Drug discrimination models in anxiety and depression

Discriminative-stimulus effects of cannabidiol oil in Sprague-Dawley rats

Cannabidiol (CBD) is one of the major centrally active phytocannabinoid components of cannabis, and has been approved by the FDA only for the treatment of seizures associated with three rare disorders. It has also been touted as a potential treatment for anxiety in place of more traditional treatments like benzodiazepines. Although there is some evidence of anxiolytic effects of CBD, its suitability as a substitute for benzodiazepines is unknown. This experiment was designed to assess the extent to which CBD shares interoceptive discriminative-stimulus properties with the anxiolytic drug chlordiazepoxide (CDP), a benzodiazepine. In the present experiment, a range of doses (0-1569 mg/kg) of over-the-counter CBD oil was administered (i.g.) in male Sprague-Dawley rats trained to discriminate 5.6 mg/kg CDP from saline. Due to the long time-course effects of CBD, generalization tests were conducted at 90 and 120 min post-CBD administration. The two highest doses of CBD tested (1064 and 1569 mg/kg) were found to partially substitute for 5.6 mg/kg CDP, with mean percent responding on the CDP-associated lever reaching above 20% at time 2 (120 min post-CBD administration), suggesting that high doses of the over-the-counter CBD oils used in this experiment share interoceptive discriminative-stimulus properties to some degree with CDP. These results are novel in comparison to existing research into stimulus effects of CBD, in which substitution for benzodiazepines has not previously been observed.

Evaluation of antianxiety and sedative effects of essential oil of Ducrosia anethifolia in mice

INTRODUCTION

We investigated the antianxiety and sedative effects of the essential oil of Ducrosia anethifolia. Boiss. (Apiaceae).

METHODS

We used elevated plus maze, spontaneous motor activity and ketamine-induced sleep tests in mice. In addition, the essential oil was analyzed by GC/MS. Twenty compounds were identified, and n-decanal (70.1%) and alpha-pinene (12.4%) constituted the major components.

RESULTS

In elevated plus maze, Ducrosia anethifolia essential oil at doses of 25-200 mg/kg increased the percentage of open arm time and entries. Unlike diazepam, ducrosia anethifolia essential oil could not suppress spontaneous motor activity and did not alter ketamine-induced sleep parameters. These results are indicative of antianxiety effect of Ducrosia anethifolia essential oil without sedative effect.

The Netherlands Study of Depression and Anxiety (NESDA): rationale, objectives and methods

The Netherlands Study of Depression and Anxiety (NESDA) is a multi-site naturalistic cohort study to: (1) describe the long-term course and consequences of depressive and anxiety disorders, and (2) to integrate biological and psychosocial research paradigms within an epidemiological approach in order to examine (interaction between) predictors of the long-term course and consequences. Its design is an eight-year longitudinal cohort study among 2981 participants aged 18 through 65 years. The sample consists of 1701 persons with a current (six-month recency) diagnosis of depression and/or anxiety disorder, 907 persons with life-time diagnoses or at risk because of a family history or subthreshold depressive or anxiety symptoms, and 373 healthy controls. Recruitment took place in the general population, in general practices (through a three-stage screening procedure), and in mental health organizations in order to recruit persons reflecting various settings and developmental stages of psychopathology. During a four-hour baseline assessment including written questionnaires, interviews, a medical examination, a cognitive computer task and collection of blood and saliva samples, extensive information was gathered about key (mental) health outcomes and demographic, psychosocial, clinical, biological and genetic determinants. Detailed assessments will be repeated after one, two, four and eight years of follow-up. The findings of NESDA are expected to provide more detailed insight into (predictors of) the long-term course of depressive and anxiety disorders in adults. Besides its scientific relevance, this may contribute to more effective prevention and treatment of depressive and anxiety disorders.

