Genetic and pharmacological models of cholinergic supersensitivity and affective disorders

Repeated cannabidiol treatment affects neuroplasticity and endocannabinoid signaling in the prefrontal cortex of the Flinders Sensitive Line (FSL) rat model of depression

Delayed therapeutic responses and limited efficacy are the main challenges of existing antidepressant drugs, thereby incentivizing the search for new potential treatments. Cannabidiol (CBD), non-psychotomimetic component of cannabis, has shown promising antidepressant effects in different rodent models, but its mechanism of action remains unclear. Herein, we investigated the antidepressant-like effects of repeated CBD treatment on behavior, neuroplasticity markers and lipidomic profile in the prefrontal cortex (PFC) of Flinders Sensitive Line (FSL), a genetic animal model of depression, and their control counterparts Flinders Resistant Line (FRL) rats. Male FSL animals were treated with CBD (10 mg/kg; i.p.) or vehicle (7 days) followed by Open Field Test (OFT) and the Forced Swimming Test (FST). The PFC was analyzed by a) western blotting to assess markers of synaptic plasticity and cannabinoid signaling in synaptosome and cytosolic fractions; b) mass spectrometry-based lipidomics to investigate endocannabinoid levels (eCB). CBD attenuated the increased immobility observed in FSL, compared to FRL in FST, without changing the locomotor behavior in the OFT. In synaptosomes, CBD increased ERK1, mGluR5, and Synaptophysin, but failed to reverse the reduced CB1 and CB2 levels in FSL rats. In the cytosolic fraction, CBD increased ERK2 and decreased mGluR5 expression in FSL rats. Surprisingly, there were no significant changes in eCB levels in response to CBD treatment. These findings suggest that CBD effects in FSL animals are associated with changes in synaptic plasticity markers involving mGluR5, ERK1, ERK2, and synaptophysin signaling in the PFC, without increasing the levels of endocannabinoids in this brain region.

Vagus nerve stimulation: a new promising therapeutic tool in inflammatory bowel disease

Inflammatory bowel disease (IBD), that is Crohn's disease (CD) and ulcerative colitis, affects about 1.5 million persons in the USA and 2.2 million in Europe. The pathophysiology of IBD involves immunological, genetic and environmental factors. The treatment is medico-surgical but suspensive. Anti-TNFα agents have revolutionized the treatment of IBD but have side effects. In addition, a non-negligible percentage of patients with IBD stop or take episodically their treatment. Consequently, a nondrug therapy targeting TNFα through a physiological pathway, devoid of major side effects and with a good cost-effectiveness ratio, would be of interest. The vagus nerve has dual anti-inflammatory properties through its afferent (i.e. hypothalamic-pituitary-adrenal axis) and efferent (i.e. the anti-TNFα effect of the cholinergic anti-inflammatory pathway) fibres. We have shown that there is an inverse relationship between vagal tone and plasma TNFα level in patients with CD, and have reported, for the first time, that chronic vagus nerve stimulation has anti-inflammatory properties in a rat model of colitis and in a pilot study performed in seven patients with moderate CD. Two of these patients failed to improve after 3 months of vagus nerve stimulation but five were in deep remission (clinical, biological and endoscopic) at 6 months of follow-up and vagal tone was restored. No major side effects were observed. Thus, vagus nerve stimulation provides a new therapeutic option in the treatment of CD.

Amygdala response to explicit sad face stimuli at baseline predicts antidepressant treatment response to scopolamine in major depressive disorder

The muscarinic antagonist scopolamine produces rapid antidepressant effects in individuals with major depressive disorder (MDD). In healthy subjects, manipulation of acetyl-cholinergic transmission modulates attention in a stimulus-dependent manner. This study tested the hypothesis that baseline amygdalar activity in response to emotional stimuli correlates with antidepressant treatment response to scopolamine and could thus potentially predict treatment outcome. MDD patients and healthy controls performed an attention shifting task involving emotional faces while undergoing functional magnetic resonance imaging (fMRI). We found that blood oxygenation level dependent (BOLD) signal in the amygdala acquired while MDD patients processed sad face stimuli correlated positively with antidepressant response to scopolamine. Amygdalar response to sad faces in MDD patients who did not respond to scopolamine did not differ from that of healthy controls. This suggests that the pre-treatment task elicited amygdalar activity that may constitute a biomarker of antidepressant treatment response to scopolamine. Furthermore, in MDD patients who responded to scopolamine, we observed a post-scopolamine stimulus processing shift towards a pattern demonstrated by healthy controls, indicating a change in stimulus-dependent neural response potentially driven by attenuated cholinergic activity in the amygdala.

The flinders sensitive line rat model of depression--25 years and still producing

Approximately 25 years have passed since the first publication suggesting the Flinders sensitive line (FSL) rat as an animal model of depression. At least 6 years of research on these rats was completed before that seminal paper, and there has been a steady stream of publications (130+) over the years. The present review will focus on several issues not previously covered in earlier reviews, summarize the several lines of ongoing investigations, and propose a novel mechanism that accounts for a number of previously unexplained observations. A key observation in the FSL rat relates to the antidepressant (AD)-like effects of known and putative antidepressants. The FSL rat typically exhibits an AD-like effect in behavioral tests for AD-like activity following chronic (14 days) treatment, although some studies have found AD-like effects after fewer days of treatment. In other observations, exaggerated swim test immobility in the FSL rat has been found to have a maternal influence, as shown by cross-fostering studies and observations of maternal behavior; the implications of this finding are still to be determined. Ongoing or recently completed studies have been performed in the laboratories of Marko Diksic of Canada, Aleksander Mathé of Sweden, Gregers Wegener of Denmark, Brian Harvey of South Africa, Paul Pilowsky and Rod Irvine of Australia, and Gal Yadid of Israel. Jennifer Loftis of Portland, Oregon, and Lynette Daws of San Antonio, Texas, have been working with the FSL rats in the United States. A puzzling feature of the FSL rat is its sensitivity to multiple chemicals, and its greater sensitivity to a variety of drugs with different mechanisms of action. It has been recently shown that each of these drugs feeds through G protein-coupled receptors to potassium-gated channels. Thus, an abnormality in the potassium channel could underlie the depressed-like behavior of the FSL rats.

