Models for depression in drug screening and preclinical studies: Future directions

The basic consideration in the field of antidepressants is that tests to model depression do not exist, as depression etiopathology is unknown. So far, any kind of proposed model for depression needs to satisfy construct, face and predictive validities. In the present editorial, this idea is challenged, based on the fact that “old” methods can only reveal therapeutical “me-too” drugs and that there is no longer a need of therapeutical “me-too” drugs in the field of antidepressants. Since reduction in the number of antidepressant non-responders is a real medical need, the predictive validity of animal models will be challenged in the future, as the new methods should be based on antidepressant-insensitive animals. Moreover, antidepressants exert similar effects in depressed and non-depressed subjects, but mood normalization is only induced in depressed patients. This implies that the use of normal cells and animals only involves pharmacological rather than therapeutical actions of drugs. Therefore, the use of environmental-induced changes, in the hope that these can evidence antidepressant-insensitive animals, will predominantly be used in the future. In the choice of experimental settings, other factors need to be taken into consideration: (1) gender of animals, as depression affects females more than males, (2) natural rhythmicity in drug effects; (3) pharmacokinetics; and (4) possible biomarker(s) to be measured. There are no golden recipes to discover new antidepressants but the experimental long-term strategy should very clearly be declared before starting the experiments.

Rodent models of depression: learned helplessness using a triadic design in rats

Certain types of human depression are precipitated by stressful life events, and vulnerable individuals experiencing these stressors may develop clinical depression. Understanding the neurobiology of stress vulnerability (depression) as well as stress resiliency (coping) is critical for guiding the development of novel pharmacotherapeutic agents for stress-related disorders such as depression in humans. The use of a triadic design (escapable shock, yoked-inescapable shock and restrained control) allows the investigator to examine the various sequella of stress exposure, while manipulating and quantifying the impact of psychological dynamics of stress such as active behavioral coping (i.e., stress control). Both escape and yoked subjects are exposed to the identical amount, intensity, pattern and duration of stress. The critical distinction between these two groups is that the escape group has the opportunity to terminate the shock stress by turning a wheel at the front of a chamber, while wheel-turning for the yoked subject is of no consequence. Any difference observed between the escape and yoked subjects is a result of the effects of coping, rather than stress exposure per se. The restrained group is included to control for the effects of handling. Any differences between this group and the escape and yoked subjects reflects the impact of stress per se.

Single and repeated stress-induced modulation of phospholipase C catalytic activity and expression: role in LH behavior

PI-PLC, a critical enzyme of the phosphoinositide (PI) signaling pathway, mediates many physiological functions in the brain, including cellular plasticity. Stress-induced learned helplessness (LH) in animals serves as a model of behavioral depression. Recently, we observed that repeated stress prolongs the duration of LH behavior in rats, enabling us to compare neurobiologic abnormalities in acute and chronic depression. Here we examine whether LH behavior is associated with alterations in phospholipase C (PLC), and whether repetition of inescapable shock has similar or dissimilar effects on PLC to those of the single-stress paradigm. Rats were exposed to inescapable shock either once on day 1, or twice, on days 1 and 7. Rats were tested for escape latency on days 2 and 4 after day 1 inescapable shock or on days 2, 8, and 14 after day 1 and 7 inescapable shock. PI-PLC activity and mRNA and protein expression of three different PLC isozymes were determined in the frontal cortex and hippocampus. Higher escape latencies were observed in LH rats tested on day 2 after single inescapable shock and on day 14 after repeated inescapable shock. Single inescapable shock reduced PI-PLC activity in the frontal cortex and hippocampus of LH rats. On the other hand, repeated inescapable shock not only reduced PI-PLC activity in these brain areas of LH rats but also selectively decreased the expression of PLC beta1 and PLC gamma1 isozymes. Our results suggest different responsiveness at the level of PI-PLC after single vs repeated stress, and that reductions in PLC may be critical in the pathophysiology of depression and other stress-related disorders.

