Baclofen blocks yohimbine-induced increases in ethanol-reinforced responding in rats.

Chronic or repeated stress increases alcohol consumption. The GABA-B agonist baclofen decreases alcohol consumption and may be most effective for individuals with comorbid anxiety/stress disorders. The present study sought to determine if baclofen blocks stress-induced increases in ethanol self-administration as modeled by repeated yohimbine injections in rats. Rats were trained to respond for 15% w/v ethanol in operant chambers using a method that applies neither water deprivation nor saccharin/sucrose fading. Following training, the rats received 6 injections of 1.25mg/kg yohimbine were given immediately prior to the operant sessions during a 2-week time period. Subsequently, some rats were pair-matched to receive either 1.25mg/kg yohimbine or saline in the presence of 0.3, 1, and 3mg/kg baclofen prior to sessions. Acquisition of ethanol self-administration was poor. Pretreatment with yohimbine consistently increased responding across repeated injections. Yohimbine's effect on ethanol intake unexpectedly diverged from the effect on responding as the rats failed to consume all reinforcers earned. Smaller doses of baclofen paired with saline injections had no effect on ethanol responding; only 3mg/kg baclofen reduced ethanol self-administration. The smallest baclofen dose of 0.3mg/kg failed to block the yohimbine-induced increase in self-administration. The large baclofen dose of 3mg/kg continued to suppress ethanol self-administration when given with yohimbine. Baclofen 1mg/kg blocked the effect of yohimbine even though it had no effect when given in the absence of yohimbine. Exposure to high ethanol concentrations may induce self-administration only in certain conditions. The dissociation between responding and intake suggests that repeated yohimbine injections may initiate other behavioral or physiological mechanisms that confound its effects as a pharmacological stressor. Furthermore, an optimal baclofen dose range may specifically protect against stress-induced alcohol self-administration, highlighting a specific contribution of GABA-B receptors and a potential therapeutic efficacy of GABA-B agonists at a non-sedating dose.

Influence of morphine on medial prefrontal cortex alpha2 adrenergic system in passive avoidance learning in rats.

The prelimbic region of the medial prefrontal cortex (mPFC) is a brain area crucial for memory, attention, and decision making. It has been shown that α2-adreneoceptors (α2-ARs) play a powerful role in regulating memory and attention functions in this region. Since many studies have demonstrated the impairment effect of morphine on memory through mPFC, we aimed to investigate the possible interaction between α2-ARs of the mPFC and morphine induced amnesia in passive avoidance learning in rats. Animals were bilaterally implanted with chronic cannulas in the mPFC, trained in the step-through type passive avoidance task, and tested 24h after training; step-through latencies were measured. Our data indicate that post-training i.p. administration of morphine (2.5, 5 and 7.5mg/kg) dose-dependently reduced the step-through latency, showing an amnesic effect. Post-training intra-mPFC administration of yohimbine (an α2-adrenergic antagonist, 0.125, 0.25 and 0.5µg/rat) and clonidine (an α2-adrenergic agonist, 0.001, 0.01 and 0.2µg/rat), dose dependently impaired memory retrieval. Furthermore, post-training intra-mPFC microinjection of ineffective doses of yohimbine or clonidine significantly reversed the inhibitory effect of morphine on memory retrieval. Furthermore, SKF96365 (a presynaptic calcium channel blocker) reduced yohimbine and showed slight inhibition of clonidine effect. These results suggest that α2-ARs of the mPFC may play an important role in morphine-induced amnesia.

The role of alpha-2 adrenoceptors in the anticonvulsant effects of adenosine on pentylenetetrazole-induced seizure threshold in mice.

