Cytokine-Purine Interactions in Behavioral Depression in Rats

THOMAS R. MINOR 1, QINGJUN HUANG 2, AND ELIZABETH A. FOLEY 1

lDepartment of Psychology, University of California, Los Angeles, Los Angeles CA 90095-1563 2Psychoneuroimmunology Laboratory, Bethune Military Medical College, 450 West Zhongshan

Road, Shijiazhuang, Hebei 050081, China

This paper reviews recent findings from our laboratories concerning metabolic and im- mune mediators of behavioral depression in rats. Specifically, a single injection of 6 mg/kg of reserpine substantially increases behavioral depression, as evidenced by an increase in the amount of time spent floating by independent groups of rats tested for swim perfor- mance at various times during the next week. The behavioral impairment consists of two components. An early component emerges one hour after reserpine treatment and persists for about 24 hours. The deficit is not reversed by intracranial ventricular infusion of the receptor antagonist for interleukin-l{I (IL-I~). A second, late-component deficit appears approximately 48 hours after reserpine treatment and recovers within a week. Late-component depression is reversed by central infusion of the IL-16 receptor antagonist, and is mim- icked by central infusion of the proinflanunatory cytokine. Importantly, both early and late components of reserpine-induced depression and IL-1 ~ induced depression are reversed by a systemic injection of the highly selective A2A adenosine receptor antagonist 8-(3- Chlorostyryl) caffeine. These data are discussed in terms of the overlap in the conservation- withdrawal reaction during sickness, traumatic stress, and major depression and the regional contribution of purines and cytokines to the organization of this reaction in the brain.

RESERPINE IS AN alkaloid extract from the root of Rauwolfia serpentina, a climbing shrub indigenous to India. The compound had a long history of use in the Orient as a tranquiliz- ing agent before being introduced in the United States in the early 1950s as a treatment for hypertension (Cooper, Bloom & Roth, 1978; Gerber & Nies, 1990; Rech & Moore, 1971). The extract reduces both cardiac output and peripheral vascular resistance by depleting stores of biogenic amines in the central and autonomic nervous systems. Specifically, reserpine binds irreversibly to storage vesicles in monoaminergic neurons (Norn & Shore, 1999; Schwartz, 1981). The vesicles become "leaky," resulting in seepage of transmitter into the cytoplasm, where it is either destroyed by intraneuronal monoamine oxidase or diffuses into the synaptic cleft. The end result is that little or no active transmitter is released at the synapse following depolarization. Recovery from the effects of reserpine requires synthesis of new storage vesicles, which can take several days to accomplish after discontinuing treatment (Cooper, Konkol, & Breese, 1978; Ponzio, Achilli, Calderini & Ferretti, 1984).

Address for Correspondence: Thomas R. Minor, Ph.D., Department of Psychology, University of California, Los Angeles, Los Angeles, CA 90095-1563, E-mail: Minor@psych. ucla. edu.

Integrative Physiological & Behavioral Science, July-September 2003, Vol. 38, No. 3, 189-202.

189

190 MINOR, HUANG, AND FOLEY

The historic importance of reserpine is more related to its unwanted side effects than to its efficacy as an antihypertensive or tranquilizing agent. Unfortunately, a significant por- tion of the population undergoing reserpine treatment for hypertension developed symp- toms of major depression. These symptoms were severe enough to require antidepressant drug treatment and, at times, hospitalization. This observation, along with findings a few years later that monoamine oxidase inhibitors and tricyclic antidepressants enhance brain biogenic amines, severed as the empirical cornerstone of catecholamine (Schildkraut, 1965), and later, monoamine theories of depression (Akiskal & McKinney, 1975; Bunney & Davis, 1965).

This paper reviews recent research from our laboratories (Haung & Minor, 2003 a, b, c; Jiang, Minor, & Huang, 2003). We revisited the animal literature on reserpine to ask whether it is the depletion of brain monoamines per se, or some downstream consequence of reserpine treatment, that precipitates a depressive episode. Despite the promise of the original catecholamine theory, empirical evidence linking any of the biogenic amines and major depression is less than convincing. As an alternative, we focused on the potential contribution of two brain-signaling pathways, one mediated by the purine nucleoside ad- enosine and the other involving the proinflammatory cytokine interleukin-1 ~ (IL-1 ~). Both of these pathways are capable of producing a conservation-withdrawal reaction (Engle & Schmale, 1971) and both are plausibly engaged by reserpine treatment. This reaction corresponds to the fatigue component of major depression (Field & Reite, 1984; Minor et al., 1994; Woodson et al., 1998). A conservation-withdrawal reaction unconditionally fol- lows periods of intense catabolic output (traumatic psychological stress, injury, or severe illness). The sensory unresponsiveness, cognitive dullness, and behavioral depression that characterize this state are assumed to be adaptive responses that husband limited resources and facilitate the recovery of metabolic homeostasis.

