Influence of potassium concentration in microdialysis perfusate on basal and stimulated striatal dopamine release: effect of ceruletide, a cholecystokinin-related peptide

Complex 33-beam simulated galactic cosmic radiation exposure impacts cognitive function and prefrontal cortex neurotransmitter networks in male mice

Astronauts will encounter extended exposure to galactic cosmic radiation (GCR) during deep space exploration, which could impair brain function. Here, we report that in male mice, acute or chronic GCR exposure did not modify reward sensitivity but did adversely affect attentional processes and increased reaction times. Potassium (K+)-stimulation in the prefrontal cortex (PFC) elevated dopamine (DA) but abolished temporal DA responsiveness after acute and chronic GCR exposure. Unlike acute GCR, chronic GCR increased levels of all other neurotransmitters, with differences evident between groups after higher K+-stimulation. Correlational and machine learning analysis showed that acute and chronic GCR exposure differentially reorganized the connection strength and causation of DA and other PFC neurotransmitter networks compared to controls which may explain space radiation-induced neurocognitive deficits.

Monitoring Dopamine Responses to Potassium Ion and Nomifensine by in Vivo Microdialysis with Online Liquid Chromatography at One-Minute Resolution

Recently, our laboratory has demonstrated the technical feasibility of monitoring dopamine at 1 min temporal resolution with microdialysis and online liquid chromatography. Here, we monitor dopamine in the rat striatum during local delivery of high potassium/low sodium or nomifensine in awake-behaving rats. Microdialysis probes were implanted and perfused continuously with or without dexamethasone in the perfusion fluid for 4 days. Dexamethasone is an anti-inflammatory agent that exhibits several positive effects on the apparent health of the brain tissue surrounding microdialysis probes. Dopamine was monitored 1 or 4 days after implantation under basal conditions, during 10 min applications of 60 mM or 100 mM K⁺, and during 15 min applications of 10 μM nomifensine. High K⁺ and nomifensine were delivered locally by adding them to the microdialysis perfusion fluid using a computer-controlled, low-dead-volume six-port valve. Each day/K⁺/dexamethasone combination elicited specific dopamine responses. Dexamethasone treatment increased dopamine levels in basal dialysates (i.e., in the absence of K⁺ or nomifensine). Applications of 60 mM K⁺ evoked distinct responses on days one and four after probe implantation, depending upon the presence or absence of dexamethasone, consistent with dexamethasone's ability to mitigate the traumatic effect of probe implantation. Applications of 100 mM K⁺ evoked dramatic oscillations in dopamine levels that correlated with changes in the field potential at a metal electrode implanted adjacent to the microdialysis probe. This combination of results indicates the role of spreading depolarization in response to 100 mM K⁺. With 1 min temporal resolution, we find that it is possible to characterize the pharmacokinetics of the response to the local delivery of nomifensine. Overall, the findings reported here confirm the benefits arising from the ability to monitor dopamine via microdialysis at high sensitivity and at high temporal resolution.

Repeated perfusion with elevated potassium in in vivo microdialysis--A method for detecting small changes in extracellular dopamine

As a great deal of variability between subjects is often seen when using the microdialysis technique to measure the effects of depolarizing agents on extracellular neurotransmitter levels, we have developed a technique to account for the variability inherent in this method. High potassium (50 or 100 mM) artificial cerebrospinal fluid (ACSF), perfused through the probe for 10 min, significantly increased extracellular dopamine (DA) concentration during both an initial and second perfusion, and the two responses were highly correlated. However, extracellular DA returned to normal following the first perfusion with 50 mM K+ but not 100 mM K+ perfusion. The slope of the regression line obtained by plotting the response of the second K+ perfusion as a function of the first K+ perfusion for all K+ concentrations was 1.03 (not significantly different from unity). Similarly, when the time between two 50 mM potassium perfusions was varied from 30-150 min, the responses were highly correlated. This technique was used to demonstrate an interaction between N-methyl-D-aspartate (NMDA) and 50 mM K+. Perfusion of 0.1 mM NMDA alone had no effect on extracellular DA, but NMDA paired with a 50 mM K+ perfusion significantly increased extracellular DA over that increase by 50 mM K+ alone. We propose that a first stimulation with 50 mM potassium may characterize an individual animal's responsiveness to a depolarizing stimulus, and may be used as a control for testing drug effects by coupling drug treatments with a second 50 mM potassium stimulation to give a more accurate measure of small changes in extracellular dopamine.

