Discriminative and affective stimulus effects of dihydropyridine calcium channel modulators: relationship to antialcohol effects.

Voltage-operated calcium channels (VOCCs) have been implicated in alcoholism. Thus, dihydropyridine (DHP) VOCC antagonists, such as nimodipine, reduce ethanol (EtOH) intake and preference in a variety of animal models of alcoholism. Paradoxically, the DHP VOCC agonist BAY k 8644 also demonstrates antialcohol effects in such models. The antialcohol effects of BAY k 8644 are stereoselective [the "agonistic" (-)-enantiomer being more potent than the "antagonistic" (+)-enantiomer], and are not blocked by pretreatment with nimodipine. The present review summarizes studies on the effects of DHPs in drug discrimination (DD), conditioned taste aversion (CTA), and conditioned place preference (CPP) paradigms, and discusses the possibility that the apparent antialcohol effect of these compounds is related to their discriminative and/or affective stimulus effects. In rats trained to discriminate nimodipine from vehicle, (-)-BAY k 8644 completely generalizes to the nimodipine cue; whereas, in rats trained to discriminate (-)-BAY k 8644, nimodipine completely generalizes to, and is unable to block, the (-)-BAY k 8644 cue. The same stereoselectivity is obtained for BAY k 8644 in DD paradigms and models of alcoholism. The apparent similarity of these profiles of activity suggests that a common neurobiological mechanism underlies the discriminative stimulus and antialcohol effects of DHPs. It appears unlikely, however, that the antialcohol effects of DHPs are based on substitution for, or blockade of, the EtOH cue, as these compounds were not found to generalize to, or block, the EtOH cue. Comparison of the effects of DHPs in CTA and CPP paradigms suggests that the affective stimulus effects of these compounds are dissimilar, and that the mechanism underlying the latter effects is probably not related to the mechanism underlying the antialcohol effects of DHP VOCC modulators.

Hormones of the hypothalamo-pituitary-gonadal axis in drug discrimination learning.

More than 30 years ago, T-maze studies with progesterone indicated that sex hormones have the potential to act as a discriminative stimulus in rats. Despite these early positive findings, the interest in discriminative stimulus properties of sex hormones remained low; few studies were dedicated to the investigation of discriminative stimulus properties of hypothalamo-pituitary-gonadal axis hormones (i.e., LHRH, LH/FSH, sex steroids). Nevertheless, the few studies that were published showed some interesting, and often sex-dependent results. Applying various methodologies (T-, or Y-maze, two-lever drug discrimination, taste aversion procedures), it was found that not only progesterone but also the two other principal sex steroids estradiol and testosterone can serve as discriminative stimuli in rodents. In addition to these gonadal hormones, the hypothalamic peptide LHRH (having a key role in the neuroendocrine regulation of steroid release from the gonads) appears to generate discriminative stimulus properties. Interestingly, recent (but preliminary) studies in postmenopausal women suggest that estradiol (and possibly progesterone) may also function as a discriminative stimulus in human subjects.

5-HT2A and 5-HT2C/5-HT1B receptors are differentially involved in alcohol preference and consummatory behavior in cAA rats.

The present study aimed to investigate the role of serotonin 5-HT2A and 5-HT2C receptors in the control of alcohol preference and consummatory behavior in alcohol-preferring cAA rats. Effects of the 5-HT(2A/2C) receptor agonist, DOI, the 5-HT(2C/1B) receptor agonist, mCPP, the 5-HT(2A/2C) receptor antagonist, ritanserin, and the 5-HT2A receptor antagonist, MDL 100,907, on ethanol (EtOH, 10% v/v) preference and intake, as well as total fluid and food intake were evaluated in a 12-h limited-access two-bottle paradigm. DOI (0.3-3 mg/kg, i.p.) reduced EtOH intake and preference, but not total fluid or food intake; whereas mCPP (1-10 mg/kg, s.c.) reduced EtOH, total fluid, and food intake. Therefore, it is concluded that DOI induces a specific and selective antialcohol effect, whereas mCPP rather induces a general suppressive effect on consummatory behavior. Ritanserin (1-10 mg/kg, i.p.) did not affect EtOH intake and preference, nor total fluid and food consumption. MDL 100,907 (0.1-1 mg/kg, i.p.) induced only a small reduction of food intake at the highest dose tested. Pretreatment with ritanserin (3 mg/kg, i.p.) and MDL 100,907 (0.3 mg/kg, i.p.) blocked the effects of DOI (3 mg/kg, i.p.), but not those of mCPP (10 mg/kg, i.p.). It is suggested that activation of 5-HT2C and/or 5-HT1B receptors results in a general decrease of consummatory behavior, whereas activation of 5-HT2A receptors selectively decreases EtOH intake and preference, as assessed in the cAA rat model of alcoholism.