Behavioral analysis of stress controllability effects in a new swim stress paradigm

Previous animal stress studies have illustrated the marked impact of coping on subsequent behavior and physiology by using shock as the stressor. The current study evaluates the generality of shock stress controllability effects in a new swim stress paradigm on several dependent measures: behavioral despair, analgesia, shuttlebox escape, and alcohol reactivity. In this new paradigm, rats in the escape group are able to learn the behavioral response as evidenced by significant reduction in the acquisition of a lever press response. Both escape and yoked subjects showed "behavioral despair" in comparison to both restrained and home cage controls when tested 24 h later. In the standard shuttlebox escape task 24-h post-stress, no group differences emerged, although a trend for poorer performance in the yoked subjects was evident. No group differences were observed in pain sensitivity after the first or second forced swim exposure. Finally, stress controllability effects were observed in behavioral reactivity to alcohol 2-h post-stress as measured by rotarod performance. This effect is opposite to the previous observations with the tailshock stress controllability paradigm. These results suggest that (1) there are certain similarities, but some fundamental differences between the behavioral endpoints measured following intermittent swim stress in comparison to the well-established effects of the intermittent tailshock stress model and (2) the qualitative nature of a stressor may markedly influence the behavioral and physiological consequences of stress and coping.

Selective effects of 8-OH-DPAT on social competition in the rat

Previous research has demonstrated that dominant-subordinate relationships measured in small groups of rats competing for access to palatable food or fluids can be disrupted by both anxiolytic and anxiogenic drugs, and it has been proposed as a possible animal model of anxiety. The present study investigated the effects of the selective 5-HT1A agonist 8-OH-DPAT on the rank order of triads of rats measured in terms of access to sweetened milk. The effect of 8-OH-DPAT on locomotor activity and intake of sweetened milk was also determined. 8-OH-DPAT (25 and 37.5 micrograms/kg) significantly increased the subordinate animals position in the social hierarchy without effect on the individual intakes of sweetened milk or locomotor activity. The same doses administered to dominant animals had no effect on any of the parameters measured. The 8-OH-DPAT-induced increase in social competition in subordinate rats was dissociable from effects on feeding behavior and locomotor activity. The results from this study provide further evidence that social competition in groups of rats may represent a model that can be used to detect drugs acting via receptor mechanisms believed to be implicated in anxiety.

The effect of 5-HT1A receptor ligands in a chronic mild stress model of depression

Antidepressant properties of 5-HT1A receptor ligands (the full agonist 8-OH-DPAT, the partial agonists ipsapirone and buspirone, and the selective antagonist WAY 100135) were studied in a chronic mild stress model of depression. In this model, rats subjected to a variety of mild stressors for a prolonged period of time show a substantial decrease in the consumption of a 1% sucrose solution (anhedonia), an effect being sensitive to repeated treatment with antidepressant drugs. In the present study we found that the stress-induced deficit in the sucrose intake was gradually reversed by chronic (3-5 weeks) administration of buspirone (2.5 and 5 mg/kg, i.p., b.i.d.) or WAY 100135 (10 mg/kg, s.c., b.i.d.), but not 8-OH-DPAT (0.5 mg/kg, s.c., b.i.d.) or ipsapirone (5 mg/kg i.p., b.i.d.). The magnitude of the effect of buspirone and WAY 100135 was comparable to that observed following similar administration of the antidepressant drugs imipramine (10 mg/kg i.p.) or citalopram (10 mg/kg i.p.). Increases in the sucrose intake following chronic treatment with buspirone, WAY 100135, imipramine and citalopram were specific to the stressed animals; the behaviour of control non-stressed animals was unchanged by any drug. These results suggest that buspirone and WAY 100135 may have antidepressant properties. Possible links between the anti-anhedonic effect of these drugs and their interaction with 5-HT1A receptors and/or the dopamine system are discussed.

