CSF GABA is reduced in first-episode psychosis and associates to symptom severity.

Schizophrenia is characterized by a multiplicity of symptoms arising from almost all domains of mental function. γ-Aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the brain and is increasingly recognized to have a significant role in the pathophysiology of the disorder. In the present study, cerebrospinal fluid (CSF) concentrations of GABA were analyzed in 41 first-episode psychosis (FEP) patients and 21 age- and sex-matched healthy volunteers by high-performance liquid chromatography. We found lower CSF GABA concentration in FEP patients compared with that in the healthy volunteers, a condition that was unrelated to antipsychotic and/or anxiolytic medication. Moreover, lower CSF GABA levels were associated with total and general score of Positive and Negative Syndrome Scale, illness severity and probably with a poor performance in a test of attention. This study offers clinical in vivo evidence for a potential role of GABA in early-stage schizophrenia.

Repeated LPS Injection Induces Distinct Changes in the Kynurenine Pathway in Mice.

The immune system has been recognized as a potential contributor to psychiatric disorders. In animals, lipopolysaccharide (LPS) is used to induce inflammation and behaviors analogous to some of the symptoms in these disorders. Recent data indicate that the kynurenine pathway contributes to LPS-induced aberrant behaviors. However, data are inconclusive regarding optimal LPS dose and treatment strategy. Here, we therefore aimed to evaluate the effects of single versus repeated administration of LPS on the kynurenine pathway. Adult C57BL6 mice were given 0.83 mg/kg LPS as a single or a repeated injection (LPS + LPS) and sacrificed after 24, 48, 72, or 120 h. Mice receiving LPS + LPS had significantly elevated brain kynurenine levels at 24 and 48 h, and elevated serum kynurenine at 24, 48 and 72 h. Brain kynurenic acid and quinolinic acid were significantly increased at 24 and 48 h in mice receiving LPS + LPS, whereas serum kynurenic acid levels were significantly decreased at 24 h. The increase of brain kynurenic acid by LPS + LPS was likely unrelated to the higher total dose as a separate group of mice receiving 1.66 mg/kg LPS as single injection 24 h prior to sacrifice did not show increased brain kynurenic acid. Serum quinolinic acid levels were not affected by LPS + LPS compared to vehicle. Animals given repeated injections of LPS showed a more robust induction of the kynurenine pathway in contrast to animals receiving a single injection. These results may be valuable in light of data showing the importance of the kynurenine pathway in psychiatric disorders.

Plasticity of the central nervous system (CNS) following perinatal asphyxia: does nicotinamide provide neuroprotection?

We have investigated the idea that nicotinamide, a non-selective inhibitor of the sentinel enzyme Poly(ADP-ribose) polymerase-I (PARP-1), provides neuroprotection against the long-term neurological changes induced by perinatal asphyxia. Perinatal asphyxia was induced in vivo by immersing foetuses-containing uterine horns removed from ready-to-deliver rats into a water bath for 20 min. Sibling caesarean-delivered pups were used as controls. The effect of perinatal asphyxia on neurocircuitry development was studied in vitro with organotypic cultures from substantia nigra, neostriatum and neocortex, platted on a coverslip 3 days after birth. After approximately one month in vitro (DIV 25), the cultures were treated for immunocytochemistry to characterise neuronal phenotype with markers against the N-methyl-D-aspartate receptor subunit 1 (NR1), the dopamine pacemaker enzyme tyrosine hydroxylase (TH), and nitric oxide synthase (NOS), the enzyme regulating the bioavailability of NO. Nicotinamide (0.8 mmol/kg, i.p.) or saline was administered to asphyctic and caesarean-delivered pups 24, 48 and 72 h after birth. It was found that nicotinamide treatment prevented the effect of perinatal asphyxia on several neuronal parameters, including TH- and NOS-positive neurite atrophy and NOS-positive neuronal loss; supporting the idea that nicotinamide constitutes a therapeutic alternative for the effects produced by sustained energy-failure conditions, as occurring during perinatal asphyxia.

