Neurophysiology of Pain Pathways and Non-opioid analgesics Flashcards
What is Pain?
Unpleasant sensory and emotional experience associated w/ actual or potential tissue damage.
What are the two basic types of pain?
- Nociceptive
2. Neuropathic
What is Nociceptive pain and what kinds are there?
Transduction of noxious stimuli, irrespective of cognitive awareness.
- Somatic (cutaneous or deep tissue)
- Visceral (internal organs)
What is Neuropathic pain?
- Caused by a primary lesion or dysfunction in the nervous system.
What kinds of nerve fibers mediate itch and pain?
BOTH by thin, unmyelinated nerve fibers from the skin but mediated by different mechanisms.
What is an itch?
- Unpleasant sensation that elicits the desire or reflex to scratch.
What is an itch induced by:
- Pruritogens
What are examples of pruritogens?
- Histamine
- Environmental chemicals
- Drugs
What was an itch considered as long ago?
- a sub-modality of pain.
What is MrgprA3?
- Member of the family of Mas-related G protein-coupled receptors.
- Shown to mediated itch.
What are Acute pain examples?
- HA & Migraine
2. From minor injuries, labor, dysmenorrhea, kidney stones.
What is Fibromyalgia?
CNS pain state, that might be accompanied by peripheral pain states.
- Osteoarthritis
- RA
- Lupus
Stimuli that initiate chronic pain also provoke what?
- Inflammatory and immune responses.
What are examples of neuroimmune pain mediatiors?
- TNF-alpha
2. IL-1beta
What to neuroimmune pain mediators act on?
- Peripheral nociceptive nerve terminals
- promote abnormal discharge and hyperexcitability. - Peripheral nerves
- Produce neuropathic hyperalgesia.
- Allodynia
What is neuropathic hyperalgesia?
- Exaggerated response to a painful stimulus.
What is allodynia?
- Pain from a stimulus that does not normally cause pain.
- light touch or temp variation.
What kinds of cells also release neuroimmune mediators?
- Cancer cells.
What is central synaptic sensitization?
Spinal cord neurons become more responsive to nociceptive input due to persistent firing, changes in descending controls.
What does central synaptic sensitization do?
- Enhances efficacy of nociceptive transmission through the spinal cord dorsal horn and perception of spontaneous and breakthrough pain.
Where is the site of nociception (noxious stimulus)?
1.
What is the anterolateral spinothalamic pathway?
Neurons snapse IN the dorsal horn then cross and ascend to the brain.
What are 3 important aspects of pain inhibition?
- Site of drug action.
- Timing
- Nature of pain.
What is more effective pre-emptive analgesia or after sensitization develops?
Pre-emptive (b/c of timing aspect)
What is the difference between nociceptor drug action and CNS drug action?
- Nociceptor NSAIDs & APAP.
2. CNS Opioids.
What are the target sites to treat pain?
- Site of pain initiation.
- Transmission Pathway.
- Spinal Cord
- Spinothalamic Tract Fibers
- Spinal Cord & Brain.
What is used at the site of pain initiation?
- Methylene Blue
2. ASA, NSAIDs.
What is used at the transmission pathways of pain?
- Local Anesthetics.
What is used at the spinal cord target site?
- Topical counterirritants.
2. Transcutaenous electrical nerve stimulation.
What is done at the target of spinothalamic tract fibers?
- Kill the fiber (ethanol)
2. Anterolateral cordotomy.
What are spinal segments also called?
- Dermatomes.
What are the 5 dermatomes?
- Cervical (C1-C8)
- Thoracic (T1-T12)
- Lumbar (L1-L5)
- Sacral (S1-S5)
- Sacrococcygeal.
What are the chemical mediators of pain?
- Bradykinin in sensory neurons.
- 5-HT from platelets.
- Histamine from mast cells
- Neuropeptides
- ATP, K+
- Chemokines
- Ion Channels.
What are the sex differences in pain and analgesic response?
- Women demonstrate lower thresholds
- Suffer more from fibromyalgia, joint shit, migraine, IBS.
- Women respond better to opioid kappa agonists.
- Red-headed women have lower threshold to cold and heat pain.
What are 3 methods to evaluate analgesia in animals?
- Tail flick assay
- Hot plate assay
- Injection of irritant.
- Phenylquinone or acetic acid.
What is the difference between nonopioid and opioid analgesics?
- Structurally heterogenous.
- No Tolerance
- No Physical Dependence.
- Less maximal efficacy.
- Good for Mild-moderate pain.
- Inflammation and fever
- Different MOA
- Different AE.
What are examples of Salicylates.
