Selegiline as immunostimulant--a novel mechanism of action?

Disease mechanisms as subtypes: Immune dysfunction in Parkinson's disease

In recent years, the contraposition between inflammatory and neurodegenerative processes has been increasingly challenged. Inflammation has been emphasized as a key player in the onset and progression of Parkinson disease (PD) and other neurodegenerative disorders. The strongest indicators of the involvement of the immune system derived from evidence of microglial activation, profound imbalance in phenotype and composition of peripheral immune cells, and impaired humoral immune responses. Moreover, peripheral inflammatory mechanisms (e.g., involving the gut-brain axis) and immunogenetic factors are likely to be implicated. Even though several lines of preclinical and clinical studies are supporting and defining the complex relationship between the immune system and PD, the exact mechanisms are currently unknown. Similarly, the temporal and causal connections between innate and adaptive immune responses and neurodegeneration are unsettled, challenging our ambition to define an integrated and holistic model of the disease. Despite these difficulties, current evidence is providing the unique opportunity to develop immune-targeted approaches for PD, thus enriching our therapeutic armamentarium. This chapter aims to provide an extensive overview of past and present studies that explored the implication of the immune system in neurodegeneration, thus paving the road for the concept of disease modification in PD.

The Immune System as a Therapeutic Target for Old and New Drugs in Parkinson's Disease

Parkinson's disease (PD) is a common neurodegenerative disease characterized by loss of dopaminergic neurons and intraneuronal accumulation of protein aggregates. The exact mechanisms leading to neuronal death in PD are not fully understood, but several different molecular pathways are involved, leading to the concept that molecular subtypes may coexist in the nosological spectrum of PD. To this respect, immune system activation, both in the periphery and inside the central nervous system, was detected as a common trait of several pathogenic pathways of PD. The current working hypothesis implies that immune cells shift towards a proinflammatory phenotype and trigger the production of neurotoxic cytokines, ultimately contributing to neurodegeneration. While it is very important to understand how commonly used antiparkinson drugs interact with such changes, the search for treatments that may directly or indirectly modulate immune function is a great opportunity for disease modification.

Safety and immunogenicity of the α-synuclein active immunotherapeutic PD01A in patients with Parkinson's disease: a randomised, single-blinded, phase 1 trial

BACKGROUND

Robust evidence supports the role of α-synuclein pathology as a driver of neuronal dysfunction in Parkinson's disease. PD01A is a specific active immunotherapy with a short peptide formulation targeted against oligomeric α-synuclein. This phase 1 study assessed the safety and tolerability of the PD01A immunotherapeutic in patients with Parkinson's disease.

METHODS

We did a first-in-human, randomised, phase 1 study of immunisations with PD01A, followed by three consecutive study extensions. Patients aged 45-65 years with a clinical diagnosis of Parkinson's disease (≤4 years since diagnosis and Hoehn and Yahr Stage 1 to 2), imaging results (dopamine transporter single photon emission CT and MRI) consistent with their Parkinson's disease diagnosis, and on stable doses of Parkinson's disease medications for at least 3 months were recruited at a single private clinic in Vienna, Austria. Patients were randomly assigned (1:1), using a computer-generated sequence with varying block size, to receive four subcutaneous immunisations with either 15 μg or 75 μg PD01A injected into the upper arms and followed up initially for 52 weeks, followed by a further 39 weeks' follow-up. Patients were then randomly assigned (1:1) again to receive the first booster immunisation at 15 μg or 75 μg and were followed up for 24 weeks. All patients received a second booster immunisation of 75 μg and were followed up for an additional 52 weeks. Patients were masked to dose allocation. Primary (safety) analyses included all treated patients. These four studies were registered with EU Clinical Trials Register, EudraCT numbers 2011-002650-31, 2013-001774-20, 2014-002489-54, and 2015-004854-16.

FINDINGS

32 patients were recruited between Feb 14, 2012, and Feb 6, 2013, and 24 were deemed eligible and randomly assigned to receive four PD01A priming immunisations. One patient had a diagnosis change to multiple system atrophy and was withdrawn and two patients withdrew consent during the studies. 21 (87%) of 24 patients received all six immunisations and completed 221-259 weeks in-study (two patients in the 15 μg dose group and one patient in the 75 μg dose group discontinued). All patients experienced at least one adverse event, but most of them were considered unrelated to study treatment (except for transient local injection site reactions, which affected all but one patient). Serial MRI assessments also ruled out inflammatory processes. Systemic treatment-related adverse events were fatigue (n=4), headache (n=3), myalgia (n=3), muscle rigidity (n=2), and tremor (n=2). The geometric group mean titre of antibodies against the immunising peptide PD01 increased from 1:46 at baseline to 1:3580 at week 12 in the 15 μg dose group, and from 1:76 to 1:2462 at week 12 in the 75 μg dose group. Antibody titres returned to baseline over 2 years, but could be rapidly reactivated after booster immunisation from week 116 onwards, reaching geometric group mean titres up to 1:20218.

