Bath Administration Research Articles

Effect of clonidine on colonic motility in rats

Clonidine, an α 2-adrenoceptor agonist, has been reported to inhibit gastric and small intestinal motility in rats. Whether clonidine also inhibits colonic motility is still not clear. The aim of this study was to examine the effect of clonidine on colonic motility and its possible site of action in adult Wistar rats. Colonic motilities in anesthetized rats in vivo or motilities of the isolated colon of rats in vitro were recorded. Clonidine was administered intravenously (i.v.) and intracerebroventricularly (i.c.v.) in vivo while bath administration was used in in vitro study. Clonidine i.v. or i.c.v. significantly inhibited colonic motility. This inhibitory effect was antagonized by pre-administration of yohimbine, an α 2-adrenoceptor antagonist, but not by pre-administration of prazosin, an α 1-adrenoceptor antagonist. Also, we have unpublished data indicating that the sympathectomy antagonized the inhibitory effect of systemically administered clonidine. A significant depression of colonic motility on the isolated colon was induced by bath administration of noradrenaline, while no such inhibition was seen by clonidine. The results of the present study suggested that clonidine inhibits colonic motility in rats through activation of central α 2-adrenergic receptor.

Protein kinase c is involved in clozapine’s facilitation of n-methyl- d-aspartate- and electrically evoked responses in pyramidal cells of the medial prefrontal cortex

We have previously shown that the atypical antipsychotic drug clozapine facilitates N-methyl- d-aspartate (NMDA)- and electrically evoked responses in pyramidal cells of the medial prefrontal cortex (mPFC). In the present study, we investigated the role of protein kinase C (PKC) in the action of clozapine. Bath administration of the PKC activator phorbol-12-myristate 13-acetate (PMA), but not the inactive isomer 4α-PMA, significantly enhanced the NMDA-evoked inward current and electrically evoked excitatory postsynaptic currents. Chelerythrine, a selective blocker of PKC, completely prevented the potentiating action produced by either clozapine or PMA on these currents in the mPFC cells. Intracellular injection of the PKC inhibitor PKC-I, but not the control substance PKC-S, through the recording electrode totally blocked clozapine’s potentiating effect, indicating that a post-synaptic expressed PKC is critically involved in the augmenting action of clozapine on NMDA-evoked currents. Of the PKC inhibitor PKC-I, but not the control substance PKC-S, through the recording electrode totally blocked clozapine’s potentiating effect, indicating that a post-synaptic expressed PKC is critically involved in the augmenting action of clozapine on NMDA-evoked currents. To further test the role of PKC in mediating the augmenting action of clozapine, we performed experiments in PKCγ mutant and wild-type mice. In contrast to results in pyramidal cells from rats or wild-type mice, neither clozapine nor PMA was able to potentiate NMDA-induced currents in the mPFC from the PKCγ mutant mice. Taken together, these results suggest that the PKC signal transduction pathway is critically involved in the facilitating action of clozapine on the NMDA-induced responses in pyramidal cells of the mPFC.

PH sensitivity of spinal cord rhythm in fetal mice in vitro.

The spinal respiratory rhythm generator is a neural network localized at the segments C4-C6 able to produce synchronous long lasting rhythmic bursts of action potentials on both phrenic nerves (Dubayle and Viala, 1996). As other spinal cord rhythms, the spinal respiratory rhythm can be induced in curarized rabbits after cervical transection by administration of nialamide-DOPA (Viala and Freton, 1983). This spinal rhythm can also be induced in the in vitro brainstem-spinal cord preparation from newborn rat or mouse by deep diethyl ether anaesthesia, or bath administration of glutamic acid, N-methyl-D-aspartic acid, amphetamine, 5-hydroxytryptophan, or high potassium concentration (Dubayle and Viala, 1998). It has been shown that the spinal respiratory rhythm recorded from neonatal rat brainstem-spinal cord preparation is sensitive to H+ and CO2 (Dubayle and Viala, 1998). However, in most of the experiments the extent to what this chemosensitive response depends directly on the spinal respiratory generator was undefined, because chemical stimulation or the pharmacological inductors affect also the activity of the medullary respiratory pattern generator and both, the medullary and spinal respiratory oscillators, are coupled (Dubayle and Viala, 1996). Uncoupling of these generators by spinal transection at C2 segment caused the disappearance of the spinal respiratory rhythm in most of the neonatal preparations (Dubayle and Viala, 1998).KeywordsVentral RootFetal MouseRespiratory RhythmHigh Potassium ConcentrationSuperfusion MediumThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Antiprotozoals effective in vitro against the scuticociliate fish pathogen Philasterides dicentrarchi.

