G protein signaling and the molecular basis of antidepressant action

Life Sciences 73 (2003) 1 – 17 www.elsevier.com/locate/lifescie

Minireview

G protein signaling and the molecular basis of antidepressant action Robert J. Donati a, Mark M. Rasenick a,b,* a

Department of Physiology and Biophysics, University of Illinois at Chicago, College of Medicine, 835 S. Wolcott Ave. M/C 901 Rm. E202, Chicago, IL 60612-7342, USA b Department of Psychiatry, University of Illinois at Chicago, College of Medicine, Chicago, IL 60612-7342, USA Received 26 June 2002; accepted 4 December 2002

Abstract Over the past four decades, a variety of interventions have been used for the treatment of clinical depression and other affective disorders. Several distinct pharmacological compounds show therapeutic efficacy. There are three major classes of antidepressant drugs: monoamine oxidase inhibitors (MAOIs), selective serotonin reuptake inhibitors (SSRIs), and tricyclic compounds. There are also a variety of atypical antidepressant drugs, which defy ready classification. Finally, there is electroconvulsive therapy, ECT. All require chronic (2– 3 weeks) treatment to achieve a clinical response. To date, no truly inclusive hypothesis concerning a mechanism of action for these diverse therapies has been formed. This review is intended to give an overview of research concerning G protein signaling and the molecular basis of antidepressant action. In it, the authors attempt to discuss progress that has been made in this arena as well as the possibility that some point (or points) along a G protein signaling cascade represent a molecular target for antidepressant therapy that might lead toward a unifying hypothesis for depression. This review is not designed to address the clinical studies. Furthermore, as it is a relatively short paper, citations to the literature are necessarily selective. The authors apologize in advance to authors whose work we have failed to cite. D 2003 Elsevier Science Inc. All rights reserved. Keywords: Depression; Fluoxetine; Tricyclic antidepressants

Introduction Several theories regarding the mechanisms of antidepressant and lithium action have been proposed (Manji et al., 1995; Rasenick et al., 1996; Duman et al., 1997; Lenox et al., 1998; Berman and Charney, * Corresponding author. Department of Physiology and Biophysics, University of Illinois at Chicago, College of Medicine, 835 S. Wolcott Ave. M/C 901 Rm. E202, Chicago, IL 60612-7342, USA. Tel.: +1-312-996-6641; fax: +1-312-996-1414. E-mail address: [email protected] (M.M. Rasenick). 0024-3205/03/$ - see front matter D 2003 Elsevier Science Inc. All rights reserved. doi:10.1016/S0024-3205(03)00249-2

2

R.J. Donati, M.M. Rasenick / Life Sciences 73 (2003) 1–17

1999; Popoli et al., 2001). It is possible that, at the level of the whole brain, elements of the hypothalamic/pituitary/adrenocortical axis are key in the ontogeny of depression and important targets of its therapeutic efficacy (Valentino and Curtis, 1991; Owens and Nemeroff, 1999; Holsboer, 2000). Nonetheless, in order to act, antidepressants are likely to have one or more primary molecular targets. Those targets may be at or near the membrane, and altered intracellular signaling (modified neurotransmitter response or responsiveness) is often among the initial effects of antidepressant treatment (Fig. 1). More specifically, the various mechanisms proposed for antidepressant action are consistent with an increase in cAMP production. Consistent with these theories, it was found that guanylyl-5V-imidodiphoshate [Gpp(NH)p]-or forskolin-stimulated adenylyl cyclase activities were significantly lower in brain membranes of suicide cases with a history of depression than that in control groups (Cowburn et al., 1994). These observations suggested that enhancement of adenylyl cyclase activity induced by antidepressants may be a relevant therapeutic effect. It is possible that the product of the G protein-adenylyl cyclase axis, cAMP, and the myriad proteins phosphorylated by cAMP-dependent protein kinases could be altered by antidepressant treatment. Consistent with the idea of chronic affective drug treatments increasing G protein-effector coupling, is the observation that lithium treatment increased protein kinase C (PKC) activation without

Fig. 1. Possible targets of chronic antidepressant (AD) treatment. 1) Increased duration neurotransmitter in the synaptic cleft. 2) h-adrenergic and serotonin receptor down regulation. 3) Increased Gsa-adenylyl cyclase coupling. 4) Increased PKA translocation and protein phosphorylation.

R.J. Donati, M.M. Rasenick / Life Sciences 73 (2003) 1–17

3

Fig. 2. Possible targets of antidepressant action. 1) Receptor sensitivity or number. Receptors might show decreased affinity for agonist or they may be internalized 2) Receptor-G protein coupling. The ability of an agonist to activate a G protein might be decreased. 3) G protein expression. Either the activation of G protein genes or the stability of G protein at the cell surface could be affected; Intrinsic properties of the G protein. The rate or extent of GTP binding or GTPase activity could be changed. 4) Coupling between G protein and effector. In this case, the ability of Gsa to activate (or Gia to inhibit) adenylyl cyclase might be modified. Effector (adenylyl cyclase) expression or activity could be altered by antidepressant treatment.

altering the amount of PKC, Gq or G11 in rat cerebral cortex (Li et al., 1993). Finally, it has been suggested that immediate early genes and other transcription factors might be activated as a result of chronic antidepressant action (Nibuya et al., 1996). The possibility exists that other gene related changes might occur as well (Hyman and Nestler, 1996). Antidepressant treatment may alter neurotransmitter function indirectly through the regulation of intracellular signaling. Antidepressant agents may be effective because they modulate converging postsynaptic signals generated in response to multiple endogenous neurotransmitters, including norepinephrine and serotonin. In this context, the signal-transducing G proteins, which play a major role in the amplification and integration of signals in the central nervous system, are in a unique position to affect the functional balance between neurotransmitter systems. There are several possible sites for antidepressant action via G proteins (Fig. 2): 1) the number or affinity of receptors could be altered; 2) the coupling between receptor and G protein could be changed; 3) the number of G proteins could be changed; or the intrinsic properties of a given G protein (e.g. affinity for GTP or rate of GTP hydrolysis) could be modified, 4) the coupling between G proteins and their effectors could be altered or 5) the effectors themselves could be increased in number or intrinsic activity. B-adrenergic receptors and antidepressant action In recent years, research has focused on the effects of chronic administration of antidepressants on various aspects of neuronal function. Many of these studies demonstrated alterations in the density and/

4

R.J. Donati, M.M. Rasenick / Life Sciences 73 (2003) 1–17

or sensitivity of several neurotransmitter receptor systems (Banerjee et al., 1977; Hertz and Richardson, 1983; Sulser, 1984; Honegger et al., 1986; Okada et al., 1986; Fishman and Finberg, 1987; Fowler and Brannstrom, 1990; Manji et al., 1991, 1992; Duman et al., 1997). In the noradrenergic system, down regulation of the h-adrenergic receptors appears to be a common effect of most tricyclic and atypical antidepressants, as well as ECT (reviewed by Sulser (1984)). This effect had been regarded as a secondary effect to the increased concentration of norepinephrine in the synaptic cleft. However, in vitro cell culture systems, like C6 glioma cells which display both h1- and h2-adrenergic receptors (Zhong and Minneman, 1993) tightly coupled to adenylyl cyclase (Homburger et al., 1980; Fishman et al., 1981), have allowed for the examination of antidepressant effects in the absence of neurotransmitter accumulation. Cultured astrocytes treated with amitryptiline showed a reduced accumulation of cAMP in response to isoproteronol (Hertz and Richardson, 1983). Other studies using cultured human fibroblasts

Fig. 3. Antidepressant-mediated h-receptor desensitization is uncoupled from increased coupling between Gsa and adenylyl cyclase. Cell membranes were assayed under stimulatory conditions for adenylyl cyclase activity. A. [Gpp(NH)p] (0.2 AM) or isoproteronol (1 AM) + [Gpp(NH)p]. B. The net effect of each ‘‘stimulant’’. [Gpp(NH)p] stimulated activity (subtracted from basal) and isoproteronol stimulated activity (subtracted from [Gpp(NH)p] stimulated activity) were calculated and compared to corresponding controls (*P < 0.05; **P < 0.02; ***P < 0.01 by one-way ANOVA and Scheffe’s tests).

