Tolerance to 3,4-methylenedioxymethamphetamine in rats exposed to single high-dose binges.

3,4-Methylenedioxymethamphetamine (MDMA or ecstasy) stimulates the transporter-mediated release of monoamines, including 5-HT. High-dose exposure to MDMA causes persistent 5-HT deficits (e.g. depletion of brain 5-HT) in animals, yet the functional and clinical relevance of such deficits are poorly defined. Here we examine functional consequences of MDMA-induced 5-HT depletions in rats. Male rats received binges of three i.p. injections of MDMA or saline, one injection every 2 h; MDMA was given at a threshold pharmacological dose (1.5 mg/kgx3, low dose) or at a fivefold higher amount (7.5 mg/kgx3, high dose). One week later, jugular catheters and intracerebral guide cannulae were implanted. Two weeks after binges, rats received acute i.v. challenge injections of 1 and 3 mg/kg MDMA. Neuroendocrine effects evoked by i.v. MDMA (prolactin and corticosterone secretion) were assessed via serial blood sampling, while neurochemical effects (5-HT and dopamine release) were assessed via microdialysis in brain. MDMA binges elevated core temperatures only in the high-dose group, with these same rats exhibiting approximately 50% loss of forebrain 5-HT 2 weeks later. Prior exposure to MDMA did not alter baseline plasma hormones or dialysate monoamines, and effects of i.v. MDMA were similar in saline and low-dose groups. By contrast, rats pretreated with high-dose MDMA displayed significant reductions in evoked hormone secretion and 5-HT release when challenged with i.v. MDMA. As tolerance developed only in rats exposed to high-dose binges, hyperthermia and 5-HT depletion are implicated in this phenomenon. Our results suggest that MDMA tolerance in humans may reflect 5-HT deficits which could contribute to further dose escalation.

Intravenous administration of the serotonin agonist m-chlorophenylpiperazine (mCPP) increases extracellular serotonin in the diencephalon of awake rats.

The serotonin (5-HT) agonist 1-(m-chlorophenyl)piperazine (mCPP) has been widely used as a pharmacological probe to assess 5-HT function. Although mCPP is known to interact with 5-HT receptors, this drug is also reported to exhibit presynaptic actions that increase extraneuronal 5-HT in vitro. In the present study, we used in vivo microdialysis to examine the effects of mCPP on extracellular 5-HT in the ventromedial diencephalon of awake rats. Intravenous mCPP (1.0 and 2.0 mg/kg) increased dialysate 5-HT in a dose-related manner, with extracellular 5-HT levels rising 8-fold above baseline after the high dose of drug. The stimulatory effect of mCPP on dialysate 5-HT was abolished by pretreatment with the 5-HT uptake blocker fluoxetine (10 mg/kg, i.p.). In complementary experiments, mCPP elevated plasma prolactin at doses equivalent to those that increased dialysate 5-HT, and fluoxetine pretreatment caused a partial, though significant, attenuation of mCPP-induced prolactin release. These results indicate that mCPP increases extracellular 5-HT in rat brain by a presynaptic mechanism involving 5-HT transporters. Moreover, the plasma prolactin response to mCPP is at least partially mediated by the presynaptic actions of the drug. Our data further suggest the possibility that mCPP exhibits indirect agonist properties in human brain. Therefore, clinical studies designed to evaluate postsynaptic 5-HT receptor sensitivity based on responsiveness to mCPP should be interpreted with caution.

Substituted piperazine and indole compounds increase extracellular serotonin in rat diencephalon as determined by in vivo microdialysis.

In vivo microdialysis was used to examine the effects of 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole (RU24969) and 1-(m-trifluoromethylphenyl)piperazine (TFMPP) on extracellular 5-hydroxytryptamine (5-HT) in the diencephalon of unanesthetized rats. Both RU24969 and TFMPP are potent 5-HT autoreceptor agonists but both compounds caused a dose-dependent increase in extracellular 5-HT, when infused into the diencephalon at micromolar concentrations. The piperazine compound, TFMPP, also caused an increase in 5-HT when administered peripherally (2.5-10 mg/kg i.p.). In contrast, peripheral administration of RU24969 (2.5 mg/kg i.p.) caused a decrease in extracellular 5-HT. Since the effects of local infusion with RU24969 and TFMPP were not additive with the increase produced by the inhibitor of the uptake of 5-HT, fluoxetine, these compounds may be acting at the site of the membrane carrier. These results suggest that direct 5-HT1 agonist activity is not the only factor involved in the physiological and behavioral consequences of peripheral administration of TFMPP.

Systemically administered cocaine alters stimulus-evoked responses of thalamic somatosensory neurons to perithreshold vibrissae stimulation.

