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.

On the synthesis of neurotransmitter receptor agonists with antagonist potential

The synthesis of a new type of antagonist is described, capable of inactivating neuroreceptors with heretofore unattainable selectivity and permanence. These antagonists are referred to as mazek agonists (i.e. direct, inhibitory agonists) as they have the high receptor affinity and initial receptor-stimulatory effect of direct agonists and are positively coupled to effector systems. However, like direct antagonists, they have a high receptor affinity and the potential to inhibit or prevent receptor stimulation. The synthesis of the present compounds consisted of the covalent attachment of a tethered dye to three different neurotransmitter analogues, resulting in dye-neuropeptide conjugates with a high affinity for the FMRFa receptor. The dye was prepared from azure B (Az), the neurotransmitter was the neuropeptide FMRFamide (FMRFa), and the dye-neuropeptide conjugates synthesized were Az-CFMRFa; Az-CFMRF and Az-CLRFa. In this procedure, the analogues serve as carrier molecules, bound at one end to the receptor and at the other end to the dye, which is thereby brought into close contact with the receptor. The receptor can then be inactivated by singlet oxygen generated by laser irradiation of the photosensitized receptor.

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.

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.

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.

Lack of effect of gamma-hydroxybutyrate on mu, delta and kappa opioid receptor binding

gamma-Hydroxybutyrate (GHB) and morphine induce a number of similar effects. Moreover, the effects they elicit can be reversed by the opiate antagonist naloxone (NX), suggesting that GHB may produce at least some of its central effects by acting as an opiate agonist. The present study considered this possibility by examining the effect of GHB on mu, delta, and kappa-opioid receptor binding in concentrations of 1 nM-0.1 mM. GHB was inactive in each instance, at every dose examined. GHB is consequently not a direct opiate receptor agonist. It is also unlikely to be an indirect (enkephalin or dynorphin release-stimulating) agonist. The mechanism of action involved whereby NX can reverse the effects of GHB must therefore not involve opioid mechanisms; at least not directly.

Receptor inactivation by dye-neuropeptide conjugates: 2. Characterization of the quantum yield of singlet oxygen generated by irradiation of dye-neuropeptide conjugates

Different neuropeptide analogues of the neurotransmitter FMRFamide were covalently attached to a tethered dye, forming dye-neuropeptide conjugates capable of stably binding to the FMRFamide receptors. Singlet oxygen (1 delta O2) generated by laser irradiation of the conjugates bound to this receptor should inactivate it if (a) the distance 1 delta O2 must diffuse to reach the photo-sensitized receptor is less than 1000 A, (b) the conjugate binds the receptor with the same affinity as the indigenous neurotransmitter, and (c) the quantum yield (phi) of 1 delta O2 is sufficient. Previous studies determined that the first two constraints are satisfied. The results of the present study confirm that the third constraint is also satisfied, as the phi of 1 delta O2 resulting from the laser irradiation of the conjugates were uniformly large, exceeding those for the dye itself, ranging from 0.25 at pD 6.0 to 0.93 at pD 9.0.

Receptor inactivation by dye-neuropeptide conjugates. 3. Comparative binding of dye-neuropeptide conjugates to FMRFamide receptors of Helix aspersa and Loligo pealei

Three neuropeptide analogues of FMRFamide (FMRFa) were covalently attached to a tethered derivative of methylene blue to form dye-neuropeptide conjugates. The comparative binding of the latter to FMRFa receptors was subsequently examined in both Helix aspersa (circumesophageal ganglia) and squid (optic lobe membrane). In Helix, the FMRFa analogue CFMRFamide (CFMRFa) inhibited the specific binding of the FMRFa ligand [125I]daYFnLRFa in a dose-dependent manner. Az-CFMRFa, one of the dye-neuropeptide conjugates, also dose-dependently inhibited the specific binding of [125I]daYFnLRFa. Moreover, their potencies equaled or exceeded that of FMRFamide. In squid, the binding of CFMRFa and FMRFa was similar. However, the dye-neuropeptide conjugate (IC50 of 14 nM) was about 44-fold less potent than FMRFa. The conjugates were synthesized as part of a study seeking to target and inactivate preselected receptors with heretofore unattainable selectivity and permanence.

