Effects of melatonin receptor ligands on swim test immobility.

The Flinders Sensitive Line (FSL) rats have been proposed as a genetic animal model of depression because their innately high immobility in the forced swim test is counteracted by antidepressants. This model was used to test the acute and chronic effects of two novel compounds which have either an agonist or antagonist-like effect at melatonin receptors. In the acute study FSL and control Flinders Resistant Line (FRL) rats were unaffected by either the agonist S 20304 (1 or 20 mg/kg) or the antagonist S 20928 (1 or 10 mg/kg) given 1 h prior to a single 5 min swim test. In the chronic study a reduction in immobility in the FSL rats at the highest dose of the agonist was the only significant drug effect. These findings suggest that the melatonin receptor agonist S 20304 may have antidepressant potential.

Insulin-like growth factor I expression alters acute sensitivity and tolerance to ethanol in transgenic mice.

We compared some biobehavioral effects of ethanol in transgenic mice that overexpress insulin-like growth factor I (IGF-I) in brain and in those that exhibit ectopic: brain expression of IGF binding protein I with those in non-transgenic littermate controls. Ethanol-induced sleep in IGF-I transgenic mice was significantly shorter, and in IGF binding protein 1 transgenic mice significantly longer, than in controls. A similar tendency, though not significant, was observed for ethanol-induced hypothermia. The groups did not differ in the degree of ethanol-induced ataxia. IGF-I transgenic mice did not acquire tolerance to either the hypothermic or hypnotic effects of ethanol following 7-day ethanol treatment. In contrast, tolerance in IGF binding protein 1 transgenic mice was significantly more pronounced than in controls. There were no significant differences among the three groups in the peak blood alcohol concentrations or the overall blood alcohol curves following acute ethanol challenge. In general, these data support the prediction made that chronically elevated exposure to IGF-I in IGF-I transgenic mice renders them less susceptible to the effects of ethanol than their non-transgenic siblings, and that overexpression of IGF binding protein 1 has the opposite effect.

Role of taste aversion in calcium channel inhibitor-induced suppression of saccharin and alcohol drinking in rats.

L-type calcium (Ca2+) channel inhibitors suppress drinking of highly preferred solutions of simple carbohydrates, saccharin, or alcohol. The present study was designed to examine whether this decrease in drinking behavior can be explained by the development of consummatory aversion. In the first experiment, the propensity of Ca2+ channel inhibitors to induce conditioned taste aversion (CTA) to 0.1% saccharin was examined using two saccharin/drug injection pairings in saccharin-naive rats. We compared three chemically different drugs: diltiazem, isradipine, and nicardipine. A dose-dependent CTA was observed after both conditioning sessions for all three drugs tested. Interestingly, the lowest dose of nicardipine (i.e., 1.25 mumol/kg), significantly increased saccharin intake. A nonsignificant trend to increase saccharin intake was also observed with the lowest dose of isradipine. We then examined whether nicardipine could similarly induce CTA to a novel taste of alcohol (6%, v/v). The drug failed to produce a significant effect. In the third experiment, we found that nicardipine did not induce CTA (or preference) if the saccharin taste was familar to rats. In the final experiment, the interaction of nicardipine (1.25 and 2.5 mumol/kg) with the ethanol (1.5 g/kg)-induced CTA to saccharin was investigated. The higher dose of nicardipine potentiated the aversive effect of ethanol in the test. Overall, the present results suggest that CTA does not play a major role in Ca2+ channel inhibitor-induced suppression of drinking behavior.

Administration of antidepressants, diazepam and psychomotor stimulants further confirms the utility of Flinders Sensitive Line rats as an animal model of depression.

