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Morphine analgesia in the formalin test: evidence for forebrain and midbrain sites of action
B H Manning1, M J Morgan, K B Franklin
1Department of Psychology, McGill University, Montreal, Quebec, Canada.
Abstract:
A mapping study was performed to determine where in the rat brain morphine acts to produce analgesia in the formalin test, which is an animal model of prolonged pain associated with tissue injury. A single dose (5 nmol) of morphine was bilaterally microinjected into a wide range of brain areas throughout the midbrain and forebrain. Strong analgesia was elicited from the posterior hypothalamic area, the periaqueductal gray and ventral tegmental area. Other sites from which analgesia was elicited were the nucleus accumbens and a few sites in the retrorubral field and caudate-putamen. Analgesia from the periaqueductal gray or nucleus accumbens was accompanied by decreased locomotor activity and catalepsy, whereas analgesia from the posterior hypothalamic area or ventral tegmentum was accompanied by a noticeable increase in locomotor activity and rearing. Morphine into various thalamic nuclei had no effect. These results indicate that the primary sites of action of morphine in the formalin test are probably the posterior hypothalamic area and periaqueductal gray, with an additional contribution from regions innervated by tegmental dopamine cells.
Insights
Morphine produces pain relief in rats by acting on specific brain areas like the posterior hypothalamus and periaqueductal gray. These actions influence movement and behavior, offering insights into pain management.
Area of Science:
- Neuroscience
- Pharmacology
- Pain Research
Background:
- The formalin test is a widely used animal model for studying prolonged pain, particularly pain associated with tissue injury.
- Understanding the precise brain regions targeted by analgesics like morphine is crucial for developing effective pain management strategies.
Purpose of the Study:
- To map the specific brain areas in rats where morphine administration results in analgesia during the formalin test.
- To investigate the behavioral effects associated with morphine-induced analgesia in different brain regions.
Main Methods:
- A mapping study involving bilateral microinjections of morphine (5 nmol) into various midbrain and forebrain areas of rats.
- Assessment of analgesic effects and associated behavioral changes (locomotor activity, catalepsy, rearing) in response to microinjections.
Main Results:
- Significant analgesia was observed following microinjections into the posterior hypothalamic area, periaqueductal gray, and ventral tegmental area.
- Analgesia was also noted in the nucleus accumbens, retrorubral field, and caudate-putamen.
- Different brain regions exhibited distinct behavioral effects: periaqueductal gray/nucleus accumbens injections led to decreased activity and catalepsy, while posterior hypothalamic area/ventral tegmentum injections increased locomotor activity and rearing. Thalamic injections had no effect.
Conclusions:
- The posterior hypothalamic area and periaqueductal gray are identified as primary sites of morphine action for analgesia in the formalin test.
- Regions innervated by tegmental dopamine cells likely contribute to morphine's analgesic effects.
- The study highlights the complex central nervous system mechanisms underlying morphine's pain-relieving properties.