Serotonin agonist actions of p-chlorophenylalanine

Neither serotonin nor adenosine-dependent mechanisms preserve ventilatory capacity in ALS rats

In rats over-expressing SOD1G93A, ventilation is preserved despite significant loss of respiratory motor neurons. Thus, unknown forms of compensatory respiratory plasticity may offset respiratory motor neuron cell death. Although mechanisms of such compensation are unknown, other models of respiratory motor plasticity may provide a conceptual guide. Multiple cellular mechanisms give rise to phrenic motor facilitation; one mechanism requires spinal serotonin receptor and NADPH oxidase activity whereas another requires spinal adenosine receptor activation. Here, we studied whether these mechanisms contribute to compensatory respiratory plasticity in SOD1G93A rats. Using plethysmography, we assessed ventilation in end-stage SOD1G93A rats after: (1) serotonin depletion with parachlorophenylalanine (PCPA), (2) serotonin (methysergide) and A2A (MSX-3) receptor inhibition, (3) NADPH oxidase inhibition (apocynin), and (4) combined treatments. The ability to increase ventilation was not decreased by individual or combined treatments; thus, these mechanisms do not maintain breathing capacity at end-stage motor neuron disease. Possible mechanisms giving rise to enhanced breathing capacity with combined treatment in end-stage SOD1G93A rats are discussed.

Effects of 8-OH-DPAT and fluoxetine on activity and attack by female mice towards lactating intruders

1. The impact of the serotoninergic receptor on the attack directed by female mice towards lactating intruders was assessed by studying the effects of 8-OH-DPAT (a serotoninergic agonist) and fluoxetine (an inhibitor of serotonin reuptake) on this paradigm at a range of doses and post-injection durations. 2. The specificity of these drug actions behaviour were examined by studying their effects on wheel running activity and performance in the open field. Non-sedative doses of 200 and 250 micrograms/kg of 8-OH-DPAT reduced attack by resident females on lactating intruders. 3. Higher doses (12-16 mg/kg) of fluoxetine reduced activity measures whereas lower non-sedative doses (up to 8 mg/kg) were without action on this aggression paradigm. 4. Additional studies with specific serotoninergic drugs are needed to clarify the role of this transmitter in attack by female mice on lactating intruders.

The role of serotonin release and autoreceptors in the dorsalis raphe nucleus in the control of serotonin release in the cat caudate nucleus

Using a push-pull cannula technique and an isotopic method for estimating [3H]serotonin continuously synthesized from [3H]tryptophan, the effects of changes in the release of serotonin in the dorsalis raphe nucleus on in vivo release of [3H]serotonin in the cat caudate nucleus were investigated. The increase in the release of serotonin in the dorsalis raphe nucleus caused by local application of parachlorophenylethylamine (10(-6) M) reduced striatal [3H]serotonin release. This inhibition in serotonin release in the striatum was blocked by the prior and continuous local superfusion of the dorsal raphe with methiothepin (10(-6) M), a serotonin autoreceptor antagonist. GABA (5 x 10(-5) M) applied to the dorsalis raphe reduced both local and striatal release of [3H]serotonin. However, picrotoxin (10(-5) M), a GABA A receptor antagonist applied locally in the dorsalis raphe nucleus increased [3H]serotonin release while decreasing striatal [3H]serotonin release. This decrease in serotonin release in the striatum was again blocked by continuous superfusion of the raphe with methiothepin. Furthermore, superfusion of serotonergic cell bodies of the dorsalis raphe nucleus with methiothepin alone never altered local release or striatal release of [3H]serotonin. These data strongly suggest that the release of serotonin from the cell body in the dorsalis raphe nucleus phasically controls release of the amine at the axonal nerve ending through serotonergic autoreceptors located on serotonergic nerve cell bodies in the dorsalis raphe nucleus. The origin of the serotonin released in the dorsalis raphe nucleus and the possibility that this type of regulation could be related to changes in nerve impulse conduction of the serotonergic raphe-striatal system are discussed.

