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K M Michels

Publications and source records attributed to K M Michels.

13 recordsLinked to original sources
PubMed1991

Differential development of insulin-like growth factor-I binding in the hypothalamus of hamster and rat.

We investigated the binding of [125]insulin-like growth factor-I ([125I]IGF-I) within the hamster suprachiasmatic nucleus (SCN) and median eminence (ME) by quantitative autoradiography and compared the development of binding to the same regions in the rat. Binding in the hamster SCN was to a single class of sites (estimated Kd = 6 x 10(-10) M). Binding in the ME was approx. 1.5-fold that of the SCN. Full displacement of binding from the SCN and ME of adults and neonates was achieved by 10(-8) M IGF-I, 10(-8) M IGF-II or 10(-6) M insulin. Binding within the hamster SCN was evident by E15, peaked between P5 and P7 and decreased to adult levels by P20 while binding in the rat SCN peaked perinatally and declined to adult levels by P6. Binding in the hamster ME was evident at P4, peaked by P12 and decreased to adult levels by P20 while binding in rat ME was present by P2, peaked by P7 and declined to adult levels by P9. These results demonstrate a different developmental time course for [125I]IGF-I binding between the SCN and ME of hamster and rat. The peak binding in the SCN of each species correlates with previously reported time courses for onset of retinohypothalamic innervation of the SCN. Further study of IGFs in these regions may help elucidate the developmental role of brain IGFs.

Animals↗
PubMed1991

Muscarinic cholinergic control of vasopressin secretion from the acute hypothalamoneurohypophysial explant.

Much of the afferent input thought to modulate vasopressin release from the magnocellular neuroendocrine cells of the supraoptic nucleus terminates in the region dorsal to the supraoptic nucleus. Cholinergic cells within this region may participate in the local processing of these afferent signals via synapses onto muscarinic cholinergic receptors. To investigate the role of these local synapses in vasopressin secretion, we characterized the muscarinic cholinergic influence on vasopressin secretion from the acute hypothalamoneurohypophysial explant in vitro. Acetylcholine induced a small dose-related secretion of vasopressin which could be totally blocked by atropine but not the nicotinic cholinergic antagonist, hexamethonium. Nicotine failed to release vasopressin from the explant, whereas alpha-bungarotoxin elicited a hypothalamic release of vasopressin which was atropine insensitive. Thus, local muscarinic receptors in the hypothalamus appear to participate in the control of neurohypophysial vasopressin secretion. The small magnitude of effect, however, is consistent with an indirect modulatory role rather than a major driving force for activation of the magnocellular neurons.

Acetylcholine↗
PubMed1991

Differential development of insulin-like growth factor I binding in the suprachiasmatic nucleus and median eminence of the rat hypothalamus.

We investigated the in vitro binding of [125I]insulin-like growth factor I ([125I]IGF-I) within the suprachiasmatic nucleus (SCN) and median eminence (ME) of the rat by quantitative autoradiography. Binding of [125I]IGF-I within the adult SCN was saturable, reversible and to a single class of sites with an estimated Kd of 3.01 x 10(-10) M and Bmax of 131 fmol/mg protein. Competition studies revealed [125I]IGF-I binding within the SCN and ME to have the pharmacological specificity characteristic of the brain IGF-I receptor. The most potent competitors were IGF-I and IGF-II, while insulin displaced binding with a much lower potency and nerve growth factor 7S had no effect. Binding within the SCN was evident by embryonic day 18, peaked perinatally, declined to adult levels by day 6 and remained constant through middle age. The potency of IGF-I to displace [125I]IGF-I binding from SCN was similar among all ages. Binding in the ME was not evident until postnatal day 2, peaked near the end of the 1st week, declined to adult levels by day 9 and remained constant thereafter. There was no difference in binding in SCN or ME between day and early night, in animals with acute or long-term eye enucleation, or in animals fed a high sucrose diet. These results demonstrate differential development of [125I]IGF-I binding within the SCN and ME and support a role for IGFs in both these nuclei during development and maturity.