R.J. Donati, M.M. Rasenick / Life Sciences 73 (2003) 1–17

5

(Honegger et al., 1986) and C6 glioma cells (Fishman and Finberg, 1987; Fowler and Brannstrom, 1990; Manji et al., 1991; Chen and Rasenick, 1995a) clearly demonstrated that antidepressants were able to down regulate h-adrenergic receptors in the absence of agonist. While down-regulation of receptors occurs, we have been able to uncouple this from the increased activation of adenylyl cyclase induced by treatment with antidepressants (Fig. 3) (Chen and Rasenick, 1995a). Finally, antidepressant induced increases in steady-state h1-adrenergic receptor mRNA levels in C6 cells have been shown (Manier et al., 1992) demonstrating that changes in transcriptional events can be elicited by antidepressants in these cells. These studies demonstrate that antidepressants have an effect on h-adrenergic receptors that is independent of the inhibition of norepinephrine uptake. It is also noteworthy that both tricyclic antidepressants and SSRI’s have been found to uncouple the regulation of adenylyl cyclase activity from 5HT1A receptors (Li et al., 1997; Hensler, 2002).

Adenylyl cyclase, cAMP and antidepressant action We first reported that long-term administration of various antidepressants enhanced [Gpp(NH)p]- and fluoride-stimulated adenylyl cyclase activity in rat cortex and hypothalamus membranes (Menkes et al., 1983). This suggested that the stimulatory A-subunit of the G protein, Gs, was a target of antidepressant action and that antidepressant treatment facilitated the activation of adenylyl cyclase by Gs. No change in the intrinsic activity of the adenylyl cyclase occurred subsequent to antidepressant treatment, since the enzyme activity in the presence of Mn2 + (without Mg2 +) was identical in the treated and control groups. No change in the dose-response to [Gpp(NH)p] was seen (the EC50 was identical for treated and control groups) and no effects of acute (1 day to 1 week) treatment were seen. Further, addition of any of the antidepressant drugs to the adenylyl cyclase assay medium was without effect (Ozawa and Rasenick, 1989; Chen and Rasenick, 1995b). Earlier studies also showed that membranes prepared from liver and kidney of antidperessant-treated rats did NOT show enhanced GsA-stimulated adenylyl cyclase activity (Menkes et al., 1983; Ozawa and Rasenick, 1989). This suggested some property unique to neurons or glia was involved in these antidepressant effects. While attempts to determine that unique factor have not yielded an answer, it has been observed that HEK293 cells (a kidney epithelial cell line) did not respond to antidepressant treatment unless they were transfected with type VI adenlylyl cyclase, a subtype of the enzyme enriched in brain and heart tissue (Chen and Rasenick, unpublished observations). The notion that increased Gsa-mediated activation of adenylyl cyclase resulted from any type of antidepressant treatment was bolstered by the finding that ECT showed a similar effect to any of the antidepressant drugs (Menkes et al., 1983; Ozawa and Rasenick, 1991). It was at this time that we suggested that Gsa enjoyed a more facile coupling to adenylyl cyclase as a result of the antidepressant treatment. Consistent with this idea, it was seen that the chronic antidepressant treatment increased ‘‘membrane fluidity’’ as determined by increased fluorescence polarization with two lipid probes (ANS and DPH). This increase was seen in membranes from animals treated with the antidepressant drugs or ECS. It was not seen in membranes from rats treated with chlorpromazine, which has a similar structure to the tricyclic antidepressants but is without antidepressant effect. These initial findings involving the stimulation of adenylyl cyclase via Gsa after antidepressant treatment have been substantiated by subsequent studies (Ozawa and Rasenick, 1989; De Montis et al., 1990; Ozawa and Rasenick, 1991; Kamada et al., 1999). Similar antidepressant-induced increases in [Gpp(NH)p]-stimulated adenylyl cyclase activity have been observed in vitro using C6 glioma cells (Chen and Rasenick, 1995a).

6

R.J. Donati, M.M. Rasenick / Life Sciences 73 (2003) 1–17

Chronic antidepressant-induced increases in stimulation of adenylyl cyclase by GTP analogs was seen by other groups as well (Andersen et al., 1984; Newman et al., 1987) however, this was not seen by all investigators (Duman et al., 1985; Okada et al., 1986). The failure to see effects in these studies was probably due to the dosage of imipramine used, since, unlike some of the other drugs, 10 mg/kg was not an effective dose. Increasing the imipramine dosage to 20 mg/kg increased Gsa stimulation of adenylyl cyclase in a manner similar to the other antidepressant drugs (Ozawa and Rasenick, 1989). There is little relationship between doses of antidepressant used in preclinical studies and those used therapeutically. Studies in rats and in cells employ doses well in excess of those used in patients. This is true even when chronic administration is required to observe effects. It does appear that animals achieve high ‘‘synaptic concentration’’ of SSRIs and that there is a concentration dependence (measured by microdialysis) for effects on 5HT uptake (Bymaster et al., 2002). Curiously, these higher doses are used in both behavioral and biochemical studies, suggesting some consistency.

G-proteins and antidepressant action G protein expression is not altered by antidepressant treatment Some groups have suggested that treatments for affective disorder might alter G protein content on the synaptic membrane. This possibility was first raised in a study where GTP binding to brain membranes was performed in rats treated chronically with lithium (Avissar et al., 1988). Changes in isoproteronolinduced GTP binding to membranes were attributed to changes in G protein content in those membranes. Given the multitude of proteins capable of binding GTP (and the likelihood that the moles of GTP bound far exceeded the molar G protein content), these results were difficult to interpret. There have also been reports that the number of G protein subunits increases subsequent to chronic antidepressant treatment (Lesch et al., 1991; Avissar and Schreiber, 1992). The ratio of receptor to cognate G protein is usually about 1:10 (Kim et al., 1994), thus it was difficult to fit changes of 10–25% in G protein content into a hypothetical framework consistent with a change in G protein signaling. Further, we and others (Lason and Przewlocki, 1993; Ozawa et al., 1994; Chen and Rasenick, 1995b; Emamghoreishi et al., 1996; Dwivedi and Pandey, 1997) have not observed any consistent antidepressant-induced changes in G protein a or hg subunit content. In our studies of G protein content, three series of antidepressant treatments (drugs or ECS) were done (Chen and Rasenick, 1995b). For any given G protein, 20–25% differences between antidepressant and control groups were observed in a single experiment. It is noteworthy, however, that in one treatment series, a 25% increase in GsA was noted for the amitrypilinetreated group and in the next, a 15% decrease was observed. Similar variations were seen for Gia 1,2 or Goa. Small changes in G protein content were seen by some investigators after certain antidepressant or antimanic treatments (Manji et al., 1995; Chen and Rasenick, 1995a,b), but no clear consensus has arisen. Thus, it appears that the effects of antidepressants on G protein signaling do not include changes in G protein gene expression. GTP-binding properties of G proteins are unaffected by antidepressant treatment Another possible site for antidepressant action is the alteration of the GTP-binding properties of Gsa (Fig. 2). This was investigated by exposing synaptic or plasma membranes to the photoaffinity GTP

