Comparison of the effects of recently developed alpha 2-adrenergic antagonists with yohimbine and rauwolscine on monoamine synthesis in rat brain

Facilitation of cognitive functions by a specific alpha2-adrenoceptor antagonist, atipamezole

The present experiments investigated the effects of a specific and potent alpha2-adrenoceptor antagonist, atipamezole (as a stimulator of the noradrenergic system) on cognitive performance in rats. Atipamezole enhanced the acquisition of a linear-arm maze test and also improved the choice accuracy of poorly performing rats in a delayed (20 min) three-choice maze test. Furthermore, atipamezole improved the achievement of a one-trial appetite-maze when injected immediately after teaching, thus having an effect on consolidation. Atipamezole clearly impaired the acquisition of the active avoidance test. The present results indicate that stimulation of noradrenergic system by atipamezole improves the performance of animals in tasks assessing relational learning and memory, possibly affecting attention, short-term memory and the speed of information processing. It has also an effect on a consolidation process unrelated to attentional or motivational mechanisms. In a stressful test. stimulation of noradrenaline release leads to impairment of performance.

Corynanthine inhibits, while idazoxan potentiates, cardiotoxic effects of ouabain

1. Ouabain, infused intravenously to anaesthetized guinea-pigs induced ventricular premature beats, ventricular tachyarrhythmias and lethality. 2. Corynanthine (1, 2 and 4 mg kg-1), an alpha 1-adrenoceptor antagonist and idazoxan (100, 200 and 400 micrograms kg-1), an alpha 2-adrenoceptor antagonist were administered 10 min prior to ouabain. Corynanthine (2 and 4 micrograms kg-1) showed significant increase in the amount of ouabain required to cause arrhythmia and lethality, whereas idazoxan (200 and 400 micrograms kg-1) decreased it. 3. Corynanthine inhibited the ouabain-induced pressor response while idazoxan potentiated it. 4. Effects of these agents on the sympathetic nervous system appear to have played a significant role in its anti- and proarrhythmic actions.

Sauvagine-induced hypothermia: evidence for an interaction with the dopaminergic system

In rats kept at an ambient temperature of 22 degrees C, centrally and peripherally administered sauvagine induces a dose-dependent hypothermia. To clarify the regulatory mechanisms and to ascertain which neurotransmitter systems mediate sauvagine-induced hypothermia, we administered sauvagine intracerebroventricularly and subcutaneously in rats pretreated with antagonists of muscarinic receptors (atropine), opiate receptors (naloxone), alpha-adrenoceptors (phentolamine, yohimbine and prazosin), beta-adrenoceptors (propranolol) and dopamine receptors (haloperidol and spiperone). Systemic pretreatment of rats with atropine, naloxone, prazosin and propranolol left sauvagine-induced hypothermia unaltered. Pretreatment with phentolamine (4 mg/kg, s.c.), a non-selective alpha-adrenoceptor antagonist, and yohimbine (3 mg/kg, s.c.), a selective alpha 2-adrenoceptor antagonist, enhanced the hypothermic action of sauvagine. Pretreatment with haloperidol (2 mg/kg, s.c.), a non-selective dopamine receptor antagonist, and spiperone (80 micrograms/kg, s.c.), a selective dopamine D2 receptor antagonist, significantly reduced the temperature fall induced by centrally (4 micrograms/rat) and peripherally (20 micrograms/kg) administered sauvagine. Thus, sauvagine-induced hypothermia appears not to be mediated by interactions with cholinergic, endogenous opiate or noradrenergic systems, but rather D2 dopaminergic pathways alone are involved in the inhibitory effect of sauvagine on body temperature in the rat.

Effects of alpha-2 adrenoceptor antagonists on rough-and-tumble play in juvenile rats: evidence for a site of action independent of non-adrenoceptor imidazoline binding sites

The pharmacological specificity of alpha-2 adrenoceptor involvement in the modulation of rough-and-tumble play behavior was assessed in juvenile rats. The alpha-2 adrenoceptor antagonists idazoxan and RX821002 both increased the frequency of pinning in individually housed rats that were given a brief opportunity to play. Dorsal contacts, a measure of play solicitation, were not consistently affected by these compounds. Since RX821002 shows little affinity for non-adrenoceptor imidazoline binding sites, it is likely that the facilitation of play following administration of these two compounds is due to blockade of alpha-2 receptors. The effect of RX821002 and idazoxan is unlikely to be an artifact associated with using rats that are reared in isolation, as RX821002 also increased pinning, as well as dorsal contacts, in group-housed rats that were isolated for a short period (4h) before the play session. The alpha-1 adrenoceptor antagonist prazosin, which also binds to alpha-2B receptors, reduced the frequency of both pinning and dorsal contacts. There was a strong trend for St 587, a centrally active alpha-1 agonist, to attenuate the effect of prazosin on play. While this leaves open the possibility that prazosin may be reducing play through alpha-1 blockade, antagonist activity at alpha-2B receptors cannot be ruled out. From these data, we conclude that the facilitation of play following idazoxan and RX821002 is likely due to blockade of alpha-2A adrenoceptors. These findings add further support for a specific role of alpha-adrenoceptors in the modulation of playfulness in the juvenile rat.

Stereoselective effects of central α2-adrenergic agonist medetomidine on in vivo catechol activity in the rat rostral ventrolateral medulla (RVLM)

The stereoselective central effects of a novel, highly potent and selective alpha 2-agonist medetomidine on adrenergic neuronal activity, reflected by changes in catechol oxidation current, in the rostral ventrolateral medulla of the halothane-anesthetized rat were examined using in vivo differential normal pulse voltammetry. Dexmedetomidine, the active isomer, significantly decreased catechol oxidation current to 33.4 +/- 4.5% of baseline when given centrally (1 microgram, i.c.v.) and to 10.3 +/- 3.9% of baseline when given systemically (50 micrograms/kg, i.v.). Dexmedetomidine also significantly reduced mean arterial blood pressure by 19.9% following central administration but significantly increased mean arterial blood pressure by 59.9% following systemic administration. Levomedetomidine, the inactive isomer, had no effect on catechol oxidation current or blood pressure. The depressant effects of dexmedetomidine on catechol oxidation current were reversed by the selective alpha 2-adrenoceptor antagonist atipamezole (2 micrograms, i.c.v. or 200 micrograms/kg, i.v.). The results of the present study demonstrate, to our knowledge, for the first time the central stereoselective effects of medetomidine and antagonism by atipamezole on rostral ventrolateral medulla activity in the anesthetized rat.

