Student Theses and Dissertations

Date of Award

1979

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

RU Laboratory

Pfaff Laboratory

Abstract

When applied during the sustained lordosis response of female hamsters, gentle probing at points on the perineal surface elicited stereotyped rump and tail movements. The rump movements always served to rapidly center the vaginal opening beneath the continuing stimulation. Tail deflections were only elicited by stimulation applied above and lateral to the vaginal opening. The tail was deflected away from the side stimulated. These observations help to complete the description of the stimulus-response relations of the component reflexes which comprise the complete lordosis response in hamsters. The movements would serve to facilitate vaginal access during copulation and foster accurate targeting of penile thrusts to the vaginal opening. Puffs of air directed to the lateral flank surface triggered lordosis in some estrogen-progesterone treated, ovariectomized female hamsters. This stimulation covered only a small area of the flank, and exerted very little pressure on deep structures, indicating that the deflection of a few hairs within a small area of the flank is sufficient to trigger lordosis in some hamsters. Prior exposure to the ministrations of a sexually active male hamster facilitated the triggering of lordosis by unilaterally applied air puffs, and potentiated the intensity of the lordoses exhibited. The expression of this effect did not require the activation of rump displacement reflexes by perineal stimulation during the post-male test. Light brushing of flank hairs, on one or both flanks elicited lordosis as reliably as post-male, unilateral air puffs. Increasing the area or intensity of artificially applied stimulation triggered more intense lordosis responses. These observations help to define the minimal sensory stimulus required to trigger lordosis in hamsters, providing Information useful in the neurophysiological analysis of this hormone dependent reflex. In estrogen-replaced, ovariectomized rats, selective transections were used to interrupt, together or separately, the medial and lateral pathways by which efferent fibers from the ventromedial nucleus of the hypothalamus reach the lower brainstem. Transections interrupting both projections reduced or eliminated lordosis performance. Transections which intercepted all of the medially descending fibers, but spared the lateral pathway, did not reduce lordosis performance in mating or manual stimulation tests. The lateral pathway was interrupted at two different locations. The lateral pathway, as a whole, was not required for lordosis when the medial pathway was left intact. Also, no particular subset of fibers assuming a lateral trajectory from the VMN to the brainstem were required for the display of lordosis. However, the fibers running through the lateral brainstem do play some role in the expression of the reflex - their transection bilaterally did reduce lordosis performance. The failure of lordosis to occur in mating tests was not a result of a systematic increase in rejection behavior. The observation of intermittent lordosis responses, or increased lordosis, performance following additional estrogen and progesterone treatment revealed that these transected animals were still able to produce the motor behavior required for lordosis. The deficits seen were attributed to the interruption of fibers mediating the control of lordosis by the hypothalamus. This control of lordosis by the ventromedial nucleus can be described as a tonic, estrogen-dependent facilitation of supraspinal lordosis control mechanisms located more caudally in the brainstem. The laterally, rather than the medially, descending efferent fibers from the ventromedial nucleus may play a quantitatively more important role in the control of lordosis.

Comments

A thesis submitted to the Faculty of the Rockefeller University in partial fulfillment of the requirements for the degree of Doctor of Philosophy

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