Reversibility of para-chlorophenylalanine-induced insomnia by intrahypothalamic microinjection of L-5-hydroxytryptophan

Para-chlorophenylalanine, a blocker of serotonin biosynthesis by inhibiting tryptophan hydroxylase, induced total insomnia which was accompanied in cat by a permanent discharge of ponto-geniculo-occipital activity. L-5-Hydroxytryptophan microinjection (1-4 micrograms/0.5 microliters) in the anterior hypothalamus 72 h after para-chlorophenylalanine administration, restored both slow wave sleep and paradoxical sleep with variable latencies for each state of sleep. On the contrary, ponto-geniculo-occipital activity was never suppressed. The hypnogenic effects of L-5-hydroxytryptophan were always followed by a return of the para-chlorophenylalanine-induced insomnia. On the other hand, the temperature recording did not show any alteration of the cerebral temperature after para-chlorophenylalanine treatment but the subsequent L-5-hydroxytryptophan microinjection was followed by hyperthermia. Using immunohistochemistry for serotonin after intrahypothalamic L-5-hydroxytryptophan microinjection in parachlorophenylalanine-pretreated cat, we defined a restricted region of the anterior hypothalamus possibly responsible for the hypnogenic effect. This region included the lateral hypothalamus and the anterior hypothalamic area. It is suggested that the reversible hypersomnia after L-5-hydroxytryptophan microinjection in the anterior hypothalamus in para-chlorophenylalanine-pretreated cat is due to a neurohormonal action of serotonin: serotonin could act upon the anterior hypothalamus which secondarily inhibits a waking system located in the ventrolateral hypothalamus leading to the appearance of paradoxical sleep.

Changes in the rat sleep-wake cycle produced by D,L-beta-(1-naphthyl)alanine, a tryptophan analog

The effects of i.p. injections of D,L-beta-(1-naphthyl)alanine, a synthetic analog of tryptophan were tested on the rat sleep-waking cycle. Administration of 30, 60 and 120 mg/kg resulted in a significant increase in slow-wave sleep (SWS) and a decrease in paradoxical sleep (PS). These modifications were dose-dependent. These results were compared with those previously obtained with L-tryptophan and several analogs and discussed in relation with possible changes in central serotonergic activity.

Effects of p-chlorophenylalanine on hypothalamic indoleamine levels and the associated changes which occur in catecholamine dynamics and LH surges in estrogen-treated ovariectomized rats

Recent studies have demonstrated that the serotonergic and noradrenergic systems are functionally and anatomically linked and both systems have been implicated as contributors to the regulation of the phasic release of LH. Consequently, perturbations within the serotonergic system could secondarily affect noradrenergic system activity and result in a loss of phasic LHRH secretion. In the present studies we examined the effects of p-chlorophenylalanine (PCPA) on LH surges and the associated changes which occur in hypothalamic serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations. We also evaluated the effects of this drug on norepinephrine (NE) and dopamine (DA) initial concentrations, rate constants and turnover rates in the medial preoptic area (MPN), suprachiasmatic nuclei (SCN), paraventricular nuclei (PVN) and median eminence (ME). Seven days after ovariectomy, rats received estradiol (E2) capsules (day 0) and on day 1 some animals also received PCPA (250 mg/kg b. wt., i.p.) while the remainder served as controls. LH surges occurred in control animals but not in PCPA-treated rats on days 2, 3 and 4. PCPA produced a significant decline in 5-HT and 5-HIAA concentrations in all microdissected hypothalamic regions at 09.00 and 15.00 h on day 2. In control rats, there were no significant changes in initial concentrations of NE in the MPN, PVN and ME between 09.00 and 15.00 h with the exception of the SCN where a slight decline had occurred by 15.00 h. NE rate constants and turnover rates increased during the afternoon in controls in the MPN, SCN and ME and declined in the PVN concomitant with LH surges. PCPA had variable effects in suppressing NE initial concentrations depending upon the hypothalamic area studied and the time of day. More importantly, the drug abolished the diurnal rhythm in rate constants observed in controls and consequently, neither the MPN, SCN nor ME showed any increase in NE turnover rates in the afternoon of day 2. In contrast, a significant decline in rate constants and turnover rates occurred in the PVN of both control and PCPA-treated rats during the afternoon of day 2. DA initial concentrations declined in controls between 09.00 and 15.00 h in the MPN and ME but not in the SCN or PVN.(ABSTRACT TRUNCATED AT 400 WORDS)