Receptor inactivation by dye-neuropeptide conjugates: 1. The synthesis of Cys-containing dye-neuropeptide conjugates

In an attempt to attenuate specifically identified receptors through photolysis, a four-step synthesis is of a useful tethered derivative of Azure-B (Az) was developed After characterization, this derivative was covalently attached to CFMRFamide, CFMRF, and CLRFamide (i.e., three different neuropeptide analogues of the putative neurotransmitter FMRFamide. This resulted in the formation of three dye-neuropeptide conjugates: Az-CFMRFamide, Az-CFMRF, and Az-CLRFamide.

Suppression of neuropathic pain behavior in rats by a non-psychotropic synthetic cannabinoid with NMDA receptor-blocking properties

HU211 is a novel synthetic derivative of tetrahydro-cannabinol (THC), the active marijuana ingredient. The stereochemistry of HU211 is enantiomeric to that of THC. In contrast to THC, HU211 is not psychotropic. This agent exhibits other types of biological activities; it is a non-competitive NMDA receptor blocker and has antinociceptive activity when injected with cupric chloride. This study examined its effects in autotomy, a behavioral model of neuropathic pain. Autotomy, a behavior of self-mutilation of denervated areas, was induced in Sabra rats by cutting the sciatic and saphenous nerves. We found that injections of HU211 (2.5 mg/kg) with cupric chloride (0.8 mg/kg) every 2nd day markedly suppressed autotomy during the injection period by delaying its average onset day and reducing the incidence of severe autotomy. Moreover, suppression of autotomy was retained in the postinjection period (for at least 30 days) but only when the drug was injected intraperitoneally. Lesser effects were achieved by subcutaneous injections. Cupric chloride or HU211 alone were ineffective. The general behavior and open field motor activity indicated that the effects of HU211 with Cu++ on autotomy were not due to sedation or ataxia but presumably due to antinociception mediated by NMDA receptor blockade.

Nonpsychotropic cannabinoid acts as a functional N-methyl-D-aspartate receptor blocker

Binding studies using the enantiomers of the synthetic cannabinoid 7-hydroxy-delta 6-tetrahydrocannabinol 1,1-dimethylheptyl homolog in preparations of rat brain cortical membranes reveal that the (+)-(3S,4S) enantiomer HU-211 blocks N-methyl-D-aspartate (NMDA) receptors in a stereospecific manner and that the interaction occurs at binding sites distinct from those of other noncompetitive NMDA antagonists or of glutamate and glycine. Moreover, HU-211 induces stereotype and locomotor hyperactivity in mice and tachycardia in rat, effects typically caused by NMDA receptor antagonists. HU-211 is also a potent blocker of NMDA-induced tremor, seizures, and lethality in mice. This compound may therefore prove useful as a nonpsychoactive drug that protects against NMDA-receptor-mediated neurotoxicity.

Inhibition of cisplatin-induced emesis in the pigeon by a non-psychotropic synthetic cannabinoid

The (+) enantiomer of the synthetic cannabinoid, 7-hydroxy-delta-6-tetrahydrocannabinol, dimethylheptyl homolog (HU-211), possesses significant antimetic efficacy in the pigeon. However, unlike all anti-emetic cannabinoids tested in the past, it is devoid of psychotropic (cannabimimetic) activity. The anti-emetic activity of HU-211 was determined in pigeons given 10 mg/kg i.v. cisplatin, a widely used antitumour agent, which is also a potent emetogenic agent at this dose. This activity was compared with that of delta-1-tetrahydrocannabinol (delta-1-THC). HU-211 pretreatment elicited a dose-related inhibition of cisplatin vomiting, with the optimal dose of HU-211 (2.5 mg/kg) inhibiting emesis by nearly 90%. Delta-1-THC in doses up to 5 mg/kg caused only an insignificant reduction in vomiting. The activity was increased in the presence of cupric chloride (0.8 mg/kg). The optimal dose of delta-1-THC (5.0 mg/kg) with CuCl2 very significantly diminished the total amount of vomitus expelled (up to 90%). However, it failed to inhibit emesis in 50% of all animals tested, did not significantly affect the time of onset of emesis and was highly psychotropic. The optimal dose of HU-211 (2.5 mg/kg) with CuCl2 inhibited emesis by 97%, significantly delayed the time on onset of emesis in the very few animals that did vomit and was completely non-psychotropic. The curve for the antiemetic effect of HU-211 was U-shaped over a narrow dose range. The present report demonstrates that complete separation of psychotropic and antiemetic activities is possible in the cannabinoid series.