Blunted response to cocaine in the Flinders hypercholinergic animal model of depression.

The Flinders sensitive line (FSL) rat is a proposed genetic hypercholinergic animal model of human depression. Considering the strong comorbidity between depression and cocaine dependence we investigated the well-documented behavioral and molecular effects of cocaine in the FSL and their control Flinders resistant line (FRL) rats. First, we found no difference between the two lines to establish cocaine self-administration; both lines reached stable responding within 10 days of training at a fixed ratio-1 schedule of reinforcement (1.5 mg/kg/injection). However, the FSL rats exhibited reduced cocaine intake at a dose of 0.09 mg/kg/injection in a within-session dose-response curve (0.02, 0.09, 0.38, 1.5 mg/kg/injection). Second, we examined the effects of repeated cocaine administration on locomotor activity, dopamine overflow and striatal prodynorphin mRNA expression. We found the FSL rats to be low responders to novelty and to exhibit less locomotor activation after repeated cocaine administration (30 mg/kg, i.p., daily injections for 10 days) than their controls. Microdialysis sampling from the nucleus accumbens shell revealed no significant difference in the dopamine overflow between the rat lines, neither during baseline nor after cocaine stimulation. Postmortem analyses of striatal prodynorphin mRNA expression (using in situ hybridization histochemistry) revealed a differentiated response to the cocaine exposure. In contrast to control FRL rats, the FSL rats showed no typical cocaine-evoked elevation of prodynorphin mRNA levels in rostral subregions of the striatum whereas both strains expressed increased prodynorphin mRNA levels in the caudal striatum after cocaine administration. In conclusion, the FSL animal model of depression demonstrates marked blunting of the locomotor and dynorphin neuroadaptative responses to cocaine in accordance with its enhanced cholinergic sensitivity.

On the release of glutamate and aspartate in the basal ganglia of the rat: interactions with monoamines and neuropeptides.

Using highly sensitive analytical procedures, glutamate (Glu), aspartate (Asp) and several putative neurotransmitters and metabolites can be monitored simultaneously in the extracellular space of neostriatum, substantia nigra and cerebral cortex of the rat by in vivo microdialysis. Glu and Asp are found at sub-micromolar concentrations in all investigated brain regions. In order to ascertain their neuronal origin, we have extensively studied the sensitivity of extracellular Glu and Asp levels to: (i) K(+)-depolarization, (ii) Na(+)-channel blockade, (iii) removal of extracellular Ca2+, (iv) depletion of presynaptic vesicles, and (v) integrity of neuronal pathways. The relevance of these criteria for several neurotransmitters monitored simultaneously or in parallel experiments has also been examined. The functional interactions among different neuronal pathways in the basal ganglia are studied by using selective pharmacological treatments, administered systemically, or locally via intracerebral injections or the microdialysis perfusion medium. Immunohistochemical evidence for the existence of Glu and/or Asp neuronal pathways in the basal ganglia of the rat is presented, discussing especially new findings indicating the existence of a Glu-independent Asp system, intrinsic to the neostriatum of the rat. The clinical relevance of these interactions is discussed, focusing on the implications for the treatment of neurodegenerative disorders affecting the basal ganglia.

Neurocircuitry of the basal ganglia studied by monitoring neurotransmitter release. Effects of intracerebral and perinatal asphyctic lesions.

The neurocircuitries of the basal ganglia are studied with in vivo microdialysis, with special consideration to dopamine transmission and its interaction with other neurotransmitter systems. The aim is to develop experimental models to study the pathophysiology and therapy of neurodegenerative disorders of the basal ganglia, as well as to develop models to study the short- and long-term consequences of perinatal asphyctic lesions. A main goal of these studies is to find and to characterize new treatments for these disorders.

Induction of c-FOS immunoreactivity in the hippocampus following potassium stimulation.

