Protein synthesis

Figure 16.1

Schematic representation of the N-methyl-D-aspartate (NMDA) synapse. The receptor contains binding sites for the endogenous agonist glutamate (Glu), for glycine (Gly) and for polyamines (Pol). The receptor-associated channel is blocked by Mg2+ ions in a voltage-dependent manner. Noncompetitive antagonists bind to the PCP recognition site within the channel. Ca2+ activates the enzyme nitric oxide synthase (NOS), which synthesizes NO from L-arginine. NO acts as a retrograde messenger on the transmitter release of the presynaptic terminal.

seems safe to assume that both anesthetic and psychoactive effects are caused by their NMDA antagonism. First, all substances known so far to bind to the PCP receptor produce a similar anesthetic state. Among them is the selective high-affinity antagonist MK-801 (Koek et al. 1989; Scheller et al. 1989; Daniell 1990; Loscher et al. 1991; Irifune et al. 1992; Perkins and Morrow 1992a). Second, the anesthetic potency of these compounds is highly correlated with their relative affinity to the PCP receptor (Koek et al. 1989; Perkins and Morrow 1992b). Ketamine exists in two enantiomers, R(—) and S(+) ket amine, for which the PCP receptor displays stereoselectivity. S(+) ketamine has a three times higher affinity than R(—) ketamine. This corresponds to the anesthetic potency, S(+) ketamine being three times more potent (Marietta et al. 1977; Ryder et al. 1978; White et al. 1980; Lodge et al. 1982; Schüttler et al. 1987; Zeilhofer et al. 1992). The same is trüe for the sübjective psychoactive effects of sübanesthetic doses, as indicated by animal discrimination stüdies (Martin and Lodge 1988), and for the psychotomimetic effects in hümans, inclüding the characteristic ego disorders (Vollenweider 1998). Third, the anesthetic effects of the noncompetitive NMDA antagonists can be modified by changing their binding kinetics. Agonists of the glütamate and glycine receptors increase the open time of the NMDA receptor channel and thereby accelerate the dissociation rate of ligands boünd within it. Both agonists antagonize the anesthetic effects of the noncompetitive antagonists (Irifune et al. 1992). This antianesthetic effect of glütamate and glycine can be reversed by selective glütamate and glycine antagonists.

From these observations one can draw the following conclüsions:

1. The normal fünctioning of the NMDA synapse is a necessary condition for conscioüsness.

2. All other physiological or compütational brain processes that remain intact after a selective blockade of the NMDA receptor are, taken together, not süfficient for the occürrence of conscioüs states.

3. If, however, the normal fünctions of the NMDA receptor are restored by removing the channel blockade, a physiological sitüa-tion will arise that is a süfficient condition for conscioüsness.

A loss of conscioüsness, it appears, is not necessarily düe to an ünspecific, global depression of all neüral activity, büt to the disrüption of a specific sübset of processes that depend on the normal fünctioning of the NMDA receptor.

If these assumptions are correct, it will follow that not only noncompetitive antagonists, like ketamine, but all other drugs that interfere with this complex molecular entity, should have similar anesthetic and psychotomimetic effects. Figure 16.2 summarizes what is currently known in this respect. In fact, it appears that any of the possible direct interventions which inhibit the activation of the receptor or the subsequent processes triggered by Ca2+ inevitably produce anesthetic effects. This is true not only for a large number of noncompetitive antagonists that bind to the PCP receptor, but also for all competitive NMDA antagonists that block the glutamate receptor, such as AP5, CPP, CGS 19755 and D-CPP-ene (Koek et al. 1986; Boast and Pastor 1988; Woods 1989; France et al. 1990; Daniell 1991; Irifune et al. 1992; Perkins and Morrow 1992a; Kuroda et al. 1993); for glycine antagonists such as ACEA (McFarlane et al. 1995); and for polyamine antagonists such as spermidine and spermine (Daniell 1992). All these drugs have been shown to decrease the minimum alveolar concentration (MAC) for halothane and/or to increase the sleeping time in a standard barbiturate narcosis. Recently, it was found that nitrous oxide (laughing gas) inhibits the ionic currents mediated through the NMDA receptor, possibly by a mixed competitive/noncompetitive mechanism (Jevtovic-Todorovic et al. 1998). This interesting finding would explain the long-known specific psychopharmacological profile of laughing gas, which is very similar to that of ketamine.

A role for the NO-signaling pathway in general anesthesia is suggested by an increasing number of studies. The inhibition of NO synthesis with the unspecific NOS inhibitor nitroG-L-arginine methyl ester (L-NAME) potentiates the effects of halothane, isoflurane, and alcohol (Johns et al. 1992; Adams et al. 1994; Ichinose et al. 1995). The brain selective NOS inhibitor 7-nitro-indazole (7-Ni) dose-dependently prolongs the duration of a barbiturate narcosis (Motzko et al. 1998) and also reduces the isoflurane MAC

(Pajewski et al. 1996). Moreover, the involvement of this pathway has been shown for a number of anesthetics, such as halothane, enflurane, and isoflurane (Zuo and Johns 1995; Zuo et al. 1996; Tonner et al. 1997), barbiturates (Morgan et al. 1991; Terasako et al. 1994) and alpha 2-adrenergic agonists (Vuilliemoz et al. 1996).

