3 Hormones and Neurotransmitters

This lecture has three goals: to understand the steps of synaptic transmission, to describe the main functions of key neurotransmitters, and to critically evaluate outcomes associated with neurotransmitter dysregulation. It opened with a discussion question about myelin, which is collected with the other questions at the end.

Synaptic transmission

The action potential from the last lecture travels down the axon of one neuron, the pre-synaptic (“sending”) cell, to its axon terminals. There the axon terminals meet the dendrites of the next neuron, the post-synaptic (“receiving”) cell.

Steps of synaptic transmission

1. The neurotransmitter, or the enzymes needed to make it, is made in the cell body.
2. It is transported down the axon to the axon terminals and stored there in small sacs called vesicles.
3. The vesicles release the neurotransmitter into the synapse.
4. The neurotransmitter binds to receptors on the post-synaptic cell, which triggers a response in that cell.
5. The excess is taken back into the pre-synaptic neuron through transporters in its membrane.

The lecture’s diagram also shows two enzymes that break neurotransmitter down: a degradation enzyme in the synapse itself, and a metabolizing enzyme inside the sending neuron’s terminal.

The lecture also shows a photo of an actual synapse, taken with an electron microscope (the scale bar is 0.5 μm). The axon terminal at the top is packed with small round synaptic vesicles. Below it, separated by a thin dark line where the two membranes meet (the synapse), is a dendrite.

What is a neurotransmitter?

Neurotransmitters and hormones

GABA and glutamate

GABA is inhibitory: it decreases action potentials. When GABA binds to GABA receptors, Cl (chloride) flows into the receiving cell more readily.

Glutamate is excitatory: it increases action potentials. When glutamate binds to glutamate receptors, Na flows into the receiving cell more readily.

The lecture shows the chemical structure of each of the next three neurotransmitters. Notice how much dopamine and norepinephrine resemble each other.

Dopamine

Dopamine is mainly produced in a region called the ventral tegmental area, and it is processed throughout the limbic system. Dopamine is often described as the brain’s “reward” chemical. The lecture’s more careful description is that dopamine signals motivational salience.

Dopamine is also involved in learning and memory, and in motor function.

Serotonin

Serotonin is also known as 5-HT (5-hydroxytryptamine). About 90% of the body’s serotonin is produced in the gastrointestinal tract; the remaining 10% is produced in the brainstem’s raphe nuclei.

Serotonin is involved in:

• mood
• anxiety
• memory formation
• digestion
• sleep
• sexual inhibition

Norepinephrine

Norepinephrine is produced by very few cells, primarily in a small cluster of neurons called the locus coeruleus. Only about one millionth of the neurons in the brain produce it, but those neurons send projections everywhere.

Norepinephrine is involved in physiological arousal and wakefulness, including:

• the fight-or-flight response
• stress and anxiety
• attention

Acetylcholine

The lecturer introduced this one as “a neurotransmitter we’ll probably never talk about again. But it does EVERYTHING”: nearly every function you can think of has acetylcholine involved.

Acetylcholine is highly prevalent in the peripheral nervous system (the nerves outside the brain and spinal cord). It is released at neuromuscular junctions, where nerves meet muscles, to control movement.

It is also produced in several brain regions involved in:

• learning, memory and attention
• arousal
• muscle movement
• cardiac function and blood pressure

The lecture’s brain diagram shows acetylcholine pathways fanning out from the septal nuclei and the substantia innominata, near the front of the brain, to the cortex, cingulate gyrus, hippocampus and amygdala, and from a cluster in the brainstem up to the thalamus and down the brainstem.

Alzheimer’s disease and dementia have been associated with low acetylcholine levels. Acetylcholine signaling is also the target of many poisons: some natural toxins work by blocking it, while nerve agents work by stopping acetylcholine from being broken down, so that it builds up at the synapse.

Oxytocin

Oxytocin is involved in:

• pair-bonding
• affiliation
• reproduction
• smooth muscle contraction

Oxytocin is a small peptide: a chain of nine amino acids (the building blocks of proteins), Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH, closed into a ring by a sulfur–sulfur (S–S) bond between its two cysteines (Cys).

Vasopressin

Vasopressin is involved in:

• water retention
• vasoconstriction (narrowing of blood vessels)
• aggression and dominance
• pair-bonding and monogamy

The lecture also shows vasopressin’s chemical structure: like oxytocin, it is a short chain of amino acids closed into a ring by a sulfur–sulfur bond.

A video followed, showing three connected cages with a small rodent in each. It appears to be a partner-preference test of the kind used to study pair-bonding.

Testosterone and estrogen

Testosterone and estrogen are not just “male” and “female” hormones! Estrogen is “processed” testosterone: the body makes estrogen from testosterone. Both hormones are involved in the development of the reproductive organs and in reproductive endocrine (hormonal) processes: puberty, spermiogenesis (the final stage of sperm production) and ovulation.

The two hormones share most of their functions:

Testosterone	Estrogen
Sexual development	Sexual development
Stereotyped male sexual behavior	Stereotyped female sexual behavior
Dominance & aggression	Dominance & aggression
Stress regulation	Stress regulation
Memory	Memory
Puberty (voice deepening, male genital development)	Puberty (physical growth, muscle development, ova & sperm maturation)

Estrogen is essential for memory

Testosterone is also essential

Check yourself