Sympathetic alpha-adrenergic regulation of blood flow and volume in hamsters arousing from hibernation.

Mammals arousing from hibernation display pronounced regional heterothermy, where the thoracic and head regions warm faster than the abdominal and hindlimb regions. We used laser-Doppler flowmetry to measure peripheral hind foot blood flow during hibernation and arousal and gamma imaging of technetium-labeled albumin to measure whole blood volume distribution in hamsters arousing from hibernation. It was discovered that the hibernating hamster responds to physical but not to sound or hypercapnic stimulation with rapid, 73% reduction of hind foot blood flow. Hind foot blood flow vasoconstriction was maintained from the onset of arousal until late in arousal when rectal temperature was rapidly increased. alpha-Adrenergic blockade early in arousal increased hind foot blood flow by 700%, suggesting that vasoconstriction was mediated by activation of sympathetic tone. Gamma imaging revealed that, by the early phase of arousal from hibernation, the blood volume of the body below the liver is greatly reduced, whereas blood volumes of the thorax and head are much greater than corresponding volumes in anesthetized hamsters. As arousal progresses and cardiac activity increases and regional heterothermy develops, this regional blood volume distribution is largely maintained; however, blood volume slowly decreases in the thoracic region and slowly increases in the shoulder and head regions. The rapid increase in rectal temperature, characteristic of mid- to late- arousal phases, is probably mediated, in part, by reduction of adrenergic tone on abdominal and hindlimb vasculature. Warm blood then moves into the hind body, produces an increase in temperature, blood flow, and blood volume in the hind body and compensatory reductions of blood volume in the neck, head, and thoracic regions.

Determination in vivo of newly synthesized gene expression in hamsters during phases of the hibernation cycle.

This study measured in vivo synthesis of total RNA and protein from cortex, cerebellum and midbrain/brainstem and 6 major organs from Syrian hamsters (Mesocricetus auratus) during (a) 33 h of torpor (body temperature 5-6 degrees C); (b) 90 min of the early arousal; (c) 90 min of the middle arousal; (d) 90 min in cold adapted cenothermic (CEN) hamsters of the same circannual period. Appropriate physiological parameters were used to confirm the phase of the hibernation cycle during infusion and incorporation of [3H]-uridine and [14C]-leucine. In torpor, RNA synthesis was 5-25% of CEN levels depending upon tissue. In brain and heart mRNA was not preferentially synthesized. Protein was synthesized at low, tissue specific levels during torpor. Initiation of arousal and the warming of anterior organs via non-shivering thermogenesis during the early arousal occurred without measurable synthesis of RNA or proteins. Tissue specific levels of RNA and protein synthesis occurred later after shivering thermogenesis had been recruited and was strongly influenced by thermal gradients in the body. In the middle arousal phase, protein synthesis is most active in the brain despite modest synthesis of RNA and mRNA. The majority of molecular processing required for the induction and maintenance of torpor and the arousal from torpor up until the onset of shivering thermogenesis occurs during the cenothermic period before the hamster initiates the hibernation cycle.

State-dependent regulation of cortical blood flow and respiration in hamsters: response to hypercapnia during arousal from hibernation.

Hibernation is characterised by a global reduction of metabolism, body temperature and blood flow, while arousal from hibernation is achieved by the reversal of these processes. Our experiments were performed on Syrian hamsters that had been chronically implanted with a cortical thermocouple and an optical fibre over the contralateral cortex, and acutely implanted with thermocouples in the rectal, cheek pouch and interscapular brown adipose tissue (BAT). Measurements revealed large thermal gradients in the body of the arousing animals. Maximum whole-body metabolic rate, which was 2.4 times normal cenothermic resting metabolic rate, coincided not with rectal temperature but more closely with respiratory rate (RR) or BAT temperature. Regional cortical blood flow (rCBF), as measured by laser-Doppler flowmetry, changed in parallel with whole-body metabolic rate, peaking at 3.8 times the normal cenothermic resting levels, when rectal temperature was 15 degrees C. When BAT temperature was less than 25 degrees C, RR, rCBF and heart rate (HR) were decreased by breathing hypercapnic gas, but these parameters were unresponsive to hyperoxic gases. At cenothermia the RR and rCBF of anaesthetised hamsters was increased by exposure to hypercapnic gases. Exposure to hyperoxic gas decreased RR but had no effect on rCBF. The mechanisms regulating rCBF, HR and RR exhibit state-dependent sensitivities to hypercapnic and hyperoxic stimuli. The large increase in rCBF observed during arousal implies that cerebral autoregulation is temporarily suspended and suggests that hamsters effectively use endogenous mechanisms to minimise the pathology normally associated with dramatic increases in rCBF.