R.J. Donati, M.M. Rasenick / Life Sciences 73 (2003) 1–17

7

analog, azidoanilido GTP. This compound, which is labeled with 32P on the a-phosphate, forms a covalent bond (upon UV irradiation) with the protein to which it is bound (Rasenick et al., 1994). It has previously been demonstrated that the apparent Kd of AAGTP for Gsa is about 5 AM and that the potency for AAGTP displacement by the different GTP analogues tested is as follows: [GPP(NH)P]> GDPhS > GTPgS > AAGTP > GTP (Gordon and Rasenick, 1988). Subsequent to SDS electrophoresis, individual G proteins can be identified and AAGTP binding quantified (Rasenick et al., 1994). This was done with membranes prepared from brains of rats subjected to various chronic antidepressant treatments. No changes in intrinsic AAGTP binding were noted. This was consistent with the notion that expression of G proteins was unaltered by antidepressant treatment. Curiously, interaction among G proteins as measured by the direct transfer of nucleotide (transactivation), was facilitated by chronic antidepressant treatment (Ozawa and Rasenick, 1991). Note that in membranes prepared from cerebral cortex, receptors couple poorly to G proteins, thus these studies did not address changes in the coupling between receptor and Gsa. The coupling between receptor and Ga can be addressed by the ability of GTP or GTP analogs to decrease high affinity binding of agonists to receptor. Chronic antidepressant treatment did not alter this (O’Donnell et al., 1984). Facilitated interaction between Gs and adenylyl cyclase results from antidepressant treatment Increased interaction among G proteins on the synaptic membrane suggested that increased activation of Gsa might result from increased physical coupling between Gsa and adenylyl cyclase. This was investigated using immunoprecipitation of Gsa/adenylyl cyclase complexes with anti-Gsa antibodies. This study also provided independent means to verify that there was no increase in Gsa content after antidepressant treatment. The study was done with cerebral cortex membranes from rats treated chronically with amitryptiline or ECS. In order to measure significant adenylyl cyclase activity in the immunoprecipitate, Gsa was activated with the hydrolysis-resistant GTP analog, guanylylimidodiphosphate. Presumably, this is in order to confer stability to that molecule during the immunoprecipitation process (Halliday et al., 1984). Preactivation with [Gpp(NH)p] increases the total number of Gs-adenylyl cyclase complexes (independent of antidepressant treatment). At maximum, 50% of the Gsa is immunoprecipitated, and this is independent of the association of Gsa with adenylyl cyclase. Thus, if there were an increase in the total Gsa after antidepressant treatment, the amount of Gsa in the supernatant would increase relative to that in the pellet. It did not, offering independent confirmation that antidepressant treatment did not increase the amount of Gsa. Nonetheless, the total amount of adenylyl cyclase immunoprecipitated by anti Gsa increased after antidepressant treatment, consistent with the idea that antidepressant treatment increases coupling between Gsa and adenylyl cyclase (Chen and Rasenick, 1995b). Although effects of G protein signaling are consistent for tricyclic and some atypical antidepressants as well as ECT and SSRIs, monoamine oxidase ihibitors have not been shown to increase the coupling between Gsa and adenylyl cyclase. Further studies with these antidepressants need to be done to characterize their effects, if any, on G proteins and cAMP signaling. As mentioned previously, we did not see changes in total adenylyl cyclase (Mn2 +-activated) activity (Menkes et al., 1983). Changes in the amount of type I or type VI adenylyl cyclase as measured by immunoblotting (Toki and Rasenick, unpublished observations) were also not observed. In fact, data accumulated by us over the past 18 years are consistent with chronic antidepressant treatment increasing the coupling between Gsa and adenylyl cyclase. In addition to some other parameters, this would be expected to evoke an increase in the generation of cAMP. Chronic increases in cAMP wrought by

8

R.J. Donati, M.M. Rasenick / Life Sciences 73 (2003) 1–17

antidepressant treatment might be associated with changes in cell shape or synaptic display. Unfortunately, despite significant data in two model systems (rats and cultured cells) which support this model for antidepressant action, no clinical data are currently available. As such, this must remain an intriguing hypothesis.

Antidepressants, growth factors and nerve growth It is evident from the above mentioned studies that there is an increase in Gsa/adenylyl cyclase coupling following chronic antidepressant treatment, but events downstream of adenylyl cyclase must be considered in this context as well. Long term increases in cAMP dependent protein kinase activity have been demonstrated in rat cerebral cortex in response to antidepressant treatment (Perez et al., 1989; Perez et al., 1991). Consistent with these findings, it has been reported that chronic antidepressant treatment increases the expression and activity of cAMP response element binding protein (CREB) in the rat brain (Nibuya et al., 1996; Duman et al., 1997; Takahashi et al., 1999; Thome et al., 2000). Concurrent studies by Nagakura et al. (2002) found that microsphere-embolised rats showed a decrease in phosphorylated CREB (pCREB) that was attenuated by chronic rolipram treatment (Nagakura et al., 2002). These data implicate an increase in cAMP following chronic antidepressant treatment leading to an increase in CREB activity. Note, however, that this is not a universal finding. Some studies have shown that chronic antidepressant treatment leads to a decrease in nuclear pCREB and pCREB-mediated gene transcription (Schwaninger et al., 1995; Rossby et al., 1999; Manier et al., 2002). Antidepressants and the development of new neurons The studies demonstrating increased CREB and pCREB were followed by findings where chronic antidepressant treatment led to increased neurogenesis in the adult rat hippocampus (Malberg et al., 2000). Recent evidence from Duman’s lab idicates that chronic rolipram treatment in mice increases the expression of pCREB and neurogenesis in the hippocampus (Nakagawa et al., 2002). These studies suggest that antidepressant induced increases in cAMP may play a role in neurogenesis. Over the past few years it has become increasingly evident that there is indeed birth of new cells in the brain (neurogenesis) (Gage, 2002) and that this new growth may play an important role in learning and memory (Gould and Gross, 2002). Chronic antidepressant treatment has been shown to increase the expression of both BDNF and its receptor, TrkA, in the rat hippocampus (Nibuya et al., 1996). Increased CREB expression was also observed in this same brain region (Nibuya et al., 1996) raising the possibility that CREB could act as a downstream target of antidepressant treatment, leading to increased expression of specific trophic factors. There is evidence that the promoter of the BDNF gene contains a CRE (Shieh et al., 1998; Tao et al., 1998) and this supports the possibility that antidepressants can lead to increased expression of certain trophic factors. These data were enhanced by the finding that chronic antidepressant treatment increased CREB phosphorylation and CRE-mediated gene transcription in several limbic regions of the rat brain (Thome et al., 2000). Behavioral studies using a CREB mutant mouse corroborate these data by demonstrating that CREB acts upstream of BDNF and is essential for the alterations in BDNF after chronic antidepressant treatment (Conti et al., 2002). As mentioned above, chronic antidepressant treatment has been demonstrated to increase neurogenesis (Malberg et al., 2000). Consistent with

