Potassium-evoked Overflow Research Articles

Neurturin Effects on Nigrostriatal Dopamine Release and Content: Comparison with GDNF

Neurturin (NTN) is a member of the glial cell line-derived neurotrophic factor (GDNF) family; and, while GDNF has been shown to increase dopamine (DA) release in normal animals, the ability of NTN to alter DA release has not been previously reported. The purpose of the present study was to determine if NTN could alter striatal DA release, and to compare the effects of NTN to GDNF. Male Fischer-344 rats were given a single injection of vehicle or 5 microg NTN or GDNF into the right substantia nigra. Three weeks later microdialysis experiments were conducted to assess striatal DA release. Basal extracellular levels of striatal DA were not affected by either NTN or GDNF. However, both NTN and GDNF led to increases in amphetamine-evoked overflow of DA from the ipsilateral striatum, and there was a trend for potassium-evoked overflow to be augmented. Postmortem tissue levels of DA were decreased by approximately 20% in the striatum, and increased by approximately 100% in the substantia nigra, on the ipsilateral side of the brain compared to the contralateral side following both NTN and GDNF injection. Thus, NTN, like GDNF, can augment striatal DA release, and the magnitude of the NTN effects are similar to those of GDNF.

HIV-1 protein Tat potentiation of methamphetamine-induced decreases in evoked overflow of dopamine in the striatum of the rat

HIV-1 infection of the brain can lead to the development of clinical syndromes reminiscent of Parkinson’s disease, suggesting that HIV infection may damage nigrostriatal dopamine (DA) neurons. Although the responsible mechanisms have not been well defined, neurotoxic viral proteins, such as Tat, released from infected cells may be involved. Drug abuse is a major risk factor for contracting HIV infection. Methamphetamine (METH), a psychostimulant with high abuse potential, may also be toxic to brain DA neurons. Thus, the combination of METH abuse and HIV infection may lead to substantial alterations in DA neuron functioning. The present experiments examined how Tat, alone and with METH, affects DA release in the striatum. Male rats were given an intrastriatal injection of Tat (25 μg) or vehicle 24 h before treatment with saline or neurotoxic doses of METH. Seven days later microdialysis studies were carried out to measure potassium- and amphetamine-evoked overflow of DA from the striatum. The Tat treatment alone led to no change in potassium-evoked overflow of DA, a 20% decrease in amphetamine-evoked overflow of DA, and a 16% decrease in striatal DA content. The METH alone led to a 37–42% decrease in striatal DA overflow and content. The combined treatment with Tat and METH led to significantly greater 70–78% decreases in striatal DA overflow and content. These results indicate that Tat enhances METH-induced reductions in striatal DA release and content, possibly in a synergistic manner, and suggest that METH abusers infected with HIV may be at increased risk for basal ganglia dysfunction.

Enhanced effects of 6-hydroxydopamine on evoked overflow of striatal dopamine in aged rats

Nigrostriatal dopamine neurons degenerate during aging, and the excessive loss of dopamine neurons that occurs with Parkinson’s disease is usually confined to older individuals. Although 6-hydroxydopamine lesioning of the nigrostriatal dopamine system is a common method for producing animal models of dopamine neuron degeneration, there have been relatively few studies that have examined the effects of 6-hydroxydopamine on the dopamine systems of aged animals. The present experiments were designed to determine if nigrostriatal dopamine neurons in aged rats are more sensitive to the neurotoxic effects of 6-hydroxydopamine than those of younger rats. Young (4-month-old), middle-aged (14-month-old) and aged (24-month-old) Fischer-344 rats were given a single injection of vehicle, 50 or 100 μg 6-hydroxydopamine into the right lateral ventricle. Three to four weeks later in vivo electrochemistry was used to measure potassium-evoked overflow of dopamine in the striatum. In the young rats the 50-μg dose had no significant effect on evoked overflow of dopamine in the striatum or on post-mortem levels of dopamine in the striatum or substantia nigra. The higher dose in the young animals diminished evoked overflow of dopamine as well as tissue levels of dopamine. In the aged rats both doses of 6-hydroxydopamine led to significant decreases in evoked overflow of striatal dopamine and in tissue levels of dopamine in the striatum and substantia nigra. These results suggest that dopamine neurons of aged Fischer-344 rats are more susceptible to the toxic effects of 6-hydroxydopamine than those of younger animals.

