Naloxone evokes a nutritionally dependent LH response in post partum beef cows but not in mid-luteal phase maiden heifers

Ovulation of the first dominant follicle arising after day 21post partumin suckling beef cows

The effects of body condition score (BCS) of 2·0 or 3·0 units at calving (low v. moderate), post-partum energy intake at 0·6 or 1·0 MJ metabolizable energy (ME) per day per kg M0·75 (low v. high) and unrestricted or restricted (once daily) suckling on the ability of cows to ovulate were studied in a 2 ✕ 2 ✕ 2 factorial design with each treatment replicated eight times. Calf isolation and restricted suckling were imposed shortly after selection of the first dominant follicle (DF) to emerge after day 21 post partum. The episodic release of LH (sampled at 15-min intervals for 10 h) was determined 48 h before and 48 h after the day calf isolation and restricted suckling commenced. Additional blood samples were collected weekly for plasma insulin determination. The mean interval from calving to first ovulation was shorter for cows in moderate than low BCS at calving (47·8 v. 57·1 days, s.e.d. = 4·50, P < 0·05), and for cows suckling once daily than for those with unrestricted suckling (42·9 v. 62·0 days, s.e.d. = 4·50, P < 0·001). Post-partum nutrition did not affect this interval. Mean LH pulse frequency prior to the start of restricted suckling was higher for cows of moderate than low BCS at calving (3·2 v. 1·6 pulses per 10 h, s.e.d. = 0·60, P < 0·05). Subsequently, LH pulse frequency was higher for cows suckling once daily than for those with unrestricted suckling (4·0 v. 2·2 pulses per 10 h, s.e.d. = 0·82, P < 0·05). More cows in moderate than low BCS ovulated the first DF to emerge after day 21 post partum (within 4 to 6 days) in response to restricted suckling (69 v. 25%, P < 0·05). LH pulse frequency prior to restricted suckling increased (P < 0·05) with plasma insulin concentration (categorized as low, < 5; moderate, 5 to 8; and high, >8 mIU per l). There were indications of interactions between suckling treatment and BCS (P < 0·08), and suckling treatment and plasma insulin concentration (P < 0·06), on LH pulse frequency, which suggested that calf restriction could alleviate the suppressive effects of under nutrition on episodic LH release. Amongst cows suckling once daily, the non-ovulating animals had fewer LH pulses prior to restricted suckling and smaller, slower growing DF, indicating an inability of the DF to respond to increased LH pulse frequency following calf restriction. Cows of moderate BCS, particularly those with moderate to high levels of plasma insulin (³ 5 mIU per l), responded favourably to restricted suckling. In contrast, excessively thin cows with low plasma insulin concentrations (<5 mIU per l), that had most to gain from restricted suckling, responded poorly.

Annual energy intake and the metabolic and reproductive performance of beef cows differing in body size and milk potential

The metabolic status and reproductive performance of four pure breeds of beef cow (small size and low milk potential, Aberdeen Angus; small size and high milk potential, Welsh Black; large size and low milk potential, Charolais; and large size and high milk potential, Simmental) were monitored during their first two parities. Heifers from each breed were allocated to one of two levels of annual energy intake relative to metabolic body weight (M075) (mean daily intakes equivalent to 705 and 820 kj/kg M0·75) in a factorial design. In the 1st year 64 heifers (eight per treatment) which had calved as a consequence of first or second service were selected for the experiment. In the 2nd year 40 of these animals (five per treatment) which held to first or second service remained on experiment. Animals were housed all year round and given diets designed to represent energy intakes while grazing during the summer and conserved feeding during the winter. Pregnancy was established in late July of each year using a combination of oestrous synchronization and artificial insemination. Blood samples were collected at monthly intervals from weaning in November until calving in May; three times weekly from calving until oestrous synchronization (11 weeks later); and fortnightly thereafter until weaning.