In microdialysis procedures high potassium ion concentrations are generally used to induce neurotransmitter release. However, the widespread effects, if any, of such a treatment have not been described. In order to establish a possible link between c-fos expression and stimulating conditions for neurotransmitter release in microdialysis procedures we administered KCl (100 mM) into the hippocampus. Proto-oncogene c-FOS-like immunoreactivity is upregulated in granule cells of the dentate gyrus, pyramidal cells of the hippocampus, cingulate, piriform and frontoparietal cortices at 2 h, but not 24 h after K+ administration. Neither implantation of the probes nor perfusion with artificial cerebrospinal fluid resulted in similar patterns of c-FOS immunoreactivity. In addition, we investigated whether the impairment of the cholinergic septohippocampal pathway would modify the K(+)-induced expression of the immediate early gene c-fos in the hippocampus. The expression of c-fos induced by KCl was not altered in the animals with fimbria-fornix lesion despite the marked decrease in acetylcholine release in the hippocampus. Glutamate concentrations measured in the same superfusates showed that a significantly greater glutamate release occurs in denervated hippocampi. Furthermore, abolishment of seizure-like activity (induced by KCl) in anesthetized animals did not alter expression of c-FOS immunoreactivity in the K(+)-stimulated hippocampi. The results from these studies confirm that most of the releasable acetylcholine of the hippocampus is linked to the fimbrial input and may suggest that c-FOS upregulation in this model does not respond to any cholinergic input from the medial septum via the fimbria-fornix.

The striatonigral dynorphin pathway of the rat studied with in vivo microdialysis--I. Effects of K(+)-depolarization, lesions and peptidase inhibition.

Extracellular levels of dynorphin B were analysed with in vivo microdialysis in the neostriatum and substantia nigra of halothane-anaesthetized rats. Dopamine and its metabolites, 3,4-dihydroxyphenyl-acetic acid and homovanillic acid, as well as GABA were simultaneously monitored. Chromatographic analysis revealed that the dynorphin B-like immunoreactivity measured in perfusates collected under basal and K(+)-depolarizing conditions co-eluted with synthetic dynorphin B. Dynorphin B, GABA and dopamine levels were Ca(2+)-dependently increased by K(+)-depolarization, while 3,4-dihydroxyphenylacetic acid and homovanillic acid levels were decreased. Dopamine and its metabolites, but not dynorphin B or GABA levels, were significantly decreased after a unilateral 6-hydroxydopamine injection into the left medial forebrain bundle. In contrast, following a unilateral injection of ibotenic acid into the striatum, dynorphin B and GABA levels were decreased by > 50% in striatum and substantia nigra on the lesioned side, whereas no significant changes were observed in basal dopamine levels. The inclusion of the peptidase inhibitor captopril (50-500 microM) into the nigral perfusion medium produced a concentration-dependent increase in nigral extracellular levels of dynorphin B. In the striatum, a delayed increase in dynorphin B and GABA levels could be observed following the nigral captopril administration, but this effect was not concentration-dependent. Thus, we demonstrate that extracellular levels of dynorphin B, dopamine and GABA can simultaneously be monitored with in vivo microdialysis. Extracellular dynorphin B appears to originate from neurons, since the levels were (i) increased in a Ca(2+)-dependent manner by K(+)-depolarization, and (ii) decreased by a selective lesion of the striatum, known to contain cell bodies of dynorphin neurons in the striatonigral pathway. Furthermore, (iii) the increase in nigral dynorphin B levels by peptidase inhibition suggests the presence of clearance mechanisms for the released dynorphin peptides.

The striatonigral dynorphin pathway of the rat studied with in vivo microdialysis--II. Effects of dopamine D1 and D2 receptor agonists.