Enhanced expression of the neuronal K+/Cl- cotransporter, KCC2, in spontaneously depressed Flinders Sensitive Line rats

We used Flinder Sensitive Line (FSL) rats, a genetic model of unipolar depression, to examine whether changes in central GABAergic transmission are associated with a depressed phenotype. FSL rats showed an increased behavioral response to low doses of diazepam, as compared to either Sprague Dawley (SD) or Flinder Resistant Line (FRL) rats used as controls. Diazepam at a dose of 0.3 mg/kg, i.p., induced a robust impairment of motor coordination in FSL rats, but was virtually inactive in SD or FRL rats. The increased responsiveness of FSL rats was not due to changes in the brain levels of diazepam or its active metabolites, or to increases in the number or affinity of benzodiazepine recognition sites, as shown by the analysis of [(3)H]-flunitrazepam binding in the hippocampus, cerebral cortex or cerebellum. We therefore examined whether FSL rats differed from control rats for the expression levels of the K(+)/Cl(-) cotransporter, KCC2, which transports Cl(-) ions out of neurons, thus creating the concentration gradient that allows Cl(-) influx through the anion channel associated with GABA(A) receptors. Combined immunoblot and immunohistochemical data showed a widespread increase in KCC2 expression in FSL rats, as compared with control rats. The increase was more prominent in the cerebellum, where KCC2 was largely expressed in the granular layer. These data raise the interesting possibility that a spontaneous depressive state in animals is associated with an amplified GABAergic transmission in the CNS resulting from an enhanced expression of KCC2.

DHEA lessens depressive-like behavior via GABA-ergic modulation of the mesolimbic system

Alterations in the levels of dehydroepiandrosterone (DHEA) in the brain can allosterically modulate gamma-aminobutyric-acid-type-A (GABA(A)R), N-methyl-D-aspartate (NMDAR), and Sigma-1 (sigma 1R) receptors. In humans, DHEA has antidepressive effects; however, the mechanism is unknown. We examined whether alterations in DHEA also occur in an animal model of depression, the Flinders-sensitive-line (FSL) rats, with the intention of determining the brain site of DHEA action and its antidepressant mechanism. We discovered that DHEA levels were lower in some brain regions involved with depression of FSL rats compared to Sprague-Dawley (SD) controls. Moreover, DHEA (1 mg/kg IP for 14 days)-treated FSL rats were more mobile in the forced swim test than FSL controls. In the NAc and VTA, significant changes were observed in the levels of the delta-subunit of GABA(A), but not of sigma 1R mRNA, in FSL rats compared to SD rats. The delta-subunit controls the sensitivity of the GABA(A)R to the neurosteroid. Indeed, treatment (14 days) of FSL rats with the GABA(A) agonist muscimol (0.5 mg/kg), together with DHEA (a negative modulator of GABA(A)), reversed the effect of DHEA on immobility in the swim test. Perfusion of DHEA sulfate (DHEAS) (3 nM and 30 nM for 14 days) into the VTA and NAc of FSL rats improved their performance in the swim test for at least 3 weeks post-treatment. Our results imply that alterations in DHEA are involved in the pathophysiology of depression and that the antidepressant action of DHEA is mediated via GABA(A)Rs in the NAc and VTA.

Cholinergic pathways modulate experimental dinitrobenzene sulfonic acid colitis in rats

Recent studies have suggested that neuroimmune interactions modulate intestinal mucosal immune responses. In the current study, we examined the role of cholinergic pathways in modulating the severity of acute dinitrobenzene sulfonic acid colitis, using pharmacological agents to suppress acetylcholinesterase in Sprague-Dawley rats, and evaluating the colitis in the cholinergic hyperresponsive Flinder's sensitive line rats and their control counterparts, the Flinder's resistant line. Colitis was induced by intrarectal dinitrobenzene sulfonic acid (80 mg x ml(-1) in 50% ethanol); controls received intrarectal saline. Sprague-Dawley rats received an acetylcholinesterase inhibitor, physostigmine (50 microg x kg(-1) s.c.) or neostigmine (50 microg x kg(-1) s.c.), 30 min prior to intrarectal dinitrobenzene sulfonic acid; controls received saline vehicle. On day 5, the macroscopic damage score, myeloperoxidase activity (an estimate of granulocyte infiltration) and smooth muscle thickness were evaluated in the inflamed colonic segment. Significant increases in macroscopic damage score and colonic smooth muscle thickness were observed in Sprague-Dawley and Flinder's Resistant Line rats on day 5 following intrarectal dinitrobenzene sulfonic acid compared to saline controls. Increased myeloperoxidase activity was also observed in dinitrobenzene sulfonic acid-treated Sprague-Dawley rats and Flinder's Resistant Line rats. In contrast, Flinder's Sensitive Line rats failed to demonstrate a significant rise in macroscopic damage, smooth muscle layer thickness, or myeloperoxidase activity on day 5 following intrarectal dinitrobenzene sulfonic acid when compared to saline-treated Flinder's Sensitive Line controls. Neostigmine and physostigmine treatment prior to intrarectal dinitrobenzene sulfonic acid significantly attenuated macroscopic damage score, myeloperoxidase activity and smooth muscle thickness on day 5 compared to colitic Sprague-Dawley controls. Significantly greater reductions in myeloperoxidase activity were observed with physostigmine vs. neostigmine pretreatment. These data suggest that cholinergic pathways modulate the acute colonic inflammatory response associated with the dinitrobenzene sulfonic acid model, with central pathways exerting a greater protective effect relative to peripheral pathways. Further studies are required to determine the contributions of sites in the nervous system and neuro-effector junctions.

