Targeting acute ischemic stroke with a calcium-sensitive opener of maxi-K potassium channels.

During ischemic stroke, neurons at risk are exposed to pathologically high levels of intracellular calcium (Ca++), initiating a fatal biochemical cascade. To protect these neurons, we have developed openers of large-conductance, Ca++-activated (maxi-K or BK) potassium channels, thereby augmenting an endogenous mechanism for regulating Ca++ entry and membrane potential. The novel fluoro-oxindoles BMS-204352 and racemic compound 1 are potent, effective and uniquely Ca++-sensitive openers of maxi-K channels. In rat models of permanent large-vessel stroke, BMS-204352 provided significant levels of cortical neuroprotection when administered two hours after the onset of occlusion, but had no effects on blood pressure or cerebral blood flow. This novel approach may restrict Ca++ entry in neurons at risk while having minimal side effects.

Dinapsoline: characterization of a D1 dopamine receptor agonist in a rat model of Parkinson's disease.

Dinapsoline is a new potent, full agonist at D1 dopamine receptors with limited selectivity relative to D2 receptors. The efficacy of this compound was assessed in rats with unilateral 6-hydroxydopamine lesions of the medial forebrain bundle, a standard rat model of Parkinson's disease. Dinapsoline produced robust contralateral rotation after either subcutaneous or oral administration. This rotational behavior was attenuated markedly by the D1 receptor antagonist SCH-23390, but not by the D2 receptor antagonist raclopride. During a chronic 14-day treatment period in which rats received dinapsoline either once or twice a day, dinapsoline did not produce tolerance (in fact, some sensitization of the rotational response was observed in one experiment). Because dinapsoline shows less D1:D2 selectivity in vitro than other D1 agonists, the contribution of D2 activity to tolerance was assessed. Chronic daily cotreatment with dinapsoline and raclopride did not enable the development of tolerance to chronic dinapsoline treatment. In contrast, when dinapsoline was administered by osmotic minipump, rapid tolerance was observed. To explore further the contribution of D1 and D2 receptors to tolerance, experiments were performed with the selective D1 agonist A-77636. Daily dosing with A-77636 rapidly produced complete tolerance, as previously observed, whereas coadministration of the D2 agonist quinpirole plus A-77636 failed to either delay or prevent tolerance. Taken together, these results indicate that the development of tolerance to D1 receptor agonists is influenced by the pattern of drug exposure but not by the D1:D2 selectivity of the agonist.

Differential effects of coadministration of fluoxetine and WAY-100635 on serotonergic neurotransmission in vivo: sensitivity to sequence of injections.

Serotonin 5-HT(1A) receptor antagonists potentiate the effects of serotonin reuptake inhibitors on extracellular serotonin levels in a variety of brain regions. These effects are quite variable, however, with reports indicating potentiations of anywhere from 100-1900%. One factor that might impact the magnitude of such potentiations is the timing of administration of the two agents; reports in which the reuptake inhibitor is given prior to the serotonin receptor antagonist consistently report larger potentiations than reports in which the antagonist is given first. To test this relationship directly, microdialysis and electrophysiology studies were performed to assess the magnitude of increase in extracellular serotonin and changes in cellular activity produced by the serotonin reuptake inhibitor fluoxetine and the 5-HT(1A) receptor antagonist WAY-100635 under various dosing regimens. In microdialysis studies, when WAY-100635 (0.5 mg/kg s.c.) was administered 80 min after fluoxetine (10 mg/kg i.p.) the increase in serotonin was more than twice that observed when the compounds were coadministered. In electrophysiology studies in vivo, WAY-100635 reversed the depression of cell firing produced by fluoxetine when administered 30 min after fluoxetine, but when the two compounds were coadministered, a depression in firing rate was observed comparable to that produced by fluoxetine alone. In contrast, slice recording studies showed that WAY-100635 blocked the effects of fluoxetine regardless of the order of administration. These results indicate that fluoxetine and WAY-100635 can interact in a fashion not predicted by the currently accepted model. It is likely that neuronal circuitry outside of the raphe nuclei underlies this relationship.

The augmentation hypothesis for improvement of antidepressant therapy: is pindolol a suitable candidate for testing the ability of 5HT1A receptor antagonists to enhance SSRI efficacy and onset latency?

