Central and peripheral endocrine correlates of the immobility reaction in the toad Bufo bufo.

The immobility reaction, which may be regarded as an innate fear response, has been observed in several animal species including Amphibians. Its endocrine correlates were established in the newt Triturus cristatus and in the toad Bufo bufo, in which the duration of immobility is influenced by sex and mating. The aim of the present study was to investigate the possibility of a relation between immobility reaction and central hormonal receptors. Testosterone (T) binding capacity in the brain and plasma sex steroids were studied in four groups of male and female sexually active and inactive toads in which the immobility reaction was induced by inversion (A) and by inversion +pressure (B). Females were found to be more susceptible than males to immobility A, whereas males were more susceptible to B. Sexually active animals with higher plasma levels of sex steroids were less susceptible than inactive ones. Lower T binding capacity in the brain and higher affinity is associated with sexual activity. Susceptibility to the immobility was negatively correlated with plasma sex steroids and positively correlated with T binding capacity. A reciprocal effect was found to exist between endocrine parameters at central and peripheral level, both controlling immobility behaviour. Mating may influence susceptibility to immobility through an endocrine mechanism.