Suppression of conditioned avoidance behavior by the local application of (-)sulpiride into the ventral, but not the dorsal, striatum of the rat.

The local application of (-)sulpiride, 200 ng side-1, into the nucleus accumbens produced a suppression of conditioned avoidance behavior in male rats, 10 and 90 min after injection. The decrease in avoidance behavior was accompanied by a decrease in motor activity, as evidenced by changes in the number of intertrial crosses. When injected into the dorsolateral neostriatum, or the amygdala, (-)sulpiride produced a suppression of conditioned avoidance behavior at the 90-min time interval only. Considering diffusion from the injection site, as indicated by an increase in local dopamine turnover [(DO-PAC + HVA) DA-1], the effects obtained in the dorsolateral neostriatum, and possibly also the amygdala, 90 min after injection could be due to diffusion to the nucleus accumbens. The local application of (-)sulpiride into the posterior neostriatum, or into the prefrontal cortex, produced no statistically significant effect on conditioned avoidance behavior 10 or 90 min after injection. It is concluded that the performance of conditioned avoidance behavior in the rat is critically dependent on an intact dopaminergic neurotransmission in the nucleus accumbens or adjacent areas of the ventral striatum.

Persistent changes in behaviour and brain serotonin during ageing in rats subjected to infant nasal virus infection.

Suckling rats were infected intranasally with the temperature-sensitive mutant G41 strain of vesicular stomatitis virus. The rats survived but demonstrated lifelong learning deficits in the Morris maze and impaired exploratory behaviour in the open field test. When examined at 18 months of age they had a severe loss of neurons in the medial and dorsal raphe nuclei in the brain stem and reduced levels of serotonin and its metabolite 5-hydroxyindole acetic acid in the cerebral neocortex and hippocampus. The levels of noradrenaline, dopamine, homovanillic acid, 3,4-dihydroxyphenylacetic acid, choline acetyltransferase and glutamate decarboxylase were largely unaffected. The permanent disturbance in brain serotonin metabolism did not cause any histological changes in the cerebral cortex. Thus there were no neurofibrillary tangles or amyloid plaques as has been reported as a late effect of chemically induced lesion to the cholinergic system in the rat brain. It is concluded that the brain serotonergic system is especially vulnerable to an episode of virus attack along olfactory pathways and that the neurochemical and behavioural alterations caused by such an episode persist during a major part of the animal's life span.

Distribution of D1- and D2-dopamine receptors, and dopamine and its metabolites in the human brain.

Densities and distribution of D1-dopamine and D2-dopamine receptors were investigated in vitro using [3H]SCH 23390 and [3H]raclopride in receptor binding assays and autoradiography on human post mortem whole hemisphere slices to serve as anatomical correlates to PET studies using [11C]SCH 23390 and [11C]raclopride. In addition, the levels of dopamine and its metabolites were determined by HPLC in various brain regions. Both dopamine receptor subtypes, as well as dopamine, HVA and DOPAC, were primarily found in the basal ganglia. Very high densities of D1-dopamine receptors were found particularly in the medial caudate nucleus, whereas D2-dopamine receptors were evenly distributed throughout the caudate. The densities of D1- and D2-dopamine receptors were similar in the caudate nucleus and the putamen, whereas there were 4 to 7 times higher densities of the D1- than of the D2-dopamine receptors in several limbic and neocortical regions. The receptor distribution in the autoradiographic study was consistent with that demonstrated in the living human brain using [11C]SCH 23390 and [11C]raclopride.

Dopamine D2 receptors and dopamine metabolism. Relationship between biochemical and behavioural effects of substituted benzamide drugs.

The effect of the substituted benzamide dopamine D2 receptor antagonists sulpiride, raclopride, FLA 966(-), FLA 988(-), eticlopride and remoxipride as well as the "classical" dopamine antagonists, haloperidol and chlorpromazine, on the concentrations of dopamine, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the brain of the rat was investigated. All compounds increased the turnover of dopamine, as defined by increased concentrations of DOPAC and HVA (without change in the concentration of dopamine) in the striatum in a dose-dependent manner. The doses of the compounds producing increased turnover of dopamine in the striatum were in the same range as those displacing the in vivo binding of [3H]spiperone in the striatum. In addition, for all the compounds tested in the study, an increase in the turnover of dopamine to about 300% of control was observed for doses antagonising the stereotypy produced by the dopamine agonist, apomorphine. On the other hand, no consistent relationship between increased turnover of dopamine and the doses of the compounds required to antagonise apomorphine-induced hyperactivity was found. This result was also found in limbic areas. Remoxipride and haloperidol had little or no effect on the turnover of dopamine in either the hypothalamus or substantia nigra at the ED50 doses of these compounds for antagonism of apomorphine-induced stereotypy.