INTERPRETATION

Repeated administrations of PD01A were safe and well tolerated over an extended period. Specific active immunotherapy resulted in a substantial humoral immune response with target engagement. Phase 2 studies are needed to further assess the safety and efficacy of PD01A for the treatment of Parkinson's disease.

FUNDING

AFFiRiS, Michael J Fox Foundation.

Clomipramine, a tricyclic antidepressant, and selegiline, a monoamine oxidase-B inhibitor, modulate the activity of phagocytic cells after oral administration in mice

OBJECTIVES

Our aim was to find out whether clomipramine, a tricyclic antidepressant, and selegiline, a monoamine oxidase-B inhibitor, influence the activity of phagocytic cells after in-vivo administration in mice.

METHODS

Clomipramine and selegiline were administered to Balb/c mice orally at a dose of 1 mg/kg, 7 or 14 times. IL-1β and nitric oxide (NO) levels were measured in supernatants of the peritoneal macrophage cultures stimulated in vitro with lipopolysaccharide from Escherichia coli. The phagocytic activity of the granulocytes and monocytes was determined using a commercial Phagotest 24 and 72 h after the last dose of the investigated drugs.

KEY FINDINGS

Seven doses of clomipramine or selegiline decreased IL-1β production, while a rise in its synthesis was observed after 14 doses of selegiline. Clomipramine administered 14 times increased NO production. Clomipramine and selegiline administered seven times reduced the percentage of phagocytosing granulocytes. The drugs administered 14 times increased the percentage of phagocytosing granulocytes and decreased the percentage of phagocytosing monocytes.

CONCLUSIONS

Both clomipramine and selegiline administered in vivo changed the phagocytic activity of blood cells and IL-1β and NO production by murine peritoneal macrophages. This effect depended on the drug, the number of doses and the type of phagocytic cells.

Immunity and longevity

The role of immune system is to protect the organism from the not built-in program-like alterations inside and against the agents penetrating from outside (bacteria, viruses, and protozoa). These functions were developed and formed during the evolution. Considering these functions, the immune system promotes the lengthening of lifespan and helps longevity. However, some immune functions have been conveyed by men to medical tools (e.g., pharmaceuticals, antibiotics, and prevention), especially in our modern age, which help the struggle against microbes, but evolutionarily weaken the immune system. Aging is a gradual slow attrition by autoimmunity, directed by the thymus and regulated by the central nervous system and pineal gland. Considering this, thymus could be a pacemaker of aging. The remodeling of the immune system, which can be observed in elderly people and centenarians, is probably not a cause of aging, but a consequence of it, which helps to suit immunity to the requirements. Oxidative stress also helps the attrition of the immune cells and antioxidants help to prolong lifespan. There are gender differences in the aging of the immune system as well as in the longevity. There is an advantage for women in both cases. This can be explained by hormonal differences (estrogens positively influences both processes); however, social factors are also not excluded. The endocrine disruptor chemicals act similar to estrogens, like stimulating or suppressing immunity and provoking autoimmunity; however, their role in longevity is controversial. There are some drugs (rapamycin, metformin, and selegiline) and antioxidants (as vitamins C and E) that prolong lifespan and also improve immunity. It is difficult to declare that longevity is exclusively dependent on the state of the immune system; however, there is a parallelism between the state of immune system and lifespan. It seems likely that there is not a real decline of immunity during aging, but there is a remodeling of the system according to the claims of senescence. This is manifested in the remaining (sometimes stronger) function of memory cells in contrast to the production and number of the new antigen-reactive naive T-cells.

L-Deprenyl reverses age-associated decline in splenic norepinephrine, interleukin-2 and interferon-γ production in old female F344 rats

Aging in female rats is associated with cessation of reproductive cycles, development of mammary cancer, and increased incidence of autoimmune diseases. Previously, we demonstrated an age-related decline in sympathetic noradrenergic (NA) innervation in the spleen and lymph nodes of female F344 rats accompanied by significantly reduced natural killer cell activity, interleukin (IL)-2 and interferon (IFN)-γ production, and T- and B-cell proliferation, suggesting possible links between sympathetic activity and immunosenescence.