Hydroalcohol extract and fractions ofStachys lavandulifolia vahl: effects on spontaneous motor activity and elevated plus-maze behaviour

The present study aimed to investigate the anxiolytic effects of four fractions of Stachys lavandulifolia Vahl. The aerial parts of the plant were extracted with petroleum ether (PF), ethyl acetate (EF), butanol (BF) and water (AF) and tested for spontaneous motor activity and elevated plus-maze (EPM) behaviour in mice. The hydroalcohol extract (HE) and different fractions of S. lavandulifolia were administered intraperitoneally to male Syrian mice, at various doses, 30 min before the behavioural evaluation. The HE of S. lavandulifolia (at 50 mg/kg) increased the percentage of time spent (39%) and the percentage of arm entries in the open arms (53%). The HE (50 mg/kg), PF (25 and 50 mg/kg), EF (25 and 50 mg/kg) and AF (50 mg/kg) of S. lavandulifolia significantly increased the percentage of time spent and the percentage of arm entries in the open arms. The BF up to a dose of 50 mg/kg had no significant effects on any of the measured parameters in the EPM. The spontaneous locomotor activity was significantly decreased in animals injected with each plant fractions, compared with that of saline. The EF and AF showed the least and the most reduction in the activity, respectively. The anxiolytic effects of EF, PF and AF could be related to their content of flavonoids, phenylpropanoids or terpenoids.

Anxiolytic effects of Echium amoenum on the elevated plus-maze model of anxiety in mice

The ethanolic extract of Echium amoenum flowers at the dose of 50 mg/kg increased the percentage of time-spent and the percentage of arm entries in the open arms of the elevated plus-maze (EPM) and decreased the percentage of time-spent in the closed arms of EPM. Moreover, it prolonged the ketamine-induced latency to sleep but had no significant effects on total sleeping time induced by ketamine. Also, the locomotor activity was affected but not to the same extent as observed for diazepam. These results suggested that the extract of E. amoenum seems to possess anxiolytic effect with lower sedative activity than that of diazepam.

Anxiolytic effects of Stachys lavandulifolia Vahl on the elevated plus-maze model of anxiety in mice

Interest in alternative medicine and plant-derived medications that affect the "mind" is growing. The aim of the present study was to investigate the effects of a hydroalcoholic extract and essential oil of Stachys lavanduifolia Vahl on the elevated plus-maze (EPM) model of anxiety. The Stachys lavandulifolia extract or its essential oil was administered intraperitoneally to male TO mice, at various doses, 30 min before the behavioral evaluation. The extract of Stachys lavandulifolia at the dose of 100 mg/kg increased the percentage of time spent and the percentage of arm entries in the open arms of the EPM and decreased the percentage of time spent and the percentage of arm entries in the closed arms of the EPM. The plant extract at doses lower than 100 mg/kg had no significant effects on any of the parameters measured on the EPM. This dose of the plant extract prolonged the ketamine-induced sleeping time, and decreased the locomotor activity in mice. These results suggested that the extract of Stachys lavandulifolia possessed anxiolytic effect with relatively lower sedative activity than diazepam. The essential oil of Stachys lavandulifolia, however, at doses of up to 100 mg/kg did not have any significant effects on the mice behaviour on the EPM.

A concept of welfare based on reward evaluating mechanisms in the brain: anticipatory behaviour as an indicator for the state of reward systems

In this review we attempt to link the efficiency by which animals behave (economy of animal behaviour) to a neuronal substrate and subjective states to arrive at a definition of animal welfare which broadens the scope of its study. Welfare is defined as the balance between positive (reward, satisfaction) and negative (stress) experiences or affective states. The state of this balance may range from positive (good welfare) to negative (poor welfare). These affective states are momentary or transient states which occur against the background of and are integrated with the state of this balancing system. As will be argued the efficiency in behaviour requires that, for instance, satisfaction is like a moving target: reward provides the necessary feedback to guide behaviour; it is a not steady-state which can be maintained for long. This balancing system is reflected in the brain by the concerted action of opioid and mesolimbic dopaminergic systems. The state of this system reflects the coping capacity of the animal and is determined by previous events. In other words, this integrative approach of behavioural biology and neurobiology aims at understanding how the coping capacity of animals may be affected and measured. We argue that this balancing system underlies the economy of behaviour. Furthermore we argue that among other techniques anticipation in Pavlovian conditioning is an easy and useful tool to assess the state of this balancing system: for estimating the state of an animal in terms of welfare we focus on the conditions when an animal is facing a challenge.

How The Hierarchical Organization of the Brain and Increasing Cognitive Abilities May Result in Consciousness

Defining perception, awareness, consciousness and reflexive or self-reflexive consciousness is difficult. I will not linger on definitions of fuzzy concepts but will attempt to put forward evidence for the rationale that awareness is likely to emerge as a consequence of how the brain processes information. Efficiency in information processing has resulted in a limited number of preferential (motivational) states of the brain and, in fact, of the whole organism. In addition, animals have the ability to internally represent external conditions and, through interactions with the motivational state, generate expectations.

