Beta-adrenergic receptor-mediated regulation of extracellular adenosine in cerebral cortex in culture

Abstract

Adenosine is an important inhibitory neuromodulator in the CNS, yet the sources of extracellular adenosine have yet to be well characterized. In this study we show that beta-adrenergic stimulation of cortical cultures results in the extracellular accumulation of cAMP as well as adenosine, and that the extracellular adenosine derives from extracellular cAMP. The concentration dependence of isoproterenol in evoking cAMP secretion was determined by radioimmunoassay, and the EC50 for this effect was found to be approximately 100 nM. In order to investigate the effect of beta-adrenergic stimulation on the regulation of extracellular adenosine, the effect of isoproterenol in stimulating the extracellular accumulation of adenine-containing compounds was examined by HPLC. Isoproterenol stimulated cAMP secretion in both astrocyte cultures and astrocyte-rich mixed cultures of astrocytes and neurons. However, no extracellular cAMP was detectable in neuron- enriched astrocyte-poor cultures. Extracellular adenosine increased in response to isoproterenol in the astrocyte-rich mixed cultures, but not in the neuron-enriched astrocyte-poor cultures. After 30 min exposure to isoproterenol, the concentration of adenosine in the extracellular medium increased by 47% in 56 experiments in the mixed astrocyte-rich cultures. In order to establish whether the adenosine that accumulates in response to isoproterenol stimulation actually derives from extracellular cAMP, phosphodiesterase inhibitors were tested for their ability to block isoproterenol-stimulated adenosine accumulation. Isobutylmethylxanthine (IBMX; 100 microM), RO 20–1724 (180 microM), carbazeran (10 microM), dipyridamole (10 microM), and trifluoperazine (10 microM) had no inhibitory effect on the isoproterenol-stimulated accumulation of extracellular adenosine. However 100 microM IBMX plus 180 microM RO 20–1724 effectively blocked isoproterenol-stimulated adenosine accumulation and, as expected, increased extracellular cAMP. As a further test of the origin of isoproterenol-stimulated adenosine accumulation, we attempted to block this phenomenon by blocking cAMP secretion itself. For this purpose probenecid, a known inhibitor of cAMP secretion in many different cell types, was used. We found that probenecid at 1 mM blocked isoproterenol-stimulated adenosine accumulation. These studies suggest that one potentially important source of extracellular adenosine in the cerebral cortex is endogenous extracellular cAMP, secreted from astrocytes in response to beta- adrenergic receptor stimulation. Since the receptors of neuromodulators other than norepinephrine may also be coupled to adenylyl cyclase in the cerebral cortex, there may be several neuromodulatory systems that regulate extracellular adenosine levels by this mechanism.

Adenosine released by astrocytes contributes to hypoxia‐induced modulation of synaptic transmission

“…Because the extracellular conversion of cAMP to adenosine requires first an extracellular phosphodiesterase capable of converting cAMP into adenosine-5 0 -monophosphate (AMP), we performed experiments with the phosphodiesterase inhibitor Ro 20-1724 (Brundege et al, 1997;Rosenberg et al, 1994), and cAMP transport inhibitor probenecid (Henderson and Strauss, 1991;Rosenberg et al, 1994). In agreement with previous studies (Brundege et al, 1997), Ro20-1724 significantly increased the amplitude of the EPSC (126.3 6 13%, P < 0.05).…”

ERK-Dependent Modulation of Cerebellar Synaptic Plasticity after Chronic Δ9-Tetrahydrocannabinol Exposure

“…In THC-tolerant mice, activation of the cAMP/PKA pathway by forskolin or by brief tetanic stimulation induces an adenosine A1R-mediated inhibition of synaptic release, instead of triggering LTP. Adenylyl cyclase activation by forskolin causes intracellular cAMP accumulation, transport, and conversion into adenosine by extracellular phosphodiesterase activity (Rosenberg and Dichter, 1989;Rosenberg et al, 1994;Rosenberg and Li, 1995). Adenosine, originating from the metabolism of intracellular cAMP, may also be released after synaptic stimulation (Mitchell et al, 1993).…”

Docosahexaenoic acid facilitates cell maturation and β-adrenergic transmission in astrocytes

“…Of the neurotransmitter receptors present in astrocytes, b-ARs occupy a special place, because unequivocal evidence suggests that astroglial cells may be the targets of the central noradrenergic system (48). These receptors have multifarious functions that include regulating astroglial glycogen metabolism (41), stimulating the expression of cytokines (49), inducing gene expression of neurotrophic factors (50), modulating glial inwardly rectifying potassium channels (51), and regulating the extracellular concentration of adenosine (52). Additionally, the b-AR system has a profound effect on the differentiation and maturation of astrocytes (53,54).…”