“Anxiolytic” and “anxiogenic” benzodiazepines and β-carbolines: effects on aggressive and social behavior in rats and squirrel monkeys

Ethopharmacological studies on the behavior of socially housed rats and squirrel monkeys were conducted to explore the role of the benzodiazepine GABA_A-coupled ionophore receptor complex in aggressive and social interactions. Benzodiazepine receptor (BZR) antagonists, ZK 93426 (1–10 mg/kg) and flumazenil (3–10 mg/kg), the partial agonist, ZK 91296 (1–10 mg/kg) and the partial inverse agonists RO 15-4513 (0.3–10 mg/kg), were administered to (1) squirrel monkeys prior to 1 h focal observations within established social groups or to (2) resident male rats before confrontations with a naive male intruder in their home cage for 5 min. Aggression was modified in a similar manner in both species, although squirrel monkeys were more sensitive to BZR challenges. Specifically, resident male rats showed dose dependent reductions in attack bites directed at intruder males that were significant at the highest dose of ZK 93426 (10 mg/kg). In squirrel monkeys, ZK 93426 (3 and 10 mg/kg) reduced aggressive grasps, threats and displays, as well as reducing the duration of being the target of aggression from untreated group members (1–10 mg/kg). The BZR partial agonist, ZK 91296 and the antagonist, flumazenil produced few effects on social behavior, low and high intensity aggression and motor activity in both species. Flumazenil (10–30 mg/kg) and ZK 91296 (10 mg/kg), but not ZK 93426, produced significant increases in foraging and feeding behaviors in squirrel monkeys. The hyperphagic effects of ZK 91296 and flumazenil, that are typical of BZR agonists compounds, were not observed in rats. Similarly, the inverse agonist-like reductions in social interactions produced by ZK 93426 (3–10 mg/kg) were observed only in squirrel monkeys. The partial inverse agonist Ro 15-4513 reduced aggression in rats, but low doses (1 mg/kg) produced tremors or seizures in 80% of the monkeys tested. Decreases in aggressive and social behaviors are often interpreted to reflect anxiogenic drug properties, whereas increased feeding has been associated with anxiolytic actions. The concurrent emergence of these apparent opposites suggests independent actions on social and alimentary functions.     

Effects of benzodiazepine receptor ligands on the performance of an operant delayed matching to position task in rats: opposite effects of FG 7142 and lorazepam

The effects of a series of benzodiazepine (BZ) receptor ligands, ranging from a full agonist through to partial inverse agonists, were examined on short term working memory in the rat. The behavioural paradigm used was a discrete trial, operant delayed matching to position task, as originally described by Dunnett (1985), with delays of 0, 5, 15 and 30 s. The benzodiazepine receptor (BZR) full agonist lorazepam (0.25, 0.375 and 0.5 mg/kg) dose and delay dependently impaired matching accuracy. Lorazepam also increased the latency to respond and decreased the number of nose pokes made into the food tray during the delays. In contrast, the BZR partial agonist ZK 95 962 (1, 3, 10 mg/kg) did not affect matching accuracy, but did increase the speed of responding. The BZR antagonist ZK 93 426 (1.25, 5, 25 mg/kg) had no effects in this paradigm. The BZR weak partial inverse agonists Ro 15-4513 (0.1, 1 and 10 mg/kg) and ZK 90 886 (1, 3 and 10 mg/kg) did not affect accuracy of performance. However, both of these drugs increased the latency to respond and decreased nose poke responses. These motoric effects were particularly strong following 10 mg/kg Ro 15-4513. This shows that the effects of drugs on the accuracy of responding and on the speed of responding can be dissociated. The BZR partial inverse agonist FG 7142 had effects on matching accuracy that were dependent upon dose. The lowest dose of FG 7142 (1 mg/kg) significantly improved accuracy, whereas the highest dose (10 mg/kg) impaired accuracy. This impairment induced by FG 7142 (10 mg/kg) was accompanied by an increase in the latency to respond and a decrease in the number of nose pokes. Taken together, these results show that the accuracy of delayed matching performance can be modulated in opposite ways by the BZR full agonist lorazepam and a low dose of the BZR partial inverse agonist, FG 7142.     