Dopamine turnover and glutamate decarboxylase activity in the rat brain after acute and chronic treatment with raclopride, a dopamine D2-selective antagonist.

The effect of acute and chronic (94, 141 and/or 199 days) oral treatment of rats with the dopamine D2-selective antagonist raclopride (0, 5, 15, 45 and 135 mumol/kg) upon the turnover of dopamine in the striatum and limbic system and upon the activity of glutamate decarboxylase (GAD) in the s. nigra, striatum and frontal cortex has been investigated. A dose-dependent tolerance to the effect of raclopride on the turnover of dopamine was observed after chronic treatment, although the degree of tolerance was marginal at the 5 and 15 mumol/kg doses. Acute treatment with raclopride was without effect on the activity of GAD in the s. nigra, striatum or frontal cortex, whereas chronic (199 days) treatment with 45 mumol/kg of raclopride produced an increase in the activity of GAD in the s. nigra and striatum.

Behavioural deficits and serotonin depletion in adult rats after transient infant nasal viral infection.

Dysfunction of subcortical serotoninergic neurons has been implicated in some behaviour disturbances. The serotoninergic neurons in the dorsal and median raphe project widely in the brain. They innervate the olfactory bulbs and can be targets for exogenous agents attacking the olfactory epithelium and bulbs. We report here an injury to the serotoninergic neurons after intranasal infection in 12-day-old rats with a temperature-sensitive mutant of vesicular stomatitis virus. The brain infection was focal and transient. Viral antigens could no longer be detected 13-15 days after infection. In spite of this the animals, as adults, had a severe serotonin depletion in the cerebral cortex and hippocampus, and showed abnormal locomotor and explorative behaviour as well as learning deficits. The neocortex was histologically intact and parameters related to other neurotransmitters such as dopamine, noradrenaline, GABA and acetylcholine showed no marked changes. A relatively selective damage to serotoninergic nuclei as a result of virus neuroinvasion through a natural portal of entry, may constitute a new pathogenetic mechanism for cortical dysfunction and behavioural deficits.

Increased total activity in the rat after L-tryptophan plus the monoamine oxidase-A inhibitor amiflamine but not after L-tryptophan plus clorgyline.

The effect of pretreatment with either saline or the monoamine oxidase-A inhibitors clorgyline and amiflamine upon the total activity, locomotion and rearing behaviour of the rat induced by various doses of the monoamine precursor L-tryptophan was studied by use of automated activity boxes. Amiflamine (2.5 and 5.0 mg kg-1, i.p.) increased in a dose-dependent manner total activity and to a lesser extent, locomotion when given 60 min before L-tryptophan (100 mg kg-1, i.p.). The increased activity was seen after amiflamine plus either 25 or 75 mg kg-1 L-tryptophan. Rearing behaviour was not affected. Analysis of 5-hydroxytryptamine (5-HT) and its deaminated metabolite 5-hydroxyindoleacetic acid (5-HIAA) by high performance liquid chromatography with electrochemical detection indicated that in both frontal cortex and hypothalamus, amiflamine (at both doses) increased 5-HT and reduced 5-HIAA concentrations. Combination of amiflamine with L-tryptophan (100 mg kg-1, i.p.) resulted in a higher 5-HT concentration being found than after amiflamine alone. L-Tryptophan treatment alone did not change 5-HT concentrations but increased 5-HIAA concentrations. Clorgyline, at a dose of either 1 or 5 mg kg-1 i.p. plus L-tryptophan (25 or 100 mg kg-1, i.p.) did not increase total activity, locomotion or behaviour. A number of possible explanations for the differences in the behavioural effects of clorgyline and amiflamine when given with L-tryptophan are discussed. It is concluded that in addition to monoamine oxidase-A inhibition, other pharmacological effects of the drugs, such as 5-HT release (amiflamine) and inhibition of tryptophan hydroxylation (clorgyline) may be of importance in determining the magnitude of the increase in activity when the compounds are given together with L-tryptophan.

