In vitro and in vivo measures of evoked excitatory and inhibitory conductance dynamics in sensory cortices.

In order to better understand the synaptic nature of the integration process operated by cortical neurons during sensory processing, it is necessary to devise quantitative methods which allow one to infer the level of conductance change evoked by the sensory stimulation and, consequently, the dynamics of the balance between excitation and inhibition. Such detailed measurements are required to characterize the static versus dynamic nature of the non-linear interactions triggered at the single cell level by sensory stimulus. This paper primarily reviews experimental data from our laboratory based on direct conductance measurements during whole-cell patch clamp recordings in two experimental preparations: (1) in vitro, during electrical stimulation in the visual cortex of the rat and (2) in vivo, during visual stimulation, in the primary visual cortex of the anaesthetized cat. Both studies demonstrate that shunting inhibition is expressed as well in vivo as in vitro. Our in vivo data reveals that a high level of diversity is observed in the degree of interaction (from linear to non-linear) and in the temporal interplay (from push-pull to synchronous) between stimulus-driven excitation (E) and inhibition (I). A detailed analysis of the E/I balance during evoked spike activity further shows that the firing strength results from a simultaneous decrease of evoked inhibition and increase of excitation. Secondary, the paper overviews the various computational methods used in the literature to assess conductance dynamics, measured in current clamp as well as in voltage clamp in different neocortical areas and species, and discuss the consistency of their estimations.

Opioid involvement in TRH-induced antinociception in the rat following intracerebral administration.

Thyrotropin-releasing hormone (TRH) has an antinociceptive action in the rat. Antinociception was observed using a thermal stimulus (tail-flick test) after TRH administration into lateral ventricle, nucleus raphe magnus, nucleus reticularis paragigantocellularis and amygdaloid nuclei. This effect was short-lived since it was completely abolished 60 min after intracerebroventricular administration. TRH may interact with the opioid systems as its antinociceptive effect was blocked by pretreatment with naloxone.

Tolerance to the behavioural effect of serotonergic (5-HT1B) agonists in the isolation-induced social behavioural deficit test.

In mice, isolation-induced social behavioural deficits are attenuated by stimulants of 5-HT1B receptors, such as TFMPP or CGS 120 66B. Repeated treatment with RU 24969 (5 mg/kg, daily, for 3 days) reduced the effect of TFMPP and that of other 5-HT1B agonists (CGS 120 66B, m-CPP, RU 24969). Similarly, repeated treatment with CGS 120 66B (8 mg/kg, twice a day for 3 days) abolished the effect of a test-dose of the same drug. Desensitization of the 5-HT1B receptors involved in this effect is suggested to have occurred. Such a desensitization may be therapeutically relevant, since acute administration of benzodiazepines and chronic administration of antidepressants both reversed the effect of TFMPP.

Increase in the isolation-induced social behavioural deficit by agonists at 5-HT1A receptors.

The effect of 5-HT1A agonists was studied in the isolation-induced social behavioural deficit test. The drugs 8-OH-DPAT (0.125 mg/kg), buspirone (16 mg/kg) and ipsapirone (8 mg/kg) further increased the deficit. Unlike 8-OH-DPAT, the other two drugs may act non-specifically since they reduced spontaneous motor activity at 16 mg/kg, as measured in an activity meter. In addition, 8-OH-DPAT (0.25 mg/kg), buspirone (8 mg/kg) and ipsapirone (8 mg/kg) decreased exploratory activity in the open-field test. Since the smallest active doses were very close in the behavioural deficit and in the open-field tests, it is suggested that a common phenomenon, increased emotionality or reactivity, sustained both these reductions in activity. The increase in the behavioural deficit induced by 8-OH-DPAT, was likely to have resulted from stimulation of 5-HT1A receptors, since it was impaired by pretreatment with penbutolol, a beta-adrenergic-blocking drug, also known to bind to 5-HT1 receptors. Since it was previously shown that the behavioural deficit was reduced by agonists at 5-HT1B receptors, it is proposed that the behavioural inhibition, resulting from an isolation-induced increase in reactivity is bi-directionally modulated by serotonergic drugs, where 5-HT1A agonists increase and 5-HT1B agonists decrease this inhibition.

Benzodiazepines reverse the anti-immobility effect of antidepressants in the forced swimming test in mice.

