GABAA receptor

The gamma amino butryric acid A (GABAA) receptor is not only a receptor for many terpenes but is also a chloride channel. For this reason it will have its own page. Before even contemplating chloride channels in neurons, one must watch this video on action potentials from the Harvard Extension School.

How GABAA modulates the action potential

This team from Harvard tells us how electro-chemical potential gradients of sodium and potassium are used to generate action “potentials” that travel down axons. We are told that there is a net positive charge on the outside of the neuron and a net negative charge on the inside. In a mere 10 seconds of this excellent video we learn that there is more chloride outside the cell than inside. We are left to wonder what opening of GABAA chloride channels does to the membrane potential and how it affects the action potentials. This GABAA receptor youtube video does an excellent job of addressing how opening chloride channels hyperpolarizes the neuron. Also recommended is the Wikipedia page on the action potential.

A In watching both of these videos remember that the action potential starts in the dendrites on the left hand side. The GABAA chloride channel lives in this region too.

B The action potential graphed out…. from Wikipedia. Both videos will show this little graph. Remember that the values on the Y- axis reflect the potential of the inside of the cell relative to the outside. An inside negative means that the outside has a positive from all of those three sodiums (Na⁺) being pumped out for every two potassiums (K⁺) being pumped in by the Sodium Pump. K⁺ flowing from inside the cell to the outside are responsible for restoring the inside negative membrane potential. Notice the low dip at the end called the “refractory period.”

C. Now it is time to watch the action potentials video. You will learn about the interplay between Na⁺ channels (green tubes) and K⁺ channels (blue tubes). When Na+ channels open too long, the change in voltage opens K+ channels and the membrane potential is restored to inside negative. Keep your eyes open to that low dip at the end. This refractory period is a resting period between action potentials.

D. Now that we’ve watched a video covering the interplay between Na⁺ and K⁺ channels we can understand how GABAA Cl^– channels can create a “refractory period” This GABAA receptor video explains how Big Pharma drugs work with amino butyric acid reduce anxiety by inhibiting action potentials.

Phasic and Tonic GABA_A receptors

This particular image came from a publication on epilepsy. The following is from a review on tonic vs phasic [7]

Tonic

• contain α4, α5 or α6 sub units
• extra synaptic

Phasic

• a γ2 subunit in association with α1, α2 or α3 subunits
• found in synapse

Menthol like terpenes and GABAA

Yes, menthol is a ligand for the GABAA receptor. [1] The GABAA receptor was expressed in Xenopus (frog) oocytes. 100 μM menthol was found to decrease the concentration of gamma amino butyric acid needed to achieve 50% of maixmal GABAA ochannel opening. . [1]

Pinene like terpenes and GABAA

β carophyllene is a popular terpene in cannabis. Does it activate GABAA? In a mouse model mice were given all sorts of tests to induce anxiety. 200mg/kg carophyllene reduced outward signs of anxiety. [2] Flumazenil, a competitive inhibitor of the benzodiazepine site on the GABAA receptor, reestablished anxiety. [2] Reversal of anti-anxiety affects of by bicuculline was viewed as evidence of acting on the GABA site. [2] (-) α-Pinene has been shown to have sleep inducing properties of a mouse in vivo and ex vivo model α-Pinene acts on the benzodiazepine site of GABAA .[3] In these mice 100mg/kg seemed to be the most effective dose. [3]

A truly insightful publication presented data on perfume terpenes interacting with GABAA receptors expressed in Xenopus (frog) oocytes.

Note that 10 uM GABA (0.01 mM) is considered a response of 100% Adding 0.65mM α-pinene increases the GABAA current to 149%. Let’s take a pause of skepticism. The α-pinene concentration exceeds the GABA concentration by 63x. Is it possible to achieve a concentration of α-pinene in the blood high enough to achieve a concentration of 0.63 mM in the synaptic cleft?

Chemists like to use a technique called dimensional analysis. Numbers in the top and bottom rows are multiplied together. The numerator is divided by the denominator. The units cancel each other out until we are left with a volume unit for liquid. Values in this table were obtained from https://en.wikipedia.org/wiki/Alpha-PineneWikipedia.

1	2	3	4	5	6
5 liter	0.63 mmol	1 mole	136 g	cm3	0.50 cm3
	liter	1000 mmol	mole	0.858 g

1. The blood volume of the average human is 5 liters.
2. The desired concentration of α-pinene in mmoles per liter. The liter unit cancels out.
3. There are 1000 mmoles per 1 mole.
4. Each mole of α-pinene is 136 g.
5. One cubic centimeter (cm³) of α-pinene, also known as a milliliter (mL) , weighs 0.8586 g.
6. The only unit that does not cancel out is cm³, or mL. It would take 1.8 mL of pure α-pinene to achieve a blood concentration of 0.65 mM.

Just a general note of caution in these studies is that α-pinene is not very soluble in water. In one case a detergent was added to dissolve it. [3]. To be transported in the blood of the mouse or human, it must partition to usual carriers for steroid hormones such as serum albumin. A light weight human of 50 kg would have to consume 50 x 0.1 g = 5 g α-pinene.

“Linear” terpenes and GABAA

In a trip back to terpenes that that are not far removed from geranyl pyrophosphate in the terpene Origami, another study examined linalool and its metabolites on GABAA Cl- currents. This time GABAA was expressed in in human embryonic kidney cells. [5] These authors tested established metabolites in conjunction with 10 μM GABA to induce Cl^– currents in the HEK cells. [5] 10 μM GABA induced about 30% of the maximal current achieved with about 1 mM GABA. To the same cell… linalool and metabolites (2 mM) were co-applied for 500 ms together with GABA at EC₁₀₋₃₀ of GABA. GABA was added first then the linalool compound with GABA still in the mixture.

