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Descending projections from the nucleus retroambiguus to the iliopsoas motoneuronal cell groups in the female golden hamster: Possible role in reproductive behavior

In the cat, the nucleus retroambiguus (NRA) projects to expiratory motoneurons in the brainstem and spinal cord. Recently, it has been demonstrated that the NRA sends fibers to a specific set of motoneurons in the lumbosacral cord, which pathway is thought to play a crucial role in mating behavior. The question is whether such projections exist in the hamster, because the female of this species displays a very distinctive receptive behavior. In the hamster, lumbosacral cord injections of wheat germ agglutinin‐horseradish peroxidase (WGA‐HRP) combined with hemisection 1 or 2 segments rostral to injection sites in three of the five cases demonstrated retrogradely labeled neurons in the NRA at levels 1.0–2.25 mm caudal to the obex, contralateral to the injection sites. Injections of WGA‐HRP into the NRA and adjoining reticular formation revealed that NRA fibers crossed the midline in the caudal medulla and descended in the contralateral lateral and ventrolateral funiculi to terminate bilaterally, but mainly contralaterally, in the motoneuronal cell groups of the abdominal wall and iliopsoas muscles. NRA projections to levels caudal to lumbar segment 5 were virtually absent. Electron microscopic examination revealed that, of the 162 labeled NRA terminal profiles found in the ultrathin sections, 144 (89%) made monosynaptic contacts with retrogradely labeled dendrites of iliopsoas motoneurons. These NRA terminals formed asymmetrical synapses and contained spherical vesicles indicative of an excitatory function. The results indicate that, in the hamster, direct contralateral NRA projections exist to iliopsoas motoneurons. A concept is discussed in which this pathway plays a crucial role in mating behavior. J. Comp. Neurol. 403:219–228, 1999. © 1999 Wiley‐Liss, Inc.