Preclinical models: status of basic research in depression

Approximately one half-century ago several classes of medications, discovered by serendipity, were introduced for the treatment of depression and bipolar disorder. These highly effective medications revolutionized our approach to mood disorders and helped launch the modern era of psychiatry. Yet our progress since those serendipitous discoveries has been disappointing. We still do not understand with certainty how those medications produce their desired clinical effects. We have not introduced newer medications with fundamentally different mechanisms of action than the older agents. We have not identified the genetic and neurobiological mechanisms underlying depression and mania, nor do we understand the mechanisms by which nongenetic factors influence these disorders. We have only a rudimentary understanding of the circuits in the brain responsible for the normal regulation of mood and affect, and of those circuits that function abnormally in mood disorders. In approaching these gaps in our knowledge, this workgroup highlighted four major areas for future investment. These include developing better animal models of mood disorders; identifying genetic determinants of normal and abnormal mood in humans and animals; discovering novel targets and biomarkers of mood disorders and treatments; and increasing the recruitment of investigators from diverse backgrounds to mood disorders research.

Economic consequences of comorbid depression, anxiety, and allergic rhinitis

The present study extends prior work on the association between allergic rhinitis (AR) and common mental disorders by testing three related hypotheses: 1) that AR is associated with increased rates of depression and anxiety disorders in a large insured population, 2) comorbid AR, depression, and anxiety are associated with increased health and mental health expenditures, and 3) allergy treatment moderates the association between increased expenditures and comorbid AR, depression, and anxiety. Data are from MARKETSCAN, a large health care claims database of over 600,000 privately insured persons. Results indicate that AR is associated with higher rates of depression and anxiety disorder. Outpatient health care expenditures were increased by an average annual amount of $207 when AR and anxiety disorder were comorbid and $363 when AR and depression were comorbid. Finally, prescription treatment of AR moderated the increased expenditures associated with comorbidity.

Functional sex differences ('sexual diergism') of central nervous system cholinergic systems, vasopressin, and hypothalamic-pituitary-adrenal axis activity in mammals: a selective review

Sexual dimorphism of the mammalian central nervous system (CNS) has been widely documented. Morphological sex differences in brain areas underlie sex differences in function. To distinguish sex differences in physiological function from underlying sexual dimorphisms, we use the term, sexual diergism, to encompass differences in function between males and females. Whereas the influence of sex hormones on CNS morphological characteristics and function of the hypothalamic-pituitary-gonadal axis has been well-documented, little is known about sexual diergism of CNS control of the hypothalamic-pituitary-adrenal (HPA) axis. Many studies have been conducted on both men and women but have not reported comparisons between them, and many animal studies have used males or females, but not both. From a diergic standpoint, the CNS cholinergic system appears to be more responsive to stress and other stimuli in female than in male mammals; but from a dimorphic standpoint, it is anatomically larger, higher in cell density, and more stable with age in males than in females. Dimorphism often produces diergism, but age, hormones, environment and genetics contribute differentially. This review focuses on the sexual diergism of CNS cholinergic and vasopressinergic systems and their relationship to the HPA axis, with resulting implications for the study of behavior, disease, and therapeutics.

Comparison of rats selectively bred for high and low ethanol intake in a forced-swim-test model of depression: effects of desipramine

This investigation examined if there is a relationship between selective breeding for high or low alcohol intake and immobility in a force-swim-test (i.e., "behavioral despair") model of depression. Time spent immobile in a water-filled cylinder was measured in the alcohol-preferring (P and nonpreferring (NP) lines of rats, and in the high-alcohol-drinking (HAD) and low-alcohol-drinking (LAD) lines. Each rat was tested for 2 10-min trials administered 24 h apart, and pretreatment with saline or desipramine (10.0 or 20.0 mg desipramine/kg b.wt. i.p.) also was evaluated. Drug was administered immediately after Trial 1 and again 1 h before Trial 2. When tested without pretreatment in Trial 1 or with saline pretreatment in Trial 2, NP rats spent significantly more time immobile than did P rats, but no comparable line differences were found when HAD and LAD rats were tested. Desipramine pretreatment reduced the time spent immobile in rats of the 2 alcohol-nonpreferring lines (i.e., the NP and LAD rats), but had no significant effect in rats of the 2 alcohol-preferring lines (the P and HAD rats). These findings do not support the hypothesis that there is a functional relationship between high alcohol drinking and susceptibility to "behavioral despair" as measured by the forced-swim test. The results with desipramine suggest that selection for high alcohol intake may be associated with insensitivity to desipramine.

