Reprint of: Locus coeruleus neuronal activity determines proclivity to consume alcohol in a selectively-bred line of rats that readily consumes alcohol.

Sprague-Dawley rats selectively-bred for susceptibility to stress in our laboratory (Susceptible, or SUS rats) voluntarily consume large amounts of alcohol, and amounts that have, as shown here, pharmacological effects, which normal rats will not do. In this paper, we explore neural events in the brain that underlie this propensity to readily consume alcohol. Activity of locus coeruleus neurons (LC), the major noradrenergic cell body concentration in the brain, influences firing of ventral tegmentum dopaminergic cell bodies of the mesocorticolimbic system (VTA-DA neurons), which mediate rewarding aspects of alcohol. We tested the hypothesis that in SUS rats alcohol potently suppresses LC activity to markedly diminish LC-mediated inhibition of VTA-DA neurons, which permits alcohol to greatly increase VTA-DA activity and rewarding aspects of alcohol. Electrophysiological single-unit recording of LC and VTA-DA activity showed that in SUS rats alcohol decreased LC burst firing much more than in normal rats and as a result markedly increased VTA-DA activity in SUS rats while having no such effect in normal rats. Consistent with this, in a behavioral test for reward using conditioned place preference (CPP), SUS rats showed alcohol, given by intraperitoneal (i.p.) injection, to be rewarding. Next, manipulation of LC activity by microinfusion of drugs into the LC region of SUS rats showed that (a) decreasing LC activity increased alcohol intake and increasing LC activity decreased alcohol intake in accord with the formulation described above, and (b) increasing LC activity blocked both the rewarding effect of alcohol in the CPP test and the usual alcohol-induced increase in VTA-DA single-unit activity seen in SUS rats. An important ancillary finding in the CPP test was that an increase in LC activity was rewarding by itself, while a decrease in LC activity was aversive; consequently, effects of LC manipulations on alcohol-related reward in the CPP test were perhaps even larger than evident in the test. Finally, when increased LC activity was associated with (i.e., conditioned to) i.p. alcohol, subsequent alcohol consumption by SUS rats was markedly reduced, indicating that SUS rats consume large amounts of alcohol because of rewarding physiological consequences requiring increased VTA-DA activity. The findings reported here are consistent with the view that the influence of alcohol on LC activity leading to changes in VTA-DA activity strongly affects alcohol-mediated reward, and may well be the basis of the proclivity of SUS rats to avidly consume alcohol.

Locus coeruleus neuronal activity determines proclivity to consume alcohol in a selectively-bred line of rats that readily consumes alcohol.

Sprague-Dawley rats selectively-bred for susceptibility to stress in our laboratory (Susceptible, or SUS rats) voluntarily consume large amounts of alcohol, and amounts that have, as shown here, pharmacological effects, which normal rats will not do. In this paper, we explore neural events in the brain that underlie this propensity to readily consume alcohol. Activity of locus coeruleus neurons (LC), the major noradrenergic cell body concentration in the brain, influences firing of ventral tegmentum dopaminergic cell bodies of the mesocorticolimbic system (VTA-DA neurons), which mediate rewarding aspects of alcohol. We tested the hypothesis that in SUS rats alcohol potently suppresses LC activity to markedly diminish LC-mediated inhibition of VTA-DA neurons, which permits alcohol to greatly increase VTA-DA activity and rewarding aspects of alcohol. Electrophysiological single-unit recording of LC and VTA-DA activity showed that in SUS rats alcohol decreased LC burst firing much more than in normal rats and as a result markedly increased VTA-DA activity in SUS rats while having no such effect in normal rats. Consistent with this, in a behavioral test for reward using conditioned place preference (CPP), SUS rats showed alcohol, given by intraperitoneal (i.p.) injection, to be rewarding. Next, manipulation of LC activity by microinfusion of drugs into the LC region of SUS rats showed that (a) decreasing LC activity increased alcohol intake and increasing LC activity decreased alcohol intake in accord with the formulation described above, and (b) increasing LC activity blocked both the rewarding effect of alcohol in the CPP test and the usual alcohol-induced increase in VTA-DA single-unit activity seen in SUS rats. An important ancillary finding in the CPP test was that an increase in LC activity was rewarding by itself, while a decrease in LC activity was aversive; consequently, effects of LC manipulations on alcohol-related reward in the CPP test were perhaps even larger than evident in the test. Finally, when increased LC activity was associated with (i.e., conditioned to) i.p. alcohol, subsequent alcohol consumption by SUS rats was markedly reduced, indicating that SUS rats consume large amounts of alcohol because of rewarding physiological consequences requiring increased VTA-DA activity. The findings reported here are consistent with the view that the influence of alcohol on LC activity leading to changes in VTA-DA activity strongly affects alcohol-mediated reward, and may well be the basis of the proclivity of SUS rats to avidly consume alcohol.

