A neurohormone is a type of hormone produced and released by neurons (nerve cells) rather than by endocrine glands.
Instead of being released into a synapse to directly affect another neuron (as a neurotransmitter does), a neurohormone is secreted into the bloodstream (or hemolymph in invertebrates) to travel to distant target cells and exert its effects. This allows neurohormones to influence a wider range of physiological processes throughout the body.
Key Characteristics of Neurohormones:
- Production Site: Synthesized within neurons.
- Release Mechanism: Secreted into the bloodstream/hemolymph from nerve cells.
- Target Cells: Affects distant target cells with appropriate receptors.
- Mode of Action: Similar to other hormones, binding to receptors and triggering intracellular signaling cascades.
- Examples: Vasopressin (ADH), oxytocin, epinephrine (adrenaline), norepinephrine (noradrenaline), and releasing/inhibiting hormones from the hypothalamus.
Neurohormones vs. Neurotransmitters
| Feature | Neurotransmitter | Neurohormone |
|---|---|---|
| Release Site | Synapse | Bloodstream/Hemolymph |
| Target | Adjacent Neuron | Distant Cells |
| Distance Traveled | Short | Long |
| Examples | Dopamine, Serotonin, GABA, Glutamate, Acetylcholine | Vasopressin, Oxytocin, Epinephrine, Norepinephrine |
Examples of Neurohormones and their Functions:
- Vasopressin (ADH): Released from the posterior pituitary gland, it regulates water reabsorption in the kidneys.
- Oxytocin: Also released from the posterior pituitary gland, it plays a role in social bonding, reproduction, and childbirth.
- Epinephrine (Adrenaline) and Norepinephrine (Noradrenaline): Released from the adrenal medulla (under neuronal control), they mediate the "fight-or-flight" response.
- Hypothalamic Releasing and Inhibiting Hormones: These hormones (e.g., gonadotropin-releasing hormone (GnRH), corticotropin-releasing hormone (CRH)) are released from hypothalamic neurons into the hypophyseal portal system and regulate the secretion of hormones from the anterior pituitary gland.
Significance of Neurohormones
Neurohormones are crucial for coordinating physiological processes that require widespread and sustained effects, such as stress responses, fluid balance, and reproduction. They bridge the gap between the nervous and endocrine systems, allowing for integrated control of bodily functions.