Beta-adrenergic receptor subtypes in stress-induced behavioral depression

The purpose of this study was to examine the role of beta-adrenergic receptors in an animal model of stress-induced behavioral depression. beta-Adrenergic receptors in several brain regions and leukocytes of rats were determined by receptor binding techniques using 125I-cyanopindolol (cyp) as ligand and propranolol as displacer for total beta-adrenergic receptors, and ICI 86,406 for beta 1- and ICI 118,551 for beta 2-adrenergic receptors. We observed that the maximum number of binding sites (Bmax) and the apparent dissociation constant (Kd) of 125I-cyp binding to total beta-adrenergic receptors were increased in hippocampus of stressed rats with escape deficits (48 h after training) as compared to control rats. This increase was due to an increase in Bmax and Kd of 125I-cyp binding to beta 1-adrenergic receptors but not to beta 2-adrenergic receptors. There was no significant difference in beta 1-adrenergic receptors in cortex and cerebellum or beta 2-adrenergic receptors in hippocampus, cortex, cerebellum, or leukocytes of stressed (48 h after training) rats with escape deficits as compared to control rats. Interestingly, it was observed that beta 1- and beta 2-adrenergic receptors in various brain regions (cortex, cerebellum, and hippocampus) and beta 2-adrenergic receptors in leukocytes of stressed rats (10 days after training) were not significantly different from control rats, although escape deficits were still present. These results suggest that abnormalities in adrenergic neurotransmission are associated with an upregulation of beta 1-adrenergic receptors, which in turn may be involved in the early stages of behavioral deficits caused by uncontrollable shock.

Learned helplessness sensitizes hippocampal norepinephrine to mild restress

A proportion of rats exposed to inescapable tailshock stress displayed a performance deficit, termed learned helplessness, in a subsequent shuttlebox avoidance task. The technique of in vivo microdialysis was used to determine hippocampal norepinephrine levels in learned helpless, nonhelpless and nonprestressed control rats. Similar basal norepinephrine levels were detected in samples between rat groups. Following an exposure to a milder form of inescapable shock, an increase in norepinephrine output was detected in learned helpless rats, which was significantly greater than nonhelpless, nonprestressed, or control animals. Thus, inescapable stress appears to sensitize the hippocampus to increase norepinephrine release in response to a subsequent smaller stressor. This hypersensitivity might underlie the avoidance impairment of learned helplessness. Therefore, the possibility exists that similar neurochemical changes may also be responsible for some of the symptoms of human posttraumatic stress disorder (PTSD), such as the poor coping associated with seemingly mild stress.

In vivo serotonin release and learned helplessness

Learned helplessness, a behavioral depression caused by exposure to inescapable stress, is considered to be an animal model of human depressive disorder. Like human depression, learned helplessness has been associated with a defect in serotonergic function, but the nature of this relationship is not entirely clear. We have used in vivo microdialysis brain perfusion to measure serotonin (5-hydroxytryptamine, 5HT) in extracellular space of medial frontal cortex in conscious, freely moving rats. Basal 5HT levels in rats perfused before exposure to tail-shock stress did not themselves correlate with subsequent learned helplessness behavior. However, 5HT release after stress showed a significant increase with helpless behavior. These data support the hypothesis that a cortical serotonergic excess is causally related to the development of learned helplessness.

Decreased forebrain [35S]TBPS binding and increased [3H]muscimol binding in rats that do not develop stress-induced behavioral depression

Recent evidence suggests that anxiety and its biological concomitants may be involved in the pathophysiology of depression. In the present study, the in vitro radioligand binding of [3H]flunitrazepam, [3H]muscimol and [35S]t-butylbicyclophosphorothionate (TBPS) sites on the benzodiazepine/GABA chloride ionophore receptor complex (BGRC) was examined using the learned helplessness paradigm. Only rats which did not develop the syndrome showed a significant increase in [3H]muscimol binding in cerebral cortex and a decrease in [35S]TBPS binding in cerebral cortex and hippocampus in comparison to naive controls. For both ligands, this represented a change in Bmax rather than a change in affinity. Adrenalectomy had no impact on these alterations indicating that critical endogenous factors are not manufactured by the adrenal glands. These findings suggest that the BGRC in the forebrain may be a site mediating the 'coping' ability of rats that do not develop the learned helplessness syndrome. The possible involvement of neurosteroids in this effect is discussed.

Learned helplessness and in vivo hippocampal norepinephrine release

Hippocampal norepinephrine release was measured using in vivo microdialysis in rats before and after exposure to inescapable tail shock stress and after testing for learned helplessness. Rats that did not develop learned helplessness after stress had higher basal norepinephrine release after stress than rats developing learned helplessness or than control rats. After the shuttlebox test for learned helplessness, K(+)-stimulated norepinephrine release was lower in learned helpless than in nonhelpless or control rats. These results confirm an important role for the hippocampal noradrenergic system in differential behavioral responses to stress.