Comparison of hypericum extracts with imipramine and fluoxetine in animal models of depression and alcoholism.

Clinical evidence suggests that hypericum extracts (Hypericum perforatum L., St. John's wort) have antidepressive properties and may offer an interesting alternative for the treatment of mood disorders. In addition, hypericum extracts, as well as standard antidepressants such as the tricyclic, impramine, and the selective serotonin reuptake inhibitor, fluoxetine, have been reported to be of therapeutic benefit in the treatment of alcoholism, as these compounds may reduce alcohol craving and/or intake in particular subgroups of patients. It was the aim of the present study to compare the effects of hypericum extracts with those of imipramine and fluoxetine in the rat forced swimming test (RFST), a model of depression, as well as in cAA rats, a genetic model of alcoholism. In the RFST, triple i.p. administration of imipramine (3-30 mg/kg) and fluoxetine (3-30 mg/kg) induced a dose-dependent reduction in immobility: the minimal effective dose (MED) being 30 and 10 mg/kg, and the maximal effect being 50% and 57% immobility reduction, for imipramine and fluoxetine, respectively. In this test, the hypericum extracts Ze 117 (Remotiv) and LI 160 (Jarsin) also induced a statistically significant reduction of immobility when administered under the same application schedule (5-40 mg/kg, i.p., triple application). In the case of the hypericum extracts the dose-response relationship was inverted U-shaped with a MED value of 20 mg/kg and a maximal effect of 41% and 32% immobility reduction, for Ze 117 and LI 160, respectively. Interestingly, the anti-immobility effects tended to be more pronounced after subacute (1 week, B.I.D.) treatment with 10 mg/kg of imipramine, fluoxetine, or Ze 117, as compared with acute treatment. This phenomenon is in accordance with clinical experience and suggests that repeated treatment is required for full development of antidepressive effects. In the alcohol-preferring cAA rats, acute i.p. administration of imipramine (3-30 mg/kg), fluoxetine (1-10 mg/kg) and Ze 117 (10-40 mg/kg) dose-dependently reduced alcohol intake in a 12-h limited access two-bottle [ethanol 10% (v/v) versus water] choice procedure: with MED values of 30, 5 and 20 mg/kg, respectively. The anti-alcohol effects of fluoxetine and Ze 1-17 appeared to be specific, as reductions in alcohol intake coincided with reductions in alcohol preference. The present study suggests that hypericum extracts have antidepressant-like properties which resemble those of clinically established antidepressants, and that Remotiv may be an interesting adjunct for the treatment of alcoholism.

Long-term alcohol self-administration and alcohol withdrawal differentially modulate microtubule-associated protein 2 (MAP2) gene expression in the rat brain.

Chronic alcohol intoxication is known to produce neuronal degeneration in the central and peripheral nervous system of experimental animals and of humans. It is suggested that various components of the cytoskeleton undergo profound changes following chronic alcohol use and misuse. Here we studied the expression of the neuronal cytoskeletal microtubule-associated protein 2 (MAP2) following long-term alcohol consumption and subsequent alcohol withdrawal. Alcohol-preferring AA (Alko Alkohol) rats with a high voluntary alcohol consumption for a period of 16 months were compared with age-matched control rats without prior experience with alcohol. For comparison, in a second experiment, heterogeneous Wistar rats that also had voluntary access to alcohol for 8 months were examined following alcohol consumption and withdrawal. In situ hybridization and subsequent dot blot and Northern blot analysis for further quantification revealed that chronically alcoholized animals exhibit markedly decreased MAP2 mRNA levels in several parts of the extrapyramidal system (mainly in the caudate putamen, the substantia nigra pars compacta and the globus pallidus), the mesolimbic system, in several hypothalamic nuclei and in the nucleus inferior colliculus. Other areas such as the hippocampus, frontoparietal cortex and cerebellum were less affected by chronic alcohol intake, however, in these regions the MAP2 mRNA levels were increased during alcohol withdrawal. These results suggest that long-term alcohol self-administration affects central neurons involved in motor control via the influence on the integrity of the cytoskeleton and may thus induce motor dysfunction.