Maternal aggression in mice: effects of treatments with PCPA, 5-HTP and 5-HT receptor antagonists

Drug treatments which influence brain serotonergic systems were administered to lactating female mice during the early postpartum period, and their effects on aggressive behavior, locomotor activity and brain monoamines were examined. P-chlorophenylalanine (200 and 400 mg/kg) and 5-hydroxytryptophan (100 mg/kg) inhibited fighting behavior of postpartum mice toward unfamiliar male intruder mice. These drug-treated postpartum females showed increased latencies to attack male intruders and also reduced frequencies of attack. In addition, postpartum mice treated with the serotonin receptor antagonists, mianserin (2 and 4 mg/kg), methysergide (4 mg/kg) and methiothepin (0.25 and 0.5 mg/kg), displayed significantly less aggressive behavior than control mice, as measured by reduced number of attacks. Whole brain monoamine and monoamine metabolite levels were measured after drug treatments. The behavioral results are discussed in terms of drug-induced changes in brain chemistry and indicate a possible role for serotonin in the mediation of maternal aggressive behavior of mice.

Intracerebroventricular administration of cholecystokinin inhibits the activity of the dopaminergic and serotoninergic systems of the brain

The behavioral and biochemical effects of intracerebroventricular administration of cholecystokinin were investigated in experiments on male Wistar rats. Cholecystokinin induced specific dose-dependent changes in the behavior of the animals. At low doses the inhibiting influence on behavior predominated; at high doses stereo-typed behavior, shaking of the head and increased reactivity to pain stimuli were observed. Cholecystokinin appreciably inhibited the circulation of serotonin and dopamine in the brain structures in comparison with physiological saline solution. Administration of cholecystokinin against a background of phenamine and 5-hydroxy-tryptophan briefly entirely inhibited the behavioral effects induced by these substances. On the basis of the data obtained it can be assumed that cholecystokinin is an endogenous modulator of the activity of the monoaminergic systems of the brain.

Studies into the dual effects of serotonergic pharmacological agents on female sexual behaviour in the rat: preliminary evidence that endogenous 5HT is stimulatory

The potential stimulatory and inhibitory effects on female sexual behaviour of five 5HT antagonists and five agents that increase 5HT activity, were noted in ovariectomised rats primed with various steroid regimes such that they were either "receptive" (LQ greater than 50%) or "non-receptive" (LQ less than 50%). The 5HT antagonists cinanserin, mianserin, ketanserin and metergoline all inhibited behaviour in receptive rats. Methysergide and cinanserin stimulated behaviour in non-receptive rats. All the drugs which increased 5HTP activity, i.e., 5HTP, zimelidine, alaproclate, WY 26002 and quipazine stimulated sex behaviour in non-receptive rats. In rats that had been ovariectomised only, part of this effect was probably due to stimulation of adrenal progesterone, but a significant stimulatory effect could still be observed in ovariectomised-adrenalectomised rats. 5HT also had a significant inhibitory effect on receptive rats, and the other agonists showed a similar but non-significant tendency. In view of the fact that 4 out of 5 of the 5HT antagonists inhibited sexual behaviour, we hypothesise that 5HT has a stimulatory role in the control of female sexual behaviour. The possible mechanisms mediating the dual action of 5HTP on female sexual behaviour are discussed.

Tryptophan hydroxylase and 5-HTP-decarboxylase activity in cockroach brain and the effects of p-chlorophenylalanine and 3-hydroxybenzylhydrazine (NSD-1015)

The activities of two enzymes of the serotonin biosynthetic pathway, namely tryptophan hydroxylase (TPH) and 5-hydroxytryptophan-decarboxylase (5-HTP-D), were determined in the cockroach, Periplaneta americana, brain using radiometric techniques, significant levels of both enzymes were found. p-Chlorophenylalanine (PCPA) inhibited brain TPH activity strongly both in vitro and in vivo. In vitro, 10 mM PCPA reduced the TPH activity by 76% and in vivo either 15 micrograms/g or 30 micrograms/g of PCPA inhibited brain TPH activity by 80-85%. The activity of 5-HTP-D enzyme was inhibited significantly in vivo by 3-hydroxybenzylhydrazine, commonly known as NSD-1015. A recovery in the activity of 5-HTP-D enzyme was observed 12 h after 50 micrograms/g of NSD-1015 but 100 micrograms/g caused an inhibition lasting for the 24 h period studied.

