Microdialysis combined with a sensitive radioimmunoassay. A technique for studying in vivo release of neuropeptides

The role of substance P in stress and anxiety responses

Substance P (SP) is one of the most abundant peptides in the central nervous system and has been implicated in a variety of physiological and pathophysiological processes including stress regulation, as well as affective and anxiety-related behaviour. Consistent with these functions, SP and its preferred neurokinin 1 (NK1) receptor has been found within brain areas known to be involved in the regulation of stress and anxiety responses. Aversive and stressful stimuli have been shown repeatedly to change SP brain tissue content, as well as NK1 receptor binding. More recently it has been demonstrated that emotional stressors increase SP efflux in specific limbic structures such as amygdala and septum and that the magnitude of this effect depends on the severity of the stressor. Depending on the brain area, an increase in intracerebral SP concentration (mimicked by SP microinjection) produces mainly anxiogenic-like responses in various behavioural tasks. Based on findings that SP transmission is stimulated under stressful or anxiety-provoking situations it was hypothesised that blockade of NK1 receptors may attenuate stress responses and exert anxiolytic-like effects. Preclinical and clinical studies have found evidence in favour of such an assumption. The status of this research is reviewed here.

Kainate seizures increase nociceptin/orphanin FQ release in the rat hippocampus and thalamus: a microdialysis study

The neuropeptide nociceptin/orphanin FQ (N/OFQ) has been suggested to play a facilitatory role in kainate seizure expression. Furthermore, mRNA levels for the N/OFQ precursor are increased following kainate seizures, while its receptor (NOP) density is decreased. These data suggest increased N/OFQ release. To obtain direct evidence that this is the case, we have developed a microdialysis technique, coupled with a sensitive radioimmunoassay, that allows measurement of N/OFQ release from the hippocampus and thalamus of awake, freely moving animals. In both these brain areas, the spontaneous N/OFQ efflux decreased by approximately 50% and 65% when Ca2+ was omitted and when tetrodotoxin was added to the perfusion medium, respectively. Perfusion of the dialysis probe with high K+ increased N/OFQ release (approximately threefold) in a Ca2+-dependent and tetrodotoxin-sensitive manner. Kainate seizures caused a twofold increase in N/OFQ release followed, within 3 h, by a return to baseline levels. Approximately 5 h after kainate, a late increase in N/OFQ release was observed. On the following day, when animals were having only low grade seizures, N/OFQ release was not significantly different from normal. These phenomena were observed with similar patterns in the hippocampus and in the thalamus. The present data indicate that acute limbic seizures are associated with increased N/OFQ release, which may prime the molecular changes described above, i.e. cause down-regulation of NOP receptors and activation of N/OFQ biosynthesis.

Substance P microinjected into the periaqueductal gray matter induces antinociception and is released following morphine administration

The aims of the present study were to investigate, in rats, the behavioral effects of substance P (SP) microinjected into the ventrolateral periaqueductal gray (PAG) and the effects of the neurokinin 1 (NK-1) receptor antagonist [d-Arg1, d-Trp7, 9, Leu11]-substance P (Spantide). The effect of morphine administration on the release of SP in the ventrolateral PAG was also investigated using microdialysis in awake rats. SP microinjected into the ventrolateral part of the PAG induced significant increases in the hindpaw withdrawal latencies (HWLs) to thermal and mechanical stimulation as an antinociceptive response. The NK-1 receptor antagonist blocked these effects but exhibited no antinociceptive effect alone. Subcutaneous administration of morphine increased basal SP-like immunoreactivity (SP-LI) release in the microdialysate obtained from the ventrolateral PAG of freely moving rats. Our results demonstrate that SP injected into the ventrolateral PAG induces an antinociceptive effect via activation of NK-1 receptors. Morphine administered systemically induces the release of SP in the ventrolateral PAG. We suggest that an increased release of SP in the PAG may contribute to opioid antinociception.

Microdialysis of neuropeptide Y in human muscle tissue

Microdialysis of neuropeptide Y (NPY) has been reported to be difficult, which partly may be due to the adhesive nature of the molecule. The aim of this study was to determine the optimal probe and perfusion medium for microdialysis of NPY and to investigate if microdialysis can be used to sample NPY from human muscle tissue. Three different probe types with a 10mm membrane were used for experiments in vitro. They were perfused with a modified Krebs-Henseleit buffer (KHB), with or without the addition of 0.5% human serum albumin (HSA). Dialysate samples were collected at different flow rates. Ten healthy subjects participated in the clinical microdialysis. Microdialysis samples were obtained by probes inserted intramuscularly in the right masseter and trapezius muscle and perfused with KHB+HSA at a flow-rate of 4 microl/min. The relative recovery of NPY was significantly higher in the dialysates with HSA added to the perfusion medium, while there was no difference between the three probe types. NPY was detectable in 90% of the dialysates from the masseter and in 40% of the dialysates from the trapezius muscle. In conclusion, this study shows that NPY can be detected in microdialysis samples from human skeletal muscle and that addition of HSA to the perfusion medium increases the relative recovery in vitro.

Substance P in the medial amygdala: emotional stress-sensitive release and modulation of anxiety-related behavior in rats

Increasing evidence implicates the substance P (SP)/neurokinin-1 receptor system in anxiety and depression. However, it is not known whether emotional stimulation alters endogenous extracellular SP levels in brain areas important for processing of anxiety and mood, a prerequisite for a contribution of this neuropeptide system in modulating these behaviors. Therefore, we examined in rats whether the release of SP is sensitive to emotional stressors in distinct subregions of the amygdala, a key area in processing of emotions. By using in vivo micropush-pull superfusion and microdialysis techniques, we found a pronounced and long-lasting increase (150%) in SP release in the medial nucleus of the amygdala (MeA), but not in the central nucleus of the amygdala, in response to immobilization stress. SP release in the MeA was transiently enhanced (40%) in response to elevated platform exposure, which is regarded as a mild emotional stressor. Immobilization enhanced the anxiety-related behavior evaluated in the subsequently performed elevated plus-maze test. Bilateral microinjections of the neurokinin-1 receptor antagonist [2-cyclopropoxy-5-(5-(trifluoromethyl)tetrazol-1-yl)benzyl]-(2-phenylpiperidin-3-yl)amine into the MeA blocked the stress-induced anxiogenic-like effect, supporting a functional significance of enhanced SP release. In unstressed rats, the neurokinin-1 receptor antagonist displayed no significant anxiolytic effect but reversed the anxiogenic effect of SP microinjected into the MeA. Our findings identify the MeA as a critical brain area for the involvement of SP transmission in anxiety responses and as a putative site of action for the recently discovered therapeutic effects of SP antagonists in the treatment of stress-related disorders.

