Mechanism of general anaesthesia at molecular level

MECHANISM OF GENERAL

ANAESTHESIA AT MOLECULAR LEVEL

DR.P. NARASIMHA REDDY

PROFESSOR OF ANAESTHESIA,NARAYANA MEDICAL COLLEGE,

NELLORE.

INTRODUCTIONGeneral anaesthetics have been in use since

mid 19th century, ether and chloroform were the first two agents to come, later in mid 20th century the Halothane was synthesized. GA’s are most useful, most dangerous, and least specific & least understood drugs. GA’s comprise one of the most important and dangerous groups of drugs in medicine because of which surgery flourished and flourishing

Since 160 years scientists are trying to explain how GA drugs work in the midst of an era of molecular neurology. It has been stated that mystery of anaesthesia like that of consciousness has not yielded to molecularity. How and where anaesthetics act remains a mystery, and that has instigated and frustrated many physicians for many decades.

ONE OF THE UNSOLVED MYSTERIES OF

PHARMACOLOGY

WHY

CHANGE IN THE RESPONSE

NO STRUCTURE ACTIVITY RELATIONSHIP AMONG ANAESTHETICS

WORKS AT VERY HIGH CONCENTRATION ONLY

AIM

WHAT IS ANAESTHESIA?

HOW IS ANAESTHESIA MEASURED?

WHAT ARE THE ANATOMICAL SITES OF ANAESTHETIC ACTION IN THE CNS?

WHAT ARE THE MOLECULAR TARGETS OF ANAESTHESIA?

ANAESTHESIA DEFINITION

UNCONSCIOUS

AMNESIA

ANALGESIA

IMMOBILITY

ATTENUATION OF AUTONOMIC RESPONSE TO NOXIOUS STIMULI

SLEEP- there are two main pathways in determining the sleep & wakefulness state. Ascending arousal pathway includes penduculo-pontine and lateral dorsal tegmental nuclei which send projections to thalamus and cortex. This pathway also includes ascending projections from locus coeruleus, raphe and tubero mamillary nucleus. The descending pathways includes projections from the ventrolateral preoptic nucleus and lateral hypothalamic orixinergic neurons to the TMN, raphe, LC & PP, Lateral hypothalamic nucleus.

ANAESTHESIA MEASUREMENT

AIM IS TO RANK THE ORDER OF POTENCY

ANAESTHESIA MEASUREMENT

MAC- MINIMAL ALVEOLAR CONCENTRATION

THE ALVEOLAR PARTIAL PRESSURE OF A GAS AT WHICH

50% OF HUMANS DO NOT RESPOND TO A SURGICAL

INCISION

SITE OF ACTION

PLAUSIBLE TARGETS ARE PERIPHERAL RECEPTORS, SPINAL CORD, BRAINSTEM, CEREBRAL CORTEXSPINAL CORD

> DIRECT EFFECT ON THE SPINAL MOTOR NEURONS

> ENDPOINT FOR ANAESTHETIC POTENCY CONT….

SITE OF ACTION

BRAINSTEM > RETICULAR ACTIVATING SYSTEM > INVOLVED IN AROUSAL BEHAVIOUR

CEREBRAL CORTEX > MAJOR SITE FOR INTEGRATION, STORAGE, RETRIEVAL > INTERFERE WITH COMPLEX FUNCTIONS CONT….

SITE OF ACTIONBOTH PRESYNAPTIC & POSTSYNAPTIC EFFECTS

GLUTAMATE- MAIN EXCITATORY NEUROTRANSMITTER IN CNS

GABA- MAIN INHIBITIORY NEUROTRANSMITTER IN THE CNS

GLYCINE- MAIN INHIBITORY NEUROTRANSMITTER IN THE SPINAL CORD

MOLECULAR TARGETS

MEYER-OVERTON HYPOTHESIS

ACTION ON ION CHANNELS

1. EFFECT ON VOLTAGE GATED

ION CHANNELS

2. EFFECT ON LIGAND GATED

ION CHANNELS

MEYER-OVERTON HYPOTHESIS

HYDROPHOBIC TARGET

POTENCY OF ANAESTHETICS AND LIPID SOLUBILITY

PRODUCE ANAESTHESIA WHEN THEY REACH A CRITICAL CONCENTRATION IN THE MEMBRANE

MEYER-OVERTON HYPOTHESIS

UNITARY THEORY OF ANAESTHESIA

> STRUCTURALLY UNRELATED

COMPOUND ARE LIKELY TO ACT

AT THE SAME MOLECULAR SITE

> ANAESTHESIA IS PRODUCED BY

DISTURBANCE OF THE PHYSICAL

PROPERTIES OF CELL MEMBRANES

MEYER-OVERTON HYPOTHESIS

LIMITATIONS

> Applies to gases and volatile mixture

> Olive oil is a poor mixture of oil

Theory fails to explain how the proposed disturbance of the lipid bilayer would result in a dysfunctional membrane protein