For the annual levels of dietary energy intake offered, two breeds (Welsh Black and Charolais) exhibited relatively high growth rates and had the longest post-partum anovulatory periods; and Charolais cows also had the poorest conception rates of all breeds. Relatively lean cows at calving (body condition score < 2·5 units) were sensitive, in terms of the duration of the anovulatory period, to live-weight loss during the early post-partum period, particularly when blood glucose levels were low, whereas relatively fat cows at calving (body condition score > 2·5 units) were not. The data suggest: (i) that mechanisms controlling the anabolic processes governing maternal growth are antagonistic towards those that control reproduction; and (ii) the catabolism of lean tissue rather than fat tissue during the early post-partum period is also antagonistic towards the mechanisms that govern reproductive function in cows.

Influência da presença do bezerro no momento da ordenha sobre o desempenho reprodutivo de vacas mestiças Holandês-Zebu

Avaliou-se a influência da presença do bezerro no momento da ordenha sobre os parâmetros reprodutivos de 75 vacas primíparas e multíparas mestiças Holandês-Zebu, divididas em três grupos experimentais: GI - vacas ordenhadas sem a presença do bezerro; GII - vacas ordenhadas com a presença momentânea dos bezerros, sendo estes retirados da sala de ordenha logo após a descida do leite; e GIII - vacas ordenhadas com a presença constante dos bezerros, sendo estes amarrados ao lado da vaca durante toda a ordenha. O escore médio da condição corporal ao parto foi de 3,1. O período de serviço, dias para o retorno ao cio e as taxas de manifestação de cio e gestação até os 120 dias pós-parto não foram influenciados (P>0,05) pelo tipo de manejo de ordenha.

Effects of nutrition and metabolic status on circulating hormones and ovarian follicle development in cattle

Nutrition is a major factor affecting cow reproductive efficiency. Long-term moderate or chronic dietary restriction results in a gradual reduction in dominant follicle (DF) growth rate, maximum diameter and persistence. Animals become anoestrus when they lose on average 22-24% of their initial body weight. There is evidence of significant animal-to-animal variation in the interval from the imposition of dietary restriction to onset of anoestrus and from the recommencement of re-alimentation to resumption of ovulation. In contrast, acute dietary restriction to 40% of maintenance requirements rapidly reduces dominant follicle growth rate and maximum diameter and induces anoestrus in a high proportion (60%) of heifers within 13-15 days of dietary restriction. In lactating dairy and beef cows negative energy balance or reduced dietary intake in the early post-partum period, while not affecting the population of small-to-medium size follicles, adversely affects the size and ovulatory fate of the dominant follicle. Re-alimentation of nutritionally induced anoestrous heifers results in an initial gradual increase in dominant follicle growth rate and maximum diameter, followed by a more accelerated increase in dominant follicle growth rate and maximum diameter as the time of resumption of ovulation approaches. Increased dominant follicle growth rate and maximum diameter are associated with increased peripheral concentrations of IGF-I, pulsatile LH and oestradiol. Direct nutritional effects on ovarian function appear to operate through hepatic rather than follicular regulation of IGF-I, and on systemic concentrations of IGF-I BPs and insulin; cumulatively reducing follicular responsiveness to LH and ultimately shutting down follicular oestradiol production. Indirect nutritional effects are apparently mediated through altering the GnRH pulse generator and in-turn selectively reducing pulsatile LH secretion without any apparent adverse effect on FSH secretory patterns. Endogenous opioid peptides, NPY and glucose appear to play a role in the nutritional regulation of GnRH release and in turn pulsatile LH secretion.