Enantiomeric cannabinoids: stereospecificity of psychotropic activity

The 1,1-dimethylheptyl homolog of (-)-(3R,4R)-7-hydroxy-delta-6- tetrahydrocannabinol (compound II) is highly psychotropic in mice, rats and pigeons. The (+)-(3S,4S) enantiomer (III) was found to be psychotropically inactive at doses up to several thousand times those of the ED50 of (II).

Diethylstilbestrol counteracts barbiturate narcosis and hypothermia in male mice

Treatment with DES one hour prior to pentobarbital injection resulted in diminution of the narcotic sleep and hypothermia usually found after pentobarbital (50 mg/kg 1) injection in male mice. The effect was biphasic: significantly countered relative to saline pretreated controls at low (.001-.10 mg/kg 1) and at high (10-50 mg/kg -1) DES doses only. Giving DES alone did not change core body temperature compared to saline injected controls, at either 25 degrees C or at 33 degrees C. At 33 degrees C, neither PeB narcosis nor body temperature loss was significantly inhibited by DES. Possible cytoplasmic, nonsex differentiated bases for these estrogen effects on barbiturate action in the brain is discussed.

Implications of dopamine agonist-induced hypothermia following increased density of dopamine receptors in the mouse

An investigation was made of hypothermia induced by dopamine (DA) agonists as a model of the effect of various treatments or conditions on the sensitivity of central postsynaptic DA receptors. Selective supersensitivity of these receptors (defined as an increase in Bmax) was induced by means of intraventricular injections of 6-hydroxydopamine (6-OHDA) in animals pretreated with desmethylimipramine (DMI). Supersensitivity was also produced by the chronic administration of haloperidol. The supersensitivity of DA receptors induced by 6-OHDA was found to be associated with a reduced hypothermic response to apomorphine. Supersensitivity elicited by the chronic administration of haloperidol, which very probably did not produce a specific effect on the density of DA receptors but also affected serotonergic receptors, did not elicit any change in hypothermia induced by apomorphine. The results of the present study are not consistent with the view that DA receptors mediate hypothermia per se, but rather suggest that hypothermia induced by DA agonists is more complex, probably involving serotonergic receptors primarily, though other factors may also be contributory. Furthermore, the results of the present study suggest that the functional significance of supersensitivity of DA receptors induced by 6-OHDA versus chronic treatment with haloperidol may be quite different, depending upon the effector system examined.

Effect of naloxone on dopamine uptake and release in vitro in the striatum

The opiate antagonist naloxone (NX) can inhibit the stereotyped behavior elicited by the directly acting dopamine (DA) agonist apomorphine, while potentiating the same behavior induced by amphetamine, an indirect DA agonist. Since NX does not appear to bind to striatal 3H-DA receptors, these effects may be mediated by an indirect action exerted by naloxone on the nigrostriatal pathway. To confirm this, the effect of NX on DA uptake in striatal homogenates was assessed in vitro, and also the effect of NX on DA release in both striatal synaptosomes and slices. While NX had no effect on its own in either of these preparations, it was able to significantly enhance the release of DA induced by d-amphetamine. Since NX has no effect by itself on either behavior or DA release, it appears that NX can exert an active effect on the release of striatal DA only if the nigrostriatal pathway is in a state of arousal (e.g. by amphetamine).

