PubMed Health⌕ Search

Biomedical subjects

J T Laitinen

Publications and source records attributed to J T Laitinen.

49 records · Page 3Linked to original sources
PubMed1990

Expression of melatonin receptors in arteries involved in thermoregulation.

Melatonin binding sites were localized and characterized in the vasculature of the rat by using the melatonin analogue 2-[125I]iodomelatonin (125I-melatonin) and quantitative in vitro autoradiography. The expression of these sites was restricted to the caudal artery and to the arteries that form the circle of Willis at the base of the brain. The arterial 125I-melatonin binding was stable, saturable, and reversible. Saturation studies revealed that the binding represented a single class of high-affinity binding sites with a dissociation constant (Kd) of 3.4 x 10(-11) M in the anterior cerebral artery and 1.05 x 10(-10) M in the caudal artery. The binding capacities (Bmax) in these arteries were 19 and 15 fmol/mg of protein, respectively. The relative order of potency of indoles for inhibition of 125I-melatonin binding at these sites was typical of a melatonin receptor: 2-iodomelatonin greater than melatonin greater than N-acetylserotonin much much greater than 5-hydroxytryptamine. Norepinephrine-induced contraction of the caudal artery in vitro was significantly prolonged and potentiated by melatonin in a concentration-dependent manner, suggesting that these arterial binding sites are functional melatonin receptors. Neither primary steps in smooth muscle contraction (inositol phospholipid hydrolysis) nor relaxation (adenylate cyclase activation) were affected by melatonin. Melatonin, through its action on the tone of these arteries, may cause circulatory adjustments in these arteries, which are believed to be involved in thermoregulation.

Animals↗
PubMed1990

Enhanced phosphoinositide hydrolysis in the pineal gland of spontaneously hypertensive rats.

We studied the phosphoinositide (PI) turnover in the pineal gland of spontaneously hypertensive rats (SHR) and compared it to that in Wistar-Kyoto and Wistar rats. Both basal and norepinephrine-stimulated PI turnover were significantly higher in the SHR than in the other strains. Prazosin (1 mumol/L) completely blocked the norepinephrine-induced PI hydrolysis in all groups. However, prazosin failed to lower the enhanced basal PI turnover in SHR. Pineal response to the cholinergic agonist carbachol was similar in all groups. These results suggest enhanced membrane phospholipid turnover in the pineal gland of SHR. Moreover, signaling via the alpha 1-adrenoceptor might be sensitized in the SHR.

Animals↗
PubMed1990

Differential sensitivity to cations of the melatonin receptors in the rat area postrema and suprachiasmatic nuclei.

We studied the effects of cations on the binding of the melatonin (MT) agonist, 125I-MT, to MT receptors in the rat area postrema (AP) and suprachiasmatic nuclei (SCN), by using quantitative autoradiography in vitro. Ca2+ promoted agonist binding in the SCN but was without effect in the AP. Na+ induced a dose-dependent loss of agonist binding in both areas. This effect was more drastic in the SCN and also in the absence of divalent cations. The presence of 0.1-4.0 mM Ca2+ or Mg2+ partially and nonselectively reversed this Na(+)-elicited inhibition. The data agree with known cationic effects on agonist binding to other G protein-coupled receptors and deepen our understanding of the mammalian MT receptor.

Animals↗
PubMed1990

Characterization of melatonin receptors in the rat area postrema: modulation of affinity with cations and guanine nucleotides.

We have localized and characterized the binding of the melatonin agonist, 2-[125I]iodomelatonin, in the rat area postrema (AP), by using quantitative autoradiography in vitro. At equilibrium conditions, Scatchard analysis revealed saturable high-affinity binding to a single class of sites (Kd 45.9 +/- (SE) 6.0 pM and Bmax 30.8 +/- 4.6 fmol/mg protein, n = 4 experiments with a total of 18 rats). Melatonin and 6-hydroxymelatonin were potent displacers of 2-[125I]iodomelatonin binding in the AP (IC50 20 and 500 pM, respectively) while N-acetylserotonin exhibited only a modest potency (IC50 25 nM). Micromolar concentrations of guanine nucleotides dose-dependently and specifically inhibited agonist binding at 22 degrees C. Saturation studies revealed that this was due to a decrease in binding affinity. Divalent cations (4 mM CaCl2 or 2 mM MgCl2) had no detectable effect on the affinity of the binding site, whereas physiological concentrations of NaCl significantly decreased the binding affinity. These results demonstrate specific high-affinity binding sites for 2-[125I]iodomelatonin in the rat AP and suggest coupling of these putative receptors to guanosine nucleotide binding regulatory protein(s).

