The serotonergic 5-HT(2C) agonist m-chlorophenylpiperazine increases weight-supported locomotion without development of tolerance in rats with spinal transections

The direct serotonergic agonist, m-chlorophenylpiperazine (m-CPP), displays high efficacy at 5-HT(2C) receptors. Systemic administration of m-CPP increased dramatically the percentage of weight-supported steps made on a treadmill by rats with complete midthoracic spinal transections. The improvement in motor function occurred in rats with grafts of fetal spinal cord into the site of transection (transplant rats) and in spinal rats without grafts (spinal rats). m-CPP produced a therapeutic action with its first administration and after 14 single daily injections. In contrast, the serotonin and norepinephrine reuptake inhibitor, chlorimipramine (CMI), failed to enhance weight support during 21 days of treatment. The results imply that stimulating directly 5-HT(2C) receptors restores postural support after spinal injury. Thus, 5-HT(2C) agonists are candidates for treating spinal patients chronically without the development of tolerance.     

m-CPP-induced self-grooming is mediated by 5-HT2C receptors

m-Chlorophenylpiperazine (m-CPP), a potent 5-HT receptor agonist, is known to induce self-grooming in rats and exacerbate symptoms in patients with obsessive-compulsive disorder (OCD). To characterise the possible role, 5-HT(2B) and 5-HT(2C) receptors play in m-CPP-induced self-grooming, subtype-selective receptor antagonists were used. m-CPP significantly increased the amount of self-grooming in male Sprague-Dawley rats. This effect followed a bell-shaped dose-response curve with a peak at 0.6 mg/kg, i.p. Pretreatment with SB-242084, a subtype-selective 5-HT(2C) receptor antagonist (0.1-0.5 mg/kg, i.p.), reversed m-CPP-induced self-grooming. In contrast, pretreatment with the subtype-selective 5-HT(2B) receptor antagonist SB-215505 (1 mg/kg, i.p) did not block the effect of m-CPP. Two days after depletion of brain 5-HT by p-chlorophenylalanine (p-CPA, 2 x 50, 2 x 100 mg/kg, i.p.) m-CPP-induced responses were significantly enhanced compared to controls. Our studies provide evidence that direct activation of 5-HT(2C) receptors mediate m-CPP-induced self-grooming and the depletion of brain 5-HT sensitizes these receptors.     

Improvement of the isolation-induced social behavioural deficit involves activation of the 5-HT1B receptors

1. Mice were isolated for 7-9 days. An isolated mouse and a mouse reared in group showed a difference in their behaviour when observed together under an inverted beaker. The isolated mouse makes one half escape attempts in regard to the grouped mouse. This is considered as a social behavioural deficit. 2. 1- 3-(trifluoromethyl)phenyl piperazine (TFMPP), 1-(3-chlorophenyl)piperazine (m-CPP) and 5-methoxy-3 (1,2,3,6-tetrahydropyridin-4-yl) 1-H indole (RU-24969) activating preferentially the 5-HT1B receptors increased the number of escape attempts of the isolated mice up to the level of grouped mice. 3. Penbutolol, a beta-blocking drug acting also at 5-HT1 receptors, devoid of effect when given alone, antagonized significantly and dose-dependently the effects of TFMPP, m-CPP and RU-24969. 4. The interaction between TFMPP and five various serotonin antagonists was examined. Neither the 5-HT2 receptor antagonist ritanserine, the 5-HT3 receptor antagonist ICS 205-930, the 5-HT1C receptor antagonists mianserin and cyproheptadine antagonized the effect of TFMPP. The neuroleptic spiperone decreased by itself the number of escape attempts and opposed the TFMPP effect. 5. Taken together, these results suggest that the isolation-induced social behavioural deficit may be considered as a behavioural model responsive to 5-HT1B agonists.     