Potential animal model of multiple chemical sensitivity with cholinergic supersensitivity

Multiple Chemical Sensitivity (MCS) is a clinical phenomenon in which individuals, after acute or intermittent exposure to one or more chemicals, commonly organophosphate pesticides (OPs), become overly sensitive to a wide variety of chemically-unrelated compounds, which can include ethanol, caffeine and other psychotropic drugs. The Flinders Sensitive Line (FSL) rats were selectively bred to be more sensitive to the OP diisopropylfluorophosphate (DFP) compared to their control counterparts, the Flinders Resistant Line (FRL) rats. The present paper will summarize evidence which indicates that the FSL rats exhibit certain similarities to individuals with MCS. In addition to their greater sensitivity to DFP, the FSL rats are more sensitive to nicotine and the muscarinic agonists arecoline and oxotremorine, suggesting that the number of cholinergic receptors may be increased, a conclusion now supported by biochemical evidence. The FSL rats have also been found to exhibit enhanced responses to a variety of other drugs, including the serotonin agonists m-chlorophenylpiperazine and 8-OH-DPAT, the dopamine antagonist raclopride, the benzodiazepine diazepam, and ethanol. MCS patients report enhanced responses to many of these drugs, indicating some parallels between FSL rats and MCS patients. The FSL rats also exhibit reduced activity and appetite and increased REM sleep relative to their FRL controls. Because these behavioral features and the enhanced cholinergic responses are also observed in human depressives, the FSL rats have been proposed as a genetic animal model of depression. It has also been reported that MCS patients have a greater incidence of depression, both before and after onset of their chemical sensitivities, so cholinergic supersensitivity may be a state predisposing individuals to depressive disorders and/or MCS. Further exploration of the commonalities and differences between MCS patients, human depressives, and FSL rats will help to elucidate the mechanisms underlying MCS and could lead to diagnostic approaches and treatments beneficial to MCS patients.

Chronobiotics--drugs that shift rhythms

A chronobiotic is defined and levels of action within the mammalian circadian pacemaker system, such as the retina, retinohypothalamic tract, geniculohypothalamic tract, suprachiasmatic nuclei, output and feedback systems are identified. Classes of drug that include the indoleamines, cholinergic agents, peptides, and benzodiazepines, which might act as chronobiotics within these levels, are evaluated. Particular emphasis is placed on the indole, melatonin (MLT). The clinical circumstances for use of chronobiotics in sleep disturbances of the circadian kind, such as jet lag, shift work, delayed sleep-phase syndrome, advanced sleep-phase syndrome, irregular and non-24-hr sleep-wake cycles, are described under reorganized headings of disorders of entrainment, partial entrainment, and desynchronization. Specific attention is given to the blind and the aged. Both human and animal studies suggest that MLT has powerful chronobiotic properties. MLT shows considerable promise as a prophylactic and therapeutic alternative or supplement to the use of natural and artificial bright light for resetting the circadian pacemaker. Throughout this discussion, the hypnotic and hypothermic versus the chronobiotic actions of MLT are raised. Finally, problems in the design of delivery systems for MLT are discussed.

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.

Is cholinergic sensitivity a genetic marker for the affective disorders?

The recent literature on the involvement of cholinergic muscarinic mechanisms and adrenergic/cholinergic balance in affective disorders is reviewed and integrated with the older literature. There is strong evidence supporting the presence of exaggerated responses (behavioral, neuroendocrine, sleep) to cholinergic agents in affective disorder patients relative to normal controls and certain other psychiatric patients. There is also some, albeit less, conclusive evidence that these exaggerated responses may occur in euthymic individuals with a history of affective disorders, or in children at risk for development of affective disorders. Despite these promising results, suggesting a role for acetylcholine in the genetics of the affective disorders, further work in biochemistry and genetics is needed to link specific muscarinic receptors or other cholinergic variables to affective illness.

Free-running period of circadian rhythms is shorter in rats with a genetically upregulated central cholinergic system

The free-running circadian rhythm of drinking activity was monitored in constant darkness in a genetic line of rats, the Flinders Sensitive Line (FSL), which show increased muscarinic receptors in several brain regions. Compared to control rats, the Flinders Resistant Line (FRL), that do not show increased muscarinic receptors, the free-running period of drinking activity was shorter in the FSL rats (FSL period = 24.02 +/- 0.01 vs FRL period = 24.12 +/- 0.02; p < .001). In an attempt to determine whether other rhythms were similarly affected, we simultaneously monitored drinking activity, gross motor activity, and core temperature in free-running constant darkness conditions. The results from three FSL and FRL rats showed that the circadian periods of all three rhythms were shorter in FSL rats. These findings indicate that an animal model with an upregulated central cholinergic system demonstrates an accelerated circadian pacemaker.

Autonomic and behavioral responses of selectively bred hypercholinergic rats to oxotremorine and diisopropyl fluorophosphate

The hypercholinergic Flinders Sensitive Line (FSL) rat was significantly more sensitive than the Flinders Resistant Line (FRL) rat to the biotelemetrically recorded hypothermic effects of oxotremorine, a direct-acting muscarinic agonist, and diisopropyl fluorophosphate (DFP), an anticholinesterase agent. The effects of these agents on heart rate and motor activity, also recorded biotelemetrically, indicate either small differences (DFP) or no significant effect (oxotremorine) between the lines. These findings confirm the dramatic differences in temperature responses to cholinergic compounds between FSL and FRL rats, for which they were selectively bred, but suggest that a general increase in the sensitivity of the FSL rats to all muscarinic-mediated responses may not occur.

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.

NMDA glutamatergic receptors, labelled with [3H]MK-801, in brain samples from drug-free depressed suicides

Glutamate receptors of the NMDA-subtype were quantitated by binding of [3H]dizocilpine maleate (MK-801) in nine brain regions from 22 suicide victims (20-60 yr), with a firm retrospective diagnosis of depression, who had not recently received antidepressant drugs, and 20 age- and sex-matched controls. [3H]MK-801-binding did not differ between suicides and controls in any region studied. Suicides who died violently did not differ from non-violent suicides and controls. A significative negative correlation was found between age and NMDA receptor-binding in the frontal cortex of suicide victims, but not in controls. This preliminary study provides little evidence for an important role of NMDA-binding sites in the pathophysiology of depression.

