Biomedical subjects

S Karanth

Publications and source records attributed to S Karanth.

61 records · Page 4Linked to original sources

PubMedDate not supplied

The nitric oxide hypothesis of aging.

Nitric oxide (NO), generated by endothelial (e) NO synthase (NOS) and neuronal (n) NOS, plays a ubiquitous role in the body in controlling the function of almost every, if not every, organ system. Bacterial and viral products, such as bacterial lipopolysaccharide (LPS), induce inducible (i) NOS synthesis that produces massive amounts of NO toxic to the invading viruses and bacteria, but also host cells by inactivation of enzymes leading to cell death. The actions of all forms of NOS are mediated not only by the free radical oxidant properties of this soluble gas, but also by its activation of guanylate cyclase (GC), leading to the production of cyclic guanosine monophosphate (cGMP) that mediates many of its physiological actions. In addition, NO activates cyclooxygenase and lipoxygenase, leading to the production of physiologically relevant quantities of prostaglandin E2 (PGE2) and leukotrienes. In the case of iNOS, the massive release of NO, PGE2, and leukotrienes produces toxic effects. Systemic injection of LPS causes induction of interleukin (IL)-1 beta mRNA followed by IL-beta synthesis that induces iNOS mRNA with a latency of two and four hours, respectively, in the anterior pituitary and pineal glands, meninges, and choroid plexus, regions outside the blood-brain barrier, and shortly thereafter, in hypothalamic regions, such as the temperature-regulating centers, paraventricular nucleus containing releasing and inhibiting hormone neurons, and the arcuate nucleus, a region containing these neurons and axons bound for the median eminence. We are currently determining if LPS similarly activates cytokine and iNOS production in the cardiovascular system and the gonads. Our hypothesis is that recurrent infections over the life span play a significant role in producing aging changes in all systems outside the blood-brain barrier via release of toxic quantities of NO. NO may be a major factor in the development of coronary heart disease (CHD). Considerable evidence has accrued indicating a role for infections in the induction of CHD and, indeed, patients treated with a tetracycline derivative had 10 times less complications of CHD than their controls. Stress, inflammation, and infection have all been shown to cause induction of iNOS in rats, and it is likely that this triad of events is very important in progression of coronary arteriosclerosis leading to coronary occlusion. Aging of the anterior pituitary and pineal with resultant decreased secretion of pituitary hormones and the pineal hormone, melatonin, respectively, may be caused by NO. The induction of iNOS in the temperature-regulating centers by infections may cause the decreased febrile response in the aged by loss of thermosensitive neurons. iNOS induction in the paraventricular nucleus may cause the decreased nocturnal secretion of growth hormone (GH) and prolactin that occurs with age, and its induction in the arcuate nucleus may destroy luteinizing hormone-releasing hormone (LHRH) neurons, thereby leading to decreased release of gonadotropins. Recurrent infections may play a role in aging of other parts of the brain, because there are increased numbers of astrocytes expressing IL-1 beta throughout the brain in aged patients. IL-1 and products of NO activity accumulate around the plaques of Alzheimer's, and may play a role in the progression of the disease. Early onset Parkinsonism following flu encephalitis during World War I was possibly due to induction of iNOS in cells adjacent to substantia nigra dopaminergic neurons leading to death of these cells, which, coupled with ordinary aging fall out, led to Parkinsonism. The central nervous system (CNS) pathology in AIDS patients bears striking resemblance to aging changes, and may also be largely caused by the action of iNOS. Antioxidants, such as melatonin, vitamin C, and vitamin E, probably play an important acute and chronic role in reducing or eliminating the oxidant damage produced by NO.

Aging↗

PubMedDate not supplied

Hypothalamic control of FSH and LH by FSH-RF, LHRH, cytokines, leptin and nitric oxide.