The comparative roles of dopaminergic and serotonergic mechanisms in mediating quipazine induced increases in locomotor activity

The effect of dopaminergic and serotonergic agonists and antagonists on quipazine induced locomotor activity was investigated in rats. Though quipazine is generally considered to be a relatively pure serotonergic agonist, its effects on locomotor activity were inhibited by small doses of a centrally acting DA receptor blocking agent (haloperidol), while three different serotonergic (5-HT) antagonists were without effect on this behavior. Moreover, quipazine induced locomotor activity was markedly inhibited by the 5-HT substrate 5-hydroxytryptophan. The data suggest that quipazine induced locomotor activity primarily involves dopaminergic mechanisms, with 5-HT playing either no role in stimulating this behavior, or a subsidiary one, requiring intact central dopaminergic receptors for its expression.

Evidence for the involvement of central dopaminergic receptors in the acute and chronic effects induced by barbiturates

There is close agreement in the literature concerning the effect of specific and directly acting dopaminergic (DA) agonists and antagonists on acute barbiturate-induced responses; with DA agonists inhibiting and DA agonists potentiating these responses. On the other hand, there are presently no studies (and hence no evidence) regarding the effects of direct and specific alterations of DA receptor arousal on chronic barbiturate-induced tolerance or withdrawal. Concerning the effect of acute barbiturate administration on central dopaminergic responses; while some studies report no effect, there is some evidence that barbiturates block DA reuptake after their acute administration. This is consistent with and may explain findings that these drugs also decrease striatal DA turnover acutely, decrease DA concentration in synaptosomes, and decrease postsynaptic DA receptor arousal. In noting the potentiation of acute barbiturate-induced responses elicited by DA antagonists, it is interesting to observe that barbiturates and DA antagonists both apparently decrease receptor sensitivity and inhibit DA reuptake presynaptically. Moreover, the supersensitivity to DA agonists induced by chronic DA antagonist administration can be potentiated by barbiturates. Thus, barbiturates appear to block the arousal of postsynaptic DA receptors, though probably not those coupled to adenylate cyclase and, unlike neuroleptics, indirectly. It is likely that the inhibitory effect on DA receptor arousal exerted by barbiturates accounts for at least some of the central effects produced by these drugs.

Possible dopamine agonist properties of quipazine maleate

Though quipazine is widely regarded as a relatively pure serotonergic (5-HT) agonist and has been reported to have no dopamine (DA) agonist properties, it has produced stereotyped behavior (SB) associated with DA agonist arousal of striatal DA mechanisms. Since we observed a dose-related inhibition of quipazine induced stereotypy (QISB) by a centrally acting DA antagonist (haloperidol) that could not be mimicked by a central 5-HT receptor blocking agent (methysergide), it appeared likely that QISB is mediated by striatal DA mechanisms. This was further supported by our observing that QISB could be potentiated by a subthreshold dose of the central DA agonist apomorphine. In light of this, and the presence of abnormal movements seen concomitantly with QISB that are typically produced by intrastriatal injections of 5-HT agonists, it appears that QISB is a complex phenomenon. While QISB seems to be primarily due to the stimulation of DA mechanisms, the effect of quipazine on behavior appears to be a combined result of its effects on both DA and 5-HT mechanisms. Specifically, central striatal DA receptors appear to mediate QISB per se, while serotonergic mechanisms stimulated by quipazine inhibit its further development and produce extrapyramidal-like abnormal movements.

Adrenal glucocorticoids as a required factor in the development of ethanol tolerance in mice

Male C57BL/6J mice were given chronic ethanol treatment by two procedures. One was by a liquid diet containing 6.4% (v/v) ethanol; the other was by repeated injections of ethanol (3.5 g/kg, i.p. twice daily). After 5 days of the liquid diet treatment, sleep time following a challenge dose of ethanol (3.0 g/kg i.p.) was reduced to 10 min as compared with 30 min in the controls not previously exposed to ethanol. After 10 days, none of the ethanol-treated animals slept. Bilateral adrenalectomy (Adx) had no effect on sleep time (34 min). However, the reduction of sleep time in ethanol-treated Adx animals was much less: 19 min after 5 days, and 20 min after 10 days. Replacement with corticosterone in Adx animals restored the reduction of sleep time to the same levels as intact animals, indicating that glucocorticoids are the hormonal factor involved in the Adx effect. By the injection procedure for short-term administration of moderate doses of ethanol, sleep time following the injection of 3.5 g/kg ethanol was reduced from 95 min on day 1 to 57 min on day 3. There was no difference between intact and Adx animals. The results from both treatment procedures suggest that Adx abolished the later, severe stage of tolerance development but not the initial stage. Functional tolerance to ethanol occurring in this advanced stage may be a glucocorticoid-dependent process.

