Low-sexually performing rams but not male-oriented rams can be discriminated by cell size in the amygdala and preoptic area: a morphometric study

Brain regions of male sheep behaviorally classified as high-sexually perfor ming (n = 10), low-sexually performing (n = 8) or male-oriented (n = 9) wer e examined to determine if differences in reproductive behavior were associ ated with differences in density or sizes of neurons. High-sexually perform ing rams actively mounted estrous ewes, low-sexually performing rams failed to mount or had long latencies to mounting estrous ewes, and male-oriented rams mounted other rams in preference to ewes in estrus. Cell densities an d sizes were quantified in Nissl stained sections through the medial amygda la (meAMY), preoptic area (POA), bed nucleus of the stria terminalis (BNST) , ventromedial hypothalamic nucleus (VMH), lateral geniculate nucleus (LG) and medial geniculate nucleus (MG). Multivariate discriminant analysis base d on soma sizes within nuclei of known importance for reproductive behavior and/or gonadotropin release (meAMY, POA, BNST and VMH) discriminated (Wilk s Lambda P < 0.05) low-performing rams from high-performing and male-orient ed rams, but did not discriminate (Wilks Lambda P = 0.14) between high-perf orming and male-oriented rams. Cell size in the parvocellular and magnocell ular layers of the LG along with cells of the MG, structures without a spec ific role in reproduction, did not discriminate any of the three behavioral ly defined groups of rams (Wilks Lambda P = 0.57). Density of cells present in structures important for the display of reproductive behavior (POA, meA MY, BNST) and/or gonadotropin release (POA, VMH) had no discriminating powe r nor did density of cells in structures important for the processing of vi sual (LG) or auditory (MG) stimuli. In conclusion, significant differences in sizes of cells located within nuclei that are specifically important for the display of male reproductive behavior were found in low-sexually perfo rming rams compared to high-sexually performing and male-oriented rams. The se differences may result from neuron development in utero or occur later a s a consequence of endocrine factors or behavioral experience. Neuronal cel l size is a critical variable that determines excitability to synaptic inpu ts because cell surface area varies exponentially with cell diameter. Relat ively small differences in neuron diameter could relate to functionally imp ortant differences in neuronal excitability. (C) 2001 Elsevier Science B.V. All rights reserved.