Endocannabinoids are derived from fatty acids in a very complex process. They serve as critical signals for a wide variety of brain functions. “Endo” refers to cannabinoids made in the brain, as opposed to “phytocannabinoids” made in plants. For simplicity, this post will use the general term “cannabinoids” when referring to the two endocannabinoids critical for brain function—AEA and 2-AG.
A previous post described the extremely elaborate system of fatty acid derivatives that are critical signaling molecules. Two specific fatty acids, DHA and ARA, enter the brain and cellular membranes where they attach to phospholipids for storage and await release by special enzymes. One in 10,000 of these fatty acids are unattached to the membrane lipoproteins and are actively metabolized into many important signals through the mysterious Land cycle. One well-known derivative of ARA is prostaglandin, a vital part of the inflammation response. Perhaps, the most important derivatives are cannabinoids that are critical for neuron and glia function. In the fetus, cannabinoid signals determine the fate of neuronal stem cells and glia and then switch to become the critical factors in the creation of the synapses for the developing brain. They are vital for every aspect of the creation of the structure of the fetal brain including directing neuron migration and axons and dendrite development.
A tremendous amount of progress has been made in understanding production of proteins from DNA and RNA. Because of this, genes can be “knocked out” (eliminated) and the effects of specific proteins can be studied. No such technology exists to study fatty acids, which are commonly part of many very complex metabolic cycles. As a result, it is extremely difficult to study how these thousands of different molecules are used in metabolic energy metabolism, as structural molecules, and as signals for vital brain functions. While known to be extremely important for all brain functions, fatty acid signaling is just now being described.
This post will outline what is currently known about these vital brain cannabinoids.
Two Vital Cannabinoids
The most important and abundant cannabinoid for the neuronal system is 2-AG (2-arachidonoylglycerol) and the second is AEA (anandamide or N-arachidonoylethanolamine.) Both of these, and many other fat signals, stimulate the two cannabinoid receptors CB1 and CB2. They, also, stimulate other important families of fatty acid receptors. AEA has variable effects on receptors both partially stimulating and partially antagonist.
Neurotransmitters dopamine, acetylcholine, serotonin and glutamate NMDA stimulate enzymes that cut ARA and DHA free from their attachment to membrane lipoproteins in neurons and glia. Then, they are metabolized in the not well-understood Land cycle to make derivatives and mediators. This process is highly regulated to make many vital brain signals. The Land cycle utilizes 5% of all of the brain’s energy. It is stimulated whenever these molecules are needed, such as in brain development, memory in adults, ischemia, brain injury and inflammation.
Mediators from DHA and ARA
DHA and ARA metabolites regulate changes in the cell membrane (see post on the Amazing Complexity of the Cellular Membrane). They stimulate complex protein kinase C (PKC) and inhibit the nuclear factor, NF-??.
One mediator derived from ARA is prostaglandin by the enzyme COX – (cyclooxygenases) as part of the inflammation response. Many other enzymes work in this cycle, such as LOX and P450, which call for leukocytes and the cleaning of debris. Prostaglandin is a very strong inflammation signal and is inhibited by the well-known NSAID medications for inflammation and pain. Other derivatives regulate the hypothalamus to produce glucose from the liver related to food intake.
Cannabinoids are very important derivatives of ARA and DHA. Most often, they travel from the postsynaptic neuron to the pre synaptic in retrograde fashion and regulate processes in the synapse. Cannabinoids and their receptors are manufactured by neurons, astrocytes and microglia. These bind to the CB1 and CB2 receptors, which regulate release of glutamate, GABA, monoamines, opiods and acetylcholine. They are critical in neuroplasticity for excitatory and inhibitory synapses.
Wide Range of Cannabinoid Effects
The details are just being discovered, but cannabinoids are known to be important for pain sensation, appetite, memory, motor learning, neuroplasticity and mood. CB1 and CB2 trigger large G proteins that lie just below the cell membrane of cells in the brain, peripheral neurons and immune cells. These send signaling cascades to the nucleus to trigger genetic networks.