Assaying protein unbound drugs using microdialysis techniques

Compared with traditional sampling methods, microdialysis is a technique for protein unbound drug sampling without withdrawal of biological fluids and involving minimal disturbance of physiological function. Conventional total drug sample consists of unbound drugs and protein bound drugs, which are loosely bound to plasma proteins such as albumin and alpha-1 acid glycoprotein, forming an equilibrium ratio between bound and unbound drugs. However, only the unbound fraction of drug is available for absorption, distribution, metabolism and elimination, and delivery to the target sites for pharmacodynamic actions. Although several techniques have been used to determine protein unbound drugs from biological fluids, including ultrafiltration, equilibrium dialysis and microdialysis, only microdialysis allows simultaneous sampling of protein unbound chemicals from plasma, tissues and body fluids such as the bile juice and cerebral spinal fluid for pharmacokinetic and pharmacodynamic studies. This review article describes the technique of microdialysis and its application in pharmacokinetic studies. Furthermore, the advantages and limitations of microdialysis are discussed, including the detailed surgical techniques in animal experiments from rat blood, brain, liver, bile duct and in vitro cell culture for unbound drug analysis.

Immunohistochemistry as a tool for topographical semi-quantification of neurotransmitters in the brain

Immunohistochemistry is a powerful tool to detect neurotransmitter (NT) presence in different brain structures with a high spatial resolution. However, it is only scarcely used in quantitative approach due to lack of reproducibility and sensitivity. We developed a protocol of NT detection based on immunohistochemistry and image analysis to show that this approach could also be useful to evaluate NT content variations. We focused our study on the GABAergic system in the cerebellum and measured different accurate parameters, namely the optical density (O.D.), the stained area and the number of immunoreactive cells in each cerebellar cell layer. In order to modify the GABA content, we used gamma-vinyl-GABA (GVG), an inhibitor of GABA-transaminase, known to dramatically increase GABA concentration in the central nervous system (CNS) and especially in the cerebellum. We observed a significant increase in the three parameters measured in the molecular and the granular layers of the cerebellum after treatment with GVG, reflecting the well-established increase in GABA content after such a treatment. Therefore, our technical approach allows not only a precise determination of the effects in particular cell layers but also a semi-quantification of GABA content variations. This technique could be suitable for monitoring NT variations following any treatment.

Effects of radiofrequency exposure on the GABAergic system in the rat cerebellum: clues from semi-quantitative immunohistochemistry

The widespread use of cellular phones raises the problem of interaction of electromagnetic fields with the central nervous system (CNS). In order to measure these effects on neurotransmitter content in the CNS, we developed a protocol of neurotransmitter detection based on immunohistochemistry and image analysis. Gamma-vinyl-GABA (GVG), an inhibitor of the GABA-transaminase was injected in rats to increase GABA concentration in the CNS. The cellular GABA contents were then revealed by immunohistochemistry and semi-quantified by image analysis thanks to three parameters: optical density (O.D.), staining area, and number of positive cells. The increase in cerebellar GABA content induced by GVG 1200 mg/kg was reflected in these three parameters in the molecular and the granular layers. Therefore, control of immunohistochemistry parameters, together with appropriate image analysis, allowed both the location and the detection of variations in cellular neurotransmitter content. This protocol was used to investigate the effects of exposure to 900 MHz radiofrequencies on cerebellar GABA content. Both pulsed emission with a specific absorption rate (SAR) of 4 W/kg and continuous emission with high SAR (32 W/kg) were tested. We observed a selective diminution of the stained processes area in the Purkinje cell layer after exposure to pulsed radiofrequency and, in addition, a decrease in O.D. in the three cell layers after exposure to continuous waves. Whether this effect is, at least partly, due to a local heating of the tissues is not known. Overall, it appears that high energetic radiofrequency exposure induces a diminution in cellular GABA content in the cerebellum.

Analytical considerations for microdialysis sampling

Adaptations in microdialysis probe designs have made it possible to obtain samples from the extracellular fluid of a variety of tissues with high temporal resolution. The resulting small volume samples, often with low concentration of the analyte(s) of interest, present a particular challenge to the analytical system. Rapid separations can be coupled on-line with microdialysis to provide near real-time data. By combining microdialysis sampling with a liquid chromatographic or capillary electrophoretic separation and a highly sensitive detection method, a separation-based sensor can be developed. Such sensors have been applied to the investigation of drug entities as well as to study endogenous analytes.

Measurement of cholecystokinin release in vivo in the rat spinal dorsal horn

The microdialysis technique, used to monitor extracellular levels of transmitter substances in the central nervous system of laboratory animals as a reflection of transmitter release, is based on the ability of neurotransmitters to diffuse in the extracellular fluid from the site of release and to cross a semipermeable dialysis membrane. Even though the surgical procedure is not very complicated, the detection of released substances in the recovered dialysate may be difficult. Especially, the measurement of neuropeptide release is limited by the low extracellular concentration and of low recovery as compared to, for example, monoamines. Thus, for example, cholecystokinin (CCK), which is the most abundant neuropeptide in the central nervous system, is found at concentrations that are several orders of magnitude lower than those of classical transmitters. Therefore a highly sensitive detection method is of utmost importance. In the dorsal horn of the spinal cord CCK is found mainly in interneurons and in terminals of descending fibers. CCK seems to be involved in nociceptive transmission and CCK attenuates morphine-induced antinociception. We here describe in vivo microdialysis in the lumbar dorsal horn of the rat with subsequent quantification of the level of CCK-like immunoreactivity (-LI) by a highly sensitive radioimmunoassay.