The histophagous ciliate Philasterides dicentrarchi causes fatal scuticociliatosis in farmed turbot Scophthalmus maximus and sea bass Dicentrarchus labrax. The present study screened 52 candidate antiprotozoals for activity against this pathogen in vitro. Of these compounds, 14 were effective (i.e. killed all ciliates within a 24 h assay period). In descending order of efficacy (minimum lethal concentration 100 to 0.8 ppm), these were niclosamide, oxyclozanide, bithionol sulfoxide, toltrazuril, N-(2'-hydroxy-5'-chloro-benzoyl) 2-chloro-4-nitroaniline, furaltadone, doxycycline hyclate, formalin, albendazole, carnidazole, pyrimethamine, quinacrine hydrochloride and quinine sulfate. Administration in filtered seawater rather than phosphate-buffered saline inactivated doxycycline hyclate and albendazole, and markedly reduced that of bithionol sulfoxide and toltrazuril, suggesting that these compounds may not be effective in bath administration. In view of these findings, we discuss the potential utility of chemotherapy as a strategy for the control of scuticociliatosis in farmed turbot and sea bass.

Localization of functional calcitonin gene-related peptide binding sites in a subpopulation of cultured dorsal root ganglion neurons

In this study we investigated whether cultured dorsal root ganglion (DRG) neurons from the adult rat express binding sites for calcitonin gene-related peptide (CGRP). These were identified on fixed cells by using CGRP labeled at the N-terminal site with 1.4-nm gold particles. After 1 day in culture, about 20% of small to medium-sized DRG neurons showed CGRP-gold binding. Binding of CGRP-gold was dose-dependently reduced by coadministration of CGRP. The calcium imaging technique in living cells revealed that the bath administration of CGRP evoked an increase of the intracellular calcium in up to 30% of the DRG neurons tested. Both depletion of intracellular calcium stores by thapsigargin or using a calcium-free medium blocked the CGRP-mediated increase of cytosolic calcium in most neurons. Thus intracellular and extracellular sources of calcium are relevant for the CGRP response. Using the whole-cell patch-clamp technique, about 30% of the neurons were found to exhibit an inward current and a depolarization upon administration of CGRP close to the neurons. Immunocytochemical double-labeling techniques showed that most of the CGRP-gold binding sites were expressed in unmyelinated (neurofilament 200-negative) DRG neurons. Most of the neurons with CGRP-gold binding sites also expressed the tyrosine kinase A receptor, and all of them showed CGRP-like immunoreactivity. This study shows, therefore, that a subpopulation of unmyelinated, peptidergic primary afferent neurons express CGRP binding sites that can be activated by CGRP in an excitatory direction. The binding sites may serve as autoreceptors because all of these neurons also synthesize CGRP. The activation of CGRP binding sites may sensitize primary afferent neurons and influence the release of mediators.

Stimulation of D1-type dopamine receptors enhances excitability in prefrontal cortical pyramidal neurons in a state-dependent manner.

