Methods to Assess Melatonin Receptor-Mediated Phase-Shift and Re-entrainment of Rhythmic Behaviors in Mouse Models

The neurohormone melatonin facilitates entrainment of biological rhythms to environmental light-dark conditions as well as phase-shifts of circadian rhythms in constant conditions via activation of the MT₁ and/or MT₂ receptors expressed within the suprachiasmatic nucleus of the hypothalamus. The efficacy of melatonin and related agonists to modulate biological rhythms can be assessed using two well-validated mouse models of rhythmic behaviors. These models serve as predictive measures of therapeutic efficacy for treatment of circadian phase disorders caused by internal (e.g., clock gene mutations, blindness, depression, seasonal affective disorder) or external (e.g., shift work, travel across time zones) causes in humans. Here we provide background and detailed protocols for quantitative assessment of the magnitude and efficacy of melatonin receptor ligands in mouse circadian phase-shift and re-entrainment paradigms. The utility of these models in the discovery of novel therapeutics acting on melatonin receptors will also be discussed.

Impact of endogenous melatonin on rhythmic behaviors, reproduction, and survival revealed in melatonin-proficient C57BL/6J congenic mice

The hormone melatonin is synthesized from serotonin by two enzymatic reactions (AANAT and ASMT/HIOMT) in the pineal gland following a circadian rhythm with low levels during the day and high levels at night. The robust nightly peak of melatonin secretion is an output signal of the circadian clock to the whole organism. However, so far the regulatory roles of endogenous melatonin in mammalian biological rhythms and physiology processes are poorly understood. Here, we establish congenic mouse lines (>N10 generations) that are proficient or deficient in melatonin synthesis (AH+/+ or AH-/- mice, respectively) on the C57BL/6J genetic background by crossing melatonin-proficient MSM/Ms with C57BL/6J. AH+/+ mice displayed robust nightly peak of melatonin secretion and had significantly higher levels of pineal and plasma melatonin vs AH-/- mice. Using this mice model, we investigated the role of endogenous melatonin in regulating multiple biological rhythms, physiological processes, and rhythmic behaviors. In the melatonin-proficient (AH+/+) mice, the rate of re-entrainment of wheel-running activity was accelerated following a 6-hour phase advance of dark onset when comparted with AH-/- mice, suggesting a role of endogenous melatonin in facilitating clock adjustment. Further in the AH+/+ mice, there was a significant decrease in body weight, gonadal weight and reproductive performance, and a significant increase in daily torpor (a hypothermic and hypometabolic state lasting only hours during adverse conditions). Endogenous melatonin, however, had no effect in the modulation of the diurnal rhythm of 2-[¹²⁵ I]-iodomelatonin receptor expression in the SCN, free-running wheel behavior in constant darkness, life span, spontaneous homecage behaviors, and various types of social-emotional behaviors. The findings also shed light on the role of endogenous melatonin in mice domestication and provide new insights into melatonin's action in reducing energy expenditure during a food shortage. In summary, the congenic mice model generated in this study offers a significant advantage toward understanding of the role of endogenous melatonin in regulating melatonin receptor-mediated rhythm behaviors and physiological functions.

Food-induced reinforcement is abrogated by the genetic deletion of the MT1 or MT2 melatonin receptor in C3H/HeN mice

Palatable food is known for its ability to enhance reinforcing responses. Studies have suggested a circadian variation in both drug and natural reinforcement, with each following its own time course. The goal of this study was to determine the role of the MT1 and MT2 melatonin receptors in palatable snack food-induced reinforcement, as measured by the conditioned place preference (CPP) paradigm during the light and dark phases. C3H/HeN wild-type mice were trained for snack food-induced CPP at either ZT 6 - 8 (ZT: Zeitgeber time; ZT 0 = lights on), when endogenous melatonin levels are low, or ZT 19 - 21, when melatonin levels are high. These time points also correspond to the high and low points for expression of the circadian gene Period1, respectively. The amount of snack food (chow, Cheetos®, Froot Loops® and Oreos®) consumed was of similar magnitude at both times, however only C3H/HeN mice conditioned to snack food at ZT 6 - 8 developed a place preference. C3H/HeN mice with a genetic deletion of either the MT1 (MT1KO) or MT2 (MT2KO) receptor tested at ZT 6 - 8 did not develop a place preference for snack food. Although the MT2KO mice showed a similar amount of snack food consumed when compared to wild-type mice, the MT1KO mice consumed significantly less than either genotype. We conclude that in our mouse model snack food-induced CPP is dependent on time of day and the presence of the MT1 or MT2 receptors, suggesting a role for melatonin and its receptors in snack food-induced reinforcement.