The present study provides evidence that, in mice subjected to the forced swimming test, the anti-immobility effect of the tricyclic antidepressants, desipramine and imipramine (16-32 mg/kg) was antagonized by the acute co-administration of a benzodiazepine, diazepam (0.25-2 mg/kg) and lorazepam (0.125 mg/kg). This effect cannot be accounted for by variations in plasma and/or brain levels of each compound since brain and plasma concentrations of desipramine and plasma levels of diazepam and desmethyldiazepam, measured immediately after the swimming test, were not significantly modified by the co-administration. Diazepam (2 mg/kg) also counteracted the reduction of time spent immobile induced by the MAO inhibitors, toloxatone (256 mg/kg) and selegiline (4 mg/kg) and the 5-HT1A receptor agonist, 8-OH-DPAT (1 mg/kg), but not by the psychostimulant, caffeine (32 mg/kg). The sedative neuroleptic, thioridazine (4 mg/kg) was also found to reverse the anti-immobility effect of desipramine whereas the non-benzodiazepine anxiolytics, alpidem (8 mg/kg) and buspirone (0.5 mg/kg) did not. These results indicate that the observed interactions were unlikely to be accounted for by a reduction of the stressful aspect of the situation whereas the participation of some motor or sedative component could not be totally ruled out.

Isolation impairs place preference conditioning to morphine but not aversive learning in mice.

Morphine (8-100 mg/kg IP) induces place preference conditioning in mice. The effect of two different periods of isolation (15 and 30 days) was examined. Mice isolated for 15 days but not 30 days exhibited place preference conditioning to morphine (8 mg/kg). After 30 days of isolation morphine could not induce place preference conditioning with the following doses (8, 16, 64, 100 mg/kg). Social regrouping of male mice previously isolated for 30 days with naive female mice for 15 or 30 days resulted in a reappearance of the conditioned place preference to morphine (16 mg/kg). The specificity of this associative deficit was examined by testing learning in isolated compared to non-isolated mice in two distinct settings: escape learning in the Morris water maze and passive avoidance acquisition and retention. On the Morris water maze isolated mice did not differ from non-isolated mice regarding place learning, the probe trial or extinction. Isolated mice were unimpaired in passive avoidance acquisition and retention. It was concluded that the deficits in place preference conditioning were not the result of a global learning impairment in isolated mice.

Effect of social isolation on the metabolism of morphine and its passage through the blood-brain barrier and on consumption of sucrose solutions.

RATIONALE: We have previously shown that place preference conditioning to morphine was observed in social mice at the dose of 8 mg/kg, whereas 4 weeks of isolation impairs the place preference conditioning to morphine (8-100 mg/kg). OBJECTIVE: The present study, aimed at explaining this phenomenon, tested three hypotheses: firstly, a reduced sensitivity to reinforcers induced by isolation; secondly, a difference in morphine disposition in isolated and social mice; thirdly, an altered blood-brain barrier transport of morphine in isolated mice. METHODS: In the sucrose experiments, mice had the choice (for 24 h) between a bottle containing tap water and a bottle containing a sucrose solution. Three sucrose concentrations were used: 0.5, 1 and 2% (weight/weight). In the morphine disposition experiments, the plasma levels of morphine and of morphine-3-glucuronide (M3G) were measured for 240 min. The brain concentrations of morphine was measured at 15 and 30 min. The passage of morphine through the blood-brain barrier was measured using a method modified from that of Takasato (1984). RESULTS: The preference for the sucrose solutions was significantly greater in isolated than in social mice for the concentration of 2%. Isolation reduced the plasma levels of morphine and of M3G, but did not alter the brain concentration of morphine. The passage of morphine through the blood-brain barrier was altered by isolation in neither of the eight structures examined. CONCLUSIONS: We conclude that the behavioural effect of isolation observed in the conditioned place preference to morphine may depend on changes both in morphine disposition and in the sensitivity to reinforcers in isolated mice.

Voltage-clamp measurement of visually-evoked conductances with whole-cell patch recordings in primary visual cortex.

Whole cell patch recordings have been realized in the primary visual cortex of the anesthetized and paralyzed cat, in order to better characterize input resistance and time constant of visual cortical cells in vivo. Measurements of conductance changes evoked by visual stimulation were derived from voltage clamp recordings achieved in continuous mode at two or more different subthreshold holding potentials. They show that the magnitude of the conductance increase can reach up to 300% of the mean conductance at rest. The observation of similar changes for the preferred and antagonist responses, when flashing ON and OFF, a test stimulus in pure ON and OFF subfields supports the hypothesis of a role for shunting inhibition in the spatial organization of simple receptive fields.

The visual cortical association field: a Gestalt concept or a psychophysiological entity?

