The role of astrocytic signaling systems in behavior and disease remains essentially unexplored in spite of the fact that these cells exhibit a wide variety of G-protein coupled receptors (GPCRs) that are activated during neuronal activity. The lack of information in this area stems, in large part, from our inability to selectively activate tese cells while measuring behavioral parameters. To circumvent this problem, we developed a transgenic line of mice that expresses a novel Gq-coupled GPCR (Gq-DREADD) in astrocytes that responds to a ligand, clozepine-N-oxide (CNO), which crosses the blood-brain barrier (bbb). All studies indicate that Gq- DREADD functions similarly to native Gq-GPCRs. We have performed extensive immunohistochemical and calcium imaging studies throughout the CNS and find that the expression and activity of Gq-DREADD are restricted to astrocytes. The scope of the behavioral/physiological phenotype observed following an ip injection of CNO in GFAP-Gq- DREADD mice is stunning. A single ip injection of CNO leads to a marked increase in blood pressure, heart rate, saliva formation, and piloerection, and a decrease in body temperature;each of these parameters are regulated by the autonomic nervous system. Importantly, CNO has no effect in littermate wt mice, and astrocytic Gq-DREADD mice are not different from littermate control mice when injected with vehicle. As exciting as these findings are, several important questions need to be addressed before proceeding to mechanistic studies. First, it is essential to define the anatomical location of the GFAP+ cells modulating ANS activity. While we have not ruled out the possibility that GFAP+ Schwann cells mediate the effect of Gq- DREADD activation on ANS activity, it seems more likely that the effect is due to the activation of astrocytic signaling cascades given the extensive literature involving astrocytic modulation of neuronal activity. Second, while all of our immunocytochemical and calcium imaging experiments suggest that Gq-DREADD signaling is restricted to GFAP+ glia, using these methods it is nearly impossible to rule out the possibility that a small population of neurons express Gq-DREADD and are responsible for the observed phenotype. Experiments described in Specific Aim 1 are designed to define the anatomical location of Gq-DREADD cells modulating ANS function and further examine the possibility that the modulation of ANS activity is the result of the activation of a small population of neurons. Third, a question that arises whe using a pharmacogenetic approach to activate signaling cascades in vivo is whether the resulting phenotype reflects a physiological process or our ability to over-stimulate a signaling cascade.
In Specific Aim 2 we propose to use a recently identified dominant/negative (d/n) mutation in phospholipase ?1 (PLC ?1) to develop an approach for selectively and reversibly blocking glial Gq-GPCR signaling cascades in vivo.
Astrocytes are the predominant cell type in brain which closely associate with all other neural cell types, express a wide variety of receptors, and release neuroactive molecules enabling them to interact with other cellular constituents. In spite of this we know little about the role of astrocytes in physiology or behavior. Our recent findings indicate that astrocytes are playing an important role in physiology and behavior including modulation of blood pressure/heart rate and a reduction in pain sensitivity. Understanding the role of astrocytes in physiology and behavior could lead to new therapeutic treatments.
| Khakh, Baljit S; McCarthy, Ken D (2015) Astrocyte calcium signaling: from observations to functions and the challenges therein. Cold Spring Harb Perspect Biol 7:a020404 |
