Interaction central GABA-B receptors with serotoninergic brain system during regulation of the hypothalamo-hypophyseal-testicular axis by the negative feedback mechanism in rats

The findings suggest that the GABA inhibits testosterone compensation after hemicastration, due to activation of the mediobasal hypothalamus serotoninergic system in Wistar rats. The GABA seems also to exert an activating effect via the GABA-receptors upon the hypothalamo-pituitary-testicular feedback regulation.     

Evidence of a negative feedback system regulating the total beta-cell volume in nondiabetic rats that received pancreas transplants

BACKGROUND: The aim of the present study was to investigate the long-term regulation of pancreatic beta-cell volume after pancreas transplantation into adult rats. METHODS: A syngeneic pancreaticoduodenal transplantation was made in normoglycemic Wistar-Furth rats. By this means, the recipients doubled their pancreatic islet volume. Nine months after transplantation, the total beta-cell volume was measured in serial pancreatic sections immunostained for insulin from both the native and transplanted pancreata, and in the native pancreas of age-matched Wistar-Furth rats that did not receive transplants. RESULTS: No changes in the volume of individual beta-cells were seen. A 50% decrease in total beta-cell volume was observed in both the native and transplanted pancreas when compared with that of age-matched controls. However, the combined beta-cell volumes of the native and transplanted pancreas in the rats that received transplants were similar to those of the native pancreas in control animals. No signs of increased apoptosis in any of the glands could be seen during the first postoperative week or after 9 months. CONCLUSIONS: These findings provide evidence of a negative feedback system, which regulates the total beta-cell volume to the levels seen in age-matched rats that did not receive transplants. The underlying mechanisms for the decreased beta-cell volume are unknown, but may involve a diminished replicatory rate of the beta-cells.     

The NMDA and GABA-A receptors in behavioral activity of rats

The effects of bicuculline (0.25 mg/kg ip) and AP-7 (5 nmols icv) on the processes of retrieval, consolidation of conditioned reflexes, object recognition and locomotor activity were tested in rats. AP-7, bicuculline and AP-7 with bicuculline increased (but not significantly) locomotor and exploratory activity in the open field test. Only coadministration of AP-7 with bicuculline facilitated retrieval of passive avoidance in rats, but was without effect on consolidation in this test. AP-7 or bicuculline also did not influence on consolidation, when they were given alone. Object recognition was impaired (but not significantly) in groups of rats treated with bicuculline and bicuculline with AP-7.     

Seasonal changes in the negative feedback regulation of the secretion of the gonadotrophins by testosterone and inhibin in rams

Three experiments were conducted with castrated Romney Marsh rams (wethers) to investigate the ability of testosterone and inhibin to suppress the secretion of LH and FSH during the breeding and the non-breeding seasons. In Experiment 1, wethers (n=5/group) were treated every 12 h for 7 days with oil or 16 mg testosterone propionate (i.m.) and were then given two i.v. injections either of vehicle or of 0.64 microg/kg human recombinant inhibin A (hr-inhibin) 6 h apart. Blood samples were collected for 4 h before inhibin or vehicle treatment and for 6 h afterwards for the assay of LH and FSH. In Experiments 2 and 3 wethers underwent hypothalamo-pituitary disconnection (HPD) and were given 125 ng GnRH i.v. every 2 h. In Experiment 2, HPD wethers (n=3/group) were injected (i.m.) every 12 h with oil or testosterone and blood samples were collected over 9 h before treatment and 7 days after treatment. In Experiment 3, HPD (n=5/group) wethers were treated with vehicle or hr-inhibin, as in Experiment 1, after treatment with oil, or 4, 8 or 16 mg testosterone twice daily for 7 days. Blood samples were collected over 4 h before treatment with vehicle or hr-inhibin and for 6 h afterwards. Treatment of wethers with testosterone (Experiment 1) resulted in a significant decrease in the plasma concentrations of LH and number of LH pulses per hour but the magnitude of these reductions did not differ between seasons. Testosterone treatment had no effect on LH secretion in GnRH-pulsed HPD wethers in either season and treatment with hr-inhibin did not affect LH secretion in wethers or HPD wethers in any instance. Plasma concentrations of FSH were significantly (P<0.05) reduced following treatment with testosterone alone during the breeding season but not during the non-breeding season. FSH levels were reduced to a greater extent by treatment with hr-inhibin but this effect was not influenced by season. During the non-breeding season, the effect of hr-inhibin to suppress FSH secretion was enhanced in the presence of testosterone. These experiments demonstrate that the negative feedback actions of testosterone on the secretion of LH in this breed of rams occurs at the hypothalamic level and is not influenced by season. In contrast, both testosterone and inhibin act on the pituitary gland to suppress the secretion of FSH and these responses are affected by season. Testosterone and inhibin synergize at the pituitary to regulate FSH secretion during the non-breeding season but not during the breeding season.     