Effects of serotonergic 5-HT1A and 5-HT1B ligands on ventral pallidal neuronal activity

To clarify the role of the 5-HT system in limbic outputs, the present study compared the effects of the 5-HT1A agonist 8-OH-DPAT and the 5-HT1B agonist CP-94253 with the non-selective 5-HT agonist TFMPP on the firing rate of ventral pallidal (VP) neurons recorded in chloral hydrate-anesthetized rats. 8-OH-DPAT (0.25-256 microg/kg i.v.) dose-dependently enhanced (9/26 neurons) or suppressed (8/26) activity, and the 5-HT1A antagonist (+)WAY-100135 often attenuated these responses. TFMPP (0.011-1.453 mg/kg i.v.) dose-dependently reduced the firing rate of 7/8 VP neurons tested. In contrast, CP-94253 (0.013-12.8 mg/kg i.v.) had little or no effect. In sum, these data suggest that the 5-HT1A receptor appears to be particularly important in influencing limbic outputs mediated via the VP.     

5-HT2C receptors inhibit and 5-HT1A receptors activate the generation of spike-wave discharges in a genetic rat model of absence epilepsy

The present study was conducted to investigate the role of 5-HT(2C) and 5-HT(1A) receptors in the generation of spike-wave discharges (SWD) in the genetic absence epilepsy model Wistar Albino Glaxo rats from Rijswijk, Netherlands (WAG/Rij rats). We have determined the effects of the 5-HT(2C) receptor preferring agonist m-chlorophenyl-piperazine (m-CPP), the selective 5-HT(2C) receptor antagonist SB-242084, the selective 5-HT(1A) receptor antagonist WAY-100635, two selective serotonin re-uptake inhibitors (SSRI, fluoxetine and citalopram) and their combinations in this model. The 5-HT(2C) agonist m-CPP caused marked, dose-dependent decreases in the cumulative duration and number of SWD administered either intraperitoneally (0.9 and 2.5 mg/kg) or intracerebroventricularly (0.05 and 0.1 mg/kg). Treatment with SB-242084 (0.2 mg/kg, ip) alone failed to cause any significant change in SWD compared to vehicle. Pretreatment with SB-242084 (0.2 mg/kg, ip) eliminated the effects of m-CPP on SWD. Fluoxetine (5.0 mg/kg, ip) alone caused moderate increase in SWD. After pretreatment with SB-242084, the effect of fluoxetine was significantly enhanced. The combination of SB-242084 and citalopram (2.5 mg/kg, ip) caused a similar effect, namely an increase in SWD. In contrast, pretreatment with WAY-100635 significantly attenuated the effect of fluoxetine. In conclusion, these results indicate that the increase in endogenous 5-HT produces a dual effect on SWD; the inhibition of epileptiform activity is mediated by 5-HT(2C), the activation by 5-HT(1A) receptors.     

Stimulation of 5-HT1A, 5-HT1B, 5-HT2A/2C, 5-HT3 and 5-HT4 receptors or 5-HT uptake inhibition: short- and long-term memory

In order to determine whether short- (STM) and long-term memory (LTM) function in serial or parallel manner, serotonin (5-hydroxtryptamine, 5-HT) receptor agonists were tested in autoshaping task. Results show that control-vehicle animals were modestly but significantly mastering the autoshaping task as illustrated by memory scores between STM and LTM. Thus, post-training administration of 8-OHDPAT (agonist for 5-HT(1A/7) receptors) only at 0.250 and 0.500 mg/kg impaired both STM and LTM. CGS12066 (agonist for 5-HT(1B)) produced biphasic affects, at 5.0 mg/kg impaired STM but at 1.0 and 10.0 mg/kg, respectively, improved or impaired LTM. DOI (agonist for 5-HT(2A/2C) receptors) dose-dependently impaired STM and, at 10.0 mg/kg only impaired LTM. Both, STM and LTM were impaired by either mCPP (mainly agonist for 5-HT(2C) receptors) or mesulergine (mainly antagonist for 5-HT(2C) receptors) lower dose. The 5-HT(3) agonist mCPBG at 1.0 impaired STM and its higher dose impaired both STM and LTM. RS67333 (partial agonist for 5-HT(4) receptors), at 5.0 and 10.0 mg/kg facilitated both STM and LTM. The higher dose of fluoxetine (a 5-HT uptake inhibitor) improved both STM and LTM. Using as head-pokes during CS as an indirect measure of food-intake showed that of 30 memory changes, 21 of these were unrelated to the former. While some STM or LTM impairments can be attributed to decrements in food-intake, but not memory changes (either increase or decreases) produced by 8-OHDPAT, CGS12066, RS67333 or fluoxetine. Except for animals treated with DOI, mCPBG or fluoxetine, other groups treated with 5-HT agonists 6 h following autoshaping training showed similar LTM and unmodified CS-head-pokes scores.     