Gonadotropin secretion by the pituitary gland is under the control of luteinizing hormone-releasing hormone (LHRH) and the putative follicle-stimulating hormone-releasing factor (FSHRF). Lamprey III LHRH is a potent FSHRF in the rat and appears to be resident in the FSH controlling area of the rat hypothalamus. It is an analog of mammalian LHRH and may be the long-sought FSHRF. Gonadal steroids feedback at hypothalamic and pituitary levels to either inhibit or stimulate the release of LH and FSH, which is also affected by inhibin and activin secreted by the gonads. Important control is exercised by acetylcholine, norepinephrine (NE), dopamine, serotonin, melatonin and glutamic acid (GA). Furthermore, LH and FSH also act at the hypothalamic level to alter secretion of gonadotropins. More recently, growth factors have been shown to have an important role. Many peptides act to inhibit or increase release of LH, and the sign of their action is often reversed by estrogen. A number of cytokines act at the hypothalamic level to suppress acutely the release of LH but not FSH. NE, GA and oxytocin stimulate LHRH release by activation of neural nitric oxide synthase (nNOS). The pathway is as follows: oxytocin and/or GA activate NE neurons in the medial basal hypothalamus (MBH) that activate NOergic neurons by alpha1 receptors. The NO released diffuses into LHRH terminals and induces LHRH release by activation of guanylate cyclase (GC) and cyclooxygenase. NO not only controls release of LHRH bound for the pituitary, but also that which induces mating by actions in the brain stem. An exciting recent development has been the discovery of the adipocyte hormone, leptin, a cytokine related to tumor necrosis factor-alpha (TNF-alpha). In the male rat, leptin exhibits a high potency to stimulate FSH and LH release from hemipituitaries incubated in vitro, and increases the release of LHRH from MBH explants by stimulating the release of NO. LHRH and leptin release LH by activation of NOS in the gonadotropes. The NO released activates GC that releases cyclic GMP which induces LH release. Leptin induces LH release in conscious, ovariectomized estrogen-primed female rats, presumably by stimulating LHRH release. At the effective dose of estrogen to activate LH release, FSH release is inhibited. Leptin may play an important role in induction of puberty and control of LHRH release in the adult as well.

Animals↗

PubMedDate not supplied

Effects of cholinergic agonists and antagonists on interleukin-2-induced corticotropin-releasing hormone release from the mediobasal hypothalamus.

In previous research we found that interleukin-2 (IL-2)-induced corticotropin-releasing hormone (CRH) release in vitro is mediated by cholinergic activation of nitric oxidergic (NOergic) neurons. The NOergic neurons release nitric oxide that stimulates CRH release. To further characterize the mechanism of IL-2-induced CRH release, the possible role of nicotinic as well as muscarinic receptors in IL-2-stimulated CRH release was evaluated. Medial hypothalamic (MH) explants from adult male rats were preincubated in Krebs-Ringer (KRB) buffer for 45 min followed by incubation for an additional 30 min in fresh KRB or KRB containing various compounds. As previously reported, acetylcholine (ACH) stimulated CRH release in a dose-related fashion. IL-2 (10(-13) M) stimulation of CRH release was unaffected by the lower concentration of ACH (10(-9) M), but surprisingly was inhibited by a 100-fold higher concentration. Atropine (ATR) (10(-7) M) blocked CRH release induced by ACH (10(-7) M) and the release of CRH induced by IL-2. The cholinergic agonist carbachol (CAR) (10(-7) M) also released CRH and this action was blocked by ATR (10(-7) M). CRH release in the presence of CAR was lowered below basal when the concentration of ATR was increased to 10(-6) M. In contrast to ACH, CAR had an additive effect to release CRH when combined with IL-2 (10(-13) M). Nicotine (10(-7) M) also stimulated CRH release and this stimulation was completely blocked by 10(-6) M but not by 10(-7) M of the nicotinic receptor blocker, hexamethonium (HEX). The lower concentration of HEX blocked the stimulatory effect of ACH (10(-7) M) and IL-2 on CRH release. Combined blockade with ATR plus HEX completely blocked the action of ACH and even reduced the CRH concentration to below basal values. Furthermore, combined blockade completely blocked the release of CRH induced by IL-2. We conclude that nicotinic as well as muscarinic receptors play an important role in CRH release, and that they both act to mediate IL-2-stimulated CRH release.

