Insertion Of Microdialysis Probe Research Articles

Extended (10-Day) Real-Time Monitoring by Dexamethasone-Enhanced Microdialysis in the Injured Rat Cortex.

Intracerebral microdialysis has proven useful for chemical monitoring in patients following traumatic brain injury. Recent studies in animals, however, have documented that insertion of microdialysis probes into brain tissues initiates a foreign-body response. Within a few days after probe insertion, the foreign body response impedes the use of microdialysis to monitor the K+ and glucose transients associated with spreading depolarization, a potential mechanism for secondary brain injury. Herein, we show that perfusing microdialysis probes with dexamethasone, a potent anti-inflammatory glucocorticoid, suppresses the foreign body response and facilitates the monitoring of spontaneous spreading depolarizations for at least 10 days following controlled cortical injury in the rat. In addition to spreading depolarizations, results of this study suggest that a progressive, apparently permanent, decline in pericontusional interstitial glucose may be an additional sequela of brain injury. This study establishes extended dexamethasone-enhanced microdialysis in the injured rodent cortex as a new paradigm for investigating trauma-induced metabolic crisis.

Investigation of biomarkers alterations after an acute tissue trauma in human trapezius muscle, using microdialysis

Alterations in muscle milieu are suggested as important activity of peripheral drive in patients with chronic musculoskeletal pain (CMP). Microdialysis (MD) has been used in monitoring altered metabolic response pattern in muscles. However, the insertion of MD probe causes a local tissue trauma. Whether and how metabolites in trapezius muscle are affected by acute tissue trauma is unknown. Hence, this study investigated the metabolic response and nociceptive reaction of the tissue following MD probe insertion in patients with CMP and healthy individuals. Fifty-nine patients and forty pain-free volunteers were recruited. Pressure pain thresholds (PPTs) were obtained at the trapezius and tibialis muscles. Pain questionnaires determined the levels of pain related aspects. MD (20 kDa cut-off) was performed in the trapezius and samples were collected within 40 min. Interstitial concentration of the metabolites was analyzed by a two-way-mixed-ANOVA. The metabolic response pattern changed over time and alterations in the level of metabolites could be seen in both CMP and healthy controls. Pain questionnaires and pain intensities manifested clinical aspects of pain closely to what CMP patients describe. Analyzing metabolites due to acute tissue trauma by aid of MD may be a useful model to investigate altered metabolic response effect in CMP.

Biomarkers alterations in trapezius muscle after an acute tissue trauma: A human microdialysis study

Abstract Aims Alteration in muscle milieu has been proposed as one of the main contributors underlying chronic musculoskeletal pain (CMP). Microdialysis (MD) provides real-time information on release of pain and metabolic biomarkers in muscle. However, insertion of MD probe causes a local tissue trauma, which may affect the tissue milieu. Whether an acute tissue trauma alter pain and metabolite biomarkers in trapezius muscle of patients with CMP is not known. Hence, this study investigated changes in muscle metabolites following MD probe insertion in patients with CMP in comparison with healthy controls. Methods Nineteen patients (11 women and 8 men; 41.4 years) with CMP and 20 pain-free volunteers (10 women and 10 men; 36.5 years) were recruited (project approval number: 2013/151- 31). Baseline pressure pain thresholds (PPT) at trapezius muscle were obtained bilaterally with a reference point at the tibialis muscle. Pain questionnaires were used for determining levels of anxiety and depression and catastrophizing impact. Interstitial samples were collected from trapezius muscles by aid of MD (20 kDa cut-off) during a period of 40 min. Collected dialysates at 2 time-points of 20 and 40 min were stored at −70 °C until analysis. Concentrations of glucose, lactate, pyruvate, glycerol, and glutamate were analyzed by ISCUSSflex. P ≤ 0.05 was considered significant. Results No potential case with respect to anxiety, depression or catastrophizing impact was found. Lower PPTs were found in CMP group (P ≤ 0.05). Significantly lower levels of pyruvate were found in CMP group at both 20 min (P = 0.003) and 40 min (P = 0.006). Gender-based analysis indicated higher concentrations of glutamate in female patients with CMP. Conclusions This study was first to demonstrate metabolite alterations during trauma phase of MD in trapezius muscles of CPM compared with healthy controls. This model proved beneficial for investigating pain and metabolic biomarkers during acute phase of MD.

