Opposing effects of APP/PS1 and TrkB.T1 genotypes on midbrain dopamine neurons and stimulated dopamine release in vivo.

Brain derived neurotrophic factor (BDNF) signaling disturbances in Alzheimer׳s disease (AD) have been demonstrated. BDNF levels fall in AD, but the ratio between truncated and full-length BDNF receptors TrkB.T1 and TrkB.TK, respectively, increases in brains of AD patients and APPswe/PS1dE9 (APP/PS1) AD model mice. Dopaminergic (DAergic) system disturbances in AD and detrimental effects of BDNF signaling deficits on DAergic system functions have also been indicated. Against this, we investigated changes in nigrostriatal dopamine (DA) system in mice carrying APP/PS1 and/or TrkB.T1 transgenes, the latter line modeling the TrkB.T1/TK ratio change in AD. Employing in vivo voltammetry, we found normal short-term DA release in caudate-putamen of mice carrying APP/PS1 or TrkB.T1 transgenes but impaired capacity to recruit more DA upon prolonged stimulation. However, mice carrying both transgenes did not differ from wild-type controls. Immunohistochemistry revealed normal density of tyrosine hydroxylase positive axon terminals in caudate-putamen in all genotypes and intact presynaptic machinery for DA release and reuptake, as shown by unchanged levels of SNAP-25, α-synuclein and DA transporter. However, we observed increased DAergic neurons in substantia nigra of TrkB.T1 mice resulting in decreased tyrosine hydroxylase per neuron in TrkB.T1 mice. The finding of unchanged nigral DAergic neurons in APP/PS1 mice largely confirms earlier reports, but the unexpected increase in midbrain DA neurons in TrkB.T1 mice is a novel finding. We suggest that both APP/PS1 and TrkB.T1 genotypes disrupt DAergic signaling, but via separate mechanisms.

Spreading depression in the cortex differently modulates dopamine release in rat mesolimbic and nigrostriatal terminal fields.

The effects of cortical spreading depression (SD) on evoked dopamine release in mesolimbic (nucleus accumbens) and nigrostriatal (nucleus caudatus) terminal fields were studied by in vivo voltammetry in anesthetized rats. Dopamine release was evoked by electrical stimulation of medial forebrain bundle (20 Hz, 100 pulses). Local application of 3 M KCl on the dura initiated SD in the cortex. It was found that SD modulated evoked dopamine release in subcortical structures at the same time when the wave of depression of cortical activity reached reciprocally connected subcortical areas. This cortical depression increased stimulated dopamine release in the nucleus accumbens and decreased dopamine release in the nucleus caudatus. In agreement with these results, electrical stimulation of the prefrontal cortex at 20 Hz, synchronized with medial forebrain bundle stimulation, decreased evoked dopamine release in the nucleus accumbens. Areas of the cortex which modulated dopamine release in these two terminal fields were spatially separated by at least 5 mm from each other. It is proposed that depression and activation of evoked dopamine release in the nucleus caudatus and nucleus accumbens following SD are indicative of tonic activation of the nigrostriatal and tonic inhibition of the mesolimbic dopaminergic terminals by cortex in normal conditions. SD in the cortex, modulating neurotransmitter release in subcortical structures, may have a general impact on redistribution of oxygen supply in these subcortical areas and on behavior associated with brain trauma, migraine, insult or seizures, i.e. the kind of neuropathology which may cause SD type phenomena also in human brain.

Patterns of dopamine overflow in mouse nucleus accumbens during intracranial self-stimulation.

Dopamine (DA) overflow in the mouse nucleus accumbens during intracranial self-stimulation (ICSS) of the median forebrain bundle was estimated by chronoamperometry with removable carbon fibre electrodes. The specificity of the voltammetric signal was confirmed pharmacologically. The parameters of stimulation (50 Hz, 0.5 s train length) allowed us to obtain measurable DA release and to maintain ICSS. Continuous (CR) and fixed-ratio (FR8) schedule of reinforcement showed differing correspondence of the patterns of DA release with the patterns of stimulation/nose-poking. The CR schedule induced a high rate nose-poking and tonic increase in dopamine overflow, which became decreased following the first periods of self-stimulations. The FR schedule induced stable peaks of DA overflow during the entire period of ICSS. We conclude that the availability of a readily-releasable pool of DA in presynaptic terminals determined the pattern of dopamine overflow in the nucleus accumbens during ICSS in mice.

Ethanol modulates evoked dopamine release in mouse nucleus accumbens: dependence on social stress and dose.