Prefrontal cortex neurons recorded in vivo exhibit bistable activity states, consisting of a depolarized phase (-55mV) and a hyperpolarized phase (-85mV). These "up" and "down" states have durations ranging from 800ms to 1s and a periodicity of approximately 1Hz. This study examines the state-dependency of prefrontal cortical neuron responses to dopamine, in which the bistable-state was approximated in vitro by intracellular current injection. At resting membrane potential (n=10), dopamine caused a significant depolarization of the membrane potential without altering any of the other electrophysiological characteristics tested. In contrast, both dopamine (30 microM, 5min) and the D1 receptor agonist SKF 38393 (5 and 10 microM) increased cell excitability when the cell was in the depolarized state (i.e., -55mV) but not the hyperpolarized state (i.e., -85 mV; n=10). This increase in excitability was accompanied by a decrease in the rheobase current. The SKF 38393-enhanced excitability was dose-dependent and could be blocked by bath administration of the D1 receptor antagonist SCH 23390 (5 and 10 microM). Administration of the GABA antagonist bicuculline (7 microM) plus the N-methyl-D-aspartate channel blocker CPP (10 microM) produced an additional increase in the excitability of prefrontal cortex neurons that was not dependent on the membrane potential. From these data we suggest that dopamine exerts state-dependent modulatory effects on the excitability of neurons in deep layers of the prefrontal cortex.

Single-dose pharmacokinetics of flumequine in the eel (Anguilla anguilla) after intravascular, oral and bath administration.

Knowledge of the pharmacokinetic properties of drugs to combat bacterial infections in the European eel (Anguilla anguilla) is limited. One antimicrobial agent likely to be effective is flumequine. The aim of this study was to investigate the pharmacokinetic properties of flumequine in European eels in fresh water. Flumequine was administered to eels (Anguilla anguilla) intravenously (i.v.) and orally (p.o.) at a dose of 10 mg/kg body weight, and as a bath treatment at a dose of 10 mg/L water for 2 h. The study was performed in fresh water with a temperature of 23 + 0.3 degrees C, pH 7.15. Identical experimental designs were used. Two additional bath treatments were also performed, one in which the pH in the water was lowered by approximately 1 unit to 6.07 (dose: 10 mg/L) and one at a dose of 40 mg/L for 2 h in a full-scale treatment. Following i.v. administration, the volume of distribution at steady state was 3.4 L/kg. Total body clearance was 0.012 L/h per kg and the elimination half-life (t1/2lambda z) was calculated to be 314 h. Mean residence time was 283 h. Following oral administration, the t1/2lambda z was 208 h. Maximal plasma concentration (Cmax) was 9.3 mg/L, at 7 h after administration (Cmax). The oral bioavailability (F) was calculated to be 85%. Following bath administration in 10 mg/L for 2 h, maximal plasma concentration was 2.1 mg/L, observed immediately after the end of the bath. The 'bioavailability' in eel following a 2-h bath treatment was 19.8%. Reducing the pH in the bath to 6.07 produced a maximal plasma concentration of 5.5 mg/L, observed immediately after the end of the bath. The 'bioavailability' was increased to 41% by the lowering of the pH. A similar effect was observed in a full-scale treatment (1 kg eels/L water). The CO2 produced by the eel lowered the pH and increased 'bioavailability' to 35%.

Muscarinic receptors regulate two different calcium-dependent non-selective cation currents in rat prefrontal cortex.

Pyramidal neurons of layer V in rat prefrontal cortex display a prominent fast afterdepolarization (fADP) and a muscarinic-induced slow afterdepolarization (sADP). We have shown previously that both of these ADPs are produced by the activation of calcium-dependent non-selective cation currents. In the present report we examine whether they represent two distinct currents. In most pyramidal neurons recorded with caesium gluconate-based intracellular solution, a calcium spike is followed by a fast decaying inward aftercurrent (IfADP). The decay of IfADP is monoexponential with a time constant (t) of approximately 35 ms. Administration of carbachol (10-30 microm) increases the time constant of this decay by approximately 80% and induces the appearance of a much slower inward aftercurrent (IsADP). IfADP recorded in control conditions and in the presence of carbachol increases linearly with membrane hyperpolarization. In contrast, the carbachol-induced IsADP decreases with membrane hyperpolarization. When the sodium driving force across the cell membrane was reduced, IfADP was found to reverse at around -40 mV whereas IsADP remain inward over the same voltage range tested. Finally, bath administration of flufenamic acid (100 microm-1 mm) selectively blocks the carbachol-induced IsADP without a significant effect on the amplitude of IfADP. These differences in the electrical and pharmacological properties of IfADP and IsADP suggest that they were mediated by two distinct non-selective cation currents.