Flinders Sensitive Line (FSL) rats have been proposed as an animal model of depression because they resemble depressed humans in that they have elevated REM sleep, reduced activity, and increased immobility and anhedonia after exposure to stressors. The present paper reviews experiments on the drug treatment of FSL and control Flinders Resistant Line (FRL) rats related to their utility as an animal model of depression, and presents new information. FSL rats exhibited exaggerated immobility in the forced swim test which is counteracted by the tricyclic antidepressants imipramine and desipramine and the serotonin reuptake blocker sertraline; the low immobility exhibited by the FRL rats is generally unaffected by these compounds. In contrast to these "therapeutic" effects of well recognized antidepressants, lithium and bright light treatment did not alter the exaggerated immobility of FSL rats. Novel data indicated that neither FSL nor FRL rats exhibited alterations in swim test immobility following chronic administration of the psychomotor stimulant amphetamine (2 mg/kg) and the anticholinergic scopolamine (2 mg/kg), which typically reduce immobility after acute administration. However, it was found that the calcium channel blockers verapamil (5 and 15 mg/kg) and nicardipine (10 mg/kg) did reduce the exaggerated immobility in FSL rats following chronic administration, suggesting that these compounds need to be evaluated further in humans. Previous studies have indicated no differences between FSL and FRL rats evaluated in the elevated plus maze, either at baseline or after the administration of diazepam, suggesting that the FSL rat may not differ from controls in anxiety-related behavior. Another recently published study showed that the FSL rat also did not differ from normal Sprague-Dawley rats in startle tests, indicating that the FSL rats do not exhibit behaviors shown in animal models of schizophrenia. These findings confirm the utility of FSL rats as an animal model of depression because the FSL rats do not appear to exhibit behaviors analogous to anxiety or schizophrenia and because they respond "therapeutically" to antidepressants and not psychomotor stimulants.

Isradipine suppresses amphetamine-induced conditioned place preference and locomotor stimulation in the rat.

The locomotor activating and the reinforcing effects of psychomotor stimulants are considered to be correlated with and responsible for the development and maintenance of stimulant addiction. Experiments were conducted to examine the effects of isradipine, the L-type calcium channel inhibitor, on the d-amphetamine-induced (1 mg/kg IP) reinforcement (conditioned place preference) and locomotor stimulation. Isradipine dose-dependently (0.6, 1.2, 2.5 mg/kg IP) attenuated the reinforcing effect of amphetamine. Two higher doses completely blocked the induction of place preference. At these doses isradipine also prevented the increase in the number of intercompartment crosses that was observed in both amphetamine- and vehicle-treated controls. In an acute experiment, isradipine failed to affect locomotor activity on its own either in the place preference boxes or in the open field. Amphetamine increased the open field activity but did not change the number of crosses in the place preference boxes. Only the highest (2.5 mg/kg) dose of isradipine significantly suppressed amphetamine-induced hyperactivity in the open field. The present results suggest that isradipine interferes with amphetamine-derived reinforcement at doses lower than those needed to block the acute locomotor effects of amphetamine. Given the qualitatively similar, previously reported results with verapamil, we conclude that the antireinforcing effects of the L-type calcium channel blockers cannot be exclusively explained by the suppression of psychomotor stimulation. The present results further support the notion that the L-type calcium channel blockers may be effective against stimulant addiction.

Susceptibility of Flinders sensitive and resistant rats to pharmacologically induced seizures.

The Flinders sensitive (FSL) and Flinders resistant (FRL) line rats have been selectively bred for hyper- and hyposensitivity to the hypothermic effect of cholinergic agonists respectively. In this study, pilocarpine (250 mg/kg) and physostigmine (0.8 mg/kg) doses that are subconvulsant to outbread Sprague-Dawley rats were systemically injected to the FSL and FRL rats and a heterogenous F2 cross. All of the FRL rats developed severe motor limbic seizures in response to pilocarpine, while none of the FSL animals did. The F2 crosses showed intermediate reaction. The FRL rats were also more affected by physostigmine than the other two groups. However, the FSL rats were confirmed to be more sensitive to the hypothermic effects of pilocarpine (20 mg/kg) and physostigmine (0.6 mg/kg). Picrotoxin and kainic acid produced similar responses in the both lines, i.e., induced clonic convulsions in some animals when applied in subthreshold doses (2 and 10 mg/kg, respectively). Thus, the normally cholinergic-insensitive rats are more sensitive to the convulsant effects of high doses of cholinergic agonists, but this increased sensitivity does not extend to noncholinergic convulsants.

