72 MOLECULAR AND CELLULAR MECHANISMS IN DEPRESSION ALAN F. SCHATZBERG STEPHEN J. GARLOW CHARLES B. NEMEROFF Over the past three decades, considerable progress has been agent, elevated depressed mood and inhibited monoamine made in our understanding of the biology of depressive degradation by the enzyme, monoamine oxidase; imipra- disorders. Still, there are a great number of unanswered mine, a tricyclic compound originally studied as an antipsy- questions regarding the relative roles specific biological sys- chotic, had potent antidepressant effects and blocked the tems may play in pathogenesis. This debate in part reflects reuptake of norepinephrine (and to some extent serotonin) a number of methodologic factors: a possibly over broad into presynaptic neurons. definition of the clinical syndrome of major depression; lim- These observations led two groups of investigators (3,4) itations inherent in studies using indirect measurement of to argue that norepinephrine (NE) activity was decreased brain neuronal activity; problems inherent in postmortem in depressive disorders and elevated in manic or excited studies; and an overemphasis on cross-sectional rather than states. Although a low norepinephrine state was the corner- longitudinal studies. In this chapter, we review the current stone of Schildkraut’s catecholamine hypothesis (30), he status of the neurochemical and cellular features of depres- also argued for other types of dysregulation, including al- sive disorders. tered receptor functioning. Indeed, more recent data have pointed to biological heterogeneity of norepinephrine activ- ity in depression with some patients demonstrating low and BACKGROUND others apparently elevated activity (5). Serotonin (5-HT) theories, in contrast, have emphasized decreased production Although Freud put forth a hypothesis for understanding or reuptake in depression. the psychological causes of depression in his classic paper, As research has continued, investigators have noted a ‘‘Mourning and Melancholia,’’ he noted that some depres- number of other alterations in depressed patients, including sions were clearly biological in etiology. Research over the among others: elevated corticotropin-releasing hormone past 40 years has done much to point to likely ‘‘culprits’’ (CRH); elevated acetylcholine activity; increased ␥amino- that are involved in the etiology of the disorder as well as butyric acid (GABA) levels; excessive glucocorticoid activity in the mediation of treatment response; these have been in psychotic major depression; hippocampal volume loss, reviewed several times recently (1,2). perhaps reflecting the effects of excessive glucocorticoids on Early research revolved around monoaminergic theories neurogenesis, and so on. These have in turn led to or been with particular emphasis first on norepinephrine and later associated with a number of new biological hypotheses re- serotonin. The basis for invoking these systems rested largely garding why some individuals become depressed or develop on a number of pharmacologic observations that have been specific symptoms. In the following sections we review the termed ‘‘the psychopharmacologic bridge.’’ These observa- current status of these approaches. tions included: reserpine, an early antihypertensive, caused depression in some patients and depleted monoamine stores in rat brain; iproniazid, a drug studied as an antitubercular NOREPINEPHRINE Norepinephrine is a catecholamine that is found in various Stephen J. Garlow and Charles B. Nemeroff: Departments of Psychiatry tissues, including brain, plasma, sympathetic nervous sys- and Behavioral Sciences, Emory University, Atlanta, Georgia. Alan F. Schatzberg: Departments of Psychiatry and Behavioral Sciences, tem, heart, and so on. It is synthesized from the amino Stanford University, Palo Alto, California. acid tyrosine, which forms L-Dopa via the enzyme tyrosine 1040 Neuropsychopharmacology: The Fifth Generation of Progress hydroxylase. L-Dopa is converted to dopamine via dopa de- collecting 24-hr urine samples; the need for patients being carboxylase and then in turn is converted to norepinephrine drug free when studied; and the lack of surety of the optimal via dopamine ␤-hydroxylase. In the adrenal and other tis- treatment for high-MHPG patients. sues, norepinephrine is converted to epinephrine via phenyl- N-methyltransferase (PNMT). NE is degraded by the en- Tyrosine Hydroxylase/Locus Ceruleus zymes catechol-o-methyltransferase and monoamine oxi- dase. The locus ceruleus (LC) is the nucleus of the NE system Norepinephrine measured in urine or plasma is largely in brain. Neurons project from the LC to various parts of derived from non-central nervous system (CNS) sources. the brain, particularly the frontal cortex. The LC has