Dopamine Modulates the Responsivity of Mediodorsal Thalamic Cells RecordedIn Vitro

The mediodorsal thalamic nucleus (MD) receives convergent inputs from subcortical limbic structures that overlap with a dopaminergic (DA) innervation. In this study, we describe the effects of DA agonists on the basal and evoked electrophysiological activity of identified thalamic cells of rats recorded in vitro. Administration of the D1 agonist SFK 38393 (10 microM) did not produce a clear effect on the physiological properties of the thalamic cells recorded. In contrast, bath administration of the D2 agonist quinpirole (10 microM) resulted in an enhancement of membrane excitability, facilitation of the occurrence of low-threshold spikes (LTSs), and changes in the resting membrane potential of the thalamic cells tested. The quinpirole-mediated responses were reversed by administration of the D2 antagonist haloperidol. Results from experiments performed with different [K+] and K+ channel blockers suggest that the effects of quinpirole are mediated at least in part by changes in K+ conductances. The results from this study suggest that DA can modulate the excitability of thalamic cells and in turn may influence the way that the thalamocortical system integrates information.

Cytochrome P450: a novel system modulating Ca2+ channels and contraction in mammalian heart cells.

1. Cytochrome P450 (P450) is a ubiquitous enzyme system that catalyses oxidative reactions of numerous endogenous and exogenous compounds. The modulatory effects of P450 on the L-type Ca2+ current (ICa), intracellular free Ca2+ signals and cell shortening were assessed in adult rat single ventricular myocytes. 2. Bath administration of the imidazole antimycotics, clotrimazole, econazole and miconazole, which are potent P450 inhibitors, significantly suppressed cardiac ICa. While the Ca2+ channel antagonist nifedipine blocked ICa within 30 s, clotrimazole-induced suppression of ICa required 5.1 +/- 0.4 min (n = 14) to reach a steady low level. The suppression of ICa was dose dependent and recovered after washout of clotrimazole. Intracellular dialysis with the P450 antibody anti-rat CYP1A2 also significantly reduced cardiac ICa. 3. Additional administration of the beta-adrenergic agonist isoprenaline (1 microM) or the membrane-permeable 8-bromo-cAMP (2 mM) completely reversed the suppressant effects of clotrimazole and NaCN on ICa. In addition, intracellular dialysis with 2 mM cAMP abolished the P450 inhibitor-induced suppression of ICa. Phosphorylation of the channel with hydrolysis-resistant ATPgammaS prevented the suppressant effect of clotrimazole on ICa. Furthermore, dephosphorylation of the Ca2+ channel with intracellular dialysis with phosphatase types I and II reduced ICa by 85 +/- 3 % and abolished clotrimazole-induced suppression of ICa. 4. Extracellular administration of the phospholipase A2 inhibitors mepacrine and 4-bromophenacyl bromide significantly suppressed ICa. 5. Clotrimazole, econazole, miconazole and CN- also significantly inhibited intracellular free Ca2+ signals and cell shortening in rat single ventricular myocytes. 6. Intracellular cAMP content was significantly reduced in isolated ventricular myocytes incubated with clotrimazole or CN-. Extracellular administration of 11, 12-epoxyeicosatrienoic acid, one of the P450-mediated metabolites of arachidonic acid, enhanced ICa and intracellular cAMP content. The epoxyeicosatrienoic acid also restored the amplitude of the reduced ICa in P450 antibody-dialysed myocytes. 7. The present data suggest that cytochrome P450 modulates cardiac ICa and cell contraction, and the modulation may result from changes in intracellular levels of cAMP by P450- mediated metabolites of arachidonic acid.