In vivo microdialysis studies of age-related alterations in potassium-evoked overflow of dopamine in the dorsal striatum of Fischer 344 rats

Intracerebral microdialysis was used to measure basal levels and potassium (K+)-stimulated overflow of dopamine (DA), homovanillic acid (HVA) and dihydroxyphenylacetic acid (DOPAC), in the dorsal striatum of young (6 months) and aged (24 months) Fischer 344 (F344) rats. Basal levels of HVA were lower in aged rats whereas basal DA and DOPAC did not differ significantly between the two groups. The administration of three low to moderate doses of K+ (10, 25, and 50 mM) through the microdialysis probe for one collection period revealed differences between the two age groups of F344 rats. DA overflow increased in a dose-dependent manner in the young but not aged rats. Extracellular levels of DOPAC and HVA decreased during the K+ stimulation and there was a significant difference in the changes in HVA produced by K+ stimulation in the young vs aged animals. These data support the hypothesis that low to moderate doses of K+ may be necessary to demonstrate age-related differences in K+-evoked DA overflow, since previous microdialysis studies using higher doses have not reported age-related differences in DA overflow.

GDNF Protection against 6-OHDA–Induced Reductions in Potassium-Evoked Overflow of Striatal Dopamine

Glial cell line-derived neurotrophic factor (GDNF), when administered before 6-hydroxydopamine (6-OHDA), has been shown to prevent the reduction in nigral dopamine (DA) levels and tyrosine hydroxylase-positive neurons normally observed after 6-OHDA lesions. The present study examined the ability of GDNF to prevent 6-OHDA-induced reductions in striatal DA release and reductions in striatal and nigral DA levels. GDNF (10 micrograms), or vehicle, was injected into the right nigra of anesthetized male Fischer-344 rats and was followed 6 hr later by intranigral 6-OHDA or saline. Three to four weeks later the animals were anesthetized with urethane and prepared for in vivo electrochemistry. Potassium-evoked overflow of DA was dramatically decreased in the right striatum of the vehicle + 6-OHDA-treated animals. GDNF appeared to prevent the reduction in evoked overflow of DA in the right striatum of the 6-OHDA-treated animals. However, in comparison with that in animals that received GDNF + saline, the overflow of DA was significantly reduced in the GDNF + 6-OHDA animals. Similarly, although nigral levels of DA were above normal in the GDNF + 6-OHDA-treated animals, they were below DA levels found in GDNF + saline-treated rats. Striatal DA levels were partially protected by GDNF. In animals examined 10-12 weeks after the GDNF and 6-OHDA treatments, the apparent protective ability of GDNF on the evoked overflow of DA in the striatum was diminished. Thus, although intranigral GDNF can prevent 6-OHDA-induced reductions in nigral DA levels, long-term protection of the evoked overflow of DA in the striatum is minimal.

Collateral Sprouting of Central Noradrenergic Neurons during Aging: Histochemical and Neurochemical Studies in Intraocular Triple Transplants