OBJECTIVES

The aim of this study is to investigate the effects of L-(-)-deprenyl, a monoamine oxidase-B inhibitor, on the sympathetic nervous system and cell-mediated immune responses in old female rats.

METHODS

Low doses of L-deprenyl (0.25 or 1.0 mg/kg body weight, BW) were administered intraperitoneally to 19- to 21-month-old female F344 rats for 8 weeks. To assess the stereoselectivity of the effects of deprenyl on splenic sympathetic activity and immune responses, the D-enantiomer (D-(+)-deprenyl; 1.0 mg/kg BW) was also included in the studies. Norepinephrine (NE) concentration and content, and mitogen-induced T-cell proliferation and cytokine production were assessed in the splenocytes after deprenyl treatment.

RESULTS

Treatment with L-deprenyl reversed the age-related decrease in NE concentration and content and IFN-γ production, and increased IL-2 production in the spleen while D-deprenyl did not affect the age-associated reduction in splenic NE levels or cytokine production.

CONCLUSIONS

These findings demonstrate that L-deprenyl exerts neurorestorative and immunostimulatory effects on the sympathetic nervous system and cell-mediated immune responses during aging and provides evidence for a causal relationship between some aspects of immunosenescence and the age-related decline in sympathetic nerves in the spleens of female F344 rats.

MAO-B inhibitors: multiple roles in the therapy of neurodegenerative disorders?

Monoamine oxidases play a central role in catecholamine catabolism in the central nervous system. The biochemical and pharmacological properties of inhibitors of the monoamine oxidase type B are reviewed. The evidence for biochemical activities distinct from their ability to inhibit MAO-B is discussed, including possible antioxidative and antiapoptotic activities of these agents. The significance of these properties for the pharmacological management of Parkinson's disease and the evidence for a neuroprotective effect of one such agent (selegiline) is also discussed.

Increase of monoamine oxidase-B activity in the brain of scrapie-infected hamsters

In the present study, the purpose is to determine activities of monoamine oxidases (MAO) in the brain of 263K scrapie-infected hamsters during the development of this experimental prion disease. Indeed, MAO activity modifications which have already been related in aging and neurodegenerations is suspected to be involved in the neuron loss process by elevated hydrogen peroxide formation. Monoamine oxidase type A (MAO-A) and B (MAO-B) activities were followed in the brain at different stages of the disease. MAO-A activity did not change significantly during the evolution of the disease. However, concerning the MAO-B activity, a significant increase was observed from 50 days post-infection and through the course of the disease and reached 42.9+/-5.3% at its ultimate stage. Regarding these results, MAO-B could be a potential therapeutic target then we have performed a pre-clinical treatment with irreversible (Selegiline or L-deprenyl) or and reversible (MS-9510) MAO-B inhibitors used alone or in association with an anti-scrapie drug such as MS-8209, an amphotericin B derivative. Our results show that none of the MAO-B inhibitors used was able to delay the onset of the disease. Neither these MAO-B inhibitors nor R-NMDA inhibitors (MK-801) can enhance the effects of MS-8209. The present findings clearly indicate a significant increase of cerebral MAO-B activity in scrapie-infected hamsters. Furthermore, inhibitors of MAO-B do not have any curative or palliative effect on this experimental model indicating that the raise of this activity is probably more a consequence rather than a causal event of the neurodegenerative process.

L-deprenyl protects against rotenone-induced, oxidative stress-mediated dopaminergic neurodegeneration in rats

The present study investigated oxidative damage and neuroprotective effect of the antiparkinsonian drug, L-deprenyl in neuronal death produced by intranigral infusion of a potent mitochondrial complex-I inhibitor, rotenone in rats. Unilateral stereotaxic intranigral infusion of rotenone caused significant decrease of striatal dopamine levels as measured employing HPLC-electrochemistry, and loss of tyrosine hydroxylase immunoreactivity in the perikarya of ipsilateral substantia nigra (SN) neurons and their terminals in the striatum. Rotenone-induced increases in the salicylate hydroxylation products, 2,3- and 2,5-dihydroxybenzoic acid indicators of hydroxyl radials in mitochondrial P2 fraction were dose-dependently attenuated by L-deprenyl. L-deprenyl (0.1-10mg/kg; i.p.) treatment dose-dependently attenuated rotenone-induced reductions in complex-I activity and glutathione (GSH) levels in the SN, tyrosine hydroxylase immunoreactivity in the striatum or SN as well as striatal dopamine. Amphetamine-induced stereotypic rotations in these rats were also significantly inhibited by deprenyl administration. The rotenone-induced elevated activities of cytosolic antioxidant enzymes superoxide dismutase and catalase showed further significant increase following L-deprenyl. Our findings suggest that unilateral intranigral infusion of rotenone reproduces neurochemical, neuropathological and behavioral features of PD in rats and L-deprenyl can rescue the dopaminergic neurons from rotenone-mediated neurodegeneration in them. These results not only establish oxidative stress as one of the major causative factors underlying dopaminergic neurodegeneration as observed in Parkinson's disease, but also support the view that deprenyl is a potent free radical scavenger and an antioxidant.