been In contrast, much early work in this area emphasized 3- the focus of several postmortem studies of depressed patients methoxy-4-hydroxyphenylglycol (MHPG), 20% to 30% of or suicide victims. Tyrosine hydroxylase activity has been which is derived from brain. The earliest studies on urinary reported to be up-regulated in brains of suicide victims, MHPG reported significantly lower levels in depressed pa- perhaps reflecting the effect of chronic stress (19). In an- tients than healthy controls (6). Further research revealed other study, NE neurons were reported to be modestly de- low urinary MHPG levels were seen, particularly in bipolar creased in suicide victims relative to controls (20). A third depressives and a subgroup of unipolar patients. As diagnos- study reported that NE transporter sites were decreased in tic nomenclature differentiated bipolar Ifrom IIpatients, depressed subjects who committed suicide but NE neurons investigators reported low MHPG levels were characteristic were not (21). These studies are somewhat contradictory in of bipolar Iand not IIpatients(5,7,8). direction of NE changes in suicide but suggest the system Unipolar depressed patients are heterogeneous in their is altered in suicide. A possible unifying hypothesis revolves MHPG levels. As indicated previously, a subgroup of unipo- around up-regulation of TH in some neurons in an attempt lar patients demonstrate low MHPG levels, similar to those to compensate for loss of neurons or transporter sites. seen in bipolar Ipatients. Incontrast, some unipolar pa- tients demonstrate elevated MHPG levels (9). In these pa- Receptors tients, urinary free cortisol is similarly elevated (10). ␣ ␣ ␤ ␤ Catecholamine levels have been reported to parallel the Receptors for NE are grouped into 1, 2, 1, and 2 sub- ␣ state of the disorder in bipolar patients. Bipolar patients types. 2 Receptor numbers and activity can be studied demonstrate significantly lower plasma NE and E levels using platelets; ␤ receptors, using leukocytes. Both have also ␣ when depressed than when euthymic or manic. Manic bipo- been explored in postmortem brain. 2 Receptors are found ␣ lar patients demonstrate elevated CSF, urine, or plasma both presynaptically and postsynaptically. Presynaptic 2 MHPG levels than depressives or healthy controls (11–13). receptors act as thermostats to regulate NE production and ␣ These data provide a rationale for measuring catecholamine release. 2 Receptors are universally connected to adenylate output in mood disorder and invoking NE as playing an cyclase second messenger systems such that agonists inhibit etiologic role; however, critics argue that some of the cAMP formation. In contrast, ␤ receptors, which are en- changes in levels may be secondary to such features as activ- tirely postsynaptic, stimulate adenylate cyclase and cAMP ity or agitation. formation. ␣ Urinary MHPG levels have been explored as possible 2 Receptor numbers and activity have been reported in tests for predicting antidepressant response. The earliest multiple studies to be increased in the platelets of depressed studies (14,15) pointed to low MHPG levels predicting pos- patients (22,23), although there is also at least one negative ␣ itive responses to imipramine but not amitriptyline. High- study (24). 2 Receptor activity can be explored by measur- MHPG patients responded to amitriptyline (14,15). These ing cAMP responses to challenges with agonists. Mooney findings led Maas (14) to hypothesize that there were two and associates (25) reported that epinephrine suppression forms of depression—one a low MHPG state reflected a of prostaglandin-E induced cAMP formation is decreased norepinephrine depression; another characterized by high in the platelets of depressed patients. Siever and colleagues MHPG levels signified a serotonin depression. This hypoth- (26) reported norepinephrine stimulation results in blunted ␣ esis, although heuristic, has not been borne out. Subsequent adenylate cyclase responses in an E1– 2 prostaglandin cou- ␣ studies failed: (a) to demonstrate that high MHPG levels pled model. Platelet aggregation that results from 2 stimu- predicted amitriptyline response (16); and (b) in the light of lation has also been reported to be altered in depressed pa- the development of selective serotonin reuptake inhibitors tients (27). Mooney and colleagues (25) using stimulation ␣ (SSRIs) the serotonergic potency of amitriptyline was also of 2 receptors with a variety of agents, including NaF, thrown into question. In contrast, several studies have re- which directly affects G1 coupled proteins have hypothe- ported that low urinary MHPG levels do indeed predict sized that this down-regulation is not agonist specific and response to noradrenergic agents—nortriptyline,