The development of selective serotonin reuptake inhibitors (SSRIs) provided a major advancement in the treatment of depression. However, these drugs suffer from a variety of drawbacks, most notably a delay in the onset of efficacy. One hypothesis suggests that this delay in efficacy is due to a paradoxical decrease in serotonergic (5-HT) neuronal impulse flow and release, following activation of inhibitory presynaptic 5-HT1A autoreceptors, following acute administration of SSRIs. According to the hypothesis, efficacy is seen only when this impulse flow is restored following desensitization of 5-HT1A autoreceptors and coincident increases in postsynaptic 5-HT levels are achieved. Clinical proof of this principal has been suggested in studies that found a significant augmenting effect when the beta-adrenergic/5-HT1A receptor antagonist, pindolol, was coadministered with SSRI treatment. In this article, we review preclinical electrophysiological and microdialysis studies that have examined this desensitization hypothesis. We further discuss clinical studies that utilized pindolol as a test of this hypothesis in depressed patients and examine preclinical studies that challenge the notion that the beneficial effect of pindolol is due to functional antagonism of the 5-HT1A autoreceptors.

Opioid receptor modulation of feeding-evoked dopamine release in the rat nucleus accumbens.

Feeding is associated with increases in the activity of the mesolimbic dopamine (DA) system which originates in the ventral tegmental area (VTA) and projects heavily to the nucleus accumbens. The present study used in vivo brain microdialysis to assess the contribution of opioid receptors in feeding-evoked DA release in the nucleus accumbens. Feeding in 18 h food-deprived rats increased DA release by about 50% above baseline. Systemic injection of the opioid receptor antagonist naltrexone (1 mg/kg, s.c.) blocked the effect of feeding on DA release and reduced the amount of food consumed. Unilateral application of naltrexone (100 microM) in the VTA via a microdialysis probe failed to affect the DA response to feeding, the amount of food consumed, or the latency to eat. In contrast, intra-VTA naltrexone significantly reduced the effect of systemic heroin (0.5 mg/kg, s.c.) on accumbal DA release. These results indicate that: (1) opioid receptor activation is a component of the neural substrates of deprivation-induced feeding: (2) opioid receptors in the VTA do not contribute significantly to feeding-associated increases in DA release in the nucleus accumbens; and (3) heroin-induced increases in accumbal DA release are mediated, at least in part, by opioid receptors in the VTA.

Acute myocardial infarction caused by a myocardial bridge treated with intracoronary stenting.

The clinical significance of myocardial bridges (MBs) is variable, and most patients are asymptomatic. However, angina, myocardial infarction, and sudden death have been reported in association with MBs. Here we describe the use of intracoronary stenting for the treatment of a patient with an anterior myocardial infarction due to an MB.

Activation of the mesocortical dopamine system by feeding: lack of a selective response to stress.

There is wide agreement that catecholamine systems in the prefrontal cortex are activated by stressful stimuli. To date, however, the extent to which other stimuli can increase the activity of these systems has received little attention. In the present study, the effects of tail pinch stress and feeding on dopamine and noradrenaline release in the prefrontal cortex of rats were examined using in vivo brain microdialysis. Both stimuli increased dopamine release, with peak effects reaching 212% above baseline for tail pinch and 165% above baseline for feeding. The effects of the two stimuli on peak dopamine release were not significantly different. Both stimuli also significantly increased noradrenaline release, with peak effects reaching 128% above baseline for tail pinch and 98% above baseline for feeding. The effects of the two stimuli on peak noradrenaline release were not significantly different. These results indicate that activation of catecholaminergic afferents to the prefrontal cortex is not specific to stress, but also occurs in response to non-stressors with positive motivational valence.

Feeding-evoked dopamine release in the nucleus, accumbens: regulation by glutamatergic mechanisms.

