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Reproductive behavior in most mammalian species consists of a highly stereotyped pattern of movements, is elicited by specific sensory stimuli and is sex steroid dependent. The present paper describes a concept of the pathways in the midbrain, brainstem and spinal cord which control the receptive posture of the female cat. The midbrain periaqueductal gray (PAG), which is an important structure in the Emotional Motor System (EMS), receives direct input from a distinct group of neurons in the dorsal horn of the lumbosacral cord. This cell group overlaps with the location of pelvic and to lesser extent, pudendal nerve primary efferents, which convey information from the pelvic viscera and sex organs to the central nervous system. The FAG, in turn, controls various motor components of female receptive behavior using different pathways. For example, immobility, which is one of the characteristics of receptive behavior, might be mediated by a diffuse pathway from the FAG, via the ventral part of the medial medullary tegmentum, to all parts of the spinal ventral hem. More specific components, such as hindlimb treading, lateral deviation of the tail and elevation of the lower back, are thought to be controlled by a circumscribed projection from the FAG to the nucleus retroambiguus (NRA). The NRA is a group of interneurons at the transition between brainstem and spinal cord and projects directly to distinct lumbosacral motoneuronal cell groups, which innervate muscles that are likely to be involved in the female receptive posture. Estrogen induces axonal sprouting of the NRA-lumbosacral pathway in adult female cats, which explains why female cats only display receptive behavior when estrogen levels are high. (C) 1998 Elsevier Science B.V. All rights reserved.

Contains fulltext : 70380.pdf (Publisher’s version ) (Closed access) During the short four-day estrous cycle of the female hamster various behavioral (lordosis, vocalization and aggression) and autonomic adaptations occur. Presumably, these changes are under ovarian control. Recently, we described a distinct estrogen receptor-alpha immunoreactive (ER-alpha-IR) cell group, now called nucleus para-retroambiguus (NPRA), in the caudal ventrolateral medulla (Gerrits et al., 2008). Neurons of this group project to the ipsilateral intermediolateral cell column in the thoracic and upper lumbar cord. Clearly, the NPRA is part of an estrogen-sensitive neuronal network and the same applies to the region containing the commissural part of the solitary tract nucleus (NTScom) and the A2 group, here called NTScom/A2. Estrogen is known to modulate neuronal ultrastructure in various brain areas and spinal cord, but not in the caudal brainstem. Because we assumed that the NPRA plays a role in estrous cycle related adaptations, we hypothesized the occurrence of plasticity in this nucleus. In the present study we examined morphological changes of axo-dendritic relationships in NPRA and NTScom/A2 in estrous, diestrous and ovariectomized (OVX) hamsters, using immuno-electron microscopy and the 1D5 anti-ER-alpha antibody. Ultrastructural analysis revealed that the ratio "axon terminals surface/dendrite surface" was significantly increased in both the NPRA and NTScom/A2 during the estrous phase compared to the OVX and diestrous conditions. Remodeling of axon terminals due to axonal sprouting into large terminal fields filled up with pleomorphic vesicles resulted in contacts with more dendrites, and was the main cause for the "axonal terminal-dendritic-ratio" shift. In conclusion: Estrous cycle-induced axonal and dendritic plasticity is present in the NPRA, and in the NTScom/A2 group. Our findings support our hypothesis that estrogen-sensitive neuronal networks in the caudal brainstem display structural plasticity, probably to modulate steroid hormone dependent behaviors or autonomic adaptations.

In 1995, we discovered a new pathway in the cat, which originates from the nucleus retroambiguus (NRA) and terminates in a distinct set of lumbosacral hindlimb, axial, and pelvic floor motoneuronal cell groups [VanderHorst VGJM, Holstege G (1995) Caudal medullary pathways to lumbosacral motoneuronal cell groups in the cat: evidence for direct projections possibly representing the final common pathway for lordosis. J Comp Neurol 359:457-475]. The NRA is a compact group of interneurons located laterally in the caudal medulla oblongata. Its projection to lumbosacral motoneurons is thought to represent the final common pathway for male mounting and for female receptive or lordosis behavior. However, females only display lordosis behavior. However, females only display lordosis behavior when they are in estrus, which suggests that the NRA-lumbosacral pathway is only active during estrus. This raised the question of whether estrogen affects this pathway. The effect of estrogen on the NRA-lumbosacral projection was studied light microscopically, using wheat-germ agglutinin horseradish peroxidase (WGA-HRP) as a tracer. The rubrospinal pathway served as control. The density of labeled NRA fibers in their target hindlimb motoneuronal cell groups appeared abundant in estrous and very weak in nonestrous cats. Such differences were not found in the rubrospinal pathway. For electron microscopical study, the NRA projection to the semi-membranosus motoneuronal cell group was selected. In this cell group, an almost ninefold increase of labeled profiles was found in estrous versus nonestrous cats. Moreover, the semimembranous motoneuronal cell group contained labeled growth cones in estrous, but not in nonestrous, cats. The present study is the first to show that estrogen induces axonal outgrowth of a precisely identified pathway in the adult mammalian central nervous system. The possible mechanisms underlying this outgrowth are discussed.