Animals↗
PubMed1990

Characterization of melatonin receptors in the rat suprachiasmatic nuclei: modulation of affinity with cations and guanine nucleotides.

We have characterized melatonin (MT) receptors in the rat suprachiasmatic nuclei by using quantitative autoradiography in vitro at equilibrium conditions for a picomolar affinity site. Binding of the MT agonist 2-[125I]iodomelatonin (30 pM to 6 nM) was saturable, of high affinity (Kd, 52.8 pM), of high specificity, and to a single class of sites (binding capacity, 16.5 fmol/mg protein). However, by shortening the washing time for bound and free ligand, we were able to demonstrate an additional low affinity form of this receptor (Kd, 761 pM; binding capacity, 72.2 fmol/mg protein). Micromolar concentrations of guanine nucleotides and millimolar concentrations of monovalent cations (Na+ and Li+) dose-dependently and specifically inhibited agonist binding at 22 C. Saturation studies revealed that this was due to a decrease in receptor affinity in both cases. Ca+2 promoted high affinity agonist binding, as omission of this cation decreased the affinity of the suprachiasmatic MT receptor. These results suggest coupling of the rat suprachiasmatic MT receptors to guanine nucleotide-binding regulatory protein(s). Moreover, our data demonstrate specific modulation of the affinity of these receptors with cations.

Animals↗
PubMed1989

Pineal muscarinic phosphoinositide response: pertussis toxin resistant signaling with very low receptor number.

Autoradiography revealed very low density of muscarinic receptors in the rat pineal gland. Yet, the magnitude of phosphoinositide hydrolysis elicited with 0.1 mM carbachol was similar to that seen with 1 mM norepinephrine. The cholinergic and adrenergic phosphoinositide responses were fully additive. The cholinergic signal was insensitive to pertussis toxin both in vivo and in vitro and persisted in pineals cultured for 5 days. The data expand our previous finding on functional muscarinic acetylcholine receptors in the rat pineal gland.

Animals↗
PubMed1989

Cholinergic stimulation of phosphoinositide hydrolysis in the rat pineal gland.

Carbachol induced a dose-dependent accumulation of inositol monophosphates in the Wistar rat pineal gland. The relative potency of muscarinic antagonists (atropine greater than pirenzepine much greater than gallamine) in blocking this response suggests the involvement of muscarinic M1 receptors. Following bilateral superior cervical ganglionectomy, the carbachol-induced phosphoinositide response was similar to that seen in sham-operated controls, while adrenergic response was enhanced by 50%. The results indicate that the pineal has functional muscarinic receptors which are not affected by removal of the sympathetic innervation to the gland.

Animals↗
PubMed1989

Diurnal rhythm of melatonin binding in the rat suprachiasmatic nucleus.

We used quantitative in vitro autoradiography to localize and characterize 2-125I-melatonin binding sites in the rat suprachiasmatic nuclei in relation to pineal melatonin production. In a light:dark cycle of 12:12 h, binding density exhibited significant diurnal variation with a peak at the dark-light transition and a trough 12 hours later. Saturation studies suggested that the decreased binding at light-dark transition might be due to a shift of the putative melatonin receptor to a low affinity state.

Animals↗
PubMedDate not supplied

The prediction of the adaptation of circadian rhythms to rapid time zone changes.

The aim of the study was to find out which factors could explain individual differences in the resynchronization speed of circadian rhythms of salivary melatonin and subjective alertness after transmeridian flights over 10 time zones. The mean age of the 40 female subjects was 33.0 +/- 6.9 years. The data were gathered by measurements of the circadian rhythms of melatonin excretion and altertness at 2 h intervals in Helsinki (Finland) two days before westward flight to Los Angeles (USA), where the measurements were repeated on the 2nd day after the flights and on the 2nd day in Finland after return flight. This shift in the acrophases of the two circadian rhythms were used as dependent variables in regression analyses. The predictors used were length of day, marital status, amount of physical exercise, age, neuroticism, extroversion, and eveningness. Age, day length, marital status, and physical exercise explained the acrophase adaptation of the melatonin rhythm after westward flight, and day length, neuroticism, and extroversion after eastward flight. Marital status, neuroticism, and physical exercise explained the variation of the acrophase adjustment of the alertness rhythm after westward flight, and age and eveningness after eastward flight. It is concluded that the amount of daylight and personality are the best predictors of the circadian rhythm adaptation after transmeridian flights.

