Birds do have cannabinoid receptors that are similar to the endocannabinoid system found in mammals. The endocannabinoid system is involved in regulating a variety of physiological processes and while the details are still being uncovered, evidence suggests birds also use cannabinoids to mediate things like appetite, pain-sensation, and muscle relaxation.
What are cannabinoid receptors?
Cannabinoid receptors are protein molecules on the surface of cells that receive chemical signals from cannabinoids. There are two main types of cannabinoid receptors found in vertebrates:
- CB1 receptors – Mostly found in the nervous system, organs, glands, and tissues
- CB2 receptors – Mostly found in immune system cells
When cannabinoids bind to these receptors, they trigger a cellular response and facilitate communication between cells. The effects depend on the type of cannabinoid and where the receptors are located. For example, THC, the psychoactive compound in cannabis, binds to CB1 receptors in the brain leading to the “high” feeling.
Evidence birds have cannabinoid receptors
Several lines of evidence indicate birds have a functional endocannabinoid system:
- The avian CB1 receptor gene has been cloned and studied. Birds have CB1 receptors with close homology to mammalian CB1 receptors.
- CB1 receptor proteins have been found throughout bird nervous systems using immunohistochemistry. Receptors detected in areas like the hippocampus, cerebellum, brainstem, and spinal cord.
- Administering THC causes behavioral effects in birds indicating cannabinoids are acting on central nervous system receptors. Effects include suppressed motor activity and delayed sensory discrimination responses.
- Anandamide and 2-AG, two naturally produced endocannabinoids, have been found in bird tissues.
- Enzymes that synthesize and break down endocannabinoids are present and active.
- CB1 receptor agonists increase food intake in chickens, as they do in mammals.
Multiple species have been examined including zebra finches, chickens, pigeons, and parrots. Taken together, these findings provide strong evidence birds have a functional endocannabinoid system that is analogous to what is seen in mammals.
Possible functions in birds
Research into the endocannabinoid system in birds is still relatively new, but initial studies suggest some similar roles to mammals:
- Appetite regulation – Activating CB1 receptors increases food consumption in birds. Endocannabinoids may provide hunger signals.
- Stress response – Lower CB1 expression seen in chickens after chronic stress exposure. May reflect changes in endocannabinoid tone due to stress.
- Pain modulation – THC reduces pain-related behaviors and vocalizations in parrots. Endocannabinoids likely involved in pain perception pathways.
- Thermoregulation – Inhibiting CB1 affects body temperature regulation in zebra finches. Cannabinoids involved in controlling metabolic rate.
- Muscle relaxation – Drugs that enhance endocannabinoid signaling reduce muscle spasticity in a neurological disorder model in finches.
The behavioral effects of THC administration also suggest the avian endocannabinoid system modulates motor function, memory, and discrimination like the mammalian system. There is still much to uncover but the current evidence points to important parallels between birds and mammals when it comes to cannabinoid biology.
Evolutionary origins
Since birds and mammals share similar endocannabinoid physiology, the system likely evolved in a common ancestor. By mapping the distribution of cannabinoid receptors across vertebrate clades, researchers can trace the evolutionary origins:
Group | CB1 receptors | CB2 receptors |
---|---|---|
Mammals | Present | Present |
Birds | Present | Present* |
Reptiles | Present | Present* |
Amphibians | Present | Absent |
Fish | Absent | Absent |
* Some studies have failed to conclusively identify CB2-like receptors in birds and reptiles. But evidence still indicates both receptors were present in a common ancestor of birds and mammals around 300-350 million years ago.
Evolution of endocannabinoid signaling
This points to endocannabinoid signaling evolving first in primitive tetrapods and persisting as a conserved behavioral modulation system in land vertebrates. It possibly initially developed to mediate appetite or stress responses. The receptors likely multiplied and diversified their signaling roles as vertebrates became larger, more active land animals. Their early origins explain why cannabinoids mediate such a wide range of physiological processes spanning metabolism, pain, reproduction, learning, and social behaviors.
Conclusion
In summary, substantial evidence demonstrates birds have a functional endocannabinoid system similar to mammals. Their cannabinoid receptors respond to the same exogenous and endogenous cannabinoid compounds. This system evolved over 300 million years ago and persists among tetrapods as an important neurological signaling mechanism regulating appetite, pain, temperature, muscle control, and various behaviors. While details of endocannabinoid biology in birds require more research, current data unambiguously shows this neuromodulatory system is conserved across avian and mammalian species.