?2 Autoreceptor-mediated modulation of tyrosine hydroxylase activity in noradrenergic regions of the rat brain in vivo

The physiological importance of brain alpha 2-adrenoceptors in controlling the activity of tyrosine hydroxylase in noradrenergic regions was investigated using the accumulation of 3,4-dihydroxyphenylalanine (DOPA) after decarboxylase inhibition as a measure of the rate of tyrosine hydroxylation (and synthesis of noradrenaline) in vivo. In the hypothalamus and cerebral cortex, clonidine (0.025-1 mg/kg, i.p.) decreased (18%-43%) and idazoxan (0.1-80 mg/kg, i.p.) increased (20%-73%) the synthesis of DOPA in a dose-dependent manner. Moreover, pretreatment with idazoxan (0.1 mg/kg) antagonized the effect of clonidine (0.1 mg/kg) in the hypothalamus. After treatment with reserpine (5 mg/kg, s.c., 18 h before decapitation) and depletion of noradrenaline, clonidine (0.5 mg/kg) continued to decrease (50%-55%) but idazoxan (20 mg/kg) failed to increase the synthesis of DOPA, which suggested the involvement of an alpha-auto-receptor mechanism. Acute treatments of rats (not exposed to reserpine) with a wide variety of adrenoceptor agonists such as guanfacine 6, B-HT920, xylazine, bromoxidine (1 mg/kg) and antagonists such as yohimbine, phentolamine, prazosin (10 or 20 mg/kg) resulted in significant decreases (15%-55%) or increases (21%-99%) in the synthesis of DOPA in both brain regions. However, other agonists (oxymetazoline, azepexole, tramazoline, methoxamine) and antagonists (tolazoline, dihydroergotamine, phenoxybenzamine, propranolol) did not modify the synthesis of DOPA. In the hypothalamus and cerebral cortex the effects of the drugs were consistent with the selectivity of alpha-adrenoceptor agonists and antagonists (except prazosin) for an alpha 2-adrenoceptor. The results also suggest that the alpha 2-autoreceptor that modulates the synthesis of noradrenaline in the rat brain appears to belong to the prazosin-sensitive alpha 2B-subtype.

Dose-response relationship for oral idazoxan effects in healthy human subjects: comparison with oral yohimbine

The effects of oral administration of the alpha 2 adrenergic receptor antagonists idazoxan (20 mg, 40 mg, 80 mg) and yohimbine (20 mg) were compared using a placebo-controlled within-subjects design. Healthy subjects completed 5 test days during which medication effects on mood and anxiety states, physiologic indices, plasma cortisol levels, and plasma levels of the norepinephrine metabolite 3-methoxy-4-hydroxy-phenylethylene glycol (MHPG) were assessed. Idazoxan dose-dependently increased plasma MHPG, plasma cortisol, systolic and diastolic blood pressure, and Panic Attack Symptom Scale scores in healthy subjects. Overall, yohimbine and idazoxan produced a similar pattern of behavioral and neuroendocrine responses. Since idazoxan possesses relatively greater receptor specificity compared to yohimbine, it may be a more useful alpha 2 antagonist in humans.

Effects of atipamezole, a novel alpha 2-adrenoceptor antagonist, in open-field, plus-maze, two compartment exploratory, and forced swimming tests in the rat

The behavioral effects of atipamezole (0.5-4.5 mg/kg), a new and highly selective alpha 2-adrenoceptor antagonist, were studied in four behavioral models: open-field, elevated plus-maze, two compartment exploratory test and forced swimming test. Atipamezole (1.5 and 4.5 mg/kg) produced a dose-dependent suppression of locomotor activity in the open field test. In the two compartment exploratory test, the same doses of atipamezole decreased locomotor activity in a 5 min test but not in a 10 min test. The numbers of transitions between the compartments were not significantly affected by atipamezole. Doses of 0.5-4.5 mg/kg did not significantly change the time spent in the open arms or the total number of arm entries in the plus-maze, and doses of 1.5 and 4.5 mg/kg decreased defecation marginally. Vocalization during the forced swimming test was increased by atipamezole (1.5 mg/kg) but the duration of immobility was not increased over the dose range (0.5-1.5 mg/kg) of atipamezole used. Our results suggest that, in the rat, atipamezole decreases motor activity in the early phase of the exploration of new surroundings. In the doses used, atipamezole may suppress defecation and increase vocalization in rats.

Interactions between adrenergic systems, anaesthetic and TRH analogue induced analeptic effects on VBT transmission

Previous behavioural and electrophysiological studies have indicated an antinarcotic action of thyrotropin-releasing hormone (TRH) and its analogues in antagonizing the action of CNS depressant drugs, including baclofen and a variety of anesthetics. While beta-adrenergic receptors are implicated in the level of anaesthesia/arousal, whether the analeptic action of TRH involves adrenergic systems for its expression is uncertain. The object of the present experiments, therefore, was to examine interactions between adrenergic systems and the anti-anaesthetic effects of TRH analogue CG3703. It was found that pretreatment with the beta-block (+/-)propranolol did not abolish or reduce the ability of CG3703 to antagonize urethane-induced depression of VBT transmission. These results suggest therefore, that beta-adrenergic systems are unlikely to be involved in the anti-anaesthetic effect of the tripeptide.

Yohimbine and naloxone: effects on male rat sexual behavior

We evaluated the effects of yohimbine (2 mg/kg) and naloxone (5 mg/kg), separately and in combination, on copulatory behavior in male rats. In Experiment 1, yohimbine evinced decrements in intromission frequency, ejaculation latency, and copulatory efficiency, whereas naloxone administration was followed by an increased ejaculation latency, and the combination of yohimbine plus naloxone was without effect. In Experiment 2, yohimbine evinced decreases in intromission frequency, ejaculation latency, copulatory efficiency in the first, but not subsequent, copulatory series, as well as a decreased latency to sexual exhaustion. Further, treatment with yohimbine alone, naloxone alone, or yohimbine plus naloxone was followed by a reduction in the number of ejaculation prior to sexual exhaustion. Thus, at the doses tested, no synergistic effects were observed for the combination of yohimbine plus naloxone.