The extent to which glutamate receptors in the nucleus accumbens and ventral tegmental area regulate feeding-evoked increases in dopamine release in the nucleus accumbens was determined using in vivo brain microdialysis in the rat. In some animals a second dialysis probe was implanted in the ventral tegmental area ipsilateral to the nucleus accumbens probe. The feeding protocol involved access to standard rat chow after 18 h of food deprivation. Under these conditions rats began eating approximately 30 s after the introduction of food and consumed 7-8 g, resulting in a 50% increase in dopamine release. Application of the glutamate receptor antagonist kynurenate (1 mM) in the nucleus accumbens potentiated the feeding-evoked increase in dopamine release by 80%. Application of the metabotropic glutamate receptor agonist trans-1S,3R-1-amino-1,3-cyclopentanedicarboxylic acid (100 microM) in the nucleus accumbens blocked the feeding-evoked increase in dopamine release. Application of a combination of the ionotropic glutamate receptor antagonists 2-amino-5-phosphopentanoic acid (200 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (50 microM) through the dialysis probe in the ventral tegmental area reduced basal dopamine output in the nucleus accumbens by 20% and markedly attenuated (by 70%) the effect of feeding on dopamine release. None of the treatments affected the latency to eat or the volume of food consumed. These results indicate that glutamatergic afferents to the ventral tegmental area mediate feeding-induced increases in dopamine release in the nucleus accumbens. In contrast, at physiological concentrations, glutamate in the nucleus accumbens appears to decrease dopamine release via actions on ionotropic and metabotropic receptors.

Effects of chronic haloperidol on stress- and stimulation-induced increases in dopamine release: tests of the depolarization block hypothesis.

There is considerable neurophysiological evidence that chronically administered neuroleptics can, under certain circumstances, decrease the activity of mesencephalic dopaminergic neurons. This finding, referred to as depolarization inactivation or depolarization block, has led to the hypothesis that the delayed therapeutic effects of neuroleptic drugs are due to a graduate silencing of mesolimbic dopaminergic neurons. One prediction of depolarization inactivation is that dopamine neurons in this state should be resistant to activation by excitatory stimuli. As a test of this prediction, rats that had been treated chronically with either saline or haloperidol (0.5 mg/kg x 21 days) were exposed to either acute mild stress or electrical stimulation of the prelimbic region of the prefrontal cortex while extracellular levels of dopamine in the nucleus accumbens were monitored by in vivo microdialysis. A 10-minute exposure to acute stress via tail pinch increased dopamine release by 20% and 18% in the saline and haloperidol groups, respectively. Similarly, 20 minutes of cortical stimulation increased dopamine release by 51% and 56% in rats treated chronically with saline or haloperidol, respectively. These results indicate that contrary to a prediction of the depolarization block hypothesis, mesolimbic dopaminergic neurons can be activated in neuroleptic-treated animals.

Glutamate receptor agonists decrease extracellular dopamine in the rat nucleus accumbens in vivo.

Intracerebral microdialysis was used to investigate the effects of local application of L-glutamate, N-methyl-D-aspartate, and the glutamate uptake inhibitor 1-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) in the nucleus accumbens (NAc) on extracellular dopamine (DA) concentrations. The effects of locally applied PDC on extracellular glutamate concentrations were also examined. Glutamate produced a concentration-dependent decrease in extracellular DA that could be blocked by concurrent, local application of the broad spectrum ionotropic glutamate receptor antagonist kynurenic acid (KYN:1 mM). N-Methyl-D-aspartate had a concentration-dependent effect on DA release, with a low concentration (0.1 mM) producing a decrease and a higher concentration (1.0 mM) resulting in an increase. Both effects were blocked by KYN. PDC (1 mM) increased extracellular glutamate concentrations to 102% above baseline. The same concentration of PDC decreased extracellular DA concentrations, and coapplication of KYN attenuated this effect. These results indicate that glutamate receptor agonists can have both facilitatory and inhibitory effects on extracellular DA concentrations. However, the effects of PDC indicate that inhibition of DA release is the more physiologically relevant effect. Furthermore, the results of these and other experiments suggest that glutamate's inhibitory effects on DA release in the NAc are not due to direct actions of this excitatory amino acid on DA terminals. A multisynaptic model that accounts for glutamate's actions on DA release is proposed.

Cortical regulation of subcortical dopamine release: mediation via the ventral tegmental area.

