Thyroid hormone manipulation influences development of cardiovascular regulation in embryonic Pekin duck, Anas platyrhynchos domestica

Cardiovascular physiology of embryonic neotropic cormorants (Phalacrocorax brasilianus)

Cardiovascular maturation in avian species has primarily been studied in precocial species of birds, with few studies conducted on altricial species, which make up the majority of avian species. In the precocial species of birds studied to date, cardiovascular regulation is derived primarily from an adrenergic receptor stimulation that is present from approximately 50% to 60% of incubation until hatching. Conversely, the cholinergic modulation of heart rate differs in its timing of activation, as it is reported to be present in some studies at 60% of incubation to as late as after hatching in others. This has led to the speculation that, although adrenergic stimulation is critical to cardiovascular homeostasis, cholinergic stimulation prior to hatching in birds is species-specific and therefore is not critical for cardiovascular homeostasis in embryonic birds. In this work, we conducted a series of studies on an altricial species, the neotropic cormorant (Phalacrocorax brasilianus), to gain novel data regarding cardiovascular development in a largely unstudied group of birds. We investigated cholinergic and adrenergic receptor mediated control of both arterial blood pressure and heart rate. We predicted that, given the state of this altricial species at hatching, both cholinergic and adrenergic tone on the cardiovascular system would be functional in the embryo. Our findings indicate that cholinergic tone was present at 90% of incubation. However, there was a pronounced adrenergic tone on the cardiovascular system that was relatively greater than that reported in the other studies of avian embryos. Therefore, our findings support our prediction regarding the function of cholinergic tone and adrenergic tone prior to hatching.

Thyroid hormone manipulation influences development of endothermy and hatching in white leghorn chickens (Gallus gallus)

Chickens experience rapid change in their physiology and metabolism during hatching. We propose that thyroid hormones play a major role in regulating the developmental changes associated with attaining endothermy. To better understand the role thyroid hormones play in hatch timing and development of thermogenic capacity and metabolic rate we manipulated plasma thyroid hormone levels in chicken embryos beginning at 80% development (day 17 of a 21-day incubation) with either a single dose of triiodo-L-thyronine (T₃) or the thyroperoxidase inhibitor methimazole (MMI). Manipulation of thyroid hormones altered the timing of hatching, accelerating hatching under hyperthyroid conditions, and prolonging hatching with hypothyroid conditions. Effect sizes comparisons of morphological variables between treatment groups revealed larger heart and body masses in hyperthyroid 1-day post hatch animals. Thyroid hormone manipulation influenced the thermal neutral zone for O₂ consumption and body temperature during gradual cooling from 35 to 15 °C of externally pipped embryos and 1-day post hatch chicks. Hyperthyroid EP animals had a wider thermal neutral zone during cooling when compared to control animals. At the temperatures tested, the hypothyroid animals did not exhibit a thermal neutral zone. Similar differences between treatments in the breadth of the thermal neutral zone carried through to 1-day post hatch chickens. These findings suggest that thyroid manipulations influence the timing and development of the animal's thermogenic response to cooling.

Effect of pre-hatch incubator lights on the ontogeny of CNS opsins and photoreceptors in the Pekin duck

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Incubated eggs with and without light had no effect on post-hatch production.

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Light does not influence the ontogeny of retinal rod and cone photoreceptors.

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Brain OPN4 mRNA is increased the later stages of embryonic development.

The Pekin duck is a valuable agricultural commodity globally and in the United States. Pekin ducks are seasonal breeders; they are sensitive to light and thus, research on the neuroendocrine and behavioral responses are needed to maximize production and to improve their welfare. There is compelling evidence that specific wavelengths of light may adversely alter the growth and welfare of meat (grow out) ducks. However, despite a birds’ dependence upon light, in commercial poultry hatcheries, incubators almost exclusively hold eggs in the dark. Therefore, our objective was to determine the effects of lighting on the expression of retina photoreceptors (RPs) and deep brain photoreceptors (DBPs) during duck embryological development. Two groups of ducks were raised with and without light over 21 d from egg laying, embryonic day 0. Brain and retinal tissues were collected at embryonic days 3, 7, 11, 16, and 21 of a 24 d incubation period. qRT-PCR was performed on RPs (OPN1LW, OPN2SW, OPN1SW, MAFA, RHO, and RBP3) and the DBP OPN4M from retinal and brain samples, respectively. We find that the presence and absence of light during pre-hatch incubation, had no influence on the expression of any retinal photoreceptor. However, a late embryological increase in DBP OPN4M expression was observed. Taken together, the impact of light during pre-hatch incubation does not impact the overall post-hatch production. However, future directions should explore how OPN4M pre-hatch activation impacts Pekin duck post-hatch development and growth.