Possible resolution of a paradox concerning the use of p-chlorophenylalanine and 5-hydroxytryptophan: evidence for a mode of action involving adrenaline in manipulating the surge of luteinizing hormone in rats

Various functions involving the central nervous system can be manipulated by the sequential administration of p-chlorophenylalanine and 5-hydroxytryptophan, compounds which respectively inhibit and restore the synthesis of 5-hydroxytryptamine in the brain. An involvement of 5-hydroxytryptamine in the control of a particular function has been considered established when the effect of p-chlorophenylalanine on that function can be overcome by treatment with 5-hydroxytryptophan. This assumption is not, however, invariably substantiated when the functional consequences of other methods of depleting 5-hydroxytryptamine are considered; studies on the control of the daily surge of luteinizing hormone in oestrogen-treated ovariectomized rats present such a paradox. The surge can be prevented by p-chlorophenylalanine and restored by 5-hydroxytryptophan. Nevertheless, neurotoxin-induced lesions of the 5-hydroxytryptamine projections from the raphe nuclei are compatible with a normal occurrence of the surge. We have therefore examined the effects of p-chlorophenylalanine and 5-hydroxytryptophan on hypothalamic monoamines in oestrogen-treated ovariectomized rats and find that the drugs respectively suppress and elevate the concentration of adrenaline in addition to that of 5-hydroxytryptamine. Phenylethanolamine N-methyltransferase, the enzyme responsible for converting noradrenaline to adrenaline, is shown to be inhibited in vivo by p-chlorophenylalanine and in vitro by its metabolite, p-chlorophenylethylamine. The reciprocal effects of p-chlorophenylalanine and 5-hydroxytryptophan on the concentration of adrenaline are of particular interest since drugs which inhibit adrenaline synthesis can block the luteinizing hormone surge. It is proposed that when the 5-hydroxytryptophan-reversible effects of treatment with p-chlorophenylalanine are not reproduced by other procedures which deplete 5-hydroxytryptamine, the significant action of these compounds may involve adrenaline.

Lack of central 5-hydroxytryptamine influence on the anticonflict activity of diazepam

This study examined the effects of various drug treatments (IP injections) proposed to modify central 5-hydroxytryptamine (5-HT) activity on a conditioned suppression of drinking behavior in water-deprived rats. The subjects were trained to drink their daily water requirement during a 10-min session. Intermittent tone periods of 7 s were then introduced, the last 5 s of which the drinking tube was electrified. The animals gradually suppressed tube contacts during the tone to a low constant level within 2 weeks of training. Diazepam increased punished responding dramatically. The 5-HT antagonists methysergide (1 - 18 mg/kg), cyproheptadine (1 - 18 mg/kg), metergoline (0.25 - 2.0 mg/kg) and cinanserin (10 - mg/kg) failed to induce large, reliable increases in punished responding. When a low dose of diazepam was combined with 5-HT antagonists, only one treatment, methysergide at 3 mg/kg, potentiated the anticonflict activity of diazepam. Acute or chronic treatment with PCPA increased behavior suppressed by punishment, but this effect was weak, brief, and poorly related to the depletion of brain 5-HT. LSD (0.3 - 100 microgram/kg) administered 1, 10, or 30 min before the test was ineffective in overcoming suppression by punishment. Mescaline (6 - 30 mg/kg) had no significant effect on punished responding. 5-HTP (18 mg/kg) decreased the number of shocks accepted, but not after pretreating with carbidopa. Pretreatment with carbidopa plus 5-HTP potentiated the anticonflict effect of diazepam. The 5-HT agonist mCPP (0.25 - 2.0 mg/kg) enhanced suppression due to punishment, but only in doses that interfered with unpunished responding. The 5-HT-releasing agent fenfluramine (0.25 - 1.0 mg/kg) did not affect this behavior. Amitriptyline pretreatment in a dose not affecting unpunished behavior (5.6 mg/kg) potentiated the diazepam-induced increase in punished responding. These results are difficult to reconcile with the proposal that suppression of behavior consequent to punishment is related to brain 5-HT activity.