Contribution of spinal 5-HT1A and 5-HT7 receptors to locomotor-like movement induced by 8-OH-DPAT in spinal cord-transected mice

Growing evidence from in vitro studies suggests that spinal serotonin (5-HT) receptor subtypes 5-HTR(1A) and 5-HTR(7) are associated with an induction of central pattern generator activity. However, the possibility of a specific role for these receptor subtypes in locomotor rhythmogenesis in vivo remains unclear. Here, we studied the effects of a single dose (1 mg/kg, i.p.) of 8-hydroxy-2-(di-N-propylamino)-tetralin (8-OH-DPAT), a potent and selective 5-HTR(1A/7) agonist, in mice spinal cord transected at the low-thoracic level (Th9/10). The results show that 8-OH-DPAT acutely induced, within 15 min, hindlimb movements that share some characteristics with normal locomotion. Paraplegic mice pretreated with the selective 5-HTR(1A) antagonists, WAY100,135 or WAY100,635, displayed significantly less 8-OH-DPAT-induced movement. A similar reduction of 8-OH-DPAT-induced movements was found in animals pretreated with SB269970, a selective 5-HTR(7) antagonist. Moreover, a near complete blockade of 8-OH-DPAT-induced movement was obtained in wild-type mice pretreated with 5-HTR(1A) and 5-HTR(7) antagonists, and in 5-HTR(7)-/- mice pretreated with 5-HTR(1A) antagonists. Overall, these results clearly demonstrate that 8-OH-DPAT potently induces locomotor-like movement in the previously paralysed hindlimbs of low-thoracic-transected mice. The results, with selective antagonists and knockout animals, provide compelling evidence of a specific contribution of both receptor subtypes to spinal locomotor rhythmogenesis in vivo.     

Effects of selected serotonin 5-HT(1) and 5-HT(2) receptor agonists on feeding behavior: possible mechanisms of action

Serotonin (5-HT) receptor agonists with high affinity for the different subtypes (i.e. 5-HT(1A-1F), 5-HT(2A-2C)) of the 5-HT(1)- and 5-HT(2) receptor families have been shown to affect ingestive behavior. It has been assumed that: (1) stimulation of hypothalamic 5-HT(2C) or 5-HT(1B) receptors leads to a behaviorally specific hypophagic effect by accelerating satiety processes; (2) stimulation of 5-HT(2A) receptors leads to a disruption of the feeding cascade; and (3) stimulation of 5-HT(1A) and 5-HT(2B) receptors leads to a hyperphagic effect. The present paper reviews studies performed with the relatively selective receptor agonists ipsapirone (5-HT(1A)), CP-94,253 (5-HT(1B)), BW 723C86 (5-HT(2B)) and ORG 37684 (5-HT(2C)), as well as the nonselective receptor agonists TFMPP (5-HT(1B/2C)), m-CPP (5-HT(2C/1B)) and DOI (5-HT(2A/2C)) in a variety of feeding paradigms in rats, both after systemic and local injection. These studies support a role for other neuroanatomical regions (i.e. brain stem) and behavioral mechanisms (i.e. appetitive processes) in the hypophagic effects of these compounds, possibly as a function of the administered dose. Studies with 5-HT receptor antagonists indicate that the proposed role of particular 5-HT(1/2) receptor subtypes in the hypophagic effects of these 5-HT receptor agonists may be more complicated than originally thought. Further characterization of the role of 5-HT(1/2) receptor subtypes in the control of ingestive behavior will require extensive pharmacological and behavioral studies, using more selective receptor agonists and antagonists and different behavioral procedures, as well as verification in transgenic animals.     