R.J. Donati, M.M. Rasenick / Life Sciences 73 (2003) 1–17

9

antidepressants up-regulating cAMP, both cAMP and BDNF increase neuronal differentiation and neurite outgrowth of progenitor cells in vitro (Palmer et al., 1997; Takahashi et al., 1999). In addition, increased content of the neurotrophic factor S100 was seen in the hippocampus of fluoxetine-treated rats (Manev et al., 2001), while hippocampal neurogenesis was seen in mice treated with the mood stabilizer, lithium (Chen et al., 2000). Thus, facilitation of cAMP generation or increased neurogenesis could prove to be targets of antidepressant action. cAMP and Neuritogenesis Recently, a role for cAMP in neuritogenesis and growth cone guidance has been postulated (Cai et al., 2001; Mizuhashi et al., 2001; Tojima and Ito, 2001). The growth of mossy fibers from dentate granule cells in hippocampal (CA1 and CA3) cell slices was used to demonstrate a potential role of cAMP in growth cone targeting (Mizuhashi et al., 2001). Decreased cAMP levels led to errant mossy fiber growth into the CA1 region with ectopic synapse formation. It was concluded that cAMP is likely to play a role in the repulsive responses of the mossy fibers away from inappropriate targets. Two other studies demonstrated a direct role of cAMP in neuritogenesis using isolated rat neurons (Cai et al., 2001) or the neuroblastoma/glioma hybrid cell line NG108-15 (Tojima and Ito, 2001). Endogenous levels of cAMP are much higher in young neurons in which axonal growth is promoted compared to older neurons in which growth is inhibited and this growth stimulation/ growth inhibition is promoted by both myelin and myelin-associated glycoprotein (Cai et al., 2001). It was demonstrated that the switch from promotion to inhibition by these 2 proteins is mediated by a developmentally regulated decrease in endogenous cAMP levels (Cai et al., 2001). NG108-15 cells were shown to extend neurites after db-cAMP and this neuritogenesis was inhibited by high K+ (Tojima and Ito, 2001). This group also demonstrated that increased cAMP leads to an increase in the expression of voltage dependent calcium channels (VDCC) resulting in increased Ca+ + influx and suppression of neurite outgrowth. Thus, cAMP is likely to play multiple roles in contributing to synaptic shape.

Effect of antidepressants on G protein organization of synaptic signaling G protein signaling complexes at the plasma membrane have been identified as associated with specific components of the membrane and cytoskeleton (Huang et al., 1997). These domains, which contain receptors, G proteins, effector enzymes such as adenylyl cyclase and other membrane associated proteins are likely to be constrained from lateral mobility within the plane of the plasma membrane (Neubig, 1994) in part by cytoskeletal structures which form ‘‘corrals’’ on the inner membrane face (Kuo and Sheetz, 1993). The localization of G proteins to specific membrane domains such as caveolae (Li et al., 1995) and rafts has generated interest in these cholesterol and sphingolipid-rich detergent resistant membrane domains and how they effect G protein targeting and function (Bayewitch et al., 2000; Brown and London, 2000; Moffett et al., 2000). Bayewitch et al. (2000) have shown that chronic exposure to agonists of Gia- coupled receptors leads to a decrease in the cholate solubility of these G protein subunits and a ‘‘superactivation’’ of adenylyl cyclase. It is noteworthy, however, that different G protein alpha subunits are found in distinct plasma membrane domains (Oh and Schnitzer, 2001). While both Gsa and Gia can move in and out of caveolae, they are predominantly found in lipid rafts complexed with Ghg. On the other hand, Gqa couples to caveolin and is found predominantly in caveolae without

10

R.J. Donati, M.M. Rasenick / Life Sciences 73 (2003) 1–17

Ghg. Recent studies have demonstrated that Gsa is localized to detergent resistant membrane domains and the tips of elongated processes of C6 glioma cells, and this localization is altered after chronic antidepressant treatment (Toki et al., 1999; Donati et al., 2001). These experiments have raised the possibility that antidepressant treatment alters the interaction between Gsa and some element of the plasma membrane or cytoskeleton. G proteins and cytoskeletal interaction Initial studies suggesting an interaction between the cytoskeleton and G protein signaling were carried out in leukocytes, where it was seen that disruption of microtubules by drugs such as colchicine or vinblastine increased adenylyl cyclase activity in the presence of beta adrenergic agonists (Insel and Kennedy, 1978; Kennedy and Insel, 1979; Rudolph et al., 1977, 1979). Rat cerebral cortex synaptic membranes treated with microtubule disrupting drugs increased Gsa-stimulated adenylyl cyclase activity due to an apparent release of Gsa from a cytoskeletal ‘‘tether’’ which prevented it from a facile interaction with adenylyl cyclase (Rasenick et al., 1981). Later experiments using rat cerebral cortex synaptic membranes revealed that tubulin dimers, rather than intact microtubules, were the active agent (Rasenick and Wang, 1988). Tubulin has been shown to interact directly with (Rasenick et al., 1990; Wang et al., 1990; Wang and Rasenick, 1991; Yan et al., 1996; Popova et al., 1997) and transfer GTP to G protein a subunits (Hatta et al., 1986; Rasenick and Wang, 1988; Roychowdhury et al., 1993; Popova and Rasenick, 1994; Hatta et al., 1995). Experiments with tubulin-AAGTP and tubulin[Gpp(NH)p] suggest that tubulin forms complexes with the a subunits of Gs, Gi1 or Gq and activates them through a direct nucleotide transfer (transactivation) mechanism (Rasenick and Wang, 1988; Popova and Rasenick, 2000). Activation of G proteins has also been shown to modify the microtubule cytoskeleton. Approximately 50% of stably transfected CHO cells expressing the Gia-coupled, melatonin mt1 receptors were shown to undergo melatonin-induced shape change from highly flattened fibroblast morphology to a bipolar shape containing long filamentous processes. This shape change was prevented by the microtubule disrupting agent colcemid (Witt-Enderby et al., 2000). At the same time, there is mounting evidence that synaptic spines rapidly reorganize in response to neurotransmitter (Maletic-Savatic et al., 1999). The mechanism for this is unknown, and microtubules do not appear to inhabit the spine tip. Nonetheless, significant tubulin appears to be associated with synaptic membrane fractions. It is hypothesized that G protein trafficking in the cell, subsequent to activation, might provide a mechanism for activity-induced reorganization. We suggest that Gsa moves between specialized membrane domains and the cytoskeleton in response to agonist, and that this movement plays a role in the rapid reorganization of the dendrite. It is hypothesized that a possible hallmark of depression is altered association of Gsa with components of the membrane or cytoskeleton, or altered movement of Gsa in response to agonist. Thus, chronic antidepressant treatment might alter the interface between Gsa and some component of the membrane or some element of the cytoskeleton. Membrane localization of G protein signaling Previous studies demonstrated that Gsa from C6 rat glioma cells migrates from a Triton X-100 (TTX100) insoluble membrane domain to a TTX-100 soluble membrane domain in response to chronic antidepressant treatment (Toki et al., 1999). Our initial biochemical data demonstrate that Gsa localizes