Prevention of learned helplessness: in vivo correlation with cortical serotonin

Learned helplessness (LH) is prevented by pretreatment with acute benzodiazepines (BDZs), subchronic tricyclic antidepressants (TCAs), or escapable stress (ES). We have investigated the role of serotonin (5-HT) in LH prevention by these three prevention paradigms, using microdialysis to measure in vivo 5-HT release in frontal cortex (FC) after LH testing. Rats receiving pretreatment before inescapable stress with any of the three methods of prevention--BDZs, TCAs, or ES--showed escape behavior in the shuttle-box test for LH comparable to naive unstressed controls. K(+)-stimulated 5-HT release in all three groups receiving pretreatment was also similar to naive unstressed controls. Rats receiving saline before inescapable stress showed significantly more LH behavior in the shuttle-box task and had significantly lower 5-HT release as well. This suggests that LH correlates with a significant decrease in intracellular releasable 5-HT in FC, and that three different techniques for LH prevention, acute BDZs, subchronic TCAs, and ES all similarly prevent this 5-HT depletion.

5-Hydroxytryptamine1A receptor agonists in animal models of depression and anxiety

The effects of different doses of buspirone, 3-dipropyl-amino-5-hydrochromar (NDO 008) and 8-hydroxydipropyl-aminotetralin (8-OH-DPAT) (administered intraperitoneally) were studied in tests of anxiolytic and antidepressant action in rats. These tests included the elavated plus maze test, the forced swim test, stress-induced suppression of open-field behavior, and the differential-reinforcement-of-low-rates-of-behaviour-72 sec (DRL 72 s) test. Buspirone (0.125 mg/kg) and NDO 008 (1.0 to 2.0 mg/kg) produced anxiolytic activity in the elevated plus maze, whereas 8-OH-DPAT did not in the doses employed. All three compounds increased activity in the forced swim test, although buspirone did so at a lower dose than NDO 008 and 8-OH-DPAT. In the stress-induced suppression test of open field activity all three compounds induced an antidepressant-like effect at different doses dependent on whether footshock (stressor) was presented 24 hr before or just prior to the open-field test. All three compounds even caused some reduction of activity in the non-shocked rats. 8-OH-DPAT (1.0 mg/kg) produced a significant and reliable increase in the Reinforcement/Response rate quotient in the DRL 72s test. These diverse results may provide an indication of potential clinical efficacy of the 5-HT1A agonists in the treatment of anxiety and depression.

Effects of an extract of Ginkgo Biloba (EGB 761) on "learned helplessness" and other models of stress in rodents

The effects of repeated oral administration of an extract of Ginkgo Biloba (EGB 761) on various behavioral models of stress in rodents were investigated. The models in rats included "learned helplessness," shock-suppressed licking (Vogel conflict test) and forced swimming-induced immobility ("behavioral despair"). The models in mice included shock-suppressed exploration (four plates test), spontaneous exploration (staircase test) and food consumption in a novel situation (emotional hypophagia). Further tests in rats examined the effects of EGB 761 on memory (passive avoidance test) and responsiveness to shock to determine whether the preventive effects observed with EGB 761 in the learned helplessness procedure were due either to drug-induced impairment of memory or to reduced shock sensitivity. In all experiments EGB 761 was administered over 5 days at daily doses of 50 and 100 mg/kg PO. In some experiments (Vogel test, four plates test, staircase test, emotional hypophagia) the effects of acute administration were also investigated. The results showed that repeated administration of EGB 761 (50 and 100 mg/kg/day) before exposure to unavoidable shock (preventive treatment) clearly reduced the subsequent avoidance deficits in the learned helplessness procedure but was less effective when first administered after "helplessness" induction (curative treatment). EGB 761 did not affect performance in the passive avoidance task or alter the animals' response to electric shock, suggesting that the effects observed in the learned helplessness procedure were not due to impaired memory or reduced shock sensitivity. Anxiolytic-like activity was also seen in the emotional hypophagia test in mice where repeated administration of EGB 761 increased the amount of food consumed.(ABSTRACT TRUNCATED AT 250 WORDS)

Antidepressant-like activity of ipsapirone, buspirone and gepirone in the forced swimming test in rats pretreated with proadifen

