TESTOSTERONE BINDING-SITES IN THE BRAIN, PLASMA SEX-HORMONES AND REPRODUCTIVE-BEHAVIOR IN MALES OF THE TOAD BUFO-BUFO

The central action of steroid hormones on the regulation of reproducti ve behaviour has been recognized in Amphibia, as in, other Vertebrates . Central and peripheral endocrine aspects of reproductive behaviour w ere studied in the toad Bufo bufo during the breeding season by determ ining testosterone (T) binding in the brain and plasma T, dihydrotesto sterone (DHT) and estradiol (E) levels. In the first experiment the ef fect of early and late entrance into the breeding pond and the differe nce between animals entering and leaving was established: DHT and E pl asma levels, as well as T binding capacity in the brain were lower in animals leaving the pond at the end of the breeding season; animals en tering early in the season showed higher levels of E than those enteri ng late. In the second experiment the hormonal effect of amplexus and spawning was established: single males showed lower plasma DHT, higher plasma E and higher T binding values in the brain than males paired w ith a female. Males in amplexus with a non-spawning female showed high er plasma E levels than those with a spawning female. These results sh ow that it is possible to relate the different reproductive success to different T binding in the brain. The effect of amplexus on potential responsiveness to steroid hormones at the central level and on periph eral hormone concentrations suggests the presence of a regulatory mech anism which is more active when both amplexus and spawning occur. On t he other hand, the data concerning animals entering and leaving the po nd indicate that the hormonal variations are not due solely to the end of sexual behaviour since the difference is already significant betwe en groups entering early and late in the breeding season.