The calcium channel agonist BAY k 8644 reduces ethanol intake and preference in alcohol-preferring AA rats.

Applying a 12-h limited access, two-bottle choice procedure, antialcohol effects of the 1,4-dihydropyridine (DHP) L-type calcium (Ca2+) channel agonist BAY k 8644 were investigated in alcohol-preferring AA rats. In this Wistar line, selectively bred for a high 10% v/v ethanol (EtOH) preference in a free choice situation, effects on EtOH preference and intake, as well as on food and total fluid intake were evaluated for racemic BAY k 8644 (0.1-1 mg/kg IP; 0.25-2 mg/kg PO), its agonistic (-)-enantiomer (0.1-1 mg/kg IP and PO) and its antagonistic (+)-enantiomer (10-50 mg/kg IP and PO). Irrespective of route of application, BAY k 8644 was found to be effective in reducing both EtOH intake and preference (minimal effective dose: 0.5 mg/kg; maximum effect: approximately 60% of baseline levels). The (+)-enantiomer, acting as a low-potency Ca2+ channel antagonist, also reduced EtOH intake and preference, but the effects were not very selective as food intake was also substantially reduced. Moreover, the effects were only obtained at relatively high doses (50 mg/kg). The essential enantiomer involved in the antialcohol effects of BAY k 8644 seems to be the (-)-enantiomer, acting as a strong Ca2+ channel agonist. This latter compound was potent (minimal effective dose: 0.3 mg/kg), very effective in reducing EtOH intake (maximum effect: 29% of baseline level) and preference (26% of baseline) and apparently more selective. Although slightly decreasing over days, effects of (-)-BAY k 8644 on EtOH intake and preference were shown to remain after repeated treatment (10 successive days, 0.3 mg/kg IP). Interestingly, the acute antialcohol effects of (-)-BAY k 8644 (0.3-1 mg/kg IP) could not be antagonized with the DHP L-type Ca2+ channel antagonists nimodipine (0.01-1 mg/kg IP) and (-)-nimodipine (1-30 mg/kg IP). The present results suggest that a mechanism of action other than L-type Ca2+ channel agonism is involved in the antialcohol effects of (+/-)- and (-)-BAY k 8644. Alternatively, it is possible that the previously described antialcohol effects of DHP Ca2+ channel antagonists are not related to antagonistic activity at Ca2+ channels. Finally, it cannot be excluded that a mechanism unrelated to Ca2+ channels is responsible for the antialcohol effects of both DHP Ca2+ channel agonists and antagonists.

Conditioned taste aversion and place preference induced by the calcium channel antagonist nimodipine in rats.

It has become clear that various calcium channel antagonists are able to suppress excessive intake of ethanol in rats. With respect to these findings, it has become of interest whether these drugs can act as rewarding and/or aversive stimulus. Therefore, such affective stimulus effects of the L-type calcium channel antagonist nimodipine and its enantiomers were studied in Wistar rats in a series of conditioned taste aversion (CTA; two-bottle choice procedure) and conditioned place preference (CPP; two-compartment procedure) experiments. Racemic nimodipine (0.95-15 mg/kg IP) was found to induce a dose-dependent CTA, 7.5 mg/kg being the lowest effective dose. Subsequent studies with both enantiomers revealed that the CTA effects of nimodipine are completely dependent on the activity of (-)-nimodipine. With (+)-nimodipine (0.25-90 mg/kg IP), none of the doses tested induced a significant CTA, whereas with (-)-nimodipine clear and dose-dependent CTA effects were noted (0.5-30 mg/kg IP). For this enantiomer, the lowest effective dose was 15 mg/kg. In additional CPP experiments, it was confirmed that (+/-)-nimodipine and (-)-nimodipine have affective stimulus properties, whereas (+)-nimodipine was again an ineffective stimulus (dose used for all drugs: 15 mg/kg IP). Interestingly, the affective stimulus effects as measured with CPP of (+/-)- and (-)-nimodipine turned out to be rewarding, as it was found that both drugs produced a significant place preference. It is concluded from these studies that nimodipine possesses intrinsic affective stimulus effects which are rewarding in nature. Furthermore, these stimulus effects are mediated by the activity of the (-)-enantiomer. Possibly, these rewarding effects of nimodipine may play a role in the reported attenuating effects of this drug on voluntary ethanol intake in rats.