Strain-dependent escape deficit in two rat models of learned helplessness

The shuttle box escape deficit produced by prior inescapable shock (IS) or by escapable shock (ES) was investigated in Koltushi high- (KHA) and low-avoidance (KLA) rat strains, genetically selected on the basis of divergent acquisition of a conditioned avoidance response in a two-way shuttle box. IS enhanced escape failures only in KHA rats. In contrast, ES produce escape deficit only in KLA rats. These data suggest that the learned helplessness interpretation of escape deficit might be relevant only when IS is administered to KHA rats. In the case of KLA rats exposed to ES, escape deficit represents a coping behavior.

Muscarinic and nicotinic influences on masculine sexual behavior in rats: effects of oxotremorine, scopolamine, and nicotine

In this study, the role of cholinergic systems in the regulation of male sexual behavior was analyzed by different approaches. Both muscarinic agonists and antagonists, as well as a nicotine agonist, were administered to sexually experienced male rats. In Experiment 1, oxotremorine (OXO), a muscarinic agonist, decreased the intromission frequency and ejaculatory latency in a dose-dependent way. Moreover, an increase in ejaculatory frequency was observed. In Experiment 2, the muscarinic antagonist scopolamine (SCO) produced a dose-related impairment of sexual behavior, decreasing the percentage of sexually active males. The smaller doses of SCO delayed the initiation of sexual behavior and decreased ejaculatory frequency. In an attempt to analyze the effect of muscarinic supersensitivity on sexual behavior, in Experiment 3 a long-term blockade of muscarinic receptors (SCO for 17 days) was followed by OXO administration. Animals displayed a significant increase of mount frequency, which results in the decrease of both the hit rate and ejaculatory frequency. In Experiment 4, six doses of nicotine were acutely administered. Only the higher doses (0.4, 0.8, and 1.6 mg/kg) induced a decrease in intromission frequency, although no significant differences were found in any other parameter. These results strongly suggest that cholinergic participation in masculine sexual behavior regulation is mediated mainly through muscarinic system.

Strain comparisons of DFP neurotoxicity in rats

The purpose of this study was to assess intraspecies differences in behavioral and autonomic function in three strains of rat following administration of diisopropyl fluorophosphate (DFP), an irreversible inhibitor of acetylcholinesterase activity. Male rats of the Long-Evans (LE), Fischer 344 (F344), and Sprague-Dawley (SD) strains wer administered DFP at doses of 0-1.5 mg/kg (sc). The animals were placed 60 min later into one of two motor activity chambers and tested for 30 min. Motor activity was measured using either a Doppler-based system or a commercial photocell device. Following measurement of motor activity in the Doppler system, body temperature (Tb) was measured and blood was then withdrawn by cardiac puncture and analyzed for serum cholinesterase activity (ChE). The remaining rats were retested 1 d after DFP administration in the photocell device. The results showed a significant influence of strain on the effects of DFP. Motor activity of LE rats was reduced by DFP at doses of 1.0 and 1.5 mg/kg, whereas the activity of F344 rats was reduced only at 1.5 mg/kg. The relative sensitivity of SD rats depended on the device used to measure motor activity. The SD rats resembled F344 rats in their response to DFP when motor activity was measured in the photocell device, and LE rats when motor activity was measured in the Doppler system. The Tb of F344 rats was unaffected by DFP, while the LE and SD rats became hypothermic at 1.5 mg/kg. The DFP-induced inhibition of serum ChE activity was significantly less in F344 rats. All three strains retested the day after DFP still showed significant decreases in motor activity. Overall, it appears that the F344 strain is relatively resistant to the behavioral and autonomic effects of DFP. This intraspecies variability should be considered in selecting appropriate experimental models for assessing the neurotoxicological hazards of cholinesterase-inhibiting pesticides.

The Flinders sensitive line rats: a genetic animal model of depression

The Flinders Sensitive Line (FSL) rat, selectively bred for increased responses to the anticholinesterase DFP, was originally proposed as an animal model of depression because, like depressed humans, it is supersensitive to the behavioral and hormonal effects of cholinergic (muscarinic) agonists. The present review critically examines earlier and recent data collected on FSL rats to assess whether the model has good face, construct and/or predictive validity. With respect to face validity, FSL rats resemble depressed humans, at least superficially, in that they demonstrate: (a) reduced locomotor activity, (b) reduced body weight, (c) increased REM sleep, and (d) cognitive (learning) difficulties. So far, studies designed to assess the presence of anhedonia, a cardinal symptom of melancholic depression, have been inconclusive, but there are trends for the FSL rats to be more anhedonic than their control counterparts, the Flinders Resistant Line (FRL) rats, when exposed to chronic mild stress. Thus, FSL rats fulfill the criterion of face validity. Because FSL rats also are more sensitive to cholinergic agonists and have phase advanced circadian rhythms, they meet the criteria for the cholinergic and circadian rhythm models of depression and, therefore, have good construct validity. A key behavioral symptom exhibited by the FSL rat is demonstration of an exaggerated immobility when exposed to stressors such as foot shock and forced swimming. This behavioral abnormality has been normalized by a number of well-recognized antidepressant drugs such as imipramine and desipramine, as well as newer generation antidepressants with promising clinical effects such as sertraline and rolipram. However, several treatments that have not been routinely used to treat depression (lithium, exposure to bright light, the anticholinesterase DFP) have been ineffective in reversing the exaggerated immobility. Thus, the evidence in the present review indicates that the FSL rat model of depression fulfills the criteria of face, construct, and predictive validities.

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.