Learned motivation drives circadian physiology in the absence of the master circadian clock

The suprachiasmatic nucleus (SCN)-often referred to as the master circadian clock-is essential in generating physiologic rhythms and orchestrating synchrony among circadian clocks. This study tested the hypothesis that periodic motivation induced by rhythmically pairing 2 reinforcing stimuli [methamphetamine (Meth) and running wheel (RW)] restores autonomous circadian activity in arrhythmic SCN-lesioned (SCNX) C3H/HeN mice. Sham-surgery and SCNX mice were treated with either Meth (1.2 mg/kg, i.p.) or vehicle in association, dissociation, or absence of an RW. Only the association of Meth treatment and restricted RW access successfully reestablished entrained circadian rhythms in mice with SCNX. RW-likely acting as a link between the circadian and reward systems-promotes circadian entrainment of activity. We conclude that a conditioned drug response is a powerful tool to entrain, drive, and restore circadian physiology. Furthermore, an RW should be recognized as a potent input signal in addition to the conventional use as an output signal.-Rawashdeh, O., Clough, S. J., Hudson, R. L., Dubocovich, M. L. Learned motivation drives circadian physiology in the absence of the master circadian clock.

Melatonin receptor activation increases glutamatergic synaptic transmission in the rat medial lateral habenula

Melatonin (MLT) is secreted from the pineal gland and mediates its physiological effects through activation of two G protein-coupled receptors, MT1 and MT2 . These receptors are expressed in several brain areas, including the habenular complex, a pair of nuclei that relay information from forebrain to midbrain and modulate a plethora of behaviors, including sleep, mood, and pain. However, so far, the precise mechanisms by which MLT control the function of habenula neurons remain unknown. Using whole cell recordings from male rat brain slices, we examined the effects of MLT on the excitability of medial lateral habenula (MLHb) neurons. We found that MLT had no significant effects on the intrinsic excitability of MLHb neurons, but profoundly increased the amplitude of glutamate-mediated evoked excitatory post-synaptic currents (EPSC). The increase in strength of glutamate synapses onto MLHb neurons was mediated by an increase in glutamate release. The MLT-induced increase in glutamatergic synaptic transmission was blocked by the competitive MT1 /MT2 receptor antagonist luzindole (LUZ). These results unravel a potential cellular mechanism by which MLT receptor activation enhances the excitability of MLHb neurons. The MLT-mediated control of glutamatergic inputs to the MLHb may play a key role in the modulation of various behaviors controlled by the habenular complex.

Genetic deletion of MT1 melatonin receptors alters spontaneous behavioral rhythms in male and female C57BL/6 mice

Behaviors vary over the 24h light/dark cycle and these temporal patterns reflect in part modulation by circadian neural circuits and hormones, such as melatonin. The goal of this study was to investigate the involvement of MT1 melatonin receptors in behavioral regulation by comparing male and female C57 wild type (WT) mice with C57 mice with genetic deletion of the MT1 receptor (MT1KO). A comprehensive array of fifteen distinct spontaneous behaviors was recorded continuously in the homecage over multiple days using the HomeCageScan system. Behaviors assessed were activity-like (i.e. come down, hang, jump, walk), exploration-like (i.e. dig, groom, rear up, sniff, stretch), resting-like (i.e. awake, remain low, rest, twitch) and ingestion-like (i.e. drink, eat). Phenotypic array and temporal distribution analysis revealed distinct behavioral rhythms that differed between WT and MT1KO mice. The rhythms were consistent from day to day in males and varied with the estrous cycle in females. We also studied the role of MT1 receptors on depressive and anxiety-like behaviors. Genetic deletion of MT1 receptors increased immobility time in the forced swim test and decreased the number of marbles buried in the marble burying test in both male and female C57 mice. We conclude that MT1 melatonin receptors are involved in neural pathways modulating diurnal rhythms of spontaneous behavior in the homecage as well as pathways regulating depressive and anxiolytic-like behaviors.

Genetic deletion of the MT1 or MT2 melatonin receptors abrogates methamphetamine-induced reward in C3H/HeN mice

The drug of abuse methamphetamine (METH) is known for its ability to enhance reward responses. The rewarding properties of psychostimulants have been shown to vary across time of day in mice. The goal of this study was to determine the role of the MT1 and MT2 melatonin receptors in METH-induced reward, as measured by the conditioned place preference (CPP) paradigm during the light and dark phases. C3H/HeN wild-type mice were trained for METH-induced CPP at either ZT 6-8 (ZT: Zeitgeber time; ZT 0=lights on), when endogenous melatonin levels are low, or ZT 19-21, when melatonin levels are high. These time points also correspond to the high and low points for expression of the circadian gene Period1, respectively. The locomotor response to METH (1.2mg/kg, ip) treatment was of similar magnitude at both times; however only C3H/HeN mice conditioned to METH at ZT 6-8 developed a place preference. C3H/HeN mice with a genetic deletion of either the MT1 (MT1KO) or MT2 (MT2KO) receptor tested at ZT 6-8 or ZT 19-21 did not develop a place preference for METH, though both showed a similar increase in locomotor activity following METH treatment when compared to wild-type mice. We conclude that in our mouse model METH-induced CPP is dependent on time of day and the presence of the MT1 or MT2 receptors, suggesting a role for melatonin in METH-induced reward.