Adenosine has anticonvulsant effects in various models of seizures. Alpha-2 adrenoceptors have also demonstrated different effects in different models of epilepsy. In this study, the role of alpha-2 adrenoceptors in the anticonvulsant effects of adenosine in mice was determined according to the method of intravenous pentylenetetrazole-induced seizure. In this study, N(6)-cyclohexyladenosine (CHA) (a selective A1 receptor agonist), clonidine (an alpha-2 adrenoceptors agonist), yohimbine (an alpha-2 adrenoceptors antagonist) and 8-cyclopentyl-1,3-dimethylxanthine (8-CPT) (a selective A1 receptor antagonist) were used. CHA at doses of 0.5, 1 and 2mg/kg significantly increased seizure threshold with the maximum anticonvulsant effect at 2mg/kg. Yohimbine (0.1, 1 and 10mg/kg), clonidine (0.1, 0.5, 1 and 2mg/kg) and 8-CPT (0.5, 1, 2 and 4mg/kg) had no effect on seizure by itself. Combination of yohimbine (10mg/kg) and CHA (0.25mg/kg) increased clonic seizure latency showing that yohimbine and CHA have an additive effect. Increasing the seizure threshold created by combining ineffective doses of yohimbine (10mg/kg) and CHA (0.25mg/kg) was completely inhibited by 8-CPT (4mg/kg) or clonidine (1 and 2mg/kg). Clonidine (0.5, 1 and 2mg/kg) inhibited the anticonvulsant effects of CHA (2mg/kg). Combination of 8-CPT (1mg/kg) and clonidine (0.5mg/kg) which completely inhibited the anticonvulsant effect of CHA (2mg/kg) indicates that 8-CPT and clonidine have an additive effect. In conclusion, adenosine and yohimbine exhibit an additive effect on the enhancement of the pentylenetetrazole-induced seizure threshold in mice, indicating the interaction of alpha-2 adrenoceptors and A1 adenosine receptors.

Pharmacological evidence that dopamine inhibits the cardioaccelerator sympathetic outflow via D2-like receptors in pithed rats.

It has been suggested that N,N-di-n-propyl-dopamine (dopamine analogue) decreased heart rate in rats through stimulation of dopamine receptors. Nevertheless, the role of prejunctional dopamine D1/2-like receptors or even α2-adrenoceptors to mediate cardiac sympatho-inhibition induced by dopamine remains unclear. Hence, this study identified the pharmacological profile of the cardiac sympatho-inhibition to dopamine in pithed rats. Male Wistar rats were pithed and prepared to stimulate the cardiac sympathetic outflow or to receive i.v. bolus of exogenous noradrenaline. I.v. continuous infusions of dopamine (endogenous ligand) or quinpirole (D2-like agonist) dose-dependently inhibited the tachycardic responses to sympathetic stimulation, but not those to exogenous noradrenaline. In contrast, SKF-38393 (100 µg/kg∙min, D1-like agonist) failed to modify both of these responses. The sympatho-inhibition to dopamine (1.8 µg/kg∙min) or quinpirole (100 µg/kg∙min): i) remained unaltered after saline or the antagonists SCH-23390 (D1-like, 300 µg/kg) and rauwolscine (α2-adrenoceptors, 300 µg/kg); and ii) was significantly antagonized by raclopride (D2-like, 300 µg/kg). These antagonists, at the above doses, failed to modify the sympathetically-induced tachycardic responses. The above results suggest that the inhibition of the cardiac sympathetic outflow to dopamine and quinpirole is primarily mediated by prejunctional D2-like receptors but not D1-like receptors or α2-adrenoceptors.

Differential effect of orexin-1 and CRF-1 antagonism on stress circuits: a fMRI study in the rat with the pharmacological stressor Yohimbine.

Translational approaches to study the neural substrates of stress and assess the mechanistic efficacy of novel anti-anxiety agents necessitate the use of stressors with a similar degree of saliency across species. The alpha-2 adrenoreceptor antagonist yohimbine represents an attractive experimental tool owing to its well-documented stress-inducing properties in humans and laboratory species. We investigated the neural substrates engaged by yohimbine in the rat brain by using functional magnetic resonance imaging and mapped their modulation by neurotransmitter systems involved in stress responses. Yohimbine elicited a composite pattern of brain activation, highlighting the recruitment of cortico-striato-thalamic regions and extra-hypothalamic stress neurocircuits. This effect was strongly attenuated by the α-2-adrenoceptor agonist medetomidine and by the dopamine (DA) D1 receptor antagonist SCH23390, thus revealing a primary contribution of both norepinephrine and DA on the neurofunctional cascade elicited by the drug. Pretreatment with the corticotrophin-releasing factor type-1 receptor (CRF1R) antagonist CP154,526 produced a region-dependent inhibition of yohimbine-induced activation in the amygdala, striatum, and cingulate cortex, while the orexin type-1 receptor (OX1R) antagonists GSK1059865 robustly inhibited the response in fronto-hippocampal regions as well as in several key components of the extended amygdala. CP154,526 and GSK1059865 did not prevent yohimbine-induced plasma corticosterone release, a finding that corroborates a central origin of the effects mapped. Our findings provide novel insight into the brain substrates and neurochemical mediators engaged by the stress-inducing agent yohimbine. The differential pattern of inhibition produced by CRF1R and OX1R antagonists suggests that these two neuropeptide systems can modulate the functional response to stress via distinct central neural pathways.

