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Biomedical subjects

N T Maidment

Publications and source records attributed to N T Maidment.

At least 55 records · Page 3Linked to original sources
PubMed1996

Intracerebroventricular orphanin FQ/nociceptin suppresses dopamine release in the nucleus accumbens of anaesthetized rats.

Cloning studies have identified a novel seven transmembrane receptor displaying high sequence homology to the three classical opioid receptors (mu, delta and kappa). This receptor is widely distributed throughout the CNS. 1 Recently, an endogenous ligand for this receptor was isolated (termed either "orphanin FQ" or "nociceptin") and identified as a heptadecapeptide showing sequence homology with the endogenous opioids. Surprisingly, in contrast to known opioids, orphanin FQ displays hyperalgesic rather than analgesic properties. Furthermore, in contrast to enkephalins and endorphins, but similarly to dynorphins, this peptide has inhibitory actions upon locomotor activity. These preliminary data suggest that orphanin FQ systems may act in an opposing manner to the previously well-described enkephalin and endorphin systems. Since numerous studies have implicated activation of the mesolimbic dopamine pathway to be central to the rewarding actions of opiates such as morphine and heroin, as well as several other abused drugs, and also to mediate the hyperlocomotory action of such drugs, we sought to determine the effect of orphanin FQ on this pathway. In accordance with the inhibitory effect of this peptide on locomotor activity, we now report that orphanin FQ suppresses dopamine release in the nucleus accumbens in a dose-dependent manner, providing the first neurochemical evidence for a modulatory role of this recently described peptide in the CNS.

Anesthesia↗
PubMed1996

Pentylenetetrazol-induced kindling: early involvement of excitatory and inhibitory systems.

Alterations in the brain of rats receiving a single non-convulsive administration pentylenetetrazol (PTZ), 30 mig/kg, i.p. (single PTZ group) were investigated and compared with those detected in fully PTZ kindled rats (chronic PTZ group). In vitro receptor autoradiography experiments showed that both single and chronic PTZ groups presented mu opioid and benzodiazepine (BDZ) receptor binding in specific brain areas. Using an antibody generated against the delta opioid receptor (DOR-1), it was found that DOR-1 like immunoreactivity was reduced in cortex and amygdala in mice following single and chronic PTZ administration. Microdialysis experiments revealed that the administration of PTZ 30 mg/kg, i.p. in freely moving rats without previous experience with the drug, induces a rise in glutamate release, detected in the first and second 10 min dialysates collected from amygdala (138% and 50%, respectively) and frontal cortex (70% and 45%, respectively) as well as aspartate in frontal cortex in the first and second PTZ-dialysates (143% and 80%, respectively). Subsequently, values returned to basal conditions. It may be speculated that decreased BDZ receptor binding results from enhanced release of GABA. On the other hand, the decrease of mu receptor binding and DOR-1 immunoreactivity observed after PTZ administration may be the result of enhanced levels of opioid peptides probably released over the kindling procedure. In conclusion, the present study indicates that PTZ-kindling is associated with an imbalance between excitatory and inhibitory systems which is apparent early in the epileptogenic process.

Amygdala↗
PubMed1996

Comparison of seizure related amino acid release in human epileptic hippocampus versus a chronic, kainate rat model of hippocampal epilepsy.

Recent microdialysis studies of excitatory and inhibitory amino acid release associated with paroxysmal hippocampal activity have found significant increases in the hippocampus of epileptic patients, but minimal or variable increases in animal models. One possible reason for the difference is that the animal models employed in these studies have not adequately reflected the pathophysiology of human epilepsy. The present study sought to verify the amino acid release reported in human epileptic hippocampus and then employs animal studies using a chronic rat model of epilepsy, in which rats exhibit spontaneous seizure activity 3 to 4 months after injection of kainic acid into the hippocampus. In agreement with earlier reports, we found increases in glutamate, aspartate and GABA during seizures in human hippocampus. In addition we found increases in taurine which have not previously been reported. The chronic rat model shows increases in the same amino acids as in the human epileptic hippocampus, both during spontaneous seizures and stimulation evoked after-discharges (ADs). In contrast, minimal increases are elicited by hippocampal stimulation in control (non-kainate injected) animals. These results correlate with the degree of mossy fiber reorganization found in the dentate gyrus of kainate rats or epileptic humans.