Lack of coactivator interaction can be a mechanism for dominant negative activity by mutant thyroid hormone receptors

We studied the interactions of two natural thyroid hormone receptor (TR) mutants from patients with resistance to thyroid hormone (RTH) and an artificial TR mutant with a nuclear receptor corepressor, N-CoR, and a steroid receptor coactivator, SRC-1. In electrophoretic mobility shift assays, wild-type TRbeta-1 interacted with N-CoR in the absence of ligand, whereas T3 caused dissociation of the TRbeta-1/N-CoR complex and formation of TRbeta-1/SRC-1 complex. In contrast, a natural mutant (G345R) with poor T3-binding affinity formed TRbeta-1/N-CoR complex, both in the absence and presence of T3, but could not form TRbeta-1/SRC-1 complex. Another TR mutant, which bound T3 with normal affinity and containing a mutation in the AF-2 region (E457D), had normal interactions with N-CoR but could not bind SRC-1. Both these mutants had strong dominant negative activity on wild-type TR transactivation. Studies with a TR mutant that had slightly decreased T3-binding affinity (R320H) showed a T3-dependent decrease in binding to N-CoR and increase in binding to SRC-1 that reflected its decreased ligand binding affinity. Additionally, when N-CoR and SRC-1 were added to these receptors at various T3 concentrations in electrophoretic mobility shift assays, TR/N-CoR and TR/SRC-1 complexes, but not intermediate complexes were observed, suggesting that N-CoR release is necessary before SRC-1 binding to TR. Our data provide new insight on the molecular mechanisms of dominant negative activity in RTH and suggest that the inability of mutant TRs to interact with coactivators such as SRC-1, which results from reduced T3-binding affinity, is a determinant of dominant negative activity.     

The participation of the serotoninergic system in regulating the activity of the central glutamatergic/aspartatergic and GABA-ergic synapses

Selective lesion of the serotoninergic system diminished the synaptic uptake of 3H-L-glutamic acid and 3H-DL-aspartic acid, as well as the Na+(-dependent) binding of 3H-L-glutamic acid in the cortex and the brain stem. The data obtained suggest an ability of the serotoninergic system to modify presynaptic processes in amino-acidergic neurons of the CNS.     

A negative feedback mechanism revealed by functional analysis of the alternative isoforms of the Drosophila splicing regulator transformer-2

The Drosophila sex determination gene transformer-2 (tra-2) is a splicing regulator that affects the sex-specific processing of several distinct pre-mRNAs. While the tra-2 gene itself is known to produce alternative mRNAs that together encode three different TRA-2 protein isoforms, the respective roles of these isoforms in affecting individual pre-mRNA targets has remained unclear. We have generated transgenic fly strains with mutations affecting specific TRA-2 isoforms to investigate their individual roles in regulating the alternative processing of doublesex, exuperantia and tra-2 pre-mRNA. Our results indicate that in somatic tissues two different isoforms function redundantly to direct female differentiation and female-specific doublesex pre-mRNA splicing. In the male germline, where tra-2 has an essential role in spermatogenesis, a single isoform was found to uniquely perform all necessary functions. This isoform appears to regulate its own synthesis during spermatogenesis through a negative feedback mechanism involving intron retention.     

Serum LH concentrations in hypogonadal men during transdermal testosterone replacement through scrotal skin: further evidence that ageing enhances testosterone negative feedback. The Testoderm Study Group

As a trial of the provision of community-service information to family carers, general practitioners and pharmacists from the Western region of Melbourne were supplied with tear-off pads listing suburb-specific community services, and were encouraged to discuss service needs with carers of people with disabilities and distribute the lists to carers accordingly. One hundred and nine general practitioners and 58 pharmacists participated in pretrial and post-trial interviews assessing their knowledge of community services and frequency of discussion with carers. Before the trial, general practitioners had significantly higher self-rated knowledge of community services and reportedly discussed these with carers more often than did pharmacists. After the five-month trial period, pharmacists showed a significant increase in self-rated knowledge and frequency of discussion. General practitioners' knowledge and discussion showed a nonsignificant increase. After the trial, the two did not differ in knowledge of services; however, general practitioners maintained a higher reported frequency of discussion about services with carers. General practitioners and pharmacists appear to be well placed to act as a service link for family carers. However, additional high-intensity strategies are needed to assist them in this role.     