Role of MT1 melatonin receptors in methamphetamine-induced locomotor sensitization in C57BL/6 mice

RATIONALE

Melatonin modifies physiological and behavioral responses to psychostimulants, with the MT₁ and MT₂ melatonin receptors specifically implicated in facilitating methamphetamine (METH)-induced sensitization in melatonin-proficient mice.

OBJECTIVE

The objective of the study is to assess differences in locomotor sensitization after a single dose of methamphetamine in low-melatonin-expressing C57BL/6 wild-type and MT₁ receptor knockout (MT₁KO) mice, comparing with melatonin-expressing C3H/HeN mice.

METHODS

Mice received a vehicle or methamphetamine (1.2 mg/kg, i.p.) pretreatment (day 1) during the light (ZT5-9) or dark (ZT 19-21) periods in novel test arenas. Locomotor sensitization was assessed by methamphetamine challenge after an eight-day abstinence (day 9). TH protein expression was evaluated by immunofluorescence and Western blot analysis.

RESULTS

Methamphetamine pretreatment induced statistically significant locomotor sensitization upon challenge after eight-day abstinence in C3H and C57 wild-type mice during the light period. The magnitude of sensitization in C57 mice was diminished in the dark period and completely abrogated in MT₁KO mice. No differences were observed in tyrosine hydroxylase immunoreactivity in the mesolimbic dopamine system. Additional exposures to the test arenas after methamphetamine pretreatment (nights 2-6) enhanced sensitization.

CONCLUSIONS

Deletion of the MT₁ melatonin receptor abolishes sensitization induced by a single METH pretreatment. The magnitude of sensitization is also altered by time of day and contextual cues. We conclude that the MT₁ melatonin receptor is emerging as a novel target of therapeutic intervention for drug abuse disorders.

Melatonin potentiates running wheel-induced neurogenesis in the dentate gyrus of adult C3H/HeN mice hippocampus

This study assessed the role of melatonin in modulating running wheel(RW)-induced hippocampal neurogenesis in adult C3H/HeN mice. Chronic melatonin (0.02 mg/mL, oral for 12 days) treatment did not affect cell proliferation or cell survival determined by the number of BrdU-positive cells in dentate gyrus of mice with access to fixed wheel (FW). RW activity significantly increased cell proliferation [RW (n = 8) versus FW (n = 6): dorsal, 199 ± 18 versus 125 ± 12, P < 0.01; ventral, 211 ± 15 versus 123 ± 13, P < 0.01] and newborn cell survival [RW (n = 7) versus FW (n = 8): dorsal, 45 ± 8.5 versus 15 ± 1.8, P < 0.01; ventral, 48 ± 8.1 versus 15 ± 1.4)] in the dorsal and ventral dentate gyrus. Oral melatonin treatment further potentiated RW activity-induced cell survival in both areas of the dentate gyrus [melatonin (n = 10) versus vehicle (n = 7): dorsal, 63 ± 5.4 versus 45 ± 8.5 P < 0.05; ventral, 75 ± 7.9 versus 48 ± 8.1, P < 0.01] and neurogenesis [melatonin (n = 8) versus vehicle (n = 8): dorsal, 46 ± 3.4, versus 34 ± 4.5, P < 0.05; ventral, 41 ± 3.4 versus 25 ± 2.4, P < 0.01]. We conclude that melatonin potentiates RW-induced hippocampal neurogenesis by enhancing neuronal survival suggesting that the combination of physical exercise and melatonin may be an effective treatment for diseases affecting the hippocampus neurogenesis.

Automated video analysis system reveals distinct diurnal behaviors in C57BL/6 and C3H/HeN mice

Advances in rodent behavior dissection using automated video recording and analysis allows detailed phenotyping. This study compared and contrasted 15 diurnal behaviors recorded continuously using an automated behavioral analysis system for a period of 14 days under a 14/10 light/dark cycle in single housed C3H/HeN (C3H) or C57BL/6 (C57) male mice. Diurnal behaviors, recorded with minimal experimental interference and analyzed using phenotypic array and temporal distribution analysis showed bimodal and unimodal profiles in the C57 and C3H mice, respectively. Phenotypic array analysis revealed distinct behavioral rhythms in Activity-Like Behaviors (i.e. walk, hang, jump, come down) (ALB), Exploration-Like Behaviors (i.e. dig, groom, rear up, sniff, stretch) (ELB), Ingestion-Like Behaviors (i.e. drink, eat) (ILB) and Resting-Like Behaviors (i.e. awake, remain low, rest, twitch) (RLB) of C3H and C57 mice. Temporal distribution analysis demonstrated that strain and time of day affects the magnitude and distribution of the spontaneous homecage behaviors. Wheel running activity, water and food measurements correlated with timing of homecage behaviors. Subcutaneous (3 mg/kg, sc) or oral (0.02 mg/ml, oral) melatonin treatments in C57 mice did not modify either the total 24 h magnitude or temporal distribution of homecage behaviors when compared with vehicle treatments. We conclude that C3H and C57 mice show different spontaneous activity and behavioral rhythms specifically during the night period which are not modulated by melatonin.