5,7-DHT facilitated lordosis: effects of 5-HT agonists.

The role of 5-HT (serotonin) in regulating lordosis was investigated by combining peripheral administration of the 5-HT agonists 8-OH-DPAT (8-hydroxy-2-[di-N-propylamino]tetralin) or TFMPP (1-[m-trifluoromethylphenyl]piperazine), with intrahypothalamic application of the 5-HT neurotoxin 5,7-DHT (5,7-dihydroxytryptamine). The 5-HT1A agonist, 8-OH-DPAT, significantly inhibited lordosis in 5,7-DHT-treated and non-treated rats. TFMPP, an agonist at 5-HT1B and 5-HT1C receptors, significantly facilitated lordosis in 5,7-DHT-treated and non-treated rats. Our results show that both inhibitory and facilitatory influences of hypothalamic 5-HT on lordosis, are modulated via postsynaptic receptors.     

Serotonin 5-HT2c agonists mimic the effect of light pulses on circadian rhythms

The serotonin agonist quipazine has been shown to cause phase shifts in melatonin and activity rhythms and to induce c-fos in the suprachiasmatic nucleus of rats. In this study, in vivo pharmacological characterisation of the phase shifting properties of serotonin agonists has been performed, with a view to determining the receptor sub-types involved. Agonists for the 5-HT2a/2c receptors, (±)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride (DOI, 0.1 mg/k), 1-(3-chlorophenyl)-piperazine HCl (mCPP, 2 mg/kg) and N-(3-trifluoromethylphenyl)-piperazine HCl (TFMPP, 2 mg/kg) injected at CT18 resulted in acute transient inhibition of melatonin production and delays in the onset of production on the following nights of 1.2±0.2, 1.7±0.3 and 1.4±0.8 h respectively. Drugs specific for 5-HT1a/7 and 5-HT3 receptors failed to affect melatonin production. At a dose of 0.07 μmole/kg, the serotonin antagonist, ritanserin inhibited the DOI induced phase delay whereas ketanserin was ineffective at this dose, providing strong evidence that DOI was acting through 5-HT2c receptors. DOI (0.5 mg/kg) at CT18 provoked a phase delay in the core body temperature rhythm of similar magnitude to that following a light pulse. Administration of DOI but not agonists active at other receptor sites resulted in the appearance of c-Fos in the ventrolateral division of the suprachiasmatic nucleus (SCN) at CT18 but not at CT6. Ritanserin was more potent than ketanserin at inhibiting the DOI induced increase in c-Fos labelled cells in the SCN. When rats were pre-treated with metergoline (15 mg/kg), ritanserin (3 mg/kg) or LY 53,857 (3 mg/kg) prior to a 2 lx/ 1 min light pulse, none of the drugs significantly inhibited the responses to light. The results of these experiments indicate that serotonergic agonists active at the 5-HT2c receptor mimic the effects of light on 2 independent rhythms and activate SCN neurones in the rat.     

Role of 5-hT2C receptors in the hypophagic effect of m-CPP, ORG 37684 and CP-94,253 in the rat

