EFFECT OF A NULL MUTATION OF THE C-FOS PROTOONCOGENE ON SEXUAL-BEHAVIOR OF MALE-MICE

Sexual behavior was observed in male mice that were homozygous for a n ull mutation of the c-fos protooncogene, as well as in heterozygous mu tants and wild-type controls. The onset of mounting was slower and the subsequent mounting rate was significantly lower in homozygous mutant s than in either group of controls. Even so, a similar percentage of m ales of each genotype achieved ejaculation, and ejaculation latencies were equivalent in these mice. Likewise, in males that intromitted, th e intromission efficiency and the number of intravaginal thrusts/intro mission were similar among the three genotypes. The nuclear protein pr oduct (Fos) of c-fos was visualized immunocytochemically in the brains of heterozygous male mice 1 h after they exhibited a series of mounts , with or without intromission, leading to an ejaculation. As in the m ale of several other rodent species, nuclear Fos immunoreactivity was augmented in neurons of limbic and midbrain regions thought to convey olfactory/vomeronasal and genital/somatosensory information, respectiv ely, to the medial preoptic area following contact with an estrous fem ale. One interpretation of our behavioral results is that in the absen ce of normal neuronal c-fos expression, sensory stimuli that impinge o n the male brain during mating lose their ability to initiate a cascad e of further gene transcription events that otherwise control the rate at which a male reorients towards and mounts an estrous female during an ejaculatory series. Alternatively, the c-fos null mutation may dis rupt normal neural development, leading to a structural change that me diates the observed deficit in mounting capacity.