Comparison of several benzodiazepine receptor ligands in two models of anxiolytic activity in the mouse: an analysis based on fractional receptor occupancies

This study compared the effects of the -carboline anxiolytic, abecarnil, with other benzodiazepine receptor (BZR) ligands, including the full agonists diazepam and alprazolam, and the partial agonists ZK 95962 and bretazenil (Ro 16-6028), and alpidem, in the mouse four-plate test and plus-maze. The efficacy and potency of each compound was related to the fraction of BZR occupied by the drug. Abecarnil was efficacious in both tests and showed anxiolytic effects comparable with alprazolam and diazepam. In the four-plate test, abecarnil, bretazenil, and ZK 95962 had selective effects on releasing exploratory locomotor activity suppressed by footshock (punished crossings). None of these compounds significantly altered non-punished crossings. In contrast, diazepam and alprazolam increased both unpunished and punished crossings at low to medium doses (receptor occupancies of approximately 20–60%). The number of punished and unpunished crossings fell to control levels or below at higher, more sedative doses (approximately 80% receptor occupancy). Alpidem had very weak anxiolytic-like effects in this test and markedly reduced unpunished crossings at relatively low receptor occupancies (> 15%). In the plus-maze, abecarnil increased the time spent in the open arms and the percentage open arm entries to an extent equal to that observed following diazepam or alprazolam administration. Bretazenil and ZK 95962 had weak effects on the measures of anxiolytic activity in this test. Alpidem also had little anxiolytic-like activity in the plus-maze but markedly reduced the total number of arm entries. The fractional BZR occupancies required to increase the time spent in the open arms of the maze to 250% of control levels were approximately 45% for abecarnil and alprazolam, 60% for diazepam, and 100% for ZK 95962. Bretazenil did not reach this potentiation at the doses tested (up to 89% receptor occupancy). Abecarnil appeared to act as a full agonist on the measures of anxiolytic activity in both tests (i.e. required low fractional BZR occupancies) but on the measures of stimulation or sedation was more similar to the BZR partial agonists (i.e. had no significant effects even at receptor occupancies approaching 100%). On this basis abecarnil could be described as a selective agonist. In general, the four-plate test was more sensitive than the plus-maze. For example, lower BZR occupancies were needed to produce significant anxiolytic effects in the four-plate test than in the plus-maze. In addition, the partial agonists bretazenil and ZK 95962, which both produced weak effects in the plus-maze, had similar anxiolytic potencies to the full BZR agonists, diazepam and alprazolam, in the four-plate test.     

Spontaneous exploration of a 6-arm radial tunnel maze by basal forebrain lesioned rats: effects of the benzodiazepine receptor antagonist β-carboline ZK 93 426

Nine days following ibotenic acid induced basal forebrain lesions or a sham-operation, rats were allowed to explore an automated six-arm radial tunnel maze. From each session, several measures of locomotor and exploratory activity were registered. Lesioned and sham-operated animals were treated with either the benzodiazepine receptor antagonist -carboline ZK 93 426 (5 mg/kg; IP) or vehicle (Cremofor EL 10% in saline; IP; n=10 for each group). Treatment was carried out 30 min before each session during acquisition (seven sessions) and reversals of the maze configuration (seven session). Eight days following the 14th session, the animals were retested without any further drug treatment. The main results suggest that the lesion resulted in locomotor hyperactivity, an increase in the number of blind arm entries, and of choice stereotypy. Treatment with ZK 93 426 attenuated the lesion-induced alterations of locomotor and exploratory activities. During the retest, the lesioned, previously vehicle-treated rats revisited arms which they had already explored during this session more frequently than the lesioned, previously ZK-treated rats; the latter group did not differ from the sham-lesioned controls. It is concluded that basal forebrain lesioned animals explored the tunnel maze less efficiently than sham-lesioned controls and that the lesioned animals benefited from the treatment with ZK 93 426. Although the specificity of the lesion in terms of destruction of cholinergic neurons remains unsettled, and although the psychological significances of the behavioral measures obtained from the tunnel maze are not yet fully understood, these results suggest that antagonists or partial inverse agonists at the benzodiazepine receptor may be able to normalize basal forebrain lesion-induced behavioral alterations.     

