Calving traits, milk production, body condition, fertility, and survival of Holstein-Friesian and Norwegian Red dairy cattle on commercial dairy farms over 5 lactations.

The objective of this study was to compare calving traits, BCS, milk production, fertility, and survival of Holstein-Friesian (HF) and Norwegian Red (NR) dairy cattle in moderate-concentrate input systems. The experiment was conducted on 19 commercial Northern Ireland dairy farms, and involved 221 HF cows and 221 NR cows. Cows completed 5 lactations during the experiment, unless they died or were culled or sold. Norwegian Red cows had a lower calving difficulty score than HF cows when calving for the first and second time, but not for the third and fourth time. At first calving, the incidence of stillbirths for NR cows was 4%, compared with 13% for HF cows, whereas no difference existed between breeds in the proportion of calves born alive when calving for the second time. When calving for the first time, NR cows had a poorer milking temperament than HF cows, whereas milking temperament was unaffected by breed following the second calving. Holstein-Friesian cows had a higher full-lactation milk yield than NR cows, whereas NR cows produced milk with a higher milk fat and protein content. Full-lactation fat + protein yield was unaffected by genotype. Norwegian Red cows had a lower somatic cell score than HF cows during all lactations. Although NR cattle had a higher BCS than the HF cows during lactations 1 and 2, no evidence existed that the 2 genotypes either lost or gained body condition at different rates. Conception rates to first artificial insemination were higher with the NR cows during lactations 1 to 4 (57.8 vs. 40.9%, respectively), with 28.5% of HF cows and 11.8% of NR cows culled as infertile before lactation 6. A greater percentage of NR cows calved for a sixth time compared with HF cows (27.2 vs. 16.3%, respectively). In general, NR cows outperformed HF cows in traits that have been historically included in the NR breeding program.

The effect of level of crude protein and available lysine on finishing pig performance, nitrogen balance and nutrient digestibility.

Two trials were conducted to investigate the effect of decreasing the crude protein (CP) content of diets for finishing pigs containing two levels of available lysine on nutrient digestibility, nitrogen (N) balance and production performance. Ten finishing diets containing five levels of CP (on average 144, 155, 168, 182 and 193 g/kg fresh basis) and two levels of available lysine (6.9 and 8.2 g/kg fresh basis) were formulated. The diets were offered to pigs on a performance trial (n = 800 Large White (LW)×Landrace (LR) pigs) from 10 wk of age until finish at 21 wks+5 d of age. Average daily gain (ADG), average daily feed intake (ADFI) and feed conversion ratio (FCR) were calculated. In addition, a digestibility/N balance trial was conducted using pigs (n = 80 LW×LR) housed in metabolism crates. Digestibility of dry matter (DM), CP, oil, fibre and energy was determined. N balance values were determined through analysis of N content of urine and faeces ('as determined'). N balance values were also calculated using ADG values and assuming that 16% of growth is protein deposition ("as calculated"). Pig performance was poor between 10 and 13 wk of age which indicated that the dietary treatments were nutritionally inadequate for pigs less than 40 kg. There was a significant (p<0.01) quadratic effect of increasing CP level on feed intake, ADG and FCR from 10 to 13 wk which indicated that the lower CP levels did not supply adequate levels of essential or non-essential amino acids. There was no effect of increasing available lysine level throughout the early period, which in conjunction with the response in older pigs, suggested that both 8.2 and 6.9 g/kg available lysine were insufficient to drive optimum growth. There was a positive response (p<0.05) to increasing available lysine level from 13 wk to finish which indicated that 6.9 g/kg available lysine was not adequate for finishing pigs. Energy digestibility decreased with decreasing CP level of diets containing 6.9 g/kg available lysine which may be attributed to the higher fibre content of the lower CP diets. Nitrogen excretion (g/d) was lowered when dietary CP was reduced regardless of whether the values were determined through balance or calculated using ADG. Calculated N excretion decreased linearly (p<0.001) and quadratically (p<0.001) with decreasing dietary CP content. When the N balance figures calculated in this study were compared with those quoted in the Northern Ireland and English Nitrates Directive Action Programmes, N excretion was less per pig (wean to finish) offered a 169 g/kg CP, 8.2 g/kg available lysine diet (2.39 kg vs 3.41 kg (Northern Ireland) and 2.93 kg (England)).