Brain Adenosine Signaling.

Adenosine signaling links cellular excitability to energy state and is actively engaged by challenges to metabolic homeostasis (Van Wylen et al., 1986; Phillis et al., 1987). The nucleoside exerts very potent inhibition on excitatory transmission in the brain as a com- pensatory reaction to neural energy failure (Fredholm & Dunwiddie, 1988; Newby, 1984; Meghji, 1991; Stone, 1981). Adenosine is extruded into extracellular space, or hydrolyzed from extracellular nucleotides, whenever the rate of adenosine triphosphate (ATP) hy- drolysis exceeds the synthesis rate (Meghji, 1991; Newby, 1984; White & Hoehn, 1991). Such an imbalance of the energy supply/demand ratio can result from excessive neural activation or from a shortage in brain glucose or oxygen. The extracellular nucleoside binds to specific adenosine receptors (A1, A2A, A2B, & A3), which are widely distributed on pre- and post-synaptic membranes and in the brain microvascular bed (Daly, 1990; Linden, 1991; Rail, 1990). Adenosine interacts with a number of cellular effector systems via these receptors to decrease membrane excitability and inhibit transmitter release, thereby decreasing metabolic demand in the target neuron (Burns, 1991). Adenosine also acts at the system level to produce a number of changes that protect neural tissue from the potentially excitotoxic effects of activation in the absence of sufficient energy (Dragunow, 1988; Harms, Wardeh, 1979; Marangos, 1991; Milusheva et al., 1990; Novelli, Reilly, Lyskoi, & Henneberry, 1988; Daval & Nicolas, 1998).

Adenosine signaling plays a crucial role in mediating the transition from an anxious/ agitated state to one of behavioral depression in the learned helplessness paradigm (Minor,

CYTOK1NE-PURINE INTERACTIONS IN BEHAVIORAL DEPRESSION 191

Chang, & Winselow, 1994; Minor, Winselow & Chang, 1994; Minor & Hunter 2002) and in mediating immobility in a forced swim test in mice (El Yacoubi et al., 2001). Adenosine also is plausibly activated by reserpine treatment. One of the conditions under which adenosine exerts potent compensatory inhibition is during excessive neural activation. Large amounts of transmitter are likely to be released upon initial reserpine treatment and during the process of depleting the biogenic amines. The resulting excitation might be sufficient to compromise metabolic homeostasis and provoke adenosine-mediated inhibi- tion as a compensatory mechanism. This possibility is supported by the similarity in the conservation-withdrawal responses following reserpine treatment and inescapable shock.

Cytokine Signaling

Systemic administration of lipopolysaccharide (LPS), the active fragment of endotoxin from gram-negative bacteria, induces the synthesis of proinflammatory cytokines in pe- ripheral macrophages---interleukin-ll3 (IL-I[3), IL-6, and tumor necrosis factor (TNFct) (Roitt, 1998). Kuppfer cells in the liver also express IL-I~ as a consequence of LPS administration, and may serve as the primary immune-to-brain communication pathway. This signal is transferred via the vagal nerve complex to the brain nucleus tractus solitarius (NTS) where IL-113 is then expressed. The cytokine also is expressed relatively quickly thereafter in a variety of other brain nuclei, particularly in the hypothalamus (Larson & Dunn 200; Fleshner, Goehler, Hermann, Relton, Maier, & Watkins, 1995; Wong et al., 1997).

IL-l[3 binds to specific receptors distributed throughout the brain to induce sickness behavior: lethargy, hypoactivity, decreased libido, anorexia, anhedonia, and increased sleep (Dantzer, 2001; Larson, & Dunn, 2001). This dramatic shift in ongoing activity, along with the induction of fever, is assumed to be a highly adaptive strategy to fight infection.