In vivo microdialysis was used to study the effect of intracerebral administration of dopamine agonists on dynorphin B release in the striatum and substantia nigra of rats. The release of dopamine and GABA was also investigated. Administration of the dopamine D1 agonist SKF 38393 (10-100 microM) into the striatum increased extracellular dynorphin B and GABA levels in the ipsilateral substantia nigra, in a concentration-dependent manner. After a short-lasting increase, nigral dopamine levels were significantly decreased after the highest concentration of striatal SKF 38393. An increase in striatal dynorphin B, GABA and dopamine levels was also observed. When SKF 38393 (10 microM) was administered into the substantia nigra, nigral dynorphin B and GABA, but not dopamine levels increased. No significant effects were observed on striatal levels. Administration of the dopamine D2 agonist, quinpirole (100 microM), into the striatum decreased dopamine levels in both striatum and substantia nigra, while no effect was observed on striatal or nigral dynorphin B and GABA levels. Quinpirole (10-100 microM) given into the substantia nigra, decreased striatal dopamine levels in a concentration manner. In the nigra, a short-lasting increase in dopamine levels was observed following the highest concentration of nigral quinpirole (100 microM). The effect was followed by a decrease in dopamine levels. No significant effects were observed on striatal or nigral dynorphin B and GABA levels. The results show that stimulation of D1 receptors in striatum and substantia nigra leads to activation of the striatonigral dynorphin pathway. A parallel effect could also be seen on nigral GABA release.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of neurotransmitter release by cholecystokinin in the neostriatum and substantia nigra of the rat: regional and receptor specificity.

The effect of cholecystokinin peptides on the release of dynorphin B, aspartate, glutamate, dopamine and GABA in the neostriatum and substantia nigra of the rat was investigated using in vivo microdialysis. Sulphated cholecystokinin-8S in the dialysis perfusate (1-100 microM) induced a concentration-dependent increase in extracellular dynorphin B and aspartate levels, both in the neostriatum and substantia nigra. Striatal dopamine levels were only increased by 100 microM of cholecystokinin-8S, while in the substantia nigra they were increased by 10-100 microM of cholecystokinin-8S. Extracellular GABA and glutamate levels were increased following 100 microM of cholecystokinin-8S only. Striatal cholecystokinin-8S administration also produced a significant increase in nigral dynorphin B levels. Local cholecystokinin-4 (100 microM) produced a moderate, but significant, increase of extracellular dynorphin B and aspartate levels in the neostriatum and substantia nigra. No effect was observed on the other neurotransmitters investigated. A 6-hydroxydopamine lesion of the nigrostriatal dopamine pathway did not affect the increases in dynorphin B and aspartate levels produced by local administration of cholecystokinin-8S. Basal extracellular GABA levels were increased significantly in both the neostriatum and substantia nigra ipsilateral to the lesion. Nigral glutamate and aspartate levels were also increased in the lesioned substantia nigra, but in the lesioned neostriatum aspartate levels were decreased. The cholecystokinin-B antagonist L-365,260 (20 mg/kg, s.c.), but not the cholecystokinin-A antagonist L-364,718 (devazepide; 20 mg/kg, s.c.), significantly inhibited the effect of cholecystokinin-8S on striatal dynorphin B and aspartate levels. In the substantia nigra, however, the effect of cholecystokinin-8S on dynorphin B and aspartate levels was inhibited to a similar extent by both L-365,260 and L-364,718. Pretreatment with L-364,718, but not with L-365.260, prevented the increase in nigral dopamine levels produced by nigral cholecystokinin-8S administration. Taken together, these results suggest that cholecystokinin-8S modulates dynorphin B and aspartate release in the neostriatum and substantia nigra of the rat via different receptor mechanisms. In the neostriatum, the effect of cholecystokinin-8S on dynorphin B and aspartate release is mediated via the cholecystokinin-B receptor subtype, while in the substantia nigra, cholecystokinin-8S modulates dynorphin B and aspartate release via both cholecystokinin-A and cholecystokinin-B receptor subtypes. Cholecystokinin-8S modulates dopamine release mainly in the substantia nigra, via the cholecystokinin-A receptor subtype.

Splanchnic homeostasis during endotoxin challenge in the pig as assessed by microdialysis and tonometry.