Brain injury: new insights into neurotransmitter and receptor mechanisms

The studies reviewed here represent a continuing search for mechanisms which play a role in neurological disturbances resulting from brain injury. Focal cortical freezing lesions in rats were shown to cause a widespread decrease in local cerebral glucose utilization (LCGU) in cortical areas of the lesioned hemisphere and this was interpreted as reflecting a depression of cortical activity. Such an interpretation was supported by the finding that in lesioned brain reduction of cerebral metabolism by pentobarbital and isoflurane was limited by the metabolic depression that has already occurred as a result of injury and by the demonstration that the energy status and substrate (glucose) supply in the cortical areas in the injured brain have not been compromised at the time when LCGU was decreased. Both the serotonergic and the noradrenergic neurotransmitter systems were implicated in functional alterations associated with injury. Cortical serotonin (5-HT) metabolism was increased throughout the lesioned hemisphere and complete inhibition of 5-HT synthesis with p-chlorophenylalanine ameliorated the decrease in cortical LCGU, interpreted as reflecting cortical functional depression. Cortical norepinephrine metabolism was bilaterally increased in focally injured brain, while prazosin, a selective alpha 1-noradrenergic receptor blocker, normalized cortical LCGU in the lesioned hemisphere. Low-affinity in vivo binding of [125I]HEAT, another selective alpha 1-receptor ligand, was specifically increased in cortical areas of the lesioned hemisphere at the time of the greatest depression in LCGU, suggesting that alpha 1-adrenoreceptors may be of functional importance in injured brain.(ABSTRACT TRUNCATED AT 250 WORDS)

Presynaptic alpha 2-adrenoceptor function and sleep in man: studies with clonidine and idazoxan

The effects of an alpha 2-adrenoceptor agonist, clonidine and an antagonist, idazoxan, were studied on nocturnal sleep in man. Clonidine increased non-rapid eye movement sleep and idazoxan reduced slow wave sleep and increased awake activity. Changes in the continuity of sleep with clonidine were similar to, and those with idazoxan opposite to, the effects of maprotiline, an inhibitor of the uptake of noradrenaline, used as an active control. These findings support the previous conclusion that raised levels of noradrenaline in the synapse, after inhibition of uptake, lead to increased presynaptic inhibition of release of transmitter in man. However, all three drugs decreased rapid eye movement (REM) sleep and the ratio of REM to nonREM sleep and this is believed to be due to a non-specific upset of the balance of influences which control the appearance of REM sleep.

The effect of alpha-adrenergic receptor blockers prazosin and yohimbine on cerebral metabolism and biogenic amine content of traumatized brain

Widespread decrease in local cerebral glucose utilization (LCGU) previously shown to occur 3 days after a local freezing lesion was interpreted as reflecting a depression of functional activity in the affected areas. In parallel experiments, cortical norepinephrine (NE) content of traumatized brain was found to be decreased. The effects of prazosin (PZ), an alpha 1-adrenergic receptor blocker, and yohimbine (YOH), an alpha 2-blocker, on glucose use and biogenic amine content of lesioned rat brain were studied to determine if the changes in the noradrenergic system associated with injury are of functional importance, to identify the receptors that may be involved in mediating the action of NE in injured brain, and to look for evidence of interaction between the noradrenergic and the serotonergic systems in traumatized brain. PZ (1 mg/kg) given 30 min before the lesion ameliorated the subsequent metabolic cortical depression seen in untreated animals. PZ given for 3 days starting before the lesion (3 mg/kg/day) was also effective in normalizing LCGU in areas where it was depressed by lesioning, despite the fact that this regimen induced significant global decrease in LCGU in normal animals. Once cortical metabolic depression had developed 3 days after the lesion, it could not be modified by PZ. YOH was less effective than PZ and was so only when given for 3 days (22.5 mg/kg/day in three divided doses). PZ (3 mg/kg/day in three divided doses) slightly but significantly decreased the accumulation of the serotonin (5-HT) metabolite 5-hydroxyindoleacetic acid in the traumatized hemisphere. These results provide evidence that blockage of alpha 1-adrenergic receptors prevents the development of cortical dysfunction associated with brain trauma. This implies that the noradrenergic system plays a role in the functional consequences of injury and that this effect is, at least in part, mediated by alpha 1-adrenergic receptors. Furthermore, alpha 1-adrenergic receptor blockage appears to modulate cortical turnover of 5-HT, previously also implicated in functional consequences of brain injury. The data are compatible with inhibitory effects of NE in the cortex and suggest a potential of alpha 1-adrenergic blockage in development of novel therapeutic approaches to brain injury.

Acute and chronic idazoxan in normal volunteers: biochemical, physiological and psychological effects

The acute and chronic effects of the selective a(2)-antagonist idazoxan were studied in 12 normal volunteers. Plasma 3-methoxy-4-hydroxyphenylethylene glycol (MHPG), blood pressure and psychological responses to oral challenge doses of idazoxan 40 mg were measured twice, on the first and 22nd day of treatment with idazoxan 40 mg t.d.s. Changes in nocturnal melatonin output were studied on six occasions, before, during and after idazoxan treatment. Although baseline MHPG levels were significantly reduced after chronic treatment with idazoxan, idazoxan challenge did not alter MHPG concentrations on either test day. A small rise in systolic blood pressure occurred after acute but not chronic idazoxan challenge tests. Systolic blood pressure values were significantly lower during the chronic compared with the acute test. Diastolic blood pressure and heart rate were not affected by acute or chronic treatment. Subjects reported increases in self- ratings of arousal and reductions in sedation and anxiety of similar magnitude after acute and chronic idazoxan. Nocturnal plasma melatonin secretion was not altered by drug administration or withdrawal, although urinary 6-sulphatoxymelatonin excretion was significantly reduced on acute withdrawal. The increase in systolic blood pressure and arousal self-ratings after acute idazoxan are in accordance with the reported effects of other a(2)-antagonists, although we did not find increased anxiety or elevated plasma MHPG levels. Chronic idazoxan appears to reduce or normalize activity of noradrenergic systems, indicated by reduced baseline systolic blood pressure and MHPG, and loss of the pressor response to idazoxan. Withdrawal of idazoxan leads to an abrupt fall in noradrenergic activity, as demonstrated by the fall in urinary 6-sulphatoxymelatonin.