The sprouting capacity of aged noradrenergic neurons of the brain-stem nucleus locus coeruleus (LC) was examined using intraocular transplants of fetal tissues. Fetal hippocampal tissue (E18) and LC tissue (E15) were transplanted together as a double transplant into the anterior chamber of the eye of young adult Fischer 344 rats. The double transplants were allowed to mature for 14–18 months, after which an additional fetal hippocampal transplant was placed next to the LC graft. The triple transplants were monitored for overall growth and vascularization for an additional 2–6 months. Immunohistochemical examinations showed that both young (2–6 months old) and aged (16–24 months old) hippocampal cografts contained a plexus of thin varicose tyrosine hydroxylase (TH)-immunoreactive fibers extending throughout the grafted hippocampal tissues. However, the aged hippocampal grafts contained a denser uniform plexus of TH-positive fibers compared to the young transplants. Immunohistochemistry with synapsin antibodies demonstrated that both the young and the aged hippocampal transplants contained much higher densities of synaptic elements than the LC grafts.In vivoelectrochemical measurements of potassium-evoked overflow of norepinephrine (NE) in the grafts showed that similar amounts of NE overflow were detected in both the young and the aged hippocampal grafts. HPLC–EC measurements of NE levels in the grafts revealed that there were similar amounts of NE in the young and the aged grafts, and the grafts did not contain serotonin or dopamine. In summary, the findings of the present study show that aged LC neurons are capable of undergoing collateral sprouting producing a functional NE neuronal system when introduced to an appropriate young target.

GDNF Selectively Protects Dopamine Neurons over Serotonin Neurons Against the Neurotoxic Effects of Methamphetamine

Repeated methamphetamine (METH) administration to animals can result in long-lasting decreases in striatal dopamine (DA) and serotonin (5-HT) levels. Glial cell line-derived neurotrophic factor (GDNF) has pronounced effects on dopaminergic systems in vivo, including partial neuroprotective effects against 6-hydroxydopamine and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine -induced lesions. The present study examined the ability of GDNF to prevent METH-induced reductions in potassium-evoked overflow of DA, and DA and 5-HT content, in striatum. GDNF (10 microg) or vehicle was injected into the right striatum of anesthetized rats. Twenty-four hours later, the rats were injected four times at 2 hr intervals with METH (5 mg/kg, s.c.) or saline. One week later, in vivo electrochemistry was used to monitor the overflow of DA evoked by local potassium application. Evoked overflow of DA was dramatically decreased in the striatum of METH-treated animals. GDNF prevented the reduction in evoked overflow of DA in the right striatum of the METH-treated animals. After each experiment, the animals were killed, and striatal DA and 5-HT levels determined by HPLC. The METH treatment produced significant decreases in both neurotransmitters. GDNF administration prevented the reduction in striatal DA levels on the treated side of the brain, whereas levels on the contralateral side were still decreased. In dose-response studies, 1 microg of GDNF was as protective as 10 microg, whereas 0.1 microg was only partially protective. In contrast, 5-HT levels were only minimally protected by previous administration of GDNF. These results suggest that GDNF can selectively protect DA neurons, compared with 5-HT neurons, against the neurotoxic effects of METH.

Age-related changes in potassium-evoked overflow of dopamine in the striatum of the rhesus monkey

Rapid (5Hz) chronoamperometric recordings using Nafion-coated carbon fiber electrodes (30–90 microns o.d.) combined with pressure-ejection of potassium from micropipettes were used to investigate potassium-evoked overflow of dopamine (DA) in the striatum of young (5 to 10 years old) and middle-aged (19 to 23 years old) anesthetized rhesus monkeys. The potassium-evoked DA-like signals from the 19- to 23-year-old animals were significantly lower in amplitude than those recorded in the young animals. In addition, the temporal dynamics of DA signals in the caudate nucleus of middle-aged animals were faster, while the time courses of the signals recorded in the putamen of middle-aged monkeys were significantly longer as compared to the signals recorded from young animals. Moreover, home cage activity levels of the middle-aged animals were significantly lower. Taken together, these data support age-related changes in the output of DA from DA fibers in the striatum of middle-aged monkeys.

Potentiation of potassium-evoked noradrenaline and neuropeptide Y co-release by cardiac energy depletion: role of calcium channels and sodium-proton exchange.