Investigation of growth rate variation between commercial pig herds.

The study was designed to provide quantifiable information on both within- and between-herd variation in pig growth rate from birth to slaughter and to examine how this was influenced by moving pigs at a common age to a common environment. Five litters were selected from each of eight pig herds in Northern Ireland with varying growth performance. All eight herds were offered the same nutritional regime. Five pigs (three boars and two gilts) were selected from each litter. In each herd, 22 pigs (12 boars and 10 gilts) were weighed individually, every 4 weeks, from 4 to 20 weeks of age. At 4 weeks of age (weaning) three non-sibling boars were taken from each herd and brought to a common environment where they received medication, were housed individually from 6 weeks of age and offered the same dietary regime. They were weighed and feed intakes were recorded twice weekly. A growth rate difference of 61 g/day (P < 0.001), 112 g/day (P < 0.01) and 170 g/day (P < 0.001) was observed on farm, between the top and bottom quartile of herds during 4 to 8, 8 to 12 and 12 to 20 weeks of age, respectively. This difference in growth rate equated to an average difference in cost of production of ¢13/kg carcass on a birth to bacon unit. When pigs from the different herds were housed in the common environment, large variation in growth performance (143 g/day (P < 0.01) and 243 g/day (P < 0.001) for 8 to 12 and 12 to 20 weeks, respectively) was also observed between the top and bottom quartile of herds. Although feed efficiency was similar, a significant feed intake difference of 329 g/day (P < 0.01) and 655 g/day (P < 0.001) between 8 to 12 and 12 to 20 weeks of age was observed. The variation in growth rate between pigs whether managed on farm or in the common environment was similar (variation in days to 100 kg on farm and in the common environment was 18 and 19 days, respectively). When housed in the common environment, although the top and bottom quartile of pigs converted feed equally efficiently, pigs in the top quartile had significantly higher feed intakes suggesting greater appetites. It is difficult to assess the extent to which these differences can be attributed to genetic effects or pre-weaning environment, and how much the effects of management, disease or genetics contributed to the variation between and within herds.

Short communication: Effects of feeding level on energy concentration in grass silage-based diets offered to dairy cattle.

Twelve grass silages were offered to sheep as a sole diet at maintenance and to lactating dairy cows ad libitum as mixed silage and concentrates diets (n = 13 diets). Fecal and urinary energy outputs were measured for silages and mixed diets. Digestible energy (DE) and metabolizable energy (ME) concentrations for mixed diets with sheep at maintenance were estimated based on the silage dry matter (DM) proportion obtained in the cattle trials, the silage energy utilization values (methane energy-predicted) determined using sheep, and tabulated concentrate values. A comparison of dietary mean data (n = 13) indicated that concentrations of ME (P < 0.01) and DE (P < 0.001) in mixed diets were significantly lower for cows at production feeding level than for sheep at maintenance. The reductions were proportionately 0.015 and 0.020 with each unit increase in feeding level above maintenance, respectively. These ME and DE data were also used to evaluate the feeding level correction factors previously proposed by Van Es (1975) (ME, 0.018) and Yan et al. (2002) (ME, 0.016; DE, 0.025) using the mean square prediction error technique. The ME correction factor proposed by Yan et al. (2002) had a greater prediction accuracy than that proposed by Van Es (1975) for the prediction of ME concentration in mixed diets offered to dairy cattle at production feeding level.

Evaluation of different energy feeding systems with production data from lactating dairy cows offered grass silage-based diets.