Effect of serotonin depletion by p-chlorophenylalanine upon discriminative behaviours

1. Para-chlorophenylalanine (p-CPA), a competitive inhibitor of the serotonin (5-HT) synthesis enzyme tryptophan hydroxylase, was administered to rats at a dosage (100 mg/kg daily for 3 days) that depletes 5-HT. 2. Different groups of these rats were previously trained to discriminate the interoceptive stimuli produced by amphetamine, cathinone, 3,4-methylenedioxymethamphetamine (MDMA), N-ethyl-3,4-methylenedioxyamphetamine (MDE), fenfluramine or yohimbine, and the effect of p-CPA pretreatment upon their discriminative performance was compared with the effect of saline (control) pretreatment. 3. p-CPA was shown to have no effect upon the dopaminergically-mediated stimuli produced by the stimulants amphetamine and cathinone or upon yohimbine performance. 4. p-CPA significantly decreased discriminative performance with the serotonergic releasing drugs MDMA, MDE and fenfluramine. This decrease in discriminative performance returned to pre-p-CPA (criterion) levels at a time (9-12 days) when 5-HT has been reported to replete to normal brain concentrations. 5. It is concluded that p-CPA pretreatment lowers brain 5-HT and, in turn, significantly decreases the ability of rats to discriminate centrally active drugs whose interoceptive cueing stimuli are mediated by 5-HT neurons.

Effects of idazoxan on 5-hydroxytryptamine-mediated behaviour in the mouse and rat

The α(2)-adrenoceptor antagonists idazoxan and RX811059 induced reciprocal forepaw treading, a component of the 5-HT-behavioural syndrome in rats. This response is independent of 'non-α(2)-adrenoceptor idazoxan binding sites' (NAIBS) at which RX811059 is inactive. Idazoxan pre-treatment, in rats, enhanced forepaw treading, head weaving and tremor induced by the 5-HT agonist 5-methoxy-N,N dimethyltryptamine (5-MeODMT), increased head twitches (but decreased hindlimb abduction) induced by the 5-HT releaser p- chloroamphetamine (pCA), but did not clearly alter head twitches induced by the 5-HT precursor L-5-hydroxytryptophan in mice. The α(1)-antagonist prazosin did not alter behaviour induced by either 5-MeODMT or pCA in rats. The α( 2)-agonist, guanoxabenz, did not alter 5-MeODMT-induced behaviour in rats. St587, an α(1)-agonist, selectively potentiated tremor induced by 5-MeODMT, but no other behaviour. A possible mechanism for these interactions could be through enhanced, α(2)-adrenoceptor-mediated, 5-HT release in specific brain areas. Other possibilities, e.g. direct action at subtypes of 5-HT receptors and the importance of these NA-5-HT interactions in the treatment of resistant depression, are discussed.

Protection against ischemia-induced neuronal damage by the alpha 2-adrenoceptor antagonist idazoxan: influence of time of administration and possible mechanisms of action

The protective effect of the alpha 2-receptor antagonist idazoxan against neuronal damage in the neocortex and in the hippocampal CA1 region was studied in rats exposed to 10 min of incomplete forebrain ischemia. When administered i.v. immediately after ischemia (0.1 mg/kg) and subsequently for 6 h (10 micrograms/kg/min), idazoxan significantly reduced neuronal damage in the hippocampus (from 84 to 26%) and in the vulnerable parts of the neocortex (from 15 to 1%). The bolus dose alone provided no significant protection. When idazoxan administration was delayed for 30 min, no significant protection was noticed in the neocortex, and the effect in the hippocampus was ambiguous. A transient elevation of plasma corticosterone levels was induced during ischemia. Idazoxan administration for 2 h did not affect postischemic changes in corticosterone levels compared with saline infusion. Idazoxan (10(-7)-10(-4) M) did not influence the in vitro binding to glutamate receptors in brain slices. Thus, the protective effect of idazoxan cannot be explained by suppression of the plasma corticosteroid levels or via an antagonistic effect on glutamate receptors. Idazoxan apparently protects neurons when given during the first hours of postischemic reperfusion, while histopathological necrosis of neurons becomes visible 48-72 h after ischemia. Detrimental processes causing delayed neuronal death occur in the early postischemic phase and can be influenced by adrenoceptor ligands. Idazoxan may protect by several mechanisms but probably exerts its protective postischemic effect mainly through an increased noradrenergic neuronal activity and an elevation of extracellular noradrenaline (NA) levels in the brain. The favorable effects of NA may either be due to inhibition of excitotoxic neurotransmission or activation of survival-promoting and trophic processes.

Specific α2-adrenoreceptor antagonists induce behavioural activation in the rat

The behavioural effects of the specific and selective α(2)-adrenoreceptor antagonists, idazoxan, efaroxan and RX811059, have been investigated in the rat. All three drugs induced periods of behavioural activation characterized by increased locomotion and exploration (rearing and hole dipping). However, these effects were only apparent in animals which were fully habituated to their environments and thus displayed low baseline activity. The behaviour observed lay within the normal range of activity and was not apparent under conditions when exploration was stimulated such as in a novel environment. α( 2)-Adrenoreceptor antagonist- induced activation was a weak response when compared with the intense and prolonged hyperactivity, in both novel and non-novel environments, induced by the amine releaser D- amphetamine. Possible mechanisms involving a direct action of noradrenaline at postsynaptic α( 1)-adrenoreceptors (subsequent to enhanced presynaptic α(2)-receptor feedback blockade) or an indirect action of α(2)-antagonists on dopamine function in mesolimbic pathways are discussed.

Idazoxan increases rough-and-tumble play, activity and exploration in juvenile rats

The effects of idazoxan, an alpha-2 noradrenergic antagonist, on play and open field behavior were assessed in juvenile rats. Play was assessed in two separate paradigms. Initially, juvenile rats were housed individually and given a daily 5 min opportunity to play with a responsive partner. Idazoxan (1-8 mg/kg) increased pinning, an indicator variable of play, but did not affect the frequency of dorsal contacts, an index of play solicitation. When rats were tested in a separate test for play solicitation using an unresponsive play partner, idazoxan increased all three measures of play solicitation. Idazoxan increased activity and exploration when rats were tested in an open field, suggesting that the effects of idazoxan on play may be due to an increase in behavioral arousal and/or attention. These data are consistent with a modulatory role for norepinephrine in the control of behavior.