Inhibition of nitric oxide synthesis attenuates alcohol consumption in two strains of alcohol-preferring rats.

The effect of the nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) on voluntary alcohol consumption was examined in two different strains of alcohol-preferring rats, in a continuous-access, two-bottle-choice paradigm. Compared with the vehicle, intraperitoneal injections of L-NAME significantly and dose-dependently (10, 30, and 60 mg/kg) suppressed alcohol intake and preference in both alcohol-preferring (P) and Fawn-Hooded (FH) rats. The effect of the highest dose of L-NAME was nonspecific; it caused general decreases in consumption of alcohol, water, and food. Repeated injection of L-NAME (30 mg/kg) for 4 consecutive days significantly attenuated alcohol intake, but tolerance developed after 3 days of treatment. A single administration of a high dose of L-NAME (60 mg/kg) did not influence the blood alcohol concentrations, which suggests a possible central effect. Furthermore, a moderate dose of 30 mg/kg L-NAME, which selectively inhibited alcohol intake, did not exert a significant effect on telemetrically measured heart rate, core body temperature, and gross motor activity of alcohol naive Fawn-Hooded rats. These results suggest an involvement of nitric oxide in alcohol drinking behavior. Although the true mechanism(s) of action is not yet clear, it can be speculated that L-NAME may exert its action indirectly by modulating neurotransmitters proposed to be involved in alcohol drinking and/or by influencing other neuronal factors, such as neuronal Ca2+ channels, which have been shown to be involved in alcohol drinking behavior.

Effects of chronic mild stress on serum complement activity, saccharin preference, and corticosterone levels in Flinders lines of rats.

Complement proteins and fragments participate in the induction and modulation of specific and nonspecific immune reactions. We have examined the effect of 4 weeks of chronic mild stress (CMS) on complement sheep red blood cell hemolytic activity measured in CH50 units in two selectively bred lines of rats, the Flinders resistant line (FRL) and the Flinders sensitive line (FSL), that differ in cholinergic sensitivity and behavioral characteristics. Additionally, CMS-induced hedonic deficit (decreased preference for 0.02% saccharin over water) and serum corticosterone levels were compared in FRL and FSL rats. CMS caused a significantly (p < 0.01) greater decline in CH50 responses in FSL (-15%) than in FRL (-7%) rats. This was accompanied by a significant (p < 0.01) suppression of saccharin preference over a 24 h period in both FRL and FSL rats. Both lines showed a similar, more than 2-fold (p < 0.01) increase in corticosterone levels following CMS. These results further confirm that CMS induces a depressive-like state in rats as well as the validity of the FSL rat as a genetic model of depression. They also indicate that the effect of stress on the immune system can be monitored by measuring the complement CH50 response.

Calcium channel inhibitors attenuate consumption of ethanol, sucrose and saccharin solutions in rats.