Adaptation, Physiological↗
PubMedDate not supplied

Vascular melatonin receptors.

High-affinity melatonin receptors are expressed in the tail artery of the rat and arteries forming the circle of Willis of the rat and certain primates. The characteristics of the vascular melatonin receptors seem to be similar to the ones described in the central nervous system. The expression of vascular melatonin receptors in the rat is differentially regulated by factors such as strain and age, and in the female by reproductive hormones. Functional studies using the caudal artery of the rat suggest that melatonin regulates vascular tone. The highly restricted distribution of melatonin receptors in arteries involved in regulating blood flow to areas involved in heat dissipation suggests that vascular melatonin receptors may have a specific function in thermoregulatory homeostasis.

Animals↗
PubMedDate not supplied

On the respective roles of nitric oxide and carbon monoxide in long-term potentiation in the hippocampus.

Perfusion of hippocampal slices with an inhibitor nitric oxide (NO) synthase blocked induction of long-term potentiation (LTP) produced by a one-train tetanus and significantly reduced LTP by a two-train tetanus, but only slightly reduced LTP by a four-train tetanus. Inhibitors of heme oxygenase, the synthetic enzyme for carbon monoxide (CO), significantly reduced LTP by either a two-train or four-train tetanus. These results suggest that NO and CO are both involved in LTP but may play somewhat different roles. One possibility is that NO serves a phasic, signaling role, whereas CO provides tonic, background stimulation. Another possibility is that NO and CO are phasically activated under somewhat different circumstances, perhaps involving different receptors and second messengers. Because NO is known to be activated by stimulation of NMDA receptors during tetanus, we investigated the possibility that CO might be activated by stimulation of metabotropic glutamate receptors (mGluRs). Consistent with this idea, long-lasting potentiation by the mGluR agonist tACPD was blocked by inhibitors of heme oxygenase but not NO synthase. Potentiation by tACPD was also blocked by inhibitors of soluble guanylyl cyclase (a target of both NO and CO) or cGMP-dependent protein kinase, and guanylyl cyclase was activated by tACPD in hippocampal slices. However, biochemical assays indicate that whereas heme oxygenase is constitutively active in hippocampus, it does not appear to be stimulated by either tetanus or tACPD. These results are most consistent with the possibility that constitutive (tonic) rather than stimulated (phasic) heme oxygenase activity is necessary for potentiation by tetanus or tACPD, and suggest that mGluR activation stimulates guanylyl cyclase phasically through some other pathway.

Animals↗
PubMedDate not supplied

On the respective roles of nitric oxide and carbon monoxide in long-term potentiation in the hippocampus.

Perfusion of hippocampal slices with an inhibitor of nitric oxide (NO) synthase-blocked induction of long-term potentiation (LTP) produced by a one-train tetanus and significantly reduced LTP by a two-train tetanus, but only slightly reduced LTP by a four-train tetanus. Inhibitors of heme oxygenase, the synthetic enzyme for carbon monoxide (CO), significantly reduced LTP by either a two-train or four-train tetanus. These results suggest that NO and CO are both involved in LTP but may play somewhat different roles. One possibility is that NO serves a phasic, signaling role, whereas CO provides tonic, background stimulation. Another possibility is that NO and CO are phasically activated under somewhat different circumstances, perhaps involving different receptors and second messengers. Because NO is known to be activated by stimulation of NMDA receptors during tetanus, we investigated the possibility that CO might be activated by stimulation of metabotropic glutamate receptors (mGluRs). Consistent with this idea, long-lasting potentiation by the mGluR agonist tACPD was blocked by inhibitors of heme oxygenase but not NO synthase. Potentiation by tACPD was also blocked by inhibitors of soluble guanylyl cyclase (a target of both NO and CO) or cGMP-dependent protein kinase, and guanylyl cyclase was activated by tACPD in hippocampal slices. However, biochemical assays indicate that whereas heme oxygenase is constitutively active in hippocampus, it does not appear to be stimulated by either tetanus or tACPD. These results are most consistent with the possibility that constitutive (tonic) rather than stimulated (phasic) heme oxygenase activity is necessary for potentiation by tetanus or tACPD, and suggest that mGluR activation stimulates guanylyl cyclase phasically through some other pathway.

Animals↗