A novel benzodiazepine inverse agonist, S-8510, as a cognitive enhancer

1. Pharmacological actions of a novel benzodiazepine receptor ligand, S-8510 (2-(3-isoxazolyl)-3,6,7,9-tetrahydroimidazo[4,5-d]pyrano+ ++[4,3-b] pyridine monophosphate monohydrate), were examined in in vitro and in vivo studies. 2. S-8510 was characterized as a partial inverse agonist with a modest GABA ratio and low efficacy. 3. S-8510 ameliorated memory impairment induced by cholinergic deficit in the water maze paradigm of Wistar rats. 4. S-8510 augmented LTP of the Schaffer collateral/commissural fiber-CA1 synapses in the hippocampal slice preparations of SD rat. 5. S-8510 increased the extracellular levels of acetylcholine and noradrenaline in the hippocampus of Wistar rat. 6. S-8510 selectively potentiated pentylenetetrazol-induced convulsion without affecting minimal electroconvulsive shock- or strychnine-induced convulsion in ddY mice. 7. S-8510 failed to induce any sign of anxiety in the Wistar rat pro-conflict test. 8. S-8510 showed antidepressant-like pharmacological actions in ddY mice. 9. These results suggest that S-8510 can be used as a therapeutic drug for senile dementia, including Alzheimer's disease with little risk for inducing anxiety or convulsion.

Dopamine release in the nucleus accumbens during sexual behavior in prenatally stressed adult male rats

In vivo microdialysis experiments were performed on the nucleus accumbens (NAc) during observation of sexual behavior (including motivation and copulation) to determine if there were any changes in NAc dopamine (DA) transmission in prenatally stressed (PS) adult male rats. Approximate 37% of control males and 83% of PS males did not exhibit copulation during the sexual behavior tests and no significant changes in NAc DA release were seen during exposure to estrous females. In contrast, both control and PS males that displayed copulatory behavior showed a marked increase in NAc DA release when presented with a sexually receptive female behind a screen and this increased further during actual copulation. The increase in DA release in copulatory PS males was not significantly different from that in sexually active control males. In addition, a similar extent in DA release induced by high potassium perfusate was observed in all rats. These results suggest that prenatal stress may result in a deficit in DA neurotransmission in the NAc and this deficit may possibly cause impaired male sexual behavior in rats.

Cholecystokinin [corrected] octapeptide modulates dopamine release in rat striatum

The effect of cholecystokinin octapeptide sulfated (CCK8S) on the basal and electrically evoked release of [3H]dopamine ([3H]DA) in striatal slices from the rat brain was studied. Cholecystokinin octapeptide did not influence the basal release of [3H]DA. Field electrical stimulation (FES) (2 Hz) induced an increase of dopamine release from striatal slices, which was Ca2+ dependent and was abolished by tetrodotoxin, 10(-6) M. Cholecystokinin octapeptide (10(-9) M, 10(-8) M and 10(-7) M) dose dependently reduced the electrically evoked release of [3H]DA. This effect was antagonized by the CCK-A receptor antagonists loxiglumide (10(-7) M, 10(-6) M and 10(-5) M) or proglumide (10(-5) M, 10(-4) M and 10(-3) M). The results suggest that CCK receptors type A are involved in this effect of CCK8S in the striatum.

Effects of administration of cholecystokinin into the VTA on DA overflow in nucleus accumbens and amygdala of freely moving rats

The carboxyterminal octapeptide of cholecystokinin (CCK-8) coexists with dopamine (DA) in mesolimbic neurons of the ventral tegmental area (VTA). In the present study, in vivo microdialysis in freely moving rats was used to assess the relative effects of sulfated CCK-8 (CCK-8S), unsulfated CCK-8 (CCK-8US) and CCK tetrapeptide (CCK-4), focally injected into the VTA, on DA overflow in two mesolimbic DA/CCK-8S terminal regions, the nucleus accumbens and the amygdala. Consistent with electrophysiological findings, microinjection of CCK-8S, but not CCK-8US or CCK-4, elicited increases in DA overflow in both terminal regions. In the absence of anatomical evidence of CCK-containing fibers in the VTA region, it seems reasonable to conclude that the modulation of terminal DA overflow by CCK-8S through actions at the somatodendritic region represents a form of autoregulation of these cells. Whereas CCK-8US and CCK-4 are preferential CCK-B receptor agonists, CCK-8S binds non-selectively to CCK-A and CCK-B receptors. Thus, these results implicate CCK-A receptors in the stimulatory effects of CCK-8S on VTA DA neurons.