Aging↗
PubMed1990

GABAA/benzodiazepine receptor localization in the circadian timing system.

gamma-Aminobutyric acid (GABA) and exogenous benzodiazepines are thought to play a role in the neural regulation of circadian rhythms. Because binding sites for the benzodiazepines and GABAA ligands are functionally coupled as part of the GABAA/benzodiazepine receptor complex (GABAA/BZR), we analyzed the localization of GABA neurons and GABAA/BZR within 3 nuclei involved in circadian rhythm regulation using autoradiographic and immunohistochemical techniques. Glutamic acid decarboxylase-immunoreactive axons are present in the suprachiasmatic nuclei (SCN), intergeniculate leaflet (IGL), and dorsal raphe nucleus (DR). Immunoreactivity for the GABAA/BZ receptor complex is absent from the SCN and the IGL whereas the DR shows a dense, uniform immunoreactivity. Semiquantitative analysis of autoradiograms for [3H]diazepam and [3H]flunitrazepam binding reveals a moderate level of binding in the SCN, a low level of binding in the IGL, and the highest level of the DR. Based on both the pattern of benzodiazepine binding and of receptor immunoreactivity the DR would appear to be a likely target site for GABAA and benzodiazepine action. The SCN would also appear to be a possible target site. The results suggest the IGL is not a site for direct GABAA and benzodiazepine action, but do not exclude a role for the IGL in the neural circuitry mediating GABA and benzodiazepine interactions with the circadian system.

Animals↗
PubMed1990

Vasopressin and oxytocin regulation of cyclic AMP accumulation in rat hypothalamo-neurohypophysial explants in vitro.

Addition of vasopressin to hypothalamo-neurohypophysial explants in vitro increased cyclic AMP accumulation whereas exogenous oxytocin decreased cyclic AMP. An opposite response pattern was observed in the neural lobe of the pituitary where vasopressin decreased and oxytocin increased cyclic AMP accumulation. Forskolin elicited a 3-fold greater increase in cyclic AMP in the neural lobe than in the supraoptic nucleus and enhanced the sensitivity of the tissues to both vasopressin and oxytocin. The ability of both vasopressin and oxytocin to modulate local cyclic AMP metabolism suggests the possibility of internal feedback within the hypothalamo-neurohypophysial system.

Acetylcholine↗
PubMed1990

Destruction of the hamster serotonergic system by 5,7-DHT: effects on circadian rhythm phase, entrainment and response to triazolam.

The role of the serotonergic system in the regulation of hamster circadian rhythms was analyzed using intraventricular injection of the selective neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT). Sixty days after 5,7-DHT administration, immunoreactive serotonin in the forebrain, particularly the suprachiasmatic nuclei and intergeniculate leaflets, was severely depleted in 16 animals, moderately depleted in four and only slightly affected in four. 5,7-DHT produced an immediate and sustained advance of the onset of running wheel activity relative to the 24 h light-dark (LD) cycle. Activity onset occurred 0.7 +/- 0.07 h before lights out among 5,7-DHT-treated animals compared with 0.18 +/- 0.04 h after lights out for vehicle-infused controls. This new, advanced phase angle of entrainment was maintained throughout the 60-day period of the study while the animals remained in a LD cycle, including after an 8-h phase advance of the light cycle. 5,7-DHT treatment also delayed the offset of wheelrunning in 16 of 24 animals and reduced the likelihood of a smooth pattern of reentrainment to the shifted LD cycle. The drug treatment did not affect circadian period in constant darkness, the rate of reentrainment to an 8-h phase advance or the amount of wheelrunning activity per day. In addition, 5,7-DHT treatment had no effect on the ability of triazolam, a short-acting benzodiazepine, to accelerate the rate of reentrainment to an 8-h phase advance. These observations show that ascending projections of midbrain raphe serotonin neurons participate in the regulation of the circadian activity phase but are not required for triazolam-induced acceleration of reentrainment to a phase-advanced LD cycle.