Enhancing Continuous Online Microdialysis Using Dexamethasone: Measurement of Dynamic Neurometabolic Changes during Spreading Depolarization.

Microdialysis is well established in chemical neuroscience as a mainstay technology for real time intracranial chemical monitoring in both animal models and human patients. Evidence shows that microdialysis can be enhanced by mitigating the penetration injury caused during the insertion of microdialysis probes into brain tissue. Herein, we show that retrodialysis of dexamethasone in the rat cortex enhances the microdialysis detection of K+ and glucose transients induced by spreading depolarization. Without dexamethasone, quantification of glucose transients was unreliable by 5 days after probe insertion. With dexamethasone, robust K+ and glucose transients were readily quantified at 2 h, 5 days, and 10 days after probe insertion. The amplitudes of the K+ transients declined day-to-day following probe insertion, and the amplitudes of the glucose transients exhibited a decreasing trend that did not reach statistical significance. Immunohistochemistry and fluorescence microscopy confirm that dexamethasone is highly effective at preserving a healthy probe-brain interface for at least 10 days even though retrodialysis of dexamethasone ceased after 5 days.

Evaluation of dynamic changes in interstitial fluid proteome following microdialysis probe insertion trauma in trapezius muscle of healthy women

Microdialysis (MD) has been shown to be a promising technique for sampling of biomarkers. Implantation of MD probe causes an acute tissue trauma and provokes innate response cascades. In order to normalize tissue a two hours equilibration period for analysis of small molecules has been reported previously. However, how the proteome profile changes due to this acute trauma has yet to be fully understood. To characterize the early proteome events induced by this trauma we compared proteome in muscle dialysate collected during the equilibration period with two hours later in “post-trauma”. Samples were collected from healthy females using a 100 kDa MW cut off membrane and analyzed by high sensitive liquid chromatography tandem mass spectrometry. Proteins involved in stress response, immune system processes, inflammatory responses and nociception from extracellular and intracellular fluid spaces were identified. Sixteen proteins were found to be differentially abundant in samples collected during first two hours in comparison to “post-trauma”. Our data suggests that microdialysis in combination with mass spectrometry may provide potentially new insights into the interstitial proteome of trapezius muscle, yet should be further adjusted for biomarker discovery and diagnostics. Moreover, MD proteome alterations in response to catheter injury may reflect individual innate reactivity.

High levels of endogenous lipid mediators (N-acylethanolamines) in women with chronic widespread pain during acute tissue trauma.

Although chronic widespread musculoskeletal pain is a significant health problem, the molecular mechanisms involved in developing and maintaining chronic widespread musculoskeletal pain are poorly understood. Central sensitization mechanisms maintained by stimuli from peripheral tissues such as muscle have been suggested. Lipid mediators with anti-inflammatory characteristics such as endogenous ligands of peroxisome proliferator activating receptor-α, oleoylethanolamide, and palmitoylethanolamide are suggested to regulate nociceptive transmission from peripheral locations on route towards the central nervous system. This case–control study investigates the levels of anti-inflammatory lipids in microdialysis samples collected during the first 2 h after microdialysis probe insertion and explores the association of these lipids with different pain characteristics in women with chronic widespread musculoskeletal pain (n = 17) and female healthy controls (n = 19). The levels of oleoylethanolamide, palmitoylethanolamide, and stearoylethanolamide were determined. During sampling of dialysate, pain ratings were conducted using a numeric rating scale. Pain thresholds were registered from upper and lower parts of the body. Oleoylethanolamide and stearoylethanolamide levels were significantly higher (p ≤ 0.05) in chronic widespread musculoskeletal pain at all time points. Numeric rating scale correlated with levels of stearoylethanolamide in chronic widespread musculoskeletal pain. Higher levels of lipid mediators could reflect an altered tissue reactivity in response to microdialysis probe insertion in chronic widespread musculoskeletal pain.