The receptive field of a visual neurone is classically defined as the region of space (or retina) where a visual stimulus evokes a change in its firing activity. Intracellular recordings in cat area 17 show that the visually evoked synaptic integration field extends over a much larger area than that established on the basis of spike activity. Synaptic depolarizing (dominant excitation) responses decrease in strength for stimuli that are flashed at increasing distances away from the centre of the discharge field, while their onset latency increases. A detailed spatio-temporal analysis of these electrophysiological data shows that subthreshold synaptic responses observed in the 'silent' surround of cortical receptive fields result from the intracortical spread of activation waves carried by slowly conducting horizontal axons within primary visual cortex. They also predict that a perceptual facilitation may occur when feedforward activation produced by the motion signal in the retina travels in phase in the primary visual cortex with the visually induced spread of horizontal activation. A psychophysical correlate has been obtained in humans, showing that apparent motion produced by a sequence of co-linear Gabor patches, known to preferentially activate V1 orientation selective cells, are perceived by human observers as much faster than non co-linear sequences of the same physical speed.

Effects of respiratory acidosis and alkalosis on the distribution of cyanide into the rat brain.

The aim of this study was to determine whether respiratory acidosis favors the cerebral distribution of cyanide, and conversely, if respiratory alkalosis limits its distribution. The pharmacokinetics of a nontoxic dose of cyanide were first studied in a group of 7 rats in order to determine the distribution phase. The pharmacokinetics were found to best fit a 3-compartment model with very rapid distribution (whole blood T(1/2)alpha = 21.6 +/- 3.3 s). Then the effects of the modulation of arterial pH on the distribution of a nontoxic dose of intravenously administered cyanide into the brains of rats were studied by means of the determination of the permeability-area product (PA). The modulation of arterial blood pH was performed by variation of arterial carbon dioxide tension (PaCO2) in 3 groups of 8 anesthetized mechanically ventilated rats. The mean arterial pH measured 20 min after the start of mechanical ventilation in the acidotic, physiologic, and alkalotic groups were 7.07 +/- 0.03, 7.41 +/- 0.01, and 7.58 +/- 0.01, respectively. The mean PAs in the acidotic, physiologic, and alkalotic groups, determined 30 s after the intravenous administration of cyanide, were 0.015 +/- 0.002, 0.011 +/- 0.001, and 0.008 +/- 0.001 s(-1), respectively (one-way ANOVA; p < 0.0087). At alkalotic pH the mean permeability-area product was 43% of that measured at acidotic pH. This effect of pH on the rapidity of cyanide distribution does not appear to be limited to specific areas of the brain. We conclude that modulation of arterial pH by altering PaCO2 may induce significant effects on the brain uptake of cyanide.

Lack of effect of single high doses of buprenorphine on arterial blood gases in the rat.

High dose buprenorphine, a potent semisynthetic agonist-antagonist for opiate receptors, is now used in substitution treatment of human heroin addiction. Deaths have been reported in addicts misusing buprenorphine. We determined the median lethal dose (LD(50)) and studied the effects of high doses of intravenous buprenorphine on arterial blood gases in rats. Male Sprague-Dawley rats were administered buprenorphine intravenously to determine the LD(50) using the up-and-down method. Subsequently, catheterized groups of 10 restrained rats received no drug, saline, acid-alcohol aqueous solvent (required to dissolve buprenorphine at a high concentration), or 3, 30, or 90 mg/kg of buprenorphine intravenously. Serial arterial blood gases were obtained over 3 h. The LD(50) determined in triplicate was 146.5 mg/kg (median of 3 series, range: 142.6-176.5). The mean dose received by surviving animals was 96.9 +/- 46.7 mg/kg. There was a significant effect of the acid-alcohol aqueous solvent on arterial blood gases. Excluding the solvent effect, 3, 30, and 90-mg/kg buprenorphine doses had no significant effects on arterial blood gases. The toxicity of intravenous buprenorphine in adult rats, assessed by the LD(50), is low. These data are consistent with a wide margin of safety of buprenorphine. The mechanism of death after the intravenous administration of a lethal dose of buprenorphine remains to be determined.

Influence of a dietary alpha-linolenic acid deficiency on learning in the Morris water maze and on the effects of morphine.