Attenuation of scopolamine-induced impairment of spontaneous alternation behaviour by antagonist but not inverse agonist and agonist β-carbolines

Mice were tested in a simple automated Y-maze. Total number of arm entries and alternation behaviour were measured. The latter is thought to reflect working memory capacity at a rudimentary level. During an 8-min session, vehicle-treated mice performed 32.4±7.4 arm entries, 51.0±12.4% of which were organized in alternations (triplets). The two variables showed a negative correlation. Scopolamine (1.0 mg/kg) significantly enhanced activity, reduced alternation behaviour and diminished the correlation between the two variables. The effects of benzodiazepine receptor inverse agonist, antagonist and agonist -carbolines on this spontaneous behaviour and on the effects of scopolamine were examined. The effects of inverse agonists and agonists on locomotor activity were complex in interaction with both vehicle and scopolamine. The scopolamine-induced reduction of alternation behaviour was significantly reversed by the antagonist ZK 93426 but not by inverse agonists; furthermore, partial agonists and agonists showed no effects. It is hypothesized that the interaction of antagonist -carbolines with scopolamine is based on a direct GABA-ergic control of cholinergic neurotransmission, and suggests an ability of antagonist -carbolines to antagonize amnestic properties of scopolamine.     

Dissociation between the effects of benzodiazepine receptor agonists on behavioral vigilance and responsivity

The effects of benzodiazepine receptor (BZR) full agonists chlordiazepoxide and midazolam, and the partial agonist-carboline ZK 91 296 on the rat's performance in a simple reaction time paradigm were examined. This task required the animals to respond to a rarely and unpredictably occurring brief (50 ms) visual stimulus. Non-parametric measures of signal sensitivity and response bias derived from signal-detection theory were used as a basis for the dissociation between the effects of these drugs on attentional abilities and general responsivity. The dose-dependent effects of midazolam (0.1–3.13 mg/kg) on signal sensitivity and general responsivity occurred in parallel. In contrast, the effects of chlordiazepoxide (1.56–12.5 mg/kg) on signal sensitivity were largely independent from effects on response bias. The partial agonist ZK 91 296 (0.39–25 mg/kg) in general had little effect on performance. The effects of the highest doses of chlordiazepoxide and midazolam were reversed by the co-administration of the BZR antagonist Ro15-1788 (15 mg/kg). Additionally, extension of the stimulus presentation time to 500 ms decreased the magnitude of the effect of chlordiazepoxide on signal sensitivity. These results support the hypothesis that BZR agonist-induced disruption of attentional abilities is not necessarily confounded by effects on general responsivity or sedation, and thus may represent a discrete pharmacological property of BZR-agonists.     

Effects of clonidine and some α-adrenoreceptor blocking agents on avoidance conditioned reflexes in rats: Their interactions and antagonism by atropine

Clonidine (150 g/kg s. c.) depressed avoidance conditioned reflexes in Long-Evans rats. Some -adrenoceptor blocking agents piperoxan, tolazoline, phentolamine, dibenamine and phenoxybenzamine also depressed these reflexes. Yohimbine (1–2 mg/kg) was found to have no significant effect. Atropine (10 to 15 mg/kg i. p.) antagonized the depression of conditioned avoidance reflexes induced by -adrenoceptor blocking agents as well as the depression produced by clonidine.Yohimbine (1–2 mg/kg) reduced the depressing effects of clonidine on avoidance conditioned reflexes. In rats treated with piperoxan (10 mg/kg) the effects of clonidine were reduced. In these rats the depression of conditioned reflexes was less than in rats treated with clonidine or piperoxan alone.These experiments suggest that clonidine depressed avoidance conditioned reflexes in rats by activating central -adrenoceptors. It is proposed that -adrenoceptor blocking agents might act as partial agonists.     