Changes in Blood Flow and Cellular Metabolism at a Myofascial Trigger Point With Trigger Point Release (Ischemic Compression): A Proof-of-Principle Pilot Study

ObjectiveTo demonstrate proof-of-principle measurement for physiologic change within an active myofascial trigger point (MTrP) undergoing trigger point release (ischemic compression). DesignInterstitial fluid was sampled continuously at a trigger point before and after intervention. SettingA biomedical research clinic at a university hospital. ParticipantsSubjects (N=2) from a pain clinic who had chronic headache pain. InterventionsA single microdialysis catheter was inserted into an active MTrP of the upper trapezius to allow for continuous sampling of interstitial fluid before and after application of trigger point therapy by a massage therapist. Main Outcome MeasuresProcedural success, pain tolerance, feasibility of intervention during sample collection, and determination of physiologically relevant values for local blood flow as well as glucose and lactate concentrations. ResultsBoth patients tolerated the microdialysis probe insertion into the MTrP and treatment intervention without complication. Glucose and lactate concentrations were measured in the physiologic range. After intervention, a sustained increase in lactate was noted for both subjects. ConclusionsIdentifying physiologic constituents of MTrPs after intervention is an important step toward understanding pathophysiology and resolution of myofascial pain. The present study forwards that aim by showing that proof-of-concept for collection of interstitial fluid from an MTrP before and after intervention can be accomplished using microdialysis, thus providing methodological insight toward treatment mechanism and pain resolution. Of the biomarkers measured in this study, lactate may be the most relevant for detection and treatment of abnormalities in the MTrP.

Microdialysis probes alter presynaptic regulation of dopamine terminals in rat striatum

The insertion of microdialysis probes into the rat striatum disrupts dopaminergic activity near the probe track. The present study suggests that a substantial fraction of DA terminals near the probe track (200μm) survive the probe implantation itself but that the surviving terminals experience altered presynaptic inhibition. We found that probe implantation did not just alter the amplitude of evoked dopamine responses recorded by voltammetry, but also changed their temporal profile in a fashion similar to that previously observed by quinpirole, an agonist of dopamine D2 autoreceptors. Altered presynaptic inhibition is supported by a hypersensitivity of evoked dopamine responses recorded near to microdialysis probes to raclopride, a D2 antagonist. Further, we found that evoked dopamine release was also hypersensitive to a final dose of the dopamine transporter inhibitor, nomifensine.

The Ability of Fluconazole to Penetrate into Ventilated, Healthy and Inflamed Lung Tissue in a Model of Severe Sepsis in Rats

Background/Aims: We measured the extracellular concentrations of fluconazole in lung tissue of septic and healthy rats. Methods: A single intravenous dose of 6 mg/kg total body weight of fluconazole was administered intravenously to rats following insertion of microdialysis probes into lung tissue. Another probe was inserted into skeletal muscle and served as control. Results: The mean peak concentration (C_max), time to C_max, area under the concentration-versus-time curve from 0 to 6 h (fAUC_0–6) and area under the concentration-versus-time curve from 0 to ∞ of unbound fluconazole for healthy lung were 11.0 ± 2.3 mg/l, 1.9 ± 1.5 h, 47.4 ± 8.6 mg·h/l and 233.7 ± 121.1 mg·h/l, respectively. The corresponding values for inflamed lung were 11.8 ± 1.7 mg/l, 1.5 ± 0.0 h, 52.9 ± 6.2 mg·h/l and 212.6 ± 79.7 mg·h/l, respectively. The mean apparent terminal elimination half-lives of fluconazole ranged from 12.3 to 22.4 h between compartments. The ratios of the fAUC_0–6 for lung to the fAUC_0–6 for plasma were 1.38 ± 0.39 and 1.32 ± 0.04 for healthy and inflamed lung, respectively. Conclusion: We provide evidence that free fluconazole levels in plasma, the extracellular space fluid of lung tissue and skeletal muscle are almost superimposable during inflammatory and normal conditions.