Mechanisms of stress: a dynamic overview of hormonal and behavioral homeostasis

Environmental events, both physical and emotional, can produce stress reactions to widely varying degrees. Stress can affect many aspects of physiology, and levels of stress, emotional status, and means of coping with stress can influence health and disease. The stress system consists of brain elements, of which the main components are the corticotropin-releasing hormone (CRH) and locus ceruleus (LC)-norepinephrine (NE)/autonomic systems, as well as their peripheral effectors, the pituitary-adrenal axis and the autonomic system, which function to coordinate the stress response. Activation of the stress system results in behavioral and physical changes which allow the organism to adapt. This system is closely integrated with other central nervous system elements involved in the regulation of behavior and emotion, in addition to the axes responsible for reproduction, growth and immunity. With current trends in stress research which focus on understanding the mechanisms through which the stress-response is adaptive or becomes maladaptive, there is a growing association of stress system dysfunction, characterized by hyperactivity and/or hypoactivity to various pathophysiological states. The purpose of this review is to 1) define the concepts of stress and the stress response from a historical perspective, 2) present a dynamic overview of the biobehavioral mechanisms that participate in the stress response, and 3) examine the consequences of stress on the physiologic and behavioral well-being of the organism by integrating knowledge from apparently disparate fields of science.

Genetic animal models of depression and ethanol preference provide support for cholinergic and serotonergic involvement in depression and alcoholism

The present article summarizes some comparative studies of the Fawn-Hooded (FH) rat, a potential animal model of ethanol preference, and the Flinders Sensitive Line (FSL) rat, a potential animal model of depression. Both FH and FSL rats exhibit high degrees of immobility in the forced swim test and have difficulty learning a two-way active avoidance task. However, there were no differences between the FH and FSL rats in the elevated plus maze. Studies of ethanol preference indicated high rates of ethanol intake (greater than 4 g/kg) and preference (greater than 50%) in the FH rats, but low rates of ethanol intake (less than 1.1 g/kg) and preference (less than 20%) in FSL rats. It is concluded that the FSL rats exhibit behaviors consistent with their being an animal model of depression, whereas the FH rats exhibit features consistent with their being an animal model of both depression and alcoholism. Psychopharmacological challenges indicated that both FSL and FH rats were more sensitive to the hypothermic effects of oxotremorine, a muscarinic agonist. However, FSL rats were also more sensitive to serotonergic agonists, and some of the present results and other investigators have reported serotonergic subsensitivity in the FH rats. Thus, FSL rats exhibit both cholinergic and serotonergic supersensitivity, whereas FH rats exhibit cholinergic supersensitivity but normal or reduced serotonergic sensitivity. Progeny from a genetic cross between FH and FSL rats exhibit cholinergic supersensitivity and have high ethanol preference scores. These data are consistent with genetic models suggesting that ethanol preference may be influenced by dominant genes, whereas cholinergic sensitivity may be influenced by recessive genes.

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.

Clinical and biochemical aspects of depressive disorders: III. Treatment and controversies

The present document is the final of three parts of a review that focuses on recent data from clinical and animal research concerning the biochemical bases of depressive disorders, diagnosis, and treatment. Various treatments for depression, including psychotherapy, pharmacological, and somatic treatments, will be described in this third part. Also, some of the controversies in the field, as well as a summary of the most salient points of the review, will be discussed. Previous sections of this review dealt with the classification of depressive disorders and research techniques for studying the biochemical mechanisms of these disorders (Part I) and various transmitter/receptor theories of depressive disorder (Part II).

The Flinders Sensitive Line exhibits enhanced thermic responsiveness to nicotine relative to the Sprague-Dawley rat

The Flinders Sensitive Line (FSL) was derived from the Sprague-Dawley rat by selectively breeding animals with heightened sensitivity to an anticholinesterase. The FSL now consistently exhibits enhanced behavioral and physiological responses to muscarinic agonists relative to its progenitor. The authors now report the FSL exhibits enhanced thermic responsiveness to nicotine relative to the Sprague-Dawley rat. The possible relevance of this finding to investigators interested in the disorders of mood is briefly discussed.

Clinical and biochemical aspects of depressive disorders: II. Transmitter/receptor theories

The present document is the second of three parts in a review that focuses on recent data from clinical and animal research concerning the biochemical bases of depressive disorders, diagnosis, and treatment. Various receptor/transmitter theories of depressive disorders are discussed in this section. Specifically, data supporting noradrenergic, serotonergic, cholinergic, dopaminergic, GABAergic, and peptidergic theories, as well as interactions between noradrenergic and serotonergic, or cholinergic and catecholaminergic systems are presented. Problems with the data and future directions for research are also discussed. A previous publication, Part I of this review, dealt with the classification of depressive disorders and research techniques for studying the biochemical mechanisms of these disorders. A future publication, Part III of this review, discusses treatments for depression and some of the controversies in this field.

The behavior of the homozygous and heterozygous sub-types of rats which are genetically-selected for diabetes insipidus: a comparison with Long Evans and Wistar stocks

Several aspects of spontaneous and conditioned behavior (food and water intake, locomotion and emotionality, passive and active avoidance acquisition and retention) of standard (albino and pigmented) rats, and rats heterozygous (HEDI) and homozygous (HODI) for diabetes insipidus, are reviewed. As would be expected, HODI rats have been repeatedly found to consume far more fluid than either HEDI or control rats. Pigmented rats appear to be more active than albinos. HODI rats exhibit less marked emotional responses than do control rats, among which the pigmented ones exhibit the highest emotionality. Light aversion is more evident in albino than in pigmented rats. No differences are found among HEDI, HODI and normal Long Evans rats. It is quite difficult to provide a clear-cut statement concerning inter-strain differences in passive avoidance behavior, possibly because of the variety of techniques employed. In any case, HODI rats do not perform worse than normal controls do. In one-way active avoidance paradigms, pigmented rats perform better than albinos, and the performance of HODI rats does not differ from that of controls. In two-way avoidance paradigms, albinos appear to outperform pigmented rats. Once again, there are no obvious differences between HODI and control animals. In addition to indicating that HODI rats may actually be less emotional than the other groups of rats reviewed here, the studies described once again fail to confirm the previously alleged functions of vasopressin in memory consolidation.