Female OF1 mice were fed on a diet deficient in alpha-linolenic acid or on a control diet 3 weeks before mating and throughout pregnancy and lactation. Pups fed on the same diet as their mothers were used for experiments. The effects of dietary alpha-linolenic acid deficiency were studied in a model of learning, the Morris water maze, and on the following effects of morphine: increase in locomotor activity, modifications of rectal temperature and analgesia. In the place and in the cue versions of the Morris water maze, learning occurred at the same speed in the two diet groups; however, in the place version of the test, the level of the performance was significantly lower in the deficient mice. The probe trial and the extinction procedure did not show any difference between the two diet groups. The morphine-induced increase in locomotor activity occurred significantly earlier and was greater in the deficient diet group. Morphine induced an early hypothermia followed by a late hyperthermia; the hypothermia was significantly greater and the hyperthermia significantly smaller in the deficient mice. The pain thresholds and the morphine-induced analgesia were unmodified by the dietary deficiency. The plasma levels of morphine were similar in the two diet groups.

Behavioral tolerance to one effect of the serotonergic agonist TFMPP.

1. In mice isolated for one week and observed in pairs with non-isolated mice under a reversed beaker, the serotonergic agonist: 1-3-(trifluoromethyl)phenylpiperazine (TFMPP) increased the number of escape attempts. 2. A partial tolerance to this effect has been observed in mice which were tested a first time after administration of TFMPP one week sooner. 3. An analysis of this form of tolerance was carried out by changing successively and separately each of the events concomitant of this tolerance. 4. The results show that this tolerance is not a pharmacocinetic or a pharmacodynamic one. Observation of the tolerance required the performance of the first test in a drug state. The more likely explanation is that this tolerance results from a conditioned opponent response.

Effect of isolation on behavioural models involving serotonergic 5-HT2 and 5-HT1A receptors.

1. The effect of 7 days of isolation were observed in mice on behavioural models involving 5-HT2 and 5-HT1A receptors. 2. The sensitivity of 5-HT2 receptors as assessed through L-5-HTP or 5-MeODMT induced head-twitches was reduced. 3. The sensitivity of the 5-HT1A receptors implicated in the 8-OH-DPAT induced feeding was unchanged. 4. The sensitivity of the 5-HT1A receptors involved in the 8-OH-DPAT induced hypothermia was diminished. 5. On the whole, these results show that after 7 days of isolation, the responses to the stimulation of serotonergic receptors is unchanged or diminished according to both the receptor's subtype and the model used.

Effect of isolation on pain threshold and on different effects of morphine.

1. The effect of three periods of isolation (8, 15 and 30 days) were studied in mice on the pain threshold and the sensitivity to morphine. 2. The pain threshold was unchanged after 8 and 15 days of isolation but increased after 30 days of isolation. 3. The analgesic effect of morphine was unchanged after 8 and 15 days of isolation but increased after 30 days of isolation. 4. The tolerance to morphine analgesia was unchanged after 8 and 15 days of isolation but increased after 30 days of isolation (morphine-induced analgesia was reduced). 5. The physical dependence on morphine induced by precipitated withdrawal was unchanged after 8 and 15 days of isolation but decreased after 30 days of isolation. 6. It is suggested that isolation may modify the metabolism the metabolism/absorption of morphine in a different way according as the treatment is unique or chronic.

Effect of isolation on morphine-induced running and changes in body temperature.

1. The influence of isolation of three durations 8, 15 and 30 days has been examined in mice on the effects of morphine on rectal temperature and on locomotor activity. Isolated mice were compared to non isolated mice with the same age. 2. Morphine (20 mg/kg ip) induced in mice an early hypothermia followed by a late hyperthermia. The hypothermic effect was significantly reduced following isolation, but the duration of isolation (8, 15, 30 days) had no influence. Isolation did not modify the hyperthermic effect of morphine. 3. Morphine (40 mg/kg ip) induced in mice an increase in locomotor activity called "running". The running activity was significantly increased following isolation. The duration of isolation (8, 15, 30 days) did not seem to influence this effect. 4. These results show that isolation does not modify in the same way every effects of morphine, they suggest that isolation alters the mechanism involved in the running activity and in the hypothermic effect. The nature of these mechanisms is discussed.

Effects of dietary alpha-linolenic acid deficiency on neuromuscular and cognitive functions in mice.

