Identification of the dopamine D3 receptor in oligodendrocyte precursors: potential role in regulating differentiation and myelin formation.

Expression of the dopamine D3 receptor (D3r) was found in primary mixed glial cultures from newborn brain and in the corpus callosum in vivo during the peak of myelination. Expression of the D3r mRNA, but not D2r mRNA, was detected as early as 5 d in vitro (DIV) by RT-PCR. Immunoblot studies revealed D3r protein was also expressed in the cultures. Double immunofluorescence analysis for the D3r and for surface markers of specific stages of oligodendrocyte development indicated that D3r expression occurred in precursors and in immature oligodendrocytes but not in mature oligodendrocytes (i.e. , A2B5(+) 007(-) 01(-) and A2B5(+) 007(+) 01(-) cells but not A2B5(-) 007(+) 01(+) cells). Confocal microscopic analysis indicated that D3r was associated with cell bodies and cell membranes but not with the processes emanating from cell somas. Immunohistochemistry of brain sections revealed the presence of D3r in some oligodendrocytes located mainly within the genu and radiato of the corpus callosum during the active period of myelination. Treatment of cultures with 20 microM quinpirole led to decreased numbers of O1(+) oligodendrocytes possessing myelin-like membranes as well as an increase in the number of precursors in 14 DIV cultures. This effect was prevented by the dopamine antagonist haloperidol. These results show that the D3r expression is not restricted to neurons but it is also expressed in differentiating oligodendrocytes before terminal maturation. It also suggests that dopamine or some other D3r ligand may play a role in oligodendrocyte differentiation and/or the formation of myelin by mature oligodendrocytes.

Naloxone reverses the inhibitory effect of gamma-hydroxybutyrate on central DA release in vivo in awake animals: a microdialysis study.

gamma-Hydroxybutyrate (GHB) is a 4-carbon anesthetic that acts primarily by inhibiting presynaptic dopamine (DA) release in vivo. A number of studies have reported a reversal of many of the central effects of GHB by the allegedly pure opiate antagonist naloxone (NX) but its mechanism of action is unclear. In vivo microdialysis performed in the present preliminary study disclosed a significant inhibitory effect of GHB (500 mg/kg) on striatal DA release which was completely reversed by a low dose of NX (0.8 mg/kg). The results indicate that NX likely inhibits many of the central effects produced by GHB primarily through its reversal of the GHB induced inhibition of central DA release.

Alterations in the spontaneous release of dopamine and the density of the DA D2 receptor mRNA after chronic postnatal exposure to cocaine.

The influence of cocaine administration on dopamine (DA) release and D2 dopamine receptor mRNA levels was examined in developing rat brain. In the rat pup, cocaine (25 mg/ kg SC) was administered daily from postnatal days 1-9 and extracellular DA measured 24 h after the last injection of cocaine, using in vivo micro dialysis. Twenty-four hours after discontinuing cocaine administration, a decrease in the extracellular concentration of DA of more than 100% was found in treated pups compared to control pups. Pups were tested on postnatal days 10-12, 20-21, or 35-36. After 1 month, basal release of DA returned to control levels. To examine the structural basis of the alteration in basal release of DA, in situ hybridization studies were performed to access the effect of chronic administration of cocaine on the mRNA encoding the D2 DA receptor. These preliminary studies, on postnatal day 10, indicate that drug treatment alters the developmental pattern of D2 mRNA. The changes in D2 mRNA expression were accompanied by delayed disaggregation of neostriatal cells and diminished growth of neostriatal neurons. These structural changes may lead to functional impairment in the development of dopamine target cells, thus altering the balance of synaptic and trophic effects of DA.