High extracellular levels of cefpirome in unaffected and infected lung tissue of patients

the objective of the present investigation was to measure the extracellular concentrations of cefpirome in unaffected and infected lung tissue of septic patients. a single intravenous dose of 30 mg/kg total body weight of cefpirome was administered to eight patients every 12 h prior to insertion of microdialysis probes into lung tissue. the median (minimum, maximum) peak concentration (C(max)), time to C(max) (T(max)), area under the concentration-time curve from 0 to 4 h (AUC(0-4)) and AUC(0-∞) of unbound cefpirome for unaffected lung were 48 (32, 107) mg/L, 0.83 (0.17, 3.17) h, 117 (60, 177) mg · h/L and 182 (80, 382) mg · h/L, respectively. The corresponding values for infected lung tissue were 45 (6, 122) mg/L, 1.17 (0.83, 2.83) h, 92 (17, 253) mg · h/L and 206 (49, 379) mg · h/L, respectively. The median apparent terminal elimination half-lives (t(½z)) of cefpirome were 2.61, 3.05 and 3.39 h for plasma, unaffected lung and infected lung, respectively. The median ratios of the AUC(0)(-∞) for lung to the AUC(0)(-∞) for plasma were 0.63 (0.19, 1.55) and 0.46 (0.32, 0.98) for unaffected and infected lung, respectively. we provide strong evidence that cefpirome penetrates effectively into the extracellular space fluid of lung tissue. Under steady-state conditions, the median concentrations of cefpirome in plasma, unaffected lung and infected lung exceeded the MICs of the majority of relevant bacteria over the entire dosing interval of up to 12 h after intravenous administration of a dose of 30 mg/kg total body weight.

Extracellular concentrations of fosfomycin in lung tissue of septic patients

The present investigation explored the ability of fosfomycin to penetrate lung tissue of septic patients by utilizing the microdialysis technique. After microdialysis probe insertion into healthy and infected lung tissue, a single intravenous dose of 4 g of fosfomycin was administered. The mean C(max), T(max), AUC(0-4) and AUC(0-infinity) for healthy lung were 131.6 +/- 110.6 mg/L, 1.1 +/- 0.4 h, 242.4 +/- 101.6 mgxh/L and 367.6 +/- 111.9 mgxh/L, respectively. The corresponding values for infected lung were 107.5 +/- 60.2 mg/L, 1.4 +/- 0.5 h, 203.5 +/- 118.4 mgxh/L and 315.1 +/- 151.2 mgxh/L. The half-life of fosfomycin ranged from 2.2 to 2.7 h between compartments. The magnitude of lung tissue penetration, as determined by the ratios of the AUC(0-infinity) for lung to the AUC(0-infinity) for plasma, was 0.63 +/- 0.31 and 0.53 +/- 0.31 for healthy and infected lung, respectively. We conclude that fosfomycin achieves antimicrobially effective concentrations in infected lung tissue.

Effects of corticosterone on corticotrophin‐releasing hormone and gastrin‐releasing peptide release in response to an aversive stimulus in two regions of the forebrain (central nucleus of the amygdala and prefrontal cortex)

Previous research has shown that chronic corticosterone treatment increases the expression of corticotrophin-releasing hormone (CRH) mRNA at the central nucleus of the amygdala (CeA). Like CRH, gastrin-releasing peptide (GRP) appears to be involved in mediation of the stress response and is released at the CeA during exposure to an acute stressor. Using in-vivo microdialysis, this study examined the effects of corticosterone treatment on the release of CRH and GRP in response to an airpuff challenge at two forebrain regions, the CeA and medial prefrontal cortex. Adrenally intact rats were treated with corticosterone by systemic implants over a 14-day period prior to microdialysis probe insertion. We found that, at both regions, the airpuff-induced CRH and GRP release were enhanced in the corticosterone pellet-implanted rats as compared with the release observed in the vehicle-implanted control rats. These findings suggest that chronic corticosterone exposure potentiates the stressor-elicited release of CRH and GRP. As cortisol dysregulation has frequently been reported in people with psychiatric conditions, such as anxiety disorders or depression, a better understanding of the glucocorticoid-mediating regulation of CRH and GRP may provide insight into the underlying neurochemical mechanisms involved in both adaptive fear-type responses and maladaptive responses leading to pathology.