Compounds that stimulate 5-HT2C and/or 5-HT1B receptors induce hypophagia, but the relative role of these receptors in the control of feeding behaviour remains to be unequivocally demonstrated. The objectives of the present study were: (a) comparison of the hypophagic effect of the mixed 5-HT2C/1B receptor agonist, m-CPP, with that of ORG 37684 and CP-94,253, a relatively selective 5-HT2C and 5-HT1B receptor agonist, respectively; (b) verification of the contribution of 5-HT2C receptors to the hypophagic effect of these compounds by antagonism experiments; and (c) to test whether cotreatment with ORG 37684 and CP-94,253 leads to a more pronounced reduction of food intake as compared with treatment with either compound alone. Food intake was measured in a free feeding experimental protocol employing female Wistar rats. m-CPP was more potent in suppressing food intake than ORG 37684 and CP-94,253 (ED50 values for the first hour of access: 0.45, 1.84 and 3.48 mg/kg ip, respectively). The 5-HT2C receptor antagonists, metergoline and SB 242.084, completely reversed the hypophagic effect of ORG 37684, but not that of CP-94,253 and m-CPP. The hypophagic effect of ORG 37684 was potentiated by a low (inactive) dose of CP-94,253 (ED50: 4.95 and 2.44 mg/kg ip after vehicle and CP-94,253 pretreatment, respectively) and vice versa (ED50 values: 4.02 and 0.62 mg/kg ip). It is concluded that the hypophagic effect of ORG 37684-but not that of m-CPP and CP-94,253--is exclusively mediated by activation of 5-HT2C receptors. The results further indicate that simultaneous activation of 5-HT2C and 5-HT1B receptors underlies the higher potency of m-CPP in reducing food intake, as compared with other, more selective, compounds.     

Inhibition of hippocampal 5-HT synthesis by fluoxetine and paroxetine: evidence for the involvement of both 5-HT1A and 5-HT1B/D autoreceptors

Hippocampal serotonin (5-hydroxytryptamine, 5-HT) synthesis, as determined by the accumulation of 5-hydroxytryptophan (5-HTP) following inhibition of L-aromatic amino acid decarboxylase with NSD 1015, was inhibited by systemic administration of the selective serotonin reuptake inhibitors fluoxetine (10 mg/kg i.p.) and paroxetine (3 mg/kg i.p.). Pretreatment of rats with the selective 5-HT1A receptor antagonist WAY 100635 for a period of 7 days using subcutaneously implanted osmotic minipumps (1 mg/kg/day) was sufficient to block the inhibition of 5-HT synthesis following the 5-HT 1A receptor agonist 8-OH-DPAT (0.3 mg/kg s.c.), but failed to inhibit the decrease of hippocampal 5-HT synthesis by fluoxetine (10 mg/kg i.p.) or paroxetine (3 mg/kg i.p.). Similarly, pretreatment of rats with GR 127935 (5 mg/kg i.p.), an antagonist with high affinity for 5-HT1B/D receptors, blocked the reduction of hippocampal 5-HT synthesis following the 5-HT receptor agonist TFMPP (3 mg/kg s.c.) without affecting the reduction of hippocampal 5-HT synthesis by either fluoxetine or paroxetine. In contrast, pretreatment with WAY 100635 (1 mg/kg/day, for 7 days s.c. in osmotic minipumps) in combination with GR 127935 (5 mg/kg i.p.) significantly attenuated the decrease of hippocampal 5-HT synthesis by both fluoxetine and paroxetine. These results indicate that both 5-HT1A and 5-HT1B/1D receptors, which function in the rat as inhibitory somatodendritic and nerve terminal autoreceptors, independently regulate hippocampal 5-HT synthesis and must be simultaneously blocked to prevent the inhibition of 5-HT synthesis by selective serotonin reuptake inhibitors which increase 5-HT availability at both nerve terminals in hippocampus and 5-HT cell bodies in the raphe nuclei.     

MDMA and fenfluramine reduce L-DOPA-induced dyskinesia via indirect 5-HT1A receptor stimulation

