Serotonin and tolerance to delay of reward in rats

Rationale: Tolerance to delay of gratification, taken to reflect impulsiveness, has been proposed to be under the preferential control of central serotonin (5-HT) processes. Objective: The present study further examined the effects of drugs which directly or indirectly alter 5-HT transmission, on behaviour controlled by a delayed positive reinforcer. Methods: Rats were given the choice in a T-maze between two magnitudes of reward: small (two food pellets) and immediate versus large (ten pellets) but delayed. When a 15-s waiting period was imposed in the arm leading to the large reward, rats selected this arm on 65–70% of the trials. This frequency was reduced to less than 40% when the large reward was delayed by 25 s. Results: In rats whose ascending 5-HT pathways had been lesioned by infusion of 5,7-dihydroxytryptamine (5,7-DHT) into the dorsal raphe, the introduction of the 15-s delay contingency resulted in a transient larger reduction of the frequency of choice of the now-delayed reward, compared to sham operated controls. In contrast, choice behaviour of rats given 5,7-DHT into the substantia nigra did not differ from controls. para-Chlorophenylalanine (pCPA, 150 mg/kg IP, daily for 3 days), a 5-HT synthesis inhibitor, bretazenil (0.5-8 mg/kg IP), a benzodiazepine (BZD) receptor partial agonist, and muscimol (0.25-1 mg/kg IP), a GABA_A receptor agonist, induced a shift toward immediate reward. In contrast to the other BZDs, alprazolam (1–2 mg/kg IP) enhanced the frequency of choice of the large-but-25 s-delayed reward. Similar increased preference for the large-but-delayed reward was induced by the selective 5-HT reuptake inhibitors, fluoxetine (4–8 mg/kg IP) and fluvoxamine (4 mg/kg IP). The full 5-HT_1A receptor agonist, 8-OH-DPAT (0.015–0.5 mg/kg IP) enhanced the frequency of choice of the large reward delayed by 25 s, whereas the partial agonists, buspirone (1–4 mg/kg IP), ipsapirone (0.5–1 mg/kg IP) and MDL 73005EF (1–2 mg/kg SC), and the antagonist, WAY 100635 (4 mg/kg SC), reduced the number of choices of the large reward delayed by 15 s. Unexpectedly, WAY 100635 (2 mg/kg), which had no effect on choice whatever the delay, did not counteract the increased tolerance to delay induced by 8-OH-DPAT (0.06 mg/kg) and further reduced the frequency of choice of the large-but- 15 s-delayed reward induced by ipsapirone (0.5 mg/kg). Conclusions: These effects on tolerance to delay may be accounted for by a subtle balance between the opposing functional consequences of pre- versus post-synaptic 5-HT_1A receptor activation or blockade. Overall, the present results provide further support to the idea that 5-HT processes participate in the control of impulsive-related behaviour, as assessed from tolerance to delay of reward in this particular T-maze procedure. Received: 26 April 1999 / Final version: 20 May 1999     

Reduction of alcohol selection by pargyline in mice

Drugs which increase brain levels of serotonin (5-HT) have frequently been found to cause a decrease in voluntary ethanol consumption. Results obtained with parachlorophenylalanine (pCPA), which decreases 5-HT, have been less consistent. The present investigation compared the effects of pCPA on alcohol selection with those of pargyline, a monoamine oxidase inhibitor which increases brain levels of 5-HT. Ingestion of a 10% ethanol solution was assessed in male C57BL/6J mice given daily injections of 250 or 300 mg/kg pCPA, 50 mg/kg pargyline, or saline. An additional control group received no treatment. A two-bottle preference procedure was employed, and ethanol and water intake were recorded during a pretreatment period (11 days), a treatment period (8 days), and a posttreatment period (10 days). Like other agents which increase 5-HT, pargyline produced a depression in ethanol intake which lasted beyond the time of drug administration. pCPA had no effect on ethanol ingestion either during the period of drug administration or afterwards.     