The role of the cardiac energy status in the potassium-evoked exocytosis of both noradrenaline and the sympathetic co-transmitter neuropeptide Y (NPY) was investigated in the guinea-pig perfused heart. The transmitter release was stimulated by potassium depolarization (10-80 mmol/l) during normoxic perfusion (pO2 > 100 mmHg) in the presence of glucose (11 mmol/l) and at various periods (5-40 min) of cardiac energy depletion. Energy depletion was induced either by anoxia (pO2 < 5 mmHg) or by cyanide intoxication (1 mmol/l), both in combination with glucose-free perfusion. Endogenous noradrenaline and NPY were determined in the coronary venous overflow by high-pressure liquid chromatography combined with electrochemical detection and by radioimmunoassay, respectively. Under normoxic conditions potassium depolarization evoked a co-release of both transmitters [molar ratio 862 (noradrenaline):1 (NPY)] at a threshold concentration of 40 mmol/l potassium. This transmitter overflow was characterized by its dependence on extracellular calcium and calcium influx through voltage-dependent neuronal calcium channels of the N-type. Cardiac energy depletion was accompanied by an acceleration and an enhancement of the potassium-evoked transmitter overflow. In comparison to normoxia, a 10-fold increased transmitter overflow with a comparable molar ratio [709 (noradrenaline:1 (NPY)] was evoked by 40 mmol/l potassium after 10 min of either anoxia or cyanide intoxication. This sensitization to potassium depolarization reached a peak after 10 min of energy depletion and was characterized by a markedly reduced threshold concentration (10 mmol/l potassium).(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of quinpirole and fenoldopam on the potassium-evoked overflow of endogenous dopamine and noradrenaline in dog mesenteric arteries

(1) Dopamine and noradrenaline overflow from the main trunk of the dog mesenteric artery and its proximal branches, elicited by K+ (52 mmol/l), was measured by high pressure liquid chromatography with electrochemical detection. (2) Quinpirole (0.1, 1 and 10 nmol/l) produced a concentration dependent reduction of dopamine and noradrenaline overflow in both segments of the mesenteric artery. The inhibitory effect of quinpirole (10 nmol/l) on amine overflow was antagonized by sulpiride (1 mumol/l) but not by phentolamine (0.2 mumol/l) or the selective dopamine (DA1), antagonist SK&F 83566 (1 mumol/l). (3) Fenoldopam (0.1 and 1 mumol/l) did not alter dopamine and noradrenaline overflow from both segments of the mesenteric artery; only 10 mumol/l fenoldopam was found to increase the overflow of dopamine and noradrenaline in both segments of the mesenteric artery. This effect of fenoldopam on amine overflow was not altered by the addition to the perifusion fluid of SK&F 83566 (1 mumol/l). (4) Clonidine (100 nmol/l) significantly reduced amine overflow from both segments of the mesenteric artery and this effect was antagonized by fenoldopam (10 mumol/l). (5) These results suggest that quinpirole inhibits sympathetic neurotransmission through the activation of prejunctional dopamine receptors of the DA2 subtype. The facilitatory effect of fenoldopam (10 mumol/l) on amine release appears to be mediated through the blockade of prejunctional alpha 2-adrenoceptors.

The effects of cholecystokinin (CCK-8) on dopamine-containing nerve terminals in the caudate nucleus and nucleus accumbens of the anesthetized rat: an in vivo electrochemical study

The action of cholecystokinin (CCK) on presynaptic function of dopaminergic nerve terminals has been the subject of much debate in the literature. In efforts to resolve some of the reported ambiguities, high speed in vivo electrochemical recordings were carried out in the caudate nucleus and nucleus accumbens of the urethane anesthetized rat, to determine effects of locally applied sulfated (CCK-8S) and unsulfated (CCK-8US) CCK octapeptide. Locally-applied CCK-8S and CCK-8US caused no increase in the baseline electrochemical signals recorded from either brain region. However, locally applied CCK-8S potentiated the potassium-evoked overflow of dopamine (DA) into the extracellular space in both the caudate and nucleus accumbens. In contrast, pressure ejection of CCK-8US produced no significant effects on the potassium-evoked overflow of DA in either structure. These data support a facilitatory effect of CCK-8S on potassium-evoked overflow from DA-containing nerve terminals in the urethane anesthetized rat that is likely mediated through a peripheral type CCK receptor.

Extracellular levels of amino acids in striatum and globus pallidus of 6-hydroxydopamine-lesioned rats measured with microdialysis.

