B-HT 920 and B-HT 958: presynaptic effects on electrically evoked 3H-dopamine release from slices of rat nucleus accumbens

The effect of B-HT 920, a dopamine D2 agonist, on bar-press feeding in the monkey

Although the dopamine (DA) system has been shown to regulate food intake, the function of the DA receptor subtypes on behavior still remains to be elucidated. In the present study, we examined the effect of B-HT 920, a selective agonist of DA D2 receptors that preferentially affect presynaptic autoreceptors, on both food consumption and execution of a high fixed-ratio bar-press task for food reward in monkeys. Two kinds of bar-press task were used: 1) a cue-triggered bar-press task during the first 40 trials, and 2) a self-paced bar-press task in which the monkeys freely performed bar-press trials until they were satiated. A SC injection of B-HT 920 (25 micrograms/kg) increased food consumption in the home cage. The same facilitatory effect on food consumption was also observed in the operant task condition. During the cue-triggered bar-press task, however, both the latency of the bar-press responses to a cue light and the time required to complete the bar-press trials were prolonged after the injection of B-HT 920. The results suggest that the activation of D2 autoreceptors suppresses the operant food acquisition behavior and increases food consumption through an inhibition of the satiety mechanism rather than an activation of any hunger-related drive.

Excitation by talipexole, a dopamine D2 agonist, of caudate nucleus neurons activated by nigral stimulation

An electrophysiological study using cats anesthetized with alpha-chloralose was performed to elucidate whether or not talipexole (B-HT 920 CL2: 6-allyl-2-amino -5, 6, 7, 8-tetrahydro-4H-thiazolo [4, 5 -d] -azepine-dihydrochroride), a dopamine D2 agonist, acts on postsynaptic dopamine receptors in the caudate nucleus (CN) neurons receiving excitatory input from the pars compacta of substantia nigra (SN). Extracellular neuron activities were recorded in the CN using a glass-insulated silver wire microelectrode attached along a seven-barreled micropipette, each of which was filled with talipexole, quinpirole (dopamine D2 agonist), domperidone (dopamine D2 antagonist), glutamate and 2M NaCl. These drugs were microiontophoretically applied to the immediate vicinity of the target neuron. In the same neurons in which the spikes elicited by the SN stimulation were blocked by microiontophoretically applied domperidone, microiontophoretic application of talipexole and quinpirole induced a dose-dependent increase in spontaneous firing. This increase in firing by talipexole and quinpirole was blocked during simultaneous application of domperidone, although glutamate-induced firing remained unaffected by domperidone. In the CN neurons, in which the SN stimulation-induced spikes were not blocked by domperidone, spontaneous firing was not affected by talipexole or quinpirole. These findings suggest that talipexole activates CN neurons receiving a dopaminergic input from SN via D2 receptors, as does quinpirole.

Behavioural effects induced in rats and chicks by D2 dopamine agonists

In a first series of experiments, different selective dopamine D2 receptor agonists (B-HT 920, B-HT 958, SND 919, CQ 32-084, CQP 201-403, and lisuride) and the D1/D2 agonist apomorphine were IP injected into adult male rats. At low doses, they elicited repeated episodes of penile erection and stretching-yawning: at all doses tested, B-HT 920, B-HT 958, and CQ 32-084 also induced hypomotility, a sign that, in the case of high doses of SND 919, CQP 201-403, lisuride, and apomorphine, was replaced by stereotyped behaviour. In a second series of experiments, the same D2 agonists and the mixed D1/D2 agonist apomorphine were IP injected at the same doses into chicks. The following behavioural signs were observed: hypomotility, sleep-like state, and stereotyped pecking. The results show that: 1. there are similarities between the behavioural effects induced by the DA agonists in rats and chicks; and 2. in both species some behavioural signs elicited by the DA ergic compounds are useful pointers to their specific neurochemical activity.

B-HT 920, a dopamine D2 agonist, in the treatment of negative symptoms of chronic schizophrenia

A prospective, nonblind 8-week trial of talipexole dihydrochloride (B-HT 920), a dopamine D2 agonist, was conducted in 15 schizophrenic patients with predominantly negative symptoms. B-HT 920 was initiated at 0.15 mg/day and then adjusted at 0.15-2.4 mg/day on the basis of clinical response and side effects. Dosage of concurrent neuroleptics was fixed at least 3 weeks prior to the trial and was unchanged throughout the study period. In addition to clinical assessment, levels of plasma homovanillic acid (pHVA), a potential index of central dopamine turnover, were measured. There was a small but significant (p < 0.01, Wilcoxon test) reduction in total scores of the Scale for the Assessment of Negative Symptoms or in a cluster score of three negative items (Emotional Withdrawal, Blunted Affect, and Psychomotor Retardation) of the Brief Psychiatric Rating Scale (BPRS). No change was observed in cluster scores of positive items of BPRS. There was a weak negative correlation between pHVA levels and the cluster scores of negative items of BPRS both at weeks 0 and 8 of the trial. The clinical results suggest that activation of D2 receptors was related to partial amelioration of the negative symptoms. The clinical and biochemical findings are consistent with a hypothesis that decreased dopaminergic activity may be related to the etiology of negative symptoms of schizophrenia.