Acute and delayed effect of (−) deprenyl and (−) 1-phenyl-2-propylaminopentane (PPAP) on the serotonin content of peritoneal cells (white blood cells and mast cells)

Acute and delayed (hormonal imprinting) effect of (-) deprenyl and its derivative without MAO-B inhibitory activity (-) PPAP, were studied on cells of the peritoneal fluid (lymphocytes, monocytes, granulocytes and mast cells) by flow cytometric and confocal microscopic analysis. Thirty minutes after treatment of 6-week-old female animals, deprenyl was ineffective while PPAP significantly increased the serotonin level of these cells. Three weeks after treatment at weaning, deprenyl drastically decreased the serotonin level of each cell type, while PPAP moderately but significantly increased the serotonin level of monocytes, granulocytes and mast cells. This means that the two related molecules have different effects on the immune cells, which seem to be independent of MAO-B inhibition. The experiments emphasize the necessity of studying the prolonged effects of biologically active molecules, even if they are without acute effects. As serotonin is a modulator of the immune system, the influence on immune cells of the molecules studied can contribute to their enhancing effect.

Effect of 1,2,3,4,-tetrahydroisoquinoline administration under conditions of CYP2D inhibition on dopamine metabolism, level of tyrosine hydroxylase protein and the binding of [3H]GBR 12,935 to dopamine transporter in the rat nigrostriatal, dopaminergic system

Current concepts of Parkinson's disease (PD) postulate that interaction between neurotoxins and specific genetic background may play an important role in pathogenesis of PD. Therefore, the effect of multiple administration of 1,2,3,4-tetrahydroisoquinoline (TIQ) under conditions of CYP2D blockade on the expression of key markers of PD was studied in the rat striatum (STR) and substantia nigra (SN). TIQ administered alone (50 mg/kg i.p. twice daily for 14 days) markedly decreased the level of tyrosine hydroxylase protein (TH) in the STR; however, this effect was not accompanied by reduction of dopamine (DA) concentration and [(3)H]GBR 12,935 binding to dopamine transporter (DAT). Administration of CYP2D inhibitor, quinine, jointly with TIQ lowered the levels of TH and DA in that structure, but slightly increased DAT binding. In the SN, treatment with TIQ alone did not change TH level although it enhanced DA content and decreased [(3)H]GBR 12,935 binding to DAT in the substantia nigra pars compacta (SNc). Neither the TH level nor DA concentration was affected by the combined treatment, although DAT binding was still reduced in the SN. TIQ did not change the total DA catabolism in the STR, but caused its inhibition in the SN. It strongly depressed the levels of intraneuronal DA metabolite DOPAC and enhanced that of extraneuronal 3-MT in either structure. TIQ more weakly affected the levels of both DA metabolites in the presence of quinine. Our results suggest that endogenous TIQ may act rather as neuromodulator but not as parkinsonism-inducing neurotoxin in the rat brain.

Why (--)deprenyl prolongs survivals of experimental animals: increase of anti-oxidant enzymes in brain and other body tissues as well as mobilization of various humoral factors may lead to systemic anti-aging effects

(--)Deprenyl, a monoamine oxidase B (MAO B) inhibitor is known to upregulate activities of anti-oxidant enzymes such as superoxide dismutase (SOD) and catalase (CAT) in brain dopaminergic regions. The drug is also the sole chemical which has been repeatedly shown to increase life spans of several animal species including rats, mice, hamsters and dogs. Further, the drug was recently found to enhance anti-oxidant enzyme activities not only in brain dopaminergic regions but also in extra-brain tissues such as the heart, kidneys, adrenal glands and the spleen. We and others have also observed mobilization of many humoral factors (interferone (INF)-gamma, tumor necrosis factor (TNF)-alpha, interleukine (IL)-1beta,2,6, trophic factors, etc.) and enhancement of natural killer (NK) cell functions by (-)deprenyl administration. An apparent extension of life spans of experimental animals reported in the past may be better explained by these new observations that (-)deprenyl upregulate SOD and CAT activities not only in the brain but also in extra-brain vital organs and involve anti-tumorigenic as well as immunomodulatory effect as well. These combined drug effects may lead to the protection of the homeostatic regulations of the neuro-immuno-endocrine axis of an organism against aging.