Acetylcholine↗

PubMedDate not supplied

Effects of luteinizing-hormone-releasing hormone, alpha-melanocyte-stimulating hormone, naloxone, dexamethasone and indomethacin on interleukin-2-induced corticotropin-releasing factor release.

Our previous studies have shown that the microinjection of interleukin (IL)-2 into the third ventricle of conscious rats evokes the release of adrenocorticotropin hormone (ACTH) and that its incubation with hemipituitaries in vitro was also effective in releasing ACTH. In the present experiments, we evaluated the effect of IL-2 on the release of corticotropin-releasing factor (CRF) from medial basal hypothalami (MBHs) incubated in vitro and studied the effect of other agents, whose release is altered in stress, on CRF release. IL-2 significantly stimulated CRF release at concentrations of 10(-13) and 10(-14) M, whereas increasing the concentration to 10(-12) to 10(-10) M did not produce significant release of CRF. A high concentration of potassium (55 mM) in the medium also significantly stimulated CRF release and this stimulation was not modified by IL-2. Since high-potassium-induced release of CRF is probably due to opening of voltage-dependent calcium channels, it is likely that IL-2 is releasing CRF by this mechanism. Since the release of luteinizing-hormone-releasing hormone (LHRH) is modified by stress, we evaluated the action of LHRH on CRF release and the release induced by IL-2. Although LHRH failed to alter basal CRF release, except for a slight decrease at 10(-7) M, it completely blocked IL-2-induced CRF release at this concentration. To examine a possible role for opioid peptides in CRF release, the opiate receptor blocker, naloxone (NAL), was tested. At concentrations of 5 x 10(-6) and 10(-5) M, it produced a marked increase in CRF release; however, the simultaneous exposure of MBHs to each of these concentrations of NAL plus IL-2 caused a dose-dependent decrease in IL-2-induced CRF release, suggesting that beta-endorphin or other opioid peptides may play a role in IL-2-induced CRF release. As has been previously shown for IL-1 and IL-6, IL-2-induced CRF release was blocked by alpha-melanocyte-stimulating hormone (alpha-MSH), which at high concentrations also reduced basal CRF release. As in the case of IL-1 and IL-2, dexamethasone (DEX), the highly active synthetic glucocorticoid, although not altering basal CRF release, completely blocked the response to IL-2. The inhibitor of cyclooxygenase, indomethacin (IND), also blocked IL-2-induced CRF release just as it has previously been shown to block IL-1- and IL-6-induced CRF release. The results are consistent with the hypothesis that IL-2 acts on its recently discovered receptors to induce an increase in intracellular calcium. In other experiments, we have shown that this activates nitric oxide (NO) synthase leading to production of NO by a NOergic neuron. NO diffuses to the CRF neuron and activates cyclo-oxygenase leading to generation of prostaglandin E2, which activates adenylate cyclase and increases cyclic AMP release, which then causes extrusion of CRF secretory granules. DEX presumably acts on its receptors on the CRF neuron to inhibit the increase in intracellular calcium and thereby blocks activation of phospholipase A2 necessary for activation of the arachidonic acid cascade. alpha-MSH and LHRH may similarly act on their receptors on these cells to, in some manner, block the pathway. On the other hand, beta-endorphin and/or other opioid peptides inhibit the pathway. Further experiments will be necessary to elucidate the exact points in the pathway at which these compounds are effective.

Animals↗

PubMedDate not supplied

Nitric oxide controls the hypothalamic-pituitary response to cytokines.