Retention deficits induced by acute p-chloroamphetamine following fear conditioning in the rat.

Rats were given four inescapable shocks (1.0 mA) when confined to the right-hand corner of a modified shuttlebox. p-Chloroamphetamine (PCA) injected just before the retention test 24 h later completely blocked the immobile posture that was observed after saline injections. This retention deficit was shown to be selectively associated with 5-hydroxytryptamine (5-HT) release, since the administration of the 5-HT uptake inhibitors zimelidine and citalopram 60 min prior to PCA antagonized this effect. The 5-HT specificity of the deficit was further established by the findings that 5-HT-depleted rats (PCA, 2 x 10 mg/kg, and fenfluramine, 2 x 25 mg/kg), but not NA-depleted rats (DSP4, 1 x 50 mg/kg), or rats treated with zimelidine (2 x 20 mg/kg) 60 min before PCA (2 x 10 mg/kg), showed an almost complete blockade of the retention failure. The data presented may provide a useful experimental model for investigating the efficacy of functional 5-HT activity in the treatment of phobic anxiety.

The selective dopamine D2 receptor antagonist raclopride discriminates between dopamine-mediated motor functions.

The actions on central dopamine (DA) mechanisms of raclopride, a new substituted benzamide, were studied by means of behavioural and biochemical methods in the rat. Raclopride blocked the in vitro binding of the dopamine D2 antagonist 3H-spiperone (IC50 = 32 nM), but not of the unselective D1 antagonist 3H-flupenthixol (IC50 greater than 100,000 nM) in rat striatum, and failed to inhibit striatal DA-sensitive adenylate cyclase in vitro (IC50 greater than 100,000 nM). Raclopride caused a dose-dependent increase in the DA metabolites HVA and DOPAC in the striatum and olfactory tubercle. Behavioural studies showed that raclopride discriminates between the motor behaviours induced by the DA agonist apomorphine. Thus, unlike haloperidol, raclopride blocked apomorphine-induced hyperactivity at considerably lower doses than those inhibiting oral stereotypies. Moreover, raclopride showed a high separation between the doses for blockade of apomorphine-induced hyperactivity and those inducing catalepsy in rats. Raclopride caused a dose-dependent blockade of the specific binding of 3H-spiperone and 3H-N-n-propylnorapomorphine (3H-NPA) in vivo at doses similar to those blocking the behavioural effects of apomorphine. The maximal blockade of 3H-spiperone binding in vivo was lower for raclopride than for haloperidol. Raclopride caused a greater inhibition of 3H-NPA than of 3H-spiperone in vivo binding in the striatum. It is suggested that the ability of raclopride to discriminate between different DA-mediated functions may be attributed to a preferential blockade of a subclass of functionally coupled dopamine D2 receptors in striatal as well as in extrastriatal brain regions in the rat.

Suppression of exploratory locomotor activity and increase in dopamine turnover following the local application of cis-flupenthixol into limbic projection areas of the rat striatum.

Recent neuroanatomical tracer studies have demonstrated the topography of 'limbic' (A10) projections into the striatum of the rat (see Introduction). The target areas include the nucleus accumbens and the ventromedial part of the neostriatum, whereas the dorsolateral part of the neostriatum does not receive such afferents. Taking this topography into account, the present results show that local application of cis-flupenthixol (10-40 micrograms/side) into the nucleus accumbens or the ventromedial, but not the dorsolateral, neostriatum produces suppression of exploratory locomotor activity in the rat. trans-Flupenthixol (40 micrograms/side) was completely ineffective when locally applied into the nucleus accumbens. Measurements of the concentrations of the dopamine metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) at the site of injection, and in neighboring areas at different times after cis-flupenthixol administration, indicated that there was little or no diffusion of the drug from the injection sites. Much higher concentrations of DOPAC and HVA in a given area were found after systemic administration of cis-flupenthixol as compared with local application of the drug to the same area.

Remoxipride, a new potential antipsychotic compound with selective antidopaminergic actions in the rat brain.