Stimulus properties of the L-type calcium channel agonist BAY k 8644 in rats.

Calcium (Ca(2+)) channels appear to be involved in the regulation of ethanol (EtOH) intake, as indicated by the effectiveness of both L-type Ca(2+) channel antagonists and agonists in reducing EtOH intake in animals. The present study was aimed to investigate rewarding/aversive and discriminative stimulus effects of the Ca(2+) channel agonist BAY k 8644, a compound showing pronounced anti-alcohol effects in rats. Therefore, a series of conditioned taste aversion (CTA), conditioned place preference (CPP) and two-lever drug discrimination (DD) experiments were conducted in Wistar rats, with (+/-)-BAY k 8644 and its enantiomers. After i.p. application, (+/-)-BAY k 8644 (0.0625-1mg/kg), (-)-BAY k 8644 (0.125-1mg/kg) and (+)-BAY k 8644 (2.5-20mg/kg) all induced a dose-dependent CTA. The minimal effective doses (MED) for (+/-)-, (-)- and (+)-BAY k 8644 were 0.25, 0.25 and 10mg/kg, respectively. In a CPP study, however, (+/-)-BAY k 8644 (0.25-2mg/kg, i.p.) showed neither aversive nor rewarding stimulus properties. Rats were trained to discriminate (-)-BAY k 8644 (0.3mg/kg, i.p.), the enantiomer acting as a high potency Ca(2+) channel agonist, from vehicle, in a two-lever DD procedure (ED(50)) value: 0.05mg/kg); full generalisation: 0.1mg/kg). The (-)-BAY k 8644 cue dose-dependently generalized to (+/-)-BAY k 8644 and (+)-BAY k 8644, the enantiomer acting as a low potency Ca(2+) channel antagonist, with ED(50) values of 0.06 and 0.28mg/kg, respectively. Both (+/-)- and (+)-BAY k 8644 produced full generalization at 1mg/kg, the latter compound showing an inverted U-shaped curve (i.e., this was the only dose showing >80% drug lever selection). The stimulus patterns of BAY k 8644 and its enantiomers appear to resemble the anti-alcohol profiles of these compounds. Therefore, commonalities between the stimulus properties of the agonistic and antagonistic enantiomers might provide a clue for the mechanism underlying the anti-alcohol effects of L-type Ca(2+) channel antagonists and agonists.

Comparison of the stimulus properties of ethanol and the Ca2+ channel antagonist nimodipine in rats.

A variety of L-type Ca2+ channel antagonists, including the dihydropyridine derivative nimodipine, have been shown to be effective in reducing ethanol intake and preference in animal models of alcoholism. The behavioral mechanism involved in the anti-alcohol effects of nimodipine are, however, not clear yet. The aim of the present study was to investigate the possibility that the effects of nimodipine on ethanol intake are based on stimulus substitution. Therefore, rats were trained to discriminate ethanol (12.5% w/v, 1000 mg/kg i.p.) from saline in a two-lever food-reinforced drug discrimination procedure (dose range of ethanol tested: 125-1000 mg/kg i.p., ED50 value: 488 mg/kg). In cross-generalization tests with nimodipine (0.15-15 mg/kg i.p.), stimulus substitution was not noted. In addition, a cross-familiarization conditioned taste aversion paradigm was utilized. In rats, 1000 mg/kg i.p. ethanol was used as the reference drug producing a conditioned taste aversion. Effects of preexposure to ethanol (500-1500 mg/kg i.p.) and nimodipine (7.5-30 mg/kg i.p.) on the magnitude of the ethanol-induced conditioned taste aversion were investigated as an index for stimulus similarity between preexposure and reference drug. Preexposure to both ethanol and nimodipine prevented the development of a conditioned taste aversion. Contrary to the drug discrimination results, these latter findings suggest that there may be similarities between the stimulus properties of nimodipine and ethanol. Moreover, the apparent discrepancy between the results obtained in drug discrimination and cross-familiarization conditioned taste aversion suggests that different stimulus properties of ethanol control behavior in both procedures. The finding that, under particular conditions, ethanol and nimodipine appear to share common stimulus properties needs to be further evaluated, as this may be related to the reported anti-alcohol effects of nimodipine and other Ca2+ channel antagonists.