Construction of 1 mm microdialysis probe for amino acids dialysis in rats

We describe here a microdialysis probe with 1 mm opening for precise and confined dialysis area in the awake, freely moving rat. This probe is designed to allow the local diffusion of the perfusion medium to an area approximately 175 microm high, 266 microm wide (mediolateral direction), and 305 microm in rostrocaudal direction. In addition, the probe allows the local application of drugs to the same precise area of interest. The probe was constructed from a piece of 25 gauge tubing with 1 mm hallowed opening located 0.5 mm from the distal (inserting) end. The dialysis fiber which was inserted into the stainless steel 25 gauge tubing and cemented into place has 200 microm diameter and 5000 molecular weight cut off. We tested the probe diffusion extent by direct infusion of fluorogold through the dialysis cannula. Changes in the extracellular concentrations of amino acids were measured in response to infusion of veratridine a sodium channel activator. All amino acids tested showed a significant 80% times decrease in their recovery concentration when compared to their respective concentrations recovered through 2 mm probe constructed earlier in our laboratory (Renno et al., 1992). Tests in awake rats with probes in the ventrocaudal PAG showed stable amounts of 12 different amino acids during repeated (6-8 times) 12 min samples at 3-5 microl/min collecting rate. Depolarization with 75 microM veratridine resulted in significant elevation in extracellular gamma-aminobutyric acid (GABA), aspartate, glutamate, taurine, glycine and citrulline. This design enables us to apply drugs of interest and measure the concentrations of amino acid neurotransmitters to a more precise, delineated and premeasured areas in the CNS.

Radioimmunoassay for the measurement of S9788 in serum and microdialysis samples

S9788, 6-[4-(2,2-di-(fluorophenyl)-ethylaminol-1-piperdinyl]-N,N'-d i-2-propenyl-1, 3,5-triazine- 2, 4-diamine, is a novel compound designed to reverse tumour multidrug resistance associated with cancer chemotherapy. A specific and sensitive radioimmunoassay has been developed for the analysis of S9788 in serum samples and adapted for samples obtained by microdialysis. The limit of quantitation is 0.2 ng ml-1 in perfusion medium and there is no cross reactivity of the antibody with known metabolites of the parent compound or with certain cytotoxic compounds likely to be coadministered with S9788. Maximum probe recovery during microdialysis was 66% at a flow of 1 microliter min-1, using Ringer/BSA (70 mg ml-1) as the perfusion medium. The assay has sufficient sensitivity, precision, accuracy and specificity for the analysis of rat and human serum and microdialysis perfusate samples. The assay has been successfully applied to the determination of S9788 in rat plasma (total concentration) and the microdialysate of the same samples.

Determination of substance P in human nasal lavage fluid

The determination of substance P (SP) concentrations in human nasal lavages can be used to monitor physiological and certain pathophysiological processes in human airway mucosa. But, because of the low concentrations, immunoassays of high sensitivity are needed. Two approaches to improve the sensitivity of the radioimmunological determinations of SP are compared: increasing the sample volume and miniaturizing the assay design. The characterization of SP-like immunoreactivity (SP-LIR) in human nasal lavage was performed by investigating the immunological specificity of the antibody used in the radioimmunoassays and by reversed-phase high-performance liquid chromatography separation of the SP-LIR. SP concentrations in nasal lavages can be reliably measured by each of the two introduced RIA methods. Despite the lower detection limit of the miniaturized immunoassay (0.2 in comparison to 1.3 fmol/incubate) it is advisable to increase the sample volume in order to improve the sensitivity because of the higher precision of the determinations. SP-LIR was found in nasal lavage specimens in concentrations between 2 and 10 fmol/ml and consisted of authentic SP and, to a less extent, SP-sulfoxide.

The behavioral neurochemistry of motivation: methodological and conceptual issues in studies of the dynamic activity of nucleus accumbens dopamine

Considerable experimental and clinical evidence links forebrain dopamine (DA) systems to the performance of motor activities and to motivational processes. Much of the support for this conclusion was obtained from studies utilizing lesions or drugs to manipulate aspects of central dopaminergic function. Although such experiments yield important information concerning the behavioral consequences of interference with DA systems in brain, they do not demonstrate any relation between the dynamic activity of DA neurons and the level or type of motor function exhibited by the organism. This review discusses the emerging field of behavioral neurochemistry, and provides an overview of recent studies investigating the relation between nucleus accumbens DA release and behavior. Particular emphasis is placed upon current research involving microdialysis, voltammetry and electrophysiology. These different methods are viewed as complementary techniques for investigating the activity of DA systems in behaving animals. Evidence indicates that DA activity is most reliably activated by stimuli that trigger instrumental behavior and during the preparatory or instrumental phase of motivated behavior. The effects of consummatory reactions to positive reinforcers are somewhat equivocal; with food consumption, dialysis studies have yielded inconsistent results, while some voltammetric and electrophysiological studies have shown that DA activity in accumbens or ventral tegmental area actually decreases during consumption of food reinforcement. Moreover, the responsiveness of accumbens DA activity during behavioral stimulation is not unique to appetitive conditions, as several studies have shown that aversive or stressful conditions also stimulate accumbens DA release or metabolism. It is reasonable to suggest at this time that accumbens DA neurons are activated by a variety of different motivational conditions, but that the consequence of that activation is to modulate the behavioral reactivity of the organism. This type of function is seen as representing an area of overlap between motor and motivational processes.

Repeated spinal cord stimulation decreases the extracellular level of gamma-aminobutyric acid in the periaqueductal gray matter of freely moving rats

Most of the previous experimental studies on the antinociceptive effects of electrical spinal cord stimulation (SCS) have focused on short-lasting effects mainly depending on spinal mechanisms. However, patients treated with SCS for chronic pain often report pain relief exceeding the period of stimulation for several hours. The long lasting effect of SCS might not only involve spinal, but also supraspinal mechanisms. A supraspinal region of major importance for the coordination of descending pain inhibition is the periaqueductal grey matter (PAG). The aim of the present microdialysis study, performed in awake freely moving rats, was to investigate if repeated SCS (two 30 min periods separated by a 90 min resting period) alters the extracellular neurotransmitter concentrations in the ventrolateral PAG. In a first series of experiments significantly decreased (-30%; P < 0.05; n = 7) gamma-aminobutyric acid (GABA) levels were detected immediately after the second SCS session. Neither the concentration of serotonin nor that of substance P-like immunoreactivity (SP-LI) was affected by SCS. The decrease of GABA after two SCS sessions was confirmed in a second series of experiments (-30%; P < 0.05; n = 7). No spontaneous decline of GABA was observed in sham-stimulated animals (n = 6). The glutamate concentration was also determined in this latter series of experiments and a significant decrease (-23%; P < 0.05; n = 5) was observed after the second SCS session. As GABA-neurons in the PAG exert a tonic depressive effect on the activity in descending pain inhibitory pathways, a decreased extracellular GABA level in this region, as detected following repeated SCS, might indicate an increased pain inhibition.