- Aspirin (Acetyl Salicylic Acid)
- Methyl Salicylate (oil of wintergreen)
- Triethanolamine Salicylate (Bengay)
- Bismuth Subsalicylate (Pepto-Bismol)
What is the Plasma Protein Binding of ASA?
50%
What is the Plasma Protein Binding of Salicylate?
95% (low therapeutic) to 80% (high therapeutic levels)
What is the PK of ASA?
- 40% metabolized by liver esterases in first pass.
- Metabolized by esterases in blood and RBCs
- Half Life = 15 min.
What is the PK of Salicylate?
- Half Life = 2-3 Hr at low concentrations.
2. Up to 30 Hr at high concentrations.
What is the Excretion profile of Salicylates?
- Less that pH of 5 = 0%
- Normally = 10%
- Increased pH up to 30% (ion trapping)
What is the MOA of Salicylates?
- Inhibition of prostanoid synthesis.
- Inhibits Prostaglandin H synthase & Prostaglandin G/H Synthase (Cyclooxygenase COX)
What do Prostanoids do?
- Mediate:
- Thrombosis and Homostasis
- Glomerular filtration & water balance.
- Ovulation, Implantation & development.
- Initiation of labor or abortion.
- Inflammation & modulation of immune response.
What are other possible MOAs of salicylates?
- Increased endocannabinoids.
- Inhibition of spinal nociceptive processing by 5-HT, NE & ACh.
- Reduction of NO production.
What are 3 main metabolites of Arachidonic Acid?
- Thromboxane (TxA2)
- PGE2
- Prostacycline (PGl2)
Which COX enzyme is constitutive?
- COX-1
Which COX enzyme is Induced?
- COX-2
What is the metabolism pathway of Arachidonic Acid?
- Arachidonic Acid
- Prostaglandin G2
- Prostaglandin H2
- (Tissue Specific Isomerases and Synthases produce products)
What are the 2 classes of arachidonic acid metabolites?
- Eicosanoids (20 C’s)
- Prostaglandins, Thromboxanes, Leucotrienes. - Prostanoids
- Prostaglandins, Thromboxanes.
What are the actions of Arachidonic Acid Metabolites for Pain & Inflammation?
- PGE2 produced by COX
What are the actions of Arachidonic Acid Metabolites in Blood Vessels?
- Vasodilators
- Prostaglandins & Prostacyclin (good guys) - Vasoconstrictor
- Thromboxane (bad guy)
What are the actions of Arachidonic Acid Metabolites in Blood?
- Platelet Aggregation
- Thromboxane (platelets) - Platelete Disaggregation
- Prostacyclin (endothelial cells lining blood vessels)
What are the actions of Arachidonic Acid Metabolites in the GI Tract?
- Inhibit gastric acid secretion
- Increase secretion of mucous + HCO3
- PGE2 & Prostacyclin for both.
What are the actions of Arachidonic Acid Metabolites in the Kidney?
- Maintain renal blood flow, Increase Na+ water retention.
- PGE2 & Prostacyclin.
What are properties of COX-1?
- Produces arachidonic acid metabolites that:
- Mediate protective features of Prostaglandins and Prostacyclin.
- Predominant enzyme producing Thromboxane.
What are properties of COX-2?
- Produces arachidonic acid metabolites:
- Mediate pathological inflammatory processes.
- Predominant enzyme producing prostacyclin
- PGE2 most abundant product of BOTH COX1 & COX2.
What are the COX2 selective NSAIDs
- Celecoxib
- Valdecoxib
- Rofecoxib
- Etoricoxib
- Lumiacoxib
What is the set point of temperature maintained by?
- Hypothalamic Nuclei
- Anterior (Heat)
- Posterior (Cold)
What is the Mechanism of temperature control in the hypothalamus?
- PGE2 Increase cAMP production
- Increase Temperature by Increasing Heat Generation
- Decrease Heat Loss.
How do NSAIDs and APAP work as anti-pyretic?
- Block Hypothalamic response to cytokines released from monocytes and macrophages by decreasing PGE2 Production.
- Increase Heat loss by vasodilation and sweating.
What happens in RA?
- Increased pro-inflammatory cytokines increases PGE2.
2. NSAIDs first line therapy.
What is first line treatment for Osteoarthritis?
- Oral acetaminophen, but less effective by intraarticular injection.
What are the GI effects of COX inhibition?
- Gastric Intolerance, Dyspepsia (most common)
2. Ulcer
Why is Gastric intolerance, dyspepsia caused by COX inhibition?
- Due to chemoreceptor trigger zone stimulation.
- Direct Chemical Irritant
- COX-1 Inhibition.
Why can Ulcers be produced by COX inhibition?
- Breakdown of gastric mucosal layer by COX1 inhibition.