Introduction to Behavioral Neuroscience · Basic Neurochemistry
Neurotransmitters Made from Fats
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In 30 seconds
Not every chemical messenger is a small water-soluble molecule or a peptide. A distinct group is lipid-derived — built from fats. The best known are the endocannabinoids (anandamide and 2-arachidonoylglycerol, or 2-AG), synthesized on demand from membrane phospholipids and acting at the receptors that cannabis THC Cannabis compound; partial agonist at CB1 Full entry → activates. The family also includes neurosteroids (from cholesterol) and eicosanoids such as prostaglandins (from arachidonic acid).
Lipid messengers break the rules of the classical life cycle from Topic 1: they are generally not stored in vesicles, not released by exocytosis, and not terminated by reuptake — they are produced when needed, diffuse through membranes, act locally, and are inactivated by specific enzymes. This "on-demand, backward-acting" style of signaling is essential for understanding cannabis pharmacology, appetite, pain, stress, and fever.
Why this matters
The endocannabinoid Fat-based messenger made on demand from membrane lipids (anandamide, 2-AG) Full entry → system is one of the most widespread modulatory systems in the brain and the target of THC, the main psychoactive ingredient in cannabis. It regulates appetite, pain, memory, mood, stress, and reward — why cannabis produces its characteristic effects and why endocannabinoid drugs are studied for pain, anxiety, and appetite disorders. Neurosteroids modulate the GABA-A receptor — the same channel targeted by sedatives — and have been implicated in menstrual-cycle and postpartum mood research. Prostaglandins mediate fever and pain, which is why NSAID pain relievers such as ibuprofen work. Lipid signaling also shows a general principle: messengers that are not stored can be turned on and off quickly, giving the nervous system a fast, local "volume knob."
The college version
Core Concepts
Endocannabinoids: made on demand, not stored
Unlike glutamate or dopamine, endocannabinoids are not packaged into vesicles. They are synthesized from membrane phospholipids only when a signal calls for them — on-demand synthesis Making a messenger only when a signal calls for it, not storing it Full entry →. Anandamide (AEA) comes from NAPE via NAPE-PLD; 2-AG, the more abundant brain endocannabinoid, comes from diacylglycerol (DAG) via DAG lipase. A typical trigger is a rise in calcium inside a postsynaptic neuron after strong or repeated activity.
Retrograde signaling: the message travels backward
This is what makes endocannabinoids famous. Classical transmitters travel forward (presynaptic → postsynaptic); endocannabinoids travel backward: released by the postsynaptic cell, they bind CB1 receptors on the presynaptic terminal. CB1 is a G-protein-coupled receptor and among the most abundant in the brain. When activated, it inhibits the voltage-gated calcium channels that drive release, so the presynaptic neuron releases less transmitter — a local negative-feedback brake on synaptic strength, seen in DSI/DSE (depolarization-induced suppression of inhibition/excitation) and contributing to forms of long-term depression (LTD).
CB1, CB2, and THC: the plant connection
- CB1 receptors sit mainly on presynaptic terminals throughout the brain — densely in hippocampus, cortex, basal ganglia, cerebellum, and hypothalamus — where they influence memory, movement, reward, and appetite.
- CB2 receptors are expressed mainly on immune cells and some neurons, and are more involved in immune signaling.
THC (delta-9-tetrahydrocannabinol) from the cannabis plant is a partial agonist at CB1: it occupies the receptor and produces a response, but weaker than the brain's own anandamide. By mimicking anandamide, THC stimulates appetite (the "munchies"), alters memory formation, reduces pain, and promotes relaxation. Anandamide is short-lived because FAAH Degrades anandamide Full entry → degrades it; 2-AG is degraded mainly by MAGL; FAAH inhibitors are an active research area. Educational note: chronic cannabis use can produce tolerance and dependence, and effects on the developing brain remain a public-health concern.
What the endocannabinoid system does
- Appetite: CB1 signaling in the hypothalamus promotes feeding.
- Pain: endocannabinoids act at spinal and brain sites to dampen pain signaling.
- Memory: CB1 in the hippocampus weakens synaptic transmission, interfering with new memory formation.
- Stress and emotion: endocannabinoid tone in the amygdala and stress axis modulates anxiety; low tone is linked in research to anxiety and depression.
- Reward and neuroprotection: endocannabinoids interact with dopamine reward circuits; some evidence (still debated) links exercise-related release to the "runner's high."
Neurosteroids: cholesterol's signaling role
The brain can synthesize steroids from cholesterol on its own — hence neurosteroids. The best studied, allopregnanolone Neurosteroid that positively modulates GABA-A Full entry →, is a positive modulator of GABA-A receptors: like benzodiazepines and anesthetics, it makes GABA signaling stronger, producing sedative, anxiolytic, and anesthetic effects. Its levels fluctuate with the menstrual cycle, leading researchers to investigate its role in premenstrual and postpartum mood changes; synthetic versions are studied as treatments for depression, epilepsy, and sleep disorders.
Prostaglandins and other lipid mediators
Arachidonic acid is also the precursor for prostaglandins, produced by cyclooxygenase (COX) enzymes, which mediate fever, pain, and inflammation. This is the mechanism behind common pain relievers: NSAIDs such as ibuprofen and aspirin inhibit COX, so less prostaglandin Lipid from arachidonic acid via COX enzymes Full entry → is made. Prostaglandins and endocannabinoids both trace back to arachidonic-acid derivatives, but they are distinct pathways with distinct receptors.
