Functionally-defined compartments of the lordosis neural circuit in the ventromedial hypothalamus in female rats

Sexual behavior in female rats, typified by the lordosis reflex, is depende nt upon estrogen action in the ventromedial nucleus of the hypothalamus (VM H) and its surrounding neuropil, However, the synaptic organization of this brain region remains unclear. Pseudorabies virus (PRV) was used to transne uronally label the neural network that innervates the lumbar epaxial muscle s that execute the lordosis response, PRV-labeled neurons were identified w ithin and subjacent to the VMH four days after injection of PRV into the ba ck muscles, The pattern of labeling was defined in relation to three landma rks: the VMH core, as defined by Crystal Violet staining; the shell, as def ined by the oxytocin fiber tract; and the cluster of estrogen receptor-cont aining cell nuclei. The pattern of PRV labeling in the VMH displayed a stri king rostral-caudal gradient. In general, many of the PRV-labeled neurons w ere found in the oxytocin fiber tract, with far fewer in the core of the VM H. Furthermore, PRV-labeled neurons were rarely found in the cluster of est rogen receptor-containing neurons, and less than 3% of the PRV-labeled neur ons were double labeled for estrogen receptor. The results suggest that oxy tocin may directly influence these lordosis-relevant VMH projection neurons , whereas estrogen may have transsynaptic effects. (C) 2000 John Wiley & So ns, Inc.