Symptoms of sickness behavior and major depression overlap considerably (Maes, 1995; Pollack & Yirmiya, 2002; Yirmiya, 1996; Zacharko et al., 1997). In this regard, systemic administration of endotoxin not only increases brain concentrations of IL- 1 [I, but also produces swim deficits (del Ceerro & Borrell, 1990) and other experimental indices of depression (Yirmiya, 1996). These LPS-induced ailments are reversed by chronic (but not acute) treatment with tricyclic antidepressants (Yirmiya, 1996). More important for the present purposes is the recent finding that LPS-induced swim deficits are reversed by systemic administration of an AaA receptor antagonist (El Yacoubi et al., 2002). These data suggest that there is an important interaction between purine (adenosine) and cytokine (IL- ll3) signaling in one model of behavioral depression.

Time Course for Reserpine-Induced Depression

One of the historic problems in creating a convincing case for the biogenic amines in reserpine-induced depression is that the time course for monoamine depletion does not fit the time course for behavioral impairment. For instance, Bean et al. (1989) reported that dopamine (DA) depletion in striatum, nucleus accumbens, and frontal cortex occurred rapidly and reached a maximum about six hours after an ip injection of 5 mg/kg of reserpine. DA levels remained at floor levels for about 18 hours after the injection and then increased thereafter, with complete recovery occurring within 48 hours. Behavioral depres- sion typically lasts considerably longer, although the recovery time course varies to some degree with the specifics of the test measure.

192 MINOR, HUANG, AND FOLEY

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F]6. 1. Recovery time course for reserpine-induced depression. Rats were injected with DMSO vehicle or 6 mg/kg of reserpine and then tested for swim performance 1, 24, 48, 72, or 168 hours later.

Our first objective in this line of work was to determine the specific recovery time course in our measure of behavioral depression following reserpine treatment. We injected groups of rats (n = 10 per group) with a single 6 mg/kg, intraperitoneal (ip) dose of reserpine or DMSO vehicle. We used a single large dose because prior research had shown that gradual depletion of the biogenic amines with repeated small reserpine doses does not produce behavioral depression in rats. This effect only occurs with the rapid depletion associated with a large dose (Cooper et al., 1978). The dose used in the time course experiment was determined empirically in work not shown here.

Rats were tested in a swim task one, 24, 48, 72, or 168 hours after drug treatment. The swim task consisted of placing a rat in a container of room-temperature water for 15 minutes. Behavior was videotaped and later scored for the amount of time spent floating (complete absence of movement) or swimming. We equipped each rat with a set of Huang water wings, a piece of Styrofoam attached to the rat's back with Velcro. Although floating might be seen as an adaptive response in this task, we found early on that reserpine-treated rats sank to the bottom of the container and would have drowned if not for the water wings.

Figure 1 shows mean floating time in each of the 10 groups. Vehicle controls (striped bars) consistently spent about half of the test session floating, regardless of the time of testing. Reserpine (solid bars) resulted in a large deficit in swim performance, with close to a two-fold increase in floating times, relative to controls. Impairment was evident as early as one hour after drug treatment and persisted for at least 72 hours. The deficit recovered within a week.

Early and Late Components of the Deficit

Although reserpine produced deficits that were equal in magnitude across the first 72 hours of testing (see Figure 1), recent evidence suggests that different mediators might emerge at different times to impair performance. For instance, over-activation of glutamate neurons resulting from direct infusion of the excitatory amino acid transmitter into frontal

C Y T O K I N E - P U R I N E I N T E R A C T I O N S IN B E H A V I O R A L D E P R E S S I O N 193

1-HOUR POST RESERPINE TREATMENT

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FIc. 2. Effects of the interleukin-lbeta receptor antagonist (IL-113ra) on swim performance 1 hour after reserpine treatment. Rats initially were injected with DMSO (2 groups) or 6 mg/kg of reserpine (3 groups). Forty-five minutes later one DMSO group received icv infusion of the IL-11~ra and served as a control for untoward effect of the antagonist during later swim testing. Additionally, one reserpine treated group received icy infusion saline vehicle (Res+sVeh) and one reserpine group received icv infusion of 6 txg of the IL-l~lra (Res+IL-113ra). Swim testing occurred in all groups 15 minutes later.

cortex produces helplessness behavior (Hunter, Balleine, & Minor, 2003; Petty, McChesney, & Kramer, 1985). Whereas behavioral impairment occurring one hour after the injection in this paradigm is mediated by adenosine, behavioral depression at longer post-injection times (e.g., 48 or 72 hours) is not (Hunter at al., 2003). These data suggested that it would be prudent to test potential mediators of reserpine-induced depression at different time points. We chose one hour and 48 hours, respectively, as representatives of the early and late components of the recovery time course.