What is NOT made from fats
Two "atypical" messengers are often grouped with endocannabinoids but are not lipid-derived: nitric oxide Gas made from arginine; diffuses freely, retrograde messenger Full entry → (NO), a gas synthesized from the amino acid arginine, and carbon monoxide (CO), from heme breakdown. Like endocannabinoids, NO is made on demand, diffuses freely, and acts retrograde — the "unconventional messenger" club is defined by how it signals, not its chemical family.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| "Endocannabinoids are stored in vesicles like glutamate" | On-demand synthesis from membrane lipids | Endocannabinoids are built when needed and never stored |
| "Endocannabinoids act on the postsynaptic cell" | Retrograde action on presynaptic CB1 | Released by the postsynaptic cell; they act backward on the presynaptic terminal |
| "THC is the same molecule as anandamide" | THC is a plant partial agonist that mimics anandamide | Different molecules; THC produces a weaker-than-full response at CB1 |
| "Nitric oxide is a fat-derived messenger" | Gasotransmitter made from arginine | NO is a gas from an amino acid — on-demand and diffusive, but not a lipid |
| "All lipid messengers are endocannabinoids" | Neurosteroids, prostaglandins, and others | Different precursors, enzymes, and receptors (e.g., prostaglandins via COX) |
| "Prostaglandins and endocannabinoids are the same pathway" | Distinct pathways from arachidonic-acid derivatives | Endocannabinoids act at CB receptors; prostaglandins at prostanoid receptors — NSAIDs only touch the COX arm |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Most brain messages are made ahead of time and stored in little bubbles. Fat-made messages are different: the cell bakes them fresh when they are needed, straight out of its own wall. And some travel backward to the sender — like a note that says "okay, that's enough — send less!"
Worked example
Forward direction (synapse level): A hippocampal neuron fires strongly while forming a new memory. That activity raises calcium, triggering on-demand synthesis of 2-AG from membrane DAG. The 2-AG diffuses backward and activates CB1 on the presynaptic terminal, which then releases less glutamate — the synapse is briefly weakened, one way the brain adjusts connection strength.
Behavioral direction (cannabis): When someone uses cannabis, THC enters the brain and binds the CB1 receptors anandamide normally uses. In the hypothalamus this boosts appetite (munchies); in the hippocampus it weakens synaptic transmission, impairing memory formation; in pain pathways it dampens nociceptive signaling. One receptor, several brain regions, several distinct effects — why a single system can influence eating, memory, and pain at once.
Key takeaways
- Endocannabinoids (anandamide, 2-AG) are synthesized on demand from membrane phospholipids — no storage, no exocytosis, no reuptake — and act retrograde on presynaptic CB1 receptors (DSI/DSE, LTD).
- CB1 is among the most abundant GPCRs in the brain; CB2 is mainly on immune cells.
- THC (cannabis) is a partial agonist at CB1 — appetite up, memory formation down, pain down; FAAH degrades anandamide, MAGL degrades 2-AG.
- Neurosteroids (e.g., allopregnanolone, from cholesterol) positively modulate GABA-A receptors → sedation/anxiolysis/anesthesia; implicated in cycle-related and postpartum mood research.
- Prostaglandins (from arachidonic acid via COX) mediate fever, pain, inflammation — NSAIDs (ibuprofen, aspirin) block COX.
- Nitric oxide and carbon monoxide are gasotransmitters made from arginine/heme — on-demand and retrograde, but not lipid-derived.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Where are endocannabinoids synthesized, and what triggers their production?
Show answer
In the postsynaptic cell membrane, from phospholipids (NAPE → anandamide; DAG → 2-AG), triggered by a rise in intracellular calcium — on-demand synthesis.
In what direction does endocannabinoid signaling travel, and what is the net effect on neurotransmitter release?
Show answer
Retrograde — from the postsynaptic cell back to the presynaptic terminal, where CB1 activation inhibits calcium channels and reduces transmitter release (a negative-feedback brake).
Which receptor does THC act at, and why is it called a partial agonist?
Show answer
CB1 receptors; a partial agonist produces a weaker response than the full agonist (endogenous anandamide) even when fully occupying the receptor.
Name the enzymes that degrade anandamide and 2-AG.
Show answer
FAAH degrades anandamide; MAGL degrades 2-AG.
What do allopregnanolone, benzodiazepines, and anesthetics have in common?
Show answer
All positively modulate GABA-A receptors — they strengthen GABA-mediated inhibition, producing sedative/anxiolytic/anesthetic effects.
Why do NSAIDs like ibuprofen reduce fever and pain?
Show answer
They inhibit cyclooxygenase (COX), reducing prostaglandin synthesis; prostaglandins are the lipid mediators of fever and pain.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- endocannabinoid
- Fat-based messenger made on demand from membrane lipids (anandamide, 2-AG)
- on-demand synthesis
- Making a messenger only when a signal calls for it, not storing it
- retrograde signaling
- Signaling that travels from the postsynaptic cell back to the presynaptic cell
- CB1 receptor
- Presynaptic GPCR, very abundant in the brain; activated by endocannabinoids and THC
- THC
- Cannabis compound; partial agonist at CB1
- FAAH
- Degrades anandamide
- neurosteroid
- Brain-made steroid from cholesterol (e.g., allopregnanolone)
- allopregnanolone
- Neurosteroid that positively modulates GABA-A
- prostaglandin
- Lipid from arachidonic acid via COX enzymes
- COX (cyclooxygenase)
- Enzyme that produces prostaglandins
- nitric oxide
- Gas made from arginine; diffuses freely, retrograde messenger
Sources & references
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