Effects of stimulants of abuse on extrapyramidal and limbic neuropeptide Y systems.
Neuropeptide Y (NPY), an apparent neuromodulating neuropeptide, has been linked to dopamine systems and dopamine-related psychotic disorders. Because of this association, we determined and compared the effects of psychotomimetic drugs on extrapyramidal and limbic NPY systems. We observed that phencyclidine, methamphetamine (METH), (+)methylenedioxymethamphetamine (MDMA), and cocaine, but not (-)MDMA, similarly reduced the striatal content of NPY-like immunoreactivity from 54% (phencyclidine) to 74% [(+) MDMA] of control. The effects of METH on NPY levels in the nucleus accumbens, caudate nucleus, globus pallidus, and substantia nigra were characterized in greater detail. We observed that METH decreased NPY levels in specific regions of the nucleus accumbens and the caudate, but had no effect on NPY in the globus pallidus or the substantia nigra. The dopamine D1 receptor antagonist SCH-23390 blocked these effects of METH, suggesting that NPY levels throughout the nucleus accumbens and the caudate are regulated through D1 pathways. The D2 receptor antagonist eticlopride did not appear to alter the METH effect, but this was difficult to determine because eticlopride decreased NPY levels by itself. A single dose of METH was sufficient to lower NPY levels, in some, but not all, regions examined. The effects on NPY levels after multiple METH administrations were substantially greater and persisted up to 48 h after treatment; this suggests that synthesis of this neuropeptide may be suppressed even after the drug is gone. These findings suggest that NPY systems may contribute to the D1 receptor-mediated effects of the psychostimulants. (+info)
The neuropsychopharmacology of phencyclidine: from NMDA receptor hypofunction to the dopamine hypothesis of schizophrenia.
Administration of noncompetitive NMDA/glutamate receptor antagonists, such as phencyclidine (PCP) and ketamine, to humans induces a broad range of schizophrenic-like symptomatology, findings that have contributed to a hypoglutamatergic hypothesis of schizophrenia. Moreover, a history of experimental investigations of the effects of these drugs in animals suggests that NMDA receptor antagonists may model some behavioral symptoms of schizophrenia in nonhuman subjects. In this review, the usefulness of PCP administration as a potential animal model of schizophrenia is considered. To support the contention that NMDA receptor antagonist administration represents a viable model of schizophrenia, the behavioral and neurobiological effects of these drugs are discussed, especially with regard to differing profiles following single-dose and long-term exposure. The neurochemical effects of NMDA receptor antagonist administration are argued to support a neurobiological hypothesis of schizophrenia, which includes pathophysiology within several neurotransmitter systems, manifested in behavioral pathology. Future directions for the application of NMDA receptor antagonist models of schizophrenia to preclinical and pathophysiological research are offered. (+info)
Effects of (+)-HA-966, CGS-19755, phencyclidine, and dizocilpine on repeated acquisition of response chains in pigeons: systemic manipulation of central glycine sites.
The effects of i.m. injections of (+)-HA-966, a glycine-site antagonist at the N-methyl-D-aspartate (NMDA) subtype of the glutamate receptor, its enantiomer (-)-HA-966, the competitive glutamate antagonist CGS-19755, the uncompetitive glutamate antagonists phencyclidine and dizocilpine, and the micro opioid agonist morphine were evaluated in a repeated acquisition task in pigeons. All of the drugs produced dose-dependent decreases in rates of responding. The NMDA receptor and channel blockers and (+)-HA-966 appeared to have a greater effect on acquisition than did morphine at doses that did not fully suppress responding. The rate suppression and learning impairment produced by a large dose of (+)-HA-966 (100 mg/kg) were completely prevented by coadministration of the glycine-site agonist D-serine (560 mg/kg) but not by its enantiomer, L-serine (1000 mg/kg). D-Serine, however, produced incomplete antagonism of the effects of dizocilpine and phencyclidine and failed to alter those of CGS-19755. These findings provide evidence that reducing the activity of the NMDA subtype of the glutamate receptor through pharmacological action at any of three sites produces similar decrements in acquisition, and those produced through antagonism of the glycine site are differentially sensitive to the glycine-site agonist D-serine. (+info)
Clozapine, but not haloperidol, prevents the functional hyperactivity of N-methyl-D-aspartate receptors in rat cortical neurons induced by subchronic administration of phencyclidine.
Repeated exposure of rats to the psychotomimetic drug phencyclidine (PCP) markedly increased the response of prefrontal cortical neurons to the glutamate agonist N-methyl-D-aspartate (NMDA) relative to agonist alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid. Moreover, acute challenge by PCP produced a significantly reduced block of NMDA-induced current. In addition, the subchronic administration of PCP reduced significantly the paired-pulse facilitation, accompanied by a significant increase of excitatory postsynaptic current variance. These results suggest that repeated exposure to PCP increased evoked release of excitatory amino acids. The enhanced release of excitatory amino acids evoked by NMDA could explain, at least partly, a hypersensitive response to NMDA and a reduced blockade of the NMDA responses by a PCP challenge in rats exposed repeatedly to PCP. Pretreatment with the atypical antipsychotic drug clozapine, but not the typical antipsychotic drug haloperidol, attenuates the repeated PCP-induced effect. Our results support the hypothesis that clozapine may facilitate NMDA receptor-mediated neurotransmission to improve schizophrenic-negative symptoms and cognitive dysfunction. This novel approach is useful for evaluating the cellular mechanisms of action of atypical antipsychotic drugs. (+info)
Rat strain differences in the ability to disrupt sensorimotor gating are limited to the dopaminergic system, specific to prepulse inhibition, and unrelated to changes in startle amplitude or nucleus accumbens dopamine receptor sensitivity.