Irreversible damage to rostral pontine nuclei in adult awake goats does not alter the ventilatory regulation of eupneic PaCO2 and the ventilatory response to hypercapnia

Controversies regarding the role of rostral pontine nuclei (RPN) in the control of breathing have prompted us to chronically implant microtubules into the rostral pons of 6 goats to assess the regulation of eupneic PaCO2 and the ventilatory response to hypercapnia following irreversible damage to RPN. All goats recovered uneventfully and 3 weeks after implantation PaCO2 and CO2 sensitivity were within normal limits. Whole body CO2 sensitivity was assessed by increasing inspired CO2 to 3, 5, and 7% at 5‐minute intervals, subsequent to breathing room air for 30 minutes. CO2 sensitivity was expressed as ΔVI/ΔPaCO2. Arterial blood was sampled during the control and min 4 of each hypercapnic level. As shown in the Table, CO2 sensitivity and resting PaCO2 were unaffected following sequential RPN damage (microtubule implantation, microdialysis probe insertion, and intra‐cranial ibotenic acid injection, respectively). We conclude that damage to RPN does not alter regulation of eupneic PaCO2 and the ventilatory response to hypercapnia. CO2 sensitivity and resting PaCO2 are not significantly changed subsequent to RPN damage. Pre‐implant Post‐implant Post‐microdialysis Post‐ibotenic acid CO2 sensitivity (l/min/mmHg) 2.24 2.25 2.08 2.30 Resting PaCO2 (mmHg) 40.24 39.57 38.67 40.00 No significant difference between variables by one‐way ANOVA (P<0.05). Supported by the Department of Veterans Affairs and NIH HL25739.

Nano‐LC‐MS/MS for the monitoring of angiotensin IV in rat brain microdialysates: Limitations and possibilities

To broaden our knowledge about the central role of the angiotensin IV (Ang IV) peptide, we aimed to monitor its extracellular concentration in the brain using in vivo microdialysis. Ang IV was measured in the dialysates using a previously developed nano-LC-MS/MS assay with an LOD of 50 pM. Using this assay, baseline levels of Ang IV in dialysates from different brain structures were undetectable. However, immediately after microdialysis probe insertion, Ang IV could be detected in a concentration that varied between 120 and 187 pM. Using the zero-net-flux method, the extracellular levels of Ang IV in the striatum were estimated at 46 pM. These data may indicate that Ang IV is mainly present intracellularly. In addition, Ang IV was clearly measurable after striatal perfusion of Ang II. On the other hand, our nano-LC-MS/MS method was successful for the detection of Met-enkephalin and neurotensin in dialysates from the rat. In conclusion, the nano-LC-MS/MS method coupled with microdialysis is well suited to monitor the biologically significant conversion between Ang II and Ang IV in vivo, but physiological extracellular levels of Ang IV appear too low to be detected.

NO inhibition of the magnocellular neuroendocrine system in rats is independent of cGMP signaling pathway

Our objective was to test the hypothesis that the cGMP signal-transduction mechanism mediates nitric oxide's (NO) modulation of oxytocin (OT) and vasopressin (VP) secretion from the hypothalamo-neurohypophysial system. Three studies were conducted in adult male Sprague–Dawley rats: (1a) Euhydrated rats received an intracerebroventricular (icv) infusion (1 μl/min for 30 min) of artificial cerebrospinal fluid (aCSF), vehicle (2.6% dimethyl sulfoxide [DMSO]) or 1 H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ) (0.05 μg/μl), an inhibitor of soluble guanylyl cyclase (sGC). ODQ did not affect basal levels of plasma VP or OT; (1b) Rats dehydrated for 24 h received aCSF or 8-Br-cGMP (icv), a membrane-permeable analog of cGMP, and plasma hormones were measured 2 min later. 8-Br-cGMP did not significantly change VP or OT levels; (2) Rats ingested water or 2% NaCl for 4 days, and NO synthase (NOS) and sGC activities were measured in posterior pituitaries, the anatomical site of hormone secretion. Salt loading enhanced ( P < 0.001) production of [ 14C]citrulline, the coproduct of NO synthesis, without altering cGMP; (3) One SON was microdialyzed with [ 14C]arginine and NOS and sGC activities were quantified in microdialysates during intravenous (iv) infusion of isotonic or hypertonic saline in awake and anesthetized rats. In awake rats, [ 14C]citrulline recovery, but not cGMP, increased ( P < 0.05) during intravenous infusion of both isotonic and hypertonic solutions, and after insertion of microdialysis probe itself. In anesthetized rats, however, where basal NOS activity is low, intravenous infusion of hypertonic, but not isotonic solution, increased [ 14C]citrulline recovery without affecting cGMP. Thus, in the forebrain, neither NO produced basally nor during osmotic stimulation depends on cGMP to modulate plasma vasopressin and oxytocin secretion.