Effects of nimodipine and other calcium channel antagonists in alcohol-preferring AA rats.

Several lines of evidence suggest that L-type calcium (CA2+) channels play a role in excessive ethanol (EtOH) intake. In accordance with this, a considerable amount of antagonists for these ion channels has been found to suppress EtOH intake and preference in various animal models of alcoholism. The aim of the present study was to examine antialcohol effects of L-type Ca2+ channel antagonists in alcohol-preferring AA rats. These rats, a Wistar line selectively bred for a high 10% v/v EtOH preference in a free-choice situation, have thus far not been subjected to systematic investigations with Ca2+ channel antagonists. Therefore, effects on EtOH preference and intake, as well as on food and total fluid intake, were evaluated for the 1,4-dihydropyridine (DHP) derivatives nimodipine, felodipine, isradipine, nicardipine, nifedipine, and nitrendipine, as well as for the phenylalkylamine verapamil and the benzothiazepine diltiazem, utilizing a limited access, free-choice procedure. All DHPs were found to be highly effective in reducing both EtOH intake and preference, without affecting total fluid intake. Irrespective of route of application (IP or PO), the effective dose ranges were found to be very similar across compounds (10-30 mg/kg). Nevertheless, because food intake was also reduced, the effects were not completely selective. For nimodipine, the (-)-enantiomer seemed to be more effective as its (+)-enantiomer, possibly reflecting stereoselectivity at central binding sites. Compared to the DHPs, verapamil produced a similar profile of activity, but diltiazem was found to be ineffective. These results confirm and extend previous findings with L-type Ca2+ channel antagonists obtained in other models of alcoholism and suggest that this class of compounds offers an interesting approach for the pharmacotherapy of alcoholism.

Ethanol intake-reducing effects of ipsapirone in rats are not due to simple stimulus substitution.

The present series of experiments was conducted to investigate whether the previously reported ethanol intake reducing effects of the 5-HT1A receptor agonist ipsapirone could be based on possible stimulus similarities between both compounds. Rats were trained to discriminate ethanol (12.5% w/v, 1000 mg/kg, IP) from saline in a two-lever food-reinforced drug discrimination (DD) procedure. Discrimination criterion was reached after a mean number of training sessions of 42. In subsequent generalization sessions, a dose-response curve was established for ethanol (125-1000 mg/kg, IP, ED50 value: 355 mg/kg). In additional cross-generalization tests with ipsapirone (1-30 mg/kg, IP), stimulus substitution for the ethanol cue was not noted (maximal degree of generalization: 33%, at 10 and 30 mg/kg). To confirm the DD findings that ipsapirone does not substitute for ethanol, an alternative cross-familiarization conditioned taste aversion paradigm (CF-CTA) was utilized. In rats, 1000 mg/kg IP ethanol was used as the reference drug producing a conditioned taste aversion (CTA). It was found that preexposure to ethanol (500-1500 mg/kg, IP) dose-dependently attenuates the CTA produced by this same drug. Full familiarization was noted with 1000 and 1500 mg/kg. In contrast with this, ipsapirone (1-30 mg/kg, IP) failed to abolish ethanol-induced CTA, suggesting again that the ipsapirone stimulus complex is dissimilar to that produced by ethanol. Because the present findings indicate that, in rats, ipsapirone does not substitute for ethanol, it is suggested that the reported ethanol intake-reducing effects of ipsapirone in animal models of alcoholism are not due to simple stimulus substitution.

Stimulus characterization of estradiol applying a crossfamiliarization taste aversion procedure in female mice.

