Shared structural mechanisms of general anaesthetics and benzodiazepines.

Most general anaesthetics and classical benzodiazepine drugs act through positive modulation of γ-aminobutyric acid type A (GABAA) receptors to dampen neuronal activity in the brain1-5. However, direct structural information on the mechanisms of general anaesthetics at their physiological receptor sites is lacking. Here we present cryo-electron microscopy structures of GABAA receptors bound to intravenous anaesthetics, benzodiazepines and inhibitory modulators. These structures were solved in a lipidic environment and are complemented by electrophysiology and molecular dynamics simulations. Structures of GABAA receptors in complex with the anaesthetics phenobarbital, etomidate and propofol reveal both distinct and common transmembrane binding sites, which are shared in part by the benzodiazepine drug diazepam. Structures in which GABAA receptors are bound by benzodiazepine-site ligands identify an additional membrane binding site for diazepam and suggest an allosteric mechanism for anaesthetic reversal by flumazenil. This study provides a foundation for understanding how pharmacologically diverse and clinically essential drugs act through overlapping and distinct mechanisms to potentiate inhibitory signalling in the brain.

GABAA receptor signalling mechanisms revealed by structural pharmacology.

Type-A γ-aminobutyric (GABAA) receptors are ligand-gated chloride channels with a very rich pharmacology. Some of their modulators, including benzodiazepines and general anaesthetics, are among the most successful drugs in clinical use and are common substances of abuse. Without reliable structural data, the mechanistic basis for the pharmacological modulation of GABAA receptors remains largely unknown. Here we report several high-resolution cryo-electron microscopy structures in which the full-length human α1ß3γ2L GABAA receptor in lipid nanodiscs is bound to the channel-blocker picrotoxin, the competitive antagonist bicuculline, the agonist GABA (γ-aminobutyric acid), and the classical benzodiazepines alprazolam and diazepam. We describe the binding modes and mechanistic effects of these ligands, the closed and desensitized states of the GABAA receptor gating cycle, and the basis for allosteric coupling between the extracellular, agonist-binding region and the transmembrane, pore-forming region. This work provides a structural framework in which to integrate previous physiology and pharmacology research and a rational basis for the development of GABAA receptor modulators.

A Structural Rationale for N-Methylbicuculline Acting as a Promiscuous Competitive Antagonist of Inhibitory Pentameric Ligand-Gated Ion Channels.

Bicuculline, a valued chemical tool in neurosciences research, is a competitive antagonist of specific GABAA receptors and affects other pentameric ligand-gated ion channels including the glycine, nicotinic acetylcholine and 5-hydroxytryptamine type 3 receptors. We used a fluorescence-quenching assay and isothermal titration calorimetry to record low-micromolar dissociation constants for N-methylbicuculline interacting with acetylcholine-binding protein and an engineered version called glycine-binding protein (GBP), which provides a surrogate for the heteromeric interface of the extracellular domain of the glycine receptor (GlyR). The 2.4 Šresolution crystal structure of the GBP:N-methylbicuculline complex, sequence and structural alignments reveal similarities and differences between GlyR and the GABAA receptor-bicuculline interactions. N-methylbicuculline displays a similar conformation in different structures, but adopts distinct orientations enforced by interactions and steric blocks with key residues and plasticity in the binding sites. These features explain the promiscuous activity of bicuculline against the principal inhibitory pentameric ligand-gated ion channels in the CNS.

Identifying Drugs that Bind Selectively to Intersubunit General Anesthetic Sites in the α1ß3γ2 GABAAR Transmembrane Domain.