The paradox of alpha 2 adrenergic regulation of prolactin (PRL) secretion. II. PRL-releasing action of the alpha 2 receptor antagonists

It has been suggested that the stimulation of the secretion of PRL by the alpha 2 adrenergic receptor antagonists (yohimbine, piperoxane) results from blockade of an inhibitory influence imposed on PRL release by the central alpha 2 receptors (7, 15). Our present results do not support these conclusions for the following reasons: 1) The effectiveness of the alpha 2 receptor antagonists yohimbine (YOH), rauwolscine (RAU), Wy 26392 and idazoxan (IDAZ) respectively to activate secretion of PRL was not related to their alpha 2 antagonist potencies. RAU was more effective in activation of PRL secretion than either YOH or Wy 26392 although it had a similar alpha 2 antagonist activity, while IDAZ, the most potent alpha 2 blocker among the four compounds, did not stimulate PRL secretion. 2) The PRL-releasing effect of YOH or Wy 26392 was reversed by the alpha 2 agonist clonidine but the same effect of RAU was not, speaking against a common alpha 2-mediated mechanism of action of the three antagonists. 3) The PRL-stimulating effect of YOH, RAU or Wy 26392 persisted following inhibition of NE synthesis and presumably release with FLA 63, DDC or combination of reserpine and DDC. 4) Conversely, we found no indication for an inhibiting influence of activation of the alpha 2 receptors on the secretion of PRL. We conclude that the stimulation of PRL secretion by the alpha 2 receptor antagonists is not derived from blockade of the central alpha 2 receptors but from other, not yet defined properties of the drugs.

Biochemical and functional studies on EMD 49,980: a potent, selectively presynaptic D-2 dopamine agonist with actions on serotonin systems

EMD 49,980 proved to be a potent and selectively presynaptic D-2 dopamine receptor agonist in biochemical studies with rats. Thus, the gamma-butyrolactone-induced accumulation of dihydroxyphenylalanine, used as a presynaptic model, was antagonized with ED50 values of 0.29 and 0.09 mumol/kg in striatum and t. olfactorium, respectively, with high maximal effects. In contrast, striatal acetylcholine concentrations, reflecting actions at normosensitive postsynaptic D-2 receptors, were only moderately increased by about 30% over a dose range of 2.3-68 mumol/kg. In rats with unilateral nigrostriatal lesions, EMD 49,980 induced long-lasting contralateral turning, indicative of agonistic actions at denervated postsynaptic D-2 receptors. In addition, EMD 49,980 potently inhibited serotonin (5-HT) uptake in vitro and in vivo. Binding studies confirmed D-2 activity in the nM range but, similarly potent effects were observed at 5-HT1A binding sites. Measurement of 5-hydroxytryptophan (5-HTP) accumulation in the n. raphe revealed that, in vivo, the net effect of EMD 49,980 on 5-HT systems is an agonistic one. Control experiments indicate that inhibition of 5-HTP accumulation by EMD 49,980 is induced mainly via direct activation of 5-HT1A receptors, although some contribution due to 5-HT uptake inhibition is likely. Furthermore, results with various reference compounds make it unlikely that there are indirect effects, also via alpha 2-receptors in the models used and support the view that D-2 agonistic, 5-HT uptake inhibiting and 5-HT1A agonistic actions are independent properties of EMD 49,980.

[3H]L-657,743 (MK-912): a new, high affinity, selective radioligand for brain alpha 2-adrenoceptors

L-657,743 (MK-912), a highly potent and selective alpha 2-adrenoceptor antagonist was tritiated to a high specific activity and its binding characteristics to brain tissue were determined. The specific binding of [3H]L-657,743 to rat cerebrocortex was saturable, reversible, and dependent on tissue concentration. In saturation studies, [3H]L-657,743 binding was resolved into two high affinity components exhibiting Kd values of 86 pM and 830 pM with densities of 82 fmol/mg protein and 660 fmol/mg protein, respectively. Based on the binding potencies of a variety of compounds with differing receptor selectivities, the sites labeled by [3H]L-657,743 were characteristic of alpha 2-adrenoceptors. In contrast to alpha 2-antagonists, alpha 2-agonists displayed shallow competition curves. In the presence of 100 microM GTP, Gpp(NH)p or 150 mM NaCl, the competition curve for epinephrine was shifted to the right, whereas that for yohimbine was unaffected. In studies utilizing human cerebrocortical tissue, [3H]L-657,743 also bound with high affinity to sites characteristic of alpha 2-adrenoceptors.

Prolactin (PRL) release-inhibiting properties of the alpha 2 adrenergic receptor antagonist idazoxan: comparison with yohimbine

The effects of the alpha 2 adrenergic receptor antagonists yohimbine (YOH) and Idazoxan (ID) on secretion of PRL were compared in nonanesthetized male rats bearing permanent intraatrial cannulae for i.v. drug delivery and serial blood sampling. YOH induced a dose-related elevation of basal plasma PRL levels. ID had either no effect or a tendency to lower them and effectively inhibited stimulation of PRL secretion with morphine, 5-hydroxytryptophan (5HTP), quipazine or restraint stress. YOH at low doses did not alter the PRL secretory responses to these stimuli or enhanced them at the highest dose used (1.56 mg/kg). ID inhibited the PRL-stimulating, effect of 5HTP or morphine following inhibition of NE synthesis with FLA63 or pretreatment with clonidine. It also blocked the effect of quipazine in rats pretreated with prazosin. It is concluded that ID, in a complete contrast to YOH effectively inhibits PRL secretion. The inhibitory mechanism appears to be unrelated to its interaction with the alpha adrenergic receptors.

Yohimbine's anxiogenic action: evidence for noradrenergic and dopaminergic sites

Yohimbine (2.5 or 4 mg/kg) reduced the percentage of open arm entries and the percentage of time spent on the open arms displayed by rats on an elevated plus-maze indicating anxiogenic activity. These effects were reversed by the alpha 2-adrenoceptor agonist clonidine (0.01 mg/kg) and by the dopamine receptor agonist apomorphine (0.57 mg/kg). The following failed to reverse the effects of yohimbine: the selective alpha 2-adrenoceptor agonists, guanfacine (0.25 and 1 mg/kg), B-HT920 (0.025 and 0.1 mg/kg), B-HT933 (1 and 10 mg/kg); the beta-blocker propranolol (2.5 and 10 mg/kg); the alpha 1-adrenoceptor agonist phenylephrine; the D1 agonist SK&F 38393 (5 and 10 mg/kg) and the D2 agonist LY 171555 (0.5 and 1 mg/kg). Therefore, it is unlikely that activity at only the alpha 1, alpha 2, beta, D1 or D2 sites can entirely account for the anxiogenic actions of yohimbine in the elevated plus-maze. Evidence that clonidine affects the dopaminergic system and that apomorphine affects the noradrenergic system suggest that yohimbine may produce its anxiogenic response by activity on both the noradrenergic and dopaminergic systems.