R.J. Donati, M.M. Rasenick / Life Sciences 73 (2003) 1–17

11

to a different membrane domain upon antidepressant treatment (Toki et al., 1999). The Triton X-100 detergent extraction of Gsa from C6 cell membranes or rat brain membranes was increased upon treatment with amitriptyline, desipramine, and fluoxetine. Gsa, which is normally localized to glycolipid and cholesterol rich Triton insoluble membrane domains (that include caveolae) was removed from these domains by 50% in antidepressant treated C6 cells (Toki et al., 1999). Recent evidence by Oh and Schnitzer (2001) suggest that Gsa is concentrated in lipid rafts. In addition to the above, there was a comigration of adenylyl cyclase with Gsa into the more TTX-100 soluble membrane fractions. There was no comparable shift in the localization of Gia to a more TTX-100 soluble membrane domain after antidepressant treatment, suggesting that the antidepressant effect on G protein membrane localization is Gsa specific (Toki et al., 1999). Studies involving the visualization of Gsa immunoflourescence and laser scanning confocal microscopy have demonstrated that Gsa is localized to the plasma membrane as well as the cytosol in both desipramine treated (3 days, 10 AM) and control cells (Donati et al., 2001). Control cells express Gsa throughout the entire length of the cell processes with an enrichment at the distal end. Both desipramine and fluoxetine treated cells show a reduced localization of Gsa in the processes (especially the distal ends) while displaying a more supranuclear localization (Donati et al., 2001). We have recently created a functional Gsa-GFP fusion protein that will allow us to monitor the movement of Gsa in response to antidepressant treatment in real time (Yu and Rasenick, 2002). Taken together, these studies suggest that the lipid environment of the G protein may play an important role in its localization and function, and chronic antidepressant treatment alters the membrane localization of Gsa resulting in changes in the signaling cascade, particularly increased coupling to adenylyl cyclase (Fig. 4).

Fig. 4. Hypothetical model of the effect of desipramine on Gsa. Desipramine may disrupt the lipid raft and caveolar structures by altering cholesterol content or membrane structure. This enables Gsa to move out of Triton X-100 insoluble membrane domains (caveolae or rafts) and enhances coupling with adenylyl cyclase. The total amount of Gsa and adenylyl cyclase does not change.

12

R.J. Donati, M.M. Rasenick / Life Sciences 73 (2003) 1–17

Summary Despite several decades of research, the molecular basis of depressive disorders and the mechanisms whereby antidepressant drugs alleviate some of the symptoms of those disorders is unknown. Several theories are extant, and most of them are bolstered by some scientific evidence. Nonetheless, decades of research have not allowed us to understand the biological basis of depression. The cellular mechanisms of action of many antidepressants are still unknown and many depressed patients do not respond completely to antidepressant therapy. Furthermore, the side effect profile of many antidepressant drugs reduces patient compliance. Many current efforts at pharmacological intervention for depression are based upon G protein coupled receptors or uptake sites, but novel therapies have not emerged. By understanding the mechanism(s) of antidepressant action the design of new, more efficacious therapeutic agents is made possible. The evidence provided here demonstrates that signaling through adenylyl cyclase via increased coupling to activated Gsa plays a major role in mediating the actions of chronic antidepressant treatment. These results have been shown both in vivo and in vitro which highlights the significance of this signaling cascade in depression. While Gsa is unlikely a direct molecular target of antidepressant drugs, the activity of Gsa (measured by coupling to adenylyl cyclase) and its localization are definitely altered by chronic antidepressant treatment. We hope through amalgamation of studies on cellular signaling and those on synaptic plasticity that a greater understanding of antidepressant action and the biology of depression will soon be achieved.

Acknowledgements The authors thank Jiang Chen, Hiroki Ozawa and Sadamu Toki whose work contributed to this review. Thanks are also expressed to Brian Layden, Josh Sommovilla and Bindu Shah for critical comments and assistance. The work described herein was supported by NIMH.

References Andersen, P., Klysner, R., Geisler, A., 1984. Fluoride-stimulated adenylate cyclase activity in rat brain following chronic treatment with psychotropic drugs. Acta Pharmacology and Toxicology (Copenhagen) 23, 445 – 447. Avissar, S., Schreiber, G., 1992. Interaction of antibipolar and antidepressant treatments with receptor- coupled G proteins. Pharmacopsychiatry 25 (1), 44 – 50. Avissar, S., Schreiber, G., Danon, A., Belmaker, R.H., 1988. Lithium inhibits adrenergic and cholinergic increases in GTP binding in rat cortex. Nature 331, 440 – 442. Banerjee, S.P., Kung, L.S., Riggi, S.J., Chanda, S.K., 1977. Development of beta-adrenergic receptor subsensitivity by antidepressants. Nature 268 (5619), 455 – 456. Bayewitch, M.L., Nevo, I., Avidor-Reiss, T., Levy, R., Simonds, W.F., Vogel, Z., 2000. Alterations in detergent solubility of heterotrimeric G proteins after chronic activation of G(i/o)-coupled receptors: changes in detergent solubility are in correlation with onset of adenylyl cyclase superactivation. Molecular Pharmacology 57 (4), 820 – 825. Berman, R.M., Charney, D.S., 1999. Models of antidepressant action. Journal of Clinical Psychiatry 60 (Suppl 14), 16 – 20 (discussion 31 – 15). Brown, D.A., London, E., 2000. Structure and function of sphingolipid- and cholesterol-rich membrane rafts. Journal of Biological Chemistry 275 (23), 17221 – 17224.

