Introduction

Cannabis is well known for its use both as a psychoactive drug and a therapeutic. Its main components are the psychoactive Δ9-tetrahydrocannabinol (Δ9-THC) and the non-psychotropic cannabidiol (CBD; Costa 2007). Side effects of cannabis-based drugs including sedation and dysphoria have limited their therapeutic application (McCarberg and Barkin 2007). CBD and the synthetic analogues of Δ9-THC ajulemic acid (AJA) and HU210 display a variety of actions including analgesic, anticonvulsive and anti-inflammatory effects (Mechoulam et al. 2007; Burstein et al. 2004; Burstein 2005; Dyson et al. 2005). The beneficial effects of these cannabinoids have been attributed to a greater contribution of peripheral cannabinoid receptors to the mechanism of action as well as to non-cannabinoid receptor mechanisms (Guindon and Hohmann 2008; Ashton 2007).

Inhibitory postsynaptic transmission in the spinal cord involves mainly glycine (Laube et al. 2002) and γ-aminobutyric acid (GABA; Todd et al. 1996; Geiman et al. 2002). Due to its relatively restricted expression in lower areas of the brain and the spinal cord, the strychnine-sensitive glycine receptor family has been suggested as a target site for therapeutic agents aiming at inhibiting pain sensitization without producing sedation or other central nervous effects (Zeilhofer 2005; Betz and Laube 2006; Lynch and Callister 2006). In recent years, loss of inhibitory synaptic transmission within the dorsal horn of the spinal cord has been established as one of the major mechanisms involved in the development of chronic pain following inflammation or nerve injury (Bolay and Moskowitz 2002; Betz and Laube 2006; Knabl et al. 2008). Recent studies suggest that cannabinoids modulate inhibitory synaptic transmission at the level of the spinal cord dorsal horn via glycine receptors (Ahrens et al. 2009a, b; Hejazi et al. 2006).

The glycine receptor α1 subunit shares considerable primary sequence homology with transmembrane segments of α, β and γ subunits of the GABAA receptor. Specific amino acid residues within the transmembrane domains (TM) 2 and 3 of the GABAA and the glycine receptor subunits are crucial for the allosteric effects of alcohols, and a variety of volatile and intravenous general anaesthetics (Belelli et al. 1997; Mihic et al. 1997). As the nature of the TM2 amino acid residue at position 267 of the α1 subunit influences the interaction of propofol with the glycine receptor (Ahrens et al. 2008), the purpose of the current in vitro investigation was to extend these observations by determining the impact of this residue on the allosteric actions of certain cannabinoids. An investigation of the actions of cannabinoids on glycine receptors, coupled with an investigation of the impact that mutations have on the in vivo effects in a knock in model, may lead to a better understanding of the molecular determinants of action of cannabinoids.

Methods

Site-directed mutagenesis

The cDNA encoding the human glycine α1 subunit contained within the eukaryotic expression vector pcDNAI amp (Invitrogen, Carlsbad, CA, USA), under the control of the cytomegalovirus promoter, was kindly provided by Prof. H. Betz, Frankfurt, Germany. For site-directed mutagenesis, single-stranded template cDNAs were synthesised from the M13 origin of replication, and mutations of the serine residue at position 267 to isoleucine (α1S267I) were generated using standard procedures (Kunkel 1985). The fidelity of the mutagenesis reaction was confirmed by standard dideoxynucleotide sequencing (fmol DNA Sequencing System Promega, Southhampton, UK) of mutated α1 cDNAs.

Cell culture and transfection

Mutated human α1S267I glycine receptor subunits were transiently transfected into transformed human embryonic kidney cells (HEK 293). α1 glycine receptor subunits efficiently form homomeric receptors in heterologous expression systems. Cells were cultured in Dulbecco's modified Eagle's medium (Biochrom, Berlin, Germany), supplemented with 10% foetal calf serum (Biochrom, Berlin, Germany), 100 U ml−1 penicillin and 100 µg ml−1 streptomycin at 37°C in a 5% CO2/air incubator. For transfection, cells were suspended in a buffer containing 50 mM K2HPO4 and 20 mM K-acetate, pH 7.35. The corresponding cDNA, each subcloned in the pCIS2 expression vector (Invitrogen, San Diego, CA, USA), was added to the suspension. To visualise transfected cells, they were co-transfected with cDNA encoding for green fluorescent protein (10 µg ml−1). For transfection, we used an electroporation device by EquiBio (Kent, UK). Transfected cells were replated on glass coverslips and incubated 15–24 h before recording.