Genetic deletion of MT(1) and MT(2) melatonin receptors differentially abrogates the development and expression of methamphetamine-induced locomotor sensitization during the day and the night in C3H/HeN mice

This study explored the role of the melatonin receptors in methamphetamine (METH)-induced locomotor sensitization during the light and dark phases in C3H/HeN mice with genetic deletion of the MT(1) and/or MT(2) melatonin receptors. Six daily treatments with METH (1.2 mg/kg, i.p.) in a novel environment during the light phase led to the development of locomotor sensitization in wild-type (WT), MT(1)KO and MT(2)KO mice. Following four full days of abstinence, METH challenge (1.2 mg/kg, i.p.) triggered the expression of locomotor sensitization in METH-pretreated but not in vehicle (VEH)-pretreated mice. In MT(1)/MT(2)KO mice, the development of sensitization during the light phase was significantly reduced and the expression of sensitization was completely abrogated upon METH challenge. During the dark phase the development of locomotor sensitization in METH-pretreated WT, MT(1)KO and MT(2)KO mice was statistically different from VEH-treated controls. However, WT and MT(2)KO, but not MT(1)KO mice receiving repeated VEH pretreatments during the dark phase expressed a sensitized response to METH challenge that is of an identical magnitude to that observed upon 6 days of METH pretreatment. We conclude that exposure to a novel environment during the dark phase, but not during the light phase, facilitated the expression of sensitization to a METH challenge in a manner dependent on MT(1) melatonin receptor activation by endogenous melatonin. We suggest that MT(1) and MT(2) melatonin receptors are potential targets for pharmacotherapeutic intervention in METH abusers.

Circadian periods of sensitivity for ramelteon on the onset of running-wheel activity and the peak of suprachiasmatic nucleus neuronal firing rhythms in C3H/HeN mice

Ramelteon, an MT(1)/MT(2) melatonin receptor agonist, is used for the treatment of sleep-onset insomnia and circadian sleep disorders. Ramelteon phase shifts circadian rhythms in rodents and humans when given at the end of the subjective day; however, its efficacy at other circadian times is not known. Here, the authors determined in C3H/HeN mice the maximal circadian sensitivity for ramelteon in vivo on the onset of circadian running-wheel activity rhythms, and in vitro on the peak of circadian rhythm of neuronal firing in suprachiasmatic nucleus (SCN) brain slices. The phase response curve (PRC) for ramelteon (90 µg/mouse, subcutaneous [sc]) on circadian wheel-activity rhythms shows maximal sensitivity during the late mid to end of the subjective day, between CT8 and CT12 (phase advance), and late subjective night and early subjective day, between CT20 and CT2 (phase delay), using a 3-day-pulse treatment regimen in C3H/HeN mice. The PRC for ramelteon resembles that for melatonin in C3H/HeN mice, showing the same magnitude of maximal shifts at CT10 and CT2, except that the range of sensitivity for ramelteon (CT8-CT12) during the subjective day is broader. Furthermore, in SCN brain slices in vitro, ramelteon (10 pM) administered at CT10 phase advances (5.6 ± 0.29 h, n = 3) and at CT2 phase delays (-3.2 ± 0.12 h, n = 6) the peak of circadian rhythm of neuronal firing, with the shifts being significantly larger than those induced by melatonin (10 pM) at the same circadian times (CT10: 2.7 ± 0.15 h, n = 4, p < .05; CT2: -1.13 ± 0.08 h, n = 6, p < .001, respectively). The phase shifts induced by both melatonin and ramelteon in the SCN brain slice at either CT10 or CT2 corresponded with the period of sensitivity observed in vivo. In conclusion, melatonin and ramelteon showed identical periods of circadian sensitivity at CT10 (advance) and CT2 (delay) to shift the onset of circadian activity rhythms in vivo and the peak of SCN neuronal firing rhythms in vitro.