For a large number of anesthetics, however, it is known that they do not act directly upon the NMDA synapse. For example, it is assumed that the GABAa receptor is the primary site of action for intravenous anesthetics like barbiturates, benzodiazepines, steroids, etomidate, and pro-pofol. GABAA receptors are typically located in the immediate vicinity of the NMDA receptor. Their activation causes hyperpolarization of the postsynaptic membrane. According to what has been said before, this will alter the working conditions of the NMDA receptor complex. It is therefore possible that the anesthetic action of GABAA agonists is due to an indirect inhibition of NMDA receptor activity. In fact, there is some experimental evidence that supports this assumption. Figures 16.3 and 16.4 show an attempt to visualize the activation state of the cortical NMDA synapses directly, under in vivo conditions, by means of an autoradiographic technique (Flohr et al. 1998). A radioactively labeled noncompetitive NMDA antagonist ([3H]-MK-801) that is able to associate with the open, but not with the closed, NMDA channel was used to label activated channels. The rate at which the indicator is bound to its recognition site under nonequilibrium conditions depends on the number of activated channels and on the mean opening times of the individual channels.

Figure 16.3 shows the in vivo uptake in the brain of the awake rat. The binding sites are distributed unevenly, with highest densities occurring in the cortex and the hippocampal formation. The picture obtained under awake conditions resembles conventional in vitro receptor autoradiographs that depict the regional concentration of NMDA receptors. This means

Polyamine antagonists

Spermidine Spermine

Glycine

Competitive

antagonists

NMDA

ACEA

antagonists

(+)-HA-966

AP5

CGS 19755

CPP

CPP-ene

Gly Glu

Guanyl-cyclase ^ cGMP Glutamate

Gly Glu

GABAa

iS

NMDA

Non

NOS

competitive

inhibitors

NMDA

7-Nitro-indazole

antagonists

Ketamine

L-NAME

Phencydidlne

L-NMMA

MK-801

N,0

Conantokine-G

phosphorylation Ca:t NO

Guanyl-cyclase ^ cGMP Glutamate

AMPA antagonists

NBQX GYK! 52466

protein synthesis

Figure 16.2

The NMD A synapse as a target for anesthetics. Schematic representation of the NMD A receptor channel complex with its regulatory sites and of neighboring AMPA and GABA receptors by which the working conditions of the NMDA receptor can be influenced. All agents mentioned possess anesthetic properties; arrows indicate the interaction sites.

Figure 16.3

In vivo uptake of [3H]-MK-801 in the rat brain under awake conditions. [3H]-MK-801 was dissolved in saline and injected through a vene catheter at a dose of 600 ^Ci/kg. The animals were sacrificed by swift decapitation 1 min after the administration of the tracer. The brain was rapidly removed and frozen. Frozen sections were washed with TRIS maleate buffer to minimize nonreceptor-associated bindings, and subsequently air-dried. The sections were juxtaposed to [3H] Hyperfilm (Amersham, Buchler) for 60 days. See color insert.

Figure 16.3

In vivo uptake of [3H]-MK-801 in the rat brain under awake conditions. [3H]-MK-801 was dissolved in saline and injected through a vene catheter at a dose of 600 ^Ci/kg. The animals were sacrificed by swift decapitation 1 min after the administration of the tracer. The brain was rapidly removed and frozen. Frozen sections were washed with TRIS maleate buffer to minimize nonreceptor-associated bindings, and subsequently air-dried. The sections were juxtaposed to [3H] Hyperfilm (Amersham, Buchler) for 60 days. See color insert.

that under awake conditions, most of the NMDA synapses present have been activated during the time period when the indicator was present in the extracelluar space. In ketamine narcosis (figure 16.4b), the indicator uptake is considerably reduced, which is not surprising because ketamine acts as a channel blocker. A very similar uptake pattern, however, is obtained in barbiturate (figure 16.4c) and propofol (figure 16.4f) narcosis, indicating that the cortical NMDA synapse is suppressed by GABAergic inhibition. A similar inhibitory effect can be observed with other anesthetics, such as hal-othane (figure 16.4d) and alcohol (figure 16.4e).

Antagonists of excitatory synapses in the vicinity of the NMDA receptor, such as the glutamatergic (AMPA) receptor, in theory should have similar indirect effects. In fact, it has been shown that 2,3 dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline (NBQX), a selective AMPA antagonist, possesses anesthetic properties (McFarlane et al. 1992).

Taken together, one can conclude (1) that an anesthetic potency has been shown for all kinds of agents that directly interfere with the function of the NMDA receptor channel complex or the subsequent plastic processes; and (2) that the action of anesthetics which primarily interact with other targets, such as the GABA synapse, can eventually be explained as an indirect effect on the NMDA synapse. These data support the hypothesis that NMDA receptor-dependent processes are the ultimate target of anesthetic action (Flohr 1995b).

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