R.J. Donati, M.M. Rasenick / Life Sciences 73 (2003) 1–17

13

Bymaster, F.P., Zhang, W., Carter, P.A., Shaw, J., Chernet, E., Phebus, L., Wong, D.T., Perry, K.W., 2002. Fluoxetine, but not other selective serotonin uptake inhibitors, increases norepinephrine and dopamine extracellular levels in prefrontal cortex. Psychopharmacology 160, 353 – 361. Cai, D., Qiu, J., Cao, Z., McAtee, M., Bregman, B.S., Filbin, M.T., 2001. Neuronal cyclic AMP controls the developmental loss in ability of axons to regenerate. Journal of Neuroscience 21 (13), 4731 – 4739. Chen, G., Masana, M.I., Manji, H.K., 2000. Lithium regulates PKC-mediated intracellular cross-talk and gene expression in the CNS in vivo. Bipolar Disorders 2 (3 Pt 2), 217 – 236. Chen, J., Rasenick, M.M., 1995a. Chronic treatment of C6 glioma cells with antidepressant drugs increases functional coupling between a G protein (Gs) and adenylyl cyclase. Journal of Neurochemistry 64 (2), 724 – 732. Chen, J., Rasenick, M.M., 1995b. Chronic antidepressant treatment facilitates G protein activation of adenylyl cyclase without altering G protein content. Journal of Pharmacological and Experimental Therapeutics 275 (1), 509 – 517. Conti, A.C., Cryan, J.F., Dalvi, A., Lucki, I., Blendy, J.A., 2002. cAMP response element-binding protein is essential for the upregulation of brain-derived neurotrophic factor transcription, but not the behavioral or endocrine responses to antidepressant drugs. Journal of Neuroscience 22 (8), 3262 – 3268. Cowburn, R.F., Marcusson, J.O., Eriksson, A., Wiehager, B., O’Neill, C., 1994. Adenylyl cyclase activity and G-protein subunit levels in postmortem frontal cortex of suicide victims. Brain Research 633 (1 – 2), 297 – 304. De Montis, G.M., Devoto, P., Gessa, G.L., Porcella, A., Serra, G., Tagliamonte, A., 1990. Selective adenylate cyclase increase in the limbic area of long-term imipramine-treated rats. European Journal of Pharmacology 180 (1), 169 – 174. Donati, R.J., Thukral, C., Rasenick, M.M., 2001. Chronic treatment of C6 glioma cells with antidepressant drugs results in a redistribution of Gs alpha. Molecular Pharmacology 59 (6), 1426 – 1432. Duman, R.S., Heninger, G.R., Nestler, E.J., 1997. A molecular and cellular theory of depression. Archives of General Psychiatry 54 (7), 597 – 606. Duman, R.S., Strada, S.J., Enna, S.J., 1985. Effect of imipramine and adrenocorticotropin administration on the rat brain norepinephrine-coupled cyclic nucleotide generating system: alterations in alpha and beta adrenergic components. Journal of Pharmacological and Experimental Therapeutics 234 (2), 409 – 414. Dwivedi, Y., Pandey, G.N., 1997. Effects of subchronic administration of antidepressants and anxiolytics on levels of the alpha subunits of G proteins in the rat brain. Journal of Neural Transmission 104 (6 – 7), 747 – 760. Emamghoreishi, M., Warsh, J.J., Sibony, D., Li, P.P., 1996. Lack of effect of chronic antidepressant treatment on Gs and Gi alpha- subunit protein and mRNA levels in the rat cerebral cortex. Neuropsychopharmacology 15 (3), 281 – 287. Fishman, P., Mallorga, P., Tallman, J., 1981. Catecholamine-induced desensitization of adenylate cyclase in rat glioma C6 Cells: Evidence for a specific uncoupling of beta-adrenergic receptors from a functional regulatory component of adenylate cyclase. Molecular Pharmacology 20, 310 – 318. Fishman, P.H., Finberg, J.P.M., 1987. Effect of the tricyclic antidepressant, desipramine on h adrenergic receptors in cultured rat C6 glioma cells. Journal of Neurochemistry 49, 282 – 289. Fowler, C.J., Brannstrom, G., 1990. Reduction in beta-adrenoceptor density in cultured rat glioma C6 cells after incubation with antidepressants is dependent upon the culturing conditions used. Journal of Neurochemistry 55 (1), 245 – 250. Gage, F.H., 2002. Neurogenesis in the adult brain. Journal of Neuroscience 22 (3), 612 – 613. Gordon, J.H., Rasenick, M.M., 1988. In situ binding of a photo-affinity GTP analog to synaptic membrane G-proteins. Federation of European Biochemical Societies 235, 201 – 206. Gould, E., Gross, C.G., 2002. Neurogenesis in adult mammals: some progress and problems. Journal of Neuroscience 22 (3), 619 – 623. Halliday, K., Stein, P., Chernoff, N., Wheeler, G., Bitensky, M., 1984. Limited trypsin proteolysis of photoreceptor GTPbinding protein: light- and GTP-induced conformational changes. Journal of Biological Chemistry 259, 516 – 525. Hatta, S., Marcus, M.M., Rasenick, M.M., 1986. Exchange of guanine nucleotide between GTP-binding proteins that regulate neuronal adenylate cyclase. Proceedings of the National Academy of Sciences USA 83, 5439 – 5443. Hatta, S., Ozawa, H., Saito, T., Amemiya, N., Ohshika, H., 1995. Tubulin stimulates adenylyl cyclase activity in rat striatal membranes via transfer of guanine nucleotide to Gs protein. Brain Research 704 (1), 23 – 30. Hensler, J.G., 2002. Differential regulation of 5-HT1A receptor-G protein interactions in brain following chronic antidepressant administration. Neuropsychopharmacology 26 (5), 565 – 573. Hertz, L., Richardson, J., 1983. Acute and chronic effects of antidepressant drugs on beta- adrenergic function in astrocytes in primary cultures: An indication of glial involvement in affective disorders? Journal of Neuroscience Research 9, 173 – 182.

14

R.J. Donati, M.M. Rasenick / Life Sciences 73 (2003) 1–17

Holsboer, F., 2000. The corticosteroid receptor hypothesis of depression Neuropsychopharmacology 23 (5), 477 – 501. Homburger, V., Lucas, M., Cantau, B., Barabe, J., Penit, J., Bockaert, J., 1980. Further evidence that desensitization of betaadrenergic-sensitive adenylate cyclase proceeds in two steps. Modification of the coupling and loss of beta-adrenergic receptors. Journal of Biological Chemistry 255 (21), 10436 – 10444. Honegger, U.E., Disler, B., Wiesmann, U.N., 1986. Chronic exposure of human cells in culture to the tricyclic antidepressant desipramine reduces the number of beta-adrenoceptors. Biochemical Pharmacology 35, 1899 – 1902. Huang, C., Hepler, J.R., Chen, L.T., Gilman, A.G., Anderson, R.G., Mumby, S.M., 1997. Organization of G proteins and adenylyl cyclase at the plasma membrane. Molecular Biology of the Cell 8 (12), 2365 – 2378. Hyman, S.E., Nestler, E.J., 1996. Initiation and adaptation - a paradigm for understanding psychotropic drug action. American Journal of Psychiatry 153, 151 – 162. Insel, P.A., Kennedy, M.S., 1978. Colchicine potentiates beta-adrenoreceptor-stimulated cyclicAMP in lymphoma cells by an action distal to the receptor. Nature 273 (5662), 471 – 473. Kamada, H., Saito, T., Hatta, S., Toki, S., Ozawa, H., Watanabe, M., Takahata, N., 1999. Alterations of tubulin function caused by chronic antidepressant treatment in rat brain. Cellular and Molecular Neurobiology 19 (1), 109 – 117. Kennedy, M., Insel, P., 1979. Inhibitor of microtubule assembly enhance beta-adrenergic and prostaglandin E1-stimulated cyclic AMP accumulation in S49 lymphoma cells. Molecular Pharmacology 16, 215 – 223. Kim, G.D., Adie, E.J., Milligan, G., 1994. Quantitative stoichiometry of the proteins of the stimulatory arm of the adenylyl cyclase cascade in neuroblastoma  glioma hybrid, NG108-15 cells. European Journal of Biochemistry 219 (1 – 2), 135 – 143. Kuo, S.C., Sheetz, M.P., 1993. Force of single kinesin molecules measured with optical tweezers. Science 260 (5105), 232 – 234. Lason, W., Przewlocki, R., 1993. The effect of chronic treatment with imipramine on the G proteins mRNA level in the rat hippocampus-an interaction with a calcium channel antagonist. Polish Journal of Pharmacology 45 (2), 219 – 226. Lenox, R.H., McNamara, R.K., Papke, R.L., Manji, H.K., 1998. Neurobiology of lithium: an update. Journal of Clinical Psychiatry 59 (Suppl 6), 37 – 47. Lesch, K.P., Aulakh, C.S., Tolliver, T.J., Hill, J.L., Murphy, D.L., 1991. Regulation of G proteins by chronic antidepressant drug treatment in rat brain: tricyclics but not clorgyline increase Go alpha subunits. European Journal of Pharmacology 207 (4), 361 – 364. Li, P.P., Sibony, D., Green, M.A., Warsh, J.J., 1993. Lithium modulation of phosphoinsitide signaling system in rat cortex: selective effect on phorbol ester binding. Journal of Neurochemistry 61, 1722 – 1729. Li, Q., Muma, N.A., Battaglia, G., Van de Kar, L.D., 1997. A desensitization of hypothalamic 5-HT1A receptors by repeated injections of paroxetine: Reduction in levels of Gi and Go proteins and neuroendocrine responses, but not in the density of 5-HT1A receptors. Journal of Pharmacology and Experimental Therapeutics 282 (3), 1581 – 1590. Li, S., Okamoto, T., Chun, M., Sargiacomo, M., Casanova, J.E., Hansen, S.H., Nishimoto, I., Lisanti, M.P., 1995. Evidence for a regulated interaction between heterotrimeric G proteins and caveolin. Journal of Biological Chemistry 270 (26), 15693 – 15701. Malberg, J.E., Eisch, A.J., Nestler, E.J., Duman, R.S., 2000. Chronic antidepressant treatment increases neurogenesis in adult rat hippocampus. Journal of Neuroscience 20 (24), 9104 – 9110. Maletic-Savatic, M., Malinow, R., Svoboda, K., 1999. Rapid dendritic morphogenesis in CA1 hippocampal dendrites induced by synaptic activity. Science 283 (5409), 1923 – 1927. Manev, R., Uz, T., Manev, H., 2001. Fluoxetine increases the content of neurotrophic protein S100 beta in the rat hippocampus. European Journal of Pharmacology 420 (2 – 3), R1 – R2. Manier, D.H., Bieck, P.R., Duhl, D.M., Gillespie, D.D., Sulser, F., 1992. The beta-adrenoceptor-coupled adenylate cyclase system in rat C6 glioma cells. Deamplification by isoproterenol and oxaprotiline. Neuropsychopharmacology 7 (2), 105 – 112. Manier, D.H., Shelton, R.C., Sulser, F., 2002. Noradrenergic antidepressants: does chronic treatment increase or decrease nuclear CREB-P. Journal of Neural Transmission 109 (1), 91 – 99. Manji, H.K., Chen, G., Bitran, J.A., Guskovsky, F., Potter, W.Z., 1992. Idazoxan down-regulates h-adrenoreceptors on C6 glioma cells in vitro. European Journal of Pharmacology 227, 275 – 282. Manji, H.K., Chen, G.A., Bitran, J.A., Gusovsky, F., Potter, W.Z., 1991. Chronic exposure of C6 glioma cells to

