Bicuculline and gabazine are allosteric inhibitors of channel opening of the GABAA receptor.

Ueno S, Bracamontes J, Zorumski C, Weiss DS, Steinbach JH.

Department of Anesthesiology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.

Anesthetic drugs are known to interact with GABAA receptors, both to potentiate the effects of low concentrations of GABA and to directly gate open the ion channel in the absence of GABA; however, the site(s) involved in direct gating by these drugs is not known. We have studied the ability of alphaxalone (an anesthetic steroid) and pentobarbital (an anesthetic barbiturate) to directly activate recombinant GABAA receptors containing the alpha 1, beta 2, and gamma 2L subunits. Steroid gating was not affected when either of two mutated beta 2 subunits [beta 2 (Y157S) and beta 2 (Y205S)] are incorporated into the receptors, although these subunits greatly reduce the affinity of GABA binding. These observations indicate that steroid binding and subsequent channel gating do not require these particular residues, as already shown for barbiturates. Bicuculline or gabazine (two competitive antagonists of GABA binding) reduced the currents elicited by alphaxalone and pentobarbital from wild-type GABAA receptors; however, gabazine produced only a partial block of response pentobarbital or alphaxalone, and bicuculline only partially blocked responses to pentobarbital. These observations indicate that the blockers do not compete with alphaxalone or pentobarbital for a single class of sites on the GABAA receptor. Finally, at receptors containing alpha 1 beta 2 (Y157S) gamma 2L subunits, both bicuculline and gabazine showed weak agonist activity and actually potentiated responses to alphaxalone. These observations indicate that the blocking drugs can produce allosteric changes in GABAA receptors, at least those containing this mutated beta 2 subunit. We conclude that the sites for binding steroids and barbiturates do not overlap with the GABA-binding site. Furthermore, neither gabazine nor bicuculline competes for binding at the steroid or barbiturate sites. The data are consistent with a model in which both gabazine and bicuculline act as allosteric inhibitors of channel opening for the GABAA receptor after binding to the GABA-binding site.

Source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=8987785&dopt=Abstract barbiturate Butalbital Fioricet





Spatiotemporal patterns of spindle oscillations in cortex and thalamus.

Contreras D, Destexhe A, Sejnowski TJ, Steriade M.

Laboratoire de Neurophysiologie, Faculte de Medecine, Universite Laval, Quebec, Canada.

Spindle oscillations (7-14 Hz) appear in the thalamus and cortex during early stages of sleep. They are generated by the combination of intrinsic properties and connectivity patterns of thalamic neurons and distributed to cortical territories by thalamocortical axons. The corticothalamic feedback is a major factor in producing coherent spatiotemporal maps of spindle oscillations in widespread thalamic territories. Here we have investigated the spatiotemporal patterns of spontaneously occurring and evoked spindles by means of multisite field potential and unit recordings in intact cortex and decorticated animals. We show that (1) spontaneous spindle oscillations are synchronized over large cortical areas during natural sleep and barbiturate anesthesia; (2) under barbiturate anesthesia, the cortical coherence is not disrupted by transection of intracortical synaptic linkages; (3) in intact cortex animals, spontaneously occurring barbiturate spindle sequences occur nearly simultaneously over widespread thalamic territories; (4) in the absence of cortex, the spontaneous spindle oscillations throughout the thalamus are less organized, but the local coherence (within 2-4 mm) is still maintained; and (5) spindling propagation is observed in intact cortex animals only when elicited by low intensity cortical stimulation, applied shortly before the initiation of a spontaneous spindle sequence; propagation velocities are between 1 and 3 mm/sec, measured in the anteroposterior axis of the thalamus; increasing the intensity of cortical stimulation triggers spindle oscillations, which start simultaneously in all leads. We propose that, in vivo, the coherence of spontaneous spindle oscillations in corticothalamic networks is attributable to the combined action of continuous background corticothalamic input initiating spindle sequences in several thalamic sites at the same time and divergent corticothalamic and intrathalamic connectivity.

Source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=8994070&dopt=Abstract barbiturate Butalbital Fioricet





Coadministered pentobarbital anesthesia postpones but does not block the motor and sleep EEG responses to MK-801.

Feinberg I, Campbell IG.

VA/UCD Sleep Lab, University of California, Davis 95616, USA.

In previous studies with Sprague-Dawley rats, we demonstrated that NMDA channel blockade during waking massively stimulates the delta (1-4 Hz) EEG of non-rapid eye movement (NREM) sleep. However, non-competitive channel blockers also produce neurotoxicity that is manifested by posterior cingulate vacuolization and heat shock protein production. These neurotoxic effects can be blocked by coadministering gabaergic drugs, including barbiturates and benzodiazepines, with the MK-801. To determine whether delta stimulation by MK-801 would be similarly blocked, we administered an anesthetic dose (40 mg/kg) of pentobarbital followed immediately by 0.3 mg/kg of MK-801. Neither the MK-801 motor syndrome nor the NREM delta stimulation was blocked. When the rats recovered from nearly two hours of barbiturate anesthesia, they behaved as though they had just received the MK-801 injection, exhibiting the typical motor syndrome, spikes in the waking EEG and strong stimulation of NREM delta EEG. These findings support our previous evidence that NREM delta stimulation by NMDA channel blockade does not depend on toxic brain changes. They also raise interesting questions regarding the fate of MK-801 during pentobarbital anesthesia. We propose that the drug is not metabolized during the period of anesthesia because it is sequestered within the NMDA cation channel. However, neurons do not respond to the channel block because they have been rendered inert by the anesthesia. When the neurons emerge from anesthesia, the cascade of MK-801 events unfolds. This and other possible explanations can be tested experimentally. Establishing the fate of MK-801 during barbiturate anesthesia could shed new light on the cellular processing of non-competitive NMDA channel blockers.

Source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9061055&dopt=Abstract barbiturate Butalbital Fioricet





Analysis of toxic drugs by capillary electrophoresis using polyacrylamide-coated columns.

Jinno K, Han Y, Sawada H.

School of Materials Science, University of Technology, Japan. Jinno chrom.-tut.ac.jp

Toxic drugs, including barbiturates and benzodiazepines, were analyzed using polyacrylamide-coated columns in capillary electrophoresis (CE). The separations were carried out in absence of electroosmotic flow. Seven kinds of barbiturates were successfully separated with the coated columns in free solution without further additives. Benzodiazepines, the electrically neutral solutes were introduced onto the coated column, and separated in presence of sodium dodecyl sulfate above its critical micelle concentration in the running buffer. This CE method offered fast and efficient separation of more hydrophobic solutes, such as benzodiazepines. The separation of seven barbiturates was studied in linear (noncross-linked) polyacrylamide solutions and in acrylamide/N-isopropylacrylamide (AA/IPAA) copolymer solutions to explore the effect of isopropyl groups in the AA/IPAA copolymer chain.

Source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9080138&dopt=Abstract barbiturate Butalbital Fioricet