In vivo microdialysis was used to determine the extent to which ionotropic glutamate receptors in the ventral tegmental area (VTA) regulate dopamine release in the nucleus accumbens. Coapplication of 2-amino-5-phosphonopentanoic acid (AP5; 200 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 50 microM) to the VTA via reverse dialysis decreased extracellular concentrations of dopamine in the nucleus accumbens by approximately 30%. In accordance with previous results, electrical stimulation of the prefrontal cortex increased dopamine release by 60%. Application of AP5 and CNQX to the VTA during cortical stimulation blocked the effect of stimulation on dopamine release. These results indicate that ionotropic glutamate receptors in the VTA are critically involved in basal and evoked dopamine release in the nucleus accumbens and suggest that a glutamatergic projection from the prefrontal cortex regulates the activity of dopaminergic neurons in the VTA.

Electrical stimulation of the prefrontal cortex increases dopamine release in the nucleus accumbens of the rat: modulation by metabotropic glutamate receptors.

In vivo microdialysis was used to assess the effects of electrical stimulation of the prefrontal cortex (PFC) on dopamine (DA) release in the nucleus accumbens (NAC) of awake, unrestrained rats. The PFC was stimulated bilaterally for 20 min at parameters previously shown to support intracranial self-stimulation in this structure. Stimulation at 50 microA evoked a 38% increase in DA release while 100 microA produced a 69% increase. Thus, phasic activation of the PFC increases DA release in the NAC. Additional experiments were performed to establish whether glutamate receptors in the NAC mediated these effects. The noncompetitive NMDA antagonist dizocilpine maleate (MK-801) and the broad spectrum competitive antagonist kynurenic acid were each applied locally to the NAC via reverse dialysis alone or in combination with electrical stimulation of the PFC (100 microA). Both MK-801 (10 microM) and kynurenic acid (5 mM) increased DA release when administered alone. When a "subthreshold" concentration (i.e., the highest concentration employed that did not itself increase DA release) of either compound was administered together with PFC stimulation, neither kynurenic acid (1 mM) nor MK-801 (1 microM) attenuated the effect of stimulation on DA release, thereby indicating that this effect is not mediated by ionotropic glutamate receptors located within the NAC. To examine the possible role of metabotropic glutamate receptors in regulating DA release, the metabotropic glutamate agonist trans(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) was employed. When applied locally to the NAC, ACPD had a dose-dependent effect on DA release with a high concentration (1 mM) causing an increase and a lower concentration (100 microM) causing a small decrease.(ABSTRACT TRUNCATED AT 250 WORDS)

Cortical regulation of acetylcholine release in rat striatum.

The effect of electrical stimulation of the prefrontal cortex (PFC) on acetylcholine (ACh) release in the dorsal striatum was investigated using on line in vivo microdialysis. ACh output was sampled before, during and after 20-min periods of 50 and 100 microA stimulation in awake, unrestrained rats. Both currents increased extracellular ACh by approximately 30% above baseline. ACh concentrations reached their maximum values during the last 10 min of the stimulation period and returned to baseline within 20 min. These results provide direct functional evidence for cortical modulation of cholinergic interneurons in the striatum.

Electrical stimulation of the medial prefrontal cortex increases dopamine release in the striatum.

Exogenous and endogenous glutamate has been shown to evoke dopamine (DA) release in the striatum using both in vitro and in vivo techniques. We hypothesized that stimulation of the prefrontal cortex (PFC) would phasically enhance striatal DA release via the glutamatergic corticostriatal pathway. To test this hypothesis, in vivo brain microdialysis was employed to measure extracellular concentrations of DA in the striatum during electrical stimulation of the PFC. Five rats were implanted with bilateral electrodes located in the medial PFC and dialysis probes in the dorsal striatum. Two days later the PFC of these awake, freely moving rats was stimulated first at 50 microA and then at 100 microA for 20 minutes at 2-hour intervals. Both currents significantly increased DA release. Extracellular DA rose rapidly during stimulation, peaked immediately afterward, and then slowly returned to baseline values. Dopamine reached 118% of baseline values with 50 microA stimulation and 138% with 100 microA stimulation. Histologic analysis using the fluorescent retrograde dye Fluoro Gold confirmed that cells projecting to the vicinity of the striatal dialysis probe originated in the vicinity of the PFC electrodes. These results provide direct evidence for phasic, excitatory modulation of striatal DA release by the PFC.