Berlin, Freie Universiẗat, Diss., 2003. / Dateiformat: zip, Dateien im PDf-Format.

There is strong evidence which supports a physiological role for arginine vasopressin (AVP) in the negative modulation of the febrile process within the central nervous system (CNS). This evidence arises from a variety of experimental techniques employed in a number of different animal models. The CNS locus of action for AVP-mediated antipyresis is within a rostral diencephalic site called the ventral septal area (VSA). It has become evident that the mechanism by which AVP and aspirin-like drugs transduce changes in febrile body temperature are similar. Moreover, antipyretic drugs and AVP may share a common CNS locus of action. Therefore, investigations were conducted to determine whether antipyretic drugs are functionally linked to the endogenous antipyretic system of the brain. In addition, an examination of the role for centrally acting AVP and the natural suppression of fever during pyrogenic tolerance to endotoxin was conducted. AVP receptor antagonists of the peripheral V₁ and V₂ sub-type or saline control were microinjected into the VSA of rats rendered febrile by an intracerebroventricular (icv) injection of E. coli endotoxin, to assess the effects on the antipyresis elicited by indomethacin. Blockade of central V₁ but not V₂ receptors significantly attenuated the antipyretic effects of indomethacin given intraperitoneally. This effect was even more pronounced when the V₁ antagonist was infused for 30 min before and for 60 min after indomethacin administration. The V₁ analogue alone was without thermoregulatory effects. In order to determine whether the above effects were applicable to antipyretic drugs in general, central V₁ blockade was performed in the febrile rat subsequently treated with intraperitoneal sodium salicylate or acetaminophen. Salicylate-induced antipyresis was blocked, in a dose related manner, by VSA administration of the AVP V₁ antagonist. The fever reducing capacity of acetaminophen was unaffected by central V₁ blockade. Collectively these antipyretic drug studies, suggest that some but not all antipyretic drugs activate the endogenous AVP antipyretic pathway within the brain. Moreover, these data suggest that the mechanism of action of antipyretic drugs can no longer be simply explained as an action on prostaglandin biosynthesis. Endogenous release of AVP from VSA nerve terminals during endotoxin fever and drug-induced antipyresis was examined using the technique of push-pull perfusion. The release of AVP into the perfusion fluid remained unaltered by indomethacin injected into the non-febrile rat. However, during fever indomethacin prompted both an antipyresis as well as a significant increase in AVP release. Acetaminophen injected intraperitoneally also evoked an antipyresis but with no concomitant release of AVP within the VSA. These results are consistent with the antagonist studies. The effects on central AVP release by indomethacin appear to be related to the pyrogen employed because the drug did not evoke the release of AVP when administered prior to the hyperthermia produced by icv PGE₂. Indeed, PGE₂ itself stimulated AVP release which was inhibited by indomethacin treatment. These results are not consistent with an antipyretic role for AVP and await further clarification. Analysis of the release of AVP into the plasma and cerebrospinal fluid (CSF) were conducted during the fever evoked by intravenous endotoxin and subsequent to antipyretic intervention. Intravenous endotoxin was a provocative stimulus for plasma AVP release. Endotoxin-stimulated plasma AVP levels were unaffected by intraperitoneal injections of indomethacin, sodium salicylate or acetaminophen. In non-febrile controls, indomethacin, and to some extent acetaminophen, prompted increases in plasma AVP; although the temporal course of this release was different between the two drugs. Within the CSF, endotoxin treatment did not alter the normal diurnal rhythm of AVP release. Indomethacin treatment significantly suppressed CSF AVP release in non-febrile animals. A similar but non-significant trend was observed in febrile rats. Collectively, these studies demonstrate the independent regulation of AVP release within three separate biological compartments in response to febrogenic and antipyretic stimuli. The suppression of fever after repeated daily intravenous injections of bacterial endotoxins was thought to be exclusively a hepatic phenomenon. Experiments were conducted to determine whether a central mechanism involving AVP may also contribute to the antipyretic state observed during pyrogenic tolerance. In endotoxin tolerant animals, administration of a V₁ but not V₂ AVP receptor antagonist within the VSA, resulted in a significant reversal of the tolerant pyrogenic response. These data support the hypothesis that the central endogenous antipyretic system, involving AVP, plays a role in the mechanism of endotoxin tolerance. Tolerance does not develop following repeated central injections of pyrogens. Further experiments were performed to determine whether tolerance-induced activation of the antipyretic pathway would render an animal hyporesponsive to centrally administered pyrogens. When injected icv, during active endotoxin tolerance, the thermoregulatory responses to PGE₂ or endotoxin were not significantly suppressed from non-tolerant controls. However, analysis of VSA push-pull perfusates performed during a tolerant reaction to intravenous endotoxin revealed that increased AVP activity occurs within the first 30 min after the intravenous injection, well before the time PGE₂ or endotoxin were injected into the cerebral ventricles. This suggests that the antipyretic system is only activated briefly and may explain why centrally evoked fevers were unaffected during active endotoxin tolerance. In summary, this thesis research has demonstrated a direct functional link between the mechanism of action of antipyretic drugs and the endogenous antipyretic system within the brain. These results call into question the hypothesis whereby the fever reducing properties of antipyretic drugs can be explained exclusively as a result of the inhibition of prostaglandin biosynthesis. In addition, the differential effects on AVP release by antipyretic drugs suggests a number of biological pathways that can be activated by these drugs. Finally, a role for the AVP endogenous antiypretic system in the suppression of fever during endotoxin tolerance was established. / Medicine, Faculty of / Cellular and Physiological Sciences, Department of / Graduate

