Enhanced selective 5-HT depletions in the DHT rat model: denervation supersensitivity and recovery of function

A novel ETV6-miR-429-CRKL regulatory circuitry contributes to aggressiveness of hepatocellular carcinoma

BACKGROUND

Tumor metastasis is one of the main causes of the high mortality of hepatocellular carcinoma (HCC). E-Twenty Six variant gene 6 (ETV6) is a strong transcriptional repressor, associated with the development and progression of tumors. However, the exact role and underlying mechanism of ETV6 in HCC remain unclear.

METHODS

Western blotting, quantitative real-time PCR and immunohistochemistry were used to detect the expression levels of ETV6, CRKL (v-crk sarcoma virus CT10 oncogene homologue (avian)-like) and miR-429 in HCC tissues and cells; Transwell chamber and F-actin cytoskeleton staining assay to examine the effects of ETV6 and CRKL deregulation on the migration, invasion and cytoskeleton of HCC cells; Co-immunoprecipitation assay to determine the interaction between CRKL and ETV6; Chromatin immunoprecipitation assay to investigate the interaction between ETV6 and miR-429.

RESULTS

We established a novel ETV6-miR-429-CRKL regulatory circuitry contributes to HCC metastasis. ETV6 and CRKL were frequently increased, while miR-429 was downregulated in both hepatocarcinoma tissues and hepatocarcinoma cells. Moreover, ETV6 upregulation was positively correlated with CRKL upregulation, and two negative correlations were also established for ETV6 and CRKL upregulation with miR-429 downregulation in both hepatocarcinoma patients' tumorous tissues and hepatocarcinoma cells. Functional investigations revealed that overexpression and knockdown of ETV6 was remarkably effective in promoting and suppressing HCC cell migration, invasion, cytoskeleton F-actin expression and arrangement, whereas, CRKL overexpression exhibited similar effects to the overexpression of ETV6. Mechanistically, ETV6 negatively regulates miR-429 expression by directly binding to the promoter region of miR-429; miR-429 negatively regulates CRKL expression by selectively targeting CRKL-3'-UTR; ETV6 directly binds to CRKL and positively regulates its expression, which in turn CRKL positively regulates ETV6 expression.

CONCLUSIONS

Our data demonstrated that ETV6 promotes migration and invasion of HCC cells by directly binding to promoter region of miR-429 via modulating CRKL expression. The newly identified ETV6-miR-429-CRKL regulatory circuitry contributes to the aggressiveness of HCC, which provides new clues for fundamental research on diagnosis and treatment parameters for HCC.

Mapping the physiological and molecular markers of stress and SSRI antidepressant treatment in S100a10 corticostriatal neurons

In mood disorders, psychomotor and sensory abnormalities are prevalent, disabling, and intertwined with emotional and cognitive symptoms. Corticostriatal neurons in motor and somatosensory cortex are implicated in these symptoms, yet mechanisms of their vulnerability are unknown. Here, we demonstrate that S100a10 corticostriatal neurons exhibit distinct serotonin responses and have increased excitability, compared with S100a10-negative neurons. We reveal that prolonged social isolation disrupts the specific serotonin response which gets restored by chronic antidepressant treatment. We identify cell-type-specific transcriptional signatures in S100a10 neurons that contribute to serotonin responses and strongly associate with psychomotor and somatosensory function. Our studies provide a strong framework to understand the pathogenesis and create new avenues for the treatment of mood disorders.

Serotonin, locomotion, exploration, and place recall in the rat

Intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT) led to a 90% reduction of the 5-hydroxytryptamine (5-HT) reuptake site. Behavioural symptoms were studied early (45 to 93 h) as well as late (11 to 14 days) in the postoperative period. Forty-five hours postoperatively, recall of a place navigation task in a water maze was clearly impaired in 5,7-DHT-treated animals. This impairment had disappeared by the fifth postoperative session. During the early test period, injection of scopolamine (0.5 mg/kg) or d-amphetamine (3.0 mg/kg) did not affect place recall of the vehicle-treated control group. In contrast, 5,7-DHT-treated animals were impaired by administration of scopolamine, but not d-amphetamine. During the late test period, the place recall of both groups was affected by scopolamine, but only the performance of the 5,7-DHT lesioned animals was sensitive to d-amphetamine. Locomotion was not severely affected at any time after 5,7-DHT treatment. The vertical hole-board test indicated that the exploratory activities of the animals were relatively unaffected by 5,7-DHT when measured 48 h postoperatively. At 14 days postsurgery, the 5,7-DHT-treated animals demonstrated an impaired habituation of the exploratory behaviour.