Evidence for antiserotonergic properties of yohimbine

Yohimbine (YOH) is a widely used pharmacological tool employed to produce a selective blockade of alpha 2-adrenergic receptors. In the present study operant behavior was used as a biobehavioral assay to determine the activity of YOH at serotonergic receptors, as indicated by its ability to antagonize the behavioral effects of a serotonergic agonist, lysergic acid diethylamide (LSD). Rats were trained to respond on a Fixed Ratio 15 schedule for food reinforcement. YOH (0.5-5.0 mg/kg) or vehicle and LSD (50 micrograms/kg) were administered (IP) 30 min and immediately prior, respectively, to the 30-min operant session. In a separate study, the ability of YOH (0.5-2.5 mg/kg) to antagonize a higher dose of LSD (100 micrograms/kg) was examined. Relatively low doses of YOH (0.5-1.0 mg/kg) were able to partially, but significantly antagonize the LSD-induced suppression and typical hallucinogen-induced disruption of schedule-controlled responding. These results suggest that YOH, even at moderate doses, may act nonselectively as an antagonist at 5-HT receptors, in addition to its antagonist action at alpha 2-adrenergic receptors. This study demonstrates the utility of operant behavior as a biobehavioral assay to study the receptor mediated action of drugs.

Behavioural and neurochemical effects of antipamezole, a novel alpha 2-adrenoceptor antagonist

The effects of antipamezole (MPV-1248), a novel selective and specific alpha 2-adrenoceptor antagonist, were studied on monoamine metabolism in rat brain and CSF. In addition, the ability of the drug to antagonize the behavioural and neurochemical effects of two alpha 2-adrenoceptor agonists, detomidine and medetomidine, was assessed. Atipamezole, 0.03-3.0 mg/kg, had no gross behavioral effects on the rats. Above 3 mg/kg, the rats showed increased vocalization and some hostility, rapid breathing and piloerection. The drug caused dose-dependent, rapid and relatively long-lasting increase in the central turnover of noradrenaline (NA) as reflected by increases in the levels of the major metabolites of NA in brain and CSF and an increase in the depleting effect of alpha-methyl-para-tyrosine on brain NA levels. An increase in the turnover of serotonin (5-HT) in brain was indicated by a decrease in the concentration of 5-HT and a corresponding increase in the level of its metabolite, 5-hydroxyindoleacetic acid. Atipamezole was able to antagonize the sedative, hypothermic and neurochemical effects of two potent alpha 2-agonists, detomidine and medetomidine. These results give support for the characterization of atipamezole as a potent antagonist at central alpha 2-adrenoceptors with a rapid onset of action.

Alpha 1- and alpha 2-adrenoreceptor antagonists differentially influence locomotor and stereotyped behaviour induced by d-amphetamine and apomorphine in the rat

The importance of dopamine (DA) in mediating locomotor, exploratory and stereotyped behaviour in rodents is well established. Evidence also indicates a modulatory role for noradrenaline (NA) although, due to nonspecificity. of previously available agents, a precise role remains undefined. The effects of the specific and selective alpha-adrenoreceptor antagonists idazoxan (alpha 2) and prazosin (alpha 1) on behaviour induced by amphetamine and apomorphine have been investigated in the rat. d-Amphetamine (2 mg/kg) induced hyperactive locomotion and exploration. Pretreatment with prazosin (1 mg/kg) markedly reduced these responses. In contrast, pretreatment with idazoxan (20 mg/kg) only marginally altered d-amphetamine hyperactivity. Apomorphine (0.5 mg/kg) induced biphasic locomotor and exploratory activity. Neither alpha-antagonist affected the initial burst of activity (60 min), although prazosin inhibited whereas idazoxan potentiated the secondary phase (90-180 min). At higher dosage, amphetamine (6 mg/kg) and apomorphine (2 mg/kg) induced stereotyped behaviours. Prazosin pretreatment enhanced stereotyped gnawing and decreased sniffing and locomotion, whereas idazoxan increased locomotion and decreased amphetamine-induced mouth movements. These data indicate that DA-induced locomotor and stereotyped behaviours are differentially influenced (in opposite directions) by both alpha1- and alpha 2-adrenoreceptor antagonists. NA may thus modulate the expression and character of behaviour by influencing DA function in certain brain areas.

Selective agonists and antagonists for 5-hydroxytryptamine receptor subtypes, and interactions with yohimbine and FG 7142 using the elevated plus-maze test in the rat

The effects of some 5-HT receptor ligands were investigated on measures of anxiety in an elevated plus-maze test in the rat. Quipazine (2 and 4 mg kg-1), a non-specific 5-HT agonist and ritanserin (0.25-10 mg kg-1), a 5-HT2 receptor antagonist displayed anxiogenic profiles by reducing both of the measures of anxiety used in this test. Two 5-HT1A receptor ligands, buspirone (4 and 8 mg kg-1) and ipsapirone (2.5-10 mg kg-1) and the 5-HT1 agonist, RU 24969 (0.1875-1.5 mg kg-1) significantly reduced only the percentage of time spent on the open arms. (-)-Propranolol (5 and 10 mg kg-1), a 5-HT1 receptor antagonist significantly reduced only the percentage of entries made onto the open arms. Metergoline (4 mg kg-1), a non-specific 5-HT antagonist displayed anxiolytic effects in this test by increasing both measures of anxiety. The 5-HT1A receptor agonist, 8-OH-DPAT (0.0625-0.25 mg kg-1) had no effect on either of the measures of anxiety. The results from the non-specific ligands (quipazine and metergoline) are consistent with the theory that a reduction in 5-HT function reduces anxiety. However, in spite of their more selective effects on 5-HT receptors the results in this test from the more specific ligands are not consistent with a strong involvement of any single receptor subtype. The interaction studies with yohimbine and FG 7142 (beta-carboline-3-carboxylate methylamide) provided no clear evidence for a major role of 5-HT pathways in the mediation of their anxiogenic effects.