In vivo detection of immunoreactive neurokinin A release within rat substantia nigra and its dependency on a dopaminergic input

In the striatum, the tachykinin peptide neurokinin A (NKA) is thought to coexist with substance P in the gamma-aminobutyric acid-containing spiny neurones which project to the substantia nigra. We have used in vivo antibody-coated microprobes to directly monitor the release of NKA-like immunoreactivity (NKA-LI) within substantia nigra during various pharmacological manipulations. The data clearly illustrates a basal or resting extracellular presence of NKA-LI restricted to substantia nigra reticulata which was found to be largely dependent on a dopaminergic input. Acute administration of haloperidol (0.1-0.2 mg/kg i.p.) considerably reduced this basal NKA-LI whereas depot administration (14 mg/kg i.m. released over 2 weeks) produced a less substantial reduction. Lesion of nigro-striatal dopamine neurones with the neurotoxic agent 6-hydroxydopamine produced significant reductions in the nigral NKA-LI detected. However, d-amphetamine administration (4 mg/kg i.p.) did not alter the pattern of NKA-LI release for up to 4 h posttreatment. These results indicate that changes in peptide mRNA levels do not necessarily reflect changes in peptide release and suggest that NKA may be the more physiologically relevant tachykinin within the substantia nigra of the rat.

Characterization and application of microdialysis probes with an active exchange length compatible with small-size brain nuclei in the rat

We communicated the construction, characterization and application of microdialysis probes with an active exchange length that is suitable for experiments involving small-size nuclei in rat brain. Using substance P (SP) as the test substance, we determined that probes with an active exchange length of 180-200 microns exhibited an in vitro recovery of 14.2 +/- 0.8% and in vivo recovery at the nucleus tractus solitarii (NTS) of 24.9 +/- 1.7% for the undecapeptide, calibrated at an infusion rate of 1 microliter/min. We also demonstrated that microinfusion of SP via these probes into the NTS allowed for a correlation of changes in tissue levels of both SP (exogenous substance) and norepinephrine (endogenous substance) with alterations in baroreceptor reflex responses (physiologic phenomenon).

Measurement of somatostatin release in rat brain by microdialysis

We determined the most suitable conditions for measuring the somatostatin (SRIF) level by brain microdialysis and investigated its release from the hypothalamus. The relative recovery rate of SRIF was 8.4 +/- 0.5% (mean +/- SE) using a polycarbonate (PC) membrane with the push-pull method at a flow rate of 2 microliters/min. Using tubes with an internal diameter of 0.28 mm and lengths of 5, 25, 50 and 100 cm, the relative recovery rates using a PC membrane with the push method were 8.2 +/- 0.5%, 7.3 +/- 0.6%, 6.2 +/- 0.5% and 4.1 +/- 0.6%, respectively. When using tubes with an internal diameter of 0.1 mm and lengths of 5, 25, 50 and 100 cm, the relative recovery rates were 7.3 +/- 0.7%, 5.6 +/- 1.0%, 3.5 +/- 1.1% and 1.4 +/- 0.7%, respectively. The relative recovery rate was 5.2 +/- 0.5% with a polysulfone (PS-F, Fresenius) membrane, 4.5 +/- 0.4% with a PS-H (Hospal) membrane, 2.6 +/- 0.2% with an ethylenevinyl alcohol membrane (EVAL), 5.1 +/- 0.8% with a polyvinyl alcohol (PVA) membrane and 10.4 +/- 0.8% with a PS-K (Kaneka) membrane. With the push method, the extracellular SRIF level in rat pituitary was 42.8 +/- 1.8 pg/ml with a PC membrane, 23.1 +/- 2.9 pg/ml with an EVAL membrane at a flow rate of 2 microliters/min. With the push-pull method, it was 52.7 +/- 5.2 pg/ml using a PC membrane, 33.5 +/- 2.8 pg/ml using a PVA membrane and 54.4 +/- 3.2 pg/ml using a PS-K membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

On the origin of striatal cholecystokinin release: studies with in vivo microdialysis

In the present study, extracellular levels of the neuropeptide cholecystokinin (CCK), of the monoamine dopamine and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and of the excitatory amino acids glutamate and aspartate were simultaneously monitored by microdialysis in the neostriatum of halothane-anesthetized rats under basal and K(+)-depolarizing conditions. Extracellular CCK and dopamine levels, but not glutamate and aspartate levels, were decreased by perfusion with a Ca(2+)-free medium, under both basal and K(+)-depolarizing conditions. HPLC revealed that the majority of the CCK-like immunoreactivity in the perfusates coeluted with CCK octapeptide. Striatal extracellular CCK levels were decreased by decortication plus callosotomy, with a parallel decrease in glutamate levels. Striatal extracellular levels of dopamine, DOPAC, and HVA were significantly decreased in animals treated previously with a unilateral 6-hydroxydopamine injection into the medial forebrain bundle. In these animals, however, the effect of decortication plus callosotomy on CCK and glutamate levels was not further augmented. Thus, this study supports the hypothesis of a neuronal origin of extracellular CCK and dopamine monitored with microdialysis in the striatum of the rat, and also supports the idea of a partly contralateral origin of corticostriatal CCK and glutamate inputs.

A method for estimation of extracellular concentration of compounds by microdialysis using urea as an endogenous recovery marker in vitro validation

A new, practical method called microdialysis has been developed for the estimation of free, unbound drug concentrations in the interstitial fluid. This method is based on the use of urea as an endogenous recovery marker. Urea freely distributes throughout the body water compartment, and its plasma concentration can be used as an accurate measure of its interstitial levels. Microdialysis exploits the relationship between the relative recoveries of urea and the recoveries of an analyte that are established during calibration. The calibration is carried out using different probes, varying perfusion rates, membrane surface areas, temperatures, and tissue models. After obtaining a broad range of recoveries, an empirical mathematical model of the relationship between the recovery of the respective analyte and that of urea is formed. During in vivo experiments, a microdialysis probe is placed into a tissue or fluid of interest, and the urea recovery is monitored by comparing the corresponding dialysate and plasma urea levels. From the calculated urea recoveries, correlation equation, and analyte dialysate concentrations, the extracellular concentration of the unbound analyte in the tissue or fluid can be estimated at any given time point. The purpose of this study is to describe the method of microdialysis and demonstrate its applicability in vitro using two model compounds, theophylline and 3'-azido-3'-deoxythymidine (AZT), in the rat plasma.