The splanchnic region is particularly susceptible to shock. The purpose of this study was to evaluate microdialysis of the liver and small intestine as a monitor of splanchnic metabolic deterioration (elevation of lactate and hypoxanthine) in porcine endotoxic shock. Tonometry of the small intestine was used as a reference. Microdialysis probes (liver, ileum, and artery), tonometer, and pulmonary artery catheter were inserted. Eight animals were given Escherichia coli lipopolysaccharide endotoxin (20 micrograms kg-1 h-1 for 2 h). Five animals served as controls. Measurements were made every half-hour. Three hours after onset of endotoxin challenge, there were significant differences between endotoxin and control groups in intestinal lactate and hypoxanthine, as well as liver lactate, in addition to mucosal pH obtained by tonometry. Lactate elevation in blood was first seen at 4 h, while there was no significant hypoxanthine elevation in arterial blood over 5 h. Hence, data obtained from the splanchnic region became significantly different early, when compared with data obtained from arterial blood. Microdialysis of liver and small intestine as well as intestinal tonometry are sensitive tools for detection of splanchnic metabolic deterioration during endotoxin shock.

Bromocriptine and apomorphine stimulation of cortisol secretion in conscious dogs; evidence for a stimulatory site located outside the blood brain barrier.

IV injections of the dopamine receptor agonists bromocriptine (0.1 mg/kg) and apomorphine (0.05 mg/kg) induced rapid and long-lasting increases of cortisol levels as measured by RIA in peripheral venous plasma of conscious dogs. Pretreatment with dopamine receptor antagonists which do not readily penetrate the blood brain barrier (domperidone, halopemide, sulpiride) abolished the release responses induced by the dopamine agonists. These results suggest that the dopamine receptor agonists stimulate cortisol release at a site located outside the blood brain barrier. In addition, some dopamine receptor antagonists (haloperidol, chlorpromazine, milenperone) were shown to cause a rapid and long-lasting increase of cortisol levels.

Grafting of genetically modified cells: effects of acetylcholine release in vivo.

In this study, microdialysis was used to investigate functional recovery of central cholinergic neurons in the forebrain of rats with cortical devascularizing lesions. Mature male rats were unilaterally lesioned by disruption of the pia arachnoid vessels and genetically modified fibroblasts secreting nerve growth factor (NGF) were placed at the site of the lesion. One month following surgery, microdialysis probes were installed in the remaining cortex and were perfused with artificial cerebrospinal fluid (csf) containing neostigmine (5 nM) and/or KCl (100 mM). The basal (non-stimulated) release of acetylcholine (ACh) in the cortex was similar in all experimental groups, whereas KCl stimulated release of ACh was significantly augmented (P < 0.05) in the ipsilateral remaining cortex in lesioned animals that have been implanted with fibroblasts secreting NGF. These results suggest that NGF secreted by genetically engineered fibroblasts modulates neuroplasticity in the adult mammalian CNS and may favour recovery of cortical function following injury.

Modulation of striatal aspartate and dynorphin B release by cholecystokinin (CCK-8) studied in vivo with microdialysis.

Sulphated cholecystokinin-8 (CCK-8) given into the neostriatum of the rat by in vivo microdialysis produced a concentration-dependent (1-100 microM) increase in extracellular aspartate (Asp) and dynorphin B (Dyn B), but not in glutamate, GABA or dopamine levels. The increase in Asp levels produced by 10 microM CCK-8 was approximately 10 fold and was inhibited (approximately 50%) by the CCKB antagonist L-365,260 (20 mg kg-1, i.p.), while the increase in Dyn B (approximately 2 fold) was totally abolished. Both increases were inhibited (approximately 50%) by local infusion of 10 microM of tetrodotoxin (TTX). Thus, CCK exerts modulatory effects in the basal ganglia, possibly by interacting with local neostriatal neurones releasing Asp, and with Dyn B-containing neurones projecting to the pars reticulata of the substantia nigra.