Oxygen-sensing microdialysis probe for in vivo use.

Electrochemical conditions were optimized to allow the metal tube used for the shaft of commercial microdialysis (MD) probes to be coated with gold. In in vitro tests with phosphate-buffered Ringer's solution using double differential pulse amperometry (DDPA), the gold-coated shafts were capable of specifically measuring the reduction of oxygen and the oxidation of ascorbic acid in the presence of high concentrations of potentially interfering endogenous substances. By using fixed-potential amperometry (FPA), the gold-plated shaft also measured oxygen with minimal interference from high concentrations of potentially interfering endogenous substances. Concentric design MD probes were constructed that used a metal shaft (O.D = 0.4 mm), fused silica inlet and outlet tubes, and a 1.5 mm dialyzing membrane (O.D = 0.2 mm). A 0.5-0.7 mm gold collar was electroplated onto the metal shaft approximately 0.5 mm above the dialyzing membrane. The nongold outer surface of the MD probe was coated with an insulating polymer. In vivo tests demonstrated that DDPA was not suitable for use with this gold microdialyzing electrode (GMDE). However, brain oxygen levels were satisfactorily measured using FPA. In urethane-anesthetized rats, the reduction current to oxygen in the striatum was increased by brief (1 min) inhalation of O2 or CO2 and decreased by inhalation of N2. Transient application of noxious stimuli (foot pinch) increased cerebral O2, whereas bilateral carotid artery occlusion and death decreased striatal O2. The responses of the GMDE were indistinguishable from the reduction current simultaneously measured from a conventional carbon fiber electrode implanted adjacent to the gold-plated area of the MD shaft. Basal levels of striatal O(2) were 20 +/- 5 microM (n = 4) for the GMDE and 30 +/- 11 microM (n = 3) for the carbon fiber. The GMDE was robust and could be used for at least three animals. This technique can be used to provide information about the oxygen status of the tissue adjacent to the dialyzing membrane without the need for implantation of an additional electrode.

Determination of striatal extracellular gamma-aminobutyric acid in non-hibernating and hibernating arctic ground squirrels using quantitative microdialysis.

This study determined extracellular concentrations of gamma-aminobutyric acid ([GABA](ecf)) in striatum of non-hibernating and hibernating arctic ground squirrels to test the hypothesis that an increase in [GABA](ecf) was associated with profound CNS depression during hibernation. Quantitative microdialysis procedures were employed to circumvent the effects of low temperature on the relative recovery of the analyte across the dialysis membrane and yielded for the first time quantitative in vivo estimates of [GABA](ecf) in any brain region or any species. Laboratory housed, wild caught Arctic ground squirrels (Spermophilus parryii) were implanted intraperitoneally with radio transmitters that enabled the telemetric monitoring of activity and core body temperature (T(b)) and bilaterally implanted with cranial guide tubes that enabled the implantation of microdialysis probes into the striatum. Striatal [GABA](ecf) was determined in unrestrained, non-hibernating ground squirrels (T(b) range 34.7-38.9 degrees C) and hibernating ground squirrels (T(b) range 2.9-3.9 degrees C) using extrapolation to zero flow and very slow flow microdialysis techniques. The results show that [GABA](ecf) in non-hibernating squirrels was 73 nM and this level was decreased by approximately 50% during hibernation thereby suggesting that an increase in [GABA](ecf) does not play a major role in CNS depression during hibernation. The reduction of [GABA](ecf) parallels a decrease in plasma and CSF [glucose] and may be related to a decrease in GABA synthesis or reduced voltage dependent release. This paper demonstrates that measurement of extracellular concentrations of neurotransmitters in animals with vastly different body temperatures is possible using microdialysis techniques of extrapolation to zero flow or very slow flow rates that enable 100% recovery. Such quantitative techniques may prove valuable in the study of the neurochemistry of the cerebral mechanisms of hibernation and tolerance to cerebral ischemia exhibited by hibernating animals.

Ascorbate and glutathione regulation in hibernating ground squirrels.