The antidepressant-like activity of ipsapirone, buspirone and gepirone was studied in rats in the forced swimming test (behavioural despair test). lpsapirone and buspirone administered in single doses (5-20 mg/kg) did not affect the immobility time in this test. When administered in the same doses in a three-injection course in 24 h, buspirone was also inactive, while ipsapirone slightly but significantly reduced the immobility time only after a dose of 5 mg/kg. On the other hand, gepirone administered both in single doses (2.5-20 mg/kg) and in a three-injection course (5-20 mg/kg) potently and dose-dependently shortened the immobility time. 1-(2-Pyrimidinyl)-piperazine (1-PP; 5-20 mg/kg), a common metabolite of all the three drugs, administered in single doses or in a three-injection course, was inactive in the forced swimming test. In rats pretreated with proadifen (50 mg/kg), a non-selective drug metabolism inhibitor, both ipsapirone and buspirone administered in single doses (5-20 mg/kg) reduced the immobility time in a dose-dependent manner. Proadifen also potentiated the anti-immobility effect of gepirone (5 and 10 mg/kg). The anti-immobility effect of single doses (20 mg/kg) of ipsapirone, buspirone and gepirone in proadifen-pretreated animals was completely abolished by 1-PP (4 mg/kg). These results indicate that the antidepressant-like activity of the examined drugs in the behavioural despairtest is masked (ipsapirone, buspirone) or attenuated (gepirone) by their metabolite 1-PP.

A pretest procedure reliably predicts performance in two animal models of inescapable stress

Rats exposed to inescapable tailshock fail to learn a shuttle-escape task 24 hours later, an effect referred to as "learned helplessness." However, within most rat strains only 10-50% of the animals tested develop this syndrome. In the present study a significant correlation was found between rats that displayed learned helplessness on the first test and those that displayed learned helplessness on a second test performed either 2 weeks (r = .80, p less than 0.001) or 4 weeks (r = .74, p less than 0.001) later. An analysis of the mean session latency of the shuttlebox task in these two tests suggested a bimodal distribution of animals that failed and learned. A significant correlation was found between individual rats that learned this task on the first test and those which learned this task 2 or 4 weeks later. Similarly, in the "behavioral despair" test, a significant correlation was observed for floating time for individual rats on the first test and on the second test either 2 (r = .72, p less than 0.001) or 4 weeks (r = .63, p less than 0.001) later. However, for the forced-swim test, a unimodal and rather graded response was observed across individual subjects. Thus, performance on the first round predicted performance on the second round in both models. When rats experienced the learned helplessness paradigm on round 1 and the behavioral despair paradigm in round 2, there was no correlation between rats that displayed helplessness following inescapable tailshock and the rats that demonstrated "behavioral despair" on a later test.(ABSTRACT TRUNCATED AT 250 WORDS)

Stress-induced behavioral depression in the rat is associated with a decrease in GABA receptor-mediated chloride ion flux and brain benzodiazepine receptor occupancy

Rats exposed to inescapable tailshock exhibit deficits in learning a simple shuttlebox escape task 24 h later. This syndrome has been termed 'behavioral depression' or 'learned helplessness', and is a model of stress-induced depression. In the present study a significant (25%) decrease in GABA receptor-mediated chloride ion flux as measured by muscimol-stimulated 36Cl- uptake in synaptoneurosomes was found in the cerebral cortices of rats that failed the shuttlebox task as compared to naive control rats. Rats which were exposed to tailshock and subsequently learned the escape task did not show a significant difference in muscimol-stimulated 36Cl- uptake as compared to naive control rats. Similarly, rats that failed to learn the shuttlebox escape task had significantly lower in vivo [3H]Ro15-1788 specific binding in cerebral cortex (43%), hippocampus (35%) and striatum (33%) as compared to naive control rats. In cerebellum and hypothalamus, there were significant reductions in specific [3H]Ro15-1788 binding in both animals that failed and animals that learned the shuttlebox escape task as compared to naive controls. To control the stress of the footshock associated with the shuttlebox escape task, we investigated the effect of gridshock in which total footshock received was equivalent to that received by rats who failed the shuttlebox task. There were no differences in muscimol-stimulated 36Cl- uptake or in vivo [3H]Ro15-1788 specific binding between naive controls and rats administered footshock independent of a learning task. These data suggest that the development of stress-induced behavioral depression may be associated with a decrease in GABA receptor-mediated chloride channel function.