Phencyclidine (PCP) injected in the nucleus accumbens increases extracellular dopamine and serotonin as measured by microdialysis

Possible abuse potential of the NMDA antagonist MK-801

Selective antagonists of the N-methyl-D-aspartate (NMDA) receptor such as MK-801 may have therapeutic potential in the treatment of ischemic brain injury. However, some drugs (e.g. ketamine and phencyclidine) with potent NMDA antagonist properties are also addictive. Intracranial self-stimulation (ICSS) is facilitated by drugs of abuse such as cocaine and amphetamine and thus is useful for screening compounds with potential abuse liability. Low doses of the NMDA antagonist, MK-801 were also found to facilitate ICSS, suggesting that this compound may possess abuse potential.

Haloperidol given chronically decreases basal dopamine in the prefrontal cortex more than the striatum or nucleus accumbens as simultaneously measured by microdialysis

Simultaneous microdialysis was performed in the prefrontal cortex, striatum, and the caudal region of the nucleus accumbens using implanted guide shafts with removable microdialysis probes. Extracellular dopamine (DA), dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) were measured in response to acute and chronic haloperidol, an antipsychotic drug. Six rats received haloperidol (0.5 mg/kg, IP) while changes in DA and its metabolites were monitored for six hours. Then the microdialysis probes were removed and daily injections of haloperidol were given for 28 more days. Next the probes were reinserted, and a final haloperidol challenge was given. A control group of six rats was treated the same way but with saline. The first injection of haloperidol increased extracellular DA, DOPAC and HVA proving increased turnover in all three regions. A month later, after chronic treatment, basal DA and its metabolites decreased in just the cortex. DOPAC and HVA also decreased in the cortex and striatum, suggesting a decrease in turnover at these two sites. None of these changes occurred in the accumbens. The last haloperidol challenge after chronic treatment no longer increased extracellular DA in any of the three sites, but increased production of metabolites was still detectable. This suggests a lack of DA responsiveness in all three sites. In summary, chronic haloperidol affected each of the regions differently; DA metabolites decreased in the STR and PFC; basal DA decreased in the PFC.

Fenfluramine administered systemically or locally increases extracellular serotonin in the lateral hypothalamus as measured by microdialysis

Microdialysis was used to monitor serotonin, 5-hydroxyindoleacetic acid (5-HIAA) and the metabolites of dopamine, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the perifornical lateral hypothalamus (PFH) of freely moving rats. Systemically administered d-fenfluramine (d-FEN) increased extracellular serotonin, DOPAC and HVA, while decreasing serotonin's metabolite 5-HIAA. Local application of d-FEN directly to the hypothalamic terminal region caused large increases in extracellular serotonin and had a tendency to decrease all 3 metabolites. This effect was confirmed when d-FEN was infused locally by reverse dialysis. These results provide direct evidence that the anorectic drug d-FEN can increase extracellular serotonin in the hypothalamus in vivo and suggest a serotonergic action in the perifornical region. This finding is consistent with our report that a meal also increases serotonin in this region.

Nicotine infused into the nucleus accumbens increases synaptic dopamine as measured by in vivo microdialysis

It has been postulated that addiction to nicotine is mediated by dopamine release in the mesolimbic system. It is possible that nicotine might act directly on the dopamine terminals to release dopamine. This hypothesis was tested by infusing nicotine through a microdialysis probe into the nucleus accumbens of freely moving rats. Dopamine, dihydroxyphenylacetic acid, and homovanillic acid from the extracellular space were collected by microdialysis and measured by high pressure liquid chromatography. Nicotine increased extracellular dopamine in a dose-related manner. Systemic injection of the nicotine antagonist mecamylamine blocked the dopamine increase induced by local nicotine. These results suggest that nicotine releases dopamine by a local action in the nucleus accumbens terminal area of the mesolimbic system. Presynaptic induction of dopamine release might play a role in nicotine addiction.

Serotonin release estimated by transcortical dialysis in freely-moving rats

The transcerebral dialysis method has been utilized for measuring extracellular brain concentrations of serotonin and 5-hydroxyindolacetic acid. Dialysis fibres were implanted transversally in the rat frontal cortex and perfused by Ringer. Serotonin and 5-hydroxyindolacetic acid were quantified by reverse phase high performance liquid chromatography with electrochemical detection. Experiments were performed in freely-moving rats 20-24 h after the implant of the fibre. Basal output of serotonin and 5-hydroxyindolacetic acid was 0.12 and 22.8 pmol in 20 min, respectively. The output of serotonin was calcium-dependent and tetrodotoxin-sensitive (1 micron in the Ringer) while was stimulated by veratridine (50 microns) and by high concentrations of K+ (60 and 100 mM). Serotonin output was increased in a concentration-dependent manner by chlorimipramine (1-10 microM) in the Ringer; this drug stimulated serotonin release also when administered s.c. (20 mg/kg) in a tetrodotoxin-sensitive manner. The irreversible monoamine-oxidase inhibitor pargyline (75 mg/kg, i.p.) strongly stimulated serotonin output while reduced 5-hydroxyindolacetic acid output. A proposed serotonin releaser, fenfluramine (25 mg/kg, s.c.), stimulated serotonin release and this effect was strongly potentiated by local application of tetrodotoxin (1 microM). Agonists of serotonin receptors such as lisuride (0.03 mg/kg, s.c.), 8-hydroxy-2-(di-n-propilamino)tetraline (0.25 mg/kg, s.c.) and 5-methoxy 3(1,2,3,6-tetrahydro-4-pyridinil)-1H indole succinate (1 mg/kg, s.c.) reduced serotonin release. It appears that brain dialysis is a suitable method for the study of serotonin release in the cortex of freely-moving rats.

Hypothalamic infusion of amphetamine increases serotonin, dopamine and norepinephrine

Microinjections of amphetamine into the lateral hypothalamus are known to cause anorexia and hypodipsia. These effects are thought to be mediated by an action of amphetamine on the catecholaminergic terminals to release dopamine and norepinephrine and block reuptake. Direct evidence of neurochemical release was lacking; therefore microdialysis was used to measure monoamines and their metabolites in the extracellular fluid of the lateral hypothalamus while amphetamine diffused out through the microdialysis probe. Amphetamine infusion significantly increased serotonin, dopamine and norepinephrine; it decreased dihydroxyphenylacetic acid (DOPAC), 5-hydroxyindoleacetic acid (5-HIAA), and did not change homovanillic acid (HVA). These results suggest that amphetamine releases dopamine, norepinephrine and serotonin and blocks reuptake which thereby retards neurotransmitter breakdown. The net effect was a quadrupling of extracellular monoamines which could react with postsynaptic receptors. This supports the hypothesis that the behavioral effects of amphetamine injections into the lateral hypothalamus are mediated by dopamine, norepinephrine and suggests, in addition, serotonin.