Ground squirrels withstand up to 90% reductions in cerebral blood flow during hibernation as well as rapid reperfusion upon periodic arousals from torpor. Metabolic suppression likely plays a primary adaptive role which allows hibernating species to tolerate such phenomena. However, several other aspects of hibernation physiology are also consistent with tolerance to dramatic fluctuations in cerebral blood flow, suggesting that multiple neuroprotective adaptations may work in concert during hibernation. The purpose of the present work was to study the dynamics of the low molecular weight antioxidants, ascorbate and glutathione (GSH), during hibernation. Alterations in concentrations of ascorbate during hibernation and arousal in two species of hibernating ground squirrels suggest that it could play a protective role during hibernation or arousal. Samples were collected during the hibernation season from arctic ground squirrels (AGS; Spermophilus parryii) and 13-lined ground squirrels (TLS; S. tridecemlineatus) during prolonged torpor and in squirrels that did not hibernate or had not been hibernating for several weeks. We determined antioxidant levels in plasma, cerebrospinal fluid (CSF), and in frontal cortex, hippocampus and cerebellum using high-performance liquid chromatography (HPLC). Plasma ascorbate concentrations increased dramatically (3-4-fold) in both species during hibernation and rapidly returned to prehibernation levels upon arousal. By contrast, plasma GSH concentrations fell slightly or remained stable during hibernation. Ascorbate levels in the CSF doubled in hibernating AGS (not determined in TLS), while brain ascorbate content fell slightly (10-15%) in both species. Substantial increases in plasma and CSF ascorbate concentrations suggest that this antioxidant could play a protective role during hibernation and reperfusion upon arousal from hibernation.

Disposable, enzymatically modified printed film carbon electrodes for use in the high-performance liquid chromatographic-electrochemical detection of glucose or hydrogen peroxide from immobilized enzyme reactors.

Disposable screen-printed, film carbon electrodes (PFCE) were modified with cast-coated Osmium-polyvinylpyrridine-wired horse radish peroxidase gel polymer (Os-gel-HRP) to enable the detection of the reduction at 0 mV of hydrogen peroxide (H2O2) derived from a post-column immobilized enzyme reactor (IMER) containing acetylcholinesterase and choline oxidase. In another series of experiments PFCE were initially modified with cast-coated Os-gel-HRP and then treated with glucose oxidase in bovine serum albumin (BSA) and cross-linked with glutaraldehyde to form a bi-layer glucose-Os-gel-HRP PFCE. This bi-layer glucose-Os-gel-HRP PFCE generated a reduction current at 0 mV to H2O2 derived from the reaction of glucose oxidase and glucose in solution. These enzyme-modified PFCE were housed in a radial flow cell and coupled with cation-exchange liquid chromatographic methods to temporally separate substrates in solution for the determination of acetylcholine (ACh) and choline (Ch) in the first experimental series, or glucose in the second experimental series. These two disposable enzyme-modified PFCE exhibited linear current vs. substrate relations, were durable, being usable for approximately 40 determinations, and were sufficiently sensitive to be employed in biological sampling. Both assays utilized the same HPLC equipment. The limit of detection for ACh was 16 fmol/10 microl and that for glucose was 12 micromol/7.5 microl. ACh and Ch were measured from a microdialysate from the frontal cortex of a rat. Glucose in human urine was determined using the bi-layer glucose oxidase-Os-gel-HRP PFCE.

Tolerance to catalepsy following chronic haloperidol is not associated with changes in GABA release in the globus pallidus.

In vivo microdialysis was used to investigate the relationship between extracellular GABA levels in rat globus pallidus following acute (1st injection) and chronic (29th injection) haloperidol (Hal) (0.25 mg kg-1 day-1, s.c.) with the presence and absence of catalepsy, respectively. There was no difference in basal pallidal GABA levels in the drug naive and chronically treated rats. Furthermore, pallidal GABA release was not affected following injection with Hal in either group although there was a prolonged catalepsy in the drug naive group and a tolerance to catalepsy in the chronically treated group. A previous microdialysis study employing similar experiment protocol showed that Hal increases striatal GABA release in drug naive rats and increases basal striatal GABA levels following chronic treatment. The results of the current study demonstrate that these effects are not reflected in the globus pallidus and suggest that striatal GABA interneurons and/or GABA projection neurons to extrapallidal nuclei such as the substantia nigra pars reticulata may be involved in initiating catalepsy following acute Hal and mediating the tolerance to catalepsy observed following chronic Hal.

Plastic film carbon electrodes: enzymatic modification for on-line, continuous, and simultaneous measurement of lactate and glucose using microdialysis sampling.