Extracellular aspartate, glutamate, glutamine, taurine and GABA concentrations were measured by microdialysis in the rat striatum and globus pallidus after a unilateral 6-hydroxydopamine lesion of the dopamine system. The basal and potassium-evoked overflow of GABA was increased in the ipsilateral striatum, but the evoked overflow was decreased in both contralateral striatum and pallidum. Both basal and evoked overflow of glutamate was increased in ipsilateral striatum. The basal overflow of aspartate was significantly increased in the ipsilateral side. Basal glutamine on the other hand was decreased in the ipsilateral side. Taurine remained unchanged in both regions. These results suggest that dopamine is involved in the regulation of transmission by GABA and glutamate. Since glutamine might be the precursor to glutamate, the change in glutamate might affect the glutamine level. The changed aspartate level has no obvious explanation.

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 sulpride (10 −4 M) was added to the perfusion medium, the pergolide effect on stimulated GABA overflow was abolished. The results indicate that D 2-receptors are able to modulate GABA overflow in the striatum.

Gamma-Aminobutyric acid and postganglionic sympathetic transmission in the pulmonary artery of the rabbit.

1 The effect of gamma-aminobutyric acid (GABA) on postganglionic sympathetic neurotransmission was studied in strips of the rabbit pulmonary artery. The strips were preincubated with 3H-noradrenaline and then superfused with 3H-amine-free medium. They were stimulated either electrically at 2 Hz, or by 60 mM potassium, or by 1 microM tyramine. 2 GABA (1 - 1000 microM) did not change the basal outflow of tritium, but decreased the electrically evoked overflow as well as the contractile response. GABA 1 microM decreased the evoked overflow by 12%, and GABA 1000 microM, by 42%. The effect of GABA was not changed by yohimbine, propranolol, cocaine, corticosterone, or indomethacin. It was not antagonized by picrotoxin or bicuculline methiodide. GABA 100 microM also slightly reduced the potassium-evoked overflow of tritium but did not change the tyramine-evoked overflow. 3 The results show that, in the pulmonary artery of the rabbit, GABA inhibits the release of noradrenaline. Its effect is independent of alpha- and beta-adrenoreceptors and is not mediated by prostaglandins. The effect may be due to activation of presynaptic receptors which appear to differ from conventional GABA receptors inasmuch as they are insensitive to blockade by either picrotoxin or bicuculline.

Modulation by endogenous dopamine of the release of acetylcholine in the caudate nucleus of the rabbit.

Slices of the caudate nucleus of rabbits were preincubated with 3H-choline and then superfused. Stimulation by electrical pulses at 3 Hz or by 25 mmol/l potassium elicited an increase in tritium outflow which was calcium-dependent and, in the case of electrical stimulation, tetrodotoxin-sensitive. The dopamine receptor agonist apomorphine (0.01-1 mumol/l) decreased, whereas the antagonist haloperidol increased the electrically evoked overflow of tritium. Nomifensine and cocaine, used at concentrations known to inhibit the re-uptake of dopamine, also reduced the evoked overflow of tritium, and this reduction was antagonized by haloperidol. Combined pretreatment with reserpine and alpha-methyltyrosine methylester (alpha-MT), which lowered dopamine levels by 99.5%, increased the electrically evoked overflow, as did bretylium which is shown here to block action potential-induced release of dopamine. The facilitation by haloperidol and bretylium as well as the inhibition by nomifensine and cocaine were diminished or abolished after pretreatment with reserpine plus alpha-MT. Apomorphine decreased, and haloperidol increased, the potassium-evoked overflow of tritium; the effects were not changed by tetrodotoxin. The results indicate that the striatal dopamine receptors which, when activated, depress the release of acetylcholine, are akin to the D-2 type. Endogenous dopamine also acts on the receptors as shown by several manipulations with known effects on dopaminergic transmission. A large fraction of these dopamine receptors may be located on the cholinergic axon terminals.

Presynaptic receptor systems on the noradrenergic neurones of rat brain.