Tachykinin- and leucokinin-related peptides in the nervous system of the blowfly: immunocytochemical and chromatographical diversity

We are interested in the presence and function in insects of neuropeptides related to the vertebrate tachykinins. Hence, we have used antisera raised against the tachykinins substance P and kassinin, and against the insect neuropeptide leucokinin I, for localization studies and immunochemical analysis of related peptides in the nervous system of the blowfly Phormia terraenovae. In radioimmunoassays (with antisera against kassinin and leucokinin I) used in combination with reverse-phase HPLC, it was shown that the antisera recognize immunoreactive material with distinctly hydrophobic properties and each antiserum appear to detect several forms of immunochemically related peptides. With immunocytochemistry it was shown that the kassinin and leucokinin antisera each reacted with material in a distinct set of neurons. The leucokinin-immunoreactive material is present both in interneurons and in neurosecretory cells, suggesting roles of native leucokinin-like peptides as neuromodulators in the nervous system and as neurohormones acting on peripheral targets. The kassinin immunoreactivity was seen in interneurons, but could not be conclusively localized in neurosecretory cells, possibly indicating a role only within the nervous system.

Epileptogenic spikes and seizures but not high voltage spindles are induced by local frontal cortical application of gamma-hydroxybutyrate

Combining the methods of microdialysis and EEG recording, we have examined the effect of unilaterally, intracortically applied gamma-hydroxybutyrate (GHB) on frontal cortical EEG activity in freely moving rats. GHB, a natural endogenous GABA metabolite, is known to induce rhythmic spike and wave activity, resembling generalized petit mal epilepsy. Without GHB, spontaneous high voltage spindles (HVS, 6-9 Hz) were observed during awake and immobile behavior in most of the animals (HVS rats), while others never had any HVS. In those both groups of animals intracortical application of GHB induced epileptogenic spikes (< 0.5 Hz) behaviorally accompanied by occasional myoclonic jerks and epileptic discharges (< 2 Hz) with behavioral convulsions and contraversive movements towards the left hindlimb (seizures) but did not induce HVS or spike and waves, as reported after systemic application. In the group of rats with spontaneous occurring HVS the amplitude of the HVS on the side of the microdialysis probe was suppressed by GHB and GHB-induced spikes invading the contralateral cortex frequently triggered and terminated local HVS. The results point to different neural mechanisms for the generation of HVS and spikes and epileptic discharges (seizures) induced after local intracortical application of GHB.

Neuronal release of somatostatin in the rat striatum: an in vivo microdialysis study

Extracellular levels of somatostatin in the rat striatum were studied using in vivo microdialysis and radioimmunoassay. In vitro studies were performed using three different dialysis membranes at various flow rates and temperatures to assess the optimal recovery of somatostatin. The best results were obtained when a cellulose fibre membrane was utilized at 37 degrees C with a flow rate of 0.5 microliters/min. For the in vivo studies, transcerebral cellulose probes were implanted in the striatum of chloryl hydrate-anaesthetized rats. Basal levels of somatostatin were detected in the striatum of the freely moving animals and found to be 5-15 fmol. Stimulation with 100 mM KCl increased the recovered somatostatin by 138% (P < 0.05). A second stimulation following a 3-h interval increased the somatostatin levels by approximately 60%. The addition of veratridine (100 microM) in the perfusion medium increased the somatostatin levels recovered from the striatum by 85% (P < 0.01). Following a 3-h interval, a second stimulation by veratridine also increased somatostatin levels (43%). The increases observed after the second depolarizing stimulus (KCl and veratridine) were not found to be significantly different from basal levels. Both EGTA and the sodium channel blocker tetrodotoxin attenuated the effect of KCl and veratridine, respectively. However, neither EGTA nor tetrodotoxin had an effect on the basal levels of somatostatin recovered. These results indicate that (i) the somatostatin measured is neuronally released in the striatum and (ii) microdialysis is a useful tool for examining the regulation of somatostatin release in the brain.

In vivo release of somatostatin from rat hippocampus and striatum

Rats were implanted with microdialysis probes in hippocampi and striata, and somatostatin-like immunoreactivity (SS-LI) was measured in outflows obtained from awake, freely moving animals 48 and 72 h post implantation. SS-LI was measurable in all dialysates under basal conditions; concentrations were stable and within a narrow range, about 3-6 fmol/ml. Cysteamine (300 mg/kg, s.c.) markedly reduced basal SS-LI concentrations in outflows from hippocampus (P < 0.00001). KCl (100 mM, 10 min) or veratridine (50 microM, 10 min) infusion elevated hippocampal SS-LI output by 55 and 106%, respectively (P's < 0.05). EGTA (10 mM) or tetrodotoxin (2 microM) infusion inhibited the SS-LI release elicited by KCl and veratridine, respectively, without affecting the basal SS-LI outflow. Thus, our results demonstrate that SS-LI is released from rat hippocampus and striatum in vivo, and provide evidence that the peptide may be released in hippocampus by both action potential dependent and independent processes.

Somatostatin release in rat neocortex during gamma-hydroxybutyrate-provoked seizures: Microdialysis combined with EEG recording

Gamma-hydroxybutyrate (GHB) was intracortically applied in two doses (first 10 and then 20 mg/ml) to awake Wistar rats using microdialysis. Simultaneously, EEG and the release of somatostatin-like immunoreactivity (SLI) were measured from the frontal cortex. Intracerebrally administered GHB induced cortical epileptogenic spikes, but not high voltage spindles (HVS) as reported after systemic administration, and seizures with myoclonic jerks and contraversive head movements. Compared to the basal level, GHB (10 mg/ml) initially increased the release of SLI (p < 0.05). However, when the frequency of spikes and seizures rose rapidly (p < 0.001), SLI release decreased significantly (p < 0.001). Minimum release of SLI occurred when seizures were most frequent (during perfusion with 20 mg/ml GHB), while after removal of the drug it rose above the basal level (p < 0.05). According to these results, intracortically applied GHB increases the release of SLI in the surrounding tissue. However, further exposure of GHB leads to a manifestation of epileptic spikes and seizures, during which the release of SLI is significantly attenuated. This suggests that release of somatostatin is affected during epileptic phenomena induced also by intracortical GHB application.