Ontogeny of OPN4, OPN5, GnRH and GnIH mRNA Expression in the Posthatch Male and Female Pekin Duck (Anas platyrhynchos domesticus) Suggests OPN4 May Have Additional Functions beyond Reproduction

The hypothalamic-pituitary-gonadal axis (HPG) is known to be regulated by daylength through the deep brain photoreceptor (DBP) system. The post-hatch ontogeny is not known for any of the DBPs. We set out to determine the ontogeny of OPN4 and OPN5 gene expression relative to GnRH and GnIH using qRT-PCR. Brains and serum were collected from five drakes and five hens on the day of hatching (Day 0) and again at 2, 4, 6, 10, 14, 19, 25 and 31 weeks of age and analyzed by qRT-PCR. Hen and drake serum was assayed for circulating levels of estradiol and testosterone, respectively. Data were analyzed between sexes over time using a repeated measures two-way ANOVA. Interestingly, the results show that on the day of hatching (Day 0), ducks showed adult-like levels of relative OPN4, but not OPN5, gene expression. During week 10, DBP levels increased, achieving highest relative expression levels at week 19 that maintained through week 31, typically peak fertility in ducks. GnRH mRNA levels increased following the DBP expression at the onset of puberty, and gonadal steroids increased after GnRH at week 14 while estradiol preceded testosterone. GnIH mRNA levels did not appreciably change during the time course of this experiment. These observations suggest that OPN4 may be active during the peri-hatch period and may have physiological roles beyond puberty and fertility.

Manipulating plasma thyroid hormone levels at hatching alters development of endothermy and ventilation in Pekin duck (Anas platyrhynchos domestica)

At hatching in precocial birds, there are rapid physiological and metabolic phenotypic changes associated with attaining endothermy. During the transition to ex ovo life, thyroid hormone levels naturally increase, peaking at hatching, and then decline. To better understand the role of the natural increase in thyroid hormone at hatching in regulating the developmental trajectory of the Pekin duck's endothermic phenotype, we examined development of O2 consumption (V̇O2 ) and ventilation (frequency, tidal volume and minute ventilation) while inhibiting the developmental increase in thyroid hormones that occurs at hatching via administration of the thyroid-peroxidase inhibitor methimazole (MMI) or accelerating the developmental increase via triiodothyronine (T3) supplementation. Animals were dosed only on day 24 of a 28-day incubation period and studied on incubation day 25, during external pipping (EP) and 1 day post-hatching (dph). On day 25, there was an increase in V̇O2  in the hyperthyroid treatment compared with the other two treatments. During the EP stage, there was a significant effect of thyroid status on V̇O2 , with hyperthyroid V̇O2  being highest and hypothyroid V̇O2  the lowest. By 1 dph, the supplemented T3 and control animals had similar V̇O2  responses to cooling with comparable thermal neutral zones followed by increased V̇O2 Hypothyroid 1 dph hatchlings had a lower resting V̇O2  that did not increase to the same extent as the supplemented T3 and control animals during cooling. During EP, inhibiting the rise in T3 resulted in embryos with lower ventilation frequency and tidal volume than control and supplemented T3 embryos. At 1 dph, ventilation frequency of all animals increased during cooling, but tidal volume only increased in supplemented T3 and control hatchlings. Our data support the role of the late incubation increase in T3 in regulating the systemic development of endothermic metabolic capacity and associated control of ventilation occurring at hatching of the Pekin duck.