In female mice (n = 240), the estradiol stimulus was characterized by studying preexposure effects of sex steroids and sickness-inducing drugs on estradiol-induced (50 micrograms/kg SC) conditioned taste aversion (CTA). It was established that preexposure to estradiol itself (2-50 micrograms/kg SC) attenuates the development of CTA produced by the hormone. Only partial crossfamiliarization effects were found with progesterone (50-200 micrograms/kg SC) and testosterone (250-1000 micrograms/kg SC), steroids that induce CTA themselves. Preexposure to the sickness-inducing drugs lithium chloride (22 mg/kg SC) and apomorphine (0.1-0.2 mg/kg SC) prevented or substantially reduced the development of estradiol-induced CTA, respectively. It was concluded that only a low degree of stimulus resemblance exists between estradiol and the other principal sex steroids, progesterone and testosterone. In addition, it was concluded that the estradiol stimulus resembles the stimuli produced by sickness-inducing drugs.

Cross-familiarisation conditioned taste aversion procedure as a method to reveal stimulus resemblance between drugs: studies on the 5-HT1A agonist 8-OHDPAT.

In the present study a cross-familiarisation conditioned taste aversion (CTA) paradigm was utilized to reveal stimulus resemblance between the selective 5-HT1A agonist 8-OHDPAT and a variety of serotonergic and non-serotonergic drugs. In male mice, a 0.22 mg/kg dose of 8-OHDPAT was used as the reference compound inducing CTA. Dose-dependent effects of pre-exposure to 24 different test drugs on the magnitude of the 8-OHDPAT-induced CTA were tested as a measure for stimulus similarity between these test drugs and 8-OH-DPAT (the reference compound). Pre-exposure to 8-OH-DPAT itself, ipsapirone, buspirone, RU 24969, sertraline, d-amphetamine, LSD, metergoline and idazoxan effectively prevented the development of CTA induced by 8-OHDPAT. Pre-exposure to apomorphine, diazepam, SCH 23390, LiCl, spiperone, DOI, spiroxatrine, umespirone, pindolol, mCPP, haloperidol, MK 212, clonidine, quipazine and also 5-MeODMT was not effective in completely abolishing the CTA produced by 8-OHDPAT. It is concluded from these results that the relatively simple and fast cross-familiarisation taste aversion method is a suitable paradigm to study similarities in stimulus properties of different drugs.

Intracerebroventricular LHRH may serve as a discriminative stimulus in male rats.

Male Wistar rats (N = 16) were trained to discriminate 5 micrograms/kg LHRH, injected intraperitoneally, from saline in a two-lever, food-reinforced drug discrimination procedure, with an injection-session interval of 45 min. Reliable discrimination of LHRH was acquired within 60 training sessions. Subsequent generalization tests in brain-cannulated animals showed dose-dependent and time-related partial substitution of intracerebroventricular LHRH for intraperitoneal LHRH (ventricle doses ranged from 25-400 ng, and the injection-session intervals ranged from 10-40 min). These results indicate that centrally administered LHRH may serve as a dose- and time-dependent discriminative stimulus in male rats.

Testosterone as appetitive and discriminative stimulus in rats: sex- and dose-dependent effects.

Stimulus properties of subcutaneously injected testosterone were studied in male and female rats. In a conditioned place preference procedure, dose-dependent effects (doses: 0, 0.5, and 1 mg/kg) were observed in males. In females, no place preference could be established (doses: 0, 1, and 3 mg/kg). In addition, 1 mg/kg testosterone acquired discriminative stimulus control in male rats in a taste aversion procedure. Animals injected with this hormone prior to saccharin-LiCl pairings and with its vehicle prior to saccharin-NaCl pairings suppressed fluid intake following the administration of testosterone and not following the administration of the vehicle. Subsequent generalization tests revealed dose-dependent stimulus control of this hormone (range of substitution doses: 0.125-2 mg/kg). It is concluded from the results that at least pharmacological (supraphysiological) doses of testosterone may act as appetitive stimuli in male rats, but not in female rats. Furthermore, in male rats (pharmacological doses of) testosterone also possess discriminative stimulus properties.

Temporal characteristics of appetitive stimulus effects of luteinizing hormone-releasing hormone in male rats.