Modulation of neuroendocrine--immune signaling by L-deprenyl and L-desmethyldeprenyl in aging and mammary cancer

The aging process is characterized by a decline in cellular functions of diverse systems of the body, including the neuroendocrine-immune network. One neuroendocrinological theory of aging is based on findings that the loss of hypothalamic neurotransmitter functions and an imbalance in hormonal secretion contribute to the cessation of reproductive cycles and the development of mammary and pituitary tumors. One potential cause of immunosenescence is an age-related decline in the regulatory functions of sympathetic noradrenergic nerve fibers whose neurotransmitters signal lymphoid cells in the bone marrow, thymus, spleen, and lymph nodes. In addition to impairment caused by the generation of free radicals during numerous biochemical processes, there is a shift in the pro-oxidant/anti-oxidant balance resulting in cellular oxidative stress and hastening the aging process. Altered interactions between the neuroendocrine system and the immune system are associated with increased incidence, development, and growth of breast cancer and other neoplastic diseases. We have demonstrated that the disruption in the neuroendocrine-immune interactions in old rats, and in female rats with mammary tumors, can be reversed by deprenyl, a monoamine oxidase inhibitor. Deprenyl treatment leads to enhanced central and peripheral catecholaminergic activity and a readjustment of immunological responses. In this brief review, the nature and changes in the bi-directional communication between the neuroendocrine system and immune system and the possible mechanism(s) of actions of deprenyl in restoring these interactions during aging and mammary cancer are discussed.

Pharmacological interventions in aging and age-associated disorders: potentials of propargylamines for human use

Past studies including our own have confirmed that chronic administration of deprenyl can prolong life spans of at least four different animal species. Pretreatment with the drug for several weeks increases activities of superoxide dismutase (SOD) and catalase (CAT) in selective brain regions. An up-regulation of antioxidant enzyme activities can also be induced in organs such as the heart, kidney, spleen, and adrenal gland, and all are accompanied by an increase in mRNA levels for SODs in these organs. The effect of deprenyl on enzyme activities has a dose-effect relationship of a typical inverted U shape. A similar inverted U shape also has emerged for the drug's effect on survival of animals. An apparent parallelism observed between these two effects of the drug seems to support our contention that the up-regulation of antioxidant enzymes is at least partially responsible for the life-prolonging effect on animals. Further, when a clinically applied dose of the drug for patients with Parkinson's disease was given to monkeys, SOD and CAT activities were increased in striatum of these monkeys, which suggests potential for the drug's applicability to humans. The drug was also found to increase concentrations of cytokines such as interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha) in the above rat organs. Together with past reports demonstrating that deprenyl increases natural killer (NK) cell functions and interferon-gamma, and prevents the occurrence of malignant tumors in rodents and dogs, the mobilization of these humoral factors may therefore be included as possible mechanisms of action of deprenyl for its diverse antiaging and life-prolonging effects. The potentials of propargylamines, (-)deprenyl in particular, for human use as antiaging drugs remain worthy of exploration in the future.

Different action on dopamine catabolic pathways of two endogenous 1,2,3,4-tetrahydroisoquinolines with similar antidopaminergic properties

