The Molecular Biology of General Anesthesia
Reviews: What is consciousness? Is it something other
than biological or physical? Is it purely physiological? Regardless
of ones philosophical disposition on the “mind/body” issue,
the short answer is that it must be in part physiological considering
that it can be manipulated by purely physical means. Specifically,
consciousness is manipulated thousands of times daily and nightly
around the world in the use of general anesthesia. Interestingly,
however, the molecular basis for these effects are not as well known
as one might suspect considering the widespread use and relative safety
of anesthetics.
In the early days of anesthetics, for a period of about 10 years from
1846 to 1946 three structurally diverse inhalation anesthetics were
discovered and put to widespread use. These were chloroform, ether
and nitrous oxide. The fascinating thing about these drugs is that
they are structurally different from each other. Thus, the fundamental
question in anesthesiology became, “how do volatile gases with
divers structures give rise to the state of anesthesia?” (Mashour,
350)
To answer this fundamental question several hypothesis have been formulated.
The first was the “unitary hypothesis” posited by Claude
Bernard. He based this hypothesis on the idea that the anesthetics
must have some common final mechanism since they all produced the
same final effect: the state of anesthesia. Next came the “lipid
hypothesis” with the independent discovery by Meyer and Overton
that anesthetic potency was correlated with its solubility in oil.
Thus, the lipid hypothesis proposed that anesthetics likely acted
by perturbing lipid cell membranes of neural tissue. Then Franks and
Lieb showed that a wide variety of anesthetics could inhibit the activity
of a prepared lipid-free enzyme. This gave way to the “protein
hypothesis” which posits that proteins could be the specific
and direct targets of inhaled anesthetic gases. It has been shown
since that volatile anesthetic gases act on many proteins such as
serum proteins, ion channels, and even intracellular signaling molecules
like protein kinase (Mashour, 350).
The general consensus is that ion channels provide the most likely
target of anesthetic action. This is because ion channels have diverse
roles in neural activity. They establish neuronal membrane potentials,
propagate action potentials, moderate pre-synaptic release, and activate
in response to post-synaptic binding. A few general types of ion channels
are affected; ligand-gated ion channels, calcium channels, and potassium
channels, both voltage gated and non-voltage gated (Mashour, 351).
Research is currently heavily directed towards gamma-aminobutyric
acid type A receptors (GABAa), neuronal nicotinic acetylcholine receptors
(nnAch), and Glutamate N-methyl-D-aspartate receptors, and glycine
receptors in ligand-gated ion channels. One study examined chimeric
glycine/p subunits and identified regions of the protein that contributed
to the potentiating effects of ethanol and enflurane. Particularly
Ser267 and Ala288. Other research has concluded that hydrophobic cavities
with the presence of methionine and aromatic residues are in principal
the ideal binding sites for anesthetics (Thompson, 79,80).
In addition, advances in neuroscience and technology give us greater
detail and vision into the workings of active conscious and unconscious
brains. Electro-Encephalograms and functional magnetic resonance imaging
may lead to breakthroughs in the physiological and molecular foundations
of consciousness, as well as genetic manipulation such as knock-out
and “knock-in” mice that enable assessment of the roles
of small regions within proteins (Mashour, 355).
Research: Inhalation general anesthetics have recently
been shown to inhibit neuronal nicotinic acetylcholine (ACh) receptors
(nnAChRs) expressed in Xenopus laevis oocytes and in molluscan neurons.
(Mori, 732) However, these systems are not model systems for mammalian
neurons. Thus the purpose of the experiment in question was to understand
how volatile inhalation gaseous anesthetics might modulate neuronal
nicotinic acetylcholine receptors in rat cortical neurons. Halothane
was used as the anesthetic and its action on the nnAChRs in rat neurons
in primary long-term culture was examined in detail. (Mori, 732)
The cells were prepared from fetuses removed from a 17-day pregnant
Sprague-Dawley rat under methoxyflurane anesthesia. Small wedges of
frontal cortex were excised and subsequently incubated in phosphate-buffered
saline solution. The purpose was to form a cortical neuron/glia coculture
of which cells that had cultured for 2 to 9 weeks were then used in
the electrophysiological experiments. The concentrations of halothane
used in the experiments were 7.5 to 2500 mM. The whole-cell, patch-clamp
technique was used to record ionic currents induced by ACh application
through a U-tube system. ACh and halothane were coapplied through
a U-tube, and halothane was perfused through the bath starting 2 min
before the coapplication. Two-minute preperfusion was long enough
to exchange the whole bath solution and to allow halothane to exhibit
its effect (Mori, 733). The results showed that halothane inhibited
the currents induced by 300 mM ACh in a concentration-dependent manner.
The inhibition was reversible after washout of halothane. (Mori, 735).
The study showed that halothane could reversibly inhibit a7-type and
a4b2-type currents of nnAChRs in a concentration-dependent manner
at clinically relevant concentrations. Halothane block of a7-type
currents was independent of ACh concentration, whereas the block of
a4b2-type currents became less with decreasing ACh
concentration. (Mori, 735)
Ultimately, the contribution this research makes is important since
it establishes that nnAChRs can be inhibited in mammalian neuronal
models with volatile gaseous anesthetics in a manner consisted with
clinical usage. This is relevant and important since the primary issue
in finding molecular mechanisms in the state of anesthesia may ultimately
rest on protein interactions of this type with ligand specific inhibition
and washout controlling the functional behaviors of consciousness
and amnesia in humans.
Many things are still unknown about this process. For example, it
is not known how consciousness is propagated within the central nervous
system. This type of research, however suggests that physiological
neural networks and their detailed molecular biology may lead to the
elucidation of a mechanism, which is verifiable and testable. This
is of primary importance in establishing that molecular biology and
its physical entailments are ultimately the “stuff” of
consciousness and rationality. The limitations however of this study
is that it could only involve in vitro experimentation on the neural/glial
cells and not in vivo.
Reference:
1. G. Mashour, S. Forman, J. Campagna, Mechanisms of general anesthesia:
from molecules to mind, 2005, Best Practice & Research Clinical
Anaesthesiology, Vol. 19. No. 3, pp. 349-364
2. S. Thompson, K. Wafford, Mechanism of action of general anaesthetics-
new information from molecular pharmacology, 2001, Current Opinion
in Pharmacology, 1:78-83
3. Mori, Zhao, Zuo, Aistrup, Nishikawa, Marszalec,
Yeh, Narahashi, Modulation of Neuronal Nicotinic Acetylcholine Receptors
by
Halothane in Rat Cortical Neurons, 2001, Molecular Pharmacology 59:732-743
Ray S. Magill - 11/16/05