Behavioural evidence that different neurochemical mechanisms underly stretching-yawning and penile erection induced in male rats by SND 919, a new selective D2 dopamine receptor agonist

The behavioural effects induced in male Wistar rats by SND 919, a new drug reputed to have selective agonistic activity at D2 dopamine (DA) receptors, were studied. The following aspects of behaviour were considered: motor activity, stretching-yawning (SY), penile erection (PE) and stereotyped behaviour (SB). Intraperitoneal injection (IP) of the drug (0.01-20 mg/kg) induced an SY syndrome in the form of a bell-shaped dose-response curve, the effect being maximal at the dose of 0.1 mg/kg and disappearing completely at 10 mg/kg. SND 919 also potently elicited PE; this latter effect, however, was not coincident with SY induction, being maximal at 1 mg/kg and persisting at 10 and 20 mg/kg. SND 919-induced SY was potently antagonized by pretreatment not only with the D2 antagonist, L-sulpiride (20 mg/kg), but also with the alpha 2 antagonist, yohimbine (1, 3 mg/kg), and the more selective alpha 2 antagonist, idazoxan (1, 2 and 5 mg/kg). While sulpiride also decreased SND 919-induced PE, idazoxan at all doses and yohimbine at 1 mg/kg did not affect this behaviour. Inhibition of motor activity was induced by the D2 agonist at low doses (0.05, 0.1 mg/kg), while at high doses (1, 10 and 20 mg/kg), it was actually replaced by a form of SB characterized by downward sniffing and licking. When, for comparison, the D2 agonist, RU 24213 (0.1-20 mg/kg IP), was tested for PE, SY, motor activity and SB, it displayed a behavioural pattern very similar to that obtained with SND 919. Idazoxan (2 mg/kg), administered before RU 24213 (10 mg/kg), significantly antagonized the drug-induced SY, but not PE.(ABSTRACT TRUNCATED AT 250 WORDS)

B-HT 920 activates dopamine D2 receptors coupled to inhibition of adenylate cyclase activity

In homogenates of female rat anterior pituitary, the azepine derivative B-HT 920 inhibited the forskolin-stimulated adenylate cyclase activity with an EC50 value of 0.35 microM. In male rat anterior pituitary, B-HT 920 curtailed the stimulation of adenylate cyclase activity by vasoactive intestinal peptide with an EC50 of 0.20 microM. In synaptic plasma membranes of rat striatum, B-HT 920 significantly reduced basal adenylate cyclase activity with an EC50 of 0.68 microM. Both in pituitary and striatum, the B-HT 920 inhibition was counteracted by the dopamine (DA) D2 receptor antagonist 1-sulpiride, but not by the alpha 2-adrenergic antagonist yohimbine. These results indicate that B-HT 920 is capable of activating DA D2 receptors negatively coupled to adenylate cyclase activity.

Inhibition by talipexole, a thiazolo-azepine derivative, of dopaminergic neurons in the ventral tegmental area

A microiontophoretic study using rats anesthetized with chloral hydrate and immobilized with gallamine triethiodide was carried out to compare the effect of talipexole (B-HT 920 CL2:2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo [4,5-d]-azepine-dihydrochloride), a dopamine autoreceptor agonist, on dopaminergic neurons in the ventral tegmental area (VTA) to non-dopaminergic neurons in the VTA. VTA neurons were classified into two types according to the responses to antidromic stimulation of the nucleus accumbens (Acc): type I neurons with a long spike latency (8.69 +/- 0.24 msec) upon Acc stimulation and low spontaneous firing rate (6.80 +/- 1.34/sec), and type II neurons with a short latency (2.76 +/- 0.20 msec) and high spontaneous firing rate (26.77 +/- 7.05/sec), probably corresponding to dopaminergic and non-dopaminergic neurons, respectively. In type I neurons, microiontophoretic application of talipexole and dopamine inhibited antidromic spike generation elicited by Acc stimulation, and talipexole-induced inhibition was antagonized by domperidone (dopamine D-2 antagonist). In type II neurons, however, the antidromic spikes were not affected by either talipexole or dopamine. Furthermore, spontaneous firing was also inhibited by iontophoretically applied talipexole and dopamine in most type I neurons, but rarely affected by either drug. Inhibitory effects of talipexole were antagonized by domperidone. These results suggest that talipexole acts on dopamine D-2 receptors, thereby inhibiting the dopaminergic neurons in the VTA.