GABAA receptors (GABAARs) are targets for important classes of clinical agents (e.g., anxiolytics, anticonvulsants, and general anesthetics) that act as positive allosteric modulators (PAMs). Previously, using photoreactive analogs of etomidate ([3H]azietomidate) and mephobarbital [[3H]1-methyl-5-allyl-5-(m-trifluoromethyl-diazirynylphenyl)barbituric acid ([3H]R-mTFD-MPAB)], we identified two homologous but pharmacologically distinct classes of general anesthetic binding sites in the α1ß3γ2 GABAAR transmembrane domain at ß +-α - (ß + sites) and α +-ß -/γ +-ß - (ß - sites) subunit interfaces. We now use competition photolabeling with [3H]azietomidate and [3H]R-mTFD-MPAB to identify para-substituted propofol analogs and other drugs that bind selectively to intersubunit anesthetic sites. Propofol and 4-chloro-propofol bind with 5-fold selectivity to ß +, while derivatives with bulkier lipophilic substitutions [4-(tert-butyl)-propofol and 4-(hydroxyl(phenyl)methyl)-propofol] bind with ∼10-fold higher affinity to ß - sites. Similar to R-mTFD-MPAB and propofol, these drugs bind in the presence of GABA with similar affinity to the α +-ß - and γ +-ß - sites. However, we discovered four compounds that bind with different affinities to the two ß - interface sites. Two of these bind with higher affinity to one of the ß - sites than to the ß + sites. We deduce that 4-benzoyl-propofol binds with >100-fold higher affinity to the γ +-ß - site than to the α +-ß - or ß +-α - sites, whereas loreclezole, an anticonvulsant, binds with 5- and 100-fold higher affinity to the α +-ß - site than to the ß + and γ +-ß - sites. These studies provide a first identification of PAMs that bind selectively to a single intersubunit site in the GABAAR transmembrane domain, a property that may facilitate the development of subtype selective GABAAR PAMs.

GABAρ subunits confer a bicuculline-insensitive component to GFAP+ cells of cerebellum.

GABA-A receptors mediating synaptic or extrasynaptic transmission are molecularly and functionally distinct, and glial cells are known to express a plethora of GABA-A subunits. Here we demonstrate that GFAP(+) cells of the granular layer of cerebellum express GABAρ subunits during early postnatal development, thereby conferring peculiar pharmacologic characteristics to GABA responses. Electron microscopy revealed the presence of GABAρ in the plasma membrane of GFAP(+) cells. In contrast, expression in the adult was restricted to Purkinje neurons and a subset of ependymal cells. Electrophysiological studies in vitro revealed that astrocytes express functional receptors with an EC50 of 52.2 ± 11.8 µM for GABA. The evoked currents were inhibited by bicuculline (100 µM) and TPMPA (IC50, 5.9 ± 0.6 µM), indicating the presence of a GABAρ component. Coimmunoprecipitation demonstrated protein-protein interactions between GABAρ1 and GABAα1, and double immunofluorescence showed that these subunits colocalize in the plasma membrane. Three populations of GABA-A receptors in astrocytes were identified: classic GABA-A, bicuculline-insensitive GABAρ, and GABA-A-GABAρ hybrids. Clusters of GABA-A receptors were distributed in the perinuclear space and along the processes of GFAP(+) cells. Time-lapse microscopy showed GABAρ2-GFP accumulation in clusters located in the soma and along the processes. The clusters were relatively immobile, with mean displacement of 9.4 ± 0.9 µm and a net distance traveled of 1-2 µm, owing mainly to directional movement or simple diffusion. Modulation of GABAρ dynamics may be a novel mechanism of extrasynaptic transmission regulating GABAergic control of GFAP(+) cells during early postnatal development.

Increased firing frequency of spontaneous action potentials in cerebellar Purkinje neurons of db/db mice results from altered auto-rhythmicity and diminished GABAergic tonic inhibition.

The hormone leptin, by binding to hypothalamic receptors, suppresses food intake and decreases body adiposity. Leptin receptors are also widely expressed in extra-hypothalamic areas such as hippocampus, amygdala and cerebellum, where leptin modulates synaptic transmission. Here we show that a defective leptin receptor affects the electrophysiological properties of cerebellar Purkinje neurons (PNs). PNs from (db/db) mice recorded in cerebellar slices display a higher firing rate of spontaneous action potentials than PNs from wild type (WT) mice. Blockade of GABAergic tonic inhibition with bicuculline in WT mice changes the firing pattern from continuous, uninterrupted spiking into bursting firing, but bicuculline does not produce these alterations in db/db neurons, suggesting that they receive a weaker GABAergic inhibitory input. Our results also show that the intrinsic firing properties (auto-rhythmicity) of WT and db/db PNs are different. Tonic firing of PNs, the only efferent output from the cerebellar cortex, is a persistent signal to downstream cerebellar targets. The significance of leptin modulation of PNs spontaneous firing is not known. Also, it is not clear if the increased excitability of cerebellar PNs in db/db mice results from hyperglycemia or from the lack of leptin signaling, since both conditions coexist in the db/db strain.

Anticonvulsant, Anxiolytic and Antidepressant Properties of the ß-caryophyllene in Swiss Mice: Involvement of Benzodiazepine-GABAAergic, Serotonergic and Nitrergic Systems.