Dorsal column stimulation induces release of serotonin and substance P in the cat dorsal horn

The neurohumoral mechanisms behind the pain-suppressing effect of dorsal column stimulation (DCS) still remain obscure. Experimental observations have indicated an inhibitory role for serotonin and, under certain conditions, also for substance P (SP), on nociceptive transmission in the spinal cord. Furthermore, some observations suggest that these substances might be involved in the effect of DCS. The present series of experiments was undertaken to investigate whether serotonin and SP are released in the dorsal horn by DCS. Twenty-one adult cats, in some experiments anesthetized, in others decerebrated at the midcollicular level, were used. Microdialysis probes were implanted bilaterally in lumbar dorsal horns (L5-L7) and perfused with Krebs' solution. Dialysates were analyzed for serotonin by high-performance liquid chromatography or for SP by radioimmunoassay. DCS was applied at the thoracolumbar junction with current parameters similar to those used clinically in humans. DCS induced a significant release of serotonin in the dorsal horn of decerebrated animals (173 +/- 83% increase; mean +/- standard error; n = 4; P less than 0.01), whereas the levels of the metabolite 5-hydroxyindoleacetic acid were not significantly influenced. In contrast, no release of SP could be recorded in response to DCS in the decerebrated preparation, although peripheral nociceptive stimulation (pinch) and noxious electric dorsal root stimulation induced an elevation of the SP levels. However, in intact animals DCS provoked a marked SP release in the dorsal horn (190 +/- 92% increase; n = 7; P less than 0.01). The release of serotonin and SP after DCS may indicate that these substances participate in the mediation of the pain alleviating effect of DCS.(ABSTRACT TRUNCATED AT 250 WORDS)

Amphetamine regulation of mesolimbic dopamine/cholecystokinin neurotransmission

The effects of acute and repeated amphetamine administration on mesolimbic dopamine (DA) neurons was assessed by studying DA and cholecystokinin (CCK) release in the nucleus accumbens (Acc), as well as effects on mRNA genes regulating DA and CCK synthesis in ventral tegmental area (VTA) cells in rats. Amphetamine (1.5 mg/kg) markedly increased extracellular levels of DA in the medial Acc (assessed by in vivo microdialysis) in drug-naive animals, about twice the amount released in animals repeatedly administered the drug for the previous 7 days (twice daily). CCK overflow was found to mirror the DA responses in that the very transient elevation of CCK monitored in drug-naive animals was attenuated in those with prior amphetamine use. The attenuation of both DA and CCK overflow in the medial Acc was found to be associated with a decrease in the number of CCK mRNA-positive VTA neurons (assessed by in situ hybridization histochemistry). Although the number of cells expressing CCK mRNA were decreased, the gene expression in those positive CCK and tyrosine hydroxylase mRNA cells in the VTA was significantly increased. The CCK mRNA neurons in the VTA were positively identified as those projecting to the medial Acc by the local perfusion of Fluoro-gold retrograde tracer via microdialysis probes located in the Acc.

Modulation of sympathetic activity by brain neuropeptide Y in cardiac hypertrophy

Several observations now support the view that the sympathetic system actively participates in the development of cardiac hypertrophy. Since norepinephrine (NE)-containing neurons involved in cardiovascular regulation in the brain are known to coexist with neuropeptide Y (NPY), it is possible that a functional interaction between NPY and NE exists centrally. In an effort to clarify whether or not central catecholamine systems are modulated by NPY soon after imposing an increased pressure overload on the heart, male Sprague-Dawley rats underwent aortic constriction and were examined 14 days later. Rats were anesthetized and subjected to microdialysis sampling by stereotaxically implanting a probe into the caudal ventrolateral medulla (A1). Perfusate was collected after a 1-hour stabilization period, purified, and analyzed for interstitial concentrations of NE and other catecholamines using high-performance liquid chromatography with an electrochemical detector. Extracellular NE concentrations in the A1 area were found to be decreased. These results were associated with increased rate of change in the specific activity of NE (NE turnover) in heart, indicating increased sympathetic activity and an increased left ventricular weight. Also, infusion of NPY (10(-9) mol/L) by microdialysis in the A1 area resulted in the reduction of NE concentration; epinephrine and dopamine levels were also decreased. In contrast, methionine-enkephalin, another neuropeptide, had no effect on the extracellular catecholamine concentrations in the A1 area. Since neurons of the A1 group project almost exclusively to forebrain structures inhibiting sympathetic activity, it is concluded that decreases of NE and other catecholamines in afferent pathways regulating the caudal ventrolateral medulla may lead to an enhanced sympathetic activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasopressin release within the supraoptic and paraventricular nuclei of the rat brain: osmotic stimulation via microdialysis

The combination of microdialysis and a highly sensitive radioimmunoassay was used in order to monitor the in vivo release of arginine vasopressin (AVP) within hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei of the rat brain. A dialysis probe was inserted into the SON or PVN area and microdialysis was performed in conscious or urethane-anesthetized animals before, during and after hypertonic artificial cerebrospinal fluid (aCSF, with 1 M NaCl) was delivered via the probe. The recovery of AVP in vitro was 1.60%, that of [3H]OH in vitro 14.2% and in vivo 8.44% (SON) and 9.26% (PVN), respectively. AVP was consistently detected in both SON and PVN dialysates; basal levels averaged 0.87 +/- 0.22 pg/30-min dialysate (SON, n = 51) and 0.80 +/- 0.24 pg/30-min dialysate (PVN, n = 6), respectively. Hypertonic aCSF given over a period of 30 min, 60 min or 90 min, resulted in an increased AVP release within the SON which, however, reached its peak (to 8.86-10.27 pg/sample; P less than 0.001 as compared to basal) only in the poststimulation period, i.e. after replacement of hypertonic with isotonic aCSF. An identical osmotic stimulus given 150-210 min after the first one produced similar, though slightly declined, changes in AVP release. In the PVN, AVP release patterns prior to and in response to the first hypertonic pulse were similar to those in the SON; a possible functional difference between the two nuclei is indicated by the lack of a rebound increase in AVP release following the second stimulation. The physiological significance of intranuclearly released AVP remains to be shown.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced in vivo release of substance P in the nucleus tractus solitarii during hypoxia in the rabbit: role of peripheral input

In the adult, pentobarbitone-anaesthetized rabbit, the in vivo release of substance P-like immunoreactivity was measured in the nucleus tractus solitarii using microdialysis and radioimmunoassay. Increased 160 +/- 16%) extracellular concentrations of substance P-like immunoreactivity were observed during hypoxic provocations of 9% O2 in N2 which also resulted in an increase in phrenic nerve activity. In bilateral carotid sinus nerve-denervated animals no enhanced release of substance P was seen in response to hypoxic challenges (105 +/- 6%) and the phrenic nerve activity was not significantly affected. Perfusion of the nucleus tractus solitarii region with the dopamine agonist, apomorphine (10(-5) M) resulted in a significant decrease in the extracellular level of substance P. These results provide further evidence that substance P is involved in the mediation of the hypoxic drive inputs from the peripheral chemoreceptors. The interactions of apomorphine with substance P release might also suggest a presynaptic modulation of substance Pergic neurons by dopamine in the nucleus tractus solitarii.