Genetic and environmental influences on reactive and spontaneous locomotor activities in rats

Paired groups of rats (derived from divergent, selective breeding or living in divergent environmental conditions) were compared with regard to locomotor activities. Intrapair differences were found to vary non-systematically, depending upon whether the rats were initially exposed to a test-environment with or without a slight environmental modification (reactive activities), or were allowed to habituate extensively to the environment (spontaneous activity). Since the behavioral patterns were found to represent distinct entities, this pointed to the necessity of differentiating clearly between spontaneous and reactive activities and indicated, once again, that both genetic and environmental influences are important in these behaviors and must be taken into account. Accepting and controlling for these variables makes it possible to use the factor of individual differences in laboratory animal behavior to advantage.

Effects of calcium channel inhibitors on the hypothermic response to oxotremorine in normo and hypercholinergic rats

The Flinders Sensitive Line of rats (FSL) has been selectively bred to have increased sensitivity to cholinergic drugs. Typically, these rats react with twice as great a hypothermic effect to muscarinic agonists such as oxotremorine, as do similarly bred Flinders Resistant Line rats (FRL). We compared the effects of three chemically different calcium channel inhibitors (diltiazem, nicardipine and verapamil) on the hypothermia induced in FRL and FSL rats by oxotremorine (0.2 mg kg-1 s.c.). Each drug was injected i.p. in a dose of 20 mumol kg-1 30 min before oxotremorine. Methylatropine (2 mg kg-1 s.c.) was administered 15 min before oxotremorine to block the peripheral effects of the agonist. The hypothermic effect of oxotremorine in FSL rats was antagonized by nicardipine and diltiazem. In contrast, verapamil failed to influence the hypothermic response in FSL rats. Verapamil significantly (P less than 0.05) augmented oxotremorine hypothermia in FRL rats. Diltiazem and nicardipine were without effect on oxotremorine-induced hypothermia in FRL rats. There were no significant changes in temperature in separate groups of FRL and FSL rats treated with calcium channel inhibitors alone.

Decreased hyperthermic effect of MK-801 in selectively bred hypercholinergic rats

The Flinders Sensitive Line (FSL) of rats has been selectively bred to have increased sensitivity to cholinergic agonists. However, these rats exhibit altered responsiveness to a number of noncholinergic agents, such as apomorphine, buspirone and ethanol. This study compared the FSL and control Flinders Resistant Line (FRL) rats in terms of their hyperthermic response to the phencyclidine (PCP) receptor agonist, MK-801 (0.2 mg/kg SC) and their MK-801 binding characteristics. We have found that FSL rats react with a delayed hyperthermia, having a significantly lower hyperthermia for the first 120 min of observation. Thereafter the response does not differ in FSL and FRL rats. Both groups had similar affinities and numbers of [3H]MK-801 binding sites in the hippocampus/cerebral cortex. Pretreatment with scopolamine (1 mg/kg SC) failed to affect MK-801-induced hyperthermia in either line of rats. These findings suggest that selective breeding of FSL rats attenuated the secondary mechanisms involved in the PCP receptor-mediated hyperthermic response. However, by itself cholinergic supersensitivity does not appear to be a major factor in the blunted responsiveness of FSL rats to MK-801.

Animal models as simulations of depression

The most familiar usage of animal models of depression is as antidepressant screening tests. Paul Willner reviews their usage in a different context--as simulations of depression. The behavioural features of animal models of depression are compared with clinical symptomatology, and the contribution of animal models to understanding the following aspects of depression are reviewed: sources of population variability, natural history, psychological aspects, symptomatology and mechanisms of antidepressant action. Finally, the role of animal models of depression is considered as a critical interface between basic behavioural neuroscience and the clinic.

Lack of anxiety in an animal model of depression with cholinergic supersensitivity

It has been suggested that anxiety and depression are correlated dimensions of behaviour. Consequently, this study investigated the behaviour of the Flinders Sensitive Line, an animal model of depression with cholinergic supersensitivity, in the elevated (+)-maze test of anxiety. The results indicate that anxiety responses (% open/total arm entries) do not differ between the Flinders Sensitive and Flinders Resistant (control) lines of rat (FSL vs. FRL, respectively). Treatment with 1.0 mg/kg of diazepam significantly increased % open/total scores to a similar degree in both lines, further suggesting that the lines do not differ in anxiety. It is concluded that the FSL rat is an animal model of depression without evidence for inherent alteration in anxiety-related behaviour.

Exposure to constant darkness enhances the thermic response of the rat to a muscarinic agonist

Bright artificial light is used to treat patients with major depression with a seasonal component ("Winter Depression"). Hyperactivity of muscarinic cholinergic systems is implicated in the pathophysiology of depression. Continual exposure to bright light for 7 days or during discrete portions of the photoperiod blunts the thermic response to a muscarinic agonist (oxotremorine) in the rat. Exposure to either 24 hours per day of bright light (in contrast to periods of circumscribed exposure) or darkness would tend to produce free-running. Observers have suggested that the reduced responsiveness to oxotremorine may result from the induction of free-running (the "free-running hypothesis"). The "free-running hypothesis" leads to the prediction that rats exposed to constant darkness will exhibit a reduction in thermic responsiveness to oxotremorine. The authors hypothesized that constant exposure to darkness would, contrary to the "free-running hypothesis," enhance the thermic response to oxotremorine. Rats (n = 12) exposed to constant darkness for 7 days exhibited super-sensitivity to oxotremorine 5 days after return to standard light/dark cycle in the vivarium. This argues against the hypothesis that the induction of free-running enhances sensitivity to the thermic effects of oxotremorine.