It is argued that optimal decision-making requires that possible sequences of behaviours each activate their associated neuronal networks representing cue- and context-related information. Prior to the initiation of an action, the consequences of each possible scenario are estimated. An efficient animal must have the ability to anticipate, weight and choose. This weighting occurs at a hierarchically higher level and results in signals which possess a coordinative function in activating the appropriate motivational state, response selection, activation of associated networks and maintenance of attention.

Higher cognitive executive centres perceive and recognize such signals and integrate ongoing behaviour with internal representations about the past and expectations within the context of the signal induced state. Humans experience these simultaneously-occurring processes as awareness. The nature of the subjective experience may vary from an emotional state to reflexive consciousness depending on the cognitive abilities of the species and the stage of development and the level of arousal in the individual.

Time course of discriminative stimulus effects of bretazenil and chlordiazepoxide in rats

The time course of the discriminative stimulus effects of a benzodiazepine partial agonist, bretazenil, and a benzodiazepine full agonist, chlordiazepoxide, were determined in rats after administration of two doses of either drug. As in man, bretazenil was considerably shorter-acting than chlordiazepoxide, with 0% drug-appropriate responding at the 2-h time point after the training dose of 7 mg/kg and < 20% drug-appropriate responding at 9 h after the 14 mg/kg dose. With chlordiazepoxide, responding on the water-appropriate lever did not occur in all rats until 7 h after administration of the training dose of 7 mg/kg, compared to 17 h after administration of 14 mg/kg. Although there was considerable individual variability with both drugs, it would appear that the drug discrimination procedure can be a valuable tool for studying the time course of the interoceptive effects of psychoactive drugs.

Effects of site-selective NMDA receptor antagonists in an elevated plus-maze model of anxiety in mice

NMDA receptor antagonists have been shown to be anxiolytic in animal models of anxiety, although they have not been tested extensively. These compounds bind to several specific sites within the NMDA-receptor complex, including the NMDA site itself, the phencyclidine site, and the strychnine-insensitive glycine site. The purpose of the present study was to examine potential anxiolytic effects of site-selective NMDA receptor antagonists in the elevated plus-maze. Drug-naive albino mice were placed in the center of an elevated maze shaped like a plus sign. Two opposing arms were enclosed by high walls; the crossing arms were open. Following injection with drug or vehicle, the number of entries and time spent in each type of arm were measured during 5-min tests. Analysis of results showed that the benzodiazepine, diazepam, and the competitive NMDA receptor antagonist, NPC 17742 (2R,4R,5S 2-amino-4,5-(1,2-cyclohexyl)-7-phosphono-heptanoic acid), increased number of open arm entries and open arm time. N-Nitro-L-arginine methyl ester, a nitric oxide synthase inhibitor which may interfere with the transduction of NMDA receptor activation, also increased open arm entries and time; however, the magnitude of these increases was small. The phencyclidine-site NMDA receptor antagonist, phencyclidine, increased open arm entries, but failed to significantly increase open arm time. ACEA 1021 (5-nitro-6,7-dichloro-1,4-dihydro-2,3-quinoxalinedione), a putative glycine-site antagonist, had significant effects only on open arm entries at the highest dose tested. These results suggest that NMDA receptor antagonists show promise as potential anxiolytic agents, but that differences among antagonists acting at different cellular sites may be expected.

Defeat engenders pentylenetetrazole-appropriate responding in rats: antagonism by midazolam