Morphine may not produce true catalepsy

Though catalepsy is one of the primary features classically associated with morphine injections in animals, several investigators have suggested that morphine may not produce true catalepsy. A study was therefore undertaken using the most widely accepted tests of catalepsy to determine whether a dose related catalepsy could be obtained in rats. The effect produced by morphine was then compared with the catalepsy elicited by subthreshold to suprathreshold doses of haloperidol. In the course of catalepsy assessment, it was found that half the tests employed could not distinguish between the several doses of morphine that were administered. Moreover, the cataleptoid behavior induced by morphine failed to satisfy nearly all of the criteria most widely used for catalepsy. This is in marked contrast to the results obtained with varying doses of haloperidol. These results are compatible with the suggestion that morphine may not be a true cataleptigenic agent.

Genetic factors influencing neurosensitivity to early phenobarbital administration in mice

This study was designed to assess the possible genetic determinants of neurosensitivity to early (neonatal) phenobarbital (PhB) administration and to conduct a strain comparison for the cerebellar histology of both inbred and outbred mice. HS/Ibg, C57BL/10 and DBA/1 pups were injected with 50 mg PhB/kg daily on neonatal days 2-21. On day 50, treated animals (B) of all strains had smaller brains than controls (C). Moreover, the cerebellar area was decreased in HS and C57 B mice but not in DBA mice, suggesting genotype-environment interaction. B mice from all strains had similar Purkinje cell losses. Strain comparison showed that control C57 mice had smaller brains than control HS, and DBA had smaller brains than both HS and C57. Similarly, C57 had smaller cerebellar layers than HS and DBA had smaller cerebellar layers than both HS and C57. DBA and C57 mice had fewer Purkinje cells than HS but did not differ from each other.

Lessened sensitivity to apomorphine induced climbing behavior in mice following neonatal exposure to phenobarbital

Heterogeneous HS/Ibg mice were injected daily with 50 mg/kg phenobarbital (PhB) from age 2 to 21 days (B group), while control litter mates received vehicle injections. Control and a B mice were then tested for striatal climbing behavior induced by 2 or 5 mg/kg apomorphine, at ages 22, 28, 35 or 44 days. At age 22 days B mice had reductions of climbing from control levels of 44% and 41% for 2 and 5 mg apomorphine respectively (p less than 0.01), regardless of sex differences. On day 28 the respective reductions were 16% and 32% (p less than 0.05). The differences on days 35 and 44 were small and did not reach statistical significance. Since climbing has been found to be induced primarily by postsynaptic striatal dopaminergic receptors, it is suggested that neonatal exposure may reduce the behaviors mediated by striatal postsynaptic dopamine receptors. The present findings support other studies implicating postsynaptic striatal dopamine receptors in the behavioral alterations induced by early PhB exposure.

The effect of drugs altering striatal dopamine levels on apomorphine induced stereotypy

Drugs that increase or decrease striatal dopamine levels appear to affect apomorphine induced stereotypy. This finding was unexpected, as it has previously been maintained that drugs which exert any action on striatal DA terminals exclusively would affect only indirect dopaminergic agonists, as opposed to apomorphine which induces stereotypy by acting directly on postsynaptic dopamine receptors. Specifically, inhibiting intrastriatal dopamine levels inhibits this behavior. This effect is explained in terms of apomorphine having a greater intrinsic activity and agonist affinity for striatal dopamine receptors than dopamine itself. Thus, dopamine and drugs which promote its release, may diminish the central behavioral effects induced by apomorphine relative to drugs which inhibit dopamine release centrally.

The role if dopamine and norepinephrine uptake inhibition in mediating the central effects of d-amphetamine in vivo

Male Sprague-Dawley rats were injected with 1.5 mg/kg d-amphetamine and later sacrificed at a time when the locomotor activity induced by this dose of d-amphetamine was maximal (60 minutes post injection). Subsequently, both cortical and striatal slices were measured for 3H-DA and 3H-NE uptake. It was found that in vivo, d-amphetamine does not preferentially inhibit the reuptake of DA or NE in either the striatum or cerebral cortex, suggesting that any selective effect on a specific catecholamine must result from a mechanism other than selective uptake inhibition.