Conditioned place preference, induced by intraperitoneal injections of 5 micrograms/kg luteinizing hormone-releasing hormone (LHRH), was studied by varying the interval between the injection of LHRH and the conditioning sessions. Place preference was investigated for five presession intervals (0, 15, 45, 75, and 120 min) in separate groups of gonadectomized male rats provided with a subcutaneous testosterone implant. It was shown that the presession interval is an important parameter in the development of LHRH-induced conditioned place preference. Place preference was not observed after conditioning with intervals of 0, 75, and 120 min. With 15 and 45 min, however, a reliable preference was induced by LHRH. This study provides insight into the onset and offset of the appetitive stimulus properties of LHRH in male rats.

Dose-dependent and time-related stimulus properties of LHRH in male but not in female rats.

Male and female rats (N = 32) were trained to discriminate 5 micrograms/kg LHRH from saline in a two-lever, food-reinforced drug discrimination procedure with injection-session intervals of 15 min or 45 min. When the interval was 15 min, neither males nor females were able to discriminate the stimulus conditions. With an interval of 45 min, LHRH showed sex-dependent stimulus properties. Male, but not female, rats reliably discriminated LHRH from saline within 50 training sessions. In males, generalization tests showed dose-dependent and time-related stimulus effects of LHRH (doses ranged from 62.5 ng/kg to 8 micrograms/kg, and intervals ranged from -15 min to -120 min). The results indicate that LHRH may be an essential part of the stimulus complex in male rats but could not gain control over operant behavior in female rats.

Discriminative stimulus properties of estradiol in male and female rats revealed by a taste-aversion procedure.

Gonadectomized male and female rats were trained to discriminate 50µg/kg estradiol-17B(E(2)) from its vehicle in a one-bottle forced-drinking discriminative taste-aversion procedure. The animals were injected SC with E(2) or arachidic oil, 60min prior to daily training sessions during which they had access to a saccharin (Sac) solution for 10min. Animals injected with E(2) prior to Sac-LiCl pairings and with arachidic oil prior to Sac-NaCl pairings acquired hormone discrimination, only suppressing Sac intake following the administration of E(2) and not following the administration of arachidic oil. Both males and females reliably discriminated E(2) from arachidic oil within 28 training sessions. Subsequent generalization tests revealed dose-dependent stimulus control of Sac intake by E(2) in both sexes (range of substitution doses: 0.4-250µg/kg). The results show that E(2) may act as a discriminative stimulus in male and female rats.

Estradiol-induced conditioned taste aversion and place aversion in rats: sex- and dose-dependent effects.

Effects of various doses (0-250 micrograms/kg, SC) of estradiol-17 beta (E2) in a two-bottle choice conditioned taste aversion and a two-compartment conditioned place preference procedure were studied in male and female rats. Dose-dependent taste aversion and place aversion effects of E2 were established, and the conditioned taste aversion procedure was found to be more sensitive in detecting aversive properties of E2 than the conditioned place preference procedure. Although aversive properties of E2 were found in both sexes, the effects were clearly more prominent in males as compared to females. From this study, it was concluded that E2 acts as an unconditioned aversive stimulus in both male and female rats capable of gaining control over different types of behavior by associative learning.

Luteinizing hormone releasing hormone-induced conditioned place-preference in male rats.

Conditioned place-preference induced by intraperitoneal injections of luteinizing hormone releasing hormone (LHRH) was studied in male rats. In Experiment 1, dose-dependent effects (doses: 0, 0.2, 1 and 5 micrograms/kg) were observed in gonadectomized males provided with a subcutaneous silastic implant containing testosterone. Animals injected with 1 or 5 micrograms LHRH developed reliable preference for the LHRH-associated compartment of a two-compartment preference box. The 0 and 0.2 microgram doses were without effect. Experiment 2 further studied the place-preference effects induced by 5 micrograms LHRH, by varying the sex steroid baseline condition of the animals. A significant effect of LHRH on place-preference was found in gonadectomized males with a testosterone or estradiol implant and in gonadally intact males. Differences between these groups were not found. However, in gonadectomized males without steroid substitution, LHRH did not induce place-preference. These data indicate that rewarding properties related to LHRH treatment can be observed in male rats, provided that the males are additionally exposed to sufficient levels of circulating sex steroids.