Ca(2+) channel inhibitors suppress ethanol intake in various strains of alcohol-preferring rats. To test whether that inhibitory effect involves interference with the caloric consequences of preferred fluids, we compared the effects of two dihydropyridines, nicardipine and isradipine, and diltiazem, a benzothiazepine Ca(2+) channel inhibitor, on intake of and preference for 10% ethanol (v/v), 13.7% sucrose (w/v, isocaloric to 10% ethanol) and non-caloric 0.02% saccharin (w/v) solutions. All of these Ca(2+) channel inhibitors dose-dependently (1.25-5µmol/kg isradipine, 5-20µmol/kg nicardipine, 10-40µmol/kg diltiazem) inhibited ethanol intake in alcohol-preferring P rats. Nicardipine and isradipine, but not diltiazem, simultaneously increased water intake and attenuated preference for ethanol. No change in food pellet intake was found. Similar inhibition was observed with regard to sucrose and saccharin intake in Sprague-Dawley rats, although higher doses (> 10µmol/kg) were required to achieve that effect. No significant changes in sucrose preference, as opposed to consumption, were observed with any drug, and only nicardipine at its highest dose (20µmol/kg, b.i.d.) significantly (p < 0.05) decreased preference for saccharin. In conclusion, all three Ca(2+) channel inhibitors significantly suppress consumption of caloric solutions of ethanol and sucrose, as well as of non-caloric saccharin solution. Although an accompanying decrease, in preference is evident only for alcohol, it appears that these effects may be related to decreased palatability of flavored fluids. However, the contribution of changes in ethanol- and sucrose-induced caloric satiety cannot be ruled out.

Swim test immobility co-segregates with serotonergic but not cholinergic sensitivity in cross-breeds of Flinders Line rats.

The Flinders Sensitive Line (FSL) rat, a genetic animal model of depression, was cross-bred with its normal control, the Flinders Resistant Line (FRL) rat, in order to investigate the relationship between cholinergic sensitivity, the selected variable, and two apparent genetically correlated variables, serotonergic sensitivity and swim test immobility. Cross-breeding established F1, F2 and back-cross progeny, with at least 20 rats of each sex for each group. Cholinergic sensitivity was assessed as the hypothermic response to oxotremorine (0.2 mg/kg) in 30 day old rats. Serotonergic sensitivity was assessed as the hypothermic response to 8-OH-DPAT, a serotonin (5-HT)-1A agonist, in 35-40 day old rats. Immobility was assessed as the time spent immobile in a 5 min swim test in 60-70 day old rats. For each variable, there were highly significant group differences, with the parental FSL and FRL groups being at the extremes. The segregating populations tended to be intermediate between the parental lines and were generally significantly different from both FSL and FRL groups. However, the crosses more closely resembled the FRL parent for only the cholinergic responses, the distributions for 8-OH-DPAT and immobility suggesting predominantly additive genetics. Statistical analyses with chi square to compare response distributions and regression to quantify the association between variables in the segregating populations confirmed that cholinergic sensitivity was different from serotonergic sensitivity and immobility, which were significantly correlated with each other.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid selection for serotonin-1A sensitivity in rats.

National Institutes of Health (NIH) heterogeneous stock (HS) rats were obtained and genetically selected for either larger (HI line) or smaller (LO line) hypothermic responses to the selective serotonin-1A (5-HT1A) agonist 8-hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT). A randomly bred (RA) control line was also bred in parallel. There was a rapid response to selection, with HI and LO S1 progeny already showing significantly different hypothermic responses to DPAT. The data for the S3 progeny indicated that selection was proceeding in both directions, with the hypothermic responses of the LO line being about 0.5 degrees C less than that of the RA line (p < 0.01) and the hypothermic response of the HI line being about 0.7 degrees C greater (p < 0.01). The selected lines also differed in their hypothermic responses to the cholinergic agonist oxotremorine, but these differences did not change with further selection. These findings indicate that selection for 5-HT1A sensitivity may occur quite rapidly and that changes in muscarinic sensitivity do not parallel those changes in serotonergic sensitivity.

5-HT receptors: implications for the neuropharmacology of alcohol and alcoholism.