Effect of dehydroleucodine on intestinal transit: structural basis of the interaction with the α(2)-adrenergic receptor.

The activity of dehydroleucodine, a sesquiterpene lactone obtained from Artemisia douglasiana, was studied in mice small intestinal transit. Its mechanism was evaluated in the presence of several adrenergic and cholinergic antagonist drugs and one opioid antagonist. Docking of dehydroleucodine into the homology model of the α2-adrenergic receptor allowed us to analyze the structural basis of their interactions. The experiments showed that dehydroleucodine delayed intestinal transit. The docking of dehydroleucodine showed a unique binding site, equivalent to the binding site of carozolol in the ß-adrenergic receptor. The results suggested that dehydroleucodine produced an inhibitory effect on intestinal transit. Its action could be mediated, at least in part, through the α2-adrenergic receptor.

Yohimbine relaxes the human corpus cavernosum through a non-adrenergic mechanism involving the activation of K+ATP-dependent channels.

The mechanism by which yohimbine relaxes the human corpus cavernosum remains unclear. Using the human corpus cavernosum strips immersed in isometric baths containing Krebs-Henseleit solution, this study investigates the effect of yohimbine on the relaxation of the human corpus cavernosum through nitrergic pathways involving the activation of ATP-dependent potassium channels (K(ATP)). The maximal relaxation induced by yohimbine in the human corpus cavernosum strips pre-contracted with phenylephrine was 100+/-0% and only 30.5+/-5.0% when they were pre-contracted with 60-mM potassium (K(+)) solution. The maximal relaxation induced by yohimbine in phenylephrine pre-contracted tissues was significantly inhibited by tetrodotoxin, 1H-[1,2,4] oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) or 7-nitroindazole (43.6, 36.1 and 42.6%, respectively). Neither the combination charybdotoxin-apamin nor tetraethylammonium altered the response of the human corpora cavernosa strips to yohimbine. Nevertheless, glibenclamide decreased the maximum relaxant response to yohimbine by 29.8% (P<0.05; n=12). The results suggest that yohimbine relaxes the human corpus cavernosum by a non-adrenergic, non-cholinergic mechanism, probably activating the nitrergic-soluble guanylate cyclase (NO-sGc) pathway and K(ATP).

Anxiolytics antagonize yohimbine-induced central noradrenergic activity: a concomitant in vivo voltammetry-electrophysiology model of anxiety.

It has been shown that increased cerebral noradrenergic activity is closely related to anxiety-related behaviours. Furthermore, the noradrenergic alpha-2 receptor antagonist yohimbine (YOH) increases noradrenaline- (NA) cell firing and release in discrete brain areas involved in the modulation of anxiety such as the locus coeruleus (LC) and amygdala (AMY). In addition, YOH is highly anxiogenic in both humans and laboratory animals. Here we used an in vivo dual methodology to measure concomitantly the extracellular levels of catecholamines in AMY with voltammetry and the cell firing rate in LC with field electrophysiology in anaesthetized rats to characterize the pharmacological effects of YOH. We found that systemic administration of YOH increases both noradrenergic cell firing in the LC and catecholamines release in the AMY. Prior administration of the alpha-2 agonist clonidine prevents these effects on both the electrophysiological and the voltammetric signals, suggesting that YOH effects are mainly mediated by the alpha-2 adrenoceptors. In addition, we found that pre-treatment with the behaviourally active doses of the benzodiazepine (BDZ) anxiolytic chlordiazepoxide (CDE, Librium: BDZ very similar to diazepam) and of the non-BDZ buspirone counteract the effects of YOH on catecholamines release and cell firing. This suggests that the neurochemical and physiological effects of YOH measured in this study might be related to the anxiogenic effects of the drug. Thus, the presented electrophysiological-voltammetric-YOH study may be proposed as in vivo real time model of anxiety and this approach might be suitable for the evaluation of the neurochemical and physiological effects of new anxiolytic compounds.

The role of noradrenaline and 5-hydroxytryptamine in yohimbine-induced increases in alcohol-seeking in rats.