Ethanol may modulate the activity of presynaptic terminals to increase extracellular dopamine release in the nucleus accumbens though conflicting results have been published. It has been suggested that the stress of social defeat might be a factor influencing the effects of ethanol. We investigated the effects of ethanol on the evoked dopamine overflow in the nucleus accumbens in anaesthetised mice by in vivo voltammetry. Dominant animals, subordinates which had been defeated following eight intruder-resident encounters, and subordinate nondefeated mice were used. The overflow was evoked by electrical stimulation of the median forebrain bundle (100 pulses) at low (20 Hz) and high (50 Hz) frequencies of stimulation. Ethanol at 0.1 and 2 g/kg had no effects on evoked dopamine overflow in aggressive and nondefeated mice. Ethanol increased dopamine release at 0.1 g/kg and decreased release at 2 g/kg following high frequency stimulation in defeated mice. These data suggest that the stress of social defeat may have sensitised the machinery involved in dopamine release to ethanol, a process that may increase the reinforcing properties of this compound.

Dopamine release from pharmacologically distinct storage pools in rat striatum following stimulation at frequency of neuronal bursting.

The increase of dopamine overflow in the rat caudate and nucleus accumbens following repeated stimulation of the median forebrain bundle (MFB) at short intervals and the role of alpha-methyl-p-tyrosine (AMPT) and reserpine sensitive storage pools in evoked dopamine overflow were investigated by in vivo voltammetry. In contrast to stimulation at 30-s intervals and longer, stimulation of the MFB at 5-s intervals led to increased dopamine release following each consecutive stimulation. This effect was much larger in the caudate than in the nucleus accumbens. We consider that the increase in dopamine release is due to translocation of dopamine from a reserpine-sensitive storage pool and this accounts for the increase of the rate of refilling of the readily releasable pool. The shorter the intervals between stimulation and the higher the frequency (with a plateau at 30 Hz), then the greater the fraction of dopamine which originates from the reserpine-sensitive storage pool. Exocytotic release of dopamine from the AMPT-sensitive storage pool does not seem to depend on the intervals between stimulation. We propose that the vesicles in presynaptic dopaminergic terminals near the outer membrane are more sensitive to AMPT. Distantly located vesicles have a relatively higher sensitivity to reserpine. Experiments using repeated stimulation at as low a frequency as 10-20 Hz, revealed that this phenomenon may take place under physiological conditions following bursting of dopamine neurones.

In vivo voltammetry with removable carbon fibre electrodes in freely-moving mice: dopamine release during intracranial self-stimulation.

The advantages of in vivo voltammetry at carbon fibre electrodes cannot be fully realised without the registration of neurotransmitter release in freely moving animals. Here we describe an approach to record electrically evoked dopamine release in freely-moving mice. A description of a simple in-house made micromanipulator (0.4 g) and preamplifier (0.6 g) is given. This system was used to record electrochemical signal in the nucleus accumbens four to eight times during 2 weeks following intracranial self-stimulation (ICSS) of the median forebrain bundle. High-speed chronoamperometry was found to be the best choice for recording. The most efficient parameters of electrical stimulation for training and self-stimulation (50 Hz, 0.2 s, 60 microA) were insufficient to induce a measurable voltammetric signal. Increasing the strength of stimulation (0.5 s, 160 microA) significantly decreased the rate of self-stimulation and allowed the registration of separate peaks of dopamine overflow on each stimulation and a tonic increase of electrochemical signal following higher rates of ICSS. Due to exhaustion of the readily releasable dopamine pool, the length of recording the electrochemical signal depended on the rate of self-stimulation. A fixed ratio schedule of ICSS decreased the rate of electrical stimulation and permitted the maintenance of stable peaks of dopamine overflow at a high rate of nose-poking.

A new technique for measuring the temporal characteristics of the carbon fibre microelectrodes in in vivo voltammetry at millisecond time intervals.

A common approach to test the time-response of carbon fibre microelectrodes entails the use of a flow injection system which allows delivery of a bolus of test compound in the flow stream to a detector electrode. The introduced bolus is distorted from its original form by convective and dispersive forces during the period of transport from the injector to the detector electrode in the connection tubing and couplings. Thus, the flow injection system allows one to change the medium in the vicinity of the electrode in about 100-200 ms. We describe here a simple falling drop system, which is an easy to use for calibration of the carbon fibre microelectrodes with respect to their sensitivity and time-response. This system does not have a loop injector and minimizes problems associated with convective and dispersive bolus distortion. It allows one to change the medium in the vicinity of the electrode in less than 10 ms. With this system, the sensitivity and the temporal resolution of the electrodes may be easily and rapidly estimated in a single experiment. The temporal resolution of the untreated, uncoated carbon fibre cylinder electrode of 300 microns tip length in response to dopamine was estimated as 1.49 +/- 0.1 microM/ms (M +/- SEM, n = 36, rise) and 0.051 +/- 0.003 microM/ms (n = 36, wash-out) on the half of amplitude of electrochemical response which was monitored by constant potential amperometry.