5,7-Dihydroxytryptamine↗
PubMed1986

Differential distribution of muscarinic cholinergic and putative nicotinic cholinergic receptors within the hypothalamo-neurohypophysial system of the rat.

Binding of the muscarinic cholinergic receptor probe [3H]quinuclidinylbenzilate ([3H]QNB) and the putative nicotinic receptor probe [125I]alpha-bungarotoxin ([125I]alpha BTX) to vasopressin (VP) and oxytocin (OT) neuroendocrine cells was investigated with a combination of quantitative receptor binding, autoradiography and immunocytochemistry. A single high-affinity site was labelled by [3H]QNB in the hypothalamus and pituitary (KD = 0.76-1.44 X 10(-10) M) with a mean hypothalamic density of 213 fmol/mg protein compared with only 56 fmol/mg protein in the pituitary. Analysis of autoradiographic silver grains from [3H]QNB binding revealed a relative absence of binding associated with magnocellular VP and OT cell groups in the hypothalamus. The median eminence and neural lobe of the pituitary contained low levels of [3H] QNB binding, which, however, were the highest within the hypothalamo-neurohypophysial system. The ligand [125I]alpha BTX binds with both a high and low affinity to sites within the hypothalamus and pituitary (high-affinity KD = 0.77-1.03 X 10(-10) M). In the hypothalamus the density of high-affinity binding sites (25 fmol/mg protein) is approximately 2.5 times greater than in the pituitary. In contrast to [3H]QNB, high-affinity binding of [125I]alpha BTX was found to be highly concentrated within the supraoptic nucleus, nucleus circularis, and the magnocellular areas of the paraventricular nucleus. Autoradiographic silver grains were distributed over both VP and OT immunoreactive neurons and processes. Binding within the neural lobe was very low. These data suggest that the cholinergic regulation of VP and OT release may occur via nicotinic cholinergic receptors at the level of the magnocellular cell bodies and predominantly via muscarinic cholinergic receptors within the neural lobe.

Animals↗
PubMed1986

Characteristics and distribution of high- and low-affinity alpha bungarotoxin binding sites in the rat hypothalamus.

When binding of 125I-alpha bungarotoxin (125I-alpha BTX) to hypothalamic membranes is observed over a wide range of concentrations, 3 binding sites can be identified, with estimated equilibrium dissociation constants (Kds) of 4.1 X 10(-11) M, 6.2 X 10(-10) M, and 9.1 X 10(-7) M for high-, low-, and very-low-affinity interactions, respectively. The densities of the high- and low-affinity sites were similar at 14-21 fmol/mg protein, whereas the very-low-affinity site had approximately 1000 X greater capacity. Association and dissociation kinetics predicted a biphasic binding reaction, with association rate constants of 1.38 X 10(8) M-1 min-1 and 7.53 X 10(7) M-1 min-1 and dissociation rate constants of 5.23 X 10(-3) min-1 and 1.80 X 10(-3) min-1. The presence of Na+ inhibited the binding of 125I-alpha BTX with a half-maximally effective concentration of 22 mM. This decrease in binding was associated with the observation of a single binding site with a Kd of 4.3 X 10(-10) M and a density of 12.1 fmol/mg protein. In competition binding experiments, alpha BTX, curare, nicotine, and quinacrine were the most potent competitors. Acetylcholine competed with 125I-alpha BTX binding at 2 sites with estimated affinities of 3.6 X 10(-8) and 7.4 X 10(-5) M. In the rostral hypothalamus, high-affinity binding of 125I-alpha BTX was localized to the region of the supraoptic nucleus, paraventricular nucleus, suprachiasmatic nucleus, and the nucleus circularis complex. Within magnocellular regions, binding was closely associated with neurophysin-immunoreactive neurons and processes, while in the region of the suprachiasmatic nucleus, the binding was in a perinuclear region surrounding parvocellular neurophysin-immunoreactive neurons.

Animals↗