BACKGROUND: Central nervous system disorders such as anxiety, depression and epilepsy are characterized by sharing several molecular mechanisms in common and the involvement of the L-arginine/NO pathway in neurobehavioral studies with ß-caryophyllene is still little discussed. OBJECTIVES: One of the objectives of the present study was to demonstrate the anxiolytic behavioral effect of ß-caryophyllene (ß-CBP) in female Swiss mice, as well as to investigate the molecular mechanisms underlying the results obtained. METHODS: This study evaluated the neurobehavioral effects of ß-CBP using the open field test, rota- rod test, elevated plus maze test, novelty suppressed feeding test, tail suspension test and forced swim test, as well as pilocarpine, pentylenetetrazole and isoniazid-induced epileptic seizure models. RESULTS: The results demonstrated that the neuropharmacological activities of ß-CBP may involve benzodiazepine/GABAergic receptors, since the pre-treatment of ß-CBP (200 mg/kg) associated with flumazenil (5 mg/kg, benzodiazepine receptor antagonist) and bicuculline (1 mg/kg, selective GABAA receptor antagonist) reestablished the anxiety parameters in the elevated plus-maze test, as well as the results of reduced latency to consume food in the novelty suppressed feeding test. In addition to benzodiazepine/GABAergic receptors, the neuropharmacological properties of ß-CBP may be related to inhibition of nitric oxide synthesis, since pre-treatment with L-arginine (500-750 mg/kg) reversed significantly the anxiolytic, antidepressant and anticonvulsant activities of ß-CBP. CONCLUSION: The results obtained provide additional support in understanding the neuromolecular mechanisms underlying the anxiolytic, antidepressant and anticonvulsive properties of ß-CBP in female Swiss mice.

[Studies on the alkaloids from roots of Corydalis impatiens].

OBJECTIVE: To study the alkaloids from Corydalis impatiens. METHODS: The alkaloids were isolated and purified by chromatography and their structures were identified by spectral data and others methods. RESULTS: Seven alkaloids were isolated and identified as bicuculline(1), ochotensine(2), ochotensimine(3), ochrobirine(4), tetrahydrothalifendine(5), norochotensimine(6), N-methylactinodaphnine(7). CONCLUSION: All these compounds are isolated from this plant for the first time.

Poly-HEMA as a drug delivery device for in vitro neural networks on micro-electrode arrays.

Delivery of pharmacological agents in vitro can often be a difficult, time consuming and costly process. In this paper, we describe an economical method for in vitro delivery using a hydrogel of poly hydroxyethyl methacrylate (PHEMA) that can absorb up to 50% of its weight of any water-solubilized pharmacological agent. This agent will then passively diffuse into surrounding media upon application in vitro. An in vitro test of PHEMA as a drug delivery device was conducted using dissociated rat-cortical neurons cultured on micro-electrode arrays. These micro-electrode arrays permit the real-time measurement of neural activity at 60 different sites across a network of neurons. Neural activity was compared during the application of PHEMA saturated with cell culture media and PHEMA saturated with bicuculline, a widely used pharmacological agent with stereotypical effects on neural activity patterns. Application of PHEMA saturated with bicuculline produced a gradual increase in concentration in vitro. When the minimum effective concentration of bicuculline was reached, which was found to be 0.59 microM using the diffusion properties of PHEMA, it produced the rapid almost periodic synchronized bursting characteristically associated with this agent. In contrast, the application of PHEMA saturated in culture media alone had no effect on neural activity reinforcing its inherent inert properties. Since PHEMA is nontoxic, can be molded into a variety of shapes, quickly manufactured in any laboratory and is inexpensive to produce, the material represents a promising alternative to drug delivery systems on the market today.

Differential Effects of D-Cycloserine and ACBC at NMDA Receptors in the Rat Entorhinal Cortex Are Related to Efficacy at the Co-Agonist Binding Site.