A set of data from 838 lactating dairy cows, drawn from 12 long-term feeding studies (at least 8 wk/period), was used to evaluate the energy feeding systems for dairy cows currently adopted in Australia, France, The Netherlands, the United Kingdom, and the United States. The animals were offered mixed diets of concentrates, forage [grass silages (n = 33) and corn silages (n = 5)] ad libitum. Data used in the present evaluation were either measured [dry matter (DM) intake, milk production and live weight], measured/estimated [dietary metabolizable energy (ME) concentration] or estimated [milk energy output and live weight change (LWC)]. The mean-square prediction error (MSPE) was used for the evaluation. Total ME intake, milk yields, and LWC varied from 91 to 338 MJ/d, 7.7 to 48.9, and -1.23 to 1.73 kg/d, respectively. Australian and French systems predicted total energy requirement and milk yield relatively well, while British, Dutch and American systems underpredicted total energy requirement by proportionately 0.06, 0.04, and 0.03, respectively; and overpredicted milk yield by 0.09, 0.06, and 0.04. The Agricultural and Food Research Council (AFRC) each produced a relatively larger error of the bias (predicted - actual data) over the total MSPE for ME requirement and milk yield and a relatively smaller error of random than other systems. However, an addition of proportionately 0.05 to the total predicted ME requirement of AFRC, as suggested in this system and currently used in the UK, indicated the prediction accuracy of ME requirement and milk yield is similar to Australian and French systems. Nevertheless, all the systems had a poor prediction of LWC. For each system, the total prediction error (total MSPE) was mainly derived from the line (slope; 0.49 to 0.64 of total MSPE), while less derived from the random (0.20 to 0.48 of total MSPE), indicating a large variation between the predicted and actual LWC existed among individual cows. The residual plots of the residual differences in LWC against predicted LWC revealed that the prediction error was greater with increasing LWC. It is concluded that Australian and French systems have a better prediction of total energy requirement and milk yield than other systems, and LWC is an inappropriate indicator of energy balance in lactating dairy cows.

Fertility of dairy cows in Northern Ireland.

A comprehensive database was established on the milk production and reproductive performance of dairy cows in 19 selected herds in Northern Ireland, varying in size, management system and genetic merit. Data were obtained for 2471 cows, 1775 of which calved in a second year, and 693 were culled from the herd for specific reasons. The estimated mean rate of heat detection (assessed by the interheat interval during the main breeding season) in all the herds was 71 per cent, with a range from 53 to 92 per cent The average conception rate to first insemination was 37.1 per cent (range 21 to 66 per cent). The average calving interval for the retained cows was 407.2 days (range 359 to 448 days). Twenty-eight per cent of the cows that calved were culled, with infertility being the largest single reason (26.8 per cent of the cows culled). There were major differences in reproductive performance between the herds, but heat detection rate, conception rate and calving interval did not appear to be affeded by a herd's genetic merit. The herds with shorter calving intervals were characterised by better heat detection efficiency (83 v 61 per cent, P<0.01), a shorter interval from calving to first insemination (74 v 97 days; P<0.05), a higher conception rate to first insemination (45 v 34 per cent, P>0.10) and a lower removal rate (23 v 37 per cent, P<0.01). Furthermore, the cows in these herds had lower body condition scores (BCS) in the dry period (3.0 v 3.3; P<0.05) but lost less body condition in early lactation (0.3 v 0.6 BCS units, P<0.05). These results show that dairy herd fertility in Northern Ireland is generally low and similar to that previously reported for England and the USA, but that in some herds changes in herd management practices improved the cows' fertility.

The effects of milk yield and stage of lactation on the partitioning of nutrients in lactating dairy cows.

The objective of the experiment was to examine, using indirect calorimetry, the effects of milk yield and stage of lactation on the response in milk and body tissue energy, and heat production, to a reduction (decrement) in nutrient intake (assessed as metabolizable energy intake). Eight lactating dairy cows, four representing each of two stages of lactation [either mean initial days in milk (DIM) 158 (SD 6.1) or 414 (SD 51.1)] were used. Each cow underwent four 17-d periods incorporating two physiological states [number of mammary glands milked: either four (periods 1 and 2), or two (periods 3 and 4)], and two levels of metabolizable energy intake within each physiological state [either sufficient to meet requirements for zero tissue balance plus 10 MJ/d (periods 1 and 3)] or these allowances reduced by 20 MJ/d in the subsequent period (periods 2 and 4, respectively). Partitioning was calculated from the changes in metabolizable energy intake, milk energy, tissue energy, and heat production between DIM groups and between four and two gland milking (milk yield) components of the study. Partitioning of the changes in metabolizable energy intake was not influenced by DIM, but milk yield response was greater in the early lactation cows compared with the late group. Cows milked in four glands (higher milk yield) partitioned a significantly greater proportion of decremental changes in metabolizable energy intake to milk energy and less to tissue energy, than when milked in only two glands (lower milk yield).

The influence of body condition on the fasting energy metabolism of nonpregnant, nonlactating dairy cows.