Dopamine sensitivity in rats selectively bred for increases in cholinergic function

Because of the extensive literature demonstrating an interaction between cholinergic and dopaminergic systems, the Flinders Sensitive (FSL) and Flinders Resistant (FRL) Lines of rats, selectively bred for differences in cholinergic function, were tested for differences in dopamine sensitivity. Large differences in sensitivity to dopamine agonists were detected, but the direction depended upon the function: The FSL rats were supersensitive to the hypothermic effects of dopamine agonists, but were subsensitive to the stereotypy-inducing effects. Measurement of dopamine receptors by either standard binding techniques or autoradiography failed to demonstrate any receptor differences in the FSL and FRL rats. Behavioural studies with dopamine antagonists were less clear-cut, but suggested that the FSL rats might be more sensitive to their catalepsy-inducing effects. These findings indicate that the changes in dopamine sensitivity which accompany cholinergic supersensitivity are function-dependent, but are not associated with parallel changes in dopamine receptor concentration.

Chronic swim stress enhances the motoric inhibiting effects of a muscarinic agonist

The authors previously demonstrated that chronic inescapable swim stress and footshock increase the capacity of a fixed dose of a muscarinic agonist to produce hypothermia in the rat. This project was designed to determine whether chronic inescapable swim stress in cold water would render a low dose of a muscarinic agonist, devoid of an effect on motor behavior in the naive rat (i.e., prior to subjection to the course of swim stress), an inhibitor of mobility. The study involved two groups of rats, an experimental group which received arecoline and a control group which received saline five minutes prior to being placed in an open field. Number of crossings, the dependent variable, was measured in both groups before and after a 14-day course of twice daily inescapable swim stress of 10 minutes duration at 12 degrees C. The arecoline-treated group, as hypothesized, exhibited a significantly greater reduction in number of crossings than the saline-treated groups following the course of swim stress.

Effects of bright light on responsiveness to a muscarinic agonist in rats selectively bred for endogenously increased cholinergic function

The Flinders Sensitive Line (FSL) was derived from the Sprague-Dawley rat by selectively breeding those animals exhibiting a high level of sensitivity to an anticholinesterase. The Flinders Resistant Line (FRL) was simultaneously developed as a control line. These lines exhibit nonoverlapping distributions of their thermic responsiveness to oxotremorine. Bright light prevents the development of supersensitivity to oxotremorine occurring as a result of forced stress or treatment with a muscarinic receptor antagonist in the rat. The authors now report that treatment with bright light during the regular photoperiod (i.e., a time that does not produce a phase-shift or free-running) differentially affects the hypothermic response and activity-suppressing effect of oxotremorine in both the FSL and FRL. Both lines exhibit decreased hypothermia without reduction in motor activity in response to oxotremorine following 6 days of treatment with bright light. The magnitude of blunting of the hypothermic response was greater in the FSL than the FRL. These findings suggest that (1) studies of the effects of bright light are contingent on the end point one measures and (2) the capacity of this treatment to blunt the hypothermic response to a muscarinic agonist is greater in an animal model with endogenously hyperactive muscarinic cholinergic systems.

Bright light blocks the capacity of inescapable swim stress to supersensitize a central muscarinic mechanism

Clinical and basic researchers have proposed that muscarinic cholinergic mechanisms mediate some effects of chronic stress. Chronic inescapable (forced) swim stress depletes brain biogenic amines and is used to produce learned helplessness in rats. Behavioral and biochemical characteristics of animals in the state of learned helplessness lead some investigators to believe this condition provides a useful animal model of depression. Inescapable swim stress also produces supersensitivity to the hypothermic effect of the muscarinic agonist oxotremorine in the rat. The authors previously demonstrated that bright light potently induces subsensitivity of a central muscarinic mechanism involved in the regulation of core temperature under a variety of circumstances. They now report using a repeated measures design that inescapable swim stress of five days duration produces supersensitivity to oxotremorine (increase in thermic response of 405%). This supersensitivity is reversed within five days by treatment with bright light, despite continuation of daily swim stress. Daily inescapable swim stress was continued beyond cessation of treatment with bright light. Five days later, supersensitivity to the hypothermic effect of oxotremorine was once again evident.

Antidepressant-like action of nicardipine, verapamil and hemicholinium-3 injected into the anterior hypothalamus in the rat forced swim test

Male Wistar rats, chronically implanted with cannulas into the anterior hypothalamus, were acutely injected with the calcium channel inhibitors, diltiazem, nicardipine and verapamil, or the choline uptake blocker hemicholinium-3 and tested in the forced swim test. Hemicholinium-3, nicardipine and verapamil markedly increased the duration of active swimming. This antidepressant-like effect did not appear to reflect merely a hyperactive state as the drug-treated rats did not differ from vehicle-injected controls in their open field motility scores. Diltiazem failed to influence rats' performance in either test. Since nicardipine and verapamil, but not diltiazem, share choline uptake property with hemicholinium-3, it seems that this action plays a role in the antidepressant-like effect of all three drugs in the forced swim test.

Impaired active avoidance responding in rats selectively bred for increased cholinergic function

It was found that the Flinders Sensitive Line (FSL) of rat, selectively bred for increased cholinergic function, performed poorly in a tone-cued two-way active avoidance task in comparison with the control Flinders Restraint Line (FRL) of rat. These findings are consistent with the suggestion that the FSL rats may be a genetic animal model of depression.