Effects of alpha-methyl tyrosine and adrenergic blocking agents on the facilitating action of amphetamine and nicotine on learning in rats

dl-amphetamine sulphate (2 mg/kg) and nicotine (0.2 mg/kg) showed a facilitatory action on the acquisition of a conditioned response in a shuttle-box by rats and this was reversed by pretreatment with -MT (30 mg/kg).Pretreatment with dibenamine (10 mg/kg) impaired the action either of amphetamine or nicotine. Nethalide (5–10 mg/kg) exerted a partial protection on the depressant effect produced by the interaction between dibenamine and nicotine.Animals treated with -MT (30 mg/kg) and kept in the cold (4–6° C for 3 h) also showed a depressed learning capacity. dl-Dopa (200 mg/kg) provided a partial protection on the depressive effects caused by the interaction of -MT with amphetamine, nicotine or cold. It is suggested that the facilitatory learning action of amphetamine and nicotine involves a common adrenergic mechanism. The depressant effects of amphetamine, nicotine or cold after -MT treatment are attributed to depletion of functional pools of catecholamines.This work was supported by grant N 2911/67 from the Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina (O. A. Orsingher).     

Modulation of Fas receptor proteins and dynamin during opiate addiction and induction of opiate withdrawal in rat brain

The Fas receptor is involved in the regulation of apoptosis but also can function as a non-apoptotic signal transducer. This study was mainly designed to quantitate Fas proteins in rat brain during heroin addiction and opiate withdrawal. In rat, mouse and human brains, and in SH-SY5Y cells, similar forms of Fas were immunodetected with different antibodies (i.e., 35 kDa native Fas and 48- and 51-kDa glycosylated Fas). Acute (2 h) treatments with the -opioid receptor agonists heroin (10 mg/kg) and morphine (30 mg/kg) increased the immunodensity of native Fas (124% and 36%) but not that of glycosylated Fas in the cerebral cortex. Chronic (5 days) heroin (5–30 mg/kg) and morphine (10–100 mg/kg) were also associated with increased native Fas (76% and 45%) and with different expressions of glycosylated Fas. In heroin-dependent rats, opiate withdrawal (48 h) resulted in a sustained increase in native Fas (107%) and in up-regulation of 51 kDa glycosylated Fas (51%). Acute treatments with selective -receptor (SNC-80, 10 mg/kg) or -receptor (U 50488-H, 10 mg/kg) agonists did not alter the content of native or glycosylated Fas. Chronic pentazocine (10–80 mg/kg, 5 days), a mixed opiate drug and ₁ receptor agonist, decreased native (48%) and glycosylated (38–82%) Fas proteins. Similarly, the selective ₁ agonist (+)-SKF 10047 also decreased native Fas (37%) and the effect was blocked by the ₁ antagonist BD 1063. Brain dynamin was up-regulated by acute and/or chronic heroin (30–39%), morphine (47–85%), pentazocine (51%) and heroin withdrawal (74%). The main results indicate that chronic heroin/morphine treatment and heroin withdrawal are associated with up-regulation of 35 kDa native Fas (and with different expressions of glycosylated Fas), and also with concomitant increases of dynamin in rat brain.     