The objective of this experiment was to investigate the effect of cow body condition score on fasting heat production. Twelve nonpregnant, nonlactating Holstein-Friesian cows were selected from within the dairy herd at the Agricultural Research Institute of Northern Ireland. Six of these animals (group A) had condition scores > or = 4.5, and the remainder (group B) had condition scores <2. All cows were offered dried grass pellets at estimated maintenance energy level (0.58 MJ of metabolizable energy/kg(0.75)) for a minimum of 21 d. The diet also supplied 2.5 times the metabolizable protein requirement for maintenance. Following this, each cow underwent a 5-d fast in open circuit respiration calorimeters during which fasting heat production (FHP) was measured. On completion of measurement, group A was fed to reduce condition score (CS) below 2, while group B was fed to raise each individual condition score above 4.5. When the appropriate condition scores were achieved, dried grass pellets were again offered at maintenance for a minimum of 21 d, and fasting heat production was measured. It was observed that fasting heat production (MJ/kg(0.75)) was significantly higher for cows with low body condition (<2; ultrasonic fat depth < or = 2.9 mm) compared with cows displaying high body condition (> or = 4.5; ultrasonic fat depth > or = 8.2 mm). A linear relationship between condition score and fasting heat production (MJ/kg(0.75)) was defined by regression analysis as; FHP (MJ/kg(0.75)) = 0.501(SE 0.0121) - 0.030CS (SE 0.0035).

Effect of nucleus tractus solitarius lesions on fever produced by interleukin-1beta.

Partial elimination of vagal sensory afferents by subdiaphragmatic vagal section has variously been reported to eliminate, to reduce, or to have no effect on fever produced by peripheral lipopolysaccharide and interleukin-1beta (IL-1beta). However, to adequately test the idea that vagal afferents convey immune information to the brain, all vagal input to the central nervous system must be eliminated. This was accomplished by bilateral electrolytic lesions of the nucleus tractus solitarius (NTS). Reflex bradycardia evoked by intravenous phenylbiguanide was eliminated in NTS-lesioned rats, verifying the lesion's effectiveness. IL-1beta (2 microg/kg) was given to conscious, unrestrained rats via an indwelling intraperitoneal catheter and produced rapid fever (approximately 1 degree C) with an onset latency of 15 min and peak response at 30 min, with a second, smaller peak at 130 min. NTS lesions attenuated the first fever peak, with a lesser, non-significant effect on the second peak. The thermogenic capacity of NTS-lesioned rats was evaluated using 3 different strategies: (1) thermogenesis evoked by CNS injections of prostaglandin E2, (2) 3 h exposure to a 4 degrees C environment, and (3) heat production of intrascapular brown fat produced by intravenous infusion of the beta3-adrenergic agonist BRL 37344. NTS-lesioned rats were equivalent, or even superior to control animals in their thermogenic response to these non-immune-related stimuli. Therefore, the impaired febrile response of NTS-lesioned rats to IL-1beta cannot be attributed to reduced thermogenic capacity. Finally, these results suggest that fever elicited by intraperitoneal IL-1beta is, at least in part, dependent on the integrity of NTS neurons, but also that mechanisms independent of vagal afferent projections to the NTS must also play a role in immune-to-brain signaling.

Dietary salt intake alters cardiovascular responses evoked from the rostral ventrolateral medulla.

The present experiments examined whether in rats consuming diets with either high NaCl content (8%) or low Na+ content (0.01%) for 2 wk excitatory inputs to the rostral ventrolateral medulla (RVLM) would be altered. In chloralose-anesthetized rats, injection of glutamate into the RVLM elicited a pressor response that, compared with rats fed a control diet, was 50% larger in rats fed a diet containing 8% NaCl and was 25% smaller in rats fed a diet containing 0.01% Na+. Pressor responses produced by electrical stimulation of sciatic nerve afferents, as well as by microinjections into the RVLM of L-dihydroxyphenylalanine or carbachol, were all potentiated by high dietary salt intake and reduced by low dietary salt intake. Dietary salt intake had no effect on pressor responses produced by intravenous injection of phenylephrine, indicating that salt-related alterations in cardiovascular responses produced by central activation could not be accounted for by changes in peripheral vascular reactivity. The decrease in arterial pressure produced by injection of glutamate into the nucleus of the solitary tract was also potentiated by the high salt diet, suggesting that the sensitivity of central baroreceptor reflex pathways may be altered by dietary NaCl. These results indicate that the amount of NaCl consumed in the diet can change the sensitivity of RVLM sympathoexcitatory neurons, and this change in sensitivity is not restricted to any particular class of cell surface receptors.