We assessed the potential contribution of the pro-inflammatory cytokine IL-115 to reser- pine-induced depression in two experiments. Given that a peripheral injection of endotoxin produces an IL-1 B-mediated deficit in swim performance, and that behavioral depression is a component of sickness behavior, the proinflammatory cytokine seems to be a plausible mediator of reserpine-induced swim deficits. If so, then it should be possible to reverse the behavioral malaise produced by reserpine by administering the naturally occurring receptor antagonist for IL-11~ directly into the brain.

We tested this possibility by stereotaxically implanting five groups of 10 rats each with guide cannulae in the right lateral ventricle. After recovering from surgery, three groups were injected with reserpine and two groups were injected with DMSO vehicle. Forty-five minutes later, one of the DMSO-treated groups received a 6 Ixg intracranial ventricular (icv) infusion of the IL-113 receptor antagonist (Group IL-l~ra) and served as a control for any untoward effects of the receptor antagonist on swim performance during later testing. One reserpine-treated group also received the IL-l~ra (Res+IL-1Dra) and another received saline vehicle (Res+sVeh). The other reserpine (Res) and DMSO groups received sham injections at this time. All groups were tested for swim performance 15 minutes later.

Figure 2 shows mean floating time in each of the five groups. Reserpine-treated rats (Group Res) showed a large swim deficit relative to the DMSO control one hour after the

194 MINOR, HUANG, AND FOLEY

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FI6. 3. Effects of the IL-ll3ra on swim performance 48 hours after reserpine treatment. Rats initially were injected with DMSO (1 group) or 6 mg/kg reserpine (3 groups), and then tested for swim performance 48 hours later. Fifteen minutes before swim testing one reserpine treated group received icy infusion of saline vehicle (Res+sVeh) and one reserpine group received icy infusion of the IL-1 ~ra (Res+IL-1 ~ra). Groups DMSO and Res received sham icv injections at this time.

injection. Icv injection of the IL-113ra had no untoward effect on swim performance (Group IL-1 ~ra). The antagonist also failed to improve swim performance in reserpine-treated rats (Group Res+sVeh vs Res+IL-113ra). Although we have not conducted a complete experi- ment, we have tested enough rats to be reasonably certain that the IL-l~ra also fails to improve performance 24 hours after reserpine treatment. Thus, IL-I~ does not contribute to the early component of the reserpine-induced deficit.

Next, we assessed the contribution of IL-113 to the behavioral depression occurring 48 hours after reserpine treatment. We randomly assigned rats to one of four groups of 10 rats each and implanted them with a guide cannula in the right lateral ventricle. One week after surgery, one group received an ip injection of DMSO vehicle and three groups received an ip injection of 6 mg/kg reserpine. All rats were tested for swim performance 48 hours later. Fifteen minutes before testing, one reserpine group received icv infusion of saline vehicle (Res+sVeh) and one reserpine group received icv infusion of 6 ~tg of the IL-113ra (Res+IL- ll3ra). The other two groups received sham injections (Groups DMSO & Res).

Figure 3 shows mean floating time during the swim test in each of the four groups. An injection of reserpine produced a large increase in floating time 48 hours later (Groups Res vs DMSO). Icv infusion of saline 15 minutes before testing produced a slight (but not statistically significant) increase in floating time (Group Res+sVeh). Most important, infu- sion of the IL-1 l]ra 15 minutes before testing substantially improved test performance such that there was no statistical difference between Group Res+IL-1l~ra and Group DMSO. Although we have not completed an entire experiment at the 72-hour point, we have tested enough rats to be confident that IL-I[~ also contributes to swim deficits 72 hours after reserpine treatment.

These experiments provide clear evidence for both an early and late component to reserpine-induced depression, The early component is not reversed by the IL-1 [3ra, and is not mediated by the proinflammatory cytokine. Indeed, one hour is too brief a period of time after reserpine treatment for the immune system to be activated and an immune-to- brain signal to be generated (Brandwein, 1986; Dantzer, 2001; Goehler et al., 2000). By

CYTOKINE-PURINE INTERACTIONS IN BEHAVIORAL DEPRESSION 195

contrast, the late component of the deficit is reversed by central blockade of IL-1 ~ recep- tors, and therefore, clearly depends upon the induction of IL- 1 ~ in brain.