As well as memory, appetite and neuroplasticity, cannabinoid signals are critical for balancing metabolic systems. They regulate hunger and interact with the amount of leptin in the blood. They bring into balance storage and transport of nutrients and energy storage in fat, intestinal, muscle and pancreas cells.
Cannabinoids are involved in the complex regulation of insulin, the cortisol stress response and the hypothalamus-pituitary-adrenal system. When stress continues, anandamide produces tonic secretion of cortisol. If stress is repeated many times then 2-AG is secreted in the amygdala, lowering cortisol. These changes are related to anxiety. In animals, cannabinoid receptors on glutamate neurons are related to aggression and anxiety.
In the immune system, cannabinoids modulate both neurons and immune cells. They protect against inflammation and muscle spasms in multiple sclerosis.
Cannabinoids are involved in modulating pain sensation. They decrease responses from the dorsal horn after painful stimuli, possibly by regulating the secretion of norepinephrine from the locus coeruleus in the pons. This modulation occurs by inhibiting GABA fibers in the spinal cord, causing more norepinephrine. Another fatty acid related to AEA—palmitoylethanolamide—is involved in the pain pathways through multiple receptors other than CB1 and CB2.
In adults, 2-AG is made in the postsynaptic cell near the postsynaptic density. Stimulation of the postsynaptic receptors and calcium stimulate 2-AG to form. It, then travels back across the synapse to the pre synaptic cell triggering CB1 receptors. This stops more activity in the synapse. It is then de activated by lipid enzymes. Astrocytes, also, have this lipid enzyme to stop 2-AG.
AEA function is more complex. It can inhibit the synapse in the same way as 2-AG or it can be made in the pre synaptic neuron, which then behaves like a more typical neurotransmitter and goes to the post synaptic cell.
Critical Cannabinoid Actions in The Fetus
The fetal brain builds multiple signaling pathways for neurons and glial cells that interact in very complex ways. The cannabinoid signals determine new neurons and glia from stem cells, neuron polarity for migration and axon production and the formation of the synapses in the cortex. They, also, build the pattern of neuronal connections.
In the fetus, morphogenic signals create gradients determining the structures of very complex tissues. Progress has been made in understanding these morphogens from protein signaling, but they, also, use many lipids signals that are less understood, including sphingolipids, glycolipids, prostanoids and cannabinoids.
In fetal tissue, cannabinoids can function in autocrine mode, that is, they are secreted in the same cell as the receptor that is triggered. This provides stimulus for stem cells to multiply. The manufacture of both cannabinoids and receptors are in different places in the stem cell, with the receptors within lipid rafts in the membrane.
They, also, use paracrine secretion where cannabinoids are secreted from one cell to another, often retrograde across the synapse. This is used to attract axons and dendrites to make synapses, oligodendrocytes to make myelin and to help with neuron and axon migration.
Cannabinoids Signals Create Fetal Brain Structures
The cannabinoid signals are critical to making new neurons for the developing brain and the structures that they form. There are specific regions where important signals are produced during the complex process of axon growth, including dendrite spines and growth cones of axons. Early neuron stem cells create both CB1 and CB2. In the fetus, CB2 is very common in the stem cells of the cortical subventricular zone, a region that makes new neurons in adults. Cannabinoids stimulate stem cells to make new neurons rather than more stem cells, and then trigger neuron migration.
Once the stem cell has committed to forming a neuron, then CB2 manufacture stops and only CB1 is manufactured and utilized. The CB1 makes the cell polarized, which stimulates migration and long axon growth. Pyramidal cells in the cortical plate (precursor of the cortex) make 2-AG that defines positions where the neuron will form in the structure.