Whole cell patch recordings from forebrain slices demonstrate angiotensin II inhibits potassium currents in subfornical organ neurons

Whole cell patch clamp recordings have been obtained from SFO neurons in a forebrain slice preparation. Basic electrophysiological characteristics recorded from these cells in current clamp mode showed a mean resting membrane potential of −57.0 ± 2.5 mV (± SEM, n = 7, mean input resistance of 900 ± 110 MΩ (n = 7), and a mean spike amplitude of 68.6 ± 4.1 mV (n = 7), accompanied by either irregular or no spontaneous activity. All cells also demonstrated a delayed return to baseline membrane potential following large hyperpolarizing current pulses indicative of the presence of a rapidly activated transient potassium current in these neurons. Voltage clamp recordings identified both rapid transient, and a sustained outward currents which demonstrated the characteristics of IA and IK respectively. While bath administration of angiotensin II (Ang) (10−7 M) was without effect on IK in 4 of 4 neurons tested, IA was reduced by between 20 and 100% in the same 4 neurons. These data provide the first description of the basic electrophysiological characteristics of SFO neurons recorded in forebrain slice preparations. They also provide the first direct evidence suggesting that Ang may exert its control over the excitability of SFO neurons through modulation of IA in these cells.

Whole cell patch recordings from forebrain slices demonstrate angiotensin II inhibits potassium currents in subfornical organ neurons

Whole cell patch clamp recordings have been obtained from SFO neurons in a forebrain slice preparation. Basic electrophysiological characteristics recorded from these cells in current clamp mode showed a mean resting membrane potential of −57.0 ± 2.5 mV (± SEM, n = 7, mean input resistance of 900 ± 110 MΩ ( n = 7), and a mean spike amplitude of 68.6 ± 4.1 mV ( n = 7), accompanied by either irregular or no spontaneous activity. All cells also demonstrated a delayed return to baseline membrane potential following large hyperpolarizing current pulses indicative of the presence of a rapidly activated transient potassium current in these neurons. Voltage clamp recordings identified both rapid transient, and a sustained outward currents which demonstrated the characteristics of I A and I K respectively. While bath administration of angiotensin II (Ang) (10 −7 M) was without effect on I K in 4 of 4 neurons tested, I A was reduced by between 20 and 100% in the same 4 neurons. These data provide the first description of the basic electrophysiological characteristics of SFO neurons recorded in forebrain slice preparations. They also provide the first direct evidence suggesting that Ang may exert its control over the excitability of SFO neurons through modulation of I A in these cells.

Angiotensin II and glutamate influence area postrema neurons in rat brain slices

The area postrema (AP) has been repeatedly implicated in cardiovascular regulation. Microinjection and single unit recording studies in vivo have suggested specific actions for angiotensin II (ANG) and glutamate (GLU) in controlling the excitability of AP neurons. The present study was therefore designed to examine the responsiveness of AP neurons to bath administration of these substances. Of the 133 AP neurons tested with ANG (10−8–10−6 M) 40% were excited, 13% inhibited and the remainder unresponsive. The excitatory effects of ANG on AP neurons were dose-dependent. Following blockade of synaptic transmission with a low calcium high magnesium solution excitatory responses were maintained in 12 of 15 cells tested. Pretreatment of slices with the AT1 receptor antagonist losartan blocked the excitatory effects of ANG in all cells (55) tested. The effects of GLU on AP neurons were also examined. Of the 71 AP cells tested, 40% were excited, 10% inhibited, 8% showed excitatory responses followed by periods of inhibition while the remaining cells were unaffected. Excitatory effects of GLU were maintained in all AP neurons (77) tested during perfusion with low calcium, high magnesium solutions. Similar responses to NMDA were observed in four of four cells tested, suggesting these GLU actions are mediated through NMDA receptors. These data demonstrate direct excitatory actions of ANG and GLU on AP neurons which are likely mediated through the AT1 and NMDA receptors, respectively.