Adenosine Signaling

Although the long-term consequence of reserpine treatment is an inability to release monoamine transmitters upon neuronal depolarization, the initial effect of the drug is likely to be excessive and unregulated transmitter release, resulting in neuronal over- excitation. These are precisely the conditions under which adenosine is recruited as a homeostatic and neuroprotective mechanism (Dragunow, 1988; Marangos, 1991; Meghji, 1991; Phillis et al., 1987). As noted earlier, adenosine is a well-established mediator of behavioral depression in the learned helplessness paradigm (Minor et al., 1994 a, b).

We assessed the potential contribution of adenosine signaling to the early and late components of reserpine depression in a lengthy series of experiments. Rats were treated with DMSO or reserpine and then tested for swim performance one or 48 hours later. Attempts to reverse impairment by treating rats with the selective A I antagonist 8- Phenyltheophylline (8-PT), the A2s antagonist alloxazine (AX), or the A 3 antagonist MRS-1220 15 minutes before swim testing were unsuccessful. However, the nonselective antagonist caffeine, and the moderately selective A2 antagonist 3,7-Dimethyl-1- propargylxanthine (DMPX), reversed both early and late deficits in a dose-dependent manner. Below we review evidence that swim deficits following reserpine treatment are mediated specifically at A2A receptors, regardless of the time of testing.

Rats were assigned randomly to one of six groups of 10 rats each. We injected four groups with 6 mg/kg reserpine. Two other groups were injected with DMSO vehicle. Forty-five minutes later, one DMSO-treated group was injected with 1.0 mg/kg of the selective A2A antagonist 8-(3-Chlorostyryl) caffeine (CSC) and served as a control during later swim testing for any untoward effects of the drug. Reserpine-treated groups received an ip injection of 0, 0.01, 0.1, or 1.0 mg/kg of CSC. All rats were tested for swim performance 15 minutes later.

Figure 4 shows mean floating time in each of the six groups. CSC alone had no untoward effects on swim performance relative to the DMSO control. Pretreatment with reserpine one hour before testing produced a large swim deficit, an effect that was com- pletely reversed by CSC in a dose-dependent manner. Thus the early, IL-1 E-independent deficit is mediated at A2A receptors and seems to parallel most closely the time course for depletion of brain monoamines (Bean et al., 1989).

We assessed the effects of CSC on swim performance 48 hours after reserpine treatment to determine whether the late component of the deficit also depends on activation of adenosine A2A receptors. Rats were assigned randomly to one of three groups of 10 rats each. One group was injected with DMSO vehicle and two groups were injected with 6 mg/kg reserpine. Swim testing occurred 48 hours later. Fifteen minutes before testing, one reserpine-treated group was injected with DMSO vehicle, and one reserpine group was injected with 1.0 mg/kg of CSC, the dose that was most effective in reversing the early deficit in the previous experiment.

Figure 5 shows mean floating times in each of the three groups. As in previous work, reserpine produced a large increase in floating time 48 hours later. The deficit was com- pletely reversed by pretreatment with 1.0 mg&g of the CSC. These data clearly indicate that the late component of reserpine-induced depression also depends on A2A receptor activation. The data also obviate an important cytokine-purine interaction.

196 MINOR, HUANG, AND FOLEY

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Dose of the AzA Antagonist 8-(3-Chlorostyryl)caffeine (mg/kg)

Fro. 4. Effects of the selective A2A antagonist 8-(3-Chlorostyryl)caffeine (CSC) on swim performance 1 hour after reserpine treatment. Rats were injected with DMSO (1 group), reserpine (4 groups), or nothing (1 group). Forty-five minutes later the untreated group received an ip injection of lmg/kg CSC and served as a control for any untoward effects of the drug on swim performance. Additionally reserpine treated groups received an ip injection of 0, 0.01, 0.1, or 1.0 mg/kg of CSC and swim tested 15 minutes later.