Some of the discussion in this paper included mammalian and plant cytochrome P450s in the oxidation of linalool, the affect on odor quality, and the affect on GABAA currents. The authors were a bit vague about statistical significance. The starting currents were a bit variable just because of variability in the expression of GABAA receptors in this particular cell line.

Compound	Irel [%] ± SEM	Irel [%] ± SEM
	10 μM GABA	10μM GABA + compound
Linalool	100 ± 15	160 ± 14
Linalyl acetate	100 ± 15	136 ± 16
L1	100 ± 53	68 ± 42
L2	100 ± 37	122 ± 33
L3	100 ± 12	95 ± 16
L4	100 ± 16	82 ± 26
L5	100 ± 37	144 ± 58
L6	100 ± 48	166 ± 35
L7	100 ± 35	99 ± 29

They were not really seeing a several fold increase in currents. Their focus seemed to be more in terms of linalool for aroma therapy. It is interesting to note that PubChem gives the half life of propofol as being short with a duration of 2 to 10 minutes due to reaction with oxygen.

Let us for the sake of argument assume that the volume of blood i the average human is 5 liters. How much linalool must we consume to achieve this concentration?

1	2	3	4	5	6
5 liter	2 mmol	1 mole	154 g	cm3	1.8 cm3
	liter	1000 mmol	mole	0.86g

Official human studies

It would appear that (-) α-pinene and perhaps other members of this class of terpenes actin on the benzodiazepine site of the GABAA receptor. Menthol and linalool appear to act on the profolol site. We really cannot allow us to make any clonclusions of in vitro studies in cells and rodent studies with high doses not reasonably obtainable in humans. Let’s take a look at clinicaltrials.gov

• A peppermint essential oil study evaluated this menthol containing mixture to treat anxiety, cognitive function and GABAA related functions. “This will be achieved by analysing gamma-Aminobutyric acid A (GABAA), neuronal nicotinic and N-methyl-D-aspartate receptor (NMDA) glutamate receptor binding efficacy, The investigators chose the most promising peppermint essential oils based on this property and gave participants 50 and 100 µL doses. [6] The goal was to displace ³H radioactive natural ligands of these receptors. Peppermint oil #4 was ~ 25% menthone and 37% menthol. [6] The higher dose of oil reduced mental fatigue but no statistically significant effect on anxiety. [6]
• Inhaled 5 mg α-pinene and different doses of inhaled THC will be tested on vital signs and cognitive performance in this John Hopkins University study.
• A Dysmenorrhea study with a cannabis lover’s dream has been completed. A sublingual tablet contained 30 mg of cannabidiol (CBD), 1 mg tetrahydrocannabinol (THC), 97 mg palmitoylethanolamide (PEA), and a 0.2 mg combination of myrcene, beta-caryophyllene, humulene, linalool, and limonene and peppermint oil. The study was completed in 2020. Results have not been posted.

It is kind of easy to see why FDA is cracking down on companies that make claims based on cell culture and animal studies. For a physician who has a patient who wishes to self medicate with plant based medicines, clinicaltrials.gov is a good place to start. Stay tuned as this site evolves into a platform that a patient can discuss cannabis based medicines with their health care providers. How to establish if something is working and is safe? How does a patient work with a health care provider to establish efficacy with a health care provider?

References

1. Hall AC, Turcotte CM, Betts BA, Yeung WY, Agyeman AS, Burk LA. Modulation of human GABAA and glycine receptor currents by menthol and related monoterpenoids. Eur J Pharmacol. 2004 Dec 3;506(1):9-16. PubMed
2. da Silva Oliveira GL, da Silva JCCL, Dos Santos C L da Silva AP, Feitosa CM, de Castro Almeida FR. Anticonvulsant, Anxiolytic and Antidepressant Properties of the β-caryophyllene in Swiss Mice: Involvement of Benzodiazepine-GABAAergic, Serotonergic and Nitrergic Systems. Curr Mol Pharmacol. 2021;14(1):36-51.
3. Wang H, Woo J, Pae AN, Um MY, Cho NC, Park KD, Yoon M, Kim J, Lee CJ, Cho S. α-Pinene, a Major Constituent of Pine Tree Oils, Enhances Non-Rapid Eye Movement Sleep in Mice through GABAA-benzodiazepine Receptors. Mol Pharmacol. 2016 Nov;90(5):530-539. free paper
4. Aoshima H, Hamamoto K. Potentiation of GABAA receptors expressed in Xenopus oocytes by perfume and phytoncid. Biosci Biotechnol Biochem. 1999 Apr;63(4):743-8. doi: 10.1271/bbb.63.743. PMID: 10361687. free paper
5. Milanos, S., Elsharif, S. A., Janzen, D., Buettner, A., & Villmann, C. (2017). Metabolic Products of Linalool and Modulation of GABA_A Receptors. Frontiers in chemistry, 5, 46. PMC free article
6. Kennedy, D., Okello, E., Chazot, P., Howes, M. J., Ohiomokhare, S., Jackson, P., Haskell-Ramsay, C., Khan, J., Forster, J., & Wightman, E. (2018). Volatile Terpenes and Brain Function: Investigation of the Cognitive and Mood Effects of Mentha × Piperita L. Essential Oil with In Vitro Properties Relevant to Central Nervous System Function. Nutrients, 10(8), 1029. https://doi.org/10.3390/nu10081029 PMC free article
7. Farrant M, Nusser Z. Variations on an inhibitory theme: phasic and tonic activation of GABA(A) receptors. Nat Rev Neurosci. 2005 Mar;6(3):215-29.