Angiotensin II and glutamate influence area postrema neurons in rat brain slices

The area postrema (AP) has been repeatedly implicated in cardiovascular regulation. Microinjection and single unit recording studies in vivo have suggested specific actions for angiotensin II (ANG) and glutamate (GLU) in controlling the excitability of AP neurons. The present study was therefore designed to examine the responsiveness of AP neurons to bath administration of these substances. Of the 133 AP neurons tested with ANG (10 −8–10 −6 M) 40% were excited, 13% inhibited and the remainder unresponsive. The excitatory effects of ANG on AP neurons were dose-dependent. Following blockade of synaptic transmission with a low calcium high magnesium solution excitatory responses were maintained in 12 of 15 cells tested. Pretreatment of slices with the AT 1 receptor antagonist losartan blocked the excitatory effects of ANG in all cells ( 5 5 ) tested. The effects of GLU on AP neurons were also examined. Of the 71 AP cells tested, 40% were excited, 10% inhibited, 8% showed excitatory responses followed by periods of inhibition while the remaining cells were unaffected. Excitatory effects of GLU were maintained in all AP neurons ( 7 7 ) tested during perfusion with low calcium, high magnesium solutions. Similar responses to NMDA were observed in four of four cells tested, suggesting these GLU actions are mediated through NMDA receptors. These data demonstrate direct excitatory actions of ANG and GLU on AP neurons which are likely mediated through the AT 1 and NMDA receptors, respectively.

Electrophysiological properties of paraventricular magnocellular neurons in rat brain slices: modulation of IA by angiotensin II.

Whole-cell patch-clamp recordings obtained from magnocellular neurons of the hypothalamic paraventricular nucleus in brain slice preparations of adult Sprague-Dawley rats have been utilized to examine three outward potassium conductances and the ionic mechanisms through which angiotensin II exerts its neurotransmitter actions within this region. Lucifer Yellow fills showed that neurons from which we recorded had large ovoid cell bodies 11-17 microns wide and 22-35 microns long, as well as 1-3 minimally branched processes, anatomical features in accordance with those previously described for magnocellular neuroendocrine neurons. These neurons had an average resting membrane potential of -58.3 +/- 0.9 (mean +/- S.E.M.) mV, spike amplitude of 92.8 +/- 1.4 mV, and input resistance of 788.9 +/- 50.4 M omega. Most of these cells displayed irregular or continuous spontaneous activity with a mean frequency of 2.44 +/- 0.33 Hz. Voltage-clamp recordings revealed three outward potassium currents; (1) a delayed outward current (IK), (2) a Ca(2+)-dependent outward current (IK(Ca)) and (3) a transient outward current (IA). These currents were classified according to their voltage dependence, inactivation, Ca2+ dependence and pharmacology. The IK was activated by depolarization beyond -40 mV and its amplitude consistently increased with depolarizing steps. The membrane conductance underlying this current was 27.3 +/- 3.8 nS for depolarization to +50 mV. In medium containing 2 mM Ca2+, depolarization to above -20 mV evoked a slowly-activating IK(Ca) which showed minimal inactivation. This current was suppressed in Ca(2+)-free/Co2+ medium and its membrane conductance was also smaller (19.4 +/- 3.5 nS at +50 mV) than that of IK. The IA demonstrated both fast activation and inactivation and was evoked only if depolarizing pulse steps were preceded by conditioning hyperpolarization. The activation threshold was approximately -65 mV and IA amplitude increased in non-linear fashion as test voltage steps became more positive. The 90% maximum of IA conductance was 15.7 +/- 1.1 nS, and was observed at membrane potentials around -15 mV. The reversal potentials of these currents were in accordance with the K+ equilibrium potential. Tetra-ethylammonium reversibly inhibited both the peak and steady-state currents of the IK, while 4-aminopyridine suppressed the IA. Replacement of 2 mM Ca2+ with 2 mM Co2+ in our bath solution or addition of Co2+ into Ca(2+)-free medium reduced the magnitude of IA, revealing the existence of a Co(2+)-sensitive IA. Bath administration of 10(-7) M angiotensin was without significant effect on IK, but resulted in a statistically significant reduction in IA (-31.0 +/- 4.1%) in 12 of 14 paraventricular nucleus cells tested, effects which were not observed following pretreatment with the AT1 receptor antagonist losartan. We conclude that in paraventricular nucleus magnocellular cells, like other CNS neurons, at least three sets of potassium channels contribute to the outward current evoked by depolarization. Our data also demonstrate ionic mechanisms through which angiotensin may act at AT1 receptors to influence the excitability of hypothalamic neuroendocrine cells.

