The effects of 5'-N-ethylcarboxamidoadenosine on evoked release of [3H]serotonin in the rat nucleus tractus solitarius

Adenosine receptor desensitization and trafficking

As with the majority of G-protein-coupled receptors, all four of the adenosine receptor subtypes are known to undergo agonist-induced regulation in the form of desensitization and trafficking. These processes can limit the ability of adenosine receptors to couple to intracellular signalling pathways and thus reduce the ability of adenosine receptor agonists as well as endogenous adenosine to produce cellular responses. In addition, since adenosine receptors couple to multiple signalling pathways, these pathways may desensitize differentially, while the desensitization of one pathway could even trigger signalling via another. Thus, the overall picture of adenosine receptor regulation can be complex. For all adenosine receptor subtypes, there is evidence to implicate arrestins in agonist-induced desensitization and trafficking, but there is also evidence for other possible forms of regulation, including second messenger-dependent kinase regulation, heterologous effects involving G proteins, and the involvement of non-clathrin trafficking pathways such as caveolae. In this review, the evidence implicating these mechanisms is summarized for each adenosine receptor subtype, and we also discuss those issues of adenosine receptor regulation that remain to be resolved as well as likely directions for future research in this field.

Serotonin modulates glutamatergic transmission in the rat olfactory tubercle

The olfactory tubercle (OT) is found in the brains of mammals that are highly dependent on their sense of smell. Its human analogue is the poorly understood anterior perforated substance. Previous work on rat brain slices identified two types of field potential responses from the OT. The association fibre (AF) pathway was sensitive to muscarinic modulation, whereas the lateral olfactory tract (LOT) fibre pathway was not. Here, we establish that serotonin (5-hydroxytryptamine; 5-HT) also inhibits field potential excitatory postsynaptic potentials (EPSPs) in the AF, but not in the LOT fibre, pathway. Parallel experiments with adenosine (ADO) excluded ADO mediation of the 5-HT effect. Exogenous 5-HT at 30 microm caused a long-lasting approximately 40% reduction in the amplitude of AF postsynaptic responses, without affecting the time-course of EPSP decline, indicating a fairly restricted disposition of the 5-HT receptors responsible. The 5-HT(1)-preferring, 5-HT(5)-preferring and 5-HT(7)-preferring agonist 5-carboxamidotryptamine caused similar inhibition at approximately 100 nm. The 5-HT(1A)-preferring ligand 8-hydroxy-di-n-propylamino-tetralin at 10 microm, and the 5-HT uptake inhibitor citalopram at 3 microm, caused inhibition of AF-stimulated field potential responses in the 5-10% range. Order-of-potency information suggested a receptor of the 5-HT(1B) or 5-HT(1D) subtype. The 5-HT(1D) agonist L-694,247 (1 microm) suppressed the AF response by approximately 10% when used on its own. After washing out of L-694,427, inhibition by 30 microm 5-HT was reduced to negligible levels. Allowing for a partial agonist action of L-694,427 and complex interactions of 5-HT receptors within the OT, these results support the presence of active 5-HT(1D)-type receptors in the principal cell layer of the OT.

Presynaptic adenosine and P2Y receptors

Adenine-based purines, such as adenosine and ATP, are ubiquitous molecules that, in addition to their roles in metabolism, act as modulators of neurotransmitter release through activation of presynaptic P1 purinoceptors or adenosine receptors (activated by adenosine) and P2 receptors (activated by nucleotides). Of the latter, the P2Y receptors are G protein-coupled, whereas the P2X receptors are ligand-gated ion channels and not covered in this review.