Partial agonists at the NMDA receptor co-agonist binding site may have potential therapeutic efficacy in a number of cognitive and neurological conditions. The entorhinal cortex is a key brain area in spatial memory and cognitive processing. At synapses in the entorhinal cortex, NMDA receptors not only mediate postsynaptic excitation but are expressed in presynaptic terminals where they tonically facilitate glutamate release. In a previous study we showed that the co-agonist binding site of the presynaptic NMDA receptor is endogenously and tonically activated by D-serine released from astrocytes. In this study we determined the effects of two co-agonist site partial agonists on both presynaptic and postsynaptic NMDA receptors in layer II of the entorhinal cortex. The high efficacy partial agonist, D-cycloserine, decreased the decay time of postsynaptic NMDA receptor mediated currents evoked by electrical stimulation, but had no effect on amplitude or other kinetic parameters. In contrast, a lower efficacy partial agonist, 1-aminocyclobutane-1-carboxylic acid, decreased decay time to a greater extent than D-cycloserine, and also reduced the peak amplitude of the evoked NMDA receptor mediated postsynaptic responses. Presynaptic NMDA receptors, (monitored indirectly by effects on the frequency of AMPA receptor mediated spontaneous excitatory currents) were unaffected by D-cycloserine, but were reduced in effectiveness by 1-aminocyclobutane-1-carboxylic acid. We discuss these results in the context of the effect of endogenous regulation of the NMDA receptor co-agonist site on receptor gating and the potential therapeutic implications for cognitive disorders.

Evidence for a non-GABAergic action of quaternary salts of bicuculline on dopaminergic neurones.

Intracellular recordings were made from neurones, presumed to be dopaminergic, in the rat midbrain slice preparation. Bicuculline methiodide (BMI) and methochloride (BMC) reversibly blocked the slow, apamin-sensitive component of the afterhyperpolarization in these cells. The IC50 for this effect was about 26 microM. In comparison, BMC antagonized the input resistance decrease evoked by muscimol (3 microM) with an IC50 of 7 microM. The base of bicuculline was less potent in blocking the slow afterhyperpolarization. SR95531 (2-[carboxy-3'-propyl]-3-amino-6-paramethoxy-phenyl-pyridaziniu m bromide), another competitive GABA(A) antagonist, and picrotoxin, a non-competitive GABA(A) antagonist, also blocked the action of muscimol (IC50s: 2 and 12 microM respectively), but had no effect on the afterhyperpolarization at a concentration of up to 100 microM. Moreover, 100 microM SR95531 did not affect the blockade of the afterhyperpolarization by BMC. This blockade persisted in the presence of tetrodotoxin and was apparently not due to a reduction of calcium entry, suggesting that it involved a direct action on the channels which mediate this afterhyperpolarization. These results strongly suggest that quaternary salts of bicuculline act on more than one target in the central nervous system. Thus, the involvement of GABA(A) receptors in a given effect cannot be proven solely on the basis of its blockade by these agents.

Bicuculline free base blocks voltage-activated K+ currents in rat medial preoptic neurons.

The effects of the well-known GABA(A)-receptor blocker bicuculline on voltage-gated K(+) currents were studied in neurons from the medial preoptic nucleus (MPN) of rat. Whole-cell currents were recorded using the perforated-patch technique. Voltage steps from -54 to +6 mV resulted in tetraethylammonium-sensitive K(+) currents of delayed rectifier type. The total K(+) current (at 300 ms), including Ca(2+)-dependent and Ca(2+)-independent components, was reversibly reduced (17 +/- 4%) by 100 microM bicuculline methiodide and (37 +/- 5%) by 100 microM bicuculline as free base. The Ca(2+)-independent fraction (77 +/- 2%) of K(+) current evoked by a voltage step was, however, reduced (54 +/- 6%) only by bicuculline free base, but was not affected by bicuculline methiodide. The half-saturating concentration of bicuculline free base for blocking this purely voltage-gated K(+) current was 113 microM, whereas for blocking a steady Ca(2+)-dependent K(+) current it was 36 microM. The bicuculline-sensitive voltage-gated K(+) current was composed of 4-AP-sensitive and 4-AP-resistant components with different kinetic properties. No component of the purely voltage-gated K(+) current was affected neither by 100 nM alpha-dendrotoxin nor by 100 nM I-dendrotoxin. The possible K(+)-channel subtypes mediating the bicuculline-sensitive current in MPN neurons are discussed.

Structure and molecular modeling of GABAA receptor antagonists.