The negative feedback actions of progesterone on gonadotropin-releasing hormone secretion are transduced by the classical progesterone receptor

Progesterone (P) powerfully inhibits gonadotropin-releasing hormone (GnRH) secretion in ewes, as in other species, but the neural mechanisms underlying this effect remain poorly understood. Using an estrogen (E)-free ovine model, we investigated the immediate GnRH and luteinizing hormone (LH) response to acute manipulations of circulating P concentrations and whether this response was mediated by the nuclear P receptor. Simultaneous hypophyseal portal and jugular blood samples were collected over 36 hr: 0-12 hr, in the presence of exogenous P (P treatment begun 8 days earlier); 12-24 hr, P implant removed; 24-36 hr, P implant reinserted. P removal caused a significant rapid increase in the GnRH pulse frequency, which was detectable within two pulses (175 min). P insertion suppressed the GnRH pulse frequency even faster: the effect detectable within one pulse (49 min). LH pulsatility was modulated identically. The next two experiments demonstrated that these effects of P are mediated by the nuclear P receptor since intracerebroventricularly infused P suppressed LH release but 3alpha-hydroxy-5alpha-pregnan-20-one, which operates through the type A gamma-aminobutyric acid receptor, was without effect and pretreatment with the P-receptor antagonist RU486 blocked the ability of P to inhibit LH. Our final study showed that P exerts its acute suppression of GnRH through an E-dependent system because the effects of P on LH secretion, lost after long-term E deprivation, are restored after 2 weeks of E treatment. Thus we demonstrate that P acutely inhibits GnRH through an E-dependent nuclear P-receptor system.     

Relevance of the tubuloglomerular feedback mechanism in pathophysiology

The balance between a high filtration rate and high reabsorption rate in the kidney is critical in the maintenance of extracellular fluid volume. One of the mechanisms that maintain this balance is the tubuloglomerular feedback (TGF) mechanism, which operates at the level of the macula densa assessing the load and/or solute concentration coming out of the loop of Henle and controlling this load by adjusting the GFR. This review discusses the potential role of the TGF system with respect to volume homeostasis in various conditions where GFR is maintained, decreased, or increased. In most of the states discussed, the TGF system seems to act appropriately regarding volume control; however, trade-off effects occasionally occur. After acetazolamide administration, during extracellular fluid volume contraction or expansion or acute hyperkalemia, the TGF mechanism responds appropriately with regard to volume balance. After a large reduction of renal mass, the system adjusts to function at a higher level of GFR and distal delivery. In chloride-depletion metabolic alkalosis, glomerulonephritis, diabetes mellitus, and acute renal failure, the adaptation of the TGF system appears to be appropriate with regard to volume control; however, it may lead to trade-off effects, such as maintenance of metabolic alkalosis, glomerular hypertension and sclerosis, or depression of GFR, respectively. Because the TGF mechanism often contributes to compensatory adjustments to or development of disease, it can be appreciated that any in-depth evaluation of the mechanisms responsible for various pathophysiologic conditions should include an assessment of the potential role of the TGF mechanism.     

The interaction of McN-A-343 with muscarine receptors in cardiac and smooth muscle