Dopamine-sensitive alternation and collateral behaviour in a Y-maze: Effects of d-amphetamine and haloperidol

The degree of alternation of arm choice in a Y-maze was examined on 15-min tests over 4 days in rats treated (IP) with saline, amphetamine (0.5 or 2.0 mg/kg) or pretreated with haloperidol (0.08 mg/kg) in each condition prior to test. On day 1 amphetamine-treated animals chose arms at random, but from day 2–4 those receiving the higher dose perseverated their choice. Controls maintained alternation. These effects could be prevented by haloperidol pretreatment. Amphetamine treatment increased the frequency of rearing at the middle, choice-point of the maze more than at the end of an arm. The increase at the mid-point was suppressed by haloperidol pretreatment from day 1 and at the end of an arm from day 2. Amphetamine induced an increase in head-turning/looking that was suppressed by haloperidol from day 2. The effect of haloperidol in increasing the duration of an item of looking or rearing at the end of an arm also started later in testing. Two effects are postulated to have occurred: (i) a conflict on day 1 between novelty-controlled sensory or attentional effects that leads to an alternation of arm choice and amphetamine-induced dopaminergic activity that facilitates an alternation of behavioural responses. The result was random choice and increased rearing at the choice point. (ii) On days 2–4 the drug-induced effects on switching motor responses came to control behaviour.     

Effects of microinjections of μ and κ receptor agonists into the dorsal periaqueductal gray of rats submitted to the plus maze test

Several lines of evidence have shown that aversive states are under the influence of opioid mechanisms in the dorsal periaqueductal gray (DPAG). In order to characterize the type of opioid receptors involved in these effects in this work we injected DAMGO and U50,488H, and selective agonists, respectively, directly in this structure. Rats implanted with chemitrode in the DPAG were submitted to the elevated plus maze test for 5 min. The effects of DAMGO (0.1–1 nmol/0.2l) and U50,488H (1–10 nmol/0.2 µl) following administration into DPAG were studied. Low doses of DAMGO (0.1 and 0.3 nmol) caused dose-dependent increases in the number of entries and time spent in the open arms while an overall deficit in the exploratory activity was produced by the higher dose used (1.0 nmol). Clear aversive effects were observed following the administration of U50,488H in the DPAG. The antiaversive effects of 0.3 nmol DAMGO were inhibited by the intraperitoneal administration of the receptor antagonist naltrexone (2.0 mg/kg, IP) whereas the aversive effects of 5.0 nmol U50,488H were antagonized by the selective receptor antagonist nor-binaltorphimine (1.0 mg/kg, IP). It is suggested that activation of receptors inhibit and receptors enhance the neural substrate of aversion in the DPAG.     

Effects of opiate antagonists and putative κ agonists on unpunished and punished operant behavior in the rat

The effects of naloxone and diprenorphine, opiate antagonists with different receptor-binding properties, and the putative -receptor agonists, ketocyclazocine and ethyl-ketocyclazocine (EKC), were studied on food-reinforced responding in rats. Behavior was maintained under a multiple-component 1-min variable-interval schedule in which 12-min periods of unpunished responding alternated with 4-min periods in which each response was punished by a brief electric footshock. Daily sessions were 1 h. Naloxone (0.01–10 mg/kg) decreased unpunished responding only slightly; punished responding was decreased significantly to 66% of control by 10 mg/kg. Diprenorphine (0.01–10 mg/kg) did not affect unpublished responding and increased punished responding dose-dependently to as much as 190% of control. EKC (0.01–1.0 mg/kg) decreased unpunished and punished responding dose-dependently and comparably, whereas ketocyclazocine (0.01–1.0 mg/kg)decreased unpublished responding but did not significantly affect punished responding. Diprenorphine was more potent than naloxone in blocking the decreases in responding produced by the agonists. Differences in the behavioral effects of naloxone and diprenorphine appear to reflect the different receptor-binding properties of the two opiate antagonists.     