Solutions

AJA was a kind gift of Prof. Burstein, University of Massachusetts Medical School. HU210, CBD (both Sigma, Deisenhofen, Germany) and AJA were prepared as 100 mM stock solution in dimethyl sulfoxide (DMSO; Fluka, Steinheim, Germany), light-protected and stored in glass vessels at 4°C. The stock solutions were directly dissolved in bath solution to reach the final drug concentration. Concentrations were calculated from the amount injected into the glass vials. Drug-containing vials were vigorously vortexed for 30 min. Glycine (Sigma-Aldrich, Steinheim, Germany) was dissolved directly into the bath solution.

Patch electrodes contained (in millimolar) KCl 140, MgCl2 2, EGTA 11, HEPES 10 and glucose 10; the bath solution contained (in millimolar) NaCl 162, KCl 5.3, NaHPO4 0.6, KH2PO4 0.22, HEPES 15 and glucose 5.6.

Experimental setup

Standard whole-cell experiments (Hamill et al. 1981) were performed at a holding potential of −30 mV. A tight electrical seal of several GΩ formed between the cell membrane, and a patch-clamp electrode allows inward currents, due to agonist-induced channel activation, to be resolved in the pA range. The electrical resistance of the recording pipettes was ∼6 MΩ, corresponding to a total access resistance in the whole-cell configuration of ∼10 MΩ. An ultra-fast liquid filament switch technique (Franke et al. 1987) was used for the application of the agonist, presented in pulses of 2-s duration. The agonist and/or the drug under investigation was applied to the cells via a smooth liquid filament achieved with a single outflow (glass tubing 0.15-mm inner diameter) connected to a piezo crystal. The cells were placed at the interface between this filament and the continuously flowing background solution. When a voltage pulse was applied to the piezo, the tube was moved up and down, onto or away from the cell under investigation. The correct positioning of the cell, in respect to the liquid filament, was ensured by applying a saturating (1,000 µM) glycine pulse before and after each test experiment. Care was taken to ensure that the amplitude and the shape of the glycine-activated current had stabilised before proceeding with the experiment. Test solution and glycine (1,000 µM) were applied via the same glass-polytetrafluoroethylene perfusion system, but from separate reservoirs. The contents of these reservoirs were mixed at a junction immediately before entering the superfusion chamber.

The compounds were applied either alone, in order to determine its direct agonistic effect, or in combination with a sub-saturating (EC20) glycine concentration (30 µM), in order to determine its glycine-modulatory effects. We have previously shown that the α1S267I glycine receptor shows a decreased sensitivity to the natural agonist glycine (Ahrens et al. 2008).

A new cell was used for each protocol, and at least five different experiments were performed for each condition. The concentration of the diluent DMSO corresponding to the highest drug concentration used was 0.3%. We have shown that the DMSO itself has no effect at this concentration—neither on glycine-evoked response nor on direct activation of the receptor. The lack of effect of 0.1% DMSO on glycine receptors has also been demonstrated by other investigators (Weir et al. 2004).

Current recording and analysis

For data acquisition and further analysis, we used the EPC 9 digitally controlled amplifier in combination with Pulse and Pulse Fit software (HEKA Electronics, Lambrecht, Germany). Currents were filtered at 2 kHz. Fitting procedures were performed using a non-linear least-squares Marquardt–Levenberg algorithm. Details are provided in the appropriate figure legends or in the “Results” section.

Results

A total of 38 cells was included in the study. Expression of mutated homomeric receptors (α1S267I) in HEK 293 cells resulted in the subsequent expression of glycine receptors that exhibited a glycine-activated inward current (amplitude = 0.8 ± 0.4 nA) following application of a saturating (1,000 µM) concentration of the natural agonist. The inward current transient showed a rapid increase, followed by a monophasic decay in the continued presence of the agonist. The time constant of desensitisation was 923 ± 394 ms.

AJA, CBD and HU210 were applied in a concentration range between 3 and 300 µM (Fig. 1). The investigated cannabinoids neither co-activated chloride inward currents when co-applied with a low concentration of glycine nor directly activated glycine receptors when applied without the natural agonist glycine. Figures 2 and 3 show representative current traces for the lack of receptor co-activation and direct activation by AJA, CBD and HU210.