Amino Acids↗
PubMed1995

Microdialysis reveals a morphine-induced increase in pallidal opioid peptide release.

Freely moving rats were implanted with microdialysis probes in the globus pallidus/ventral pallidum, nucleus accumbens or caudate nucleus. Morphine (10 mg kg-1 i.p.) induced an average 128% increase in extracellular opioid peptide levels in the pallidum over a 2 h period peaking 1 h after injection. No change was observed in the nucleus accumbens or caudate nucleus. Dose-response analysis showed a smaller (26%), non-significant, increase at a lower dose of 2 mg kg-1 and no effect at a higher dose of 40 mg kg-1. No evidence of significant acute tolerance was apparent following repeated administration of morphine (10 mg kg-1, i.p.) at 3 h intervals. The magnitude of the morphine effect varied greatly between animals and was dependent on the pre-injection baseline opioid peptide levels.

Animals↗
PubMed1995

Long-term expression of a reporter gene from latent herpes simplex virus in the rat hippocampus.

A problem in utilizing herpes simplex virus (HSV) as a vector for expression of foreign genes in CNS neurons has been the inability to facilitate long-term expression of the engineered genes. Previously, we showed that the murine moloney leukemia virus LTR would drive beta-galactosidase (beta-gal) transcription for extended periods from the latent viral genome in sensory, but not motor neurons. In this communication we further evaluate the utility of the LTR promoter for use in long-term expression vectors. Following stereotactic injection of 8117/43 (an ICP4 minus, non-replicating virus with the LTR driving the beta-gal gene, or KD6 (an ICP4 minus non-replicating virus not expressing beta-gal) into the hippocampus of rats, polymerase chain reaction (PCR) analysis of viral DNA after 2 months indicated that latent infections were established. Assaying by both x-gal staining and reverse transcriptase PCR we demonstrate that (1) beta-gal can be detected for at least 6 months in hippocampal neurons, and (2) although the number of beta-gal transcripts in these cells drops considerably by 2 weeks, they can be detected during the period studied. These studies indicate that the LTR promoter is active and affords long-term expression in the CNS, albeit at comparatively low levels compared to those observed at acute times.

Animals↗
PubMed1994

Microdialysis reveals changes in extracellular opioid peptide levels in the amygdala induced by amygdaloid kindling stimulation.

Enkephalins released in basolateral nucleus of the amygdala in response to electrical stimulation were determined in amygdala kindled and nonkindled freely moving rats using microdialysis. Enkephalin release was enhanced after a single and repetitive electrical stimulation (233 and 130% above control levels, respectively) in nonkindled rats. In fully kindled rats, the extracellular enkephalin levels decreased (35% below the control levels) within the first 20 min after onset of stage V kindled seizures, reaching baseline level 40 to 60 min following the generalized seizure activity. HPLC analysis identified the majority of recovered immunoreactive material from the amygdala as Met-enkephalin. On the basis of our results it is suggested that the enhanced enkephalin release in the amygdala during the early kindling stages might have a suppressive effect which may represent a homeostatic mechanism to avoid the spread of the afterdischarge to other structures. The decreased extracellular level of enkephalin in the amygdala after stage V kindled seizures could reflect a general impairment of inhibitory mechanisms in this structure with subsequent production of seizure activity.

Amygdala↗
PubMed1994

Neural-targeted gene therapy for rodent and primate hemiparkinsonism.

Expression of the rate-limiting enzyme for catecholamine biosynthesis, tyrosine hydroxylase (TH), via retroviral and plasmid expression vectors improved the efficacy of conditionally immortalized nigral neural cells in ameliorating rodent and nonhuman primate models of Parkinson's disease through neural transplantation. No improvement in rotational behavior occurred when sham transplants or nondopaminergic transplants were performed. Transplantation of the temperature-sensitive immortalized parental nigral neural line with a TH expression vector resulted in improvement for at least 2 months. Improvement was accompanied by HPLC evidence of increased L-DOPA production and immunocytochemical evidence of TH in the transfected cells increased over that of the parental line. No tumor formation was detected. These results suggest that: (1) temperature-sensitive immortalized neural cells may be genetically engineered successfully to improve their efficacy for the treatment of parkinsonism; and (2) a change in L-DOPA production, as opposed to growth factor production or other factors, is likely to account for the observed improvement, since the parental and derived lines differ by a single gene.