Effect of MT1 melatonin receptor deletion on melatonin-mediated phase shift of circadian rhythms in the C57BL/6 mouse

In the mouse suprachiasmatic nucleus (SCN), melatonin activates MT1 and MT2 G-protein coupled receptors, which are involved primarily in inhibition of neuronal firing and phase shift of circadian rhythms. This study investigated the ability of melatonin to phase shift circadian rhythms in wild type (WT) and MT1 melatonin receptor knockout (KO) C57BL/6 mice. In WT mice, melatonin (90 microg/mouse, s.c.) administered at circadian time 10 (CT10; CT12 onset of activity) significantly phase advanced the onset of the circadian activity rhythm (0.60 +/- 0.09 hr, n = 41) when compared with vehicle treated controls (-0.02 +/- 0.07 hr, n = 28) (P < 0.001). In contrast, C57 MT1KO mice treated with melatonin did not phase shift circadian activity rhythms (-0.10 +/- 0.12 hr, n = 42) when compared with vehicle treated mice (-0.12 +/- 0.07 hr, n = 43). Similarly, in the C57 MT1KO mouse melatonin did not accelerate re-entrainment to a new dark onset after an abrupt advance of the dark cycle. In contrast, melatonin (3 and 10 pm) significantly phase advanced circadian rhythm of neuronal firing in SCN brain slices independent of genotype with an identical maximal shift at 10 pm (C57 WT: 3.61 +/- 0.38 hr, n = 3; C57 MT(1)KO: 3.45 +/- 0.11 hr, n = 4). Taken together, these results suggest that melatonin-mediated phase advances of circadian rhythms of neuronal firing in the SCN in vitro may involve activation of the MT2 receptor while in vivo activation of the MT1 and possibly the MT2 receptor may be necessary for the expression of melatonin-mediated phase shifts of overt circadian activity rhythms.

Melatonin desensitizes endogenous MT2 melatonin receptors in the rat suprachiasmatic nucleus: relevance for defining the periods of sensitivity of the mammalian circadian clock to melatonin

The hormone melatonin phase shifts circadian rhythms generated by the mammalian biological clock, the suprachiasmatic nucleus (SCN) of the hypothalamus, through activation of G protein-coupled MT2 melatonin receptors. This study demonstrated that pretreatment with physiological concentrations of melatonin (30-300 pM or 7-70 pg/mL) decreased the number of hMT2 melatonin receptors heterologously expressed in mammalian cells in a time and concentration-dependent manner. Furthermore, hMT2-GFP melatonin receptors heterologously expressed in immortalized SCN2.2 cells or in non-neuronal mammalian cells were internalized upon pretreatment with both physiological (300 pM or 70 pg/mL) and supraphysiological (10 nM or 2.3 ng/mL) concentrations of melatonin. The decrease in MT2 melatonin receptor number induced by melatonin (300 pM for 1 h) was reversible and reached almost full recovery after 8 h; however, after treatment with 10 nM melatonin full recovery was not attained even after 24 h. This recovery process was partially protein synthesis dependent. Furthermore, exposure to physiological concentrations of melatonin (300 pM) for a time mimicking the nocturnal surge (8 h) desensitized functional responses mediated through melatonin activation of endogenous MT2 receptors, i.e., stimulation of protein kinase C (PKC) in immortalized SCN2.2 cells and phase shifts of circadian rhythms of neuronal firing in the rat SCN brain slice. We conclude that in vivo the nightly secretion of melatonin desensitizes endogenous MT2 melatonin receptors in the mammalian SCN thereby providing a temporally integrated profile of sensitivity of the mammalian biological clock to a melatonin signal.

Mid- and far-infrared spectroscopic studies of the influence of temperature, ultraviolet photolysis and ion irradiation on cosmic-type ices

Infrared (IR) studies of laboratory ices can provide information on the evolution of cosmic-type ices as a function of different simulated space environments involving thermal, ultraviolet (UV), or ion processing. Laboratory radiation experiments can lead to the formation of complex organic molecules. However, because of our lack of knowledge about UV photon and ion fluxes, and exposure lifetimes, it is not certain how well our simulations represent space conditions. Appropriate laboratory experiments are also limited by the absence of knowledge about the composition, density, and temperature of ices in different regions of space. Our current understanding of expected doses due to UV photons and cosmic rays is summarized here, along with an inventory of condensed-phase molecules identified on outer solar system surfaces, comets and interstellar grains. Far-IR spectra of thermally cycled H2O are discussed since these results reflect the dramatic difference between the amorphous and crystalline phases of H2O ice, the most dominant condensed-phase molecule in cosmic ices. A comparison of mid-IR spectra of products in proton-irradiated and UV-photolyzed ices shows that few differences are observed for these two forms of processing for the simple binary mixtures studied to date. IR identification of radiation products and experiments to determine production rates of new molecules in ices during processing are discussed. A new technique for measuring intrinsic IR band strengths of several unstable molecules is presented. An example of our laboratory results applied to Europa observations is included.