Serotonin receptors 5-HT1A and 5-HT3 reduce hyperexcitability of dorsal horn neurons after chronic spinal cord hemisection injury in rat

Spinal cord injury (SCI) results in abnormal pain syndromes in humans. In a rodent model of SCI, T13 spinal hemisection results in allodynia and hyperalgesia due in part to interruption of descending pathways, including serotonergic (5-HT) systems, that leads to hyperexcitability of dorsal horn neurons. To characterize further the role of 5-HT and 5-HT receptor subtypes 5-HT(1A) and 5-HT(3) in neuronal activation after hemisection, we have examined the responsiveness of dorsal horn neurons to a variety of innocuous and noxious peripheral stimuli. Male Sprague-Dawley rats, 150-175 g, were spinally hemisected (n=40) at T13 and allowed 4 weeks for development of mechanical allodynia and thermal hyperalgesia. Animals then underwent electrophysiologic recording and the results were compared with those from sham controls (n=15). Evoked responses of convergent dorsal horn neurons (n=224 total) at L3-L5 to innocuous and noxious peripheral stimuli were characterized after administration of vehicle, 5-HT (25, 50, 100, and 200 microg), 5-HT (100 microg) in conjunction with the selective 5-HT(1A) antagonist WAY 100135 (100 microg), the 5-HT(3) antagonist MDL 72222 (100 microg), the selective 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 150 microg), or the 5-HT(3) agonist 2-Me-5HT (75 microg), with or without pretreatment with antagonists; all treatments were delivered topically onto the cord adjacent to the recording electrode. In hemisected animals, increased responsiveness of convergent cells to all peripheral stimuli was observed bilaterally when compared to controls. No changes in ongoing background activity were present. In control animals, only the highest dose of 5-HT (200 microg) was sufficient to reduce evoked activity, whereas in hemisected animals a concentration-dependent decrease in response was observed. In hemisected animals, both 5-HT(1A) and 5-HT(3) receptor antagonism reduced the effectiveness of 5-HT, restoring elevated evoked activity by up to 70% at the doses tested. Administration of 5-HT(1A) and 5-HT(3) receptor agonists also decreased hyperexcitability, effects prevented by pretreatment with corresponding antagonists. These results demonstrate the development of denervation supersensitivity to 5-HT following SCI, corroborate behavioral studies showing the effectiveness of 5-HT in reducing allodynia and hyperalgesia after SCI, and contribute to a mechanistic understanding of the role of 5-HT receptor subtypes in chronic central pain.

Changes in serotonin, serotonin transporter expression and serotonin denervation supersensitivity: involvement in chronic central pain after spinal hemisection in the rat

Spinal cord injury (SCI) results in abnormal locomotor and pain syndromes in humans. In a rodent SCI model, T13 unilateral spinal hemisection results in bilateral mechanical allodynia and thermal hyperalgesia, partly by interruption of tonic descending serotonin (5-HT) inhibition. In the current study, we examined changes in density and distribution of 5-HT and 5-HT(T) in cervical (C8) and lumbar (L5) enlargements after T13 spinal hemisection and studied the effects of intrathecally delivered 5-HT (10, 21, and 63 microg), 5-HT antagonist methysergide (125 microg/kg), and 5-HT reuptake inhibitor fluvoxamine (75 microg/kg) on pain-related behaviors. Thirty-day-old male Sprague-Dawley rats were spinally hemisected and sacrificed at 3 (n = 20) and 28 (n = 20) days postsurgery for immunohistochemistry, Western blot, and ELISA analysis and compared against sham-operated animals (n = 10). At day 3, C8 5-HT levels were not significantly changed but at L5 there was a significant decrease in ipsilateral 5-HT in laminae I-II followed by incomplete recovery at 28 days postinjury. At both 3 and 28 days postinjury, C8 5-HT(T) levels were not significantly changed, but at L5 there was significant ipsilateral up-regulation of 5-HT(T) in laminae I-II. A second group of animals (n = 30) was hemisected and, starting at 28 days postinjury, behaviorally tested with intrathecal compounds. Increasing doses of 5-HT attenuated both fore- and hindlimb mechanical allodynia and thermal hyperalgesia, and effects of endogenous 5-HT were attenuated by methysergide and enhanced with fluvoxamine, all without locomotor alterations. Sham controls (n = 10) were unaffected. Thus, permanent changes occur in 5-HT and 5-HT(T) after SCI, denervation 5-HT supersensitivity develops, and modulation of 5-HT attenuates pain-related behaviors. Insight gained by these studies may aid in the understanding of dynamic 5-HT systems which will be useful in treating chronic central pain after SCI.