Operant psychostimulant self-administration in a rat model of depression.

Depression and psychostimulant addiction are co-morbid conditions; depression is a significant risk factor for psychostimulant abuse, and the rate of depression in drug addicts is higher than in the general population. Despite the prevalence of this comorbidity, there are few animal models examining psychostimulant abuse behaviors in depression. We have shown previously that while rats selectively bred for depression-like phenotypes (SwLo) have blunted mesolimbic dopamine (DA) signaling and locomotor responses to dopaminergic drugs, they voluntarily administer excessive amounts of psychostimulants compared to normal or depression-resistant (SwHi) rats in oral consumption paradigms. To determine whether this increased drug intake by depression-sensitive rats extends to operant self-administration, we assessed fixed ratio-1, progressive ratio, extinction, and reinstatement responding for cocaine and amphetamine in SwLo and SwHi rats. Contrary to the oral consumption results, we found that the SwHi rats generally responded more for both cocaine and amphetamine than the SwLo rats in several instances, most notably in the progressive ratio and reinstatement tests. Food-primed reinstatement of food seeking was also elevated in SwHi rats. These results provide further insight into the neurobiology of depression and addiction comorbidity and caution that oral and operant psychostimulant self-administration paradigms can yield different, and this case, opposite results.

Influence of chronic administration of antidepressant drugs on mRNA for galanin, galanin receptors, and tyrosine hydroxylase in catecholaminergic and serotonergic cell-body regions in rat brain.

Activity of locus coeruleus (LC) neurons and release of the peptide galanin (GAL), which is colocalized with norepinephrine (NE) in LC neurons, has been implicated in depression and, conversely, in antidepressant action. The present study examined the influence of chronic administration (for 14days, via subcutaneously-implanted minipump) of antidepressant (AD) drugs representing three different classes (tricyclic [desipramine], selective serotonin reuptake inhibitor [SSRI] [paroxetine], and monoamine oxidase inhibitor [MAOI] [phenelzine]) on mRNA for GAL, GAL receptors (GalR1, GalR2, and GalR3), and tyrosine hydroxylase (TH), the rate-limiting enzyme for NE synthesis, in four brain regions--LC, A1/C1, dorsal raphe (DRN), and ventral tegmentum (VTA) of rats. Consistent with previous findings that chronic administration of AD drugs decreases activity of LC neurons, administration of AD drugs reduced mRNA for both GAL and TH in LC neurons. GAL and TH mRNA in LC neurons was highly correlated. AD drugs also reduced GAL and TH mRNA in A1/C1 and VTA but effects were smaller than in LC. The largest change in mRNA for GAL receptors produced by AD administration was to decrease mRNA for GalR2 receptors in the VTA region. Also, mRNA for GalR2 and GalR3 receptors was significantly (positively) correlated in all three predominantly catecholaminergic brain regions (LC, A1/C1, and VTA). Relative to these three brain regions, unique effects were seen in the DRN region, with the SSRI elevating GAL mRNA and with mRNA for GalR1 and GalR3 being highly correlated in this brain region. The findings show that chronic administration of AD drugs, which produces effective antidepressant action, results in changes in mRNA for GAL, GAL receptors, and TH in brain regions that likely participate in depression and antidepressant effects.

Effects of fenfluramine, 8-OH-DPAT, and tryptophan-enriched diet on the high-ethanol intake by rats bred for susceptibility to stress.