RATIONALE AND OBJECTIVES: We previously showed that systemic administration of the prototypical alpha-2 noradrenaline (NA) receptor antagonist yohimbine increases alcohol self-administration and reinstatement. Yohimbine also acts as an agonist of 5-hydroxytryptamine (5-HT) 5-HT1A receptors, which have been shown to be involved in alcohol seeking. Here, we determined the contributions of the alpha-2 and 5-HT1A properties of yohimbine to its effects on alcohol seeking. METHODS: The effects of lesions of the dorsal or ventral NA bundles with 6-OHDA on yohimbine-induced alcohol self-administration were first determined in male Wistar rats trained to self-administer alcohol (12% w/v, 0.19 ml per alcohol delivery), and then on reinstatement induced by yohimbine after extinction of the operant response. It was then determined whether the selective alpha-2 antagonist RS-79948 (0.1, 0.2, 0.4 mg/kg) would mimic the effects of yohimbine on self-administration and reinstatement. The effects of the alpha-2 receptor agonist clonidine, or the 5-HT1A antagonist WAY 100,635 were then determined on yohimbine-induced self-administration and reinstatement. RESULTS: Lesions of the NA systems did not affect yohimbine-induced alcohol self-administration or reinstatement, and RS-79948 did not mimic the effects of yohimbine. Clonidine did not significantly affect increased alcohol self-administration induced by yohimbine, but did attenuate its effects on reinstatement. Blockade of 5-HT1A receptors reduced both yohimbine-induced self-administration and reinstatement. CONCLUSIONS: These results suggest that alpha-2 antagonist properties of yohimbine may play a role in the reinstatement of alcohol-seeking, but not self-administration. On the other hand, yohimbine's actions on 5-HT1A receptors contribute to its effects on both alcohol self-administration and reinstatement.

Synthesis of some tropane derivatives of anticipated activity on the reuptake of norepinephrine and/or serotonin.

A variety of tropane derivatives 14a-g were prepared via the reaction of the alcohol analogs 12a and 12b with substituted fluorobenzenes 13a-f. The prepared compounds were tested for their activity and selectivity toward the norepinephrine transporter (NET) and serotonin transporter (SERT) using yohimbine-induced mortality and 5-hydroxytryptophan-induced neurotoxicity in mice, respectively. All the tested compounds were found to be NE and 5-HT reuptake inhibitors except 14d which exhibited selective 5-HT reuptake inhibition activity.

Extract of Ginkgo biloba leaves reverses yohimbine-induced spatial working memory deficit in rats.

Extract of Ginkgo biloba is used to alleviate age-related decline in cognitive function, which may be associated with the loss of catecholamines in the prefrontal cortex. The purpose of this study was to verify whether alpha-2 adrenergic activity is involved in the facilitative effects of extract of Ginkgo biloba on prefrontal cognitive function. Male Wistar rats were trained to reach criterion in the delayed alternation task (0, 25, and 50-s delay intervals). A pilot study found that 3 or 4 mg/kg of yohimbine (intraperitoneal) reduced the choice accuracy of the delayed alternation task in a dose and delay-dependent manner, without influencing motor ability or perseverative behaviour. Acute oral pre-treatment with doses of 50, 100, or 200 mg/kg (but not 25 mg/kg) of extract of Ginkgo biloba prevented the reduction in choice accuracy induced by 4 mg/kg yohimbine. These data suggest that the prefrontal cognition-enhancing effects of extract of Ginkgo biloba are related to its actions on alpha-2-adrenoceptors.

Efeitos hemogasométricos da xilazina e da romifidina em cabras tratadas por ioimbina / Haemogasometric effects of xylazine and romifidine in goats treated with yohimbine

Estudaram-se as alterações produzidas por doses equipotentes de xilazina e romifidina e os efeitos da administração subseqüente de ioimbina em oito cabras mestiças. Respeitou-se um intervalo de sete dias entre os seguintes tratamentos: A- 250µg/kg/IM de xilazina e 0,1ml/kg/IV de solução fisiológica, B- 250µg/kg/IM de xilazina e 250µg/kg/IV de ioimbina, C- 25µg/kg/IM de romifidina e 0,1ml/kg/IV de solução fisiológica, D- 25µg/kg/IM de romifidina e 250µg/kg/IV de ioimbina. Foram mensurados a freqüência respiratória, o pH, as pressões parciais de oxigênio e dióxido de carbono, a concentração de íon bicarbonato, o excesso de bases e a saturação de oxigênio no sangue arterial. Utilizou-se um delineamento experimental crossover, e as médias foram comparadas pelo teste Duncan (Pmenor ou igual a 0,05). Xilazina e romifidina reduziram a pressão arterial de oxigênio e aumentaram a pressão arterial de dióxido de carbono. A ioimbina reverteu os efeitos da xilazina e da romifidina sobre as pressões parciais de oxigênio e dióxido de carbono no sangue arterial.