During infection, bacterial products, such as lipopolysaccharide (LPS), and viral products release cytokines from immune cells. These cytokines reach the brain by several routes. Furthermore, cytokines such as interleukin-1 (IL-1) are induced in central nervous system neurons by systemic injection of LPS. These cytokines determine the pattern of hypothalamic-pituitary secretion which occurs in infection. IL-2, by stimulation of cholinergic neurons, activates neural nitric oxide synthase (NOS). The nitric oxide (NO) released diffuses into corticotropin-releasing hormone (CRH)-secreting neurons and releases CRH. IL-2 also acts in the pituitary to stimulate adrenocorticotropic hormone secretion. On the other hand, IL-1 alpha blocks the NO-induced release of luteinizing-hormone-releasing hormone (LHRH) from neurons, thereby blocking pulsatile luteinizing hormone (LH), but not follicle-stimulating hormone release, and also inhibiting sexual behavior which is induced by LHRH. IL-1 alpha and granulocyte-macrophage colony-stimulating factor (GM-CSF) block the response of the LHRH terminals to NO. GM-CSF inhibits LHRH release by acting on its receptors on gamma-aminobutyric acid (GABA)ergic neurons to stimulate GABA release. GABA acts on GABA-A receptors on the LHRH neuronal terminal to block NOergic stimulation of LHRH release. This concept is supported by a blockade of GM-CSF-induced suppression of LHRH release from medial basal hypothalamic explants by the GABA-A receptor blocker, bicuculline. IL-1 alpha inhibits growth hormone (GH) release by inhibiting GH-releasing hormone release mediated by NO and stimulating somatostatin release, also mediated by NO. IL-1 alpha-induced stimulation of prolactin release is also mediated by intrahypothalamic action of NO which inhibits release of the prolactin-inhibiting hormone, dopamine. The actions of NO are brought about by its combined activation of guanylate cyclase liberating cyclic guanosine monophosphate and activation of cyclooxygenase and lipoxygenase, with liberation of prostaglandin E2 and leukotrienes, respectively. Thus, NO plays a key role in inducing the changes in the release of hypothalamic peptides induced in infection by cytokines. Cytokines, such as IL-1 beta, also act in the anterior pituitary gland, at least in part, via induction of inducible NOS. The NO produced alters the release of anterior pituitary hormones.

Animals↗

PubMedDate not supplied

Effects of azido-3'-deoxy-thymidine on luteinizing hormone, follicle-stimulating hormone and prolactin release by the pituitary-hypothalamus complex.

Azido-3'-deoxy-thymidine (AZT) is a drug extensively used in the treatment of AIDS. AZT was incubated in vitro either with the pituitary-hypothalamus complex (PHc) or the intact pituitary (PI) of male rats. The PHc is comprised of the hypothalamus and the attached pituitary gland. After a preincubation period, the PHc or PI was incubated for 1 or 2 h with Krebs-Ringer bicarbonate buffer or either of two different concentrations of AZT (1 and 10 microM). In the control incubations, the PHc released less prolactin (PRL) and more follicle-stimulating hormone (FSH) and luteinizing hormone (LH) than the PI, indicating that hypothalamic control of the pituitary was exerted in vitro, presumably by diffusion of releasing and inhibiting hormones from the neurohypophysis to the anterior lobe of the hypophysis. Both concentrations of AZT evoked a significant increase in release of PRL and a decreased release of LH and FSH from the PHc. In the case of LH, the higher concentration of AZT partially suppressed LH release within 1 h. The other effects were not dose-related and were observed after incubating the tissue with AZT for 2 h. However, incubation of the PI with AZT failed to alter anterior pituitary hormone release, illustrating that the site of action of AZT is in the hypothalamus. We hypothesize that AZT blocks DNA synthesis resulting in suppression of synthesis and consequent release of hypothalamic hormones that control release of pituitary hormones in vitro. The results raise the possibility that AZT may alter hypothalamic-pituitary function in vivo.

Animals↗

PubMedDate not supplied

Overestimation of neonatal PO2 by collection of arterial blood gas values with the butterfly infusion set.

The butterfly, or scalp vein infusion set, is a widely used and convenient way to collect arterial blood gas samples in neonates. Analyzing arterial blood gas values by using this technique in a clinical situation, we obtained elevated values of PO2 in blood samples collected and measured from butterfly tubing in comparison with tuberculin syringe controls. Diffusion of gas through the polyvinylchloride tubing was suggested and proved by a study of pairs of butterfly and tuberculin syringes filled with venous blood from the blood bank. In conclusion, the butterfly gives falsely elevated PO2 values as a result of gas diffusion and is not a reliable technique for arterial blood gas sampling in neonates.

Blood Specimen Collection↗