Benzodiazepine antagonist RO 15-1788 partly reverses some anxiolytic effects of ethanol in the mouse

The effects of the benzodiazepine (BZD) receptor antagonist RO 15-1788 (3 mg/kg) on the anxiolytic properties of ethanol in mice confronted with a light/dark choice procedure and with the staircase test were investigated. RO 15-1788 reversed the effects of ethanol on some of the behavioural parameters without eliciting intrinsic effects when given alone. These data closely resemble those we previously obtained with several BZD receptor inverse agonists such as RO 15-3505, RO 15-4513 or -CCM. Since anxiogenic-like properties of low doses of RO 15-1788 have been identified by other authors, it is suggested that the antagonistic action of this drug against some of the behavioural effects of ethanol could be due to its being a partial BZD inverse agonist.     

[11C]Ro 15-4513, a ligand for visualization of benzodiazepine receptor binding

Ro 15-4513, a partial inverse agonist at the benzodiazepine (BZ) receptor site was labelled with¹¹C and used for in vitro autoradiography on human post mortem brain sections and for positron emission tomography (PET) on Cynomolgus monkeys. The total radiochemical yield of [¹¹C]Ro 15-4513 was 30–40% with an overall synthesis time of 40 min. The specific radioactivity was about 1000 Ci/mmol at end of synthesis. In vitro autoradiography showed that [¹¹C]Ro 15-4513 bound specifically predominately in the neocortex of the human brain. Specific binding was also demonstrated in the basal ganglia and the cerebellar cortex. Flumazenil (Ro 15-1788) and clonazepam inhibited the binding in cerebral regions, but a significant proportion in the cerebellum was not inhibited by these agents. This proportion may represent ₆-containing BZ receptors. PET examination of [¹¹C]Ro 15-4513 binding in Cynomolgus monkeys demonstrated high uptake of radioactivity in neocortex. The uptake of radioactivity was markedly displaced by high doses of Ro 15-4513 or clonazepam. [¹¹C]Ro 15-4513 should be a useful ligand to examine BZ receptor characteristics in the living human brain by PET.     

Effects of Δ8-THC,and Δ9-THC on the acquisition of a discriminative positional habit in rats

Using a reversal learning paradigm the dissociative effects of two tetrahydrocannabinols (THC) on the acquisition and reversal of a discriminative positional habit in rats were studied. A T-shaped water maze was used. From these experiments it is concluded that learning under the influence of ⁸-THC (10 and 20 mg/kg), and ⁹-THC (5 mg/kg) is state-dependent (StD) in the rat.Numbering system according to IUPAC rules.Parts of the results were presented at the Symposium on the chemistry and biological activity of cannabis, Stockholm, October 26–28, 1971 and at the Symposium on medical plants of Brazil, Sao Paulo, April 17–20, 1972.     

Dissociation between the attentional effects of infusions of a benzodiazepine receptor agonist and an inverse agonist into the basal forebrain

The effects of infusions of the benzodiazepine receptor (BZR) full agonist chlordiazepoxide (CDP) or the full inverse agonist -CCM into the basal forebrain on behavioral vigilance were tested. Vigilance was measured by using a previously characterized task that requires the animals to discriminate between visual signals of variable length and non-signal events. Measures of performance included hits, misses, correct rejections, false alarms, side bias, and errors of omission. Following the infusion of saline (0.5 µl/hemisphere), the relative number of hits varied with signal length. In response to shorter signals, the number of hits decreased over time, indicating a vigilance decrement. Infusions of CDP (20, 40 µg/hemisphere) initially decreased the relative number of hits in response to shorter signals and, later in the course of the test sessions, to longer signals as well. CDP did not affect the relative number of correct rejections. In contrast, infusions of the inverse agonist -CCM (1.5, 3.0 µg/hemisphere) did not affect the relative number of hits but decreased the relative number of correct rejections (i.e., increased the number of false alarms). These data suggest that the basal forebrain mediates the attentional effects of BZR ligands. As systemic or intrabasalis administration of BZR agonists and inverse agonists was previously demonstrated to decrease and augment, respectively, activated cortical acetylcholine (ACh) efflux, their effects on behavioral vigilance are hypothesized to be mediated via their effects on cortical ACh.     

