In vitro demonstration of seasonal transition effect on acetylcholine liberation from Auerbach's plexus of non-hibernator

Sensitivity changes in guinea-pig atria and ileum following subacute administration of dyflos

1. Subacute administration of dyflos to guinea-pigs (1.2 mg/kg, s.c., daily for 10 days) led to the development of subsensitivity to carbachol in the ileum but not in the electrically stimulated left atrium. 2. Atria from dyflos-pretreated animals became supersensitive to acetylcholine but the ileum became subsensitive to this agonist despite marked inhibition of cholinesterases. The degree of subsensitivity was less than that for carbachol. 3. Subsensitivity also developed to histamine and was of a similar order to that observed for carbachol supporting the suggestion that subsensitivity is a nonspecific post-receptor mechanism. 4. Other changes which occurred in the ileum, together with the increase in the EC50 value to agonists were an increase in tissue mass, an increase in the slope of the concentration-response curve to agonists and an increase in the maximal response to agonists.

Nature of seasonal variation in development of acute tolerance to morphine

Morphine inhibits contractions of coaxially stimulated guinea pig ileum. Acute tolerance to this effect occurs on continuous perfusion with morphine for 1.5 h. This is associated with an increase in the sensitivity of the tissue to acetylcholine. Acute tolerance does not always develop in spite of identical treatment, but shows a distinct seasonal incidence. It is significantly higher in the summer (May--Nov.) than winter (Dec.--April). This seasonal variation in development of tolerance does not appear to be related to changes in diet, environmental light or temperature, or to the sensitivity of the ileum to morphine. In the winter, the acetylcholine release from stimulated ilea is lower than in the summer. In addition to inhibiting acetylcholine release, morphine also has a non-specific blocking effect in the winter. This postjunctional effect could prevent the development of supersensitivity to acetylcholine during continuous morphine treatment so that acute tolerance cannot occur.