The neurobiology of tryptophan depletion in depression: effects of intravenous tryptophan infusion

BACKGROUND

Previous work has suggested that acute depletion of the serotonin (5-HT) precursor tryptophan (TRP) causes transient compensatory changes in the 5-HT system that might be exploited for their antidepressant effects. In this study, neuroendocrine and mood responses to intravenous (i.v.) infusion of TRP were examined in order to evaluate central 5-HT function in depressed patients undergoing acute TRP depletion.

METHODS

Thirty-eight drug-free patients with DSM-III-R major depression participated. Each patient underwent two randomized, double-blind TRP depletion tests, one sham and one active. At the estimated time of maximum TRP depletion, each patient received an i.v. infusion of TRP 100 mg/kg. Blood was obtained for serum cortisol, prolactin, and growth hormone. Mood was assessed using standardized rating scales.

RESULTS

The cortisol response to i.v. TRP was significantly greater during TRP depletion than during sham depletion. Depressive symptoms showed a tendency to decrease after i.v. TRP following active, but not sham, TRP depletion.

CONCLUSIONS

These findings are consistent with the present hypothesis and previous evidence that acute TRP depletion in drug-free depressed patients induces compensatory upregulation of postsynaptic 5-HT receptors. These changes are insufficient to serve as a means of effecting clinical improvement, but suggest that the antidepressant properties of rapid, marked manipulations of 5-HT function warrant further study.

The functional significance of neonatal 5,7-dihydroxytryptamine lesions in the rat: response to selective 5-HT1A and 5-HT2,1C agonists

To study the involvement of serotonin (5-HT) receptor subtypes in behavioral supersensitivity following neonatal 5,7-dihydroxytryptamine (5,7-DHT) lesions, we measured acute behavioral responses to a single dose of selective 5-HT1A (8-OH-DPAT) or 5-HT2,1C (DOI) agonist compared to 5-hydroxytryptophan (5-HTP) in rats injected with 5,7-DHT intraperitoneally or intracisternally 14 weeks earlier. Only intraperitoneal 5,7-DHT injection resulted in brainstem 5-HT hyperinnervation, but cortical 5-HT depletions were also less. Effects of DOI, such as shaking behavior and forepaw myoclonus, were enhanced by 5,7-DHT lesions made intracisternally not intraperitoneally, whereas 8-OH-DPAT-evoked behaviors, such as forepaw myoclonus and head weaving, were enhanced more by the intraperitoneal route. The main consequence of intraperitoneal compared to intracisternal 5,7-DHT injection on supersensitivity to 5-HT agonists was increased presynaptic 5-HT1A responses and decreased 5-HT2,1C responses. In contrast, 5-HTP evoked more shaking behavior and less of the serotonin syndrome with the intraperitoneal compared to the intracisternal route of 5,7-DHT injection. Behavioral supersensitivity to 5-HTP, which was attributable to 5-HT1A, 5-HT2,1C, and possibly to other 5-HT receptors, was orders of magnitude greater than that elicited by direct receptor agonists and more clearly differentiated between rats with 5,7-DHT lesions and their controls, and between routes of 5,7-DHT injections, than responses to 5-HT agonists at the dose studied. 5,7-DHT induced dysregulation of 5-HT receptors, including both presynaptic and postsynaptic changes and altered interactions between receptor subtypes, better explains these data than postsynaptic changes alone.