IL- l fl Induced Behavioral Depression

We also have conducted experiments that provide additional evidence for a role of IL- 1~1 in reserpine depression and explore more fully the cytokine-purine interaction. The foregoing experiments suggest that treatment with reserpine produces some condition (e.g., cell damage in the periphery or brain) that increases the expression of IL-1 ~l in the brain. Adenosine was recruited, perhaps as a compensatory feedback on the proinflammatory cytokine (Bshesh et al., 2002; Trincavelli, Costa, Tuscano, Lucacchini, & Martin, 2002), and activates extracellular A2A receptors to produce a swim deficit. Although much detail is missing in this hypothetical chain of events, several outcomes should be obtained if the basic sequence is correct. For instance, it should be possible to produce an immediate swim deficit by directly infusing IL-11~ into the brain.

To test this possibility, we randomly assigned rats to one of five groups of 10 rats each. Each rat initially was implanted with a guide cannula in the right lateral ventricle. One week after surgery, one group received icy infusion of saline vehicle (3 B1) and three groups received a 2 ng infusion of IL-I~. Swim testing occurred one hour later. Ten minutes before testing, the as-yet untreated group received a 6 Bg infusion of the IL-113ra and served as a control for any untoward effects of the antagonist during later swim testing. One IL-1 ~ treated group received an infusion of saline vehicle and another IL-113 group received icv infusion of 6 Bg of the IL-1 ~ra. The other two groups received sham icv injections at this time.

Figure 6 shows mean floating time during the swim test in each of the five groups. ICV infusion of IL-1 ~ produced a large swim deficit one hour later, as evidenced in the differ- ence between Groups sVeh and IL-I~. Central infusion of the IL-l~ra 15 minutes before testing had no untoward effect on swim performance. More important, infusion of the

CYTOKINE-PURINE INTERACTIONS IN BEHAVIORAL DEPRESSION 197

15 48HOURS POST RESERPINE TREATMENT

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Fro. 5. Effects of CSC on swim performance 48 hours after reserpine treatment. Rats initially were injected with DMSO (1 group) or reserpine (2 groups), and then tested for swim performance 48 hours later. Fifteen minutes before swim testing one reserpine group received an injection of DMSO (Res+DMSO), and the other reserpine group received and injection of 1.0 mg/kg 8-(3-Chlorostyryl)caffeine (Group Res+CSC).

receptor antagonist in IL-l-treated rats completely eliminated any evidence of a swim deficit, indicating that the cytokine is capable of producing a swim deficit in the near term. We also have assessed whether the IL-113 deficit depends on an interaction with A2A adenosine receptors. Rats were assigned randomly to one of six groups of 10 rats each. All rats underwent stereotaxic surgery to implant a guide cannula in the right lateral ventricle. After one week of recovery, one group received icv infusion of saline vehicle (Group sVeh) and five groups received icv infusion of 2 ng of IL- 1 ~. Forty-five minutes later, one IL-113 group received an ip injection of DMSO (Group IL-I~+DMSO), and another IL-113 group received a 1.0 mg/kg ip injection of the A2A receptor antagonist CSC (Group IL- I~I+CSC). Because we did not have a complete adenosine receptor profile for the IL-I~ swim deficit, we also assessed the effects of the nonselective adenosine receptor antagonist caffeine. If CSC were ineffective in reversing the IL-113 deficit, the comparison to caffeine would give us an indication of whether some other adenosine receptor might be involved in the cytokine deficit. Thus, one IL-1 ~l group received an ip injection of caffeine vehicle (10% alcohol, 40% propylene glycol, 50% H20; Group IL-1 ~l+cVeh), and one IL-113 group received an ip injection of 7 mg/kg of caffeine (Group IL-1 ~+Caf). Groups sVeh and IL- l~l received sham ip injections at this time. Swim testing occurred 15 minutes later.

Figure 7 shows mean floating time in the swim test in each of the six groups. Infusion of IL-I~ produced a large increase in floating time one hour later (Groups sVeh versus IL- 1~). The magnitude of the deficit was not altered by ip injection of the caffeine vehicle (Group IL-113+cVeh) or DMSO (Group IL-113+DMSO) vehicle 15 minutes before testing. Importantly, the cytokine deficit was completely eliminated by pretest treatment with the nonselective adenosine receptor antagonist caffeine (Group IL-l~+Caf) and the highly selective A2A antagonist CSC (Group IL-I~+CSC). Thus, the IL-I~ deficit, as with the other deficits described above, requires activation of an adenosine A2A receptor.