Fig. 1
figure 1

Chemical structure of ajulemic acid, cannabidiol and HU210. Common structural features are the non-substituted phenolic hydroxyl group (circle) and the alkyl chain (ellipse) in meta-position to the hydroxyl group

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Fig. 2
figure 2

Representative current traces elicited by a 2-s co-application of 30 µM glycine and ajulemic acid (AJA; left column), cannabidiol (CBD; middle column) or HU210 (right column) with respect to the current elicited by 1,000 µM glycine in the respective control experiment (upper trace). AJA, CBD and HU210 failed to co-activate chloride inward currents via glycine receptors in every concentration tested (second and next traces from top)

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Fig. 3
figure 3

Representative current traces elicited by a 2-s application of ajulemic acid (AJA; left column), cannabidiol (CBD; middle column) or HU210 (right column) in the absence of glycine with respect to the current elicited by 1,000 µM glycine (upper trace) in the same experiment. AJA, CBD and HU210 failed to directly activate glycine receptor function in every concentration tested (second and next traces from top)

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DMSO itself has no effect at a maximum concentration of 0.3%—neither on glycine-evoked response nor on direct activation of the receptor (data not shown).

Discussion

In this study, we have shown that the mutation of the TM2 S267 serine residue at the glycine receptor α1-subunit to isoleucine abolished receptor modulation by AJA, CBD and HU210. Similarly, the nature of this TM2 residue influences the co-activating actions of propofol, ethanol and alkane anaesthetics (Mihic et al. 1997; Krasowski et al. 1998; Ahrens et al. 2008).

Glycine receptors belong to the ligand-gated ion channel superfamily, which has a common structure in which 2α and 3β subunits form an ion channel (Jentsch et al. 2002; Grudzinska et al. 2005). Upon activation, these receptors usually inhibit neuronal firing by opening the associated chloride channel and the consequent decrease in neuronal input resistance (Jentsch et al. 2002). α1 glycine receptor subunits efficiently form homomeric receptors in heterologous expression systems. Co-expression of the glycine β subunit does not affect the response of heterologously expressed α1 subunits to the anaesthetic propofol and the cannabinoids AJA and CBD (Ahrens et al. 2004, 2009a, b; Haeseler et al. 2005). We have recently shown that mutation of the S267 residue at the glycine receptor α1 subunit abolished direct receptor activation by propofol and strongly decreased its potency to co-activate the receptor (Ahrens et al. 2008). Mutations of the TM2 S267 residue of the glycine receptor α1 subunit may prove invaluable in deciphering the putative role of glycine receptors in the behavioural effects of cannabinoids.

Our results in this study on mutated glycine receptors illustrate that the nature of the amino acid residue at position 267 is crucial for the glycine-enhancing effect of cannabinoids. Presuming this TM2 residue to contribute to a binding cavity, these data might suggest that the branched amino acid isoleucine is effective in impeding the access of the investigated cannabinoids. A more detailed knowledge of the amino acid residues crucial for the lack of positive allosteric modulatory effects of cannabinoids at strychnine-sensitive glycine receptors may ultimately allow to investigate the contribution of glycinergic mechanisms to the in vivo effect of cannabinoids in a knock in animal model.

Animal experiments will be needed to show the involvement of glycinergic with respect to cannabinergic mechanisms in the analgesic effects of AJA, CBD and HU210. The assumption that the effects of these compounds at strychnine-sensitive glycine receptors might complement its cannabinergic effects is indirectly supported by studies in transgenic mice lacking CB1 receptors in peripheral neurons which revealed a major reduction in analgesia produced by systemic cannabinoids like Δ9-THC, indicating that these peripheral CB1 receptors, not those inside the central nervous system, constitute a prime target for producing cannabinoid analgesia (Agarwal et al. 2007). One potential mechanism underlying this observation is that the CB1-mediated impact on network activity might be considerably different between central nervous and peripheral cannabinoid receptors.

There is evidence that significant amino acid sequence homologies exist between glycine receptor subunits and putative ligand-binding regions of the CB2 receptor (Tao et al. 1999; Betz and Laube 2006). It is conceivable that strychnine-sensitive glycine receptors and CB2 receptors both are targets for cannabinoid receptor ligands due to structural similarities in the receptor-binding site. Glycine receptor modulation has previously been shown for Δ9-THC, the endogenous cannabinoid anandamide (Hejazi et al. 2006) and the synthetic cannabinoid WIN55,212-2 (Iatsenko et al. 2007).

In conclusion, this in vitro study shows that AJA, HU210 and CBD fail to modulate S267I glycine receptors. Our results enlarge knowledge about amino acid residues governing cannabinoid effects on glycine receptors. This offers the perspective to create knock in mouse models with reduced glycinergic stimulation by cannabinoids (e.g. α1S267I). Knock in mouse models might be an invaluable tool to distinguish between GABA-ergic and glycinergic mechanisms and provide insight into the potential contribution of glycinergic mechanisms to the actions of cannabinoids.