Regional central serotonin-2 receptor binding and phosphoinositide turnover in rats with 5,7-dihydroxytryptamine lesions

"Denervation supersensitivity" of serotonin (5-HT) receptors has been proposed to explain the behavioral supersensitivity to 5-hydroxytryptophan (5-HTP) which develops after lesions of indoleamine neurons with 5,7-dihydroxytryptamine (5,7-DHT). To examine the possible role of receptor recognition sites and second messenger activity in supersensitivity, we measured regional 5-HT2 receptor ligand binding and 5-HT-stimulated phosphoinositide turnover in adult rats with 5,7-DHT lesions made by intracisternal injection and their saline-treated controls. In [3H]ketanserin binding studies of fresh brain tissue two weeks after 5,7-DHT injection, there were no significant changes in frontal cortex, brainstem, or spinal cord in Bmax, Kd, or nH of 5-HT2 receptors, 5,7-DHT lesions did not affect basal levels of [3H]inositol phosphate (IP) accumulation but significantly increased 5-HT-stimulated [3H]IP accumulation in the brainstem (+27%) and cortex (+23%). Because brainstem rather than cortex is involved in 5-HTP-evoked myoclonus, increased 5-HT-stimulated phosphoinositide hydrolysis in brainstem following 5,7-DHT lesions in the rat may be relevant to serotonergic behavioral supersensitivity.

Brainstem serotonergic hyperinnervation modifies behavioral supersensitivity to 5-hydroxytryptophan in the rat

Rat pups were injected intracisternally (i.c.) or intraperitoneally (i.p.) with 5,7-dihydroxytryptamine (5,7-DHT) or saline and challenged 2 and 14 weeks later with the 5-HT precursor 5-hydroxytryptophan (5-HTP), which evokes behavioral supersensitivity in adult rats, 5,7-DHT induced transient postinjection convulsions in rats injected i.c. but not i.p. Rats with either type of 5,7-DHT lesions displayed supersensitive behavioral responses to 5-HTP. However, rats lesioned by i.p. injections exhibited significantly greater shaking behavior (+1445%) in response to 5-HTP than their i.c. counterparts, who instead showed more forepaw myoclonus (+250%) and head weaving (+270%), the core features of the 5-HT syndrome. Differences in 5-HT syndrome behaviors were already present 2 weeks after lesioning, whereas the difference in shaking behavior was not. After 14 weeks, 5-HT was selectively depleted (-43 to -92%) in hippocampus, spinal cord, and frontal cortex, and differences between i.c. and i.p. 5,7-DHT routes were insignificant except in frontal cortex. Brainstem 5-HT concentrations were significantly increased (+35%) after i.p. 5,7-DHT injections in contrast to reduction (-89%) after i.c. 5,7-DHT; 5-hydroxyindole acetic acid/5-hydroxytryptamine (5-HIAA/5-HT) ratios were decreased (-20%) with either route. These data suggest that brainstem 5-HT hyperinnervation following i.p. 5,7-DHT injection modifies the functional consequences of injury in abating the 5-HT syndrome, but does not result in complete recovery since shaking behavior is enhanced. Loss of presynaptically mediated autoregulation or receptor dysregulation may play a major role in behavioral supersensitivity induced by 5-HTP in rats with 5,7-DHT lesions. To the extent that the 5-HT syndrome is mediated by 5-HT1A receptors and shaking behavior by 5-HT2 sites, differential responses to injury of 5-HT1A and 5-HT2 receptors may contribute to these behavioral differences.

Chronic ACTH treatment: influence on 5-HT2 receptors and behavioral supersensitivity induced by 5,7-dihydroxytryptamine lesions

The capacity of the serotonin (5-HT) precursor 5-HIP to induce the ACTH-responsive myoclonic-convulsive disorder infantile spasms in patients with Down's syndrome has been cited as evidence for altered serotonergic neurotransmission in infantile spasms. Since there is no animal model of infantile spasms, the suitability of behavioral supersensitivity (myoclonus) evoked by 5-HTP in rats with 5,7-dihydroxytryptamine (DHT) lesions as a model was tested by determining the effect of chronic treatment with ACTH (40 IU/kg) on 5-HTP-evoked myoclonus. In rats treated with DHT as adults, ACTH administration did not alter the "serotonergic behaviors," such as myoclonus, induced by 30 mg/kg 5-hydroxytryptophan (5-HTP), but induced a small significant increase in Bmax of neocortical 5-HT2 sites of the DHT group, with no change in rats without lesions. In rats treated with DHT as neonates, there was also no significant difference in behaviors evoked by several doses of 5-HTP. These data suggest that ACTH minimally modifies the effects on 5-HT receptors of DHT lesions, but the intracisternal DHT model is not a suitable model for infantile spasms because chronic ACTH was not antimyoclonic.

