Adenosine A1Receptor-Mediated Activation of Phospholipase C in Cultured Astrocytes Depends on the Level of Receptor Expression

Abstract

Adenosine A₁receptors induce an inhibition of adenylyl cyclase via G-proteins of the G_i/ofamily. In addition, simultaneous stimulation of A₁receptors and of receptor-mediated activation of phospholipase C (PLC) results in a synergistic potentiation of PLC activity. Evidence has accumulated that Gβγ subunits mediate this potentiating effect. However, an A₁receptor-mediated increase in extracellular glutamate was suggested to be responsible for the potentiating effect in mouse astrocyte cultures. We have investigated the synergistic activation of PLC by adenosine A₁and α₁adrenergic receptors in primary cultures of astrocytes derived from different regions of the newborn rat brain. It is reported here that (1) adenosine A₁receptor mRNA as well as receptor protein is present in astrocytes from all brain regions, (2) A₁receptor-mediated inhibition of adenylyl cyclase is of similar extent in all astrocyte cultures, (3) the A₁receptor-mediated potentiation of PLC activity requires higher concentrations of agonist than adenylyl cyclase inhibition and is dependent on the expression level of A₁receptor, and (4) the potentiating effect on PLC activity is unrelated to extracellular glutamate.Taken together, our data support the notion that βγ subunits are the relevant signal transducers for A₁receptor-mediated PLC activation in rat astrocytes. Because of the lower affinity of βγ, as compared with α subunits, more βγ subunits are required for PLC activation. Therefore, only in cultures with higher levels of adenosine A₁receptors is the release of βγ subunits via G_i/oactivation sufficient to stimulate PLC. It is concluded that variation of the expression level of adenosine A₁receptors may be an important regulatory mechanism to control PLC activation via this receptor.