Failure of flumisol bath treatments during commercial transport of Atlantic salmon smolts to prevent the activation of stress inducible furunculosis

This paper reports the development of protocols for the bath administration of flumequine to Atlantic salmon ( Salmo salar L.) smolts during their transfer from a fresh water hatchery to a marine farm in buckets suspended under a helicopter. The fish were treated at a density of 365 kg m −3 in water of pH 6.3 and hardness 13.2 mg 1 −1 CaCO 3. Serum flumequine concentrations showed a linear increase with respect to time of the bath and were dose dependent. The addition of benzocaine to the baths had little effect on uptake during the first 20 min of the bath. 720 000 smolts with covert stress inducible furunculosis were transported to 20 sea cages during 180 helicopter trips in buckets containing 100 μg ml −1 flumequine and 5.7 μg ml −1 benzocaine. Furunculosis was detected in fish in all sea cages shortly after transfer and the treatment was considered to have failed to control the activation of the covert infections. The pharmacokinetics of flumequine were determined in fish that were introduced to sea water at the termination of a bath treatment in fresh water. This data was compared to that obtained from fish that remained in fresh water following a similar treatment. The introduction of the fish into sea water resulted in a very rapid excretion of flumequine via the intestine. It is argued that this rapid elimination of the antimicrobial agent may have been an important factor in the failure of the bath treatment under commercial conditions.

Dopamine modulation of synaptic transmission in rat prefrontal cortex: an in vitro electrophysiological study

The prefrontal cortex is innervated by a well-defined dopaminergic bundle originating from the brainstem and is a key structure in higher order mental processes. We have studied the effects of dopamine (DA) on layer V pyramidal cells of the prefrontal cortex using intracellular recording in rat brain slices maintained in vitro. Bath administration of DA (50–100 μM) had weak effects on membrane properties of these neurons. In contrast, DA markedly decreased all components of the synaptic responses evoked by electrical stimulation of layer I or VI, and in particular the monosynaptic excitatory postsynaptic potential (EPSP) which arises from activation of glutamatergic receptors. The afferents from layer VI seemed less affected by DA than those from layer I. The NMDA ( N- methyl- d-aspartate ) and AMPA (α-amino-3-hydroxy-5-methyl-isoxazolepropionic acid) components of monosynaptic EPSPs were equally reduced by DA. The isolated fast gabaergic potential (IPSP) resulting from GABA A receptors activation was similarly reduced by DA. The suppressive effect of DA on glutamatergic transmission was partially mimicked by the D1 receptor agonist SKF 38393 (50 μM) whereas the D2 receptor agonist quinpirole (50 μM) was ineffective. Conversely, this effect was antagonized by the D1 receptor blocker SCH 23390 (100 μM) but not by the D2 receptor antagonist sulpiride (100 μM). These findings indicate that DA decreases both glutamatergic and gabaergic synaptic transmission in neurons located in layer V of rat prefrontal cortex. These results also suggest that D1 dopamine receptor is involved in the decrement of glutamatergic transmission. These interactions between DA and glutamate are important in regard to the suspected implications of both neurotransmitters in psychiatric diseases.