Naltrindole, a selective delta-opioid receptor antagonist, potentiates the lethal effects of cocaine by a central mechanism of action.

The potentiation of the toxic and lethal effects of cocaine by the selective delta-opioid receptor antagonist naltrindole was explored in unrestrained, unanesthetized rats that received a continuous intravenous infusion of cocaine until death. The lethal dose of cocaine was lowered dose dependently in animals administered naltrindole intracisternally (3.0-30 microg), but not intravenously (30-300 microg). There was also a decrease in the lethal dose of cocaine following an injection of the nonselective opioid antagonist naltrexone, but not naloxone. However, the seizure-producing dose of cocaine was decreased dose dependently in rats that received naltrindole, regardless of the route of administration, naloxone, or naltrexone. In contrast, the effect of cocaine on heart rate was altered only by centrally administered naltrindole or intravenous naltrexone, with a dose of 30 microg naltrindole and 10 mg/kg naltrexone abolishing the bradycardic effect of cocaine. Despite this, neither naltrindole nor naltrexone changed the hypertensive effect of cocaine. Higher doses of naltrindole (100 microg i.c.) produced significant increases in heart rate and mean arterial pressure and were not tested in combination with cocaine. Because the lethal dose of cocaine was reduced only when naltrindole was administered intracisternally, the potentiation of the lethal effects of cocaine by naltrindole is through a central mechanism of action that may involve changes in cardiovascular function.

Anatomical evidence for a bi-neuronal pathway connecting the nucleus tractus solitarius to caudal ventrolateral medulla to rostral ventrolateral medulla in the rat.

Using tract tracing techniques and dual-color histochemistry, this study investigated connections between the nucleus tractus solitarius (NTS), caudal ventrolateral medulla (CVLM), and rostral ventrolateral medulla (RVLM) in the rat. The anterograde tracer biotinylated dextran amine (BDA) was deposited into the NTS. The retrograde tracer Fluoro-Gold was deposited into the RVLM. In the CVLM, Fluoro-Gold-labeled cells were found intermingled with anterogradely labeled fibers coursing from the NTS. BDA-labeled axons, varicosities, and boutons were observed in close apposition to retrogradely labeled neurons in the CVLM. These data provide anatomical evidence for a bi-neuronal pathway from NTS to CVLM to RVLM; and support the hypothesis that these connections may comprise the medullary baroreceptor reflex pathway.

Effect of forage digestibility and type of concentrate on nutrient utilization by lactating dairy cattle.

Six lactating dairy cows were used in a three period, part balanced changeover design experiment to investigate the effects of forage digestibility and concentrate composition on the efficiency of nutrient utilization in lactating dairy cows. Six treatments comprising three forage regimens and two concentrate types (starch v. fibre) were examined in a 3 x 2 factorial design. The three forage regimens were high digestibility grass silage offered ad lib. (HA) or restricted to 6.5 kg dry matter/d (HR) and a low digestibility grass silage offered ad lib. (LA). Within each forage regimen animals were offered 10 kg/d of supplements containing either high-starch or high-fibre concentrations. Experimental periods lasted 28 d with a 10 d recording period, during which animal performance, ration digestibility and nitrogen and energy utilization were measured. Respiratory exchange measurements were made over a 72 h period using indirect open-circuit calorimetry. Throughout the experiment, there were no significant forage x concentrate interactions in any of the intake, production or nutrient utilization results. Milk yield was significantly influenced by forage regimen (24.1, 21.7 and 21.9 kg/d for HA, HR and LA respectively) and concentrate type (21.6 and 23.5 kg/d for high-starch and high-fibre respectively). Concentrate type also significantly influenced milk protein concentration (32.8 and 30.9 g/kg for high-starch and high-fibre respectively). Forage regimen significantly influenced the efficiency of utilization of metabolizable energy (ME) for milk production (k1) with values of 0.62, 0.64 and 0.59 for HA, HR and LA respectively. Concentrate type had no significant effect on ME intake, heat production or k1, although animals receiving the high-fibre concentrates synthesized proportionately 0.11 more milk energy per unit of available energy (ME intake--heat production) than those receiving the high-starch concentrates. Interpolation of the values obtained with the two high digestibility forage regimens indicated that at similar ME intakes there was a trend towards a higher k1 with the diet based on high digestibility silage, and this was in line with the higher metabolizability of the overall diet with this silage.