Defeat and the threat of defeat by an aggressive conspecific is stressful and may engender an anxiety- or fear-like state in animals; the present experiment investigated whether defeat generalized to the discriminative stimulus properties of PTZ and how benzodiazepine receptors were involved in this generalization. Separate groups of male Long-Evans rats (Rattus norvegicus) were trained to discriminate 20 mg/kg pentylenetetrazole (PTZ) or 0.4 mg/kg midazolam (MDZ) from saline in a two-choice drug-discrimination task. After establishing stimulus control, PTZ- and MDZ-trained rats were exposed to an aggressive conspecific which resulted in defeat, as defined by the display of defensive and submissive postures as well as audible and ultrasonic vocalizations. Administration of saline after defeat resulted in greater than 80% PTZ lever selection in 15 out of 25 PTZ-trained rats; this effect was attenuated through pretreatment with MDZ (1 mg/kg). Furthermore, short-term defeat substitution for the PTZ discriminative stimulus was not accompanied by long-term changes in the post-defeat generalization curves for PTZ and MDZ when compared to pre-defeat generalization curves. Nor did defeat alter the antagonism of PTZ by diazepam (2.5 mg/kg) or MDZ by flumazenil (10 mg/kg). In order further to characterize the necessary features for defeat substitution for the PTZ discriminative stimulus, exposure to a threatening conspecific was also attempted by PTZ-trained rats protected from physical contact with a wire mesh cage. In these tests, saline continued to engender greater than 50% PTZ lever responding in 15 of 25 rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Recent developments in the behavioral pharmacology of benzodiazepine (omega) receptors: evidence for the functional significance of receptor subtypes

Recent research in molecular biology has demonstrated the complexity of GABAA receptors and shown that benzodiazepine (BZ-omega) receptor subtypes have a structural reality. It is therefore appropriate to ask whether the different pharmacological effects produced by benzodiazepines (anticonvulsant activity, anxiety reduction, motor incoordination, learning deficits, characteristic discriminative stimulus effects, tolerance and dependence) are associated with activity at different receptor subtypes. The present paper reviews the literature dealing with the behavioral effects of novel BZ (omega) receptor ligands relevant to the question of the functional significance of the BZ1 (omega 1) and BZ2 (omega 2) receptor subtypes. The only drugs currently available with a considerable degree of selectivity are alpidem and zolpidem. These compounds have relatively high affinity for GABAA receptors containing the alpha 1 subunit (corresponding to the BZ1 (omega 1) subtype) and very low affinity for receptors with the alpha 5 subunit (corresponding to one type of BZ2 (omega 2) receptor). Pharmacological effects observed with these, and other, less selective compounds allow several tentative conclusions to be drawn: (a) Little is known of the role of subtype selectivity in anxiolytic or amnestic effects but compounds with low intrinsic activity may reduce anxiety without giving rise to sedation or motor incoordination and BZ1 (omega 1) selective drugs appear to disrupt memory only at sedative doses; (b) Selectivity for BZ1 (omega 1) receptors may be associated with sleep-inducing activity but not with motor incoordination, suggesting that BZ2 (omega 2) receptors may be of particular importance in mechanisms of muscle relaxation; (c) The discriminative stimulus effects of different BZ (omega) receptor ligands are not identical and differences may be related to receptor selectivity; (d) Compounds with BZ1 (omega 1) selectivity and compounds with low intrinsic activity produce little or no tolerance and dependence. A wider range of selective compounds will be necessary to investigate these factors in detail and many different pharmacological profiles can be expected from drugs with selectivity and different levels of intrinsic activity.

Anxiolytic and antidepressant mechanisms of 5-HT1A drugs in the pigeon: contributions from behavioral studies

The discovery that compounds acting through 5-hydroxytryptamine (5-HT) receptor subtypes can produce anxiolytic and/or antidepressant therapeutic effects in humans has resulted in considerable interest in the role of the 5-HT receptor system in both anxiety and depressive disorders. Because many of the clinically efficacious 5-HT1A anxiolytic drugs are either ineffective or produce inconsistent results in traditional or standard types of preclinical punishment or conflict procedures with rodents and other nonhuman mammals, there is considerable need for alternative behavioral assays sensitive to and selective for these compounds. In contrast to data with nonhuman mammals, 5-HT1A drugs are quite effective in pigeons studied under a punishment procedure. This paper reviews the use of the pigeon conflict procedure as a method for the detection and analysis of potential anxiolytic drugs acting through 5-HT1A receptors. Additionally, recent studies, also with the pigeon, have indicated that, in contrast to the rat, it is possible to establish an antidepressant such as imipramine as a discriminative stimulus, and then to use this procedure to evaluate the neuropharmacological bases for the behavioral and, presumably, therapeutic actions of these drugs. Using the drug discrimination procedure, it has been possible to examine a number of selective compounds that substitute for imipramine, thereby clarifying specific substrates for the antidepressant activity of this and related drugs. The pigeon promises to be a useful species in the pharmacological analyses of novel anxiolytic drugs and provides new approaches to the analysis and understanding of traditional as well as the more recently introduced antidepressant drugs.

