Photoaffinity labelling of the benzodiazepine receptor compromises the recognition site but not its effector mechanism

When subjected to ultraviolet light flunitrazepam could be irreversibly attached to brain membrane preparations such that the affinity of the benzodiazepine receptor for certain ligands, such as diazepam, becomes markedly reduced. However, the apparent affinity for diazepam is increased by the addition of gamma-aminobutyric acid and sodium chloride, to the same extent in this membrane preparation as in membranes not so pretreated. This observation suggests that photoaffinity labelling of the benzodiazepine receptor with flunitrazepam modifies the recognition characteristics of the receptor but not its effector mechanism.

Kinetics of [3H]Ro 15-1788 binding to membrane-bound rat brain benzodiazepine receptors

Ro 15-1788 is thought to interact specifically with the benzodiazepine receptor population in the mammalian CNS as an antagonist. We have compared the kinetics of interaction of this ligand with the benzodiazepine agonist flunitrazepam in the rat cerebellum. The association of [3H]Ro 15-1788 with the benzodiazepine receptor in this brain region is monoexponential, and the dissociation, initiated either by dilution or by displacement with a number of different ligands, also obeys simple monoexponential kinetics. There is no evidence of cooperative interactions with this ligand, and its dissociation is unaffected by the presence of 100 microM gamma-aminobutyric acid and/or 150 mM sodium chloride. In contrast, the dissociation of the agonist [3H]flunitrazepam is biphasic, and the possible interpretation of this data in terms of agonist-induced conformational change is discussed. Evidence is also presented that the mechanism of interaction of Ro 15-1788 with the benzodiazepine receptor population in hippocampal membranes is distinct from that found in the cerebellum.

The benzodiazepine receptor in rat brain and its interaction with ethyl beta-carboline-3-carboxylate

[3H]Ethyl beta-carboline-3-carboxylate ( [3H] beta-CCE) binds to a homogeneous population of recognition sites in rat whole brain membranes with high affinity. The [3H]beta-CCE binding is completely displaceable by low concentrations of a number of benzodiazepines with similar potencies found when using a 3H-benzodiazepine as the ligand. This suggests that the recognition sites for beta-CCE and the benzodiazepines are identical or that they are involved in a close interaction. The binding of [3H]beta-CCE does not obey simple mass-action kinetics. [3H]Flunitrazepam dissociation from its receptor population is biphasic, and different methods of initiation of this dissociation indicate that cooperative interactions take place within the receptor population. We conclude that the benzodiazepine receptor is a single entity that can exist in two conformations, the equilibrium between which may be controlled by some as yet unidentified factor.

Photoaffinity labelling of the benzodiazepine receptor cannot be used to predict ligand efficacy

Irreversible attachment of flunitrazepam to brain membranes occurs on exposure to UV light, resulting in a loss of the number of binding sites detected by saturation binding of [3H]flunitrazepam, but not of [3H]ethyl-beta-carboline-3-carboxylate (beta CCE). Changes in the affinity of ligands for the benzodiazepine receptor produced by photoaffinity labelling, as indicated by the relative IC50 values for the displacement of [3H]beta CCE in photoaffinity labelled versus control membranes, appear to be related to their chemical structure rather than to their pharmacological profile.

Para-chlorophenylethylamine in brains of rats treated with a monoamine oxidase inhibitor and p-chlorophenylalanine

1. The accumulation of p-chlorophenylethylamine (pCPE) in rat brain after administration of pargyline plus p-chlorophenylalanine (pCPA) is demonstrated. 2. Measurements of pCPE in brain were performed at 0.25 h, 1 h and 4 h after administration of pCPA to pargyline-pretreated rats, and amounts of pCPE were shown to increase with time.

Interactions of beta-carbolines with the benzodiazepine receptor: structure-activity relationships

The effects of a number of beta-carboline derivatives on [3H]flunitrazepam binding to the benzodiazepine binding site were investigated. The potency of beta-carbolines at the benzodiazepine binding site appeared to be determined largely by the aromaticity of the molecule. Norharmane-3-carboxylic acid ethyl ester was considerably more potent (IC50 10 nM) than its tetrahydro-beta-carboline analogue (IC50 6 microM). There is essentially no difference in potency between the (+)- and (--)-forms of the tetrahydro-beta-carboline-3-carboxylate analogues.

Determination of 5-hydroxytryptamine in rat brain regions by gas-liquid chromatography with electron-capture detection

An assay method for measurement of 5-hydroxytryptamine in rat brain areas is described. The procedure involves extraction with a liquid ion exchanger followed by acetylation and reaction with pentafluoropropionic anhydride. The resultant beta-carboline is separated and quantitated using gas-liquid chromatography with electron-capture detection. The procedure also works well with 5-methoxytryptamine standards, and the tissue samples were checked simultaneously for the presence of this amine. The results indicate that concentrations of 5-methoxytryptamine in all areas studied (brain stem, hypothalamus, frontal cortex, and striatum) are less than 10 ng/g.

Bulbospinal tryptaminergic neurones. A search for the role of bulbospinal tryptaminergic neurones in the control of sympathetic activity

1. The possible role of bulbospinal tryptaminergic neurones in the control of sympathetic activity has been investigated in anaesthetised cats. 2. Depletion of spinal cord stores of 5-hydroxytryptamine was achieved by systemic administration of p-chlorophenylalanine or by intraspinal microinjections of 5,6-dihydroxytryptamine. 3. Blood pressure was little changed by these treatments, neither was the pattern of ongoing activity in sympathetic nerves (arterial pulse rhythmicity and respiratory modulation), the influence of pulmonary afferent nerves on this activity (determined by an airway occlusion technique), the sympatho-inhibitory influence of the carotid sinus baroreceptors, nor the sympatho-inhibitory or -excitatory influences of somatic afferent nerves. 4. Since both p-chlorophenylalanine and 5,6-dihydroxytryptamine treatment caused extensive disruption of the bulbospinal tryptaminergic neurones, it was concluded that these play no significant role in the mediation of the responses tested in anaesthetised cats in the present study.

Procedural difficulties in the gas-liquid chromatographic assay of the arylalkylamines

A gas-liquid chromatographic method for the estimation of 2-phenylethylamine in rat brain tissue is described. The procedure involves initial separation of the amine from homogenates with ion-exchange chromatography, subsequent acetylation of the eluate and extraction of the N-acetylated amine with ethyl acetate. The N-acetyl-2-phenylethylamine derivative is then converted to the pentafluoropropionyl derivative and quantitated with electron capture detection. Difficulties involved with alternative separative procedures and derivative formation are discussed.

Reversal learning enhanced by lysergic acid diethylamide (LSD): concomitant rise in brain 5-hydroxytryptamine levels

1 Small doses of lysergic acid diethylamide (LSD) (12.5-50 mug/kg) consistently facilitated learning of a brightness discrimination reversal.2 2-Bromo-lysergic acid diethylamide (BOL-148), a structural analogue of LSD, with similar peripheral anti-5-hydroxytrypamine activity but no psychotomimetic properties, had no effect in this learning situation at a similar dose (25 mug/kg).3 LSD, but not BOL-148, caused a small but significant increase in brain 5-hydroxytryptamine levels, but had no effect on the levels of catecholamines in the brain at 25 mug/kg.