Ring and split-disk plastic film carbon electrodes (PFCEs) were fabricated for use in thin-layer radial flow cells which were coupled to a microdialysis sampling system. PFCEs, were initially coated with osmium poly(vinylpyridine) redox polymer horseradish peroxidase (Os-gel-HRP). Then a second coat of oxidase enzyme was applied to produce enzyme bilayer (oxidase/Os-gel-HRP) PFCEs which were subsequently over-coated with cellulose acetate for use in the determination of glucose or lactate at 0 mV (vs Ag/AgCl). Split-disk electrode geometry enabled different oxidase enzymes to be immobilized on each half of a split-disk, Os-gel-HRP-coated, PFCE to facilitate the electrochemically independent yet continuous on-line determination of these two analytes from a single dialysate. In continuous-flow experiments, cellulose acetate overcoated oxidase/Os-gel-HRP cast-coated PFCEs were quick to stabilize background current and displayed linear and sensitive responses to substrates. The effect of ascorbic acid was minimal and cross talk between partner split-disk electrodes was demonstrated to be acceptable for in vivo applications. The utility of this analytical system is demonstrated by the quantitative on-line continuous assay of changes in dialysate striatal extracellular glucose and lactate from a conscious rat during (a) local stimulation of neurons by perfusion with the depolarizing agent, Veratridine, and (b) physical restraint.

Hippocampal and striatal blood flow during behavior in rats: chronic laser Doppler flowmetry study.

A technique is described for the chronic measurement of cerebral blood flow in conscious, unrestrained rodents, utilizing laser doppler flowmetry (LDF) removably coupled to an optical fiber permanently implanted into brain tissue by established stereotaxic procedures. Changes in relative blood flow in response to a range of pharmacological and behavioral challenges were measured in the hippocampus (HBF) and striatum (StBF) 24-72 h and up to 28-32 days after surgical implantation of the optical fiber. Intraseptal microinfusion of L-glutamate in artificial cerebrospinal fluid 48-96 h and 28-32 days after surgery increased HBF. Pentobarbital (Nembutal) and urethane anesthesia decreased HBF. On the day of euthanasia under urethane anesthesia, HBF was demonstrated to be responsive to alteration of blood CO2 via hyper/hypocapnia, and autoregulation was demonstrated in response to hypovolemic hypotension. In behavioral experiments, blood flow was found to increase with activity and locomotion, as well as during paradoxical (PS) and slow-wave sleep (SWS). The greatest increase in CBF was measured during PS. Although basal levels of blood flow were similar between regions, the increase in blood flow during PS was greater in the hippocampus. This simple procedure enables real-time measurement of qualitative changes in regional cerebral blood flow during behaviors in conscious, unrestrained animals. The observation that constancy of measurements was obtained for 1 month enables within-subject analysis in longitudinal studies and reduces the number of animals required for investigations.

On-line, continuous measurement of extracellular striatal glucose using microdialysis sampling and electrochemical detection.

A sensitive, enzymatic glucose electrode was coupled with the microdialysis sampling technique to enable the continuous, on-line measurement of dialysate glucose. The glucose sensitive electrode was fabricated by immobilizing glucose oxidase onto the surface of an osmium-polyvinylpyrridine horse radish peroxidase gel (Os-gel-HRP) which had been cast coated onto a glassy carbon electrode. This 'bilayer' electrode generated a reductive current to glucose at a potential of 0 mV thereby minimizing faradic oxidative interferences. The system utilized the continuous mixing of two fluids immediately prior to the 'bilayer' electrode. One fluid was the dialysate. The other was an oxygenated, low pH phosphate buffer which minimized oxidative interference, buffered the electrode from variations of pH and maximized enzyme efficiency. In practical terms, the 'bilayer' electrode was simple to manufacture, quick to reach stable basal currents (less than 60 min), sensitive (2.5 microM glucose could be detected in the dialysate) and durable (usable for up to 3 days). In vivo experiments, used the smallest commercially available microdialysis probes to demonstrate that on-line, continuous measurements of EC striatal glucose in the dialysate were receptive to pharmacological (local perfusion with veratridine (50 microM), systemic hyperglycemia (1.5 ml of 0.55 M glucose intraperitoneal (i.p.)) and anesthesia (Nembutal 40 mg/kg i.p.)) and behavioral (restraint) manipulations. This technique allows for greater temporal resolution than conventional HPLC procedures whilst requiring significantly less technical outlay or analytical expertise. The high sensitivity of the analytical technique could facilitate the study of EC glucose levels in very localized regions of the brain if coupled to microdialysis probes of small dimensions.