Mice were fed a diet deficient in alpha-linolenic acid [18:3 (n-3)] or a control diet and the effect of this deficiency was assessed by behavioral and pharmacological measurements. Three weeks before mating female mice were fed a diet containing either peanut oil poor in alpha-linolenic acid (n-3)- or peanut+rapeseed oil rich in alpha-linolenic acid (n-3)+ = controls. Pups, aged 47 to 61 days, fed the same diet as their dams, were used for behavioral experiments. Muscular function and neuromuscular coordination assessed by the traction test, the elevated rotarod test and swimming endurance were unchanged by the (n-3)- deficiency. The level of anxiety assessed by the elevated plus-maze (anxiety protocol), the light-dark transition and the neophobia tests did not differ between (n-3)- and control (n-3)+ mice. Defensive behavior was not changed by the diet. The pentobarbital-induced loss of the righting reflex had the same duration in males, females, and controls as in (n-3) deficient mice; the latency to pentobarbital-induced loss of the righting reflex was significantly shorter in females than in males but did not differ according to the diet. Mice fed the (n-3)- deficient diet showed less efficient learning in the elevated plus-maze (learning protocol) and poorer understanding of the situation (or less motivation to escape) in the low rotarod test than mice fed the control (n-3)+ diet.

Effect of dietary alpha-linolenic acid deficiency on habituation.

Three weeks before mating, two groups of SWISS OF1 mice were fed a diet that was similar but contained either peanut oil poor in alpha-linolenic acid [18:3(n-3)] (n-3 deficient = deficient mice = (n-3)-) or peanut + rapeseed oil rich in alpha-linolenic acid (n-3 nondeficient = controls = (n-3)+). Pups, fed the same diet as their dams, aged 45 to 62 days were used for brain lipid analysis and for behavioral experiments, aimed at determining whether there is a relation between the dietary intake of alpha-linolenate and a simple form of learning: habituation. The behavior of mice was compared using four models: exploration recorded in a photocell actimeter, activity in an open-field, duration of immobility in the forced swimming test and number of escape attempts from a small closed space. Habituation was measured by testing the mice in the same situation after some time had elapsed since the first test. Exploration in the photocell actimeter was significantly reduced between day 1 and 4 in nondeficient mice, but, not in deficient mice. The number of square crossings in the open-field was significantly reduced on the second test neither in the control nor in the deficient mice. In the forced swimming test, the habituation (increase in duration of immobility) was significantly greater (255%) in nondeficient than in deficient mice (163%). In the escape attempt experiment, the habituation showed a trend to be greater in controls than in deficient mice (p = 0.061) and was significantly greater in females than in males (p = 0.028). These results suggest that a simple form of learning, habituation, occurs more slowly in mice fed a diet deficient in alpha-linolenic acid.

Isolation increases a behavioral response to the selective 5-HT 1B agonist CGS 120 66B.

The effect of two serotonergic drugs, CGS 120 66B acting specifically and TFMPP acting preferentially onto 5-HT1B receptors, was compared in preisolated and in pregrouped mice. Two mice put under an inverted beaker attempt to escape. The number of escape attempts of mice preisolated for 7 days was half that of pregrouped mice. In preisolated mice, TFMPP and CGS 120 66B increased the number of escape attempts up to, respectively, 200% and 300% of that of preisolated control mice. In pregrouped mice, CGS 120 66B was nearly inactive and TFMPP exerts a smaller effect. These results suggest that isolation increases the apparent responsiveness to 5-HT1B stimulants.

Behavioral effect of beta-blocking drugs resulting from the stimulation or the blockade of serotonergic 5-HT1B receptors.

The present study was aimed at determining the relative potency of various beta-blocking drugs as agonists or antagonists at 5-HT1B receptors. The behavioral model used (increase in escape attempts of isolated mice) has been previously shown to be exclusively responsive to 5-HT1B agonists such as 1-3-(trifluoromethyl) phenylpiperazine (TFMPP). Beta-blocking drugs acted in three different ways: they were either inactive, or acted as agonists or as antagonists at 5-HT1B receptors. The specific beta-blocking drugs: atenolol and betaxolol (beta-1) and ICI 118,551 (beta-2) were inactive by themselves and in interaction with TFMPP. The mixed beta-1 beta-2 blocking drug 1-penbutolol, (but not d-penbutolol), inactive alone, behaved as an antagonist: it impaired in a dose-dependent way the effect of TFMPP. (+/-)Pindolol and (-)pindolol was inactive. None of the (-), (+), or (+/-)pindolol was able to impair TFMPP effect. The increase in escape attempts induced by (+/-)pindolol was antagonized with 1-penbutolol or after a specific desensitization. Cyanopindolol and S-tertatolol (but not R-tertatolol) acted as agonists. SDZ 21009 was inactive as agonist or antagonist. It may be concluded that all beta-blocking drugs are not equivalent regarding their effect at 5-HT1B receptors. L-penbutolol was the only drug acting as an antagonist.