Chronic L-3,4-dihydroxyphenylalanine (L-DOPA) pharmacotherapy in Parkinson's disease is often accompanied by the development of abnormal and excessive movements known as dyskinesia. Clinical and experimental studies indicate that indirect serotonin agonists can suppress dyskinesia without affecting the efficacy of L-DOPA. While the mechanism by which these effects occur is not clear, recent research suggests that serotonin 5-HT1A receptors may play a pivotal role. To test this, male Sprague-Dawley rats with unilateral 6-hydroxydopamine medial forebrain bundle lesions received 1 week of daily treatment with L-DOPA (12 mg/kg, i.p.) plus benserazide (15 mg/kg, i.p.). Beginning on the 8th day of treatment and every 3rd or 4th day thereafter, rats were pretreated with vehicle (0.9% NaCl), the serotonin and dopamine releaser 3,4-methylenedioxymethamphetamine (MDMA; 0.25 or 2.5 mg/kg, i.p.) or the serotonin releaser fenfluramine (FEN; 0.25 or 2.5 mg/kg, i.p.) 5 min prior to L-DOPA, after which abnormal involuntary movements (AIMs) and rotations were quantified every 20th minute for 2 h. Pretreatment with 2.5 mg/kg of either MDMA or FEN reduced AIMs. To determine the contribution of the 5-HT1A receptor to these effects, another group of L-DOPA-primed 6-hydroxydopamine-lesioned rats were pretreated with the 5-HT1A antagonist WAY100635 (0.5 mg/kg, i.p.), MDMA + WAY100635 (2.5 + 0.5 mg/kg, i.p.) or FEN + WAY100635 (2.5 + 0.5 mg/kg, i.p.) 5 min prior to L-DOPA and subsequent AIMs and rotation tests. The antidyskinetic effects of MDMA and FEN were reversed by cotreatment with WAY100635. These results suggest that 5-HT-augmenting compounds such as MDMA and FEN probably convey antidyskinetic properties in part via stimulation of 5-HT1A receptors.     

Involvement of 5-HT1B receptors in the anticonflict effect of m-CPP in rats

Summary The anticonflict activity of m-CPP, a non-selective agonist of 5-HT receptors, was studied in the drinking conflict test in rats. m-CPP administered in doses of 0.125–0. 5 mg/kg increased the number of punished licks, the maximum effect having been observed after a dose of 0.25 mg/kg. The anticonflict effect of m-CPP (0.25 mg/kg) was antagonized by the non-selective 5-HT antagonist metergoline (1–4 mg/kg) and by the -adrenoceptor blocker SDZ 21009 (2 and 4 mg/kg) with affinity for 5-HT_1A and 5-HT_1B receptors. On the other hand, the 5-HT_1A receptor antagonist NAN-190 (0.5 and 1 mg/kg), the 5-HT₂ receptor antagonist ritanserin (0.25 and 0.5 mg/kg), and the -blockers betaxolol (8 mg/kg) and ICI 118,551 (8 mg/kg) with no affinity for 5-HT receptors did not affect the effect of m-CPP. The effect of m-CPP was not modified, either, in animals with the 5-HT lesion produced by p-chloroamphetamine.These results suggest that the anticonflict effect of m-CPP described above results from stimulation of 5-HT_1B receptors — most probably these which are located postsynaptically.     

Evidence for 5-HT autoreceptor-mediated,nerve impulse-independent,control of 5-HT synthesis in the rat brain

To gain further insight into the operation of 5-HT autoreceptor-mediated feedback control of 5-HT biosynthesis in serotonergic nerve terminal areas, the effect of the 5-HT_1B and the 5-HT_1A receptor agonists, TFMPP and 8-OH-DPAT, respectively, were investigated in the rat central nervous system (CNS) using in vivo and in vitro neurochemical approaches. TFMPP suppressed 5-HT synthesis (5-HTP accumulation after decarboxylase inhibition) both in vivo and in vitro. In vivo, the 5-HT synthesissuppressing effect of the drug (3.0 mg/kg, s.c.) proved resistant to either acute hemitransection or reserpine (5 mg/kg, i.p.; 90 min before) pretreatment. In vitro, in cortical, hippocampal and striatal slice preparations, TFMPP (0.1–10 μM) decreased 5-HT synthesis under basal and stimulated (30 mM K⁺) conditions, an effect which was unaltered by prior in vivo reserpine-induced 5-HT depletion but was attenuated in the presence of 5-HT_1B receptor antagonists such as methiothepin, cyanopindolol or propranolol. The 8-OH-DPAT (0.1 mg/kg, s.c.)-induced decrease of 5-HT synthesis in vivo was abolished by hemitransection but resistant to acute reserpine pretreatment; 8-OH-DPAT (10 μM) did not decrease 5-HT synthesis in vitro. In conclusion, the present study confirms the importance of 5-HT autoreceptors in the feedback control of nerve terminal 5-HT biosynthesis. Specifically, our data indicate: (1) that the reduction of rat brain 5-HT synthesis after TFMPP is mediated by 5-HT_1B autoreceptors located on the serotonergic axon terminals, and (2) that the effect is directly mediated and occurs independently of 5-HT neuronal firing and intact monoamine stores. © 1995 Wiley-Liss, Inc.     