Previous studies have shown that systemically administered cocaine can transiently alter responses of primary somatosensory cortical neurons to threshold level stimulation of peripheral receptive fields. The goal of the present investigation was 2-fold: (1) characterize the effects of systemic cocaine on stimulus-evoked responses of the ventral posterior medial (VPM) thalamic neurons which relay somatosensory information to the cortex and (2) determine the time course and magnitude of changes in monoamine levels within the somatosensory thalamus following systemic administration of cocaine. Extracellularly recorded responses of single VPM thalamic neurons to whisker stimulation were monitored before and after cocaine administration in halothane anaesthetized rats. Each cell was first characterized by assessing its response profile to a range of perithreshold level deflections of the optimal whisker on the contralateral face. Drug effects on stimulus-response curves, response magnitude and latency were determined from quantitative analysis of spike train data. The results indicate that cocaine elicits a predictable augmentation or attenuation of the sensory response magnitude, with the direction of the change inversely related to the initial magnitude of the stimulus-evoked discharge. In addition, cocaine consistently reduced the response time of somatosensory thalamic neurons to peripheral receptive field stimulation. At the same dose and over the same time period, cocaine also produced marked elevation of norepinephrine and serotonin levels within the ventrobasal thalamus, as determined by in vivo microdialysis. These results suggest that cocaine-induced increases in norepinephrine and serotonin are responsible for drug-related modulation of the transfer of sensory signals through primary thalamocortical relay circuits.

TFMPP and RU24969 enhance serotonin release from rat hippocampus.

Using a batch method for incubation of hippocampal slices, we have examined the effects of 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole (RU24969) and (m-trifluoromethylphenyl)piperazine (TFMPP) on release of endogenous 5-hydroxytryotamine (5-HT). Release of 5-HT from slices was enhanced by RU24969 and TFMPP at concentrations from 1 to 10 mumols. The 5-HT uptake inhibitors imipramine and fluoxetine, but not the autoreceptor antagonist methiothepin, blocked the enhancement in 5-HT. These results suggest that RU24969 and TFMPP, previously identified as potent agonists at the nerve terminal autoreceptor, also interact at higher concentrations with the reuptake carrier to enhance extracellular levels of 5-HT.

Increase in extracellular serotonin produced by uptake inhibitors is enhanced after chronic treatment with fluoxetine.

The effect of prolonged uptake inhibition with fluoxetine (10 mg/kg/day i.p. x 14 days) on extracellular serotonin (5-HT) in the rat diencephalon was monitored using in vivo microdialysis. The increase in extracellular 5-HT after repeated administration of fluoxetine was significantly greater than the increase produced by a single injection of this uptake blocker. This difference may have been due to a decrease in somatodendritic autoreceptor sensitivity, since the response to a low dose of the 5-HT1A agonist 8-OH-DPAT (25 micrograms/kg i.v.) was abolished in the chronic rats, while the response to a high dose (100 micrograms/kg i.v.) was attenuated as compared to animals injected once with fluoxetine.

Acute uptake inhibition increases extracellular serotonin in the rat forebrain.

The effect of acute uptake inhibition on serotonin (5-HT) in the rat central nervous system was monitored by using in vivo dialysis. Peripheral administration of the selective 5-HT uptake blocker, fluoxetine, caused a dose-dependent increase in extracellular 5-HT in both the diencephalon and the striatum. Administration of fluoxetine or sertraline, another selective 5-HT uptake inhibitor, caused a prolonged (24 hr) increase in 5-HT and decrease in 5-hydroxyindoleacetic acid. In addition, fluoxetine and sertraline attenuated the 5-HT releasing effect of fenfluramine administered 24 hr later. Local infusion of fluoxetine into the diencephalon caused an increase in 5-HT that was twice as large as the effect of peripheral injection. Peripheral fluoxetine, by enhancing extracellular 5-HT in the raphe, probably resulted in activation of somatodendritic autoreceptors and inhibition of 5-HT neuronal discharge. Thus, the increase in 5-HT in the diencephalon after peripheral fluoxetine presumably reflected a balance between decreased release and inhibition of reuptake. In support of this, after first infusing fluoxetine into the diencephalon to maximally block reuptake, peripheral injection of the uptake inhibitor caused a decrease in 5-HT.

Systemic uptake inhibition decreases serotonin release via somatodendritic autoreceptor activation.

In vivo microdialysis was used to examine the effects of peripheral uptake inhibition on extracellular serotonin (5-HT). Previous results from this lab indicated that systemic fluoxetine caused a decrease in 5-HT when terminal uptake was inhibited by local infusion of the uptake blocker. We hypothesized that the decrease in 5-HT levels in the terminal region was due to an increase in 5-HT in the vicinity of the inhibitory somatodendritic autoreceptors in the dorsal raphe nucleus (DRN). To test this prediction, rats were implanted with probes in both the basal diencephalon (a nerve terminal region) and the DRN (the cell body region). Fluoxetine (10 mg/kg i.p.) increased extracellular 5-HT, in a depolarization-dependent manner, by approximately 140% in both areas. In a separate experiment, fluoxetine was infused into the diencephalon overnight to block nerve terminal uptake sites. This pretreatment caused an eight- to 10-fold increase in 5-HT levels. Subsequent systemic fluoxetine, sertraline, or paroxetine, produced a 50% decrease in extracellular 5-HT in the diencephalon, presumably due to activation of the 5-HT1A somatodendritic autoreceptors. Consistent with this hypothesis, systemic administration of the 5-HT1 antagonists spiperone, penbutolol, or WAY100135 reversed the fluoxetine-induced decrease in 5-HT to approximately 85% of the pre-fluoxetine baseline levels. Likewise, pretreatment with penbutolol, but not selective beta-adrenergic antagonists, blocked the fluoxetine-induced decrease in release. These findings suggest that the ability of acute systemic 5-HT uptake inhibition to elevate nerve terminal 5-HT is limited by autoreceptor activation following elevation of 5-HT in the DRN.