Alpha2-adrenergic control of dopamine overflow and metabolism in mouse striatum.

The effects of the alpha2-adrenoceptor drugs, medetomidine and atipamezole, on dopamine overflow evoked by low (6 Hz-10 s) and high (50 Hz-4 s) frequency electrical stimulation of the median forebrain bundle were studied in striatum of BALB/C mice anaesthetized with chloral hydrate with fast in vivo voltammetry techniques. The effects of these drugs on the basal concentrations of dopamine metabolites were also investigated by means of differential pulse voltammetry. Medetomidine dose dependently decreased dopamine overflow in nucleus accumbens in the dose range 5-100 microg/kg, s.c. This effect was seen only at low frequency stimulation and reached 85% at a dose of 100 microg/kg. Medetomidine also decreased the basal concentration of striatal homovanillic acid. This effect did not exceed 35%. Atipamezole antagonized the inhibitory effects of medetomidine on the dopamine overflow. but showed no effect itself. We suggest that alpha2-adrenoceptors in dopaminergic terminal fields in the mouse striatum are involved in the regulation of dopamine release at physiological stimulation frequencies.

Two simultaneously working storage pools of dopamine in mouse caudate and nucleus accumbens.

1. The dynamics of the decline of evoked dopamine overflow after repeated electrical stimulation (2 or 4 s train duration, 50 Hz) of the median forebrain bundle were investigated by means of in vivo voltammetry in mouse caudate and nucleus accumbens. An unexpected effect-slowing of the rate of dopamine decline after repeated stimulation at short (10 s or less) between-stimulation intervals, and an increase in the absolute amount of dopamine released at the beginning of the repeated stimulation-was found. 2. After the evoked dopamine overflow had been reduced by alpha-methyl-p-tyrosine (AMPT), repeated stimulation at intervals of 5 s increased dopamine release to each subsequent stimulation applied. 3. It is proposed that there are two compartments involved in dopamine storage. Both contribute to the release of dopamine, however, they may be separated artificially by either applying stimulation at short intervals or by depletion of intracellular dopamine. 4. The first releasable pool (newly synthesized, AMPT-sensitive) provides dopamine for the release after a single stimulation or repeated stimulation, being independent of whether the first stimulation is succeeded quickly by a second. It is also independent of between-stimulation interval. 5. The second pool (AMPT-insensitive storage pool) is progressively activated after repeated stimulation. The duration of the between-stimulation intervals is the crucial factor for the activation of this pool.

The systemic administration of tacrine or selegiline facilitate spatial learning in aged fisher 344 rats.

When compared to young Fisher 344 rats, aged Fisher 344 rats were impaired in their acquisition of the water maze task as indicated by longer escape latencies and distances to find a hidden platform. In a free swim trial which was performed after the training period, young rats had a better spatial bias, since they spent more time swimming in the previous training quadrant. Tacrine 3 mg/kg, an anticholinesterase, and selegiline 0.25 mg/kg, a MAO-B inhibitor, partially reversed the acquisition deficit in aged rats when administered on their own, and drug-treated aged rats swam more in the previous training quadrant than vehicle-treated aged rats during the free swim trial. Aged rats also swam slower than young rats. Tacrine, but not selegiline, increased swimming speed in aged rats. Taken as a whole, these data support the proposal that tacrine may be effective at alleviating age-related learning impairment and confirm the role of cholinergic dysfunction in the spatial learning deficit in aged rats.

Some unusual effects of alpha 2-adrenergic drugs on cortical high voltage spindles in rats.

The dose-response curves for a number of alpha-adrenergic drugs were investigated to estimate a possible role of the alpha 2/alpha 1 selectivity of these drugs on the incidence of cortical high voltage spindles (HVS), reflecting level of vigilance. The alpha 2 antagonists yohimbine (0.25-4 mg/kg) and idazoxan (0.5-4 mg/kg), but not atipamezole induced a biphasic effect on the incidence of HVS in rats. This effect of relatively small doses of yohimbine and idazoxan should be taken into consideration when using these drugs as alpha 2 antagonists in behavioral and neurophysiological tests. On the other hand the linearity of the dose-response curve for atipamezole (0.01-4 mg/kg) indicates that this drug is a good candidate for use in such tests.

A comparison of the effects of individual organic solvents and their mixture on brain stimulation reward.