Mechanisms of dopamine antagonism by morphine in rodents

A number of investigators have concluded on the basis of a substantial and compelling body of biochemical, pharmacological and behavioral evidence, that opiates and particularly morphine, directly block central dopamine (DA) receptors. This evidence includes the recent finding that cataleptogenic doses of morphine suppress 3H-spiroperidol binding to striatal membranes ex vivo. On the other hand, an important albeit relatively sparse literature of experimental evidence exists suggesting that morphine and other mu-receptor opiates do not directly bind to central dopaminergic receptors. The most convincing evidence to this effect are behavioral findings that morphine potentiates rather than inhibits the stereotyped behavior induced by the direct DA agonist apomorphine and biochemical evidence demonstrating a failure of 3H-morphine or 3H-dihydromorphine to specifically bind central DA receptors in striatal tissue. (Indeed, even those reports that demonstrated a morphine induced suppression of 3H-spirioperidol labelling of DA receptors failed to find a direct effect on post-synaptic receptors.) Evidence is presented in this report to show that morphine acts presynaptically to acutely inhibit DA release, and thus, that morphine inhibition of DA receptor mediated responses is indirect, being the result of an inhibition of pre-synaptic DA release rather than a direct effect exerted on post-synaptic DA receptors themselves.

Neuronal losses in mice following both prenatal and neonatal exposure to phenobarbital

Mice were given phenobarbital (PhB) during neonatal development (NeoB group) and during pre- and neonatal development (Pre- and NeoB group). Prenatal exposure was accomplished transplacentally by feeding the pregnant mothers 3 g PhB/kg milled food on gestation days 9-18. Neonatal exposure was conducted directly by injecting the neonates daily with 50 mg PhB/kg on postnatal days 2-21. The brains of these animals were studied at age 50 days using H & E staining. Consistent with previous studies, NeoB animals had a 14-20% smaller cerebellar layer area than controls, 32% fewer cerebellar Purkinje cells and 34% fewer granule cells than controls. NeoB mice had a 14-18% smaller hippocampal layer and 17% fewer hippocampal pyramidal cells than controls. The number of the hippocampal granule cells was not reduced by early PhB administration. NeoB mice had an 18% smaller cerebral cortex area and 22% fewer cortical cells than controls. The pre- and NeoB animals did not differ from controls in the area of their cerebellar layers, but they did have fewer cerebellar Purkinje and granule cells. Similarly, the Pre- and NeoB animals did not differ from controls in the area of their hippocampal layers, but they had fewer pyramidal cells. The Pre- and NeoB animals had a smaller cerebral layer area than controls but did not differ significantly from controls in the number of cortical neurons. It was suggested that exposure to PhB during prenatal and neonatal periods is not additive. On the contrary, tolerance or compensatory mechanisms may develop after prenatal exposure and lessened the effect of subsequent neonatal PhB exposure.

Morphological alterations in the medial preoptic area after prenatal administration of phenobarbital

Pregnant Sabra mice received 3 g phenobarbital per kilogram milled food from gestational days (GD) 9 to 18 as their only food source; control females received milled food. All the females were divided into three groups and injected intraperitoneally with a single dose of 3H-thymidine on GD 13, 15 or 17. At 50 days of age of the offspring were perfused with 10% neutral formalin and their brains removed and prepared for autoradiography. Matching sagittal sections of the offspring were selected for the study of the medial preoptic area (MPOA) of the hypothalamus. The total number of cells per section was not significantly different between males and females, however, there was a greater cellular packing density (cell/mm2) in females (p less than 0.05). This sex difference disappeared in treated offspring, possibly due to a 76% decrease on GD 13 in the rate of cell proliferation in females (number of labeled cells but not in males (p less than 0.001); the greatest decrease in the number of labeled cells occurred in the sexually dimorphic areas of the MPOA (p less than 0.001). The area of the sagittal section of the MPOA did not differ significantly between sexes or treatments.