MEYER-OVERTON HYPOTHESIS

EXCEPTIONS

> HALOGENATED COMPOUNDS

> NONIMMOBILIZERS

> CUTOFF EFFECT

PRESSUREREVERSALTHEORYGA can be reversed by applying pressure on to the tissues. All these

theories conclude that there is no chemical reaction but physiological perturbation occurring during anaesthesia, like changes in phase separation, changes in bilayer thickness, changes in order parameters, changes in elasticity.

Modern lipid hypothesisModern version of hypothesis states that anesthetic effect

happens if solubulisation of GA in bilayer causes re distribution of lateral pressures. Each layer has distinct profile of distribution of lateral pressure in it. Ion channels are sensitive to changes in this lateral pressure distribution profile. Changes in membrane lateral pressure profile shifts the conformational equilibrium of certain membrane proteins known to be affected by GA drugs such as Ligand gated ion channels, this is also non specific. Experiments showed that GA drugs likely involve inhibition of opening of the ion channel in post synoptic ligand gated membrane protein.

According to modern lipid hypothesis GA drugs do not act directly on their membrane protein targets, but rather perturbs specialized lipid matrices at the protein lipid interface which acts as mediators.

Oleamied is an endogenous anaesthetic found in vivo and it is known to potentiate sleep and lower the temperature of the body through the closing gap junction channel connection.

PROTEIN THEORIES OF ANAESTHESIA

HYDROPHOBIC SITES OF PROTEINS

> CORE OF WATER SOLUBLE

PROTEINS

> LINING OF BINDING SITES FOR

HYDROPHOBIC LIGANDS

FIREFLY LUCIFERASE

ACTION ON ION CHANNELS

LIKELY TARGET FOR ANAESTHETICS

PATCH CLAMP TECHNIQUE

VOLTAGE GATED ION CHANNELS

INCLUDES

> VOLTAGE DEPENDENT SODIUM

CHANNELS

> VOLTAGE DEPENDENT POTASSIUM

CHANNELS

> VOLTAGE DEPENDENT CALCIUM

CHANNELS

SODIUM CHANNELS

INSENSITIVE TO VOLATILE ANAESTHETICS

50% INHIBITION OF SODIUM CHANNEL CURRENT REQUIRED HALOTHANE CONCENTRATION 8 TIMES THOSE REQUIRED TO PRODUCE ANAESTHESIA

FEW SUBTYPES ARE SENSITIVE

RESULTS IN SIGNIFICANT REDUCTION IN SYNAPTIC FUNCTION

CALCIUM CHANNELS

LOCATED AT PRESYNAPTIC TERMINAL

RESPOND TO ACTION POTENTIAL BY OPENING

RELEASE OF CALCIUM INTO THE CELL

RELEASE OF NEUROTRANSMITTER IN THE SYNAPTIC CLEFT

CONT….

CALCIUM CHANNELS

SIX TYPES

L, N, P, Q, R, T

N, P, Q, R- MAJOR ROLE IN SYNAPTIC TRANSMISSION

L, T- MAJOR ROLE IN CARDIAC, SKELETAL AND SMOOTH MUSCLE

CONT….

CALCIUM CHANNELS

VOLATILE SUBSTANCE INHIBIT VOLTAGE DEPENDENT CALCIUM CHANNELS (50% REDUCTION OF CURRENT) AT CONCENTRATION 2 TIMES THOSE REQUIRED TO PRODUCE ANAESTHESIA IN HUMANS

BACKGROUND(LEAK) POTASSIUM

ION CHANNELS

ACTIVATED BY BOTH VOLATILE AND GASEOUS ANAESTHETICS

TEND TO BE OPEN AT ALL VOLTAGES PRODUCE A LEAK CURRENT

LIGAND GATED ION CHANNELS

WHICH OPENS WHEN THERE IS ALTERATION IN THE MEMBRANE POTENTIAL

MEDIATES EXCITATORY AND INHIBITORY NEUROTRANSMISSION

LOGICAL TARGET FOR ANAESTHETIC ACTION

GABA-ACTIVATED ION CHANNELS

MOST IMPORTANT INHIBITORY NEUROTRANSMITTER

RESPONSE TO MANY DRUGS

BARBITURATES, BENZODIAZEPINES, PROPOFOL & VOLATILE ANAESTHETICS MODULATE GABAA RECEPTOR FUNCTION

CONT….