Previous studies indicate that a variety of pharmacological agents interfere with the prepulse inhibition of the acoustic startle (PPI) response including phencyclidine (PCP), 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), amphetamine, and apomorphine. Strain differences have been observed in the ability of apomorphine to disrupt PPI, although the degree to which these strain differences occur after administration of nondopaminergic drugs or the degree to which differences can be observed in other models of dopamine (DA) receptor activation has not been elucidated. The present study tested the effects of apomorphine, amphetamine, 8-OH-DPAT, and PCP on PPI in the Sprague Dawley and Wistar rat strains. Because apomorphine disrupts PPI via activation of DA receptors in the nucleus accumbens, apomorphine-induced hyperlocomotion, also a behavioral model of nucleus accumbens DA receptor activation, was measured in both rat strains. Administration of PCP or 8-OH-DPAT attenuated PPI in both strains, whereas apomorphine and amphetamine only attenuated PPI in Wistar rats. The ability of apomorphine to increase motor activity in the absence of a startle-eliciting stimulus was similar in the two strains, as was apomorphine-induced hyperlocomotion. A time course analysis of the effects of apomorphine on startle response in Sprague Dawley rats found that changes in the magnitude of PPI followed changes in basic startle amplitude. Similarly, no apomorphine-induced attenuation of PPI was observed in Sprague Dawley rats after 6-OHDA-induced DA receptor supersensitivity in the nucleus accumbens. These data suggest a dissociation between the effects of DA receptor agonists in PPI and other behavioral models of DA receptor activation. (+info)
Effects of sustained phencyclidine exposure on sensorimotor gating of startle in rats.
Phencyclidine (PCP), a non-competitive NMDA antagonist with actions at multiple other central nervous system receptors, can cause both acute and lasting psychoses in humans, and has also been used in cross-species models of psychosis. Acute exposure to PCP in rats produces behavioral changes, including a loss of prepulse inhibition (PPI) of the startle reflex, which parallels the loss of PPI observed in schizophrenia patients. Sustained exposure to PCP in rats produces neuropathological changes in several limbic regions and prolonged behavioral abnormalities that may parallel neuropsychological deficits in schizophrenia. It is unclear whether sustained PCP exposure will also produce a loss of prepulse inhibition which parallels the decrease observed in schizophrenia patients. In the present study, we examined changes in PPI during and after sustained PCP administration, using 5-day PCP exposure via subcutaneous osmotic minipumps, or 14-day PCP exposure via repeated intraperitoneal injections. In both forms of drug delivery, PPI was disrupted during, but not after, sustained drug exposure. PPI does not appear to be sensitive to neuropathological effects of sustained PCP exposure. (+info)
Excitatory actions of NMDA receptor antagonists in rat entorhinal cortex and cultured entorhinal cortical neurons.
We have characterized excitatory effects of non-competitive NMDA receptor antagonists MK-801, PCP, and ketamine in the rat entorhinal cortex and in cultured primary entorhinal cortical neurons using expression of immediate early gene c-fos as an indicator. NMDA receptor antagonists produced a strong and dose-dependent increase in c-fos mRNA and protein expression confined to neurons in the layer III of the caudal entorhinal cortex. Induction of c-fos mRNA is delayed and it is inhibited by antipsychotic drugs. Cultured entorhinal neurons are killed by high doses of MK-801 and PCP but c-fos expression is not induced in these neurons indicating that this in vitro model does not fully replicate the in vivo effects of PCP-like drugs in the entorhinal cortex. Excitatory effects of the NMDA receptor antagonists may be connected with the psychotropic side effects of these drugs and might become a useful model system to investigate neurobiology of psychosis. (+info)
Characterization of interaction of 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester with Torpedo californica nicotinic acetylcholine receptor and 5-hydroxytryptamine3 receptor.
The widely used calcium channel antagonist 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester (TMB-8) has been identified as a noncompetitive antagonist (NCA) and open-channel blocker of both muscle- and neuronal-type nicotinic acetylcholine receptors (AChRs). To further examine the interaction of TMB-8 with the AChR, the compound was tested as a competitor for the binding of two NCAs of the Torpedo californica AChR, phencyclidine and 3-trifluoromethyl-3-(m[125I]iodophenyl)diazirine, for which the binding to the AChR has been pharmacologically well characterized and a channel binding loci has been established. TMB-8 fully inhibited specific photoincorporation of 3-trifluoromethyl-3-(m[125I]iodophenyl)diazirine into the resting AChR channel (IC50 = 3.1 microM) and inhibited high-affinity [3H]phencyclidine binding to the desensitized AChR (IC50 = 2.4 microM). We conclude that TMB-8 is a potent NCA of the nicotinic AChR, interacting with the resting, open-channel, and desensitized channel conformations. TMB-8 was next tested as an inhibitor of the structurally homologous 5-hydroxytryptamine (5-HT)3 receptor (5-HT3R). Using 5-HT3R containing Sf21 cell membranes, TMB-8 completely inhibited specific binding of the radiolabeled 5-HT3R antagonist [3H]GR65630 (Ki = 2.5 microM). Furthermore, TMB-8 antagonized 5-HT-evoked currents of both mouse and human 5-HT3Rs expressed in Xenopus laevis oocytes, and additional analysis was consistent with a competitive antagonistic mechanism of action. These results, taken together, indicate that TMB-8 antagonizes the function of the AChR and 5-HT3R by different mechanisms. Given the sequence similarity and emerging evidence of structural homology in the channels of these two receptors, these results underscore the existence of subtle yet important structural differences in each channel. (+info)