The interaction of the muscarine receptor partial agonist (4-m-chlorophenylcarbamoyloxy)-2-butynyltrimethylammonium chloride (McN-A-343) was investigated at muscarine receptors in the atria and taenia caeci of the guinea-pig to compare its interaction at the muscarine M2 receptor in the two tissues. In the smooth muscle, the muscarine M3 receptor subtype is responsible for the contractile response but the major subtype detected in binding or antibody experiments is the M2 subtype. In guinea pig atria the dissociation constant of McN-A-343 at muscarine receptors was 15.2 microM determined in functional experiments on left atria in McEwen's solution or 14.8 microM in binding experiments with [3H]-(-)-quinuclidinyl benzilate ([3H]QNB) in the same medium containing 5'-guanylylimododiphosphate (50 microM). In the taenia caeci, the dissociation constant estimated for McN-A-343 at the M3 receptor from functional experiments based on the contractile response to the agonist in McEwen's solution was 4.6 microM. This value was similar to the dissociation constant (6.2 microM) estimated from binding studies versus [3H]QNB conducted in the same medium although studies with 11-[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H- pyrido[2,3-b][1,4]benzodiazepine 6-one (AF-DX 116) versus [3H]-(-)-N-methylscopolamine suggested that 70% of the receptors were the M2 subtype. The presence of the M2 subtype in the taenia caeci was also confirmed by the ability of oxotremorine to inhibit the increase in cAMP produced by isoprenaline (10 microM) since apparent pKB values for AF-DX 116 and hexahydrosiladiphenidol were 6.95 and 6.75, respectively. McN-A-343 (100 microM) failed to inhibit the response to isoprenaline and did not antagonize the inhibitory response to oxotremorine. It is concluded that the apparent affinity of McN-A-343 for muscarine M2 receptors in the atria and the taenia caeci differs and a number of explanations are discussed.     

Nitric oxide synthase inhibition by haem oxygenase decreases macrophage nitric-oxide-dependent cytotoxicity: a negative feedback mechanism for the regulation of nitric oxide production

Nitric oxide (NO) production in macrophages by inducible nitric oxide synthase (NOS2) has multiple tissue damaging effects and is involved in the pathogenesis of inflammation and graft rejection. Haem oxygenase (HmOx) is the enzyme which degrades haem. Its inducible isoform, HmOx1, was recently shown to increase cellular resistance against oxidative stress and to decrease inflammation and graft rejection. Since haem is an essential cofactor for NOS2 activity, we investigated the effects of HmOx1-induction upon NO secretion in macrophages. We induced HmOx1 in BALB/c bone-marrow-derived macrophages by short-term exposure to haemin (20 micromol/l, 30 min); then we incubated them for 24 h to allow maximal expression of HmOx1 activity. Next, we activated the macrophages with lipopolysaccharide (LPS) and measured their NO production and their NO-dependent cytotoxicity against P815 cells. We found that HmOx induction 24 h before LPS activation in mouse macrophages suppresses their production of NO, while HmOx inhibition (with zinc protoporphyrin) increases NO secretion. NOS2 inhibition is reflected by the decrease of macrophage NO-dependent cytotoxicity against the P815 targets. We therefore propose that HmOx1 is a physiological inhibitor of NOS2 in activated macrophages because it decreases haem availability for NOS2 synthesis. NOS2 inhibition may explain the antinflammatory effects of HmOx induction which could also be used therapeutically in situations when NO hyperproduction leads to cytotoxic effects such as inflammation or transplant rejection.     

The pharmacology of adenylyl cyclase modulation by GABAB receptors in rat brain slices

GABAB receptor activation inhibits forskolin-stimulated adenylyl cyclase activity but augments noradrenaline-stimulated adenylyl cyclase activity. The present study investigated the pharmacology of these two GABAB receptor mediated responses. In a cross-chopped rat cortical slice preparation, it was confirmed that (-)baclofen inhibited forskolin-stimulated adenylyl cyclase activity and augmented noradrenaline-stimulated adenylyl cyclase activity. The potency of five further agonists was investigated (SKF97541, CGP47656, CGP44533, 3-APA and CGP44532). Of these agonists two compounds were significantly more potent as inhibitors of forskolin-stimulated adenylyl cyclase than as augmenters of noradrenaline-stimulated adenylyl cyclase activity, these were (-)baclofen (pEC50 = 6.07 +/- 0.29 and 5.04 +/- 0.17, respectively (p < 0.05)), and CGP47656 (pEC50 = 6.44 +/- 0.05 and 4.48 +/- 0.26, respectively (p < 0.05)). It is possible to explain this difference in potency by proposing that these compounds have low intrinsic efficacy, and the augmentation of noradrenaline-stimulated adenylyl cyclase has a low receptor reserve. In addition six antagonists (CGP49311A, CGP46381, CGP45024, CGP45397, CGP36742) were also tested for their ability to antagonize 10 microM (-)baclofen in these two assays. These antagonists ranged in potency as inhibitors of forskolin-stimulated adenylyl cyclase activity from CGP49311A (pEC50 = 5.45 +/- 0.30) to CGP36742 (pEC50 = 3.87 +/- 0.16). Each antagonist had similar potency in the two assays, suggesting that these two responses are mediated by pharmacologically similar receptors.     