The recently described potent and selective GABAA antagonist SR 95531 (gabazine) is compared to six other GABAA antagonists: (+)-bicuculline, (-)-securinine, (+)-tubocurarine, iso-THAZ, R-5135, and pitrazepine. Starting from ab initio molecular orbital calculations performed on crystal atomic coordinates, attempts were made to identify in each structure the functional groups that are involved in receptor recognition and binding. A molecular modeling study revealed that (a) all compounds possess accessible cationic and anionic sites separated by an 4.6-5.2 A intercharge distance, (b) the antagonistic nature of the compounds can be explained by the presence of additional binding sites, (c) the correct spatial orientation of the additional binding sites is crucial for GABAA selectivity, and (d) the criteria determining the potency of the antagonist effect are an accurate intercharge distance (greater than 5 A) and the existence of hydrogen-bonding functionalities on one of the additional ring system. The presented pharmacophore accounts also for the inactivity of closely related compounds such as (-)-bicuculline, adlumidine, virosecurinine, allosecurinine, and the 4,6-diphenyl analogue of gabazine.

Superimposition-based protocol as a tool for determining bioactive conformations. II. Application to the GABA(A) receptor.

The natural templates (NT) superimposition method is used to determine the pharmacophoric requirements of the A subtype of the gamma-aminobutyric acid (GABA) receptor. Bioactive conformations for antagonists and agonists are found by superimposing them on a relatively rigid alkaloid bicuculline, which itself is a competitive antagonist at this ligand-gated ion channel receptor. As has been usual in the application of this modeling method, consideration of available experimental data is the cornerstone for obtaining realistic models. The identification of two substructural fragments of bicuculline permitted classification of the ligands. Analysis of the antagonists and agonists with respect to the two substructural fragments revealed two bioactive conformations of the highly flexible GABA molecule, one of which is extended with the nonhydrogenic atoms roughly coplanar torsional angles of -37 and -179 degrees at N-C-C-C and C-C-C-C (carboxyl), respectively. The second bioactive compound is clearly non planar (torsional angles of -81 and -109 degrees at N-C-C-C and C-C-C-C (carboxyl), respectively).

Superimposition-based protocol as a tool for determining bioactive conformations. I. Application to ligands of the glycinergic receptor (GlyR).

The natural templates (NT) approach, which is a superimposition-based protocol that has been successfully employed in several studies, is here applied to ligands of the glycine ligand-gated ion channel receptor. Bioactive conformations for glycine and its analogs were obtained using strychnine (a natural and specific competitive antagonist) as template. Experimental evidence was used to guide the superimposition protocol. Three essential regions have been defined in strychnine's structure that serve as a pharmacophore for agonist and antagonist activities. Reasonable alignments of known ligands were found in the majority of the cases. Molecular mechanics (i.e., conformational searches for the relatively flexible ligands) and molecular dynamics (for relatively rigid ligands such as strychnine and 5,6,7,8-tetrahydro-4H-isoxazolo[3,4-d]azepin-3-ol) were used to assess the energetic accessibility of the proposed bioactive conformations.

Surface activity of thymol: implications for an eventual pharmacological activity.

In the present work, we studied the ability of thymol to affect the organization of model membranes and the activity of an intrinsic membrane protein, the GABA(A) receptor (GABA(A)-R). In this last aspect, we tried to elucidate if the action mechanism of this terpene at the molecular level, involves its binding to the receptor protein, changes in the organization of the receptor molecular environment, or both. The self-aggregation of thymol in water with a critical micellar concentration approximately = 4 microM and its ability to penetrate in monomolecular layers of soybean phosphatidylcholine (sPC) at the air-water interface, even at surface pressures above the equilibrium, lateral pressure of natural bilayers were demonstrated. Thymol affected the self-aggregation of Triton X-100 and the topology of sPC vesicles. It also increased the polarity of the membrane environment sensed by the electrochromic dye merocyanine. A dipolar moment of 1.341 Debye was calculated from its energy-minimized structure. Its effect on the binding of [3H]-flunitrazepam ([3H]-FNZ) to chick brain synaptosomal membranes changed qualitatively from a tendency to the inhibition to a clear activatory regime, up on changing the phase state of the terpene (from a monomeric to a self-aggregated state). Above its CMC, thymol increased the affinity of the binding of [3H]-FNZ (K(d-control)= 2.9, K(d-thymol)= 1.7 nM) without changing the receptor density (B(max-control)= 910, B(max-thymol)= 895 fmol/mg protein). The activatory effect of thymol on the binding of [ [3H]-FNZ was observed even in the presence of the allosteric activator gamma-aminobutyric acid (GABA) at a concentration of maximal activity, and was blocked by the GABA antagonist bicuculline. Changes in the dipolar arrangement and in the molecular packing of GABA(A)-R environment are discussed as possible mediators of the action mechanism of thymol.

