Effects of Anesthetic Drugs on CNS Function in Dogs and Cats #
Mechanism of Action of Anesthetic Drugs #
Anesthetic drugs alter CNS function by depressing neural activity, enhancing inhibitory neurotransmission, or reducing excitatory signaling. The primary targets include:
- GABA (Gamma-Aminobutyric Acid) Receptors: Enhancing GABAergic inhibition reduces neuronal excitability, inducing sedation and unconsciousness.
- NMDA (N-Methyl-D-Aspartate) Receptors: Inhibition of NMDA receptors reduces excitatory neurotransmission, contributing to analgesia and loss of consciousness.
- Voltage-Gated Ion Channels: Some anesthetics block sodium or calcium channels, reducing neuronal firing and transmission.
Inhalant Anesthetics and CNS Depression #
Inhalant anesthetics, such as isoflurane and sevoflurane, induce anesthesia by:
- Depressing cerebral metabolic activity and oxygen consumption.
- Increasing intracranial pressure due to cerebral vasodilation.
- Reducing neuronal excitability by enhancing GABA receptor activity.
Injectable Anesthetics and Their CNS Effects #
- Propofol: A GABA agonist that induces rapid CNS depression, causing unconsciousness and muscle relaxation.
- Alfaxalone: A neurosteroid anesthetic that enhances GABA-mediated inhibition, used for sedation and anesthesia.
- Ketamine: An NMDA receptor antagonist that disrupts excitatory neurotransmission, inducing dissociative anesthesia and analgesia.
Opioids and CNS Modulation #
Opioids such as morphine, fentanyl, and hydromorphone act on opioid receptors to:
- Suppress pain perception and alter nociceptive pathways.
- Induce sedation, respiratory depression, and sometimes euphoria or dysphoria in cats.
Alpha-2 Agonists and CNS Inhibition #
Drugs like dexmedetomidine and xylazine act on alpha-2 adrenergic receptors to:
- Decrease norepinephrine release, leading to profound sedation and analgesia.
- Reduce sympathetic tone, causing bradycardia and hypotension.
Local Anesthetics and Neural Blockade #
Local anesthetics, such as lidocaine and bupivacaine, block sodium channels to:
- Prevent nerve impulse transmission, providing regional anesthesia.
- Induce temporary loss of sensation and motor function at the site of administration.
Side Effects and Considerations #
- Neurotoxicity: Prolonged or high-dose anesthetic exposure can cause neuronal damage.
- Hypotension and Ischemia: Decreased cerebral perfusion may affect brain function.
- Excitatory Phenomena: Some anesthetics, like ketamine, may cause CNS excitation rather than depression.
References #
- Hall, L. W., Clarke, K. W., & Trim, C. M. (2013). Veterinary Anesthesia and Analgesia. Elsevier.
- Grimm, K. A., Lamont, L. A., Tranquilli, W. J., Greene, S. A., & Robertson, S. A. (2015). Veterinary Anesthesia and Analgesia: The Fifth Edition of Lumb and Jones. Wiley-Blackwell.
- Muir, W. W., & Hubbell, J. A. (2013). Handbook of Veterinary Anesthesia. Mosby.