After stem cells make more neurons, widespread cannabinoid signaling throughout the region stimulates production of many glia cells. The large amount of cannabinoid signals helps many neurons find their way in the migrations forming the cortex. These interactions between the various neurons are just being uncovered but are extensive. They may provide the direct guidance for movement.
To maintain the growing structure, specific enzymes are de activated, which limit cannabinoid signaling as the neuronal connection structure forms. These cannabinoid signals, also, affect the radial glia, which form the basic scaffolding structure on which the cortical neuronal network forms. The radial glia form a 3D checkered pattern, which then limits more cannabinoids that would alter the structure.
Another critical fetal structure built by cannabinoids is the basic sensory long tracts between the cortex and the thalamus. The enzymes that control cannabinoids sculpt the amount of neurons and axons needed. CB1, also, creates the pattern of interneurons.
Fetus Receptor Signals
When making dendrites and synapses, many other factors stimulate 2-AG, which alter the internal scaffolding skeleton of the neurons, which are forming the axon and dendrite. The cannabinoid receptors can be altered by specific factors to make them positive and negative. An important example is when the neurons from the thalamus and the cortex meet and CB1 positive and CB1 negative attract each other. Through this attraction, digitations are formed for the synapse by many elaborate signaling cascades.
Gradual Change from AEA to 2-AG
AEA is necessary for the embryo implantation and continued survival of the fetus. But, then, as the nervous system develops, 2-AG becomes the prominent signal. Cannabinoids determine the amount of neural stem cells and are critical to signal the need to make more neurons rather than astrocytes. This depends on a switch from CB2 to CB1.
GABA interneurons are directed by 2-AG and avoid enzymes that will degrade them. There appear to be special cannabinoid gradients that are used for migration of these interneurons. Special enzymes that make and degrade the cannabinoids direct the gradients.
CB1 receptors on the growing axons are related to internal cytoskeletal alterations necessary for the rapidly growing axons and dendrites. Locations of the 2-AG are highly specialized to provide direction for the very complex 3D travel of the axon throughout the organizing brain. The positive and negative versions of the receptors are critical for attraction and repulsion in this travel.
Once the synapse forms, then the enzymes regulating cannabinoids are dramatically altered. One is then stationed in the dendrite region and the other in the axon.
Cannabinoid lipid signaling is critical for making new astrocytes and oligodendrocytes, as well. All of the same enzymes are found in both cells. For oligodendrocytes, the mechanism uses a neurotransmitter and a receptor in the same cell. This increases the number of glia stem cells. Cannabinoid signaling between cells (cannabinoid and receptor from different cells) attract astrocytes and oligodendrocytes to particular regions. In this way glia are attracted to make myelin for the long neuronal axons as they develop.
Ingestion of Plant Cannabinoids during Pregnancy
Although cannabinoids are critical for development of the nervous system, they can react to new circumstances and change very rapidly. Studies in animals show that giving cannabinoid antagonists stop the travel of early neurons from the subventricular zone by altering significant wiring structures. Cannabinoid agonists, also, are disruptive by increasing the number of cells in particular regions.
Since cannabinoid signaling is highly regulated by neurotrophic factors, and many different enzymes, ingested cannabinoids have subtle wide range effects on different neurons and synapses that are not well understood.
In humans, babies exposed during pregnancy can develop compulsions and depression, as well as decreased visual memory. Animal studies show dramatic effects in very young babies. In mice, adolescents are affected, as well, with decreased social behavior and learning.
Cannabinoids In Adults
Cannabinoids have different functions in the adult brain and therefore affects of ingested plant cannabinoids are very different in the established adult brain.
In adults, cannabinoids stimulate the creation of new neurons in the adult brain, in the hippocampus, the subventricular and olfactory zones. Because of this, research has focused on the possibility of using cannabinoids to stimulate new brain cells after brain damage.