Effect of brain serotonin level on induced hippocampal paroxysmal activity in rats

Experiments were performed to study the involvement of brain 5-HT in an experimental model of epilepsy induced by repeated electrical stimulation of the dorsal hippocampus of rats. The experiments included: (1) systemic injections of 5-hydroxytryptophan (5-HTP) and p-chlorophenylalanine (pCPA) and (2) electrolytic lesions of the midbrain raphe nuclei. The pCPA group showed a significant increase while animals which received systemic injections of 5-HTP showed a great reduction in the electrographic seizure activity. Although several reports have shown that midbrain raphe lesions do not modify the 0pileptic susceptibility, we observed a clear enhancement in the epileptiform activity in lesioned animals. The results presented here support the view that serotonergic systems may exert a tonic inhibitory effect on hippocampal epileptic activity.     

Post-synaptic 5-HT1A receptor involvement in yawning and penile erections induced by apomorphine,physostigmine and mCPP in rats

Apomorphine and mCPP induced yawning associated with penile erections in rats, whereas physostigmine induced only yawns. Apomorphineinduced yawning and penile erections were antagonized by low doses of raclopride, whereas physostigmineinduced yawning and mCPP-induced effects were only partly inhibited at high doses of raclopride. Scopolamine as well as clozapine antagonized yawning and penile erections induced by apomorphine, mCPP and physostigmine. Similarly, the 5-HT_1A agonists 8-OH-DPAT and S 14506 inhibited yawning and penile erections induced by apomorphine, mCPP and physostigmine, and at similar doses induced lower lip retraction and hyperreactivity to handling. The /5-HT_1A antagonist tertatolol reversed the inhibitory effects of 8-OH-DPAT and S 14506 on drug-induced yawning and penile erections and increased apomorphine-and physostigmine-induced yawn frequency but not penile erection frequency. Like tertatolol, propranolol increased apomorphine- and physostigmine-induced yawn frequency, whereas ICI 118551 increased only physostigmine-induced yawning. 8-OH-DPAT-and S 14506-induced lower lip retraction and hyperre-activity to handling were also significantly antagonized by tertatolol. Finally,p-chlorophenylalanine pretreatment produced about 95% depletion in 5-HT in hypothalamus, hippocampus, striatum and frontal cortex and modified neither the responses of the inducing drugs nor the inhibitory effects of 8-OH-DPAT and S 14506 on drug-induced yawning and penile erections. These data suggest (i) that a final cholinergic mechanism blocked by scopolamine and clozapine is involved in drug-induced yawning and penile erections, (ii) that the effects of S 14506 depend upon stimulation of 5-HT_1A receptors, (iii) that the inhibitory effects of 5-HT_1A agonists on yawning and penile erections probably occurred at final steps on the pathways involved in the expression of yawning and penile erections and that these effects could be related to other effects of the 5-HT_1A agonists, (iv) that a tonic inhibitory influence could interfere with the expression of yawning but not with that of penile erections and (v) that presynaptic 5-HT_1A receptors do not seem to play an important role in the inhibitory effects of 5-HT_1A agonists on drug-induced yawning and penile erections in rats.     

Serotonin involvement in the electroacupuncture- and moxibustion-induced gastric emptying in rats