Effect of α2-adrenoceptor agonists on the expression of morphine-withdrawal in rats

Summary Previous studies using clonidine indicate that ₂-adrenoceptors are involved in suppressing opiate-withdrawal symptoms. However, clonidine may act as a partial agonist at ₂-adrenoceptors and it also possesses significant ₁-receptor agonist activity.The aim of this study was to determine the role of ₂-adrenoceptors in the expression of opiate withdrawal signs using morphine-dependent rats. A range of agonists were selected for study on the basis of their differential preferences for -adrenoceptors.Hooded Wistar rats were made physically dependent on morphine (s.c. injection of an emulsion releasing a total of 250 mg/kg of morphine base over 48 h). Test drugs were injected s.c. followed by naloxone (10 mg/kg i.p.) 20 min later. The incidence of 5 selected withdrawal signs was recorded during the following 20 min. The ₂-adrenoceptor agonists displayed different profiles of activity. Azepexole (1–10 mg/kg) reduced all signs. Clonidine (80–800 g/kg) reduced all signs except paw shakes while guanfacine (25–250 g/kg) reduced all except jumping and diarrhoea. Talipexole (0.1–1 mg/kg) reduced all signs except diarrhoea which was not affected and jumping which was markedly enhanced. UK 14,304 (80–800 g/kg) reduced jumps, potentiated paw shakes but did not affect body shakes, teeth chattering or diarrhoea. The results suggest that there are subpopulations of ₂-adrenoceptors that modulate the expression of opiate withdrawal signs and/or that some of the drugs used affect receptors other than ₂-adrenoceptors.     

α2 Autoreceptor-mediated modulation of tyrosine hydroxylase activity in noradrenergic regions of the rat brain in vivo

Summary The physiological importance of brain 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 -autoreceptor mechanism. Acute treatments of rats (not exposed to reserpine) with a wide variety of adrenoceptor agonists such as guanfacine 6, B-HT 920, 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 -adrenoceptor agonists and antagonists (except prazosin) for an ₂-adrenoceptor. The results also suggest that the ₂-autoreceptor that modulates the synthesis of noradrenaline in the rat brain appears to belong to the prazosin-sensitive _2B-subtype.Dedicated to Nils-Erik Andén, in memoriamSend offprint requests to J. A. Garcia-Sevilla at the above address at Palma de Mallorca     

Dexmedetomidine synergism with midazolam in the elevated plus-maze test in rats

The anxiolytic profile of dexmedetomidine, a novel, highly-selective ₂-adrenergic agonist, was examined in rats in the elevated plus-maze test when administered either alone or in combination with the benzodiazepine agonist midazolam. Dexmedetomidine, 0.1–10 µg/kg, was inactive in modifying the rats' behavioral response in this test. Midazolam, 0.1–10 mg/kg, dose-dependently produced an anxiolytic-like profile characterized by an increased time spent in the open arms of the elevated plus-maze. A combination of dexmedetomidine 0.5 µg/kg and midazolam 0.5 mg/kg produced a synergistic interaction. This heterergic interaction of dexmedetomidine on midazolam's anxiolytic-like profile was dose-dependently blocked by pretreatment with an ₂-adrenergic antagonist, atipamezole, 10–50 µg/kg, and a benzodiazepine antagonist flumazenil, 1.0 and 10 mg/kg, but not by the ₁-adrenergic antagonist, prazosin, 0.1–10 mg/kg. While the transmembrane signal transduction pathways for benzodiazepine- and ₂-agonist responses do not share any molecular component, there does appear to be crosstalk between these two systems. These may involve GABA or noradrenergic downstream effects of either dexmedetomidine or midazolam, respectively.