Shock Wave Lithotripsy Causes Ipsilateral Renal Injury Remote From the Focal Point: The Role of Regional Vasoconstriction

Shock wave lithotripsy induced renal damage can occur as a result of multiple mechanisms, including small vessel injury and free radical production. Previous studies have demonstrated that shock wave lithotripsy exerts a regional change in renal hemodynamics, resulting in a global reduction in the glomerular filtration rate and renal plasma blood flow. We determined if biochemical evidence of cellular damage could be identified in ipsilateral locations remote from the shock wave site or in the contralateral kidney, suggesting regional or systemic alterations in renal function. Ten juvenile female swine underwent open insertion of microdialysis probes into the renal parenchyma at the right upper and lower poles, and left lower pole. The animals were divided evenly into a sham and a treatment group. Dialysate samples were collected from all 3 sites from the sham group at 10-minute intervals for 100 minutes and quantitatively assessed for conjugated dienes, a measure of lipid peroxidation and free-radical activity, signifying renal cellular damage. The animals in the treatment group underwent shock wave lithotripsy focused on the right lower pole. Dialysate samples were collected from all 3 sites at baseline and at 1,000 shock intervals for a total of 10,000 shocks and analyzed for conjugated dienes. The results from the sham and treatment groups as well as from the different locations within each group were compared using Student's t test. The mean conjugated diene ratio for the sham group was 3.59, 3.42 and 2.7 microM. for the right upper and lower poles, and left kidney (p >0.05). A dose related increase in conjugated diene ratio levels from the right lower pole (lithotripsy site) and to a lesser degree from the ipsilateral right upper pole were observed, which were significantly different from sham group measurements or the contralateral kidney of the treatment group (p <0.05). The elevation in conjugated diene levels at the lithotripsy site was also noted to be significantly greater than that of the upper pole of the ipsilateral kidney (p <0.05). The increase in free radical activity at a site remote from the treated region suggests detrimental global effects from shock wave lithotripsy therapy. These observations could be the result of vasoconstriction throughout the treated kidney with resultant ischemia-reperfusion injury. Although these global renal effects may subject patients with baseline renal dysfunction to irreversible renal damage, the clinical significance of our findings is unclear and warrants further investigation.

In vivo Tolerance Assessment of Skin after Insertion of Subcutaneous and Cutaneous Microdialysis Probes in the Rat

The purpose of the study was to evaluate the trauma induced by insertion of the linear microdialysis probe in the subcutaneous and dermal tissue in the rat and to check if the microdialysis probe insertion affects transdermal drug delivery. Non-invasive bioengineering methods (TEWL, Laser Doppler Velocimeter, Chromameter) as well as histology were combined to characterize these effects. The results showed that the dermal and subcutaneous insertion of microdialysis probes did not change skin permeability, blood flow and color, confirming the safety of this technique. The probe depth did not influence the trauma. No significant physical damage after probe insertion was noticed. Thus, the present work validates the use of microdialysis in dermatopharmacokinetics studies after topical or systemic drug delivery.