The effect of single and multiple 1-methyl-1,2,3,4-tetrahydroisoquinoline (1MeTIQ) and 1-benzyl-1,2,3,4-tetrahydroisoquinoline (1BnTIQ) administration on concentrations of dopamine and its metabolites: homovanillic acid (HVA) and 3,4-dihydroxyphenylacetic acid (DOPAC) and 3-methoxytyramine (3MT) in three brain areas was studied HPLC with electrochemical detection in Wistar rats. The rate of dopamine catabolism in the striatum along the N-oxidative and O-methylation pathways was assessed by calculation of the ratio of appropriate metabolites to dopamine concentration. In addition, the spontaneous and apomorphine-stimulated locomotor activity, and muscle rigidity was studied after acute administration of 1MeTIQ and 1BnTIQ. We have found that 1MeTIQ did not change the level of dopamine and HVA in all investigated structures both after a single and chronic administration. However, the levels of intermediary dopamine metabolites, DOPAC and 3MT, were distinctly affected. The level of DOPAC was strongly depressed (by 60-70%) while the level of extraneuronal matabolite 3MT was significantly elevated (by 170-200%). In contrast to 1MeTIQ, 1BnTIQ depressed the level of dopamine (by approximately 60%) and increased the level of total metabolite, HVA, (by 40%) especially in the striatum, but the levels of DOPAC and 3MT remained unchanged. The paper has shown that 1MeTIQ and 1BnTIQ produced different effects on dopamine catabolism. Potential neuroprotective compound 1MeTIQ did not change the rate of total dopamine catabolism, it strongly inhibited the monoamine oxidase (MAO)-dependent catabolic pathway and significantly activated the catechol-O-methyltransferase (COMT)-dependent O-methylation. In contrast 1BnTIQ, a compound with potential neurotoxic activity, produced the significant increase of the rate of dopamine metabolism with strong activation of the oxidative MAO-dependent catabolic pathway. Interestingly, both compounds produced similar antidopaminergic functional effects: antagonism of apomorphine hyperactivity and induction of muscle rigidity. The results may explain the biochemical basis of the neuroprotective and of the neurotoxic properties endogenous brain tetrahydroisoquinoline derivatives.

Common properties for propargylamines of enhancing superoxide dismutase and catalase activities in the dopaminergic system in the rat: implications for the life prolonging effect of (-)deprenyl

(-)Deprenyl has been reported to prolong the life span of different animal species. Further, the drug effectively increases antioxidant enzyme activities such as superoxide dismutase (SOD) and catalase (CAT) in brain dopaminergic regions. We have found that the effect of the drug on antioxidant enzyme activities is highly dose dependent, increasing with an increasing dose, however, a higher dose becomes less effective and an excessive dose becomes adversely effective. Most importantly, an optimal dose for the effect varies widely depending on animal species, strain, sex, age and duration of the treatment, which may at least partly explain discrepancies reported among different studies in the past. From the parallelism of the dose-effect relationship of the drug between life span extension and increasing endogenous antioxidant enzyme activity, we have suggested that the above two effects of (-)deprenyl may be causally related. This review summarizes our past series of studies and also reports our very recent observation that other propargylamines such as rasagiline and (R)-N-(2-heptyl)-N-methylpropagylamine (R-2HMP) also share the property of enhancing antioxidant enzyme activities. Further, our most recent study has found that these propargylamines increase antioxidant enzyme activities not only in brain dopaminergic regions but in extra-brain dopaminergic tissues such as the heart and kidneys. These observations are discussed in relation to the life prolonging effect of (-)deprenyl reported in the past.

Differential effect of a single high dose of the tricyclic antidepressant imipramine on interleukin-1beta and tumor necrosis factor-alpha secretion following an in vivo lipopolysaccharide challenge in rats

Tricyclic antidepressants (TCAs) of which imipramine is one, are commonly used in the treatment of depressive disorders and other forms of psychiatric illness. There have been many reports regarding the suppressive effects of TCAs on immune function. However, information is still limited regarding the effects of TCAs on the immune system, as many of the studies conducted to date have concentrated on in vitro exposure to such drugs, or ex vivo measures of immunity following drug administration. Thus in the present investigation, an in vivo challenge with bacterial lipopolysaccharide (LPS) (100 microg/kg; i.p.) was used to assess immunocompetence following administration of a single high dose of the TCA, imipramine (100 mg/kg, p.o.). The results demonstrated that imipramine pretreatment inhibits LPS-induced increases in serum concentrations of the proinflammatory cytokine, tumor necrosis factor (TNF)-alpha both 3 and 6 h, following administration. However, LPS-induced interleukin (IL)-1beta secretion was not significantly altered following imipramine treatment at either of the timepoints examined. In addition, serum concentrations of corticosterone and the antiinflammatory cytokine IL-10 were measured, and imipramine treatment failed to alter either basal, or LPS-induced increases in these immunosuppressive agents. In conclusion, although IL-1beta and TNF-alpha are both macrophage-derived proinflammatory cytokines, the present study demonstrates a differential sensitivity of these cytokines to the suppressive effects of the TCA imipramine. Furthermore, the suppressive effects of imipramine on LPS-induced TNF-alpha secretion could not be attributed to either increased glucocorticoid levels, or increased secretion of the antiinflammatory cytokine IL-10. The relevance of these findings to antidepressant-induced immunotoxicity are discussed.