Yohimbine-induced alterations of monoamine metabolism in the spontaneously hypertensive rat of the Okamoto strain (SHR). II. The central nervous system (CNS)

Steady state levels of monoamine neurotransmitters were examined in SHR, a genetic model of hypertension and compared to its normotensive control (WKY). SHR and WKY were also challenged with alpha 2-adrenergic antagonists, (yohimbine, YOH, idazoxan) or an alpha 1-antagonist (prazosin) and alterations in CNS monoamine metabolism evaluated. SHR were found to have elevated levels of NE and 5-HT in a number of brain regions involved in cardiovascular control when compared to WKY. DA levels and metabolism were also altered in the SHR. Blockade of alpha 2-adrenoceptors and other direct and indirect actions of YOH exacerbated the abnormalities in central monoaminergic neurotransmission in SHR. Significant decreases in NE content were produced by YOH or idazoxan treatment in both SHR and WKY, presumably the result of the inhibition of alpha 2-adrenoceptor medicated presynaptic control of NE release. YOH treatment abolished the differences in steady state levels of NE between SHR and WKY, however, idazoxan did not. YOH administration resulted in significant increases in DA and 5-HT in a number of brain regions of both SHR and WKY. Idazoxan or prazosin produced few changes in DA and 5-HT metabolism except for increases in DA content in the spinal cord and brainstem of SHR given idazoxan. The YOH-induced increases in DA and 5-HT content of SHR were of a greater magnitude than the WKY in several brain regions. DOPAC levels were significantly elevated by YOH in both WKY and SHR, reflecting the antidopaminergic properties of YOH. 5-HIAA content was significantly reduced by YOH in a number of brain regions in both SHR and WKY, however, this effect was attenuated in several brain regions in SHR. The results of the present study demonstrate the multifarious nature of the alterations in CNS monoamine metabolism in SHR.

Pharmacological profile of a new potent and specific alpha 2-adrenoceptor antagonist, L-657,743

L-657,743,(2S,12bS)1',3'-dimethylspiro(1,3,4,5',6,6',7,12 b-octahydro-2H- benzo[b]furo[2,3-a]quinolizine)-2,4'-pyrimidin-2'-one, was tested in several in vitro and in vivo models for alpha 2-adrenoceptor antagonism. L-657,743 exhibited a high affinity (less than or equal to 1 nM) for alpha 2-adrenoceptors labelled by [3H] rauwolscine or [3H]clonidine with a 240-fold selectivity versus alpha 1-adrenoceptors labelled by [3H]prazosin. L-657,743 was a potent, selective, and competitive alpha 2-adrenoceptor antagonist in the rat isolated vas deferens (pA2 = 9.3 vs. clonidine; pA2 = 7.1 vs methoxamine). In vivo, L-657,743 potently blocked clonidine-induced mydriasis in the rat and stimulated cerebrocortical norepinephrine synthesis, two indices of central alpha 2-adrenoceptor antagonism. L-657,743 exhibited a comparatively low affinity for several monoamine receptor subtypes (D1, D2, 5-HT1, 5-HT2) in radioligand binding assays in vitro and a comparatively low potency to alter the synthesis of brain DA and 5-HT in vivo indicating a marked alpha 2-specificity versus other monoamine receptor mechanisms. Compared to yohimbine, L-657,743 had considerably higher alpha 2-antagonist potency and alpha 2/alpha 1 selectivity and was significantly more alpha 2-specific (i.e., vs. DA, 5-HT receptors).

Effects of the alpha 2-antagonist idazoxan on monoaminergic parameters measured in the cerebrospinal fluid of rabbits

The alpha 2-antagonist idazoxan was administered intravenously to rabbits. The increase in central noradrenergic, dopaminergic and serotonergic activity was followed as a function of time by determining neuronal parameters in the cerebrospinal fluid (CSF) and was compared with changes previously determined after yohimbine. These parameters include the enzyme dopamine-beta-hydroxylase (D beta H), the noradrenergic metabolites 3-methoxy-4-hydroxyphenylmandelic acid (VMA) and 3-methoxy-4-hydroxyphenylethylene glycol (MHPG), the dopaminergic metabolite 3-methoxy-4-hydroxyphenylacetic acid (HVA) and the serotonergic metabolite 5-hydroxyindole acetic acid (5-HIAA). Control experiments with physiological saline were also performed. D beta H activity increased to 211% in control experiments, and to 570 and 530%, respectively after yohimbine and idazoxan. Compared to the control experiments yohimbine was able to elevate VMA, MHPG and HVA concentrations, but 5-HIAA levels were reduced. Idazoxan caused increased MHPG concentrations, slight increases in VMA, little effect on HVA and no effect on 5-HIAA levels. We conclude that idazoxan was as potent as yohimbine as an alpha 2-antagonist in our in vivo experiments and that idazoxan shows a much greater selectivity with regard to the noradrenergic system.

Alpha 2-adrenoceptor antagonists and male sexual behavior: I. Mating behavior

Three alpha 2-adrenoceptor antagonists yohimbine, idazoxan, and imiloxan were compared by examining the effects of a single injection on male rat copulatory behavior. Dose ranges were: yohimbine: 0.25-8.0 mg/kg; idazoxan: 0.25-8.0 mg/kg; imiloxan: 12.5-50.0 mg/kg. Yohimbine and idazoxan administration produced significant increases in the number of animals copulating to ejaculation and all three drugs increased the rate of copulation as evidenced by reductions in ejaculation latency and intercopulatory interval. Only yohimbine significantly reduced mount latency and postejaculatory interval, but yohimbine and imiloxan significantly reduced intromission latency and idazoxan showed a similar trend. The highest yohimbine dose suppressed sexual activity. A time-course experiment with yohimbine (2.0 mg/kg) and idazoxan (4.0 mg/kg) showed stimulation at 75 min and a trend at 5. To further explore the arousal-stimulating capacity of the two more effective drugs, a mounting test with genital anesthetization was used. Yohimbine but not idazoxan showed marked increases in mounting at 1.0-4.0 mg/kg. Both drugs had a suppressive effect at the highest doses. These data support the involvement of alpha 2-adrenoceptors in the regulation of male sexual behavior, specifically by facilitating sexual arousal, with no effects on ejaculatory threshold, as measured by intromission frequency. Yohimbine is the most globally effective agent and it is likely that factors other than yohimbine's alpha 2-antagonism may play a role in its unique, consistent and broad behavioral effects.

