9603651 Dulka There are striking sex differences in the central nervous system, and in the physiology and behavior of most vertebrates. Steroid hormones derived from the gonads of males and females are primarily responsible for generating and maintaining these differences. Androgens and estrogens have been shown to both organize sex-specific neural pathways during critical periods of development, and cause structural remodeling of existing neural pathways later in adulthood. This steroid-induced neural plasticity in adulthood not only involves changes in synaptic connections and neuronal morphology, but also changes in the expression of pathway-specific neuropeptides. This is very important since neuropeptides play an important role in modulating neurotransmitter activity. Moreover, there is surprisingly little information on neuropeptide systems which either originate in, or innervate sexually dimorphic brain regions. Dr. Dulka is using a neuroethological model system to examine this issues. He will explore how steroid-induced neural plasticity of neuropeptides mediate behaviors in vertebrates. Dr. Dulka has found that the neuropeptide substance P located in several important forebrain regions is sexually dimorphic in that males, not females, have an intense network of substance P activity. He will use a multidisciplinary approach that includes neuroanatomical, neurophysiological, biochemical, and behavioral techniques to determine how androgens modulate the substance P system in the brain to influence a complex communicatory behavior. These results are crucial to understanding the basic mechanisms underlying neural plasticity. Furthermore, they will generate important new knowledge of steroid and peptide hormone interactions in the brain of vertebrates, and how such interactions play a role in establishing sex differences in neural organization, circuitry, and behavior.