Expression pattern of type 3 adenylyl cyclase in rodent dorsal root ganglion and its primary afferent terminals

Highlights

• AC3 is predominantly expressed in the cytoplasm of small to medium-sized DRG neurons.

• AC3 is preferentially colocalized with CGRP-containing DRG neurons and their primary afferent terminals.

• AC3 expression pattern in DRGs is similar between C57/BL6 J mouse and Sprague Dawley rat.

Abstract

cAMP (Cyclic Adenosine monophosphate), one of the most highly studied second messengers, is regulated by a family of adenylyl cyclase (AC) enzymes. Type 3 adenylyl cyclase (abbreviated as AC3), a subtype of adenylyl cyclase, is reported to be expressed in cilia in the olfactory and central nervous system and plays an important role in many physiological functions such as olfaction, development. However, expression of AC3 in the dorsal root ganglion (DRG) is not reported. In the present study, using immunohistochemical method, we discovered that AC3 immunoreactivity (IR) is predominantly expressed in the cytoplasm of small to medium sized DRG neurons. Double labelling revealed that the majority of AC3 IR are colocalized with CGRP (a peptidergic nociceptor marker), rarely with NF200 (a myelinated neuronal marker) or IB4 (a nonpeptidergic nociceptor marker). Furthermore, dense AC3 IR exists in the superficial dorsal horn, especially in laminaⅠand dorsal part of lamina II, where CGRP-positive DRG neurons terminate. The expression pattern of AC3 is similar between C57/BL6 J mouse and Sprague Dawley rat. For instance, AC3 is primarily expressed in the cell bodies of small to medium sized DRG neurons and the majority of AC3 IR is also in CGRP-containing neurons in rat. Taken together, our data suggest that AC3 is primarily expressed in the small to medium sized cell bodies and central terminals of CGRP-positive DRG neurons, implying AC3 enzyme might potentially function in nociception.

Introduction

Adenylyl cyclase (AC) is an enzyme widely distributed in the central nervous system. It converts ATP to cAMP, which is one of the most-studied second messengers and plays an important role in many brain functions, such as development, learning, memory and drug addiction [1]. To date, ten different isoforms of adenylyl cyclases (AC1–AC10) have been identified, each with a unique expression pattern within the central nervous system, from peripheral sensory nerves to the cerebral cortex [2]. AC3 is one of the least studied ones. AC3 mRNA was originally reported to be expressed in rat olfactory epithelium (OE) in 1990 [3]. Two years later, Xia and her colleagues found that AC3 mRNA was expressed not only in olfactory sensory neurons, but also in other tissues types including brain, spinal cord, adrenal medulla, adrenal cortex, heart atrium, aorta, lung, and retina [4]. In 2007, Bishop et al reported that AC3 localized to primary cilia throughout the adult mouse brain [5]. Whether AC3 is present in peripheral dorsal root ganglion (DRG) is unknown. Recently, AC3 mRNA was detected in rat DRGs [6]. In light of this, it is of interest to determine whether AC3 protein is expressed in DRG and whether this enzyme is localized in specific subpopulations of sensory neurons. Thus, the goals of the present study are to explore the expression pattern of AC3 protein in primary sensory neurons. The colocalization of AC3 with different putative population markers for the primary sensory neurons including neurofilament protein 200 (NF200), a marker for the myelinated DRG neurons [7], calcitonin gene-related peptide (CGRP), a marker for peptidergic DRG neurons [8], isolectin B4 (IB4), a marker for small diameter, non-peptidergic DRG neurons [9], was examined.