Determination of bicuculline in rat plasma by liquid chromatography mass spectrometry and its application in a pharmacokinetic study.

Bicuculline, a phthalide isoquinoline alkaloid is of current interest as an antagonist of gamma-aminobutyric acid (GABA). A simple and sensitive liquid chromatography mass spectrometry method for determination of bicuculline in rat plasma was developed over the range of 5-500ng/mL. After addition of midazolam as internal standard, protein precipitation with acetonitrile-methanol (9:1, v/v) was used as sample preparation. Chromatographic separation was achieved on a Zorbax SB-C18 (2.1mm×150mm, 5µm) column with acetonitrile -0.1% formic acid in water as mobile phase with gradient elution. Electrospray ionization (ESI) source was applied and operated in positive ion mode; selective ion monitoring (SIM) mode was used for quantification using target fragment ions m/z 368 for bicuculline and m/z 326 for the IS. Linear calibration was obtained with correlation coefficients r>0.99. The CV of the precision measurements was less than 13%. The accuracy of the method ranged from 93.6% to 100.5%. Mean recoveries of bicuculline in plasma were in the range of 80.5-91.8%. The method was successfully applied to the pharmacokinetic study after gavage administration of 15mg/kg bicuculline in rats.

Aminocatalytic asymmetric exo-Diels-Alder reaction with methiodide salts of Mannich bases and 2,4-dienals to construct chiral spirocycles.

An asymmetric exo-Diels-Alder reaction of α-methylene carbonyl compounds, generated in situ from stable methiodide salts of Mannich bases, with 2,4-dienals, has been developed through trienamine activation of a chiral secondary amine. A spectrum of spirocyclanes with high molecular complexity was efficiently constructed in moderate to excellent diastereo- and enantioselectivity.

Ring-shaped neuronal networks: a platform to study persistent activity.

Persistent activity in the brain is involved in working memory and motor planning. The ability of the brain to hold information 'online' long after an initiating stimulus is a hallmark of brain areas such as the prefrontal cortex. Recurrent network loops such as the thalamocortical loop and reciprocal loops in the cortex are potential substrates that can support such activity. However, native brain circuitry makes it difficult to study mechanisms underlying such persistent activity. Here we propose a platform to study synaptic mechanisms of such persistent activity by constraining neuronal networks to a recurrent loop like geometry. Using a polymer stamping technique, adhesive proteins are transferred onto glass substrates in a precise ring shape. Primary rat hippocampal cultures were capable of forming ring-shaped networks containing 40-60 neurons. Calcium imaging of these networks show evoked persistent activity in an all-or-none manner. Blocking inhibition with bicuculline methaiodide (BMI) leads to an increase in the duration of persistent activity. These persistent phases were abolished by blockade of asynchronous neurotransmitter release by ethylene glycol tetraacetic acid (EGTA-AM).

Antagonistic properties of a natural product-Bicuculline with the gamma-aminobutyric acid receptor: studied through electrostatic potential mapping, electronic and vibrational spectra using ab initio and density functional theory.

(+)-Bicuculline (hereinafter referred to as bicuculline), a phthalide isoquinoline alkaloid is of current interest as an antagonist of gamma-aminobutyric acid (GABA). Its inhibitor properties have been studied through molecular electrostatic potential (MEP) mapping of this molecule and GABA receptor. The hot site on the potential surface of bicuculline, which is also isosteric with GABA receptor, has been used to interpret the inhibitor property. A systematic quantum chemical study of the possible conformations, their relative stabilities, FT-Raman, FT-IR and UV-vis spectroscopic analysis of bicuculline has been reported. The optimized geometries, wavenumber and intensity of the vibrational bands of all the conformers of bicuculline have been calculated using ab initio Hartree-Fock (HF) and density functional theory (DFT) employing B3LYP functional and 6-311G(d,p) basis set. Mulliken atomic charges, HOMO-LUMO gap ΔE, ionization potential, dipole moments and total energy have also been obtained for the optimized geometries of both the molecules. TD-DFT method is used to calculate the electronic absorption parameters in gas phase as well as in solvent environment using integral equation formalism-polarizable continuum model (IEF-PCM) employing 6-31G basis set and the results thus obtained are compared with the UV absorption spectra. The combination of experimental and calculated results provides an insight into the structural and vibrational spectroscopic properties of bicuculline.