R.J. Donati, M.M. Rasenick / Life Sciences 73 (2003) 1–17

15

desipramine desensitizes beta- adrenoceptors, but increases KL/KH ratio. European Journal of Pharmacology 206 (2), 159 – 162. Manji, H.K., Potter, W.Z., Lenox, R.H., 1995. Signal transduction pathways: Molecular targets for lithium’s actions. Archives of General Psychiatry 52, 531 – 543. Menkes, D.B., Rasenick, M.M., Wheeler, M.A., Bitensky, M.W., 1983. Guanosine triphosphate activation of brain adenylate cyclase: enhancement by long-term antidepressant treatment. Science 219 (4580), 65 – 67. Mizuhashi, S., Nishiyama, N., Matsuki, N., Ikegaya, Y., 2001. Cyclic nucleotide-mediated regulation of hippocampal mossy fiber development: a target-specific guidance. Journal of Neuroscience 21 (16), 6181 – 6194. Moffett, S., Brown, D.A., Linder, M.E., 2000. Lipid-dependent targeting of G proteins into rafts. Journal of Biological Chemistry 275 (3), 2191 – 2198. Nagakura, A., Niimura, M., Takeo, S., 2002. Effects of phosphodiesterase IV inhibitor rolipram on microsphere embolisminduced defects in memory function and cerebral cyclic AMP signal transduction system in rats. British Journal of Pharmacology 135 (7), 1783 – 1793. Nakagawa, S., Kim, J.E., Lee, R., Malberg, J.E., Chen, J., Steffen, C., Zhang, Y.J., Nestler, E.J., Duman, R.S., 2002. Regulation of neurogenesis in adult mouse hippocampus by cAMP and the cAMP response element-binding protein. Journal of Neuroscience 22 (9), 3673 – 3682. Neubig, R.R., 1994. Membrane organization in G-protein mechanisms. Faseb Journal 8 (12), 939 – 946. Newman, M.E., Lipot, M., Lerer, B., 1987. Differential effects of chronic administration of desipramine on the cyclic AMP response in cortical slices and membranes in the rat. Journal of Neuropharmacology 26, 1127 – 1130. Nibuya, M., Nestler, E.J., Duman, R.S., 1996. Chronic antidepressant administration increases the expression of cAMP response element binding protein (CREB) in rat hippocampus. Journal of Neuroscience 16 (7), 2365 – 2372. O’Donnell, J.M., Wolfe, B.B., Frazer, A., 1984. Agonist interactions with beta adrenergic receptors in rat brain. Journal of Pharmacological and Experimental Terapeutics 228 (3), 640 – 647. Oh, P., Schnitzer, J.E., 2001. Segregation of heterotrimeric G proteins in cell surface microdomains. G(q) binds caveolin to concentrate in caveolae, whereas G(i) and G(s) target lipid rafts by default. Molecular Biology of the Cell 12 (3), 685 – 698. Okada, F., Tokumitsu, Y., Ui, M., 1986. Desensitization of beta-adrenergic receptor-coupled adenylate cyclase in cerebral cortex after in vivo treatment of rats with desipramine. Journal of Neurochemistry 47 (2), 454 – 459. Owens, M.J., Nemeroff, C.B., 1999. Corticotropin-releasing factor antagonists in affective disorders. Expert Opinion on Investigive Drugs 8 (11), 1849 – 1858. Ozawa, H., Katamura, Y., Hatta, S., Amemiya, N., Saito, T., Ohshika, H., Takahata, N., 1994. Antidepressants directly influence in situ binding of guanine nucleotide in synaptic membrane. Life Sciences 54 (13), 925 – 932. Ozawa, H., Rasenick, M.M., 1989. Coupling of the stimulatory GTP-binding protein Gs to rat synaptic membrane adenylate cyclase is enhanced subsequent to chronic antidepressant treatment. Molecular Pharmacology 36 (5), 803 – 808. Ozawa, H., Rasenick, M.M., 1991. Chronic electroconvulsive treatment augments coupling of the GTP- binding protein Gs to the catalytic moiety of adenylyl cyclase in a manner similar to that seen with chronic antidepressant drugs. Journal of Neurochemistry 56 (1), 330 – 338. Palmer, T.D., Takahashi, J., Gage, F.H., 1997. The adult rat hippocampus contains primordial neural stem cells. Molecular and Cellular Neurosciece 8 (6), 389 – 404. Perez, J., Tinelli, D., Bianchi, E., Brunello, N., Racagni, G., 1991. cAMP binding proteins in the rat cerebral cortex after administration of selective 5-HT and NE reuptake blockers with antidepressant activity. Neuropsychopharmacology 4 (1), 57 – 64. Perez, J., Tinelli, D., Brunello, N., Racagni, G., 1989. cAMP-dependent phosphorylation of soluble and crude microtubule fractions of rat cerebral cortex after prolonged desmethylimipramine treatment. European Journal of Pharmacology 172 (3), 305 – 316. Popoli, M., Mori, S., Brunello, N., Perez, J., Gennarelli, M., Racagni, G., 2001. Serine/threonine kinases as molecular targets of antidepressants: implications for pharmacological treatment and pathophysiology of affective disorders. Pharmacology and Therapeutics 89 (2), 149 – 170. Popova, J.S., Garrison, J.C., Rhee, S.G., Rasenick, M.M., 1997. Tubulin, Gq and phosphatidylinositol 4,5-bisphosphate interact to regulate phospholipase Cb1 signaling. Journal of Biological Chemistry 272, 6760 – 6765. Popova, J.S., Rasenick, M.M., 1994. Tubulin activates the G protein Gaq in a manner similar to that seen for Gas and Gai. Society of Neuroscience Abstr 20, 190.