Detection of basal acetylcholine release in the microdialysis of rat frontal cortex by high-performance liquid chromatography using a horseradish peroxidase-osmium redox polymer electrode with pre-enzyme reactor.

To determine the basal acetylcholine level in the dialysate of rat frontal cortex, a horseradish peroxidase-osmium redox polymer-modified glassy carbon electrode (HRP-GCE) was employed instead of the conventional platinum electrode used in high-performance liquid chromatography-electrochemical detection (HPLC-ED). In initial experiments, an oxidizable unknown compound interfered with the detection of basal acetylcholine release on HPLC-HRP-GCE. An immobilized peroxidase-choline oxidase precolumn (pre-reactor) was included in the HPLC system, to eliminate the interference from the unknown compound. This combination could detect less than 10 fmol of standard acetylcholine and basal acetylcholine levels in the dialysate from a conventional concentric design microdialysis probe, without the use of cholinesterase inhibitor, and may facilitate physiological investigation of cholinergic neuronal activity in the central nervous system.

GABAergic system inducing hyperthermia in the rat preoptic area: its independence of prostaglandin E2 system.

Brain temperature of conscious freely moving rats was recorded during perfusion of the preoptic area (POA) with neuroactive compounds using the microdialysis technique. Unilateral perfusion of the POA with the sodium channel blocking agent, tetrodotoxin (1 microM), induced a pronounced hyperthermia. Of the neuroactive compounds examined, the greatest thermogenic response to local perfusion of the POA was elicited by the GABAergic agonist, muscimol. Muscimol (10, 20 and 100 microM) exhibited a dose-dependent and reversible hyperthermia. This hyperthermia was attenuated by co-perfusion with the GABAergic antagonist, bicuculline (10 microM). Muscimol-induced hyperthermia was independent of prostaglandin biosynthesis, and additive with prostaglandin E2 (10 microM)-induced hyperthermia. Prostaglandin E2-induced hyperthermia was not affected by co-perfusion with bicuculline. These data suggest the existence of two independent neurochemical systems for genesis of hyperthermia colocalized within the POA.

A GABAergic mechanism in the medial septum influences cortical arousal and locomotor activity but not a previously learned spatial discrimination task.

The effect of perfusion of the medial septum (MS) with artificial cerebrospinal fluid (CSF) on three consecutive daily trials was assessed on the amount of sleep/wake as determined by measurement of electro-encephalographic activity (EEG), spontaneous locomotor activity as determined by open field test and spatial discrimination task as determined by Morris water maze performance. Perfusion of the MS on the fourth trial with the GABAA agonist, muscimol (10-100 microM) produced an increase in cortical arousal and increased spontaneous locomotor activity. Perfusion with muscimol (5 microM) had an effect not distinguishable from perfusion with CSF alone. However doses of muscimol that reduced slow wave sleep (SWS) and increased locomotion had no effect on the memory and performance of a learned spatial discrimination task.

Perfusion of the preoptic area with muscimol or prostaglandin E2 stimulates cardiovascular function in anesthetized rats.

The effect of unilateral perfusion of the preoptic area (POA) utilizing in vivo microdialysis with the GABAergic agonist, muscimol, or prostaglandin E2 (PGE2) on cardiovascular function (heart rate, blood pressure and cutaneous blood flow) was determined in halothane-anesthetized rats. Perfusion of muscimol or PGE2 increased the heart rate, 55 +/- 6 beats/min and 69 +/- 12 betas/min, respectively (P < 0.01). Cutaneous paw blood flow tended to decrease. Blood pressure did not change in response to the perfusion of muscimol, but increased 9 mmHg in response to the perfusion of PGE2 (P < 0.01). The increases in heart rate and blood pressure were blocked by systemic administration of propranolol or adrenalectomy (P < 0.01), suggesting that these effects were due to the activation of the sympathetic innervation of the heart or adrenal gland. It is proposed that the POA provides a tonic inhibitory afferent to posterior hypothalamus neurons which regulate cardiovascular function.

Acute versus chronic haloperidol: relationship between tolerance to catalepsy and striatal and accumbens dopamine, GABA and acetylcholine release.

