Objectives for Adenosine

• Understand the normal physiological functions of adenosine and its role in retaliatory and neuroprotective capacities during pathological conditions.

• Recognize the structural similarities between adenosine and its metabolites:

  • Inosine

  • Xanthine

  • Hypoxanthine

  • Uric acid

• Identify both agonists and antagonists of adenosine including caffeine and theophylline.

• Be prepared to discuss in detail the mechanisms of neurochemical transmission:

  • Synthesis

  • Storage

  • Release or alternative sources of extracellular adenosine

  • Receptor subtypes and effectors

  • Termination of effect

• Understand the distribution of A1 and A2A receptors.

Questions for Adenosine

• What genes and proteins might be up or down-regulated if adenosine signaling is increased to enhance sleepiness during the winter season?

• Explain the influence of concentrative nucleoside transporters (CNTs) and equilibrative nucleoside transporters (ENTs) on extracellular adenosine levels.

• List A1 adenosine receptors and explain their effects on adenyl cyclase activity and sleep.

• Identify the roles of adenosine in seizure activity.

Introduction to Purines

• Purines such as adenine and guanine are important components of nucleic acids, with basic structural units being purine nucleosides such as adenosine.

Chemistry of Adenosine

• Adenosine consists of an adenine molecule bound to ribose.

• ATP (adenosine triphosphate) is formed by the addition of three phosphate groups to adenosine.

Biochemistry of Purine Catabolism

• Pathway includes dephosphorylation from ATP to AMP and then further breakdown to adenosine by enzymes such as:

  • Adenylate deaminase

  • Nucleotidases

  • Through an adenosine pathway:

  • Inosine → Guanosine nucleoside phosphorylases → Guanine

  • Xanthine → Uric Acid through xanthine oxidase

• Reaction involves various enzymes, including adenosine deaminase and nucleoside phosphorylase, to produce urate.

Epigenetic Role of Adenosine

• Adenosine inhibits methylation (epigenetic modifications) via S-adenosylhomocysteine (SAH).

  • SAM (S-adenosylmethionine) is a methyl group donor.

  • Accumulation of adenosine affects the balance favoring SAH, which inhibits methyltransferases, along with adenosine kinase removing adenosine's inhibitory effect on SAM.

Purinergic Receptors

• Purinergic receptors are categorized into:

  • P1 receptors: Bind adenosine (A1, A2A, A2B, A3).

  • P2 receptors: Bind ATP (P2X and P2Y subtypes).

  • A1 receptors inhibit adenyl cyclase via Gi proteins.

  • A2A and A2B receptors stimulate adenyl cyclase via Gs proteins.

  • Roles of intracellular ion channels and metabotropic effects of P2 receptors are also noted.

Functional Roles of Adenosine in the CNS

• Functions include:

  • Neuromodulation

  • Neuroprotection against ischemia and seizure

  • Regulatory impact on cerebral blood flow

  • Involvement in neuropathic pain

  • Regulation of sleep and thermogenic processes

Flow Metabolism Coupling

• Connection between neuronal activity and increased blood flow.

• Mechanism includes:

  • NO (Nitric oxide)

  • cGMP (cyclic guanosine monophosphate)

  • Regulated by adenosine acting on A2A receptors.

Sleep Regulation and Adenosine

• Adenosine has a prominent role in sleepiness and homeostasis:

  • Increased adenosine correlates with reduced alertness.

  • Mechanisms highlighted include receptor activation for sleep onset and regulation during sleep deprivation.

• Graphical data displays adenosine levels over time relative to sleep stages, confirming its role in sleep modulation.

Seizure Suppression Mechanisms

• A1 receptor activation is crucial in seizure suppression mechanisms:

  • This involves modulation of excitatory neurotransmitter release, particularly in the hippocampus, and vasodilatory responses to maintain blood flow.

Transport Mechanisms of Adenosine

• Two transporter families are noted:

  • ENTs (Equilibrative Nucleoside Transporters): Carry nucleosides across concentration gradients.

  • CNTs (Concentrative Nucleoside Transporters): Concentrate adenosine against concentration gradients.

• Cellular mechanisms controlling the adenosine concentration via transporters are crucial for its function in the CNS.

Methods of Terminative Effect for Adenosine

• Involves degradation by enzymes such as adenosine deaminase and uptake via CNTs after transportation processes.

Conclusory Remarks on Adenosine

• Understanding the significance of adenosine in neuromodulation and neuroprotection highlights its potential as a therapeutic target for various CNS disorders.

• Recognition of the dense distribution of receptors, particularly A1 and A2A, elucidates their functional implications for brain regions involved in sleep, memory, and neuroprotection.