The swim-test susceptible (SUS) line of rats has been bred in our laboratory for the characteristic of reduced motor activity in the swim test following exposure to an acute stressor. Testing of multiple generations of SUS rats has also revealed that they consume large amounts of ethanol voluntarily. As reported for lines of rats that show a propensity for high-ethanol intake, the SUS rats show evidence of low serotonergic function. Because serotonergic function has often been shown to be involved in the regulation of alcohol consumption, here we examined the effects of manipulations of serotonin transmission on intake of ethanol by SUS rats. Fenfluramine, a serotonin-releasing drug, was injected at various doses (0.625, 1.25, 2.5, and 5.0mg/kg) twice per day and ethanol intake was measured using a two-bottle free-choice method. The 8-OH-DPAT, a 5­HT(1A) agonist, was injected at various doses (0.03125, 0.0625, 0.125, 0.25, 0.5, and 1.0mg/kg) before a 1-h session of exposure to ethanol (single-bottle test, water available the other 23h per day). A diet enriched with 3% tryptophan (TRP), the amino acid precursor for serotonin synthesis, was administered in a restricted feeding schedule (5h per day) with ethanol intake measured the last 4h. Fenfluramine decreased ethanol intake at all doses tested. The 8-OH-DPAT increased ethanol intake at lower doses, presumably acting at autoreceptors, which inhibit serotonergic neurons, and decreased intake at higher doses, presumably acting at postsynaptic 5-HT(1A) receptors. TRP-enriched diet also significantly decreased ethanol intake. Food and water intake were less or unaffected by these three manipulations. With all three manipulations, ethanol intake remained suppressed one or more days after the day of tests that decreased ethanol intake. These data suggest that SUS rats, like many other lines/strains of rodents that consume large amounts of alcohol, show an inverse relationship between serotonin transmission and voluntary intake of ethanol.

Genetic propensities to increase ethanol intake in response to stress: studies with selectively bred swim test susceptible (SUS), alcohol-preferring (P), and non-preferring (NP) lines of rats.

RATIONALE: Swim test susceptible (SUS) rats selectively bred for reduced struggling in the forced swim test (FST) following stress show high voluntary ethanol intake like alcohol-preferring (P) rats selectively bred for ethanol preference. It is unknown whether stress enhances drinking in SUS rats or FST behavior in P and non-preferring (NP) rats. OBJECTIVES: The aim of this study was to assess the response to stress in male SUS, Sprague-Dawley (SD), P, and NP rats on 10% ethanol drinking and FST behavior. METHODS: In experiment 1, SUS and SD rats had limited access to ethanol and water following white noise, rehousing, and forced swim stress. In experiment 2, P and NP rats received footshock, white noise, restraint, or no stress prior to the FST. Rats then had continuous access to ethanol and water, and the effects of weekly exposures to stress were measured. RESULTS: SUS rats drank more ethanol (M = 2.98 g/kg) than SD rats (M = 1.26 g/kg) at baseline. Stress produced sustained increases (~33% of baseline) in ethanol intake in SUS rats. NP rats spent twice as much time immobile as P rats in the FST. Stress did not alter FST behavior in P or NP rats. Only footshock produced an increase (~29%) in ethanol intake in P rats. CONCLUSIONS: Selection for stress-induced depressive-like behavior in SUS rats is associated with enhanced stress-induced ethanol drinking. However, the selection for alcohol preference is not associated with stress-induced depressive-like behavior but is associated with footshock stress-induced ethanol drinking. In these experiments, relationships among stress, depressive-like behavior, and alcohol preference were not symmetrical.

Effects of chronic antidepressant drug administration and electroconvulsive shock on activity of dopaminergic neurons in the ventral tegmentum.