Hibernation, a Model of Neuroprotection

Hibernation, a natural model of tolerance to cerebral ischemia, represents a state of pronounced fluctuation in cerebral blood flow where no brain damage occurs. Numerous neuroprotective aspects may contribute in concert to such tolerance. The purpose of this study was to determine whether hibernating brain tissue is tolerant to penetrating brain injury modeled by insertion of microdialysis probes. Guide cannulae were surgically implanted in striatum of Arctic ground squirrels before any of the animals began to hibernate. Microdialysis probes were then inserted in some animals after they entered hibernation and in others while they remained euthermic. The brain tissue from hibernating and euthermic animals was examined 3 days after implantation of microdialysis probes. Tissue response, indicated by examination of hematoxylin and eosin-stained tissue sections and immunocytochemical identification of activated microglia, astrocytes, and hemeoxygenase-1 immunoreactivity, was dramatically attenuated around probe tracks in hibernating animals compared to euthermic controls. No difference in tissue response around guide cannulae was observed between groups. Further study of the mechanisms underlying neuroprotective aspects of hibernation may lead to novel therapeutic strategies for stroke and traumatic brain injury.

Individual variability of dopamine release from nucleus accumbens induced by nicotine

Effects of subcutaneous administration of vehicle, amphetamine (1 mg/kg) or nicotine (0.4 mg/kg, injected twice, 90 min apart) on extracellular dopamine (DA) concentration in the nucleus accumbens (ACC) and ventral tegmental area (VTA) of the Sprague–Dawley rat were studied using microdialysis. Experiments were conducted at least 10 days following implantation of guide cannulae, and at least 2 h following insertion of microdialysis probes into the guides on the morning of each experiment. Probes were perfused at 2.5 μl/min and several fractions were collected every 10 min before and after the two test injections. Samples were analyzed by high-performance liquid chromatography with electrochemical detection for the major neurotransmitters and their metabolites. Significant DA release following nicotine administration was observed in ACC but not in VTA. By classifying ACC DA responses of individual rats, three major subgroups were identified which exhibited more robust responses. Nicotine appeared to be acting as a modulator of ACC DA, increasing DA output if baseline was <5 nM, but slowing release when the baseline exceeded 5 nM. These data are consistent with previous reports of modulation of arousal level by nicotine via DA.

Declines in stimulated striatal dopamine release over the first 32 h following microdialysis probe insertion: generalization across releasing mechanisms

In a recent paper [R.R. Holson, J.F. Bowyer, P. Clausing, B. Gough, Methamphetamine-stimulated striatal dopamine release declines rapidly over time following microdialysis probe insertion, Brain Res. 739 (1996) 301–307] we reported that methamphetamine-stimulated striatal dopamine release declined rapidly over the first eight hours following microdialysis probe insertion. This decline was strictly a function of time post-probe implantation, and not due to tolerance or desensitization. To further examine this phenomenon, we subjected rats to three brief pulses of several DA-releasing compounds at 2, 4 and 6 h post-probe insertion, and compared these results to those caused by a single pulse 6 h post-insertion, or in some cases to pulses given more than 24 h post-insertion. We found that when buproprion, a dopamine reuptake blocker, was infused briefly into the striatum via the microdialysis probe, there was a pronounced drop in the amount of dopamine released at 6 h vs. 2 h post-insertion; this drop was not due to repeated exposure, since dopamine release at 6 h post-insertion was the same for a single pulse, or when preceded by two earlier pulses. Twenty-four hours later, buproprion-stimulated dopamine release was still lower, but did not appear to drop further thereafter. Potassium-stimulated dopamine release, on the other hand, dropped rapidly over the first 8 h post-insertion, and this decline continued throughout the 24–32 h interval post-insertion. Similarly, a single i.p. injection of 0.5 mg/kg haloperidol released three times as much dopamine when given two compared to six hours post-implantation. Both bupropion- and potassium-stimulated dopamine release were accompanied by declines in extracellular DOPAC concentrations, and these declines were the same 2 or 26 h post-insertion. In contrast, haloperidol exposure increased extracellular DOPAC, and this haloperidol-stimulated DOPAC increase was also greatly attenuated at 6 compared to 2 h post-insertion. We conclude that there is a general decline over time post-probe implantation in the ability of the striatal dopamine system to release dopamine, and perhaps to increase dopamine synthesis, in response to pharmacological challenges.