Using in vivo microdialysis, changes in extracellular dorsolateral striatum and nucleus accumbens dopamine, GABA and acetylcholine following acute and chronic haloperidol (0.25 mg/kg, s.c.) were evaluated in rats concurrent with the measurement of catalepsy. When administered to drug-naive and chronically treated rats, haloperidol was associated with a consistent and prolonged (> 150 min) increase in dorsolateral striatum and nucleus accumbens DA release and a transient (60 min) increase in dorsolateral striatum GABA release. Haloperidol was also associated with a transient (30 min) increase in dorsolateral striatum acetylcholine release in the chronically treated rats. Basal dopamine and acetylcholine levels were similar in both brain regions; however, basal dorsolateral striatum GABA levels were two-fold higher in the chronically treated rats. Administration of haloperidol was associated with a prolonged (> 150 min) catalepsy in the drug-naive rats which was greatly diminished or absent in chronically treated rats. Additionally, serum haloperidol levels were shown to be similar 120 min following administration of haloperidol in both groups. These results indicate a marked behavioral difference in the effects of haloperidol in drug-naive and chronically treated rats which is not related to an altered bioavailability of the drug and which is dissociated from both basal and haloperidol induced effects on dopamine and acetylcholine release in both brain regions. However, the selective elevation of basal dorsolateral striatum GABA release following chronic administration of haloperidol may contribute to the development of tolerance to catalepsy as well as providing an in vivo neurochemical marker of the long-term effects of haloperidol.

Behavioral activation by stimulation of a GABAergic mechanism in the preoptic area of rat.

The locomotor activity and grooming of conscious freely moving rats were recorded during a 60-min unilateral perfusion of the preoptic area with neuroactive compounds using the microdialysis technique. The GABA agonist, muscimol (10, 20 and 100 microM) induced a dose-dependent increase in locomotor activity and grooming which was attenuated by co-perfusion with the GABA antagonist, bicuculline (10 microM), and was blocked by systemic injection of haloperidol, a preferential dopamine D2 receptor antagonist (0.25 mg/kg). Muscimol-induced hyperactivity was associated with a simultaneous increase of striatal extracellular dopamine. These data suggest that the preoptic area is functionally linked with the extrapyramidal dopaminergic system possibly via GABAergic system.

Evidence for a substrate of neuronal plasticity based on pre- and postsynaptic neurotensin-dopamine receptor interactions in the neostriatum.

The major mechanism underlying the neuroleptic action of the tridecapeptide neurotensin (NT) appears to be an interaction with dopamine receptor mechanisms based on biochemical binding and behavioral experiments. In vivo microdialysis was used in conscious rats to investigate the effects of local perfusion with NT on the sensitivity of striatal dopamine D1 and D2 receptors for their selective agonists by monitoring extracellular dopamine, 3,4-dihydroxyphenylacetic acid, homovanilic acid, and gamma-aminobutyric acid levels in the awake unrestrained male rat. Perfusion with NT (10 nM) counteracted the inhibitory effects of the dopamine D2 agonist pergolide (500 nM) on extracellular levels of dopamine and gamma-aminobutyric acid. In contrast, NT (10 mM) significantly enhanced the reduction of extracellular striatal levels of dopamine after perfusion with the D1 agonist SKF 38393 (5 microM), and this combined treatment also resulted in a significant increase in the extracellular striatal levels of gamma-aminobutyric acid. These results provide in vivo evidence that NT regulates central dopamine transmission by reducing pre-and postsynaptic dopamine D2 and enhancing D1 receptor sensitivity possibly through an antagonistic NT receptor-D2 receptor interaction. This heteroregulation has the potential to substantially increase the plasticity within the dopamine synapse.

In vivo characterisation of extracellular dopamine, GABA and acetylcholine from the dorsolateral striatum of awake freely moving rats by chronic microdialysis.

Basal extracellular (EC) DA, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), gamma aminobutyric acid (GABA) and acetylcholine (ACh) were measured in dialysates from the dorsolateral striatum (DLS) of awake rats, every 30 min for 4.5 h each day over a 4-day period. The responsiveness of basal EC DA, DOPAC, HVA and GABA to local perfusion with tetrodotoxin (1 micron) was measured 1 and 4 days after implantation. In addition EC ACh was also measured 4 days after probe implantation. The results of this study indicate that EC levels of DA, DOPAC, HVA, GABA and ACh can be reliably monitored for up to 4 days after probe implantation. In addition, we show that striatal EC levels of DA, GABA and ACh may be regarded as a reflection of ongoing neuronal activity for up to 4 days after implantation of a microdialysis probe.