GABA-ACTIVATED ION CHANNELS

ANAESTHETICS BIND TO THIS RECEPTOR

CLORIDE ION CONDUCTANCE INCRAESES

HYPERPOLARIZATION OF THE POSTSYNAPTIC CELL MEMBRANE

INHIBITION OF POSTSYNAPTIC NEURON

GABA-ACTIVATED ION CHANNELS

3 TYPES OF EFFET ON THE RECEPTOR

1. POTENTIATION

2. DIRECT GATING

3. INHIBITION

GABA-ACTIVATED ION CHANNELS

POTENTIATION ABILITY OF THE ANAESTHETICS TO INCREASE THE CURRENT ELICITED BY LOW CONCENTRATION OF GABA

DIRECT GATING ABILITY OF THE ANAESTHETICS TO ACTIVATE GABAA CHANNELS IN THE ABSENCE OF GABA

GABA-ACTIVATED ION CHANNELS

INHIBITION

ABILITY OF THE ANAESTHETICS TO

PREVENT GABA FROM INITIATING CURRENT FLOW THROUGH GABAA

CHANNELS

GLUTAMATE ION CHANNELS

MAIN EXCITATORY NEOROTRANSMITTER IN THE CNS

3 CATEGORIES

> AMPA RECEPTORS

> KAINATE RECEPTORS

> NMDA RECEPTORS

GLUTAMATE ION CHANNELS

AMPA & KAINATE RECEPTORS INVOLVED IN FAST EXCITATORY SYNAPTIC TRANSMISSION(Na & K)

NMDA RECEPTORS LONG TERM MODULATION KETAMINE & NITROUS OXIDE

GLYCINE-ACTIVATED ION CHANNELS

INHIBITORY NEUROTRANSMITTER IN SPINAL CORD & BRAINSTEM

VOLATILE ANAESTHETICS PRODUCE EFFECT BY INCREASING AFFINITY OF THE RECEPTOR

PROPOFOL, PHENTOBARBITAL ALSO POTENTIATE GLYCINE-ACTIVATED CURRENT

O2PATHWAY PERTURBATION THEORYThis theory tries to explain how membranes are not

simply permeable to O2 but actually a complex O2 transportation structure.it is postulated that all the receptors interactions seen under the effect of anaesthetics are as a result of a kind of cellular cascade initiated by O2.O2 is needed in neurons,anaesthetics may disrupt O2 pathway causing less energy production. Anaesthetics acting on membrane lipid-protein disrupt O2 transport initiating a cellular cascade via triggered o2 sensing mechanisms.mitochondria have been implicated in this proposed mode of action.

CONCLUSIONS.Observations favor the membrane has the site of action & anaesthetic

receptors seems to be of finite size & anaesthetic molecules of limited size.

It is suggested that narcosis occurs when a critical fraction of membrane is occupied by anaesthetic agent.

It is clear that all anaesthetic action cannot be localized to a specific site in the CNS.

Some Evidence Suggest That different components of anaesthetic state may be mediated by actions disparate anaesthetic site

Actions of anaesthetics cannot be localized by specific physiological process.

Some assume that anaesthetic ultimately effects synaptic function as opposed to intrinsic neuronal excitability

Some Evidence Suggest That different components of anaesthetic state may be mediated by actions disparate anaesthetic site

Actions of anaesthetics cannot be localized by specific physiological process.

Some assume that anaesthetic ultimately effects synaptic function as opposed to intrinsic neuronal excitability

The effects of anaesthetics depend on the agent and pre and post synaptic receptors.

At molecular level volatile anaesthetic agents show some selectivity but still effect multiple ion channels and synaptic proteins.

IV anaesthetics are more specific with GABA a receptors, but there are more proteins directly interact with anaesthetics.

It is agreed that narcosis is due to physical rather than chemical action of the molecule & relatively weak forces are involved.

There is a good correlation between anaesthetic potency & molar refraction polarisability of the anaesthetic agent, molal volume & solubility in olive oil.

Developments in molecular biology, genetics, and cell physiology make it possible that next generation can find some answers to the 160 yrs old pharmacological puzzle of mechanism of anaesthesia.

CURRENT NEWS

March 30, 2007

The Wall Street Journal: “FDA Wants More Research on Anesthesia Risk to Kids”

Anesthesia can be harmful to the developing brain, studies on animals suggest, raising concerns about potential risks in putting young children under for surgery

Prolonged changes in behavior; memory and learning impairments

Relevance of the animal findings to pediatric patients is unknown

REFERENCES

CLINICAL ANAESTHESIA BY PAUL G. BARASH SIXTH EDITION

A PRACTICE OF ANAESTHESIA BY

WYLIE & CHURCHILL-DAVIDSON’S SEVENTH EDITION

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