A quick screening test of competency to stand trial for defendants with mental retardation

19 terms and concepts from evaluations of competency to stand trial of 55 defendants with mental retardation were rated to examine whether a quick screening test could be devised that would differentiate those who were judged competent or not competent. A multiple regression and discriminant analysis gave four items that yielded maximum predictability (R = .84): court strategy, plead, testify, and jury. Guilty, trial, and prosecutor were also significantly more difficult for those who were not competent than those who were.

Bafilomycin-sensitive acid secretion by mantle epithelium of the freshwater clam, Unio complanatus

This study reports the effects of a series of putative inhibitors of proton translocation on electrogenic acid secretion by the isolated shell-facing mantle epithelium of the freshwater clam, Unio complanatus. In this epithelium, adenosine 3',5'-cyclic monophosphate (cAMP)-dependent acid secretion is electrogenic and equivalent to the cAMP-dependent short-circuit current (Isc) when mounted in a Ussing-type flux chamber. Bafilomycin A1, a specific inhibitor of vacuolar-type H(+)-ATPases (V-type ATPases), inhibited cAMP-dependent electrogenic acid secretion, with an IC50 of 7 nM and a maximum effect at 100 nM. Inhibition of acid secretion by bafilomycin A1 occurred only when added to the shell side and was not reversed by repeated washings. Concomitant with the inhibition of cAMP-dependent Isc by bafilomycin A1, the electrical potential difference across the shell-facing membrane depolarized, and the conductance ratio of the apical to basolateral membranes decreased. The cAMP-dependent Isc and cAMP-dependent acid secretion also were inhibited by dicyclohexyl-carbodiimide, diethylstilbestrol, N-ethylmaleimide, and tributyltin. Amiloride, omeprazole, oligomycin, venturicidin, and ouabain had no effect on either cAMP-dependent Isc or cAMP-dependent acid secretion. The cAMP-dependent Isc and cAMP-dependent acid secretion were not affected by Na(+)-free or K(+)-free solutions. Finally, the cAMP-dependent Isc was strongly dependent on the pH of the shell-facing compartment, with 50% inhibition of the maximum cAMP-dependent Isc occurring at pH 6.0 and complete inhibition at pH 5.5.(ABSTRACT TRUNCATED AT 250 WORDS)

Ion transport by the isolated mantle epithelium of the freshwater clam, Unio complanatus

Transepithelial ion transport was studied in three types of shell-facing mantle epithelia of the freshwater clam, Unio complanatus. These were 1) the epithelium isolated from near the margin of the shell; 2) the epithelium from the central region of a "normal" shell; and 3) the epithelium from the central region that was regenerating a damaged shell. Marginal and "shell-regenerating" epithelia produced a small (less than 5 mV) transepithelial voltage that was independent of PCO2 (0-4%). In the presence of 4% PCO2, the "normal" central epithelium produced a significantly larger transepithelial voltage (-8.5 +/- 0.99 mV), that, in addition, could be stimulated (-30.5 +/- 2.18 mV) by permeable analogues of adenosine 3',5'-cyclic monophosphate (cAMP) [e.g., 8-(4-chlorophenylthio)-adenosine 3',5'-cyclic monophosphate]. Voltage clamping the transepithelial voltage to zero resulted in a short-circuit current (Isc) that was consistent with positive charge moving from the hemolymph toward the shell. The measurements of transepithelial fluxes of 45Ca, 22Na, 42K, and 36Cl revealed that none of these ions, alone or in combination, could account for the stimulated Isc in "normal" mantle. In addition, similar measurements in marginal and "shell-regenerating" epithelia did not detect any significant net transepithelial flux of any of these ions. Acid extrusion at the shell-facing membrane and bicarbonate extrusion at the hemolymph-facing membrane were identified in "normal" central epithelium, but could not be found in marginal or "shell-regenerating" epithelia. These fluxes are equivalent to the cAMP-stimulated Isc in "normal" central epithelium.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium-coupled glycine uptake by Ehrlich ascites tumor cells results in an increase in cell volume and plasma membrane channel activities