Insensitivity to the alpha2-adrenergic receptor blocker yohimbine hydrochloride and occurrence of spontaneous platelet macroaggregation (SPMA) in diabetes.

We report here a study of platelet aggregation in diabetes, induced by epinephrine and its inhibition by yohimbine hydrochloride (YH), an alpha(2)-adrenergic receptor-blocking agent. Interestingly, emergence of spontaneous platelet macroaggregation (SPMA) was observed in six out of 75 cases in the absence of any agonist. The SPMA cases were strongly associated with insensitivity to YH (in contrast with non-SPMA cases) when epinephrine was used as an agonist. We suggest that the observed correlation is a result of over expression of platelet alpha(2)-adrenoceptors in such subjects. The quantitative nature of the effect is supported by the observation that addition of YH at higher concentration (more than 5 microM) led to restoration of the adrenergic receptor-blocking activity of the said agent. Eventually for non-SPMA subjects YH exhibited blocking activity even at lower concentration. The aggregation profile and the platelet morphology of the SPMA cases had distinctive features as compared to microaggregates formed in other diabetic subjects (non-SPMA cases).

Protein kinase C regulates alpha(2A/D)-adrenoceptor constitutive activity.

We investigated a possible role for protein kinases in the constitutive activity of alpha(2A/D) adrenoceptors in membranes from transfected PC12 cells, using a [35S]GTPgammaS binding assay. After treatment of intact cells with various protein kinase inhibitors, constitutive activity was assessed by the reduction in basal GTP binding caused by the inverse agonist rauwolscine (RAU). Inhibitors of protein kinase C (PKC) caused the loss of RAU-sensitive GTP binding, while inhibitors of other protein kinases were ineffective. Anti-G(alpha) antibody treatments showed that constitutive alpha(2A/D)-receptor activity is directed toward different G proteins than agonist-stimulated activity. T373A mutant receptors exhibited increased constitutive activity, including a component that was insensitive to PKC inhibition. Since T373 is located within a putative G(i/o) activator sequence, these results suggest that PKC-dependent phosphorylation of T373 increases alpha(2A/D)-adrenergic receptor constitutive activity and causes a switch in G protein preference.

Prevention by 5-HT1A receptor agonists of restraint stress- and yohimbine-induced release of cholecystokinin in the frontal cortex of the freely moving rat.

In line with the idea that cholecystokinin (CCK) is involved in anxiety-related behaviours, previous investigations showed that stressful conditions and an 'anxiogenic' drug, yohimbine, increased the cortical release of CCK like-material (CCKLM) in awake rats, and that this effect could be prevented by diazepam. Here, we investigated whether other anxiolytic drugs such as 5-HT1A receptor agonists could also affect cortical CCKLM release. Indeed, neither buspirone (1 mg/kg i.p.), alnespirone (1 mg/kg i.p.) nor lesopitron (3 mg/kg i.p.) affected, on their own, CCKLM release. However, pretreatment with the latter drugs completely abolished the stimulatory effect of restraint stress on the peptide outflow. As expected of the involvement of 5-HT1A receptors, tertatolol (10 mg/kg i.p) markedly reduced the inhibitory effect of buspirone on restraint stress-evoked CCKLM overflow. On the other hand, pretreatment with buspirone, alnespirone or lesopitron also inhibited the stimulatory effect of yohimbine (5 mg/kg i.p.) on cortical CCKLM outflow. These data support the idea that the anxiolytic action of 5-HT1A receptor agonists could be mediated, at least partly, through their inhibitory influence on cortical CCK-ergic systems.

Antagonism of kynurenic acid to anxiogens in mice.

In a dark-light chamber in mice, kynurenic acid (KYNA, 200 mg/kg, i.p.), an endogenous neuroactive metabolite of tryptophan, attenuated the most stable effect of anxiogens in this model of anxiety--a decrease in the rate of leanings-out of the dark compartment --induced by caffeine, pentylenetetrazole and yohimbine, but not by beta-phenylethylamine (PEA). KYNA by itself did not alter behavior of mice in the chamber, in contrast to what has been observed in an elevated plus-maze, another model of anxiety, where KYNA had an anxiolytic pharmacological profile.

