Ulf Tossman

Purine levels in the intact rat brain. Studies with an implanted perfused hollow fibre.

A thin dialysis tube was implanted stereotaxically under halothane anesthesia in the caudate nucleus of Sprague-Dawley rats and perfused with Ringer solution at a rate of 2 microliters/min. Initially there was a high rate of purine outflow but after 1-2 h of perfusion the rate was essentially constant (anesthetized - adenosine 0.4 +/- 0.04 microM, inosine 0.8 +/- 0.2 microM; non-anesthetized - adenosine 0.33 +/- 0.03 microM, inosine 0.21 +/- 0.07 microM). Hypoxia (9% O2) increased the levels more than 3-fold. The adenosine deaminase inhibitor erythro-2-(2-hydroxy-3-nonyl)adenine (EHNA) increased the adenosine level and decreased the inosine level. In vitro recovery of adenosine was about 30%. Therefore, we conclude that the free exchangable concentration of adenosine in the rat brain is likely to be 102 micro M. This level is high enough to potentially affect central nervous function.

Dopamine Synaptic Mechanisms Reflected in Studies Combining Behavioural Recordings and Brain Dialysis

By using a new technique of brain dialysis, it has been possible to recover endogenously released dopamine and aminoacids in awake as well as anaesthetized rats. By combining dialysis experiments with behavioural recordings we have studied the relationship between neurotransmission and behaviour. The results indicate that changes in receptor sensitivity may take place in direct response to changes in transmitter release. Studies with rotational behaviour strongly suggest the existence of dopamine receptor sites preferentially stimulated by apomorphine or pergolide and preferentially inhibited by cis-flupenthixol and sulpiride respectively. These receptor sites seem to relate differently to cholinergic and GABA-ergic mechanisms. Finally we describe the surprising finding that methylxanthines induce rotational behaviour in a way very similar to known dopamine agonists.

Tissue levels and in vivo release of tachykinins and GABA in striatum and substantia nigra of rat brain after unilateral striatal dopamine denervation.

SummaryBrain tissue levels and in vivo release of substance P (SP) and neurokinin A (NKA) and GABA were measured bilaterally in striatum and substantia nigra of the rat, after a unilateral 6-hydroxydopamine lesion of the nigro-striatal dopamine pathway. Sham injected animals served as controls. The dopamine denervation decreased the tissue levels of SP in striatum (-38%) ipsilateral to the lesion and in substantia nigra both ipsi- (-54%) and contralateral (-38%) to the lesion. NKA was not significantly changed in the striatum, but decreased (like SP) in the substantia nigra both ipsi- (-50%) and contralateral (-40%) to the lesion. GABA tissue levels increased in the denervated striatum (+20%) and remained unchanged in substantia nigra at both sides. The extracellular levels of SP, NKA and GABA were measured with microdialysis in vivo at basal conditions and during stimulation with potassium administered locally via the microdialysis probe. The stimulated release of SP and NKA in the substantia nigra ipsilateral to the lesion was compared to in sham operated animals reduced with 39% and 64%, respectively, while no change in SP or NKA release was detected in the striatum. The basal release of GABA in the striatum was increased with 296% and with 76% during stimulation in the dopamine denervated striatum, while no change in GABA basal or stimulated release was detected in the substantia nigra. We suggest that the increased GABA release in the dopamine denervated striatum may be due to a decreased dopamine mediated inhibition of local GABA neurons. Furthermore, the decreased nigral release of SP and NKA ipsilateral to the lesion is suggested to be caused by an increased GABA inhibition in striatum of SP- and NKA-containing striato-nigral neurons.

Brain Amino Acids Measured by Intracerebral Dialysis in Portacaval Shunted Rats

Abstract: Changes in brain amino acid uptake and metabolism have been proposed as a possible etiological factor in hepatic encephalopathy. By use of a brain dialysis technique (a thin tube implanted in the brain of the living animal), the extracellular amino acid concentrations in the striatum of portacaval (PC)‐shunted and shamoperated rats were measured. Leucine, phenylalanine, methionine, and glutamine were increased two‐ to sixfold in the PC‐shunted rats, whilst no changes were seen for GABA, valine, glutamate, or isoleucine, confirming previous reports. Aspartate levels were 350% higher in the PC‐shunted rats, and this rise, as well as that of phenylalanine, was significantly correlated with the lower motor activity observed in the PC‐shunted rats, suggesting a possible importance of these amino acids in the etiology of hepatic encephalopathy. The amino acid concentrations measured in whole blood demonstrated the well‐known pattern of low levels of branched‐chain amino acids and increased concentrations of phenylalanine, glutamine, and histidine.