The involvement of serotonergic mechanisms in the neuropharmacology of alcohol was appreciated before it was recognized that there were multiple subtypes of serotonin (5-hydroxytryptamine; 5-HT) receptors. Thus, it was known that manipulations of the central serotonergic system could lead to a modification of the rate of tolerance development to alcohol (Frankel et al., 1975) or to a modulation of alcohol intake (Myers and Martin, 1973; Myers and Melchior, 1975) before Peroutka and Snyder (1979) first suggested that there were at least two subtypes of 5-HT receptors. Since these early reports were written, there has been a wealth of studies which have continued to support a role for 5-HT in the regulation of alcohol intake (See McBride et al., 1993b; Sellers et al., 1992, for reviews). Simultaneously, a tremendous expansion in the number of known 5-HT receptor subtypes has occurred (See Peroutka, 1988). However, there have not been, to our knowledge, any papers which have examined the possible role of specific 5-HT receptor subtypes in the regulation of alcohol's central effects. The present review addresses this deficiency in the literature. This review will focus on three major areas: the pharmacological regulation of alcohol intake; differences in 5-HT receptor subtypes among alcohol-preferring and -nonpreferring rat strains; and alterations in 5-HT receptor subtypes following chronic exposure to alcohol.

Chronic mild stress-induced anhedonia: greater effect in a genetic rat model of depression.

The effects of acute and chronic stressors on saccharin intake and preference in the hypercholinergic Flinders Sensitive Line (FSL) rat, a putative genetic animal model of depression, were studied and compared to the control Flinders Resistant Line (FRL) rats. Overall, the FRL rats drank significantly less saccharin and water than the FSL rats when compared over a wide range of saccharin concentrations (0.01-5%) under baseline conditions. A 0.02% saccharin concentration was used in subsequent experiments. We observed a significant suppression of saccharin intake/preference at 1 h following a single 5-min exposure to cold swim stress only in FSL rats. There was a tendency to increase saccharin intake in both lines at 1 h following a scrambled foot shock stress. These effects of acute stressors disappeared upon retesting for saccharin consumption/preference 23 h after the stress. Chronic 4-week exposure to unpredictable mild stressors significantly (p < 0.01) decreased saccharin consumption in the FSL rats, but not in the FRL rats. The FSL rats also exhibited a significantly greater decrease in saccharin preference (-24% vs. prestress baseline, as compared to -7% in FRL controls, p < 0.05). In conclusion, FSL rats appear more prone than the FRL rats to chronic, as well as immediate acute, stress-induced anhedonic effects. This outcome further supports the notion that the FSL rat is a useful model of a genetic predisposition to depressive-like reactions.

Reduction of spontaneous alcohol drinking and physical withdrawal by levemopamil, a novel Ca2+ channel antagonist, in rats.

Neuronal Ca2+ channels have been shown to be involved in both alcohol drinking behavior in rats and nonhuman primates and in the manifestation of alcohol withdrawal symptoms in rodents. Experiments were performed to determine the effect of a single injection of levemopamil, a novel Ca2+ channel antagonist with antiserotonergic [5-hydroxytryptamine2 (5-HT2)] properties, on alcohol preference and alcohol withdrawal symptoms in alcohol-preferring (P) and Wistar rats, respectively. P rats were individually housed and provided free access to food, water, and a solution of 10% (v/v) ethanol. Ethanol, food, and water intakes were measured daily. After establishing a stable baseline, P rats were injected with levemopamil (0, 3.3, 10, 15, and 20 mg/kg) and their food, water, and alcohol intakes measured 24 h later. In a separate experiment, the ability of acute and chronic (12 consecutive days) administrations of levemopamil to suppress alcohol withdrawal symptoms in chronically alcohol-treated rats was studied. In addition, the effects of levemopamil on the level of monoamines in different areas of the brain, as well as its action in alcohol metabolism, were examined. Our findings showed that a single administration of levemopamil (10, 15, and 20 mg/kg) significantly and dose-dependently attenuated alcohol intake and increased water intake in P rats. Both acute and chronic treatment with levemopamil reduced the alcohol withdrawal symptoms, overall seizure scores, and proportion of rats seizing. A single injection of levemopamil produced a clear, but not significant, trend to increase the 5-HT turnover rate in certain brain areas. This drug did not influence the pharmacokinetics of alcohol.(ABSTRACT TRUNCATED AT 250 WORDS)

Verapamil suppresses d-amphetamine-induced place preference conditioning.

