Central mechanisms involved in the reproductive cycle of the ring dove (Streptopelia Risoria)

Abstract

An immunocytochemical method was employed to localize progesterone receptors in the brain, pituitary and oviduct of incubating, non-incubating and oestrogen (E 2)-treated ring doves. Progesterone receptors were localized in similar regions of the brain in both sexes: the preoptic area (POA), nucleus preopticus paraventricularis magnocellularis (PPM), nucleus hypothalami lateralis (PLH), and in the tuberal region (TR).
Quantitative analysis demonstrated an increase in PR number in the TR of the male ring dove on day I of incubation compared to non-incubating males. Changes in PR expression was investigated in different parts of the TR. This revealed an increase in PR density in the ventral region after E 2-treatment in both male and female doves. In addition, this region was devoid of PR in brooding birds. In contrast, PR expression in the POA, PPM and PLH was increased in brooding birds, compared to E 2-treated doves and those at other stages of the reproductive cycle.
High numbers of PR were localized in the developed oviduct in E 2-treated birds and on day I of incubation compared to control doves. In the anterior pituitary gland, a greater density of PR was observed in the pituitary of females compared to males at the onset of incubation. E2-treatment resulted in an increase in PR expression in both males and females, and brooding females demonstrated a lower PR density than females at other stages of the breeding cycle. The results indicated that the source of oestrogen which regulates PR expression in the brain of the male dove is central, (probably derived from
the aromatization of testosterone), and not systemic.
There was evidence from the study that PR in the ventral region of the TR are oestrogen-dependent, whilst those in other brain regions are not dependent upon oestrogen. It is proposed that PR in the TR have a neuroendocrine role, and may inhibit the expression of VIP cells when the bird is not brooding. This would prevent development of the crop sac and crop milk at an inappropriate time. In contrast, PR in the POA are thought to be involved in the expression of incubatory behaviour. Central progesterone receptors may serve to provide a general preparation for incubation.
It was also suggested that PR in the POA, PPM and PLH are involved in mediating the expression of the aggressive behaviour which is apparent during brooding. The number of PR in these brain regions increases during brooding in both sexes, when circulating oestrogen concentrations are low. It was postulated that prolactin is responsible for the increase in PR in the POA, PPM and PLH during this period of the reproductive cycle.
Another aim of the study was to determine the sites of androgen action in the brain, anterior pituitary gland and oviduct of ring doves using an immunocytochemical method. The pituitary and oviduct were devoid of androgen receptors (AR), but there was widespread staining of the nuclear receptor in the brain.
Two regions which demonstrated (AR)-immunoreactivity were the POA and the TR. This is consistent with AR in the POA being involved in the mediation of courtship behaviour in male and female doves. In the TR of the male dove, AR may be involved in the regulation of PR expression. This is thought to be controlled by oestrogen, which in the male can be derived from the aromatization of testosterone. A similar mechanism may play a past in oestrogenic regulation of PR expression in the female, although plasma oestrogen is also thought to be involved.
Finally, the role of vasoactive intestinal polypeptide (VIP) in the breeding cycle of the ring dove was investigated. Hypothalamic VIP is the releasing factor responsible for an increased concentration of prolactin in the ring dove during incubation. It is possible that in addition to this role, VIP may have central effects associated with behaviours
relevant to this stage of the reproductive cycle.
VIP cell bodies, fibres and terminals were localized in the brains of male and female non-breeding doves using an immunocytochemicat method. In addition, the distribution of VIP binding sites in the male and female dove brain was determined by autoradiography. Quantitative image analysis was employed to examine differences in central binding density between male and female, non-breeding and incubating birds. VIP neurons and binding sites were distributed extensively throughout the dove brain, and their distribution was similar in males and females. Particularly notable was the dense staining of VIP cell bodies and fibres observed in the TR, an area which also contained a moderate density of VIP binding sites. Several areas of the male brain demonstrated a significantly higher (p .c 0.05) density of VIP labelling than the female,
although the possible causes and consequences of this sex difference are not yet clear.
There was no difference in the density of VIP binding sites between incubating and control doves in any brain region. These results suggest that the predominant role of VIP in the central nervous system is the stimulation of prolactin release.
It was concluded that the studies described in this thesis have demonstrated the importance of the tuberal infundibular region and the preoptic area in the control of reproductive behaviour in the ring dove.