198 MINOR, HUANG, AND FOLEY

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Flc. 6. Effects of the IL- 1 lira on swim performance 1 hour after icy infusion of 1L- 113. One group received an icv infusion of saline vehicle (Group sVeh), and 3 groups received an icy infusion of 2 ng of IL-1I~, and one group received nothing. Forty-five minutes later, the as of yet untreated group received a 6 ktg infusion of the IL-l~ra and served as a control for any untoward effect of the drug on later swim performance. Additionally, one IL-1I] group received icv infusion of saliene vehicle (Group IL-l~+sVeh), while another IL-1I] group received an icv infusion of the 1L-libra (Group IL-l_+ra). Groups sVeh and IL-11] received sham treatment at this time. Swim testing occured 15 minutes later.

Overv iew

Even though reserpine's ability to induce depressive symptomology in human and ani- mal populations provided the empirical foundation of catecholamine/monoamine theories of depression (Akiskal & McKinney, 1975; Bunney & Davis, 1965; Schildkraut, 1965), the fit between data and theory was never particularly close. The main problem is that the time course for behavioral malaise generally does not match the time course for biogenic amine depletion.

The present data provide a clear answer as to why this is the case. Reserpine-induced depression has multiple determinants that emerge at different times following initial treat- ment. Moreover, the impairment appears to be a downstream consequence of reserpine's effect on the rnonoamines, rather than a direct result of their depletion per se.

An early component of the deficit is evident one hour after drug injection and persists for about 24 hours. The receptor antagonist for the proinflammatory cytokine IL-1I~ does not reverse the early deficit. Indeed, this deficit appears too quickly for significant cytokine expression to occur (Dunn & Swiergiel, 1998; Dunn, Young, Hall, & McNulty, 2002; Fleshner et al., 1995; Nathan, 1987) or the induction of the types of processes that might lead to significant immune activation (Basu et al., 2002). In this regard, the early deficit appears to match the time course for catecholamine depletion (Bean et al., 1989). Given the link between early deficit and adenosine signaling, the process of depleting brain catecholamines with a single large dose of reserpine may challenge energy homeostasis, leading to a compensatory adenosine response, and ultimately to behavioral depression.

Late-component depression emerges about 48 hours after reserpine treatment, persists for at least 24 hours, and then recovers within a week. This deficit is reversed by the

CYTOKINE-PURINE INTERACTIONS IN BEHAVIORAL DEPRESSION 199

15 1 HOUR POST IL-I~ TREATMENT

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Fro. 7. Effects of the selective A2A antagonist CSC on swim performance 1 hour after central infusion of IL- 1~. Rats initially were implated with guide cannula in the right lateral ventricle one week after surgery. One group received an icy infusion of saline vehicle (sVeh) and 5 groups received an icv infusion of 2 ng 1L-II3. Forty-five minutes later one IL-113 group received an ip injection of the nonselective adenosine receptor antagonist caffeine (7 mg/kg) and one IL-I_ group received an ip injection of caffeine vehicle. Other I L - 1 groups received an ip injection of the highly selective A2A antagonist CSC (1.0 mg/kg) or DMSO vehicle. Swim testing occurred 15 minutes later.

receptor antagonist for IL-1 ~ and is mimicked by direct infusion of the proinflammatory cytokine into the cerebral ventricles.

Perhaps most important is the finding that both early and late components of the deficit are reversed by systemic administration of selective A2A receptor antagonists. Although there is only a small literature on the potential interactions between adenosine and IL-1 ~l, it is clear that these pathways interact. For instance, application of endotoxin or IL-115 on PC12 or THP-1 cell upregulates the density of A2A receptors and increases extracellular concentrations of adenosine (Bshesh et al., 2002; Trincevelli et al., 2002). Several investi- gators have argued based on such data that adenosine (an anti-inflammatory agent) is induced as negative feedback on proinflammatory cytokines.

Adenosine A2A receptors also have a distinct and limited distribution in the brain. These receptors are largely confined to the olfactory epithelium and on the medium spiny neuron of the striopallidal tract in the striatum (El Yacoubi et al., 2001; Ferre, Fredholm, Morelli, Popoli, Fuxe, 1997; Svenningsson, LeMoine, Fisone, & Fredholm, 1999). The striatum is a crucial structure for the integration of sensory, motor, and motivational information. If behavioral depression can be thought of as the uncoupling of motivation from ongoing motor activity, then the striatum would seem to be the ideal structure in which such a process occurs. Thus, activation of Striatal adenosine A2A receptors may act as the switch for behavioral depression during sickness, traumatic stress, or major depression.

200 MINOR, HUANG, AND FOLEY

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