OBJECTIVE: Electroacupuncture (EA) as well as moxibustion stimulation has been reported to produce an excitatory effect on the gastrointestinal motility of the rat. Serotonergic neurons of the mioenteric and submucous plexus are major participants in the gastrointestinal physiology. Here, we compared the outcomes of the stimulation of a specific set of acupoints with either acupuncture or moxibustion on the gastrointestinal motility and the role of serotonin (5-HT) in this effect. METHODS: To analyze the role of 5-HT on the gastrointestinal motility of the rat, we studied the flow of 25 glass beads administered to the stomach, after treatment of the animals with a serotonin inhibitor (para-chlorophenylalanine [pCPA]). Acupuncture stimulation was performed on acupoints St-36 (Zusanli) and Sp-6 (Sanyinjiao), with electrical stimulation, or on acupoints Ren-10 (Xiawan), Ren-12 (Zhongwan) and St-25 (Tianshu), with moxibustion. Animals subjected to sham stimulation were used as controls in addition to naive, unstimulated animals. RESULTS: Stimulation of the hind limb (St-36 and Sp-6) and abdominal (Ren-10, Ren-12, St-25) acupoints resulted in effective gastric emptying, as compared with sham-stimulated animals. Pretreatment of animals with pCPA abolished either the response provided by acupuncture stimulation in animal groups subjected to hind limb acupoints or the response provided by moxibustion stimulation in abdominal acupoints. CONCLUSION: Our data suggest that moxibustion in the abdominal points and EA in the hind limb require an intact serotonergic pathway. In addition, we suggest that this involvement of serotonin is a general feature of the mediated effects of acupuncture on gastric emptying of the rat.     

Hypnotic action of flunitrazepam in the rat: Does 5-HT mechanism play a role?

This study examined whether pharmacological manipulation of serotonergic (5-HT) systems would affect the hypnotic action of flunitrazepam in rats. Flunitrazepam, a potent hypnotic, was used alone or combined with parachlorophenylalanine (pCPA), an inhibitor of the synthesis of 5-HT, 8-OH-DPAT, a 5-HT_1A receptor agonist and fluvoxamine, an inhibitor of the reuptake of 5-HT. Flunitrazepam increased the amount of orthodox sleep, the latency of rapid eye movement (REM) sleep and decreased the amount of REM sleep. The drug pCPA decreased the total sleep time and the amount of orthodox and REM sleep. Administration of flunitrazepam to pCPA-pretreated rats induced orthodox sleep in an identical way to that found in the controls. The drug 8-OH-DPAT increased wakefulness and the latency of REM sleep. The association of flunitrazepam with 8-OH-DPAT abolished the increase in waking seen after 8-OH-DPAT alone. In contrast, the combined treatment with flunitrazepam and 8-OH-DPAT resulted in a lengthening of the latency of REM sleep significantly greater than that observed with the same dose of each drug alone. Fluvoxamine increased the latency and decreased the amount of REM sleep. The association of fluvoxamine with flunitrazepam induced a decrease in REM sleep, equal to the sum of the effects of the two drugs alone. Fluvoxamine did not modify the other effects of flunitrazepam. The present experiments demonstrate that the association of pCPA, 8-OH-DPAT and fluvoxamine, did not alter the hypnogenic effect of flunitrazepam. The possibility of an involvement of 5-HT mechanisms in the effect of flunitrazepam on the phasic events in sleep is questionable.     

Serotonin agonist actions of p-chlorophenylalanine

p-Chlorophenylalanine (pCPA), in combination with a monoamine oxidase inhibitor, caused a behavioural syndrome shown previously to be due to serotonin receptor activation. p-Chlorophenyl-ethylamine (pCPEA), a decarboxylation product of pCPA, also caused the behavioural syndrome. Decarboxylase inhibition prevented the syndrome after pCPA administration, but not after pCPEA. Prior depletion of brain serotonin to 18% of control prevented the syndrome caused by pCPA (plus monoamine oxidase inhibitor), or by pPCEA alone. Subsequent replacement of serotonin in depleted rats by 5-hydroxytryptophan restored the behavioural effects of pCPA and pCPEA. The results show that pCPA (in combination with an inhibitor of monoamine oxidase) caused serotonin receptor activation. The behavioural syndrome is probably elicited by displacement of presynaptic serotonin by pCPEA. As pCPA apparently has a long biological half-life, continued synthesis of pCPEA may cause anomalous behavioural effects not correlated with serotonin depletion. This possibility should be considered in behavioural studies with pCPA, especially during early time periods when serotonin concentrations are not yet reduced.     