Effect of gamma-hydroxybutyrate on central dopamine release in vivo. A microdialysis study in awake and anesthetized animals.

gamma-Hydroxybutyrate (GHB) is generally considered to be an inhibitor of striatal dopamine (DA) release. However, a number of recent reports and at least one major review suggest that GHB enhances rather than inhibits striatal DA release. To examine this discrepancy, the effect of GHB on striatal DA release was monitored for 2 hr by microdialysis in awake and urethane-anesthetized rats. GHB (500 mg/kg, i.p.) significantly inhibited striatal DA release in conscious animals. However, anesthetic pretreatment completely abolished the inhibitory effect of GHB on DA release. In urethane-anesthetized animals, intraperitoneal injections of GHB resulted in a dialysis DA output that was the same as basal and saline control levels for all but the last three intervals where DA release was elevated slightly. In contrast to the intraperitoneal route, subcutaneous injections of GHB in anesthetized animals produced significant elevations of DA release above baseline levels. The increases ranged from 125 to 133% of basal levels. These results indicate that while GHB enhances striatal DA release in anesthetized animals, it inhibits rather than enhances this release in awake animals. This would explain why GHB induces an inhibition of DA-release-dependent behaviors rather than an enhancement. The results also indicate that the route of GHB administration influences its effects on striatal DA release, at least in anesthetized animals.

Effects of naloxone on amphetamine induced striatal dopamine release in vivo: a microdialysis study.

The opiate antagonist naloxone (NX) alters amphetamine (AMPH) induced behaviors including locomotor activity, rearing and stereotypy. However, the exact nature of the NX induced alteration of AMPH induced behaviors is controversial, with some studies using high (5-40 mg/kg) doses of NX reporting an inhibition, and others using low (< or = 1-2 mg/kg) doses observing a potentiation. As these behaviors are mediated by AMPH induced dopamine (DA) release, the effect of NX on the latter was examined by microdialysis in an effort to resolve the controversy. Saline and NX pretreated groups subsequently administered AMPH were compared in vivo across nine separate 10 min intervals as well as by grouped intervals. NX alone (0.8 mg/kg) and saline exerted statistically equivalent effects on striatal DA release with the exception of the fifth interval, where a small but significant increase was seen after NX. On the other hand, the same dose of NX significantly enhanced AMPH induced striatal DA release relative to saline pretreated animals during each of four separate intervals, from 30 to 70 minutes following AMPH (1.5 mg/kg), and across all nine intervals combined. NX pretreatment (0.8 mg/kg) followed by a higher dose of AMPH (3.0 mg/kg) produced a significantly greater cumulative effect on DA release than saline pretreatment over the last six combined intervals (30-90 min) and over two grouped intervals (30-50 min and 40-60 min inclusive). However, a comparison of single rather than paired or grouped intervals revealed no significant differences. Previous studies have also examined the effect of NX on AMPH induced striatal DA release using in vivo microdialysis. However, the doses used were invariably high (5 mg/kg) and the results on striatal DA release always inhibitory. The present results suggest that NX potentiates AMPH induced striatal DA release when lower doses of NX are used. These results combined with those of previous studies also suggest that NX exerts a biphasic effect on AMPH induced DA release, with lower doses potentiating release and higher doses inhibiting release. This is close agreement with behavioral observations and may be due to the effect of low versus high doses of NX on intraterminal calcium influx.

Does gamma-hydroxybutyrate inhibit or stimulate central DA release?

Gamma-hydroxybutyrate (GHB), a four-carbon fatty acid and anaesthetic, is widely considered to be a relatively specific inhibitor of central dopamine (DA) release. The inhibitory effect of GHB on the latter is thought to occur as a consequence of its diminution of impulse flow in central dopaminergic neurons. However, a number of studies have recently reported that GHB primarily stimulates rather than inhibits central DA release, with any inhibitory effect produced of a modest and transitory nature. GHB has been and continues to be widely used as an important research tool largely because it is one of only a few drugs available that acts primarily on DA release. Consequently, it is important to determine whether GHB inhibits DA release as previously thought, or stimulates DA release, as more recently suggested. Following a critical review of the literature, the present report suggests that GHB does inhibit rather than stimulate presynaptic DA release in consonance with its behavioral and pharmacological activity. Recent in vivo studies indicating that GHB stimulates DA release were done under anaesthesia or in the presence of a high concentration of calcium. Both conditions have been found to spuriously enhance striatal DA release in vivo, which may account for the failure of some studies to observe an inhibitory effect of GHB on DA release in vivo.