The novel substituted benzamide, remoxipride, preferentially blocked apomorphine-induced hyperactivity with weak effects on stereotypies. The potency of remoxipride was about 50 times higher than that of sulpiride. Remoxipride caused a weak, atypical form of catalepsy and showed a high separation between the ED50 for blockade of apomorphine-induced hyperactivity and the ED50 for induction of catalepsy (ratio 24). Remoxipride was shown to be a selective dopamine D2 receptor antagonist since it displaced [3H]spiperone (IC50 = 1570 nM) but not [3H]flupentixol (IC50 greater than 100 000 nM) in rat striatum, and did not inhibit striatal DA-sensitive adenylate cyclase in vitro (IC50 greater than 100 000 nM). Remoxipride is a potent antagonist of D2 receptors showing a dose-dependent blockade of [3H]spiperone and [3H]n-propylnorapomorphine in vivo binding with a potency equal to that of chlorpromazine. In contrast to haloperidol, remoxipride caused a preferential blockade of in vivo [3H]spierone binding in the mesolimbic DA rich areas and the substantia nigra with much less effect in the striatum. In addition, remoxipride produced a preferential increase of DA utilization following synthesis inhibition in the olfactory tubercle. Only minor changes in NA and 5-HT metabolism were observed while HVA and DOPAC levels were markedly elevated. Taken together, these results indicate that remoxipride is a potent, selective D2 receptor blocking agent with a preferential action in mesolimbic and extrastriatal dopamine-containing neurons.

The effect of selective noradrenergic lesions upon the stimulation by noradrenaline of inositol phospholipid breakdown in rat hippocampal miniprisms.

The breakdown of inositol phospholipid (PI) stimulated by hippocampal noradrenaline in rat miniprisms in vitro was used as an index of alpha 1-adrenoceptor function after selective noradrenergic denervation. Selective denervation was produced by microinjections of 6-hydroxydopamine (6-OHDA) into either the dorsal noradrenergic bundle (DNAB) or the locus coeruleus (LC), or by systemic treatment with the noradrenergic neurotoxin DSP4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine). Fourteen days after these treatments, there was a large depletion of cortical noradrenaline but no change in the stimulation of hippocampal PI breakdown by noradrenaline. It is concluded that selective noradrenergic denervation under the conditions used here does not lead to hippocampal alpha 1-adrenoceptor supersensitivity as assessed by noradrenaline-stimulated PI breakdown.

Influence of 5-HT1B/1D receptors on dopamine release in the guinea pig nucleus accumbens: a microdialysis study.

To clarify whether serotonin (5-HT) 5-HT1B/1D receptors are involved in dopamine (DA) release, extracellular levels of DA were monitored by in vivo microdialysis during various conditions. 5-HT (10 microM) alone, and together with the 5-HT1B/1D receptor antagonist. GR127935 (10 microM), or the 5-HT1B/1D agonist, sumatriptan (1 microM), were perfused into the nucleus accumbens of freely moving guinea pigs. A 10-fold increase in the extracellular concentration of DA was obtained during administration of 5-HT alone. The 5-HT-induced DA elevation was not significantly affected by co-administration of sumatriptan (MANOVA; P > 0.05) but markedly attenuated by coperfusion of GR127935 (MANOVA; P = 0.02). Neither GR127935 nor sumatriptan, when administered alone, significantly affected extracellular DA levels. These results suggest that, in the DA-rich nucleus accumbens, 5-HT1B/1D receptors are not involved in the modulation of DA release during normal tonic or basal conditions but may take part in the regulation of DA release when synaptic 5-HT levels are very high.

Forebrain serotonergic involvement in avoidance learning.

The one-way active avoidance deficit caused by the serotonergic (5-HT) releasing compound p-chloroamphetamine (PCA) was examined in rats after degeneration of 5-HT neurons in the forebrain. Injection of 5,7-dihydroxytryptamine (5,7-DHT) into the forebrain in desipramine (20 mg/kg)-pretreated rats resulted in a 65-70% decrease in 5-HT concentrations in the prefrontal cortex and hippocampus without any significant effect on striatal 5-HT. Slight reductions in noradrenaline (NA) (25%) and dopamine (DA) (34%) concentrations were observed in the prefrontal cortex only. The 5,7-DHT lesions markedly attenuated the impaired avoidance performance induced by PCA (2.5 mg/kg), suggesting that the avoidance deficit depends on intact 5-HT terminals in cortex and/or hippocampus. The 5,7-DHT lesion alone caused a slight but significant impairment of acquisition. The results suggest that 5-HT terminal systems in the forebrain play an important role in avoidance learning.

Neurochemical effects of prolonged treatment with remoxipride as assessed by intracerebral microdialysis in freely moving rats.

