Pot’s not the best thing for your memory. Yes, I know there are functional potheads who enjoy their greens and get also their work done. Still, it’s hard to ignore the legions of studies that show Δ9-THC consumption impairs spatial learning and working memory – that is, the ability to hold several pieces of information in mind and manipulate them to reach a mental goal.
Yet paradoxically, THC may benefit those with Alzheimer’s disease. Previous research in rats show that the compound breaks down clumps of disease-causing proteins (called β-amyloid plagues) by upregulating a “scissor” enzyme that chops them up. Sweeping out these junk protein plagues decreased the number of dying neurons in the hippocampus, a brain area crucial for learning and memory. THC also has powerful anti-oxidant effects and may protect the integrity of mitochondria – the “power plants” of our cells.
So here’s the dilemma: THC may potentially battle dementia, yet it also naturally impairs memory. In an unexpected turn of events, scientists from Louisiana State University discovered a key protein that mediates THC-caused memory loss, and show in mice that you can have your edibles and eat it too.
The protein in question is COX-2, a crucial player in inflammatory pain – think headaches, muscle pains and fever. Sound familiar? That’s because COX-2 is one of the targets of OTC painkillers such as Asprin and Tylenol (the other one is COX-1). Scientists have previously linked 2-AG, a THC-like substance produced endogenously in the brain, to inhibiting COX-2 signaling. Blocking COX-2 led to problems with memory retention. So naturally, they wondered whether THC impaired memory in the same way.
They found the opposite. As you can see on the left (blue bars), a single injection of THC boosted the level of COX-2 in both neurons and astroglias (“structural” non-neurons that play a role in memory and inflammation) in the hippocampus; the more THC, the more COX-2. This effect went away by 48hrs after the injection, but when the mice went on a weeklong THC binge (1 dose/day), their COX-2 levels remained chronically high cough unregulated (right graph, red bar compared to control black bar). When researchers blocked the THC/endocannabinoid receptor CB1R by either genetically deleting it or using a selective pharmaceutical blocker, the effect went away, showing that THC administration is indeed the cause of COX-2 increase.
Why would endogenous cannabinoids (2-AG) and THC have polar effects? Further molecular sleuthing revealed that it’s all in the messenger: although both 2-AG and THC activated the same receptor, 2-AG recruited Gα as courier, while THC opted for Gβγ. It’s like slapping a different address sticker on two boxes shipped to the same sorting facility; they’re now going different places. Indeed, Gβγ triggered a molecular cascade that activated several proteins previously shown to impair memory.
Naturally, researchers went on to block COX-2. After a week of THC, neurons begin to loose their spines – that is, little protrusions along the dendrite that house proteins necessary for forming and maintaining synapses (compare red bar/THC to black bar/control below). The breakdown of spines caused a decrease in the many proteins and receptors needed for normal excitatory signal transmission. Unsurprisingly, eliminating these channels of communication blunted the response of a cohort of neurons in the hippocampus after electrical stimulation. However, giving a COX-2 selective blocker concurrently with THC rescued all these deficits – structural, molecular and electrical (green bar – the spines are back!).
As for mutant mice that lack COX-2 at birth? They didn’t suffer any of these problems associated with THC. In the case of spines, as you can see above, THC (burgundy bar) had no effects compared to control (blue).
Do any of these “under-the-hood” changes lead to observable behaviour? In a fear-conditioning experiment, researchers trained mice to associate a box with electrical shocks. They then gave some of the mice 7 days of THC with or without a COX-2 inhibitor. When tested 24hrs later – presumably to weed out THC’s effect on anxiety* – stoner mice showed little fear when put back into the box. Those on the multi-drug regime, however, froze in fear. Like their sober peers, they retained and retrieved the fear memory. (The half-life of THC is ~20.1 hrs in mice, so they might have still been high at the time of testing.)
In a spatial memory task, researchers trained mice to find a hidden platform in a big tub of water. After 5 days of training, they then gave a subgroup a single injection of THC 30min before the test, which resulted in these mice taking roughly twice as long to find the platform as the controls. Once again, concurrent COX-2 administration “saved” the memory of the platform location. 24hrs later, after the mice had sobered up, they were tested again – same results.
Amazingly, inhibiting COX-2 did not destroy THC’s ability to wipeout Alzheimer’s-related protein plagues in a mice model of the disease. Treatment with THC once daily for a month, with or without the OTC COX-2 inhibitor Celebrex, significantly decreased the number of protein clumps (green below) and protected hippocampal neurons (blue).
Before you reach for the bottle of aspirin, joint in hand, maybe hold back on the self-medication just yet. For one, it’s hard to extrapolate these findings to humans, there are some interspecies differences in THC metabolism. Second, chronic COX-2 inhibition is linked to serious side effects such as ulcers and heart problems (think Tylenol is safe? Think again). Third, mice with inhibited COX-2 showed didn’t seem as couch-locked as they normally would; so if you’re after that body high, an aspirin would be rather counter-productive.
As a molecular neuroscientist, I love the detailed characterization of THC-CB1R signalling pathway, but the behaviour data could use some strengthening. Although researchers claimed that the water maze task assessed working memory, the protocol they used looks at normal spatial memory. To specifically probe working memory, they would’ve needed to move the platform to different locations and see how well the mice updated their memory. The results also directly counter those of a previous study, which showed that once the mice learn the location of the platform, THC did not impair the memory. They also didn’t report whether THC mice were simply too stoned to swim (or motivated enough) – tracking total swimming distance and speed at the time of testing would’ve helped .
This study focuses mostly on neurons*; a previous study published in March 2012 showed that THC impairs memory through a type of glia called astrocytes (the non-neuron brain cells); in fact, marijuana impaired working memory only when it was able to bind to the CB1Rs on astrocytes. That study pointed to deregulation of excitatory neurotransmitters as the cause of memory impairment; could COX-2, which is expressed in glia, also have a role?
*Edit: HT to reddit/u/superkuh. The text suggests that the authors of this paper did not consider the role of astroglia; in fact they explicitly did, when they showed that COX-2 upregulation occurred greater in astrocytes than neurons. The authors also showed that the reduction of glutamate (excitatory) receptors was due to COX-2-induced increase in glutamate release from both neurons and glia.