Hypoxia-mediated in vivo release of dopamine in nucleus tractus solitarii of rabbits.

A wide variety of neuroactive substances have been suggested to be involved in the respiratory depression observed in response to severe hypoxia. By use of the technique of microdialysis, the release of dopamine (DA) was measured in the nucleus tractus solitarii during severe hypoxic provocations (6% O2 in N2) in the adult pentobarbital-anesthetized rabbit. DA release was analyzed by high-performance liquid chromatography with electrochemical detection. Such hypoxic provocations caused pronounced phase of depression in the phrenic nerve activity and enhanced release of DA. After bilateral carotid sinus nerve denervation, acute severe hypoxia did not give rise to enhanced release of DA or to phrenic nerve depression. Mild hypoxic (9% or 12% O2 in N2) or hypercapnic (6% CO2) stimuli resulted in an increased phrenic nerve activity without any concomitant changes in DA release. Decerebration at the midcollicular level in rabbits prevented an enhanced release of DA in the nucleus tractus solitarii during severe hypoxia. The results suggest that 1) DA is involved in the central ventilatory response to severe hypoxia, 2) not only the initial excitatory but also the second depressive phase in response to severe hypoxia is mediated partially by the peripheral chemoreceptors, and 3) the depressive phase is dependent on intact connections from suprapontine structures.

Effects of dopaminomimetics on the secretion of VIP-like immunoreactivity in conscious dogs.

The effects of some dopaminomimetics on VIP levels in peripheral venous blood of conscious dogs were analysed with a radioimmunoassay. The dopamine D2 agonist pergolide, like apomorphine and bromocriptine, increased VIP levels. The putative DA autoreceptor agonist 3PPP, as well as the D1 agonist SK&F 38393 were devoid of action. The D1 antagonist SCH 23390 did not abolish the effect of apomorphine. It is suggested that monitoring of VIP levels could be an interesting screening test for activity at D2 receptors. Amphetamine did not modify VIP levels suggesting that DA neurons are not involved in the mechanism leading to a release of VIP. The VIP response to apomorphine was not suppressed by an infusion of somatostatin. Decreasing blood pressure with nitroglycerin or with the adrenergic antagonist prazosin did not release VIP. The mechanism by which administration of dopaminomimetics lead to a release of VIP is further discussed.

Release of VIP-like immunoreactivity in response to dopamine agonists, insulin hypoglycemia and feeding in conscious dogs.

VIP levels were measured by radioimmunoassay in peripheral venous blood of conscious dogs. Bolus injections of the dopamine agonists apomorphine, 0.05 mg/kg, and bromocriptine, 0.2 mg/kg, were found to increase VIP levels from approximately 5 pmol/l to 150 pmol/l. The release responses were abolished by pretreatment with dopamine antagonists (haloperidol 0.1 mg/kg or halopemid 0.1 mg/kg) and by hexamethonium (1 mg/kg) a blocker of ganglionic transmission. Vagotomy did not inhibit the dopamine agonist induced output of VIP. Vagal activation by means of feeding or insulin hypoglycemia caused only minor rises of VIP levels (5-10 pmol/l). It is concluded that dopamine agonists stimulate the release of VIP from populations of neurons other than those affected by vagal and sympathetic activation. Possible sites of action for the VIP releasing effect exerted by dopamine agonists are discussed. Furthermore, it is suggested that some of the peripheral effects exerted by dopaminergic drugs are exerted via a previous release of VIP.

Modulation of catecholamine release by alpha 2-adrenoceptors and I1-imidazoline receptors in rat brain.