5-HT depletion with 5,7-DHT, PCA and PCPA in mice: differential effects on the sensitivity to 5-MeODMT, 8-OH-DPAT and 5-HTP as measured by two nociceptive tests

Depletion of 5-hydroxytryptamine (5-HT) in mice was produced by intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT, 80 micrograms) or by systemic injections of p-chloroamphetamine (PCA, 3 X 40 or 4 X 40 mg/kg), p-chlorophenylalanine (PCPA, 5 X 400 or 14 X 400 mg/kg) or combined PCA (3 X 40 mg/kg) + PCPA (11 X 400 mg/kg). Neither of the pretreatments altered nociception in the increasing temperature hot-plate test, whereas hyperalgesia was demonstrated in 5,7-DHT lesioned animals in the tail-flick test. 5,7-DHT-pretreatment enhanced the antinociceptive effect of the 5-HT agonists 5-methoxy-N,N-dimethyltryptamine (5-MeODMT), 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and 5-hydroxytryptophan (5-HTP). This effect was observed after 2, 5 and 8 days in the tail-flick test and after 5 and 8 days in the hot-plate test. However, pretreatment with PCPA or PCA failed to alter the antinociception elicited by the 5-HT agonists, although a tendency towards enhancement of antinociception was found after combined treatment with PCA and PCPA. It is suggested that the injection of 5,7-DHT induces denervation supersensitivity of post-synaptic 5-HT receptors. The lack of such supersensitivity after PCPA-pretreatment which induces similar 5-HT depletion to 5,7-DHT, may suggest that other factors than the absence of 5-HT may contribute to the development of denervation supersensitivity. Alternatively, the three 5-HT depleting agents may produce a qualitatively different reduction of 5-HT.

The regulation of TRH and serotonin receptors: chronic TRH and analog administration in the rat

Thyrotropin releasing hormone (TRH) and serotonin (5-HT) are co-transmitters in spinal and medullary neurons. To study the functional significance of this relationship and the regulation of TRH receptors, we chronically administered TRH and analogs MK-771 and CG-3509 to rats at a dose which evoked behavioral abnormalities. TRH reduced specific binding at spinal (24%) and hypothalamic (31%) TRH receptors and decreased TRH stimulated motor behaviors, such as rearing and cage crossing (locomotion). 5-HT1 and 5-HT2 receptors were unaffected except for an 11% increase in specific binding at spinal 5-HT1 sites. 5-HT and NE concentrations measured by HPLC were not altered in brainstem or hippocampus. In contrast, spinal TRH receptor specific binding increased 11% in rats treated intracisternally with 5,7-dihydroxytryptamine, but the effect was not significant. In competition studies in vitro, MK-771, TRH, and CG-3509 had no activity at 5-HT2 receptors in neocortex, brainstem, or spinal cord, and little activity at 5-HT1 sites (IC 50s greater than 100 uM). A mixed competitive and non-competitive binding profile at 5-HT1 sites resulted in the presence of 100 uM TRH. Conversely, 5-HT agonists had minimal effect at TRH receptors in spinal cord. These data suggest reciprocal or independent regulation of 5-HT1 and TRH receptors in co-transmitter and non-cotransmitter regions, respectively, in response to chronic TRH administration.

The comparative pharmacology of the behavioral syndromes induced by TRH and by 5-HT in the rat

1. The relationship of the behavioral syndromes induced by the co-transmitters thyrotropin releasing hormone (TRH) and serotonin (5-HT) has not been previously studied with drugs selective for 5-HT receptor subtypes. 2. Both the TRH analog MK-771 (in naive rats) and 5-hydroxytryptophan (in rats with 5,7-dihydroxytryptamine [DHT] lesions) evoked reciprocal forepaw tapping, Straub tail, hunching, hindlimb abduction, and shaking behavior. Sniffing and rearing were features of the MK-771 but not the 5-HT syndrome. 3. 5-HTP potentiated MK-771-induced hyperthermia. 4. MK-771 evoked two types of shaking behavior, head shakes (HS) and wet-dog shakes (WDS). Neither independently was dose-related, unlike total shaking behaviors. 5. MK-771-induced shaking behavior was pharmacologically dissociated from other MK-771-evoked behaviors. A 5-HT1A agonist (8-OH-DPAT) blocked WDS, but like putative 5-HT1B (RU 24969) and 5-HT2 (DOI) agonists and the 5-HT antagonists methysergide (non-selective), ritanserin (5-HT2 selective), and l-propranolol (5-HT1 selective), it did not block other antagonists behavioural effects of MK-771. 6. Ipsapirone, a 5-HT1A-active drug purported both as an agonist and as an antagonist, inhibited MK-771-evoked WDS, like 8-OH-DPAT, but did not induce the serotonin syndrome, unlike 8-OH-DPAT. 7. DHT-treated rats were behaviorally supersensitive to 10 mg/kg MK-771 as indicated by a significantly shortened latency of onset of WDS and greater frequency of abnormal forepaw movements. The same rats were also supersensitive to 50 mg/kg 5-HTP to a significantly greater degree. 8. These data suggest behavioral relatedness of the TRH and 5-HT syndromes, but distinctive pharmacologic features and presumed mechanisms of action.