Antidepressant-like behavioral effects of serotonin receptor agonists

The clinical discoveries that drugs that stimulate 5-HT neurotransmission, either by inhibiting 5-HT uptake or by stimulating postsynaptic receptors directly, have antidepressant properties has stimulated interest in defining the role of the 5-HT receptor system in the clinical effects of antidepressant drugs. Two approaches are reviewed in this paper that address the neurochemical mediation of the therapeutic effects of antidepressant drugs from the standpoint of animal behavior. The first approach utilizes a behavioral response in rats, the forced swimming test, that correlates well with predicting antidepressant drugs in humans. Studies are reviewed that examined serotonergic compounds in the forced swimming test, from the standpoint of identifying better serotonergic mechanisms involved in the antidepressant response. Both 5-HT uptake inhibitors and 5-HT1A receptor agonists produce effects in the forced swimming test that are similar to those of other classes of antidepressant drugs. In contrast, agonists at other 5-HT receptors or 5-HT receptor antagonists do not produce antidepressant-like behavioral effects. Evidence for an important role of 5-HT1A receptors in the antidepressant response is supported by findings that antagonists of 5HT1A receptors prevent the ability of 5-HT1A receptor agonists to reduce immobility in the forced swimming test. The results of studies interfering with 5-HT neurotransmission, either by inhibition of 5-HT synthesis or by the destruction of 5-HT neurons, favor the idea that the effects of 5-HT1A receptor agonists are produced by the stimulation of postsynaptic 5-HT1A receptors. The second approach for studying the behavioral effects of antidepressant drugs employs drug discrimination studies, conducted using a discriminated taste aversion procedure, to provide a method for studying the discriminative stimulus effects of the antidepressant 5-HT uptake inhibitor sertraline. Rats were trained to discriminate the effects of sertraline (10 mg/kg) from saline. Other 5-HT uptake inhibitors, such as fluoxetine, fluvoxamine and paroxetine, substituted for the sertraline stimulus. High doses of norepinephrine uptake inhibitors, such as desipramine or maprotiline, were required to produce similar effects. These two behavioral approaches promise to be useful for defining the important pharmacological effects associated with the behavioral effects of antidepressant drugs.

Discriminative stimulus properties of a new anxiolytic, DN-2327, in rats

In an operant learning lever-pressing procedure on an FR10 schedule of milk reinforcement, male Wistar rats were trained to discriminate between saline and 3 mg/kg IP DN-2327, a new anxiolytic which acts on benzodiazepine receptors, 3 mg/kg IP diazepam or 15 mg/kg IP pentylenetetrazol (PTZ). More than 80% appropriate lever responding was established after 27, 38 and 44 daily training sessions with DN-2327, diazepam and PTZ, respectively, as the training drug. Although rats trained with DN-2327 dose-dependently generalized to various doses of DN-2327 and diazepam, the cue of DN-2327 was more potent than that of diazepam: ED50 values of DN-2327 and diazepam for stimulus generalization were 0.30 and 0.66 mg/kg, respectively. These animals partially generalized to pentobarbital (1-10 mg/kg) but did not generalize to buspirone (0.1-10 mg/kg). Rats trained with diazepam dose-dependently generalized to various doses of DN-2327, diazepam and pentobarbital with ED50 values of 0.51, 0.47 and 4.5 mg/kg, respectively, but did not generalize to buspirone. In rats trained with PTZ, DN-2327 and diazepam antagonized the discriminative stimulus produced by 15 mg/kg PTZ in a dose-dependent manner with ED50 values of 0.27 and 0.83 mg/kg, respectively, but buspirone neither antagonized nor was able to substitute for the PTZ-induced stimulus. The cue of DN-2327 was antagonized by flumazenil dose-dependently as was that of diazepam.(ABSTRACT TRUNCATED AT 250 WORDS)

Discriminative stimulus effects of omega (BZ) receptor ligands: correlation with in vivo inhibition of [3H]-flumazenil binding in different regions of the rat central nervous system

