[Effects of post-weaning rearing conditions on learning performance in "dwarf" mice (author's transl)]

Effect of age at the time of pituitary grafting on behavioral recovery in dwarf mice

The effects of ectopic pituitary isograft implanted in 13- or 21-day-old Snell dwarf mice on their freely expressed behavior were examined over a 14- or 31-day period. This mutant strain has severe anterohypophyseal deficiency which causes high levels of inactivity and abnormally low exploratory activity. The graft totally normalized these behaviors. Results showed that the time necessary for inducing behavioral changes is independent of age of the animals at the time of grafting, but the age at which implantation occurs influences the speed of full behavioral normalization. The possible involvement of prolactin and growth hormone in such behavioral improvement is discussed.

Recovery of exploratory capacity after ectopic pituitary homografts in Snell Dwarf mice (dw/dw)

The effects of ectopic pituitary homografts on free behavior were studied over a period of one month in adult Snell Dwarf mice. Because of anterohypophyseal deficiency, these animals exhibit dwarfism, sterility, and marked behavioral deficits. Beginning on the 16th day after surgery, in addition to a considerable weight gain, grafts result in the total disappearance of inactivity with passive social contact--the behavior most frequently observed in the mutants--to be replaced only by exploration--the most frequently observed behavior in controls. The treatment totally restores the latter behavior. Specific hormonal treatments and biochemical manipulations at different stages would enable identification of the hypophyseal hormones and/or the growth factors involved in the behavioral and morphological changes.

Cholinergic neurotransmission in the central nervous system of the Snell dwarf mouse

An unequal decrease in cholinergic activity has been evidenced in discrete brain areas in the growth hormone, thyroid-stimulating hormone and prolactin deficient Snell dwarf mouse. The effect of the mutation's pituitary deficit on central cholinergic mechanisms appears to be selective: Normally high cholinergic activity areas such as striatum, olfactory tubercles, and hippocampus show strong alterations in this neurotransmitter system. Structures which appear earlier in ontogenesis are less affected, if at all. The lack of pituitary hormones seems to have effects on choline acetyltransferase activity and/or synthesis as well as on the development of high affinity (H.A.) cholinergic uptake mechanisms, both strongly defective in hippocampus and striatum. Therefore, a lower density of cholinergic terminals can be inferred. Furthermore, our observations are consistent with a close functional coupling of the choline H.A. transport and of subsequent choline acetylation. Acetylcholinesterase activity does not seem to be affected. Moreover, a compensatory effect at the postsynaptic level may have occurred due to developmental or functional plasticity for cholinergic responsiveness. In conclusion, the dwarf mouse seems to be a useful model for a better understanding of the influences of growth hormone and thyroid hormones on the development of central cholinergic mechanisms. It also provides the possibility to attempt a functional restoration of the deficient cholinergic neurotransmission and the behavioral disturbances which may be linked to them, by hormone replacement.