Increasing attention is now focused on reduced dopaminergic neurotransmission in the forebrain as participating in depression. The present paper assessed whether effective antidepressant (AD) treatments might counteract, or compensate for, such a change by altering the neuronal activity of dopaminergic neurons in the ventral tegmental area (VTA-DA neurons), the cell bodies of the mesocorticolimbic dopaminergic system. Eight AD drugs or vehicle were administered to rats for 14 d via subcutaneously implanted minipumps, at which time single-unit electrophysiological activity of VTA-DA neurons was recorded under anaesthesia. Further, animals received a series of five electroconvulsive shocks (ECS) or control procedures, after which VTA-DA activity was measured either 3 d or 5 d after the last ECS. Results showed that the chronic administration of all AD drugs tested except for the monoamine oxidase inhibitor increased the spontaneous firing rate of VTA-DA neurons, while effects on 'burst' firing activity were found to be considerably less notable or consistent. ECS increased both spontaneous firing rate and burst firing of VTA-DA neurons. It is suggested that the effects observed are consistent with reports of increased dopamine release in regions to which VTA neurons project after effective AD treatment. However, it is further suggested that changes in VTA-DA neuronal activity in response to AD treatment should be most appropriately assessed under conditions associated with depression, such as stressful conditions.

Antidepressant drugs with differing pharmacological actions decrease activity of locus coeruleus neurons.

Previous studies suggest that all effective antidepressant (AD) drugs decrease activity of locus coeruleus (LC) neurons. However, little data exist regarding blood levels of drug in these studies, and what data do exist suggest blood levels might have been very high. To assess whether decreased LC activity is produced by drugs that selectively block reuptake for either norepinephrine or serotonin at therapeutically relevant blood levels, effects of chronic administration of desipramine, paroxetine, and escitalopram on LC activity were measured across a range of doses and blood levels of drug. Further, effects of a range of doses of mirtazapine were examined; in that mirtazapine blocks alpha2 adrenergic receptors, it might be anticipated to increase rather than decrease LC activity. Finally, to begin to assess whether the response of LC to ADs was specific to these drugs, effects of four non-AD drugs (single dose) were measured. Drugs were administered via osmotic minipump for 14 d. Electrophysiological recording of LC activity (assessment of both spontaneous firing rate and sensory-evoked 'burst' firing) then took place under isoflurane anaesthesia on the last day of drug treatment. The blood level of drugs present at the end of the recording session was also measured. All AD drugs tested decreased LC spontaneous and sensory-evoked 'burst' firing, and this was observed across a wide range of blood levels for the drugs. Non-AD drugs did not decrease LC activity. The findings of this investigation continue to support the possibility that all effective AD drugs decrease LC activity.

Intake of ethanol and reinforcing fluids in rats bred for susceptibility to stress.

Rats have been selectively bred in our laboratory based on how swim-test behavior is affected by stress. Following exposure to an acute stressor, active swim-test behavior is reduced in the swim-test susceptible (SUS) line but is not reduced in the swim-test resistant (RES) line. Earlier findings indicate that SUS rats have reduced central serotonin and dopamine levels relative to normal, random-bred (i.e., nonselected [NS]) rats and RES rats, suggesting that SUS rats might respond differently to reinforcing substances, particularly ethanol. We report here comparison of SUS, NS, and RES rats regarding consumption of ethanol. Also examined was consumption of saccharin, sucrose, and quinine. Testing involved a two-bottle, free-choice method of measuring intake of substances in a home cage. Intake of each substance was tested across a range of concentrations. The results indicate that the SUS rats, tested across 14 generations, consume markedly more ethanol than the other two lines; in fact, SUS rats consume amounts similar to that ingested by lines/strains of rats bred specifically for ethanol intake. Similar to other alcohol-preferring rats, SUS rats show an increased affinity for saccharin solutions and a marked increase in their total daily fluid intake when a sweet-tasting saccharin or sucrose solution is available. These results indicate that a propensity to drink alcohol occurs in a line of rats that were selectively bred, not for alcohol intake, but for vulnerability to stress.

Testing the hypothesis that locus coeruleus hyperactivity produces depression-related changes via galanin.

This paper reviews progress made in testing the idea that depression-related behavioral changes can arise from hyperactivity of locus coeruleus (LC) neurons which consequently inhibits activity of mesocorticolimbic dopamine neurons in the ventral tegmentum (VTA) via release of galanin from terminals on LC axons in VTA. Results from pre-clinical testing are described, including the most recent findings indicating that, in an animal model that shows long-lasting symptoms of depression, recovery to normal activity in the home cage is accelerated by infusion of a galanin receptor antagonist, galantide (M15), into VTA. Data are also described suggesting that all effective antidepressant treatments decrease activity of LC neurons.