The Suprachiasmatic Nucleus and the Intergeniculate Leaflet of the Flat-Faced Fruit-Eating Bat (Artibeus planirostris): Retinal Projections and Neurochemical Anatomy.

Nelyane N M Santana,Rovena C J G Engelberth,Marília A S Barros,Paulo L A G Morais,Fernando V L Ladd,Lara L Silva,Jeferson S Cavalcante,Miriam S M O Costa,Expedito S Nascimento,Melquisedec A D Santana,Ruthnaldo R M Lima,Helder H A Medeiros,Judney C Cavalcante

doi:10.3389/fnana.2018.00036

Abstract

In mammals, the suprachiasmatic nucleus (SCN) and the intergeniculate leaflet (IGL) are the main components of the circadian timing system. The SCN, classically known as the master circadian clock, generates rhythms and synchronizes them to environmental cues. The IGL is a key structure that modulates SCN activity. Strategies on the use of time by animals can provide important clues about how some species are adapted to competitive process in nature. Few studies have provided information about temporal niche in bats with special attention on the neural substrate underlies circadian rhythms. The aim of this study was to investigate these circadian centers with respect to their cytoarchitecture, chemical content and retinal projections in the flat-faced fruit-eating bat (Artibeus planirostris), a chiropteran endemic to South America. Unlike other species of phyllostomid bats, the flat-faced fruit-eating bat’s peak of activity occurs 5 h after sunset. This raises several questions about the structure and function of the SCN and IGL in this species. We carried out a mapping of the retinal projections and cytoarchitectural study of the nuclei using qualitative and quantitative approaches. Based on relative optical density findings, the SCN and IGL of the flat-faced fruit-eating bat receive bilaterally symmetric retinal innervation. The SCN contains vasopressin (VP) and vasoactive intestinal polypeptide (VIP) neurons with neuropeptide Y (NPY), serotonin (5-HT) and glutamic acid decarboxylase (GAD) immunopositive fibers/terminals and is marked by intense glial fibrillary acidic protein (GFAP) immunoreactivity. The IGL contains NPY perikarya as well as GAD and 5-HT immunopositive terminals and is characterized by dense GFAP immunostaining. In addition, stereological tools were combined with Nissl stained sections to estimate the volumes of the circadian centers. Taken together, the present results in the flat-faced fruit-eating bat reveal some differences compared to other bat species which might explain the divergence in the hourly activity among bats in order to reduce the competitive potential and resource partitioning in nature.

Highlights

Biological rhythms are ubiquitous in nature and occurs in all living organisms (Foster and Kreitzman, 2014)
The present work provides the characterization of retinal innervation and neurochemical signature of the suprachiasmatic nucleus (SCN) and intergeniculate leaflet (IGL) in a South America endemic chiropteran, the flat-faced fruiteating bat
glutamic acid decarboxylase (GAD)-IR is present as a plexus of axons in the SCN as well as in the IGL which probably has its origins in cells of the midbrain raphe nuclei

Summary

Introduction

Biological rhythms are ubiquitous in nature and occurs in all living organisms (Foster and Kreitzman, 2014). The CTS orchestrates body rhythms for concerted actions and entrains them to photoperiodic or non-photic stimuli (KronfeldSchor et al, 2013). This system is built from a neural network of oscillators, modulating structures, synchronizing pathways, and efferent projections (Morin, 2013) and a molecular machinery formed by CCG, proteins and the transcriptional-translational feedback loops (Reppert and Weaver, 2001; Albrecht, 2012). The SCN of the hypothalamus is the central circadian pacemaker (Moore and Lenn, 1972) This structure is a paired nucleus located in the anteroventral hypothalamus, on each side of the third ventricle, immediately dorsal to the optic chiasm (Van den Pol, 1991). Previous studies have shown that the SCN can be divided in two zones, a ventrolateral “core” region and a dorsomedial “shell” region, on the basis of the neuronal cytoarchitecture (Van den Pol, 1980), neurochemical phenotype (Moore et al, 2002; Morin, 2013; Allali et al, 2017), organization of afferent innervation (Moga and Moore, 1997), distribution of efferent projections (Leak and Moore, 2001), pattern of gene expression (Dardente et al, 2002) and electrical activity (Schaap et al, 2003)

Objectives

Methods

Discussion

Conclusion