The guinea pig myoclonic model: behavioral supersensitivity to 5-hydroxytryptophan induced by intracisternal 5,7-dihydroxytryptamine

To study the species difference of guinea pigs and rats in response to 5-hydroxytryptophan (5-HTP), we injected both animals intracisternally with 5,7-dihydroxytryptamine. In rats with 5,7-dihydroxytryptamine lesions, 5-HTP evoked the well described myoclonic-serotonergic syndrome. In the guinea pig, 5,7-dihydroxytryptamine lesions significantly increased the severity of myoclonic response to 5-HTP (150 mg/kg) compared to vehicle controls, resulting in lethal convulsions. Guinea pigs treated with 5,7-dihydroxytryptamine did not develop spontaneous myoclonus, or when treated with 5-HTP, other 'serotonergic behaviors' such as lateral head weaving, hindlimb abduction, and forepaw tapping. Guinea pigs tolerated intracisternal 5,7-dihydroxytryptamine less well than rats, with a higher mortality, although immediate post-injection convulsions were less severe and did not require phenobarbital prophylaxis. Staged lower doses of 5,7-dihydroxytryptamine (100-200 micrograms) were better tolerated than a single high dose of neurotoxin (400 micrograms). The regional profile of 5,7-dihydroxytryptamine lesions in the guinea pig resembled that of the rat, with maximal depletion of 5-HT in spinal cord and selected forebrain structures, and little effect in diencephalon and midbrain. Depletions in the guinea pig were less selective for 5-HT using desipramine pretreatment than in the rat. In naive guinea pigs and rats, regional content of 5-HT was similar. These data suggest that the functional integrity of serotonergic neurons is not requisite for the expression of myoclonus induced by 5-HTP in the guinea pig. 5,7-Dihydroxytryptamine lesions in the guinea pig resulted in behavioral and neurochemical similarities and differences in comparison with the rat.

Effects of 5-HT receptor subtype-selective drugs on locomotor activity and motor habituation in the DHT adult rat model

5-Hydroxytryptophan (5-HTP) induces biphasic time and dose dependent effects on locomotor activity (LMA) and motor habituation in rats with 5,7-dihydroxytryptamine (DHT) lesions. To identify the role of serotonin (5-HT) receptors in these responses, we studied the effects of 5-HT2 receptor antagonists on LMA occurring spontaneously and evoked by 5-HTP or putative selective 5-HT agonists in rats injected intracisternally with DHT or vehicle. Motor habituation was assessed by analysis of computer-tabulated 10 minute "bins" during hour long recording. Neuroleptic 5-HT2 antagonists prevented 5-HTP stimulation of LMA in DHT-lesioned rats in the rank order of potency pirenperone greater than pipamperone greater than ketanserin = cinanserin. The non-neuroleptic ritanserin, however, did not reduce LMA stimulated by the 5-HT1B agonist RU24969 but did reverse transient suppression of LMA induced by high dose 5-HTP and by the putative 5-HT2 agonist DOI. RU24969, like 5-HTP, induced a failure of motor habituation which differed from DOI-evoked alteration. 8-OH-DPAT did not affect motor habituation at the dose tested. These data suggest that the 5-HT1B site mediates 5-HTP-evoked locomotor hyperactivity in the DHT model, that the 5-HT2 site participates in the transient hypoactivity seen with high doses of 5-HTP, and that 5-HT1 and 5-HT2 sites may be functionally linked. Both sites differentially influence motor habituation, which appears to be under complex regulation. Bin analysis is a sensitive index of these habituation effects.