Internalization and desensitization of adenosine receptors

Until now, more than 800 distinct G protein-coupled receptors (GPCRs) have been identified in the human genome. The four subtypes of the adenosine receptor (A₁, A_2A, A_2B and A₃ receptor) belong to this large family of GPCRs that represent the most widely targeted pharmacological protein class. Since adenosine receptors are widespread throughout the body and involved in a variety of physiological processes and diseases, there is great interest in understanding how the different subtypes are regulated, as a basis for designing therapeutic drugs that either avoid or make use of this regulation. The major GPCR regulatory pathway involves phosphorylation of activated receptors by G protein-coupled receptor kinases (GRKs), a process that is followed by binding of arrestin proteins. This prevents receptors from activating downstream heterotrimeric G protein pathways, but at the same time allows activation of arrestin-dependent signalling pathways. Upon agonist treatment, adenosine receptor subtypes are differently regulated. For instance, the A₁Rs are not (readily) phosphorylated and internalize slowly, showing a typical half-life of several hours, whereas the A_2AR and A_2BR undergo much faster downregulation, usually shorter than 1 h. The A₃R is subject to even faster downregulation, often a matter of minutes. The fast desensitization of the A₃R after agonist exposure may be therapeutically equivalent to antagonist occupancy of the receptor. This review describes the process of desensitization and internalization of the different adenosine subtypes in cell systems, tissues and in vivo studies. In addition, molecular mechanisms involved in adenosine receptor desensitization are discussed.

Different modulation of inhibitory and stimulatory pathways mediated by adenosine after chronic in vivo agonist exposure

After 6 days of in vivo treatment with two selective adenosine receptor agonists, 5'-N-ethylcarboxamido adenosine (NECA) and R-N6-phenylisopropiladenosine (R-PIA), we investigated their effects on adenosine receptors/adenylyl cyclase system in synaptic plasma membranes isolated from rat brain. NECA treatment caused a significant loss of NECA-stimulated adenylyl cyclase activity, suggesting a desensitization of the adenosine A2 receptors-mediated pathway. No significant differences in total adenosine A2 receptors were observed, but Gs protein levels were decreased, suggesting Gs down-regulation as a mechanism for desensitization. On the other hand, NECA treatment caused a significant decrease in high-affinity adenosine A1 receptors population; however, no changes in CHA-inhibited adenylyl cyclase activity or Gi protein level were observed. Finally, when we studied the effects of R-PIA, a selective adenosine A1 receptor agonist, on stimulatory pathway of adenosine, low-affinity adenosine A2 binding sites were decreased without affecting the functionality of the pathway. These results show that adenosine A1 and A2 receptors are modulated in a different way after chronic agonist exposure and suggest the existence of cross-talk mechanisms between both stimulatory an inhibitory pathways mediated by adenosine.

Activation of A2a adenosine receptors in the nucleus tractus solitarius inhibits renal but not lumbar sympathetic nerve activity

The activation of adenosine A2a receptors in the nucleus tractus solitarius (NTS) via microinjection of the selective agonist CGS 21680 elicits long-lasting, dose-dependent decreases in mean arterial pressure (MAP) and heart rate (HR) and preferential dilation of the iliac vascular bed in comparison to the renal and mesenteric vascular beds. We investigated whether differential changes in regional sympathetic output occur with A2a receptor activation. In 24 chloralose/urethane anesthetized male Sprague-Dawley rats MAP, HR, renal (RSNA) and lumbar sympathetic nerve activity (LSNA) were recorded simultaneously. Data were analyzed as both the maximum decrease and the integral of the decrease over the duration of the depressor response. Microinjection of CGS 21680 (2 and 20 pmol in 50 nl volume) into the subpostremal NTS caused significant and dose-dependent decreases in MAP, HR and RSNA, however, did not significantly decrease LSNA in comparison to the effect of vehicle. Maximum responses of RSNA vs. LSNA in delta% of control values were: -32 +/- 4 vs. -9 +/- 2, and -59 +/- 4 vs. -19 +/- 5 for low (n = 9) and high (n = 8) doses of CGS 21680 respectively; integral responses of RSNA vs. LSNA in delta% x min were: -487 +/- 112 vs. -19 +/- 35 and -1258 +/- 164 vs. -175 +/- 126 for low and high doses of CGS 21680 respectively. Microinjections of vehicle (n = 7) did not alter integral hemodynamic or neural parameters. We conclude that activation of A2a adenosine receptors in the NTS evokes differential changes in visceral vs. somatic sympathetic nerve activity which cannot explain differential vascular responses in terms of simple sympathetic withdrawal. Lack of significant inhibition of LSNA combined with preferential vasodilation in hindquarter vascular bed suggests that active vasodilation may be triggered by activation of A2a adenosine receptors in the subpostremal NTS.