The addition of 10 mM glycine to a physiological saline bathing Ehrlich ascites tumor cells is followed by a slow increase in cell volume that plateaus between 15 and 30 min at a level approximately equal to 17% greater than the control volume; this increase is not observed when glycine is added to cells suspended in a Na+-free saline. The results of studies using the patch-clamp technique in the cell-attached mode indicate that, 0.5-3 min after the addition of glycine to the bathing solution, there is a marked increase in the activity of single channels, which is almost all instances were previously present and operant in the plasma membrane. Successfully excised patches of membrane that contained a channel stimulated by glycine fell into two categories. Some became inactive within 15 sec in spite of the fact that the G omega seal remained intact. Others persisted for the lifetime of the seal. All of the persistent channels had an 11-fold selectivity for Cl- over K+ and a conductance of 23 pS when bathed by symmetrical 150 mM KCl solutions. Although the ionic specificities of the other channels have not been identified, there is reason to suspect that they might be K+ channels whose activities are dependent on factors lost when the patch is excised. Swelling induced by exposing these cells to a 50% hypotonic perfusate stimulated the activities of Cl- channels whose properties closely resemble those stimulated by the addition of glycine to the perfusate, strongly suggesting that the glycine-induced stimulation of Cl- channel activity is part of a volume-regulatory response to cell swelling. If the increase in channel activity induced by the addition of glycine to the perfusate is indeed a response to cell swelling, then this perfusate is indeed a response to cell swelling, then this volume-regulatory response must be extremely sensitive inasmuch as it appears to be "triggered" by an average increase in cell volume that does not exceed 5%.

Cell Na+ activities and transcellular Na+ absorption by descending colon from normal and Na+-deprived rabbits

The relation between intracellular Na+ activities, (Na)c, determined employing Na+-selective microelectrodes, and the rates of active Na+ absorption, INa, by rabbit descending colon was examined when INa was varied over a wide range by chronic dietary Na+ deprivation. (Na)c averaged 13 mM and was independent of INa over a sixfold range. Further, the ratios of the slope resistance of the apical membrane (rm) to that of the basolateral membrane (rs) (i.e. rm/rs) in low-transporters (control diet) and high-transporters (Na+-deprived) did not differ significantly inspite of the fact that the Na+ conductance of the apical membranes of high-transporters was, on the average, three times greater than that of the low-transporters. These findings, together with the results reported by other laboratories, strongly suggest that the aldosterone-induced increase in the conductance of the apical membrane to Na+ and, in turn, the rate of entry of Na+ into the absorptive cells are followed by parallel increases in the ability of cells to extrude Na+ across the basolateral membrane in the absence of a sustained increase in (Na)c as well as the conductance of that barrier.

Current-voltage relations of sodium-coupled sugar transport across the apical membrane of Necturus small intestine

The current-voltage (I-V) relations of the rheogenic Na-sugar cotransport mechanism at the apical membrane of Necturus small intestine were determined from the relations between the electrical potential difference across the apical membrane, psi mc, and that across the entire epithelium, psi ms, when the latter was varied over the range +/- 200 mV, under steady conditions in the presence of galactose and after the current across the apical membrane carried by the cotransporter, ImSNa, is blocked by the addition of phloridzin to the mucosal solution. ImSNa was found to be strongly dependent upon psi mc over the range -50 mV less than psi mc less than EmSNa where EmSNa is the "zero current" or "reversal" potential. Over the range of values of psi mc encountered under physiological conditions the cotransporter may be modeled as a conductance in series with an electromotive force so that ImSNa = gmSNa (EmSNa - psi mc) where gmSNa is the contribution of this mechanism to the conductance of the apical membrane and is "near constant." In several instances ImSNa "saturated" at large hyperpolarizing or depolarizing values of psi mc. The values of EmSNa determined in the presence of 1, 5, and 15 mM galactose strongly suggest that if the Na-galactose cotransporters are kinetically homogeneous, the stoichiometry of this coupled process is unity. Finally, the shapes of the observed I-V relations are consistent with the predictions of a simple kinetic model which conforms with current notions regarding the mechanico-kinetic properties of this cotransport process.

Transcellular sodium transport and intracellular sodium activities in rabbit gallbladder

This study was designed to explore the relation between the rate of transcellular active Na+ transport by rabbit gallbladder epithelium, JNa, and the intracellular Na+ activity, (Na)c; the latter was determined by use of highly selective Na+ microelectrodes. The underlying strategy was based on the well-established observation that JNa is stimulated by the presence of bicarbonate in the bathing solutions. Our results confirm previous observations that the addition of bicarbonate to the bathing solutions results in a twofold increase in JNa. In the absence of bicarbonate, (Na)c averaged 16 mM. Within 2-4 min after the addition of bicarbonate to both bathing solutions, (Na)c increased to an average value of 22 mM and then gradually declined and by 15 min did not differ significantly from the value observed in the absence of bicarbonate. Thus, a twofold increase in JNa is not associated with an increase in (Na)c. These results are in accord with earlier observations on Necturus urinary bladder and small intestine and contradict the notion that an increase in the rate of active Na+ extrusion from the cell across the basolateral membrane in response to an increase in the rate of Na+ entry across the apical membrane is necessarily the result of a sustained increase in (Na)c.