Occurrence and properties of receptors modulating release of noradrenaline were studied in slices of rat occipital cortex and, less extensively, of rat hypothalamus and cerebellar cortex. The slices were preincubated with 3H-noradrenaline and then superfused and stimulated either electrically at 1–3 Hz or by 15–20 mM potassium. (1) Omission of calcium abolished the electrically and potassium-evoked overflow of tritium. Tetrodotoxin blocked the electrically evoked overflow and reduced the response to potassium. The electrically evoked overflow of total tritium consisted of 80% 3H-noradrenaline and 19% 3H-3,4-dihydroxyphenylglycol (DOPEG). Cocaine strongly decreased the electrically evoked overflow of 3H-DOPEG. The overflow of tritium evoked by 15 mM potassium also contained mainly 3H-noradrenaline with only a minor part of 3H-DOPEG. (2) Both the electrically and the potassium-evoked overflow of tritium were reduced by unlabelled noradrenaline (in the presence of cocaine) and tramazoline, and increased by phentolamine and yohimbine. The changes in the evoked overflow of total tritium reflected approximately proportionate changes of 3H-noradrenaline and 3H-DOPEG. (3) Both the electrically and the potassium-evoked overflow of tritium were reduced by morphine and methionine-enkephalin. 3H-noradrenaline and 3H-DOPEG were proportionately decreased. Dextrorphan, in contrast to its analgesically active enantiomer levorphanol, had no effect. Inhibition by morphine persisted after pretreatment with indometacin. (4) Prostaglandin E1 reduced both the electrically and the potassium-evoked overflow of tritium. 3H-noradrenaline and 3H-DOPEG were proportionately decreased. (5) Phentolamine antagonized the inhibitory effect of noradrenaline, but not that of morphine, enkephalin, and prostaglandin E1. Naloxone antagonized the effect of enkephalin, but not that of tramazoline and prostaglandin E1. (6) Acetylcholine (up to 10−5 M) and oxotremorine (up to 10−4 M) failed to reduce the electrically and potassium-evoked overflow. Moreover, atropine (up to 3×10−6 M) had no effect. Up to 10−3 M acetylcholine (in the presence of atropine) as well as up to 10−4 M nicotine did not change the basal outflow of tritium in the absence of releasing stimuli. 10−3 M nicotine caused an increase which was not changed by hexamethonium or omission of calcium. In contrast to the overflow evoked by electrical stimulation or high potassium, the overflow evoked by nicotine mainly consisted of 3H-DOPEG with only a minor part attributable to 3H-noradrenaline. (7) γ-Aminobutyric acid slightly enhanced both the basal and (maximally by 27%) the electrically evoked overflow. Histamine (in the presence of cocaine) slightly decreased the electrically evoked overflow (maximally by 17%). (8) The following drugs did not change the electrically evoked overflow of tritium: dopamine (in the presence of cocaine), isoprenaline (up to 10−6 M), propranolol (up to 10−6 M), serotonin (in the presence of cocaine), angiotensin I, angiotensin II, saralasin, and substance P. (9) It is concluded that transmitter release from the noradrenergic neurones of several brain areas is modulated by drugs acting on α-adrenoceptors, opiate receptors, and prostaglandin receptors. It seems likely, though by no means certain, that the receptors involved are located on the noradrenergic nerve endings themselves, i.e., that they are presynaptic receptors. No evidence was obtained for presynaptic dopamine, β-, muscarine, nicotine, and angiotensin receptors. This pattern differs from that found in other tissues, thus substantiating marked tissue differences in presynaptic receptor systems.

Enkephalin: A potential modulator of noradrenaline release in rat brain

In slices of rat occipital cortex preincubated with 3H-noradrenaline, methionine-enkephalin diminished the overflow of tritium evoked by electrical field stimulation or by 20 mM potassium. The effect was not modified by phentolamine, but was antagonized by naloxone. Given alone, naloxone slightly increased the potassium-evoked overflow. It is concluded that enkephalin reduces transmitter release from cerebrocortical noradrenergic nerve endings. One function of endogenous opiate receptor ligands may be presnaptic inhibition of central noradrenergic transmission.