The Organization of CRF Neuronal Pathways in Toads: Evidence that Retinal Afferents Do Not Contribute Significantly to Tectal CRF Content

Abstract

Previous work has suggested that the peptide corticotropin-releasing factor (CRF) acts to inhibit visually guided feeding in anurans, but little is known about potential targets for CRF within the subcortical visuomotor circuitry. Here we investigated the relationship between CRF neuronal organization and visual pathways in toads. CRF-immunoreactive (ir) neurons and fibers were widely distributed throughout the ventral subpallial telencephalon and hypothalamus, although few fibers were found in telencephalic areas, such as the striatum, that are known to project to the tectum in anurans. Large populations of CRF-ir cells were observed in the bed nucleus of the stria terminalis and preoptic area as well as in the ventral infundibular hypothalamus. CRF-ir neurons and fibers also were observed in several midbrain and brain stem areas. Colchicine treatment significantly enhanced CRF-ir neurons and fibers throughout the brain, and revealed CRF-ir cell groups in several brain areas (including the dorsal hypothalamus) that were not observed in untreated animals. Intrinsic CRF-immunoreactive neurons were routinely observed in cell layer 8 and sometimes in layer 6 of the optic tectum in both untreated and colchicine-treated animals. CRF was detected in toad optic tectum by radioimmunoassay, although tectal CRF content was less than that of the hypothalamus and forebrain. Unilateral eye ablation did not affect CRF content of the contralateral optic tectum. We conclude that CRF-producing neurons are widely distributed in several areas of the toad brain known to be involved in regulating the behavioral, autonomic and endocrine response to stressors, including the optic tectum and several brain areas known to project to the optic tectum. Furthermore, retinal afferents do not contribute significantly to tectal CRF content.

I'll take the low road: the evolutionary underpinnings of visually triggered fear

“…Moderate to high concentrations of CRF have been observed in the OT of several amphibian and fish species (reviewed by Carr et al, 2010 ). In African clawed frogs, in vitro release of CRF is greater from the OT than any other brain area (Carr et al, 2013 ).…”

“…In African clawed frogs, in vitro release of CRF is greater from the OT than any other brain area (Carr et al, 2013 ). CRF-ir neurons are located in tectal layers 6 and 8 (Carr et al, 2010 ), areas known to possess interneurons with dendrites extending to layer 9 in order to intercept incoming information from the retina and pretectal areas and visual thalamus. Peripheral or central administration of the peptide reduces visually-guided prey capture (Carr et al, 2002 ; Crespi and Denver, 2004 ; Morimoto et al, 2011 ), but whether this is a direct effect on tectal CRF receptors is currently being tested in my laboratory.…”

Regulation of feeding behavior and psychomotor activity by corticotropin-releasing hormone (CRH) in fish

“…CRH and its related peptides also act as regulators of feeding behavior and stress responses in vertebrates including mammals, birds, amphibians, and fish (Kalra et al, 1999; Bernier and Peter, 2001; Ohgushi et al, 2001; Hillebrand et al, 2002; Tachibana et al, 2004; Saito et al, 2005; Lowry and Moore, 2006; Carr et al, 2010; Matsuda et al, 2010b; Morimoto et al, 2011; Khan et al, 2013). It has been reported that, in the goldfish, intracerebroventricular (ICV) administration of CRH or UI exerts an anorexigenic action (de Pedro et al, 1997; Bernier and Peter, 2001; Volkoff et al, 2005; Matsuda, 2009), which is blocked by treatment with a CRH 1/CRH 2 receptor antagonist, α-helical CRH (9−41) (de Pedro et al, 1997; Bernier and Peter, 2001; Bernier, 2006; Maruyama et al, 2006).…”