Potentiation of synaptic transmission in the rat hippocampal slice by exogenous l-glutamate and selective l-glutamate receptor subtype agonists

We have investigated the effects of administration of exogenous glutamate receptor agonists on the amplitude of field excitatory post-synaptic potentials (fEPSPs) evoked in the CA1 region of the rat hippocampal slice by stimulation of the Schaffer collateral-commissural fibres. l-Glutamate applied by iontophoresis or by bath perfusion (50 μ M for 5 min) evoked a slowly rising increase in the amplitude of the fESPS which persisted for over 90 min. l-Glutamate induced potentiation was blocked by either d(−)-2-amino-5-phosphonopentanoic acid (40 μM) or by (RS)-α-methyl-4-carboxyphenylglycine (500 μM). In slices in which synaptic long-term potentiation had been saturated, iontophoretically applied l-glutamate did not induce further potentiation, but reset the fEPSP amplitude back to control levels. Iontophoretic administration of N- methyl- d-aspartate (NMDA) evoked a transient potentiation which decayed back to control levels within 90 min whereas bath perfusion of NMDA (50 μM) evoked a persistent depression. Bath perfusion of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA, 50 μM) evoked no persistent effects. Bath administration of (1S, 3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD, 50 or 100 μM) caused a short term depression of the fEPSP and no significant persistent effects. Perfusion of 100 μM ACPD in medium containing 1 μM picrotoxin caused a much smaller short term depression of the fEPSP and this was followed by a gradually developing and persistent potentiation. Bath perfusion of spermidine (250 μM) and d-serine (200 μM) merely prolonged the time course of the iontophoretic NMDA induced potentiation, whereas administration of NMDA in the presence of 50 μ M ACPD evoked a rapidly initiated potentiation which remained stable throughout the recording period. The results may reflect a requirement for coactivation of metabotropic and NMDA receptors in the induction of LTP.

Pharmacological treatments against Ichthyobodo necator (Henneguy, 1883) in rainbow trout, Oncorhynchus mykiss (Walbaum)

Abstract. Various antiparasitic drugs belonging to several pharmacological groups were tested by bath administration for in vivo activity against a natural infestation of rainbow trout. Oncorhynchus mykiss (Walbaum), by he flagellate protozoan Ichthyobodo necator. Fish were also monitored for signs of drug toxicity. Complete elimination of infestation in all fish was achieved only by bithionol (25 mgl‐1 for 3h on two consecutive days). Ketoconazole, toltrazuril, amprolium, sulphaquinoxaline, quinacrine, N‐metylglucamine, chloroquine, 1,3‐di‐6‐quinolylurea, diminazene aceturate and paromomycin were not effective. Diminazene aceturate (100 mgl‐1, 3h) was the only drug which was clearly toxic. The comparison of the response of symptomatic and asymptomatic infested fish to a formaldehyde bath indicates that poor health caused by I. necator may greatly increase the susceptibility of fish to the toxic effects of a drug or chemical.

Noradrenaline decreases transmission of NMDA- and non-NMDA-receptor mediated monosynaptic EPSPs in rat prefrontal neurons in vitro.

The effects of noradrenaline on pyramidal cells of layer V of the prefrontal cortex were examined in rat brain slices in vitro. Bath administration of noradrenaline (10 microM) reduced synaptic transmission of afferent inputs from layer 1. The decrease affected all the components of the evoked response and particularly the monosynaptic excitatory postsynaptic potential (EPSP) as evidenced by a reduction of its initial rising slope (mean slope: 71 +/- 11% of its control). Pharmacological dissociation of the NMDA- and non-NMDA-receptor components of the EPSP showed that noradrenaline reduced both (mean EPSP slopes were 71 +/- 8% and 73 +/- 10% of their control, respectively). Alpha 1-, but not alpha-2- or beta-adrenoceptor antagonists prevented the noradrenaline-induced decrease in synaptic efficacy. However, the effect of noradrenaline was not reproduced by alpha 1-adrenoceptor agonists. Lastly, noradrenaline acting through beta-adrenoceptors reduced the slow hyperpolarization that follows a train of action potentials.