Excitatory amino acid receptors in central cardiovascular regulation.

Excitatory amino acid neurotransmitters and their receptors are the principal mediators of fast synaptic transmission within the central nervous system. Accumulating evidence suggests that synaptic activation of excitatory amino acid receptors in the nucleus of the tractus solitarius, rostral and caudal ventrolateral medulla, and in the spinal cord play a key role in neural transmission of cardiovascular information in the central nervous system. Pharmacological blockade of excitatory amino acid receptors at these sites eliminates a variety of centrally-mediated cardiovascular responses. These include baroreceptor reflexes and increases in arterial pressure produced by stimulation of various brain regions as well as peripheral afferent nerves. These observations indicate that synaptic activation of EAA receptors at specific sites within the brainstem and in the spinal cord play an important role in central cardiovascular regulation.

Role of rostral ventrolateral medulla in centrally mediated pressor responses.

The region of the rostral ventrolateral medulla (RVLM) plays an important role in central nervous system regulation of cardiovascular function. The initial purpose of these studies was to determine whether synaptic activation of excitatory amino acid (EAA) receptors in the RVLM might mediate central pressor responses. Blockade of EAA receptors in the RVLM with kynurenic acid abolished pressor responses evoked by stimulation of sciatic nerve afferents but had no effect on increases in arterial pressure produced by stimulation of hypothalamic sites. To determine whether synaptic transmission in the RVLM, independent of EAA receptor activation, was a prerequisite for the production of hypothalamic pressor responses, axonal conduction and/or synaptic transmission were pharmacologically interrupted in the RVLM. Blockade of synaptic transmission with muscimol or kainic acid attenuated, but did not eliminate, hypothalamic pressor responses. Concurrent blockade of synaptic and axonal transmission in the RVLM with lidocaine produced the greatest reduction of hypothalamic pressor responses. Collectively, these results suggest that central pressor responses are not uniformly mediated by synaptic activation of neurons within the RVLM. Instead, a combination of synaptic transmission and axonal conduction within and possibly outside the region of the RVLM may be required for the production of many centrally mediated pressor responses.

A simple method to improve the reliability of iontophoretic administration of tracer substances.

Microiontophoresis is a widely used technique for depositing tracer materials into the central nervous system for neuroanatomical experiments. However, the reliability of iontophoretic injection is often less than optimal. Coating the lumen of glass micropipettes with silicone to reduce surface tension reduced the incidence of failure for iontophoretic deposition of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) from 40% to zero. Similar results were observed with other tracer substances. Siliconizing the internal surface of glass micropipettes can significantly improve the reliability of iontophoretic deposition.

Increased dietary salt sensitizes vasomotor neurons of the rostral ventrolateral medulla.

Excess dietary sodium is a major contributing factor to the incidence and severity of hypertension. However, the precise mechanism or mechanisms by which salt contributes to the severity of hypertension are unknown. The region of the rostral ventrolateral medulla (RVLM) is a principal brain stem locus critical for the regulation of arterial blood pressure by the sympathetic nervous system. The purpose of this study was to determine if excess dietary sodium chloride might alter the function or responsiveness of neurons in the RVLM. Male Sprague-Dawley rats were given either tap water or 0.9% sodium chloride solution to drink for 10 to 14 days. Excess sodium chloride did not affect baseline blood pressure. However, when neurons of the RVLM were stimulated by microinjections of L-glutamate, evoked increases in arterial pressure were potentiated in rats given sodium chloride. Augmented pressor responses could not be accounted for by increased vascular reactivity because both groups responded similarly to intravenously administered phenylephrine and norepinephrine. Additionally, electrical stimulation of descending spinal sympathoexcitatory axons produced identical pressor responses in both groups, indicating that altered synaptic transmission at central or peripheral neuroeffector junctions distal to the RVLM could not explain enhanced pressor responses produced by direct stimulation of RVLM cell somata. Finally, impaired arterial baroreceptor reflexes could not account for augmented RVLM pressor responses, as depressor and bradycardic responses produced by electrical stimulation of aortic baroreceptor afferents were not reduced in rats given excess dietary sodium chloride.(ABSTRACT TRUNCATED AT 250 WORDS)

Non-NMDA receptors in the rostral ventrolateral medulla mediate somatosympathetic pressor responses.