Typescript. / Bibliography: leaves 135-147. / Photocopy. / Microfilm. / xiv, 147 leaves, bound ill. 29 cm

Three major systems participate in the control of the peripheral circulation: the renin-angiotensin, the arginine vasopressin (AVP) and the sympathetic nervous systems. These studies examined the roles of the AVP and the sympathetic nervous systems in the regulation of blood pressure at both the central and the peripheral level. Anatomical studies have revealed that hypothalamic neurons containing AVP extend to the nucleus tractus solitarius (NTS) in the medulla. Since the NTS is the primary site of termination of the afferent neurons of the baroreceptor reflex arc, it suggests that AVP may be involved in central cardiovascular regulation. The effect of central AVP on mean arterial pressure (MAP) and sympathetic nerve activity, estimated from plasma catecholamine levels, was investigated. The injection of AVP into the fourth cerebroventricle and NTS of conscious, unrestrained rats increased MAP and plasma noradrenaline and adrenaline levels, suggesting that AVP may act centrally at the NTS to modulate sympathoadrenal outflow. However, the injection of a selective vascular antagonist of AVP, d(CH₂)₅Tyr(Me)AVP, into the fourth ventricle or NTS did not affect MAP or plasma catecholamine levels, either in normotensive rats, in rats subjected to hypotensive stress, or in neurogenically-stressed rats. This suggests that endogenously-released AVP may not have a tonic influence on central cardiovascular regulation. The role of AVP in the control of MAP, cardiac output (CO) and its distribution was investigated in anesthetized, surgically-stressed rats. The i.v. injection of d(CH₂)₅Tyr(Me)AVP decreased MAP and total peripheral resistance (TPR), did not alter CO, and increased the distribution of blood flow (BF) to the stomach and skin. The vascular role of AVP was found to be greater in the absence of influence from the renin-angiotensin and the sympathetic nervous systems. After blockade of the renin-angiotensin system by the infusion of saralasin the AVP antagonist increased BF to the skin and muscle, while after blockade of the α-adrenergic system with the infusion of phentolamine, the AVP antagonist markedly increased BF to the muscle. Thus, the amount of vasoconstriction produced by AVP in different vascular beds was found to depend on the endogenous vasomotor tone from the renin-angiotensin and α-adrenergic systems. Cross-circulation studies were conducted to concurrently observe the peripheral and central effects of α-agonists in two anesthetized rats, designated rat A and B, respectively. The i.v. injection of clonidine into rat A was found to increase MAP and decrease HR in rat A, and reduce MAP and HR in rat B. Since the stimulation of peripheral α-adrenoceptors in rat A by clonidine increased MAP, it suggests that the effects of peripheral post-junctional α₂-adrenoceptors predominate over those of peripheral pre-junctional α₂-adrenoceptors. In contrast, the i.v. injection of the α₁-agonist, methoxamine, in rat A increased MAP and decreased HR in rat A, and increased both MAP and HR in rat B. This suggests that central α₁-adrenoceptors may mediate responses in the opposite direction to those produced by α₂-adrenoceptors. To verify the results of the cross-circulation studies in animals free of the influence of surgery and anesthesia, and to determine whether the responses to a-agonists were mediated by changes in sympathoadrenal outflow, clonidine and a more selective α₂-agonist, B-HT 920, were injected centrally in conscious rats. The i.e.v. injection of clonidine (1 µg) significantly decreased MAP and HR and slightly decreased plasma noradrenaline and adrenaline levels; however, contrary to expectations, the i.c.v. injection of B-HT 920 (1, 10 µg) increased MAP, decreased HR and slightly increased plasma noradrenaline and adrenaline levels. To determine whether the responses to central injection of clonidine or B-HT 920 were due to the stimulation of α₂-adrenoceptors, i.c.v. injections of these drugs were given after pretreatment with rauwolseine, a selective α₂-antagonist. The i.c.v. injection of rauwolscine in conscious rats increased MAP and plasma noradrenaline and adrenaline levels, suggesting that central α₂-adrenoceptors may mediate tonic inhibition of the cardiovascular system. However, i.c.v. injections of clonidine or B-HT 920 produced the same responses in the absence or presence of rauwolscine. Further studies with different α-adrenergic agonists and antagonists with various selectivities are necessary before we can explain the differential effects of central clonidine and B-HT 920. / Medicine, Faculty of / Anesthesiology, Pharmacology and Therapeutics, Department of / Graduate