Effect of p-chlorophenylalanine and tryptophan on sleep, EEG and motor activity in the rat

The effects of a single injection of parachlorophenylalanine (PCPA, 300 mg/kg i.p.) on sleep, motor activity and consummatory behavior were investigated in unrestrained rats which were continuously recorded with telemetric techniques on two control days and six drug days. Slow wave sleep (SWS) was defined as the non-REM sleep (NREMS) fraction with a low predominant EEG frequency. In the 24 h following PCPA administration, motor activity and food intake were reduced and sleep was increased. SWS was massively enhanced, while REM sleep (REMS) was depressed. The initial phase of sedation was followed by a phase of partial insomnia lasting 1-2 days. SWS and REMS were particularly depressed. A rebound phenomenon was observed at the end of recovery period when some of the SWS and REMS values exceeded the control level. The administration of tryptophan (Trp, 150 mg/kg i.p.) 28 h after PCPA pretreatment, causing a significant rise in the brain serotonin (5-HT) concentration, produced a temporary increase in SWS and REMS, and a reduction of motor activity. The experiments show that the depression of SWS and REMS, and the hyperactivity 1-2 days after PCPA administration, are a consequence of the reduced 5-HT level, whereas the involvement of serotonergic mechanisms in the initial sedative phase and in the recovery phase is less clear. The persistence of the daily distribution of sleep and activity, and of the specific pattern of SWS and REMS, indicates that the circadian sleep organization is little affected by 5-HT depletion.

Para-halogenated phenethylamines: similar serotonergic effects in rats by different mechanisms

The serotonin (5-HT) behavioral syndrome in rats served as a model to test for possible acute serotonergic effects of para-halogenated phenethylamines. p-Chloro-, p-chloro-beta-methyl-, p-fluoro-, p-bromo-, and p-iodophenethylamine produced the same 5-HT behavioral syndrome as did p-chloroamphetamine, but unlike the latter did not deplete brain 5-HT 3 days after injection. Pretreatment of rats with the 5-HT depletor p-chlorophenylalanine (pCPA)( prevented the serotonergic effects of both chloro-derivatives, and partially prevented the effects of bromo- and iodophenethylamine. 5-hydroxytryptophan restored the behavioral responses to these compounds in pCPA-pretreated rats. pCPA treatment did not prevent the behavioral effects of p-fluorophenethylamine. Similarly, zimelidine, a 5-HT uptake inhibitor, prevented the serotonergic behavioral effects of all compounds tested except p-fluorophenethylamine. Taken as a group, para-halogenated phenethylamines are short-acting serotonergic compounds which, unlike p-chloroamphetamine, do not produce long-lasting depletion of brain 5-HT. p-Chlorophenethylamine and its beta-methyl analog apparently activate central 5-HT receptors indirectly, i.e., by 5-HT release; p-fluorophenethylamine is a direct 5-HT agonist. The p-bromo- and p-iodo-derivatives apparently possess both properties.

Review of the role of the central serotonergic neuronal system in blood pressure regulation

Alterations in the dynamics of brain serotonin biosynthesis can lead to changes in cardiovascular function. It appears that the activation of cerebral serotonin receptors produces a pressor effect in normotensive rats but produces a depressor effect in normotensive cats or dogs. On the other hand, reductions in the levels of serotonin can prevent the onset of hypertension in some experimental hypertensive models and lower the blood pressure of organisms with established hypertension. The ability of brain serotonin to modulate arterial blood pressure may be mediated by the influences of the serotonergic neuronal systems on efferent sympathetic activity. Finally, the reduction in sympathetic outflow produced by increasing brain serotonin levels in dogs protects the heart against ventricular fibrillation and may, therefore, constitute a reasonable adjunct in the management of high-risk, cardiac-arrest patients.