Involvement of the caudal medulla in negative feedback mechanisms triggered by spatial summation of nociceptive inputs

In the rat, applying noxious heat stimuli to the excitatory receptive fields and simultaneously to adjacent, much larger, areas of the body results in a surface-related reduction in the responses of lumbar dorsal horn convergent neurons. These inhibitory effects induced by spatial summation of nociceptive inputs have been shown to involve a supraspinally mediated negative feedback loop. The aim of the present study was to determine the anatomic level of integration of these controls and hence to ascertain what relationships they might share with other descending controls modulating the transmission of nociceptive signals. The responses of lumbar convergent neurons to noxious stimulation (15-s immersion in a 48 degrees C water bath) applied to increasing areas of the ipsilateral hindlimb were examined in several anesthetized preparations: sham-operated rats, rats with acute transections performed at various levels of the brain stem, and spinal rats. The effects of heterotopic noxious heat stimulation (tail immersion in a 52 degrees C water bath) on the C-fiber responses of these neurons also were analyzed. The electrophysiological properties of dorsal horn convergent neurons, including their responses to increasing stimulus surface areas, were not different in sham-operated animals and in animals the brain stems of which had been transected completely rostral to a plane -2. 8 mm remote from interaural line (200 micron caudal to the caudal end of the rostral ventromedial medulla). In these animals, increasing the stimulated area size from 4.8 to 18 cm2 resulted in a 35-45% reduction in the responses. In contrast, relative to responses elicited by 4.8 cm2 stimuli, responses to 18 cm2 were unchanged or even increased in animals with transections at more caudal level and in spinal animals. Inhibitions of the C-fiber responses elicited by heterotopic noxious heat stimulation were in the 70-80% range during conditioning in sham-operated animals and in animals with rostral brain stem transections. Such effects were reduced significantly (residual inhibitions in the 10-20% range) in animals with transections >500 micron caudal to the caudal end of the rostral ventromedial medulla and in spinal animals. It is concluded that the caudal medulla constitutes a key region for the expression of negative feed-back mechanisms triggered by both spatial summation of noxious inputs and heterotopic noxious inputs.     

The impact of positive and negative feedback on reaction time in brain-damaged patients.

Little is known about the impact of feedback on the reaction times (RTs) of brain-damaged (BD) patients. The authors therefore investigated the effect of positive and negative feedback on these patients, using a 4-choice RT task. Participants were 107 BD patients with different etiologies and 50 orthopedic (OG) control patients. Patients were assigned to 3 groups in which performance-independent negative, positive, and no feedback were given. Statistical analysis showed that negative feedback led to significantly shorter RTs in BD patients. Even BD patients with high depression scores were affected by negative feedback. In contrast, negative feedback had no impact on the RTs of the OG controls, and positive feedback had no influence on the RTs of any group. These results raise some interesting questions about motivational processes in BD patients. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Modulation by magnesium of N-methyl-D-aspartate receptors in developing human brain

AIM: To investigate age related alterations in glutamate N-methyl-D-aspartate (NMDA) receptor binding produced by the modulatory compounds glutamate, glycine, and magnesium (Mg2+) sulphate. METHODS: The effects produced by glutamate plus glycine, and Mg2+ on the binding of [3H]MK-801, a ligand for the N-methyl-D-aspartate ion channel phencyclidine site, were measured in membrane preparations made from prefrontal cortex from human neonate (n = 5), infant (n = 6), and adult (n = 6) necropsy brains. RESULTS: Neonatal brains had the least [3H]MK-801 binding, suggesting either a low density of NMDA receptors or a more restricted access of [3H]MK-801 to cation channel sites. Infant brains had the most [3H]MK-801 binding which was stimulated to a greater extent by L-glutamate (100 microM) and glycine (10 microM) than in neonatal and adult brains. MG2+ invariably inhibited [3H]MK-801 binding. However, the Mg2+ IC50 value was higher in neonatal brain (3.6 mM) than infant (1.4 mM) and adult (0.87 mM) brains. CONCLUSION: Infant brain may have excess NMDA receptors which are hyper responsive to glutamate and glycine. The lower potency of Mg2+ to inhibit [3H]MK-801 binding in neonatal cortex may be because newborn babies have NMDA receptors without the normal complement of Mg2+ sites. The findings suggest that therapeutic NMDA receptor block in neonates requires higher concentrations of magnesium sulphate in brain tissue.