Effect of hypertonicity on the increase in basolateral conductance of Necturus small intestine in response to Na+-sugar cotransport

Exposure of Necturus small intestine to a galactose-containing perfusate that is 20% hypertonic compared to the galactose-free (control) perfusate results in a rapid depolarization of the electrical potential difference across the apical membrane, psi mc, and a decrease in the ratio of the resistance of the apical membrane to that of the basolateral membrane, (rm/rs); however, the slow repolarization of psi mc and increase in (rm/rs), observed under isotonic conditions, is blocked. These findings are consistent with the notion that the increase in the conductance of the basolateral membrane in response to Na+-coupled sugar (or amino acid) transport across the apical membrane may be a 'volume regulatory response' to cell swelling.

Electrophysiological studies of sodium cotransport in epithelia: toward a cellular model

During the past two decades, microelectrophysiological studies of small intestine and renal proximal tubule employing conventional as well as ion-selective microelectrodes have contributed significantly to our understanding of the nature of Na-coupled entry processes at the apical membrane as well as the overall workings of the simple model illustrated in FIGURE 1. These studies have unequivocally established the rheogenic and conductive nature of the Na-coupled sugar and amino-acid entry processes across the apical membrane of small intestine (and renal proximal tubule) and have, in addition, disclosed that the properties of the basolateral membrane respond to an increase in Na-coupled solute entry with an increase in the ability of the Na-K pump to extrude Na with little or no change in (Na)c32 and a parallel increase in the conductance of that barrier to K. Although these responses may be "triggered" by cell swelling, it is unclear how a cell "recognizes" minimal swelling and how this recognition, in turn, culminates in the observed changes in basolateral membrane pump-leak properties. Clearly, these findings have brought us to the interfaces between cell physiology and cell and molecular biology and have raised a number of intriguing questions that focus on the more global question: How do epithelial cells work?

Sodium-coupled sugar transport: effects on intracellular sodium activities and sodium-pump activity

Intracellular sodium activities, (Na)c, were determined in Necturus small intestine before and after addition of galactose to the mucosal bathing solution. In the absence of galactose, (Na)c averaged 12 millimoles per liter. Within 2 minutes after the addition of galactose to the mucosal solution, (Na)c increased to a mean value of 20 millimoles per liter and then declined, in parallel with an increase in transcellular sodium transport, to a value that did not differ significantly from that observed in the absence of the sugar. The final steady state in the presence of galactose was characterized by a three- to fourfold increase in the rate of transcellular Na+ transport in the absence of a significant increase in (Na)c. Thus, the increase in steady-state basolateral pump activity cannot be attributed to an increase in the intracellular sodium transport pool.

Cell swelling increases a barium-inhibitable potassium conductance in the basolateral membrane of Necturus small intestine

Previous studies have shown that, immediately after the addition of galactose or alanine to the solution bathing the mucosal surface of Necturus small intestine, there is a rapid depolarization of the electrical potential difference across the mucosal membrane (psi mc). This is followed by a repolarization of psi mc that is paralleled by an increase in the ratio of the effective resistance of the mucosal membrane to that of the basolateral membrane (rm/rs); the latter was shown to be, at least in part, due to a marked increase in the conductance of the basolateral membrane. We now report the following. (i) Exposure of this epithelium to a 12% hypotonic solution results in a hyperpolarization of psi mc and an increase in rm/rs. These effects are blocked by metabolic inhibitors and by the presence of 5 mM Ba2+ in the bathing solution; indeed, in the presence of Ba2+, psi mc depolarizes and rm/rs decreases to low values. (ii) Addition of 15 mM galactose to the mucosal solution when the serosal solution alone contains 5 mM Ba2+ results in a depolarization of psi mc but no subsequent repolarization of psi mc or increase in rm/rs; however, psi mc repolarizes and rm/rs increases when Ba2+ is subsequently removed from the serosal bathing solution. We conclude that (i) the basolateral membrane normally possesses a Ba2+-inhibitable K conductance, which appears to be reduced in the presence of metabolic inhibitors; (ii) after exposure of the tissue to a hypotonic solution or the addition of galactose to the mucosal solution, this conductance increases; and (iii) these responses can be blocked by metabolic inhibitors. These findings suggest that the delayed response of this tissue to the addition of sugars or amino acids to the mucosal solution may be the result of cell swelling resulting from the intracellular accumulation of these solutes in osmotically active forms.

Treatment of styes and meibomian cysts. Practical procedures

Styes and meibomian cysts are common eye problems presenting to the general practitioner and both can be simply managed in the consulting rooms. Meibomian cysts, however, may present difficulties to the inexperienced and Dr Hudson has outlined a comprehensive plan of assessment and surgical management.

Surgical treatment of chronic papilloedema in children

A surgical technique for optic nerve decompression in children is described and contrasted with other techniques described in the literature. The operation was effective in relieving long-standing disc oedema in two cases in which the swelling was due to raised intracranial pressure. Photographic evidence is presented. The indications for surgery and how its effect is exerted are discussed.