Previous studies in the sheep, rabbit, cat and rat have demonstrated the ability of the neuropeptide, arginine vasopressin (AVP), to suppress endotoxin-induced fever when perfused into a discrete brain locus. Fever can also be suppressed if AVP is microinjected into the cerebral ventricles of the rat. The mechanisms by which AVP mediates antipyresis are unknown. Experiments were conducted, therefore, to examine the effect of intracerebroventricular (icv) AVP on an established fever and to assess the mechanism of action using a specific, V₁-receptor antagonist (M-AVP). Studies were also conducted to elucidate the effector mechanisms utilized to accomplish antipyresis induced by icv AVP. Finally, cerebrospinal fluid (CSF) AVP concentrations were measured in febrile and non-febrile rats to determine the role of endogenously released AVP in the CSF during fever. AVP administered icv was shown to have marked antipyretic effects at very low doses. This antipyresis was elicited in rats with an established fever but the peptide had no effect on the temperature of non-febrile rats. Thus AVP both prevented and reversed endotoxin-induced fever. Furthermore, this AVP-induced antipyresis was abolished by pretreatment with the the V₁-antagonist, M-AVP. The antipyretic effects of AVP were, therefore, receptor mediated and likely to be of physiological importance. Efforts to manipulate the endogenous AVP system by icv M-AVP were also attempted. When M-AVP was injected icv, the fever height of endotoxin-treated rats was not different from endotoxin-treated controls. In addition, M-AVP did not influence the magnitude of the antipyresis induced by indomethacin. It has become clear, however, that this method of administering the antagonist is inappropriate to block endogenous AVP effects occurring within the neuropil. Subsequent experiments in another laboratory have shown that M-AVP must be microinjected into the AVP-sensitive brain locus to effectively block endogenous activity. The antipyretic response to icv AVP was further investigated at three ambient temperatures in an attempt to identify the effector mechanisms involved. Responses of non-febrile and febrile rats to icv injections of AVP and sc injections of indomethacin were observed at cold (4°C), neutral (25°C) and warm (32°C) ambient temperatures. As in the previous experiments, AVP at 25°C decreased brain temperatures of febrile but not non-febrile rats. This antipyretic effect was also observed at the warm ambient temperature and during cold exposure. Responses to sc indomethacin were qualitatively similar to icv AVP at neutral and warm temperatures. In the cold, however, indomethacin decreased the brain temperature of both non-febrile and febrile animals, although unlike AVP, brain temperature of non-febrile animals decreased somewhat more than that of febrile animals. These data showed that AVP decreased brain temperature of febrile more so than non-febrile rats at all ambient temperatures and may therefore have been acting partially on febrile set-point. It was possible that AVP affected specific effector mechanisms since antipyretic effects were of different magnitudes at different ambient temperatures. The observation that AVP and indomethacin had qualitatively similar effects on fever at three ambient temperatures suggested that they may act via a common neural pathway. Further analysis of the mechanism of icv AVP-induced antipyresis was conducted at the three ambient temperatures while measuring specific effectors: heat loss and heat production. At 25°C, AVP-induced antipyresis was mediated by tail skin vasodilation while metabolic rate was unaffected. At 4°C, the antipyresis produced by AVP was mediated exclusively by inhibition of heat production since the metabolic rate decreased markedly following AVP. This antipyresis at 4°C was accompanied by cutaneous vasoconstriction. At 32°C, neither vasomotor tone, metabolic rate nor evaporative heat loss could be shown to contribute to the small antipyretic effect elicited by AVP. These data strongly suggest that icv AVP produced antipyresis by affecting the febrile body temperature set-point mechanism since the thermoregulatory strategy to lose heat varied at different ambient temperatures and the decrease in body temperature could not be shown to be due to changes in a single effector mechanism. As an index of endogenous AVP activity, cerebrospinal fluid (CSF) concentrations of AVP were measured in febrile and non-febrile rats in order to determine the role of CSF AVP in fever and antipyresis. The results demonstrated that the AVP release pattern was not altered in endotoxin-treated febrile compared to non-febrile rats. It was concluded that CSF AVP had no role in the febrile process. In summary, icv AVP appears to induce antipyresis by its action on febrile set-point rather than on a specific effector system. This action of AVP is mediated by a V₁-like receptor mechanism which is not affected by endogenous CSF AVP. The neural/neurochemical basis for the thermoregulatory set-point has not been clearly established so the mechanism of action by which AVP affects set-point remains to be determined. These data contribute, however, to the growing body of evidence that AVP is acting centrally as a neurotransmitter or neuromodulator to regulate body temperature during the febrile process. / Medicine, Faculty of / Cellular and Physiological Sciences, Department of / Graduate