Mechanisms linking under-nutrition and ovarian function in beef heifers

Prolonged reduction in energy intake in beef heifers has been reported to suppress ovulation but the mechanisms involved are poorly understood. The objective of this study was to examine whether changes in the pattern of LH secretion following each of three different tests predicted the functional state of the hypothalamo-pituitary-ovarian (H-P-O) axis. Test 1 examined the ratio of LH secretion during the 1h before and 2h after naloxone (NAL) administration. The other two tests assessed the LH surge following an exogenous oestradiol positive feedback signal (Test 2) or exogenous progesterone priming (Test 3). In phases 1 and 3, each of 8 weeks duration, the heifers were fed 100% of their maintenance energy requirements. In phase 2, of 9 weeks duration, they were fed 50% of their maintenance energy requirements. Oestrus was induced in all heifers by PG administration at the start of the experiment. Heifers were administered a naloxone challenge of 50, 100, 200 or 400mg naloxone hydrochloride i.v. (one dose per heifer) during the mid-luteal period of phase 1 and all four naloxone treated heifers received 400mg naloxone hydrochloride at the end of phases 2 and 3. Doses of 10, 20 or 40 mg oestradiol benzoate (EB) i.m. were each administered to two of the remaining heifers during the mid-luteal period of phase 1. One heifer on each dose of oestradiol benzoate in phase 1 had the same dose administered at the end of phases 2 and 3. The progesterone challenge was administered to three heifers by insertion of a PRID for 12 days starting in the middle of phase 2. In Test 1, the ratio of LH secretion before and after naloxone administration in phase 1 was 1:1 (50mg), 1:4 (100mg), 1:4 (200mg) and 1:9 (400mg) (50mg versus 100mg and 100mg versus 200mg doses, P<0.05); 50mg versus 400mg doses, P<0.001). In phase 2, this ratio was 1:1 and there was no response to 400mg dose of naloxone in any of the four heifers. In phase 3, the ratio depended on the ovarian activity in the heifer and ranged from 1:1 to 1:4 (P<0.05). In Test 3, a positive oestradiol feedback signal was detected in cyclic heifers in phases 1-3 but not in the acyclic heifer in phase 2. Heifers challenge with exogenous progesterone did not have oestradiol or LH values above threshold levels. We conclude that all three tests successfully predicted the functional state of the hypothalamo-pituitary-ovarian axis. In nutritionally undernourished beef heifers onset of ovarian acyclicity is either preceded or accompanied by the loss of a positive feedback signal (Test 2) and progesterone priming ability (Test 3), and that a plasma LH ratio of > or =1:2 following naloxone challenge (Test 1) is a sign of recovery of the functional state of the hypothalamo-pituitary-ovarian axis.

Reduction of luteinzing hormone secretion induced by long-term feed restriction in male rats is associated with increased expression of GABA-synthesizing enzymes without alterations of GnRH gene expression

In rats, fasting or restriction of feed intake impairs the activity of the hypothalamic gonadotropin-releasing hormone (GnRH) pulse generator which results in reduced luteinizing hormone (LH) secretion. It is still unknown which neurotransmitters are involved in this phenomenon. However, it is known that increased GABA concentrations in the hypothalamus reduce GnRH biosynthesis and release. Therefore, we examined whether 17 days of feed restriction in male rats affected the hypothalamic gene expression of GnRH and the GABA-synthesizing enzymes glutaminase (GLS) and glutamic acid decarboxylase-which exists in two forms, GAD67 and GAD65-in the mammalian brain. Furthermore, the expression of the GnRH receptor (GnRH-R) and the GABA transporter 1 (GAT-1) were investigated. Feed restriction resulted in a 75% reduction in body weight (b.w.) compared to rats fed ad libitum. Serum concentrations of LH and testosterone in the feed restricted group were significantly reduced to approximately 15% of that of rats fed ad libitum, while the FSH concentration remained unchanged. In the mediobasal hypothalamus (MBH) where GnRH is released into the portal vessels, mRNA levels of GAD67 and GLS were increased twofold compared to rats fed ad libitum while no changes were observed in the preoptic area of the hypothalamus (POA) where GnRH is biosynthesised. Neither the expression of preoptic GnRH mRNA nor the expression of GAD65 and of GnRH-R mRNA in both hypothalamic structures was affected by feed restriction. In the anterior pituitary, a significant reduction of the expression of GnRH-R, LH-beta and the alpha subunit was observed in the feed restricted rats, whereas FSH-beta mRNA levels remained constant. Thus, feed restriction selectively increased the expression of GABA-synthesizing enzymes in the MBH but did not modify GnRH expression in the POA. However, the reduced expression of the LH-beta- and alpha-subunit and of the GnRH-R in the anterior pituitary indicates that pulsatile GnRH release may have been attenuated or even abolished. We suggest, that enhanced expression of GABA-synthesizing enzymes reflects increased GABAergic neurotransmission and thereby reducing GnRH release from the MBH.