Naloxone reverses the inhibitory effect of gamma-hydroxybutyrate on central DA release in vivo in awake animals: a microdialysis study.

gamma-Hydroxybutyrate (GHB) is a 4-carbon anesthetic that acts primarily by inhibiting presynaptic dopamine (DA) release in vivo. A number of studies have reported a reversal of many of the central effects of GHB by the allegedly pure opiate antagonist naloxone (NX) but its mechanism of action is unclear. In vivo microdialysis performed in the present preliminary study disclosed a significant inhibitory effect of GHB (500 mg/kg) on striatal DA release which was completely reversed by a low dose of NX (0.8 mg/kg). The results indicate that NX likely inhibits many of the central effects produced by GHB primarily through its reversal of the GHB induced inhibition of central DA release.

Gamma hydroxybutyrate is not a GABA agonist.

Gamma hydroxybutyrate (GHB) is primarily known and used as a relatively specific inhibitor of central DA release. However, it is also widely assumed to be an agonist or prodrug of gamma-aminobutyric acid (GABA) and its central activity has been attributed to an action exerted at GABA receptors. Nevertheless, there is compelling evidence that: (1) GHB formation may occur independently of GABA; (2) GHB is behaviorally, biochemically and physiologically distinct from GABA in many ways, and does not consistently effect GABAA or GABAB agonist induced responses; (3) GHB has little effect on either GABAA or GABAB receptors at less than millimolar concentrations. Consequently, GHB does not appear to be either a GABA prodrug or a GABA agonist. However, the GHB metabolite gamma butyrolactone (GBL) may possess some limited GABA agonist activity.

The effect of naloxone on spontaneous and evoked dopamine release in the central and peripheral nervous systems.

A number of studies have reported that the opiate antagonist naloxone (NX) inhibits behaviors dependent upon central dopamine (DA) release. However, equally compelling evidence from other studies suggests that NX excites a facilitatory effect. The present review was undertaken to resolve the issue by critically evaluating the effects of NX on DA release; the substrate subserving these behaviors. Included are studies reporting an effect of NX on spontaneous as well as drug altered DA release in various central regions. In the preponderant majority of these studies, NX was found to significantly enhance DA release in the virtually every major DA pathway, irrespective of whether DA release was initially stimulated or inhibited by various agents. It is concluded that NX most probably enhances behaviors induced by DA release, especially when administered in low, specific doses. Studies finding an inhibitory effect of NX on such behaviors may inadvertently produce conditions which mask the stimulatory effects of NX on DA release-dependent behaviors.

Intracellular analysis of effects of phencyclidine on rat neostriatal neurons recorded in vitro.

The effects of phencyclidine (PCP) on the synaptic and direct membrane properties of neostriatal neurons were assessed using intracellular recordings from 62 neurons in rat neostriatal slices. Electrophysiological measurements were obtained before, during and after bath application of PCP. At all concentrations (1-300 microM) PCP raised current thresholds for evoking synaptic potentials and reduced excitability (the firing rate of directly evoked action potentials). In a smaller number of cells PCP (50-300 microM) increased input resistance. These effects were long-lasting and neurons did not show reversal with washes as long as 75 min. Some neurons were injected with biocytin for identification. All were medium-sized spiny type I neurons typical of rodent neostriatum.

The D1 agonist SKF 38393 increases dopamine release in the developing rat striatum.

Changes in extracellular levels of dopamine (DA), dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) following systemic injection of the D1 agonist SKF 38393, 10 mg/kg, the D1 antagonist SCH 23390, 0.5 mg/kg, or the mixed D1/ /D2 agonist apomorphine, 0.05 mg/kg, were monitored in the striatum of developing rats implanted with a dialysis probe. There was a significant age-related increase in DA in perfusates from adult and 35-36-day-old rats injected with SKF 38393 compared to 10-11- and 21-22-day-old rats. Also, SKF 38393 significantly decreased DOPAC and HVA in perfusates from 10-11-day-old rats when compared to the older aged rats. Pretreatment with SCH 23390 completely blocked the SKF 38393-induced increase in DA but not the SKF 38393-induced decrease in DOPAC and HVA. In contrast, there were no significant differences between ages in the response to DA to SCH 23390. However, SCH 23390 did produce a small, but significant, decrease in DOPAC and HVA at 10-11 days of age compared to the other ages. Forty minutes after injecting apomorphine, DA levels were decreased by 45% and remained near this level for the duration of the experiment. These results indicate that stimulation of DA D1, receptors can increase striatal DA release and that this ability is acquired between 21 and 35 days of age. This finding is consistent with the idea of opposing roles for DA D1 and D2 receptor subtypes.