Injection of tachykinins and selective neurokinin receptor ligands into the substantia nigra reticulata increases striatal dopamine and 5-hydroxytryptamine metabolism

Bilateral injection of endogenous tachykinins (substance P (SP), neurokinin A (NKA)) and selective neurokinin receptor ligands (senktide, [Sar9,Met(O2)11]SP, [MePhe7]NKB) into the substantia nigra reticulata increased striatal dopamine and serotonin metabolism. The increase in dopamine metabolism in the dorsal striatum at a low dose of the substances may be a direct effect on dopamine neurons in the substantia nigra reticulata via NK1 and NK3 receptors. The lack of effect at intermediate doses may be due to inhibitory mechanisms or desensitization. The changes after high doses in the dorsal and ventral striatum may be due to actions on dopaminergic neurons in the substantia nigra as well as in the ventral tegmental area, since there was considerable diffusion from the site of injection. An apparent rapid degradation of injected SP or NKA indicates that N-terminal SP fragments may participate in the SP response. The increased serotonin metabolism that occurs only at a high dose may involve all three neurokinin receptors.

A comparison between three methods for estimation of extracellular concentrations of exogenous and endogenous compounds by microdialysis

In vitro models simulating tissues were used to test the validity of three methods for determining the free concentration of drugs and endogenous compounds in the extracellular space by means of microdialysis. Theophylline was used as a model substance. Recovery is defined as the proportion of compounds extracted from the medium surrounding the probe. The water recovery method is the use of recovery, determined in a water solution, to estimate concentrations in other media. This method was shown to underestimate the surrounding concentration when the effective rate of diffusion is smaller in the other medium than in water. The difference method measures the net transport over the dialysis membrane at varying concentrations in the perfusion medium. The point of equilibrium, where no net transport takes place, is used to estimate the surrounding concentration. The perfusion rate method involves two phases. In the first phase (calibration), surrounding media with different diffusion characteristics were used as tissue models. The amount recovered at different perfusion rates was measured and a multivariate regression method was used to calculate a mathematical model. In the second phase, the mathematical model was used to predict the concentration in the surrounding medium in new experiments. The two latter methods gave satisfactory predictions of the surrounding concentrations. Protein binding did not affect the methods. It is concluded that the difference method and the perfusion rate method may be used to estimate the in vivo concentration of drugs and endogenous compounds.

RIA-linked microdialysis sampling in the awake rat: application to free-drug pharmacokinetics of hydrocortisone

The purpose of this research was to combine microdialysis sampling techniques with a highly sensitive radioimmunoassay (RIA) to study the in vivo kinetic response of pharmacologically important substances. This technique allowed for a dense sampling regimen from an awake, free-roaming experimental animal with no loss of blood and with rapid analysis of the dialysate. An important methodological criterion for accurate quantitation of a test drug in the extracellular space was knowledge of the relative recovery of the sampling system at the time of experimentation. Accordingly, the factors which influenced the recovery of drug during dense in vivo microdialysis sampling were examined and an analytical technique was developed to measure the instantaneous recovery of drug from the extracellular space. This information was applied to in vivo (iv) sampling experiments on anaesthetized and awake, free-roaming rats following bolus and multiple long-term iv administrations of the highly protein bound steroid (i.e. greater than 90%), hydrocortisone-21-phosphate. These studies indicated that unbound hydrocortisone levels as determined by the RIA-linked microdialysis (RIALM) technique fluctuated rapidly between each 2-min sampling interval, but nevertheless decreased to predose endogenous concentrations in a first-order fashion (t1/2 = 17-29 min). The rapid fluctuations of unbound hydrocortisone may reflect real pharmacokinetic or pharmacodynamic phenomena, attributed, perhaps, to reequilibration of the unbound drug pool with proteins and tissues in the blood.

Dual determination of extracellular cholecystokinin and neurotensin fragments in rat forebrain: microdialysis combined with a sequential multiple antigen radioimmunoassay

Microdialysis was combined with a highly sensitive sequential multiple antigen radioimmunoassay to simultaneously measure extracellular cholecystokinin and neurotensin fragments from discrete regions of the rat brain in vivo. The assay was conducted in 96-well plates and provided a limit of detection for both peptides of 0.1 fmol. Dialysis membranes composed of polyacrylonitrile, Cuprophan and polycarbonate were evaluated in vitro using both radiolabelled peptides and radioimmunoassay. Polycarbonate probes were implanted in the posterior medial nucleus accumbens-septum, medial caudate nucleus or medial prefrontal cortex of halothane-N2O-anaesthetized rats. Cholecystokinin immunoreactivity levels were generally above the assay detection limits (0.1-0.7 fmol) in 30-min samples from all three regions under basal conditions. Recovered basal amounts of neurotensin immunoreactivity were detectable in the nucleus accumbens-septum in approximately 50% of experiments (0.1-0.2 fmol) but were not measured in the caudate nucleus or prefrontal cortex. In the nucleus accumbens-septum, a 10-min pulse of 200 mM K(+)-containing artificial cerebrospinal fluid in the perfusion medium during a 30-min sampling period increased the recovered cholecystokinin and neurotensin immunoreactivity to 9.7 fmol +/- 1.9 S.E.M. and 5.8 +/- 1.6 S.E.M., respectively. A second stimulation following a 2.5-h interval produced similar elevations with S2:S1 ratios of 0.62 +/- 0.07 and 0.68 +/- 0.07 for cholecystokinin and neurotensin, respectively. In a separate series of experiments the second stimulation of both peptides was prevented by perfusion of a 10 mM EGTA-containing medium. Similar results were obtained in the caudate nucleus for cholecystokinin, but K(+)-induced elevations in neurotensin immunoreactivity were much smaller (0.5 fmol) in this brain region and calcium dependency was not established. Sequential K+ stimulations at 50, 100 and 200 mM produced progressively greater increases in recovered cholecystokinin and neurotensin immunoreactivity from the nucleus accumbens-septum and of cholecystokinin immunoreactivity from the prefrontal cortex. No neurotensin immunoreactivity was detected in the prefrontal cortex following K+ stimulation. Large post mortem increases in the recovered amounts of cholecystokinin and neurotensin immunoreactivity were observed. This effect was significantly attenuated by EGTA although there was a large calcium-independent component of the cholecystokinin immunoreactivity. On reverse-phase high-performance liquid chromatography the major cholecystokinin-immunoreactive peak co-eluted with sulphated cholecystokinin octapeptide. Neurotensin-immunoreactive material co-eluted with neurotensin (1-13), neurotensin (1-12), neurotensin (1-11), neurotensin (1-10) and neurotensin (1-8). These results further demonstrate the potential of microdialysis for studying neuropeptide release and metabolism in vivo when combined with sufficiently sensitive assay procedures.