5-HT precursor loading, but not 5-HT receptor agonists, increases motor function after spinal cord contusion in adult rats

Serotonergic (5-HT) receptors are upregulated following spinal cord transection. Stimulation by administration of serotonergic receptor agonists has been successful in improving hindlimb function. We tested whether this strategy would be successful in incomplete injury models (moderate or severe thoracic contusion) where descending projections are partially spared which should produce less denervation-induced receptor upregulation. Adult rats received midthoracic moderate (MOD: 25 mm drop) or severe (SEV: 50 mm drop) contusion injuries. Distribution of 5-HT and its transporter and expression of 5-HT_2C receptors were evaluated in lumbar spinal cord and motor response to 5-HT receptor activation was assessed using open field locomotion (BBB) score, percent weight supported treadmill stepping (%WS) and evaluation of hindlimb muscle activation (tremor and serotonin syndrome).5-HT immunostaining 3 months post-contusion revealed few 5-HT fibers caudal to the severe contusion, and more spared caudal to the moderate contusion. The distribution of 5-HT transporter paralleled 5-HT staining, but was more greatly reduced. Thus serotonin reuptake may be less efficient in the injured spinal cord. Immunostaining for the 5-HT_2C receptor in the dorsal and ventral horns at L5 showed significant upregulation in SEV, compared to sham or MOD rats.Neither 5-HT_2C nor 5-HT_1A receptor agonists, alone or in combination, nor the serotonin transporter inhibitor d-fenfluramine modified BBB scores or %WS in either group. Despite the increased sensitivity of post-synaptic targets, agonist treatment did not improve function in SEV rats. We conclude that selective 5-HT_2C or 5-HT_1A receptor activation was not effective in improving hindlimb function after incomplete lesions. In contrast, the 5-HT precursor 5-hydroxytryptophan (l-5-HTP), which leads to activation of all classes of 5-HT receptors, increased both %WS and hindlimb activity in the MOD group. While no side effects were observed in normal or MOD rats, SEV rats displayed hindlimb tremors and 33% mortality, indicating hypersensitivity to the precursor.     

L-dopa-induced air-stepping in developing rats

Rat pups at postnatal day (PN) 0, 5, 10, 15, and 20 were injected with 100 mg/kg L-3,4-dihydroxyphenylalanine (L-DOPA; s.c.) or the control vehicle and suspended in a sling. L-DOPA induced highly stereotyped air-stepping that varied along several dimensions across development. The latency to air-stepping was constant across age, but the duration was significantly shorter in the older pups. At all ages, air-stepping was preceded or accompanied by raising of the head and dorsiflexion of the tail. The rate of stepping increased across age and gait varied with age. At PN 0, L-DOPA induced alternating forelimb movements with little involvement of the hindlimbs. At PN 5 and 10, both forelimb and hindlimb alternation were observed. At PN 10 and 15 swimming-like behavior emerged, and at PN 10, 15, and 20, episodes of galloping were interspersed with limb alternation. Thus, highly coordinated activity can be elicited by L-DOPA at all ages studied when the rat pup needs not support its own body weight and has no contact with a substrate. Drug-induced air-stepping promises to be an important new paradigm for pharmacological and behavioral studies of the development of locomotor controls in mammals.     