In spite of the prevalence of solvent abuse, there are only a few experimental investigations on the addictive potential of household organic solvents. In the present study we attempted to investigate the influence of glue thinner, a very popular glue used by glue-sniffing children, and the four organic solvents that compose this thinner (toluene, mixture of petroleum hydrocarbons, ethyl acetate, methylene chloride) on self-stimulation of the lateral hypothalamus (ICSS) in rats. Glue thinner, toluene, a mixture of petroleum hydrocarbons, and methylene chloride had a biphasic effect on ICSS, increasing frequency of self-stimulation at lower concentrations and decreasing it at higher concentrations. Ethyl acetate decreased frequency of self-stimulation at all concentrations. In contrast to classically abused drugs, solvents increased the threshold current of self-stimulation. The differences between concentration-response curves of ICSS for glue thinner and solvents permit the proposal that the mixture of solvents can be more dangerous than the individual components in potential for inducing solvent abuse.

Experimental estimation of addictive potential of a mixture of organic solvents.

In the present study we investigated in rats the reinforcing properties of glue vapours which are a mixture of four organic solvents (toluene 25%, benzine fraction 37%, ethyl acetate 31% and methylene chloride 7%). This mixture is used as a glue thinner and is a very popular among glue-sniffing children. Immediately after inhalation at a concentration of 7200 ppm, the glue vapours increased locomotor activity in the open field and response rate of self-stimulation in the lateral hypothalamus. Contrary to classical drug abuse, glue vapours enhanced the threshold current of self-stimulation. While the response rate of self-stimulation decreased to the control level 30 min after inhalation, the threshold current was still enhanced. Increasing the concentration of the vapours produced a decrease in response rate of self-stimulation and locomotor activity. When glue vapours were inhaled at a concentration of 14,400 ppm and higher, the response rate of self-stimulation was completely suppressed. The discriminative stimulus effects of the glue vapours appeared to be similar to those of general anaesthetics such as ether and pentobarbital. At vapour concentrations of 7200 and 14,400 ppm, conditioned place preference was established. The ability of solvents to reinforce conditioning in the place preference paradigm and to activate the brain reward system in intracranial self-stimulation experiments may be useful for predicting the addictive potential of inhalants.

The interaction of L-deprenyl and scopolamine on spatial learning/memory in rats.

L-Deprenyl, a specific MAO-B inhibitor, has been reported to improve learning/memory in some cognitive tests in aged rats. The present study investigated whether L-deprenyl could alleviate the spatial learning deficit induced by muscarinic blockade and aging in OFA rats. Scopolamine (0.25 mg/kg) impaired the acquisition of a water maze task in adult rats and increased their swimming speeds. L-Deprenyl (0.25 mg/kg, 14 days) had no effect on water maze performance in saline treated adult rats, but markedly alleviated the learning deficit induced by scopolamine and increased the time and distance of swimming in the training quadrant when the platform was removed (spatial probe trial). L-Deprenyl partly reduced the effect of scopolamine on speed of swimming. Nevertheless, administration of l-deprenyl (0.25 mg/kg, 14 days) had no effect on spatial learning/memory in aged rats. We suggest that the l-deprenyl-scopolamine interaction in the water maze test may be considered as a premise for further investigations of l-deprenyl as cognition enhancer.

Fast cyclic voltammetry of oxidizable species; depth profile in the rat striatum with and without potassium stimulation of the median forebrain bundle.

Fast cyclic voltammetry was used to monitor the release of dopamine in the caudate nucleus following potassium stimulation of the median forebrain bundle and to determine the characteristics of the small basal oxidation current while dipping of an electrode with a large tip (0.3 mm) in the caudate nucleus. The oxidation current was shown to be greater just after the electrode reached the caudate area than at a time 15-20 s after its arrival. There was less current increase observed when the tip reached the cortex and no current was observed in the white matter or with the electrode returning or with repeated passage along the same track. Potassium stimulation of median forebrain bundle induced an initial low magnitude dopamine release which disappeared within minutes. The basal oxidation current increased step by step after a few potassium stimulations. This effect was not blocked by pargyline (75 mg/kg) whereas L-DOPA (200 mg/kg) was shown to increase it. It is thought that the large tip of the lowered electrode can destroy neurons and induce the release of intracellularly stored compounds into the extracellular space. We consider also dopamine to be responsible for the increase in oxidizable current just after the electrode movement stopped.

[New model for studying group learning in mice (synthesis of conditioned reflex and ethologic approaches]. / Novaia model' dlia izucheniia gruppovogo obucheniia u myshei (sintez uslovnoreflektornogo i étologicheskogo podkhodov).

In experiments on mice it has been shown that synthesis of conditioned reflex and behavioural approaches is advantageous for the more complete qualitative and quantitative characteristic of group learning and performance of conditioned avoidance reaction in rodents. The efficiency of group learning and performance of this reaction depend not only on the individual characteristics of animals (ability or inability to learning) but also on the type of hierarchic relationships in the group. The possibility is suggested to use this model for investigating the modulating effect of psychotropic agents on group learning and performance of avoidance reaction.