1. At three microdialysis sessions, dialysates were collected from the striatum of the same rats. 2. Microdialysis session 1. A single s.c. injection of remoxipride (40 mumol/kg), resulted in increased dialysate concentrations of dopamine, DOPAC and HVA. 3. Microdialysis session 2. Continuous administration of remoxipride (8.6 mumol/rat/day) for 14 days, using mini-osmotic pumps, produced maintained elevated levels of dopamine, DOPAC and HVA. 4. Microdialysis session 3. A challenge dose of remoxipride (40 mumol/kg s.c.), given to the rats after a 48-hour wash-out period following the continuous remoxipride treatment, increased the dialysate concentrations of dopamine, DOPAC and HVA to similar extent as at dialysis session 1. 5. It is concluded that after long-term treatment of remoxipride, an adaptation of the basal state of the DA system appears to take place, implying a lowering of basal DA release and DA metabolism. However, the capacity to respond with increased DA release and DA metabolism to renewed remoxipride treatment is retained, indicating little, if any, tolerance.

In vivo intrinsic efficacy of the 5-HT1A receptor antagonists NAD-299, WAY-100,635 and (S)-(-)-UH-301 at rat brain monoamine receptors.

The receptor-mediated control of brain monoamine synthesis was used to examine the in vivo intrinsic efficacy of the 5-HT1A receptor antagonists NAD-299, S(-)-UH-301 and WAY-100,635. The rate of monoamine synthesis was estimated by measuring the accumulation of DOPA and 5-HTP in the ventral neostriatum and the ventral hippocampus in rats pretreated with an inhibitor of cerebral aromatic L-amino acid decarboxylase. S(-)-UH-301 (2.0-32.0 micromol kg(-1)), but not WAY-100,635 (0.08-1.2 micromol kg(-1)), produced a decreased 5-HTP accumulation in the neostriatum and in the hippocampus. The administration of NAD-299 (0.75-12.0 micromol kg(-1)) resulted in a slight increase in neostriatal, but not hippocampal, 5-HTP accumulation. Neostriatal DOPA accumulation was decreased by S(-)-UH-301, whereas treatment with WAY- 100,635 resulted in an increase. NAD-299 did not affect neostriatal DOPA levels. There were no effects by any of these agents on DOPA levels in the ventral hippocampus. It is concluded that S(-)-UH-301, but not WAY-100,635 or NAD-299, displays intrinsic efficacy at brain 5-HT1A and DA D2/3 receptors, whereas WAY-100,635 behaves as a DA D2/3 receptor antagonist. By this comparison, NAD-299 appears to be the most selective and specific 5-HT1A receptor antagonist.

Temporal studies of the inhibition of voluntary ethanol intake in the rat induced by intermittent ethanol treatment and some long term neurochemical consequences.

Male rats were injected with ethanol (groups 3 and 5; 2.0 g/kg i.p.) or saline (groups 2 and 4) once a week for 52 weeks. The rats had access to ethanol as a voluntary choice for 24 h either once 6 days after the injection (groups 2 and 3) or twice 3 and 6 days after the injection (groups 4 and 5). At the beginning of the treatment ethanol injections inhibited voluntary ethanol intake if tested 6 days later (groups 3 and 5), but a tolerance developed to this inhibition. During tolerance development the rats in group 5 also drank less ethanol on day 3 than on day 6. No corresponding behaviour was seen in group 4. Thus part of the tolerance was a gradual reduction of the duration of inhibition. During the evaluation period (25 weeks) after the treatment, ethanol exposure (20 weeks) consisted of a continuous choice between ethanol and water. Of different ethanol concentrations both ethanol-injected groups (3 and 5) took the same voluntary dose of ethanol independent of the offered concentration. After 5 weeks without ethanol all rats were killed and a number of neurochemical variables were determined. Compared with almost unexposed rats (group 1) changes were seen in inositol phospholipid breakdown, muscarinic binding sites in hippocampus, noradrenaline concentrations in frontal cortex, hippocampus and hypothalamus, dopamine concentration in frontal cortex and 5-hydroxytryptamine concentration in hypothalamus. In most cases the largest changes were seen in group 5. None of the variables had a constant relation to ethanol intake in the total population. However, significant correlations were found in some of the groups.

Influence of age on effects induced by intermittent ethanol treatment on the ethanol drinking pattern and related neurochemical changes in the rat.