Role of presynaptic serotonergic receptors on the mechanism of action of 5-HT1A and 5-HT1B agonists on masculine sexual behaviour: physiological and pharmacological implications

Summary In order to establish whether the 5-HT1A or the 5HT1B agonists, 8-OH-DPAT or TFMPP, produce their facilitatory or inhibitory actions on masculine sexual behaviour via a mechanism involving: (a) the serotonin synthesis or release; (b) the stimulation of presynaptic receptors, or (c) the stimulation of somatodendritic receptors, three series of experiments were performed. The administration of the serotonin synthesis inhibitor, p-chlorophenylalanine (p-CPA, 300mg/kg×3 days), facilitated sexual behaviour but does not interfere neither with the inhibitory nor with the facilitatory effects of TFMPP (0.5mg/kg) or 8-OH-DPAT (0.5 mg/kg), respectively. The icv or the intraraphé administration of the serotonergic neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), slightly stimulated masculine sexual behaviour and produced a decrease in serotonin and its metabolite levels. In lesioned animals TFMPP (0.5 mg/kg) resulted in an inhibitory effect reflected as a prolongation of the ejaculation latency. The inhibitory effect of this drug on mounting behaviour was not observed in 5,7-DHT treated rats. In lesioned animals 8-OH-DPAT (0.5 mg/kg) produced the same facilitatory effect. Present data indicate that serotonergic postsynaptic receptors mediate both the inhibitory and the facilitatory actions of TFMPP or 8-OH-DPAT in copulation. All data further support the idea that endogenous serotonin acts via the stimulation of 5-HT1B receptors to induce its inhibitory effects on masculine sexual behaviour.     

Role of the 5-HT2C receptor in improving weight-supported stepping in adult rats spinalized as neonates

Loss of descending serotonergic (5-HT) projections after spinal cord injury (SCI) contributes to motor deficits and upregulation of receptors on partially denervated serotonergic targets in the spinal cord. Serotonergic agonists acting on these upregulated receptors are potential therapeutic agents that could ameliorate motor deficits. However, modification of 5-HT receptors following complete spinal cord injury results in different effects by 5-HT2C receptor agonists and antagonists. For example, administration of 5-HT2C receptor agonists suppresses locomotor activity in normal animals, but enhances it in spinalized animals. In addition, administration of 5-HT2C receptor agonists does not induce activity-dependent hindlimb tremors in normal animals, but does induce them in spinalized animals. We therefore extended our previous work with the 5-HT2C receptor agonist 1-(m-chlorophenyl)-piperazine hydrochloride (mCPP), which enhances weight-supported stepping when administered to adult rats spinalized as neonates, to identify the optimal dose for improved weight-supported stepping with minimal side effects. In order to determine whether mCPP enhances weight-supported stepping after SCI is through activation of the 5-HT2C receptor, we performed the following experiments. We determined that stimulation of the 5-HT1A receptor did not contribute to this improvement in weight-support. We reversed the increase in mCPP-induced weight-supported stepping with SB 206,553, a 5-HT2C receptor antagonist. We also provide evidence for denervation-induced upregulation of 5-HT2C receptors in the injured spinal cord. Since mCPP does not have the behavioral toxicity associated with non-selective 5-HT2 receptor agonists, targeting the 5-HT2C receptor may have clinical relevance for the treatment of SCI.     

Sensory feedback modulates quipazine-induced stepping behavior in the newborn rat

Research has shown that sensory feedback modulates locomotor behavior in intact as well as spinal adult animals. Here we examined if locomotor activity ("stepping") in newborn rats is influenced by cutaneous and proprioceptive feedback. One-day-old rats were treated with the serotonergic receptor agonist quipazine (3.0mg/kg) to induce air-stepping behavior or with saline (vehicle control). During stepping, a substrate/floor (elastic, stiff, or none) was placed beneath their limbs so that the feet could make plantar surface contact with a substrate. Pups treated with quipazine showed significantly more alternated fore- and hindlimb steps and plantar paw contact with the substrate, compared to pups treated with saline. Pups also made proportionately less contact with the stiff substrate versus the elastic substrate during stepping. Different types of movements made on the substrate (paw pushes, taps, swipes, and stances) were also characterized. These results indicate that sensory feedback modulates locomotor mechanisms and behavior in perinatal rats.