Brain cortical amino acids measured by intracerebral dialysis in portacaval shunted rats

The extracellular amino acid content was measured in the parietal cortex in portacaval and sham operated rats, using the brain dialysis technique. The amino acid content of the perfusate was determined for 10 min before and during stimulation with potassium chloride. Basal levels of aspartate, glutamine, glycine, methionie, valine, phenylalanine and leucine were 2-to 6-fold higher in the PC-shunted as compared to the sham operated rats. For glutamate, taurine, and GABA no differences were observed between the two groups. After KCl stimulation the release of glutamate and GABA increased significantly in both groups. For GABA this rise was approximately twice as high in the PC-shunted rats (+300%,P<0.01) as in the sham operated rats (+150%,P<0.01 as compared to basal). In the sham operated, but not in the PC-shunted rats, methionine and valine levels rose significantly (+200%,P<0.05) and glutamine release decreased (−50%,P<0.05). These findings suggest that the brain metabolism of amino acids is altered after a portacaval shunt. This could in turn alter the neurotransmission and partly explain the low spontaneous motor activity seen in these animals.

γ-aminobutyric acid and taurine release in the striatum of the rat during hypoglycemic coma, studied by microdialysis

Extracellular levels of striatal gamma-aminobutyric acid (GABA) and taurine were monitored during insulin-induced hypoglycemia using microdialysis. At the onset of isoelectricity in the electroencephalogram (EEG), a transient 5-fold increase in the levels of GABA occurred. Taurine levels increased 5 min following the onset of isoelectricity and continued to increase during the entire isoelectric period. The results demonstrate that events associated with the onset of isoelectricity during hypoglycemia trigger an increase in extracellular concentrations of GABA and taurine. The discrepancy in time-course of these changes may reflect differences in compartmentation, function and metabolism of the two amino acids.

γ-Aminobutyric acid release in the globus pallidus in vivo after a 6-hydroxydopamine lesion in the substantia nigra of the rat

This experiment was designed to determine whether the release of gamma-aminobutyric acid (GABA) in the globus pallidus (GP) is affected by a lesion of the nigrostriatal pathway. Rats were lesioned with 6-hydroxydopamine 4 weeks prior to study of the in vivo release of GABA. A microdialysis probe was stereotaxically implanted in halothane-anesthetized animals in a vertical position on both GPs. The perfusates were analyzed for their GABA content with a high performance liquid chromatography technique. Compared to unlesioned controls, a marked decrease in the overflow of GABA was observed in the GP contralateral to the lesion, whereas the ipsilateral GP showed a slight increase. The differences between the sides were exaggerated after KCl (100 mmol) administration. The results are discussed in terms of a possible bilateral influence of dopamine terminals in the striatum on GABA transmission.

The effect of apomorphine and pergolide on the potassium-evoked overflow of GABA in rat striatum studied by microdialysis

The extracellular GABA concentration in the rat striatum was measured by the microdialysis technique. Addition of pergolide (10(-4) M) to the perfusion medium decreased the GABA overflow induced by high K+ but apomorphine (10(-4) M) had no effect on the GABA overflow. When sulpiride (10(-4) M) was added to the perfusion medium, the pergolide effect on stimulated GABA overflow was abolished. The results indicate that D2-receptors are able to modulate GABA overflow in the striatum.

The effect of an uptake inhibitor (dihydrokainate) on endogenous excitatory amino acids in the lamprey spinal cord as revealed by microdialysis.

Microdialysis was utilized to test the effects of the uptake inhibitor dihydrokainate (DHK) on endogenous amino acid levels in the lamprey spinal cord in vitro. The level of L-glutamate increased markedly (165%) in the presence of DHK, whereas the level of the glutamate precursor L-glutamine decreased (53%). Since DHK can potentiate or evoke fictive locomotion in the lamprey spinal cord, it is suggested that L-glutamate is released by neurons which take part in the activation of the spinal locomotor network.

Behavioural and Biochemical Studies With the Benzamide Sulpiride in Rats

Abstract We have studied the effects of the benzamide, sulpiride, on dopamine transmission using a functional approach combining brain dialysis experiments with behavioural studies utilising rotational and holeboard models.