Anxiogenic action of caffeine: an experimental study in rats.

The anxiogenic action of caffeine (10, 25 and 50 mg/kg, i.p.) was investigated in rats and compared with that of yohimbine (2 mg/kg, i.p.). The experimental methods used were the open-field, elevated plus-maze, social interaction and novelty-suppressed feeding latency tests. Caffeine produced a dose-related profile of behavioural changes, which were qualitatively similar to those induced by yohimbine and which indicate an anxiogenic activity in rodents. Thus, both the drugs reduced ambulation and rears, and increased immobility and defaecation in the open-field test. They decreased the number of entries and time spent on the open arms of the elevated-plus maze, reduced social interaction in paired rats and increased the feeding latency in an unfamiliar environment in 48-h food-deprived rats. Lorazepam, a well known benzodiazepine anxiolytic agent, attenuated the anxiogenic effects of caffeine and yohimbine. Subchronic administration of caffeine (50 mg/kg, i.p.) for 21 days, in different groups of animals, induced a significant degree of tolerance in the elevated plus-maze test, which was statistically significant after 14 and 21 days' treatment. Yohimbine, however, did not induce similar tolerance. When caffeine (50 mg/kg, i.p.) was withdrawn after 21 days' administration, to a separate group of rats, significant withdrawal anxiety was observed 48 h later as noted in the elevated plus-maze test. The investigations support clinical evidence of caffeine-induced anxiety, tolerance to anxiety on continued use, and withdrawal anxiety in chronic caffeine-containing beverage users.

A schematic representation of the psychopharmacological profile of antidepressants.

1. Using simple animal tests, "behavioural" and "biochemical" aspects of depression, anxiolysis, disinhibition, psychostimulation and sedation were investigated in mice using a variety of antidepressant drugs. 2. Dothiepin and mianserin (16 and 32 mg/kg), fluoxetine (32 and 64 mg/kg), maprotiline and imipramine (16, 32 and 64 mg/kg) and viloxazine (16 mg/kg) significantly potentiated mortality following acute administration with yohimbine. 3. Dothiepin and imipramine (32 mg/kg), fluoxetine (16 and 32 mg/kg), viloxazine (8 and 16 mg/kg), maprotiline (32 mg/kg) and mianserin (32 mg/kg) reduced immobility time in the forced swimming test. 4. In the black and white box, dothiepin (32 mg/kg) significantly increased the time spent in the bright compartment: Fluoxetine (8 and 16 mg/kg) significantly increased the number of crossings between compartments, an effect indicative of desinhibition. 5. It can be concluded that dothiepin possesses both antidepressant and anxiolytic properties in these animal models. The present procedure is useful not only for the screening of compounds that may possess antidepressant properties, but is also of value in determining other properties that may contribute to the overall clinical efficacy of the drugs.

Adrenocorticotropin, prolactin and beta-endorphin stimulatory actions of alpha-2-adrenoceptor antagonists.

We studied the effect of glucocorticoid pretreatment, mediobasal hypothalamus lesion (MBHL) and the interaction between clonidine and yohimbine in male Wistar rats to elucidate the sites and/or mechanisms of endocrine actions of alpha 2-antagonists. The pretreatment of 1 mg/kg s.c. dexamethasone for 4 days effectively prevented the stimulatory effect of alpha 2-antagonists yohimbine (5 mg/kg i.p.) and CH-38083 (1 mg/kg i.p.) on adrenocorticotropin (ACTH) secretion, while the action of these antagonists on prolactin (PRL) and beta-endorphin (beta E) remained unchanged. The central (i.c.v.) pretreatment of 5 micrograms/rat clonidine failed to antagonize the prolactin (PRL) and beta E releasing effect of yohimbine. However, it inhibited the yohimbine-induced ACTH secretion. MBHL resulted in a significant enhancement in basal plasma PRL and beta-endorphin (beta E) levels. But basal plasma ACTH levels have not been changed. Yohimbine failed to stimulate ACTH secretion in MBH-lesions rats, while PRL and beta E response to the yohimbine was maintained in these animals. This study confirms that the alpha 2-antagonists stimulate ACTH secretion by a corticosteroid-sensitive mechanism which is located centrally. In contrast, alpha 2-antagonists affect PRL and beta E secretion via a corticosteroid-insensitive mechanism located at the periphery, possible within the pituitary gland.