The role of excitatory amino acid receptors in the rostral ventrolateral medulla (RVLM) in mediating a somatosympathetic pressor response (SPR) was studied. Rats were anesthetized with urethane, bilaterally vagotomized, paralyzed and respirated. Increases in mean arterial pressure were evoked by 10-s trains of electrical stimulation of sciatic nerve afferents before and after bilateral microinjections into the RVLM of the N-methyl-D-aspartic acid (NMDA) receptor antagonist D-2-amino-7-phosphono-heptanoic acid (D-AP7) or the non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX). DNQX reversed or markedly attenuated the SPR. In contrast, the SPR was not significantly altered by blockade of NMDA receptors in the RVLM with D-AP7. However, prior administration of D-AP7 prevented reversal of the SPR by DNQX, while administration of D-AP7 after DNQX partially restored the SPR. These results indicate that pressor responses evoked by electrical stimulation of sciatic nerve afferents require synaptic activation of non-NMDA receptors in the RVLM. A somatic depressor response, revealed after blockade of non-NMDA receptors within the RVLM, may be mediated by activation of NMDA receptors in this region of the brainstem.

Sympathoexcitation from the rostral ventrolateral medulla is mediated by spinal NMDA receptors.

These studies examined the role of spinal N-methyl-D-aspartic acid (NMDA) receptors in mediating sympathoexcitation evoked by stimulation of neurons in the rostral ventrolateral medulla (RVLM). In urethane-anesthetized rats, blood pressure, heart rate, and splanchnic sympathetic nerve activity (SNA) were recorded. The NMDA receptor antagonist D-2-amino-7-phosphonoheptanoic acid (D-AP7) was administered to the spinal cord via intrathecal (IT) catheter. Blockade of spinal NMDA receptors reduced arterial blood pressure, heart rate, and SNA. Spinal administration of D-AP7 markedly attenuated the pressor and sympathoexcitatory responses evoked by L-glutamate stimulation of the RVLM. The small increases in heart rate evoked by stimulation of the RVLM were not affected by IT administration of D-AP7. These results indicate that NMDA receptors in the spinal cord mediate the pressor and sympathoexcitatory responses evoked by activation of a bulbospinal pathway originating from the RVLM. Moreover, these data suggest that excitatory amino acid neurotransmitters and NMDA receptors in the spinal cord play an important role in the maintenance and regulation of SNA and cardiovascular function.

Evidence for a kynurenate-insensitive glutamate receptor in nucleus tractus solitarii.

Previous studies have shown that pharmacological blockade of ionotropic excitatory amino acid (EAA) receptors in the nucleus tractus solitarii (NTS) with kynurenate (Kyn) abolishes baroreceptor reflexes but fails to affect cardiovascular responses evoked by microinjections of L-glutamate (Glu) into the NTS. These observations have raised doubts as to whether Glu is a neurotransmitter of baroreceptor information in the NTS because the pharmacological actions of exogenously administered Glu are not identical to those of the neurotransmitter released in the NTS coincident with baroreceptor activation. One possible explanation for these results is that exogenously administered Glu might act at receptors that are not blocked by Kyn and are not accessible to synaptically released Glu in the NTS baroreflex pathway. The purpose of this study was to determine if Kyn-insensitive Glu receptors are present in the NTS. One candidate for this Kyn-insensitive receptor is the metabotropic EAA receptor that is selectively activated by trans-DL-1-amino-1,3-cyclopentane-dicarboxylic acid (ACPD). Microinjections of ACPD into the NTS of anesthetized rats produced dose-related depressor responses that were not reduced by Kyn or by pretreatment with the putative ACPD receptor antagonist L-2-amino-3-phosphonopropionate (L-AP-3). Similarly, depressor responses produced by Glu also were not affected by Kyn or by L-AP-3. These data demonstrate the presence of a Kyn-insensitive Glu receptor in the NTS. Moreover, they suggest that the failure of Kyn to reduce cardiovascular responses evoked by Glu injections into the NTS can be explained by an action of Glu at Kyn-insensitive ACPD receptors.(ABSTRACT TRUNCATED AT 250 WORDS)