Animals↗
PubMed1993

Profiles of extracellular amino acid changes in focal cerebral ischaemia: effects of mild hypothermia.

Mild hypothermia has been recently proposed as a therapeutic approach for ameliorating ischaemic cerebral damage. The protective potential of mild hypothermia, however, may be dependent on its ability to reduce the efflux of potentially excitotoxic amino acids and the severity of ischaemia. In this study, we examined the effects of mild brain hypothermia (33 degrees C) in a rabbit model of permanent focal ischaemia. In vivo microdialysis was used to measure extracellular amino acids in central and peripheral regions of the ischaemic cortex. In normothermic ischaemia (n = 7), glutamate, alanine, taurine, and phosphoethanolamine increased above baseline levels by about 2 h post-ischaemia. Mild hypothermia (n = 7) reduced glutamate efflux only in the central regions and increased alanine efflux in the peripheral regions of ischaemia. There were no significant differences in other amino acid levels between the two temperature groups. Haematoxylin-eosin histology did not demonstrate hypothermic protection in the ischaemic hemisphere. The lack of neuroprotection in this study may correspond with the sustained release of glutamate in the peripheral regions of ischaemia even with lowered brain temperature. These results suggest that hypothermic reduction of excitotoxic perturbations may be more important in the ischaemic periphery than the core.

Amino Acids↗
PubMed1993

Amphetamine and other weak bases act to promote reverse transport of dopamine in ventral midbrain neurons.

Amphetamine-like psychostimulants are thought to produce rewarding effects by increasing dopamine levels at mesolimbic synapses. Paradoxically, dopamine uptake blockers, which generally increase extracellular dopamine, inhibit amphetamine-induced dopamine overflow. This effect could be due to either inhibition of amphetamine uptake or inhibition of dopamine efflux through the transporter (reverse transport). We used weak bases and dopamine uptake blockers in ventral midbrain neuron cultures to separate the effects on blockade of amphetamine uptake from reverse transport of dopamine. Amphetamine, ammonium chloride, tributylamine, and monensin, at concentrations that produce similar reductions in acidic pH gradients, increased dopamine release. This effect was inhibited by uptake blockers. Although in the case of amphetamine the inhibition of release could have been due to blockade of amphetamine uptake, inhibition also occurred with weak bases that are not transporter substrates. This suggests that reduction of vesicular pH gradients increases cytoplasmic dopamine which in turn promotes reverse transport. Consistent with this model, extracellular 3,4-dihydroxyphenylacetic acid was increased by ammonium chloride and monensin, as would be expected with elevated cytoplasmic dopamine levels. These findings extend the weak base mechanism of amphetamine action, in which amphetamine reduces vesicular pH gradients resulting in increased cytoplasmic dopamine that promotes reverse transport.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1991

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.

Animals↗
PubMed1991

Postmortem changes in rat brain extracellular opioid peptides revealed by microdialysis.

Microdialysis combined with a solid-phase radioimmunoassay was used to monitor changes in extracellular opioid peptide levels in the rat globus pallidus/ventral pallidum as a result of terminal brain ischemia. Ischemia was induced by anesthetic overdose or by severance of blood vessels supplying the brain. In control animals the recovered immunoreactivity increased an average of 13-fold in the 30-min sample following anesthetic overdose. Perfusion of a calcium-free, 10 mM EGTA-containing medium through the dialysis probe significantly attenuated the amplitude of this response, with the average increase being only threefold. Shorter sampling intervals (5 min) indicated that release of opioid peptide material into the extracellular environment occurs within the first 5 min of ischemia resulting from severance of the blood supply to the brain. HPLC analysis identified the majority of the postmortem-induced immunoreactive material as Met- and Leu-enkephalin.

Anesthetics↗
PubMed1989

Microdialysis of extracellular endogenous opioid peptides from rat brain in vivo.