In adult synapses, AEA and 2-AG are controlled by multiple unique enzymes and transporters both at the cell membrane and inside multiple different compartments inside the cell. They are related to heat shock proteins and lipid storage organelles. There is a major intracellular system for cannabinoids that involves many different compartments that are just now being discovered. Many of the transporters have been identified, but not the exact compartments. These compartments, also, exist outside of the cell on both presynaptic and post synaptic neurons, microglia and astrocytes.
The major niche for new neurons in the adult is in the hippocampus and the subventricular zone of the lateral ventricle where the neurons travel a long distance to the olfactory bulb. Research shows that in the dentate nucleus of the hippocampus new neurons are related to new memories. After brain damage, neurons from the subventricular zone travel to the damaged regions and the striatum.
There are several powerful growth factors that are signals for new neurons and regulate stem cells division and resting periods. The stem cells are ready to go in either direction as stimulated by cannabinoids. Surprisingly, cannabinoid stimulation of new neurons by stem cellss occurs more powerfully in the elderly brain.
2-AG is the main cannabinoid stimulating new neurons in adults. Blocking it decreases new neurons in the hippocampus. This effect is either by cells stimulating their own receptors or by stimulating other cells. It is not clear how and where cannabinoids maintain the readiness of stem cells to make new neurons.
Conclusions of this research are the existence of a dynamic process in the adult brain where cannabinoids maintain neuroplasticity and keep stem cells ready to make new neurons. The system responds rapidly to injury, degenerative brain disease, inflammatory brain disease and depression. Cannabinoids provide stimulus for new neurons and directions for their travel in the brain. Research has shown that stimulating neurogenesis is part of the cure of depression. The response to damage is less certain.
The most prominent cannabinoid known in the marijuana plant is ?9 –tetrahydrocannabinol, known as THC. THC stimulates the CB receptors in stem cells that make new neurons as well as the neurons themselves. These affects appear to be different in adults, adolescents and children. It is possible that manipulating these pathways in treatments might increase new neurons and affect out of control neuronal and glioma cancers.
Glioblastoma is a malignant cancer of astrocytes, with a mechanism of cancer growth tied to lipids in the membrane. In animals, cannabinoids have stopped cancers of glia cells through stimulation of cell death of tumor cells without hurting normal cells. The stimulation of CB receptors, also, affects metastasis and blood vessel production for the cancer. In humans CB2 receptors and calcium channels are increased in the glioma. Clinical trials using cannabinoids in treatment of human cancers are now being done.
The mechanism has to do with stressing the endoplasmic reticulum until the tumor cell dies. AEA, also, causes cell death of cancers from neuroblastoma, astroglia and oligodendroglia cells
It is not clear how cannabinoids can stimulate the production of new neurons and glia stem cells, but kill glia tumor cells. The lipid cycles and pathways are too complex at present to understand this dramatic and significant difference.
Endocannabinoids Critical for Brain Function
Cannabinoids are fatty acid molecules derived from the polyunsaturated free fatty acids ARA and DHA through a very complex and poorly understood process. Among many fatty acid signals in the brain, cannabinoids are vital to the developing brain in many ways and adult brain function including memory and neuroplasticity. Cannabinoids, and their receptors, are manufactured by neuronal and glia stem cells, neurons, astrocytes, microglia and oligodendrocytes.
There is no current technology to study free fatty acids that is equivalent to the very detailed knowledge of proteins through knockout of genes. Free fatty acids are ubiquitous in cells in metabolic cycles for all types of functions. But, it is now clear that they are, also, critical neurotransmitters and neuromodulators. Lipid signaling (as well as glyco signaling) adds an enormous amount of new complexity for understanding brain function.
Surprisingly, cannabinoids stimulate neuronal stem cells to make new neurons in adults, and at the same time kill brain cancer cells. The mechanism of this major difference is not clear.
When stem cells both secret cannabinoids and then trigger their own receptors stimulating the production of more stem cells, where is the direction for this? This is a question equivalent to how cells know how to self edit their own DNA.
Cannabinoids are an exciting area for future research.