16

R.J. Donati, M.M. Rasenick / Life Sciences 73 (2003) 1–17

Popova, J.S., Rasenick, M.M., 2000. Muscarinic receptor activation promotes the membrane association of tubulin for the regulation of Gq-mediated phospholipase Cbeta(1) signaling. Journal of Neuroscience 20 (8), 2774 – 2782. Rasenick, M.M., Chaney, K.A., Chen, J., 1996. G protein-mediated signal transduction as a target of antidepressant and antibipolar drug action: evidence from model systems. Journal of Clinical Psychiatry 57 (Suppl 13), 49 – 55 (discussion 56 – 48). Rasenick, M.M., Talluri, M., Dunn III, W.J., 1994. Photoaffinity guanosine 5’triphosphate analogs as a tool for the study of GTP-binding proteins. Methods in Enzymology 237, 100 – 110. Rasenick, M.M., Wang, N., 1988. Exchange of guanine nucleotides between tubulin and GTP-binding proteins that regulate adenylate cyclase: cytoskeletal modification of neuronal signal transduction. Journal of Neurochemistry 51, 300 – 311. Rasenick, M.M., Wang, N., Yan, K., 1990. Specific association between tubulin and G proteins: participation of cytoskeletal elements in cellular signal transduction. Advances in Secondary Messenger Phosphoprotein Research 22, 381 – 386. Rasenick, M.M., Stein, P.J., Bitensky, M.W., 1981. The regulatory subunit of adenylate cyclase interacts with cytoskeletal components. Nature 294, 560 – 562. Rossby, S.P., Manier, D.H., Liang, S., Nalepa, I., Sulser, F., 1999. Pharmacological actions of the antidepressant venlafaxine beyond aminergic receptors. International Journal of Neuropsychopharmacology 2 (1), 1 – 8. Roychowdhury, S., Wang, N., Rasenick, M.M., 1993. G protein binding and G protein activation by nucleotide transfer involve distinct domains on tubulin: regulation of signal transduction by cytoskeletal elements. Biochemistry 32, 4955 – 4961. Rudolph, S.A., Greengard, P., Malawista, S.E., 1977. Effects of colchicine on cyclic AMP levels in human leukocytes. Proceedings of the National Academy of Science U S A 74 (8), 3404 – 3408. Rudolph, S.A., Hegstrand, L.R., Greengard, P., Malawista, S., 1979. The interaction of colchicine with hormone-sensitive adenylate cyclase in human leukocytes. Molecular Pharmacology 16, 805 – 812. Schwaninger, M., Schofl, C., Blume, R., Rossig, L., Knepel, W., 1995. Inhibition by antidepressant drugs of cyclic AMP response element-binding protein/cyclic AMP response element-directed gene transcription. Molecular Pharmacology 47 (6), 1112 – 1118. Shieh, P.B., Hu, S.C., Bobb, K., Timmusk, T., Ghosh, A., 1998. Identification of a signaling pathway involved in calcium regulation of BDNF expression. Neuron 20 (4), 727 – 740. Sulser, F., 1984. Antidepressant treatments and regulation of norepinephrine-receptor- coupled adenylate cyclase systems in brain. Advances in Biochemical Psychopharmacology 39, 249 – 261. Takahashi, M., Terwilliger, R., Lane, C., Mezes, P.S., Conti, M., Duman, R.S., 1999. Chronic antidepressant administration increases the expression of cAMP- specific phosphodiesterase 4A and 4B isoforms. Journal of Neuroscience 19 (2), 610 – 618. Tao, X., Finkbeiner, S., Arnold, D.B., Shaywitz, A.J., Greenberg, M.E., 1998. Ca2+ influx regulates BDNF transcription by a CREB family transcription factor-dependent mechanism. Neuron 20 (4), 709 – 726. Thome, J., Sakai, N., Shin, K., Steffen, C., Zhang, Y.J., Impey, S., Storm, D., Duman, R.S., 2000. cAMP response element-mediated gene transcription is upregulated by chronic antidepressant treatment. Journal of Neuroscience 20 (11), 4030 – 4036. Tojima, T., Ito, E., 2001. A cyclic AMP-regulated negative feedforward system for neuritogenesis revealed in a neuroblastomaxglioma hybrid cell line. Neuroscience 104 (2), 583 – 591. Toki, S., Donati, R.J., Rasenick, M.M., 1999. Treatment of C6 glioma cells and rats with antidepressant drugs increases the detergent extraction of G(s alpha) from plasma membrane. Journal of Neurochemistry 73 (3), 1114 – 1120. Valentino, R.J., Curtis, A.L., 1991. Antidepressant interactions with corticotropin-releasing factor in the noradrenergic nucleus locus coeruleus. Psychopharmacology Bulletin 27 (3), 263 – 269. Wang, N., Rasenick, M.M., 1991. Tubulin-G protein interactions involve microtubule polymerization domains. Biochemistry 30, 10957 – 10965. Wang, N., Yan, K., Rasenick, M.M., 1990. Tubulin binds specifically to the signal-transducing proteins, Gsa and Gia1. Journal of Biological Chemistry 265, 1239 – 1242. Witt-Enderby, P.A., MacKenzie, R.S., McKeon, R.M., Carroll, E.A., Bordt, S.L., Melan, M.A., 2000. Melatonin induction of filamentous structures in non-neuronal cells that is dependent on expression of the human mt1 melatonin receptor. Cell Motility and the Cytoskeleton 46 (1), 28 – 42.

R.J. Donati, M.M. Rasenick / Life Sciences 73 (2003) 1–17

17

Yan, K., Greene, E., Belga, F., Rasenick, M.M., 1996. Synaptic membrane G proteins are complexed with tubulin in situ. Journal of Neurochemistry 66, 1489 – 1495. Yu, J.Z., Rasenick, M.M., 2002. Real-time visualization of a fluorescent G(alpha)(s): dissociation of the activated G protein from plasma membrane. Molecular Pharmacology 61 (2), 352 – 359. Zhong, H., Minneman, K.P., 1993. Close reciprocal regulation of beta 1- and beta 2-adrenergic receptors by dexamethasone in C6 glioma cells: effects on catecholamine responsiveness. Molecular Pharmacology 44 (6), 1085 – 1093.