Rats can be trained to discriminate benzodiazepines (BZ) from vehicle and there is considerable evidence that the stimulus effects of these drugs are mediated by activity at omega (BZ) modulatory sites of the GABAA receptor complex. A number of recent studies, however, have indicated that differences may exist between the discriminative stimulus effects of benzodiazepines and those of certain non-benzodiazepine ligands for the omega (BZ) receptors (e.g. zolpidem, abecarnil). As it is known that several subtypes of omega (BZ) sites are found in the central nervous system, and that drugs such as zolpidem have selectivity for certain subtypes, it is possible that differential stimulus effects may be associated with receptor selectivity. In the present study, correlations were calculated between the potencies of nine compounds with affinity for omega receptors (diazepam, lorazepam, triazolam, clonazepam, alprazolam, zopiclone, suriclone, CL 218, 872 and zolpidem) to substitute for chlordiazepoxide in rats trained to discriminate a dose (5 mg/kg) of this benzodiazepine and the ability of the same compounds to inhibit the binding of [3H]-flumazenil from different structures in the rat central nervous system in vivo. The correlations obtained were: cerebellum 0.46, cortex 0.39, striatum 0.78 (P < 0.05), hippocampus 0.79 (P < 0.05) and spinal cord 0.95 (P < 0.001). These different structures are known to contain different relative concentrations of omega 1 (BZ1) and omega 2 (BZ2) sites with the spinal cord containing the greatest (80%) and cerebellum the lowest (5%) concentration of omega 2 (BZ2) sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Discriminative stimulus properties of RU 33965, a benzodiazepine receptor weak partial inverse agonist

Rats were trained to discriminate the low-efficacy benzodiazepine receptor inverse agonist RU 33965 from vehicle in a two-lever discrimination task on a fixed ratio (FR) 20 schedule. Consistent discrimination was obtained at 0.5 mg/kg PO RU 33965. Both leptazol and stronger inverse agonists (FG7142, S-135, RU 34000) substituted for the cue. The weak inverse agonists/antagonists RU 33094, RU 34030, Ro 15-1788, and ZK 93426 also substituted for the cue with the latter two compounds being particularly potent. The agonist and partial agonists diazepam, RU 33203, and RU 39419 did not substitute for the RU 33965 cue but RU 39419 antagonised it. The full agonists diazepam and loprazolam only consistently antagonised the cue when given IP 5 min pretest. These data suggest that the RU 33965 cue results from its weak inverse agonist activity at benzodiazepine receptors, but kinetic factors must be considered when interpreting drug effects in discrimination studies.

Psychopharmacological properties of calcium channel inhibitors

The previous decade has witnessed a major expansion of knowledge of the role played by voltage-sensitive calcium channels in the function of the central nervous system. Significant progress in the field has been made possible with the broadening use of organic calcium channel inhibitors (CCIs, Ca2+ antagonists), until recently considered almost exclusively as peripherally active antianginal and antiarrhythmic drugs. CCIs, however, do penetrate the blood-brain barrier from the periphery. Autoradiographic studies have established a highly heterogeneous distribution of CCI recognition sites within the brain. The existing evidence suggests that CCIs have marked psychotropic properties. The profile of their central activity is unique and spans a wide range of effects. Nevertheless, question regarding potentially confounding potent peripheral effects of these drugs remain. This paper reviews the psychopharmacology of CCIs, concentrating on preclinical data, but including supportive clinical and biochemical evidence as well. It focuses on these drugs' antidepressant, antidopaminergic (neuroleptic-like), anxiolytic and anticonvulsant effects. CCIs may also modify the reinforcing properties of some addictive drugs.

Involvement of 5-HT1A activity in the discriminative stimulus effects of imipramine

Pigeons were trained to discriminate the tricyclic antidepressant imipramine (3.0 or 5.6 mg/kg) from saline. The selective 5-HT1A agonist 8-OH-DPAT (0.03-1.0 mg/kg) resulted in dose-dependent increases in responding on the key correlated with imipramine administration. Doses of 8-OH-DPAT from 0.3 to 1.0 mg/kg substituted completely for imipramine. NAN-190 (0.3-3.0 mg/kg), a putative 5-HT1A antagonist with affinity for both 5-HT1A and alpha 1 receptors, blocked the discriminative stimulus effects of imipramine and resulted in saline-key responding. The discriminative stimulus effects of imipramine were also blocked by administration of the alpha 1-adrenoceptor antagonist prazosin, suggesting a dual mediation of imipramine through both 5-HT1A and alpha 1-adrenoreceptor systems. Although antidepressants have not been used frequently as stimuli in drug discrimination studies, it may be possible to arrive at a more complete understanding of their neurochemical and behavioral effects using this procedure.