Effect of BHT 920 on monoaminergic neurons of the rat brain: an electrophysiological in vivo and in vitro study

BHT 920 was originally described as a dopamine autoreceptor agonist. In this study, the effect of this compound on the firing rate of noradrenergic locus coeruleus, serotonergic dorsal raphe and dopaminergic ventral tegmental area neurons was examined both in the anaesthetized rat and in rat brain slices. Extracellular recordings were performed in cells whose identity was determined by electrophysiological, pharmacological and histological criteria. In vivo, BHT 920 inhibited the firing of locus coeruleus neurons (ID50: 14.5 +/- 4.7 micrograms/kg, mean +/- SEM) and ventral tegmental area neurons (ID50: 7 +/- 3 micrograms/kg) at very low doses. As a comparison, the ID50 of clonidine on locus coeruleus cells was 5.5 +/- 0.6 microgram/kg and the ID50 of apomorphine on ventral tegmental area neurons was 13 +/- 3 micrograms/kg. BHT 920 also decreased the firing of dorsal raphe cells, but this effect was obtained at higher doses (ID50: 57 +/- 11 micrograms/kg). The in vitro study confirmed the results obtained in vivo. BHT 920 potently inhibited the firing of locus coeruleus cells (IC50: 71 +/- 28 nM) and was less potent than clonidine (IC50: 5.3 +/- 0.98 nM). The compound also inhibited the firing of ventral tegmental area neurons at very low concentrations (IC50: 21 +/- 3.3 nM), being more potent than apomorphine (IC50: 56 +/- 29 nM). BHT 920 only slightly decreased the firing rate of dorsal raphe neurons at 50 microM, showing that the drug has little direct effect on these cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of a dopamine autoreceptor selective profile of B-HT 920 in functional in vitro model systems of D2 receptors in rat striatum

Based on the results of in vivo studies, the thiazoloazepine derivative B-HT 920 has been proposed to be a selective agonist of dopamine autoreceptors. In the present study, we investigated the effects of B-HT 920 in two functional in vitro model systems of D2 receptors and compared these effects with the effects of the classical D2 agonist LY 171555. B-HT 920 and LY 171555 concentration dependently inhibited the electrically evoked release of radiolabeled dopamine and acetylcholine and the forskolin-induced stimulation of adenylate cyclase activity in rat striatal tissue slices with comparable efficacies. In striatal tissue slices prepared after 6-hydroxydopamine-induced destruction of dopaminergic terminals, both drugs were still able to inhibit forskolin-stimulated adenylate cyclase activity with a efficacy similar to that in tissue obtained from unlesioned rats. It is concluded that, in vitro, B-HT 920 is an agonist at both presynaptic and 'normosensitive' postsynaptic D2 receptors showing relatively high intrinsic activity.

Potentiation of yawning responses to the dopamine receptor agonists B-HT 920 and SND 919 by pindolol in the rat

Subcutaneous injection of B-HT 920, a dopamine D2-receptor agonist, in doses ranging from 5 to 100 micrograms/kg, induced yawning behavior in rats. Yawning was also elicited by low doses (25-500 micrograms/kg sc) of SND 919, a newly synthesized dopamine receptor agonist. The yawning evoked by B-HT 920 or SND 919 was increased by the beta-adrenoceptor antagonist pindolol (20 mg/kg ip) which alone did not induce yawning. Stereotyped behavior did not appear after B-HT 920 or SND 919 given alone or in combination with pindolol. The results suggest that SND 919 as well as B-HT 920 has stimulatory activity at dopamine D2-receptors, and that pindolol may exert its enhancement of the yawning response to dopamine receptor agonists via blockade of beta-adrenoceptors.