The physiological and pharmacological effects of imidazoli(di)ne derivatives, such as clonidine, have been related not only to the interaction with alpha 2-adrenoceptors but also to their activity on non-adrenoceptor sites termed imidazoline receptors. The modulation of catecholamine release by imidazoline drugs was studied by monitoring extracellular levels of norepinephrine (NE), dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) with microdialysis in cingulate cortex of rats, with or without irreversible alpha 2-adrenoceptor blockade. NE and DA levels were in the 1 nM range whereas DOPAC and HIVA levels were approximately equal to 100 nM. NE and DA levels were increased when the uptake blocker desipramine (1 microM) or KCl (100 mM) were added to the perfusion medium. Clonidine induced a dose-dependent (0.3-1.2 mg/kg i.p.) decrease in NE (max 61%) and DA (max 40+) levels that was reversed by the alpha 2-adrenoceptor antagonist RX821002. After alpha 2-adrenoceptor irreversible blockade with the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), [3H]clonidine binding to alpha 2-adrenoceptors was reduced by 94 +/- 1%. Under such conditions, clonidine elicited a paradoxical dose-dependent (0.6-2.4 mg/kg i.p.) increase of NE (max 56%) without modifications in DA, DOPAC and HVA levels. The stimulatory effect of clonidine was prevented by the imidazoline receptor antagonist idazoxan (10 mg/kg i.p.) but not by RX821002 (5 mg/kg i.p.). In rats pretreated with EEDQ, cirazoline (I1/I2-imidazoline receptor agonist), moxonidine (I1-imidazoline receptor agonist), but not guanabenz (I2-imidazoline receptor agonist) (1.2-2.4 mg/kg i.p.) elicited an increase of NE levels in a similar manner to clonidine (11-82%). Idazoxan also abolished these responses to cirazoline or moxonidine. In contrast to systemic administration, local perfusion of clonidine (10-100 microM) through the microdialysis probe under alpha 2-adrenoceptor alkylating conditions, did not modify extracellular levels of NE and DA suggesting an indirect mechanism. The results demonstrate that clonidine and related imidazoli(di)ne drugs are able not only to inhibit NE release in rat cerebral cortex involving an alpha 2-adrenoceptor mechanism, but also to induce a paradoxical NE release through an indirect extracortical mechanism. The findings evidence that the indirect modulation of NE levels by imidazoline drugs is mainly due to a functional activity on I1-imidazoline receptors.

Effect of morphine on dynorphin B and GABA release in the basal ganglia of rats.

In vivo microdialysis was used to study the effects of systemic, as well as intracerebral administration of morphine and naloxone on dynorphin B release in neostriatum and substantia nigra of rats. The release of dopamine (DA), gamma-aminobutyric acid (GABA), glutamate (Glu) and aspartate (Asp) was also investigated. Systemic injection of morphine (1 mg/kg s.c.) induced long-lasting increases in extracellular dynorphin B and GABA levels in the substantia nigra, whereas DA, Glu and Asp levels, measured in the same region, were not significantly affected. No effect on striatal neurotransmitter levels was observed following systemic morphine administration. Local perfusion of the substantia nigra with morphine (100 microM) through the microdialysis probe also increased nigral dynorphin B and GABA levels. Perfusion of the neostriatum with morphine (100 microM) significantly increased GABA and dynorphin B levels in the ipsilateral substantia nigra, but no effect was observed locally. Naloxone blocked the effect of systemic morphine administration on nigral dynorphin B and GABA release, already at a dose of 0.2 mg/kg s.c. Naloxone alone, given either systemically (0.2-4 mg/kg s.c.) or intracerebrally (1-100 microM), did not affect dynorphin B or amino acid levels, either in neostriatum or in substantia nigra. However, naloxone produced a concentration-dependent increase in DA levels. The present results indicate that systemic morphine administration stimulates the release of dynorphin B in the substantia nigra, probably by activating the mu-subtype of opioid receptor, since the effect of morphine on nigral dynorphin B and GABA was antagonized by a low dose of naloxone. The increase in extracellular DA levels produced by high concentrations of naloxone, both in neostriatum and substantia nigra, indicates a disinhibitory effect of this drug on DA release, probably via a non-mu subtype of opioid receptors located on nigro-striatal DA neurones.

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.