Neurochemical characterization of extrinsic nerves in myenteric ganglia of the guinea pig distal colon

Extrinsic nerves to the gut influence the absorption of water and electrolytes and expulsion of waste contents, largely via regulation of enteric neural circuits; they also contribute to control of blood flow. The distal colon is innervated by extrinsic sympathetic and parasympathetic efferent and spinal afferent neurons, via axons in colonic nerve trunks. In the present study, biotinamide tracing of colonic nerves was combined with immunohistochemical labeling for markers of sympathetic, parasympathetic, and spinal afferent neurons to quantify their relative contribution to the extrinsic innervation. Calcitonin gene-related peptide, vesicular acetylcholine transporter, and tyrosine hydroxylase, which selectively label spinal afferent, parasympathetic, and sympathetic axons, respectively, were detected immunohistochemically in 1 ± 0.5% (n = 7), 15 ± 4.7% (n = 6), and 24 ± 4% (n = 7) of biotinamide-labeled extrinsic axons in myenteric ganglia. Immunoreactivity for vasoactive intestinal polypeptide, nitric oxide synthase, somatostatin, and vesicular glutamate transporters 1 and 2 accounted for a combined maximum of 14% of biotinamide-labeled axons in myenteric ganglia. Thus, a maximum of 53% of biotinamide-labeled extrinsic axons in myenteric ganglia were labeled by antisera to one of these eight markers. Viscerofugal neurons were also labeled by biotinamide. They had distinct morphologies and spatial distributions that correlated closely with their immunoreactivity for nitric oxide synthase and choline acetyltransferase. As reported for the rectum, nearly half of all extrinsic nerve fibers to the distal colon lack the key immunohistochemical markers commonly used for their identification. Their abundance may therefore have been significantly underestimated in previous immunohistochemical studies. J. Comp. Neurol. 523:742–756, 2015. © 2014 Wiley Periodicals, Inc.     

Association of N‐cadherin levels and downstream effectors of Rho GTPases with dendritic spine loss induced by chronic stress in rat hippocampal neurons

Chronic stress promotes cognitive impairment and dendritic spine loss in hippocampal neurons. In this animal model of depression, spine loss probably involves a weakening of the interaction between pre‐ and postsynaptic cell adhesion molecules, such as N‐cadherin, followed by disruption of the cytoskeleton. N‐cadherin, in concert with catenin, stabilizes the cytoskeleton through Rho‐family GTPases. Via their effector LIM kinase (LIMK), RhoA and ras‐related C3 botulinum toxin substrate 1 (RAC) GTPases phosphorylate and inhibit cofilin, an actin‐depolymerizing molecule, favoring spine growth. Additionally, RhoA, through Rho kinase (ROCK), inactivates myosin phosphatase through phosphorylation of the myosin‐binding subunit (MYPT1), producing actomyosin contraction and probable spine loss. Some micro‐RNAs negatively control the translation of specific mRNAs involved in Rho GTPase signaling. For example, miR‐138 indirectly activates RhoA, and miR‐134 reduces LIMK1 levels, resulting in spine shrinkage; in contrast, miR‐132 activates RAC1, promoting spine formation. We evaluated whether N‐cadherin/β‐catenin and Rho signaling is sensitive to chronic restraint stress. Stressed rats exhibit anhedonia, impaired associative learning, and immobility in the forced swim test and reduction in N‐cadherin levels but not β‐catenin in the hippocampus. We observed a reduction in spine number in the apical dendrites of CA1 pyramidal neurons, with no effect on the levels of miR‐132 or miR‐134. Although the stress did not modify the RAC–LIMK–cofilin signaling pathway, we observed increased phospho‐MYPT1 levels, probably mediated by RhoA–ROCK activation. Furthermore, chronic stress raises the levels of miR‐138 in accordance with the observed activation of the RhoA–ROCK pathway. Our findings suggest that a dysregulation of RhoA–ROCK activity by chronic stress could potentially underlie spine loss in hippocampal neurons. © 2015 Wiley Periodicals, Inc.