Possible involvement of differing classes of dopamine D-2 receptors in yawning and stereotypy in rats

The present experiments were performed to investigate differences in the properties of the dopamine D-2 receptors related to yawning and stereotypy. Subcutaneous injection of talipexole (B-HT 920) (10-250 micrograms/kg) or SND 919 ((S)-2-amino-4,5,6,7-tetrahydro-6-propylamino-benzothiazole) (25-500 micrograms/kg) evoked yawning behavior with bell-shaped responses. However, SK&F 38393 (1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol) (0.5-10 mg/kg SC) did not elicit yawning and decreased yawning responses to low doses of talipexole (25 micrograms/kg SC) or SND 919 (100 micrograms/kg SC). These low but effective doses for inducing yawning of talipexole or SND 919 in combination with SK&F 38393 (0.5-10 mg/kg SC) did not elicit stereotypy. In contrast, yawning behavior was not produced after very high doses of talipexole (500 micrograms/kg SC) or SND 919 (1000 micrograms/kg SC) given alone or in combination with SK&F 38393 (0.5-10 mg/kg SC). These extremely high doses of talipexole or SND 919 evoked slight stereotypy, which was enhanced by the combined treatment with SK&F 38393. The present results suggest that the dopamine D-2 receptors related to yawning are more sensitive to dopamine receptor agonists than those related to stereotypy, and that concurrent stimulation of postsynaptic dopamine D-1 receptors with D-2 receptors reduces the incidence of yawning but enhances that of stereotypy.

Involvement of adenylate cyclase inhibition in dopamine autoreceptor regulation of tyrosine hydroxylase in rat nucleus accumbens

In homogenates of rat nucleus accumbens, quinpirole, a dopamine (DA) D2 receptor agonist, inhibited the activation of tyrosine hydroxylase (TH) elicited by either forskolin, an activator of adenylate cyclase, or rolipram, a cyclic nucleotide phosphodiesterase inhibitor. The inhibition produced by 1 microM quinpirole was completely antagonized by the D2 blocker L-sulpiride (2 microM). Quinpirole failed to inhibit the stimulation of TH elicited by dibutyryl cyclic AMP (2 mM), which acts independently of adenylate cyclase. Quinpirole (10 microM) significantly inhibited the stimulation of adenylate cyclase activity elicited by 1 microM forskolin. These results indicate that mesolimbic DA autoreceptors can regulate TH activity by inhibiting a presynaptic adenylate cyclase system.

Potentiation by serotonergic inhibition of yawning induced by dopamine receptor agonists in rats

Low doses of the dopamine D2-receptor agonist, B-HT 920 (25 micrograms/kg, SC), and the dopamine D1/D2-receptor agonists, apomorphine (50 micrograms/kg, SC) and piribedil (1 mg/kg, SC), evoked yawning. However, the dopamine D1-receptor agonist, SK&F 38393 (2 mg/kg, SC), failed to induce yawning. The yawning responses induced by B-HT 920, apomorphine or piribedil were markedly increased without eliciting stereotypy by pretreatment with reserpine (5 mg/kg, IP, 24 hr). These yawning responses were also enhanced by p-chlorophenylalanine (PCPA) (300 mg/kg, IP, 72 hr), but not by alpha-methyl-p-tyrosine (300 mg/kg, IP, 6 hr). The yawning induced by B-HT 920 given alone and in combination with reserpine or PCPA was inhibited by spiperone (0.5 mg/kg, IP) or scopolamine (0.5 mg/kg, IP), but not by SCH 23390 (0.5 mg/kg, IP). The present results suggest that yawning is evoked by stimulation of dopamine D2-receptors having a high affinity and consequent muscarinic activation, and that the yawning induced by dopamine receptor agonists is potentiated by decreases in serotonergic neuron activity.

Effects of locally applied D-1 and D-2 receptor agonists and antagonists studied with brain dialysis

The effect on dopamine (DA) release of D-1 and D-2 receptor agonists and antagonists applied locally in the caudate through a trans-striatal dialysis probe was studied in freely moving rats. D-2 agonists (LY 171555 and BHT 920) reduced DA release in a concentration-dependent manner. The same local application of haloperidol abolished the effect of 10 microM LY 171555 and BHT 920. A specific D-1 agonist, the catechol benzazepine SKF 38393, reduced DA release and this effect was not modified by systemic or local administration of the D-1 antagonist, SCH 23390, nor by the D-2 antagonist, haloperidol. In contrast, the non-catechol D-1 agonist, CY 208243, failed to modify DA release. Local application of the D-1 antagonist, SCH 23390, or of the D-2 antagonist, (-)-sulpiride, stimulated DA release in a concentration-dependent manner. The D-1 agonist, CY 20843, reversed the stimulatory effect of SCH 23390 but not that of (-)-sulpiride. It is concluded that D-1 and D-2 receptors located in the caudate control DA release separately in this area.