Effects of phencyclidine on extracellular levels of dopamine, dihydroxyphenylacetic acid and homovanillic acid in conscious and anesthetized rats.

Dose-dependent effects of phencyclidine on extracellular levels of dopamine (DA), dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the neostriatum were studied in both urethane-anesthetized and conscious rats. In vivo microdialysis was used to collect 10 min samples that were analyzed for levels of DA, DOPAC and HVA, using high-performance liquid chromatography with electrochemical detection (HPLC-EC). In both the anesthetized and conscious preparations, 20 mg/kg of phencyclidine produced an increase in extracellular levels of DA, 10 mg/kg resulted in no change, while 1 mg/kg produced a slow decrease. In the anesthetized animals phencyclidine did not have a significant effect on levels of DOPAC or HVA, but in the conscious animals phencyclidine produced a dose-dependent decrease in levels of DOPAC and HVA. The increase in levels of DA could be the result of increased release of DA or inhibition of the uptake of DA. The decrease in levels of DOPAC and HVA, at the 1 mg/kg dose, could result from a decrease in the synthesis of DA that is offset at the 10 and 20 mg/kg doses by opposing mechanisms.

Effect of prenatal exposure to phencyclidine on the postnatal development of the cholinergic system in the rat.

The purpose of this study was to examine the effect of prenatal exposure to phencyclidine (PCP) on the postnatal development of the cholinergic system in the neostriatum and the neocortex. Pregnant females were injected daily with PCP (0.5 mg/kg s.c.) from conception to parturition. On postnatal day 21 or 35, pups were injected with a challenge dose of PCP (0.5 mg/kg s.c.). The levels of acetylcholine and choline were measured, as well as the incorporation of deuterium-labelled choline into newly synthesized acetylcholine in the frontal cortex and neostriatum. The data demonstrate that even at 21 days, rat pups exposed to PCP in utero were more sensitive to a challenge dose of PCP than the saline controls. Thus, a tolerance to PCP, even after a prolonged prenatal exposure did not develop.

Development of haloperidol-induced dopamine release in the rat striatum using intracerebral dialysis.

Haloperidol-induced dopamine (DA) release and metabolism were studied in the rat striatum at 10-11, 21-22, and 35-36 days of age using intracerebral dialysis and HPLC with electrochemical detection. There was an age-related increase in basal DA release and extracellular levels of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), with the greatest increases occurring between 10-11 and 21-22 days of age. Haloperidol (0.1 mg/kg, i.p.) significantly increased DA release at each age compared to control. Also, haloperidol produced a significantly greater increase in DA release at 10-11 days than at 21-22 or 35-36 days of age when expressed as percentage of predrug release. Haloperidol increased DA release over 60 min to 235%, 138%, and 158% above baseline at 10-11, 21-22, and 35-36 days of age, respectively, after which time the levels remained relatively constant. Haloperidol significantly increased extracellular DOPAC and HVA levels at each age compared to controls, but there were no significant differences in DOPAC or HVA levels between ages in response to haloperidol. The results indicate that, at 10 days of age, DA release in the striatum is physiologically functional and that the regulatory feedback control of DA release and metabolism in the striatum develops prior to 10 days of age.

Inhibition of dopamine release by methylenedioxymethamphetamine is mediated by serotonin.

(+/-)-3,4-Methylenedioxymethamphetamine (MDMA), at doses of 0.1, 1 and 10 mg/kg, produced a long-lasting decrease in extracellular dopamine concentration in the neostriatum of anesthetized rats, as measured by in vivo voltammetry. Since MDMA has been shown to release serotonin from rat brain slices and synaptosomes, we examined the possibility that increased serotonin release might be the cause of the decrease in dopamine release. Rats were treated with d,l-p-chloroamphetamine seven days prior to acute MDMA administration. Rats pretreated with p-chloroamphetamine, which produced a marked decrease in serotonin content, showed no significant decrease in extracellular dopamine concentration when administered 10 mg/kg MDMA. These data suggest that MDMA produces a significant decrease in dopamine release when administered acutely, and that this decrease is an indirect effect mediated by an increase in serotonin release.