Vasopressin, or antidiurectic hormone, has long been known to have peripheral antidiuretic and vasoconstrictor properties. However, more recently a body of research has shown that vasopressin (AVP) affects central nervous system functions by to influencing memory processes. In light of the growing evidence for the role of vasopressin (AVP) in memory, my dissertation research was designed to test the hypothesis that AVP acts as a neuromodulator in the CNS. To test this hypothesis criteria used to establish neurotransmitter status was applied to AVP. Thus, a series of experiments were carried out to investigate (1) AVP brain levels; (2) release of AVP in the CNS; (3) existence of specific AVP binding sites in brain and finally, (4) existence of AVP metabolite peptide, AVP (4-9), binding sites in brain. Results of these experiments indicate that AVP meets some of the criteria for neuromodulator status in the CNS. The detection of AVP in brain, elucidation of the modulatory influence of a CNS depressant upon the content and release of AVP in brain, demonstration and characterization of the regional distribution for putative AVP receptors in brain along with binding sites for a metabolite peptide of AVP, all suggest that AVP acts through receptors within the CNS to influence memory processes.

published_or_final_version / Biological Sciences / Doctoral / Doctor of Philosophy

No description available.

Computer modelling is a powerful tool for clarifying and testing theory. In neuroscience, this often means replicating firing patterns. Models need evaluation functions to quantify the significance of features in the firing patterns, but usually the effect of firing is insufficiently understood. The magnocellular vasopressin neurons of the hypothalamus do have an output that is both well understood and quantifiable: they secrete a hormone into the bloodstream in proportion to blood osmolarity and volume, regulating these properties within a narrow physiologically acceptable range. This response of vasopressin secretion to osmotic pressure must be maintained to defend blood pressure. The neurons display a distinctive phasic firing pattern, which a model was developed to mimic. A further, unique step was then taken of extending this model by developing a model for the effect of firing, a stimulus-secretion model. The firing pattern model and stimulus secretion model were then linked and then noisily duplicated to produce a population. This population had a measurable performance - secretion - allowing evaluation of the model in a novel fashion. The population could replicate the secretory response to osmotic pressure observed in vivo. It is possible to test the effect of features by incorporating them into the model and observing the response. A demonstration of this was conducted by changing the mix of excitatory and inhibitory PSPs, showing that inhibition was necessary for an efficient response. Effective techniques may well be reused elsewhere in the brain, so exploring their significance in a simple system may allow understanding of more complex ones. This project has constructed a model from firing to effect, offering novel possibilities for quantification and therefore evaluation. The main outcomes from this work are construction of a simple model system in which features can be benchmarked; that a integrate and fire model modified to include bistability can explain the firing of vasopressin neurons; that secretion could well also be controlled by a pool structure, similar to other secretory systems and that a population of these cells can produce a linear output. It has also confirmed that balanced excitatory and inhibitory input is necessary for the most efficient response. It shows that population performance is a trade-off between maximising efficiency, maintaining the secretory response over a wide dynamic range and maximising the maximum achievable secretion rate.

No description available.