B-HT 920 stimulates postsynaptic D2 dopamine receptors in the normal rat: electrophysiological and behavioral evidence

The putative autoreceptor-selective dopamine (DA) agonist B-HT 920 was tested using electrophysiological and behavioral models thought to reflect actions at postsynaptic D2 DA receptors. Direct iontophoretic application of B-HT 920 onto nucleus accumbens neurons caused a current-dependent inhibition of firing which could be attenuated by pretreatment with alpha-methyl-p-tyrosine (to deplete DA) and reinstated (enabled) by concurrent administration of the selective D1 DA receptor agonist SKF 38393. These findings suggest that, like other selective D2 DA receptor agonists, the postsynaptic effects of B-HT 920 require concurrent stimulation of D1 DA receptors. Behavioral indices of postsynaptic D2 DA receptor stimulation (stereotyped sniffing and rearing) were also evident following combined treatment with B-HT 920 and SKF 38393. Moreover, similar "low-level" stereotyped behaviors were also observed when B-HT 920 was administered alone following pretreatment with the alpha-2 adrenoceptor antagonists idazoxane and piperoxane, suggesting that alpha-2 agonist actions of B-HT 920, in some way, mask the expression of D2 receptor-mediated stereotyped responses. When B-HT 920 was combined with SKF 38393 following pretreatment with idazoxane, both the intensity and form (continual licking and gnawing) of stereotyped behavior was enhanced. Taken together, these electrophysiological and behavioral findings indicate that B-HT 920 possesses the properties of a selective D2 DA receptor agonist and cannot be considered as a DA autoreceptor-selective compound.

B-HT 920, B-HT 933, and B-HT 958: presynaptic effects on electrically evoked 3H-noradrenaline release from slices of rat brain cortex and hypothalamus

The effects of the three azepine compounds, B-HT 920 (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo-[4,5-d]azepine), B-HT 933 [2-amino-6-ethyl-5,6,7,8-tetrahydro-4H-oxazolo[4,5-d]azepine; azepexole] and B-HT 958 (2-amino-6-(p-chloro-benzyl)-4H-5,6,7,8-tetrahydrothiazolo[5,4-d]a zepine) on electrically evoked overflow of 3H-noradrenaline were studied. Slices from parietal cortex (Cx) or nucleus anterior hypothalami (nah) were incubated with 3H-noradrenaline, superfused at 23 degrees C or 37 degrees C in the presence of the noradrenaline uptake inhibitor desipramine (3 mumol/l) and field stimulated at frequencies of 0.3 or 3 Hz. At 37 degrees C/0.3 Hz, B-HT 920 and B-HT 933 concentration-dependently decreased the evoked overflow of tritium in both regions studied, whereas B-HT 958 had no effect. In a second set of experiments each drug was tested under three additional experimental conditions, i.e. 37 degrees C/3 Hz, 23 degrees C/0.3 Hz and 23 degrees C/3 Hz. Increasing the stimulation frequency to 3 Hz or lowering the superfusion temperature to 23 degrees C reduced the effects of B-HT 920 (1 mumol/l) and B-HT 933 (10 mumol/l) as compared to the effects at 37 degrees C/0.3 Hz. When tested at 23 degrees C/3 Hz, both drugs did not significantly affect the evoked overflow of tritium in the Cx or the nah. In contrast, B-HT 958 (10 mumol/l), caused a facilitation of evoked overflow in both regions when the higher stimulation frequency or the lower superfusion temperature was used. Its release-enhancing action was most pronounced at 23 degrees C/3 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)

D1 dopamine receptor--the search for a function: a critical evaluation of the D1/D2 dopamine receptor classification and its functional implications

The present review focuses on the hypothesized D1/D2 dopamine (DA) receptor classification, originally based on the form of receptor coupling to adenylate cyclase activity. The pharmacological effects of compounds exhibiting putative selective agonist or antagonist profiles at those DA receptors positively coupled to adenylate cyclase activity (D1 DA receptors) are extensively reviewed. Comparisons are made with the effects of putative selective D2 DA receptor agonists and antagonists, and on the basis of this work, the DA receptor classification is critically evaluated. A variety of biochemical, behavioral, and electrophysiological evidence is presented which supports the view that D1 and D2 DA receptors can interact in both an opposing and synergistic fashion. Particular attention is focused on the possibility that D1 receptor stimulation is required to enable the expression of certain D2 receptor-mediated effects, and the functional consequences of this form of interaction are considered. A hypothetical model is presented which considers how both the opposing and enabling forms of interaction between D1 and D2 DA receptors can control behavioral expression. Finally, the clinical relevance of this work is discussed and the potential use of selective D1 receptor agonists and antagonists in the treatment of psychotic states and Parkinson's disease is considered.