Insulin-like growth factor I (IGF-I) distribution in the tissue and extracellular compartment in different regions of rat brain

The regional distribution of insulin-like growth factor I (IGF-I) was examined in the tissue and extracellular compartment of rat brain. The tissue content of IGF-I was the highest in the pituitary gland, followed by the olfactory bulb, upper brainstem, cerebellum, striatum, hippocampus, lower brainstem, and cerebral cortex. The extracellular concentration was studied by intracerebral microdialysis technique, and the highest content was found in the hippocampus, followed by the olfactory bulb, hypothalamus, cerebellum, striatum, and cerebral cortex. The tissue and extracellular contents were significantly correlated in the olfactory bulb, hypothalamus, cerebellum, striatum, and cerebral cortex. IGF-I might act by paracrine and/or autocrine regulatory mechanisms in these regions.

Microdialysis measurement of in vivo neuropeptide release

The application of the microdialysis sampling technique for in vivo measurement of neuropeptide release is discussed. In vitro recoveries of different microdialysis membranes are described for a variety of neuropeptides (ranging from 400 to 4500 MW) as well as their dynamic potential for measuring external concentration changes. In vivo experiments illustrating the use of the technique for measuring release of oxytocin and met-enkephalin from the brain of the conscious sheep, and vasopressin in the blood of the anaesthetised rat are described.

Physical and chemical considerations in the in vitro calibration of microdialysis probes for biogenic amine neurotransmitters and metabolites

The object of the present study was to examine the effects of temperature, oxidation, and pH on in vitro relative recovery of catecholamine and indoleamine neurotransmitters and their metabolites using microdialysis probes. Relative recovery of norepinephrine (NE), dihydroxyphenylacetic acid (DOPAC), 5-hydroxyindoleacetic acid (5HIAA), dopamine (DA), homovanillic acid (HVA), and 5-hydroxytryptamine (5HT) increased with temperature from 0 to 46 degrees C. For each compound, the increase in the amount recovered with increasing temperature was different. The stability of norepinephrine and dopamine was not affected at any temperature using deoxygenated calibration standard solutions containing ascorbic acid but was greatly reduced when exposed to ambient air without antioxidant treatment; catecholamine metabolites and the indole compounds were less affected. No change for in vitro relative recovery was observed by varying the pH of the perfusing solution from 6 to 8. Thus, temperature control in probe calibration as well as analyte stability using antioxidant treatment are important in reducing the error when estimating extracellular concentrations of neurotransmitter and metabolites.

Hemorrhage-induced regional brain thyrotropin-releasing hormone release in conscious rats measured by microdialysis

The septum, nucleus accumbens and preoptic area in the brains of conscious, freely moving rats were perfused using microdialysis probes. The TRH concentration significantly increased in the septum after withdrawal of 30% of the total blood volume but remained at constant levels in the other brain areas. Also, high potassium dose-dependently stimulated TRH release in vivo. These results suggest that blood loss stimulates septal TRH release, probably by membrane depolarization of TRH-containing nerve terminals.

Microdialysis as an approach to quantitate the release of neuropeptides

1. In vivo microdialysis was performed on anesthetized and awake rats to measured release of cholecystokinin from the posterior nucleus accumbens. 2. Basal levels of cholecystokinin were detectable by radioimmunoassay in some, but not all animals. 3. Recovery through the microdialysis probe ranged from 0.1-2.4% for cholecystokinin, suggesting practical limitations to this approach with present technology.

Intracerebral microdialysis: I. Experimental studies of diffusion kinetics

Intracerebral microdialysis is a brain perfusion technique in which a tubular, semipermeable membrane perfused with a physiological solution is implanted into a selected brain region. Molecules in the extracellular space diffuse into the perfusate and may be recovered and their concentration determined. Hence, the level of substances such as neurotransmitters may be monitored, and the response to different treatments may be studied. The technique also allows for administration of substances locally to the region of the brain surrounding the perfused tubular membrane. Basic principles of the microdialysis technique are described, and the results from methodological experiments are examined. It is concluded that there is a direct linear relation between the concentration of a molecule in the medium surrounding the dialysis membrane and the concentration measured in the collected perfusate. Relative changes of molecular concentration in brain extracellular space may be calculated even when the molecular diffusion rate is unknown. In addition, a method is presented for calculating the real concentration of a substance in the extracellular space from its concentration in the perfusate. Applied in striatum of rat brain using microdialysis in vivo, the average extracellular concentration of the following substances is estimated to be: substance P, 0.9 nM; dopamine, 1 microM; and dihydroxyphenylacetic acid, 0.05 mM.

Subchronic haloperidol treatment decreases the in vivo release of tachykinins in rat substantia nigra

Microdialysis combined with sensitive radioimmunoassays was used to measure the in vivo release of substance P (SP) and neurokinin A (NKA) in the rat substantia nigra. The effect of acute and subchronic haloperidol treatment (0.5 mg/kg) on the basal and potassium-evoked release was studied. No significant effect was observed after a single injection. However, pretreatment with haloperidol for 10 days decreased the potassium-induced release of SP and NKA by 25 and 27%, respectively. The basal overflow of NKA was reduced by 29%, while no significant effect could be seen on the basal release of SP.

Amphetamine facilitates the in vivo release of neurokinin A in the nucleus accumbens of the rat

Microdialysis combined with radioimmunoassay was used to measure the release of neurokinin A-like immunoreactivity (NKA-LI) in the rat brain in vivo. The effect of a single dose of amphetamine (2 mg/kg s.c.) on the basal overflow and the potassium-induced release of NKA-LI was assessed in the nucleus accumbens and caudate-putamen. Amphetamine potentiated the potassium-stimulated release of NKA-LI by 71% in the nucleus accumbens, while no significant change was observed in the caudate-putamen. Amphetamine did not affect the basal NKA-LI overflow.