it may have diminished input relative to that of the entorhinal cortex, a suggestion also made for the human brain (Green & Mesulam, 1988).…”

“…The brains of the AWSDs were accessed, removed, and preserved in 4% formalin as previously described (Reid, Geula, & Darvesh, 2018). Blocks, containing the hippocampal formation in the temporal sector between a level at the uncus to the retrospenial sector, were cut from coronal slabs and cryoprotected by immersion in increasing concentrations of sucrose, ranging from 10% to 40% in 0.1 M PB (pH 7.4) for 48 hr each.…”

“…At 0.5 mm intervals, sections were stained for Nissl substance and AChE activity. A thionin‐based Nissl‐stain was used to assess cytoarchitecture as previously described (Reid et al, 2018).…”

“…A modified (Reid & Darvesh, 2015) Karnovsky–Roots method (Karnovsky & Roots, 1964) was used to demonstrate the presence of acetylcholinesterase. The AChE staining method employed in the current study was appropriate to visualize AChE‐stained neuronal profiles and neuropil, as described previously in this dolphin basal forebrain (Reid et al, 2018). All reagents were purchased from Sigma‐Aldrich (St. Louis, MO).…”