The involvement of serotonergic system in the antidepressant effect of zinc in the forced swim test

Recent preclinical data indicated the antidepressant-like activity of zinc in different tests and models of depression. The present study investigates the involvement of the serotonergic system in zinc activity in the forced swim test (FST) in mice and rats. The combined treatment of sub-effective doses of zinc (hydroaspartate, 2.5 mg Zn/kg) and citalopram (15 mg/kg), fluoxetine (5 mg/kg) but not with reboxetine (2.5 mg/kg) significantly reduces the immobility time in the FST in mice. These treatments had no influence on the spontaneous locomotor activity. Moreover, while the antidepressant-like effect of zinc (5 mg/kg) in the FST was significantly blocked by pretreatment with inhibitor of serotonin synthesis, p-chlorophenylalanine (pCPA, 3 × 200 mg/kg), 5HT-2_A/C receptor antagonist, ritanserin (4 mg/kg) or 5HT-1A receptor antagonist, WAY 1006335 (0.1 mg/kg), the zinc-induced reduction in the locomotor activity was not affected by these serotonin modulator agents. These results indicate the specific involvement of the serotonergic system in antidepressant but not the motion behavior of zinc in mice. Also, an increase in the swimming but not climbing parameter of the rat FST observed following zinc administration (2.5 and 5 mg Zn/kg) indicates the serotonin pathway participation. This present data indicates that the antidepressant-like activity of zinc observed in the FST involves interaction with the serotonergic system.     

Inhibitory effect of crotoxin on the pain-evoked discharge of neurons in thalamic parafascicular nucleus in rats.

Crotoxin (Cro), the principal neurotoxic component of Crotalus durissus terrificus, has been previously reported to have a behavioral analgesic effect in rats and mice. The present study investigated electrophysiologically the effect of Cro on pain-evoked unit discharge of neurons in thalamic parafascicular nucleus (Pf) and underlying mechanisms of its effect. The electrical discharge of Pf neurons was recorded with the microelectrode technique in rats. Intracerebroventricular (i.c.v.) injection of Cro at 0.25, 0.45 and 0.65 microg/kg resulted in a dose-dependent inhibitory effect on the pain-evoked discharge of Pf neurons. The discharge frequency and the discharge duration significantly (P<0.05) decreased after Cro administration. This inhibitory effect was significantly (P<0.05) attenuated after pretreatment with para-chlorophenylalanine (pCPA), or electrolytic lesion of dorsal raphe (DR) nucleus. In contrast, i.c.v. injection of atropine (muscarinic receptor antagonist, 5 microg) or naloxone (opioid receptor antagonist, 4 microg) had no effect on Cro-induced inhibition of discharge of Pf neurons. The results suggested that Cro has an analgesic effect, which is mediated, at least partially, by the central serotonergic system.     

Effects of 5-hydroxytryptophan, iproniazid and p-chlorophenylalanine on lidocaine seizure threshold of mice

The effects of 5-hyroxytryptophan (5-HTP), iproniazid and p-chlorophenylalanine (p-CPA) on the lidocaine seizure threshold of mice were studied. The median i.p. convulsant dose (CD₅₀) of lidocaine in control (saline-treated) mice was 76.0 mg/kg. Pretreatment with 5-HTP (100 mg/kg i.p.) lowered the CD₅₀ to 61.0 mg/kg. Similar results (CD₅₀ of 58.0 mg/kg) were observed in the iproniazid (100 mg/kg i.p.) pretreated group. In the iproniazid—5-HTP treated group, a more pronounced and significant (p < 0.01) drop of the CD₅₀ to 45.0 mg/kg was observed. In p-CPA pretreated mice (300 mg/kg i.p. for 3 days), the CD₅₀ increased slightly to 90.0 mg/kg. The duration of seizures increased in 5-HTP and iproniazid pretreated groups. These results suggest the possibility of an involvement of 5-HT in the lidocaine-induced convulsive process.