The effect of pretreatment with (+/-)-verapamil (5, 10 or 15 mg/kg, i.p.) on place preference induced with d-amphetamine (1 mg/kg, i.p. 40 min after verapamil) was studied in male rats. Place preference conditioning was performed using two-compartment shuttle boxes and 8 alternating stimulant/saline sessions. Verapamil dose-dependently suppressed amphetamine-induced place preference. No significant changes in place preference were observed following 8 alternating verapamil (no stimulant)/saline sessions, irrespective of whether verapamil injections were paired with the originally less or the originally more preferred compartment. It appears that verapamil effectively suppresses the reinforcing properties of d-amphetamine in the paradigm used.

Effect of chronic antidepressant treatment on responses to apomorphine in selectively bred rat strains.

The purpose of this study was to verify the dopamine-sensitizing behavioral effect of chronic antidepressant treatment in two selectively bred rat strains: the hypercholinergic Flinders Sensitive Line (FSL) and control Flinders Resistant Line (FRL). Two antidepressants, desipramine HCl (DMI) and sertraline HCl, were injected IP in separate groups of FSL and FRL rats in a dose of 16.5 mumol/kg twice daily for 16 days. Twenty-four hours after withdrawal, locomotor and hypothermic responses to 0.2 mg/kg of apomorphine, SC, were examined. Attenuation of the effect of apomorphine was observed in the open field: FRLs withdrawn from sertraline were significantly less mobile than control FRLs, and the same trend was found in FSL rats. Chronic DMI resulted in similar changes in the locomotor activity. Sertraline treatment decreased apomorphine-induced hypothermia by almost half in FSLs, whereas slight hyperthermia was induced in FRL rats instead. The present results suggest that in these selectively bred strains, a serotonergic antidepressant such as sertraline may have sensitized dopaminergic autoreceptors and/or desensitized postsynaptic receptors. Apomorphine-induced hypothermia could be mediated by serotonergic neuron function that may have been altered by chronic sertraline but not DMI treatment.

Immobility-reducing effects of antidepressants in a genetic animal model of depression.

Chronic treatment with the tricyclic antidepressants imipramine (15 mg/kg) and desmethylimipramine (5 mg/kg) significantly reduced the exaggerated immobility normally exhibited by the Flinders Sensitive Line (FSL) rats in the Forced Swim Test. The control group, Flinders Resistant Line (FRL) rats were only slightly affected. In contrast, chronic treatment with the anticholinesterase diisopropyl fluorophosphate at doses known to down regulate muscarinic receptors did not alter swim test immobility in either FSL or FRL rats. Our findings support the validity of the FSL rats as an animal model of depression and suggest that serotonergic and/or noradrenergic, but not cholinergic mechanisms, may underlie the exaggerated immobility of the FSL rats.

Ethanol and complement hemolytic activity of selectively bred hypercholinergic rats.

Depression and alcoholism are associated with impaired immune responses. Complement proteins and fragments participate in the induction and modulation of both specific and non-specific immune reactions. This study examined the effect of prolonged ethanol ingestion on complement CH50 levels in two strains of rats, the Flinders Resistant Line (FRL) and the Flinders Sensitive Line (FSL), that differ in cholinergic sensitivity and depressive tendencies. Chronic ethanol exposure given as either the source of drinking fluid or as a liquid diet had a significant inhibition on mean CH50 unit responses in both FSL (41-48%) and FRL (23-24%) rats. The difference in group response to ethanol was confirmed by a significant interaction of ethanol treatment versus group in the two-way ANOVA. The FSL rats appear to be more easily affected than FRLs. Genetic differences in the neurotransmitter systems, therefore, may play a role in susceptibility to immunosuppression resulting from ethanol exposure.

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.