Dye-coupling in the neostriatum of the rat: I. Modulation by dopamine-depleting lesions.

Evidence from experiments performed in turtle and fish retina suggests that dopamine (DA) modulates the permeability of gap junctions. The present experiment was aimed at determining if DA has a similar role in the mammalian neostriatum. Adults rats received one of four treatments: unilateral electrolytic substantia nigra lesions, unilateral injection of 6-hydroxydopamine (6-OHDA) into the substantia nigra, unilateral neocortical aspiration, or no treatment. After 3-5 weeks, neostriata from both sides of the brain were prepared for in vitro intracellular recordings. Recorded neurons (N approximately 150) were filled with Lucifer Yellow (LY), a low molecular weight dye that crosses gap junctions. In animals with electrolytic nigral lesions, dye-coupling in the ipsilateral neostriatum occurred after 38% of the intracellular injections. After 6-OHDA lesions, 19% of the injections produced dye-coupling in the ipsilateral neostriatum. This difference may have been accounted for by the fact that electrolytic lesions produced a greater degree of DA loss than 6-OHDA injections. Both of these percentages contrast with the very small percentage of dye-coupling found in intact animals or in animals with neocortical lesions. Dye-coupling occurred only between medium-sized spiny cells. No morphological differences between dye-coupled and non-dye-coupled cells were observed with light microscopy. Overall, passive and active electrophysiological properties of dye-coupled and single neurons were similar. The results suggest that DA may function in the neostriatum to control permeability of gap junctions.

Phenylcyclohexylamine: effect of a metabolite of phencyclidine on the efflux of dopamine in the rat.

The effect of phenylcyclohexylamine (PCA) on the efflux of dopamine (DA) in the neostriatum was examined using in vivo electrochemical techniques. Phenylcyclohexylamine produced a long-lasting dose-dependent biphasic effect on the efflux of DA in the rat. This response, to one of the major metabolites of phencyclidine, was similar in duration to but less potent than that seen with phencyclidine.

The ontogeny of amphetamine-induced dopamine release in the caudate-putamen of the rat.

Amphetamine-induced dopamine (DA) release in the caudate-putamen of adult rats was compared with that in the 35-36-day-old and 21-22-day-old rat pup, using in vivo voltammetry. In the adult and 35-36-day groups, 1.0 mg/kg amphetamine (AMP) produced a significant increase in DA release, while 0.1 mg/kg produced no significant change in DA release. In the 21-22-day group, 1.0 mg/kg AMP produced a slight increase, followed immediately by a significant decrease in DA release. Similarly, at a dose of 0.1 mg/kg, AMP produced a significant decrease in DA release. This decrease was greater than that seen after the 1.0 mg/kg dose of AMP. Tyramine produced no significant change in DA release, however, it served as a control for peripheral cardiovascular effects. These data suggest that AMP-induced DA release in the caudate-putamen is mature by postnatal day 35. The AMP-induced decrease in DA release found in the 21-22-day group is not due to either the cardiovascular effects of AMP or to a depletion of DA content. This decrease in neostriatal DA release may be due to a decrease in the neuronal firing of nigrostriatal DA neurons that is caused by an AMP-induced increase in dendritic DA release in the substantia nigra, exerting an inhibitory effect through DA autoreceptors.

Effects of electrical stimulation on acetylcholine synthesis in cat caudate nucleus.

The concentration of endogenous- and deuterium-labeled acetylcholine (ACh) in the cat caudate nucleus was determined after stimulation of either the substantia nigra or the precruciate cortex. In this procedure the caudate nucleus is exposed surgically, and a coring device is used to obtain biopsy specimens which are immediately frozen in liquid nitrogen. Samples are collected at rest, 5 min after stimulation, and again 5 min after a resting period. An infusion of 2H9-choline is maintained during these manipulations to provide a label for ACh synthesis. Electrical stimulation of the substantia nigra, which increases the release of dopamine, produced a decrease in endogenous ACh and the newly synthesized deuterium-labeled ACh. Stimulation of the precruciate cortex produced no significant effect on the levels or synthesis of ACh, but attenuated the effect of subsequent nigral stimulation. These preliminary results indicate that stimulation of the substantia nigra has a net excitatory effect on ACh synthesis in the caudate. This stimulation apparently is modulated by input from the cortex.