Effect of barbitone sodium and thiopental sodium on brain dopamine, noradrenaline, serotonin and 5-hydroxyindoleacetic acid content in Arvicanthis niloticus

The quantitative estimation of total dopamine (DA), noradrenaline (NE), serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) content in the whole brain tissue of normal Nile grass rat, Arvicanthis niloticus, gives and average of 631 +/- 12 ng DA/g, 366 +/- 12 ng NE/g, 617 +/- 15 ng 5-HT/g and 431 +/- 10 ng 5-HIAA/g fresh brain tissue. The effect of barbitone sodium and thiopental sodium on the total DA, NE, 5-HT and 5-HIAA content in the brain tissue of the Nile grass rat, Arvicanthis niloticus, was studied. The total DA, NE, 5-HT and 5-HIAA contents were determined 5 hr after i.p. injection of different doses of barbitone sodium (20, 40 and 80 mg/ml/100 g body wt) and thiopental sodium (5, 10 and 20 mg/ml/100 g body wt). The effect of different time intervals (1, 10, 30 min, 1, 2.5, 5, 8, 16, 24 and 48 hr) on the total brain DA, NE, 5-HT and 5-HIAA content was investigated after i.p. injection of 40 mg of barbitone sodium and 10 mg of thiopental sodium/ml/100 g body wt. Both barbitone sodium and thiopental sodium caused an increase in DA, NE and 5-HT content and a decrease in 5-HIAA content in the brain tissue of Arvicanthis niloticus. The increase in the whole brain contents of DA, NE and 5-HT after the administration of barbitone sodium and thiopental sodium may be due either to inhibition of transmitter release by an action at the monoamine nerve terminal or to effects causing a decrease in nerve impulse flow. On the other hand, the decrease in 5-HIAA may be due to the decrease in the turnover of 5-HT.

Effects of yohimbine and idazoxan on monoamine metabolites in rat cerebrospinal fluid

Effects of two alpha 2-adrenoceptor antagonists, idazoxan and yohimbine, on the concentrations of monoamine metabolites in cisternal cerebrospinal fluid (CSF) of freely moving rats were investigated. Both drugs caused a dose-dependent, up to 250% increase in the concentration of 3-methoxy-4-hydroxyphenylglycol (MHPG) in CSF indicating enhanced release, metabolism and turnover of noradrenaline in the central nervous system (CNS). In addition, a similar increase in homovanillic acid (HVA) in CSF was observed, while the level of 5-hydroxyindoleacetic acid was unchanged. The present results demonstrate the usefulness of monitoring drug-induced alterations in noradrenergic activity in the CNS by measurement of free MHPG in repeatedly collected cisternal CSF samples from awake rats. The possibility that the observed increase in the concentration of HVA after the highly specific alpha 2-antagonist idazoxan reflects increased noradrenergic rather than dopaminergic neuronal activity is discussed.

Enhanced noradrenergic neuronal activity increases homovanillic acid levels in cerebrospinal fluid

Idazoxan, a highly specific and selective alpha 2-adrenoceptor antagonist, caused a dose-dependent increase in the concentration of homovanillic acid (HVA) a metabolite of 3,4-dihydroxyphenylethylamine, in cisternal CSF of freely moving rats. This increase in HVA level could be antagonized by the alpha 2-adrenoceptor agonist medetomidine. The increase was directly proportional to the concurrent elevation in level of 3-methoxy-4-hydroxyphenylglycol, a metabolite of noradrenaline, in the CSF of individual rats and followed a similar time course. It is suggested that the HVA level in CSF may be increased under conditions of enhanced noradrenergic activity and that, in such situations, it reflects noradrenergic rather than dopaminergic neuronal activity. Care should be taken, therefore, when changes in central dopaminergic activity are assessed by measurements of HVA level in CSF.

L-654,284 a new potent and selective alpha 2-adrenoceptor antagonist

L-654,284 [(2R, 12bS)-N-(1,3,4,6,7,12b-hexahydro-2H-benzo[b]-furo[2,3-a] quinolizine-2-yl)-N-methyl-2-hydroxyethanesulfonamide) was tested in several in vitro and in vivo models for alpha 2-adrenoceptor antagonist activity and compared to several reference agents. In vitro L-654,284 completed for the binding of 3H-clonidine or 3H-rauwolscine (Ki's 0.8 nM, 1.1 nM) and blocked the presynaptic effects of clonidine in the rat isolated vas deferens (pA2, 9.1). L-654,284 exhibited marked alpha 2-vs. alpha 1-adrenoceptor selectivity in vitro, inhibiting 3H-prazosin binding with a Ki of 110 nM and blocking the effects of methoxamine on the vas deferens with a pA2 of 7.5. In vivo L-654,284 at 22 nmoles/kg i.v. doubled the ED50 of clonidine to produce mydriasis in rats. Given orally, the potency of L-654,284 in this test was reduced by a factor of 5.5. L-654,284 also potently increased cerebrocortical NE turnover in the rat, another in vivo index of alpha 2-adrenoceptor blockade in the central nervous system. In the periphery, L-654,284 demonstrated alpha 2-adrenoceptor selectivity by preferentially blocking the pressor effects of UK 14304 versus those of methoxamine in the pithed rat. Overall, L-654,284 was generally a more potent alpha 2-adrenoceptor antagonist than RX 781094 with comparable alpha 2/alpha 1 selectivity and was several times more potent and alpha 2-selective than WY 26703 or yohimbine. In addition, L-654,284 had better (5-6 times) oral bioavailability than RX 781094 or WY 26703.

An automated direct-injection HPLC-method for the electrochemical/fluorimetric quantitation of monoamines and related compounds optimized for the screening of large numbers of animals

An automated, highly stable HPLC system is described allowing the quantitation of monoamines and related compounds with virtually online data processing and statistical evaluation of the mean values of various differently treated groups of animals. The system is highly suited to the screening of drugs involving large numbers of samples. To ensure long-term, uninterrupted performance, two robust chromatographic systems were developed optimized to separate neighboring peaks as widely as possible. This was achieved by using mobile phases of relatively low pH to retard acidic compounds, and optimal concentrations of the ion-pairing reagent to manipulate the retention times of amines on the RP-18 column, resulting also in clearcut separations from the solvent/tissue peak. Direct injection of deproteinized tissue samples, requiring no clean up procedures is used. One faster system allows measurement of the main biogenic amines, metabolites and TRP, whereas another system allows the simultaneous quantitation of DOPA, 5HTP, NA, MOPEG, NMN, A, MN, VMA, DA, 3MT, DOPAC, HVA, 5HT, 5HIAA and TYR. By pretreatment of animals with a decarboxylase inhibitor the latter system offers the possibility of detecting drug effects in the same animal on either dopamine and serotonin turnover or noradrenaline and serotonin turnover, depending on the brain area chosen. The system described has been running for over a year without major disturbance and with minimal technician attendance.