The combination of microdialysis and a highly sensitive radioimmunoassay was developed in order to monitor the in vivo extracellular levels of endogenous opioid peptides from discrete regions of the rat brain. The radioimmunoassay cross-reacts 100% with peptides with alpha N-acetyl Tyr.Gly.Gly.Phe-Met or -Leu at the N terminus and thus recognizes all known endogenous opioid peptide fragments following acetylation of the sample. The assay was conducted on solid phase with antibody bound via protein A to 96-well plates and provided a limit of detection of approximately 0.2 fmol. A variety of dialysis membranes were evaluated with respect to their efficiency in recovering opioid peptides in vitro. Custom-made probes (4 mm active length) manufactured from polyacrylonitrile membranes and commercially available polycarbonate membrane probes proved most suitable with relative recoveries for [Met]- and [Leu]enkephalin in the range 6-10% at a flow rate of 2.7 microliters/min. Probes implanted in the globus pallidus/ventral pallidum of halothane/N2O anaesthetized rats recovered approximately 1.5 fmol of immunoreactive opioid material per 30-min sample in the absence of peptidase inhibitors. The majority of this immunoreactivity co-eluted with [Met]- and [Leu]enkephalin on reverse-phase high-performance liquid chromatography. A 2-min pulse of 100 mM K(+)-containing artificial cerebrospinal fluid in the perfusion medium during a 30-min sampling period increased the recovered immunoreactive material to 43.9 fmol +/- 12.4 S.E.M. A second stimulation 3 h later also resulted in elevated levels with an S2:S1 ratio of 0.64 +/- 0.03. The second stimulation was completely blocked by perfusion of a 10 mM EGTA-containing medium, basal release on average remaining unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1988

Excitability properties of medial forebrain bundle axons of A9 and A10 dopamine cells.

A9 and A10 units identified as dopaminergic were recorded with extracellular micropipettes. The units were antidromically activated by electrical stimulation at the level of the preoptic area. The absolute refractory periods ranged from 1.2 to 2.5 ms. During the 2-8 ms of the relative refractory period, conduction was slower than normal by up to 1.5 ms. The time constant, C, of the strength-duration curve ranged from 0.4 to 0.6 ms. The current (I)-distance (D) relationship, tested by moving the stimulating electrode past the axon, was approximately parabolic (I = K D exp 2), with the constant of the equation, K, ranging from 900 to 2000 microA/mm exp 2, for 0.5 ms pulses. This relationship allows calculation of the radius of the field of dopamine axon excitation at any current. These high K values show that axons of dopamine cells cannot be activated unless high current densities are delivered, even when electrodes are placed near the axons. These data allow determination of the extent to which dopamine axons can be the directly activated substrates for behaviors, such as self-stimulation and circling, which are evoked by electrical stimulation of the medial forebrain bundle or internal capsule.

Action Potentials↗
PubMed1987

Repeated atypical neuroleptic administration: effects on central dopamine metabolism monitored by in vivo voltammetry.

Changes in extracellular DOPAC levels were monitored simultaneously in the nucleus accumbens and striatum of halothane/N2O anaesthetised rats using in vivo differential pulse voltammetry with carbon fibre electrodes following repeated administration of the atypical neuroleptics thioridazine and clozapine. Thioridazine (20 mg/kg s.c.) increased the DOPAC peak in the nucleus accumbens and striatum of rats treated with saline for the previous 21 days by 66% +/- 5 S.E.M. and 91% +/- 16 respectively. No such increase was recorded in the nucleus accumbens of rats previously treated with thioridazine (20 mg/kg s.c.) for 21 days. Similarly the increase in the striatum produced by a challenge dose on day 22 was markedly attenuated compared to controls although analysis of absolute DOPAC peak heights revealed extracellular DOPAC to be elevated above basal levels in this region (but not the nucleus accumbens) indicating a possible selective action of this drug to induce absolute tolerance to its acute effects in the nucleus accumbens after repeated administration. Administration of increasing doses of apomorphine (0.05, 0.1, 0.25 mg/kg s.c.) 1 h after a challenge dose of thioridazine (20 mg/kg s.c.) on day 22 to rats treated with the neuroleptic for the previous 21 days produced a progressive decrease in extracellular DOPAC levels both in the nucleus accumbens and striatum. Repeated administration of clozapine (50 mg/kg s.c.) for 21 days failed to induce tolerance to the acute effects of this drug, extracellular DOPAC levels increasing by 60% +/- 8 and 90% +/- 18 in the nucleus accumbens and striatum respectively following challenge with the drug on day 22.(ABSTRACT TRUNCATED AT 250 WORDS)

3,4-Dihydroxyphenylacetic Acid↗