Male rats were treated with one ethanol (2.0 g/kg i.p.) or saline injections once a week for 50 weeks. During this treatment period the rats had in addition access to ethanol (10% in drinking fluid) as a choice against water for 24 h prior to the injection. During the following evaluation period, animals had a continuous choice between ethanol and water and the concentration of the ethanol solution increased every 3rd week from 5 to 10, 15 and 25%, with 10% as a reference tested between the other concentrations. The animals were killed after an abstinence of 4 weeks, whereupon the concentrations of noradrenaline (NA), dopamine (DA), serotonin (5-HT) and 5-hydroxyindole acetic acid (5-HIAA) were determined in the frontal cortex. In the remaining cerebral cortex, activity of monoamine oxidase, reuptake of NA and stimulated inositol phospholipid (PI) breakdown was also determined. Muscarinic binding sites were determined in the striatum. During treatment, saline injected rats had a constant voluntary 24 h ethanol intake. There was a decrease in the corresponding intake in the animals given the ethanol injections. The diminishing of the intake was more marked in rats starting treatment at an age of 19.4 weeks when compared to rats starting at an age of 5.4 weeks. In the evaluation period the ethanol intake was fairly constant for all groups. However, the regressions between intake of the reference concentration when plotted against the different tested concentrations were most marked in the group where ethanol injections started at an early age. In the total material there were significant F-values when concentrations of NA, 5-HIAA, 5-HT/5-HIAA in the cortex and muscarinic binding sites in the striatum were tested. Age could not be excluded as a contributing factor, but for muscarinic binding sites in the striatum, concentrations of DA and 5-HIAA in the cortex, and potassium stimulated PI breakdown in the cortex significant regressions with voluntary ethanol intake as dependent variable could be established. Since these intakes are stable, a causal relation with dependence may be involved.

Impaired performance of rats in the Morris swim-maize test late in abstinence following long-term sodium barbital treatment.

Rats were tested for place learning in the Morris swim maze on days 110-114 of abstinence following 48 weeks of treatment with sodium barbital. A retarded acquisition of the swim-maze task, that could not be ascribed to motor impairments, was found in the barbital-treated rats. There was a significant difference in brain weight, but there were no significant differences between the control and barbital-treated rats in the frontal cortical concentrations of noradrenaline (NA), dopamine (DA), 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA), nor in the intra- and extrasynaptosomal activities of cerebral cortical monoamine oxidase towards NA and 5-HT. Postsynaptically, neither the cerebral cortical inositol phospholipid breakdown responses to carbachol and NA (mediated by muscarinic and alpha 1-adrenergic receptors, respectively), nor the striatal and cortical densities of muscarinic receptors labelled by [3H]quinuclidinyl benzilate [( 3H]QNB) were found significantly to be altered in the barbital-treated rats. A strong correlation between the density of striatal and cortical [3H]QNB binding sites was seen for the barbital-treated (r = 0.91) but not for the control (r = -0.05) rats. It is suggested that the deficit in performance of the barbital-treated rats in the Morris maze may be related to a cholinergic dysfunction.

Tissue and microdialysate changes after repeated and permanent probe implantation in the striatum of freely moving rats.

Neurochemical and morphological effects of repeated microdialysis or permanent microdialysis probe implantations in striatum were studied. The extracellular levels of dopamine did not change between a first and a second probe insertion separated by 2 weeks or at a third dialysis session 2 days later. The 3,4-dihydroxyphenylacetic acid and homovanillic acid levels were similar at the first and second microdialysis session, but decreased at the third. Probes implanted permanently for 2 weeks clogged, and the recovery varied markedly after insertion of new probes. Tyrosine hydroxylase-stained dopamine fibers appeared unaffected after all dialysis sessions, although some swollen fibers were observed surrounding the probes. No change in the glial fibrillary acidic protein staining was seen immediately after the first dialysis session, although 2 weeks later gliosis was observed. After the second and third dialysis a diffuse gliosis was observed, while a glial barrier was seen surrounding the permanently implanted probes. Immediately after the first dialysis session enlarged laminin-stained blood vessels were seen, whereas repeated probe implantation also increased the blood vessel density. Thus, chronic in vivo microdialysis with permanently implanted probes is limited by severe technical problems and marked tissue changes. On the other hand, repeated probe insertion in the same brain site appears to be acceptable for performing chronic microdialysis studies in the same subject, provided neurochemical and morphological changes are taken into consideration.