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European NeuropaLvchopharmacoh~gg", 2 (I992) 433-.441 t , 1992 Elsevier Science Publishers B.V. All rights reserved 0924-977X/92/$05.00 NEUPSY 00068

Mode of action of the triazolobenzodiazepines in the treatment of panic attacks: a hypothesis Dominique Van Gool, Paul lgodt and Hugo De Cuyper Department o/P,sychiatry, Catholic L)fiversitv q/'Leuven, Leuven, Belgium (Received 3 December, 1991) (Revised, received 16 June, 1992) (Accepted 30 June, 1992)

Key words." Triazolobenzodiazepines; Locus coeruleus: Corticotropin-releasing hormone; Platelet-activating factor

Summary Alprazolam (Xanax) or 8-chloro-1-methyl-6-phenyl-4H-S-triazolobenzodiazepine is a potent drug for the treatment of anxiety disorders. The chemical structure differs from the classical benzodiazepines by incorporation of the triazoloring. Due to the triazolo ring, the drug can have additional modes of action than the normal benzodiazepines. The triazolobenzodiazepines are potent inhibitors of the platelet-activating factor. This factor is a potent stimulator of the corticotropin-releasing hormone. This hormone has an effect on the hypothalamo pituitary adrenal axis but the corticotropin-releasing hormone is also known to be a stimulator of the locus coeruleus. The corticotropin-releasing hormone in patients with panic attacks is elevated. This could be a result of the hyperactive metabolism which is observed by positron emission tomographic (PET) studies of the right parahippocampal area.

Introduction In the treatment of panic attacks, different therapeutic regimens are used: the behaviour therapy, the antidepressant drugs, and the classical benzodiazepines in high doses. There are also, more recently, the triazolobenzodiazepines. They have, as the other benzodiazepines, a sedative, anxiolytic, myorelaxant and anti-convulsive effect, but they are also potent anti-panic drugs and antidepressant qualities are present as well (Fawcett et al., 1982: Dawson el al., 1984). Alprazolam can act as an anxiolytic drug because of its effect on different sites in the brain which are Correspondence to." D. Van Gool, M.D., Department of Psychiatry, University Hospital St.-Raphael, Capucijnenvoer, 3000 Leu ven, Belgium. Tel.: (16) 215792.

known to be related to the origin of anxiety. There is the action from the classical benzodiazepines on the benzodiazepine receptors, related to the gamma-aminobutyric acid (GABA) receptors (Clow et al., 1985), but due to the triazoloring, there is also an effect on the platelet-activatmg factor (Kornecki et al., 1984) and on the G-protein regulated adenylate cyclase complex (Mooney et al., 1985), which is related to antidepressant effects of tricyclic antidepressants and electroshock-therapy (Menkes et al., 1983). In relation to the effect on the platelet-activating factor, the link can be made to the corticotropin-releasing hormone and the hypothalamo-pituitary-adrenal axis (Dilsaver, 1989). In addition there is an effect on the extracerebral benzodiazepine receptors that can be found intracellularly on the mitochondria where they play a role in the regulation of the

434

steroidogenesis (Papadopoulos, 1991). The mitochondrial benzodiazepine receptor has also a corticosteroid binding site. The steroids are potent inhibitors of benzodiazepine binding (Deckert el al., 1991 ). We intend to describe the rnode of action of the triazolobenzediazepines, especially the actions due to the triazolo group. Finally, we will try to formulate a hypothetical model of the etiology of panic attacks and the rationale of treatment with triazolobenzodiazepines.

Mode of action of the triazolobenzodiazepines Alprazolam acts as a benzodiazepine and has no important metabolites (Fawcet ct al.. 1982; Dawson et al,, 1984). Binding studies showed that the affinity for the benzodiazepine receptor by alprazolam is two times higher than the affinity seen with diazepam (Sethy el al., 1982). The ratio BZ1/BZ2 remains unknown. Alprazolam significantly enhances the chloride uptake in neurons and that effect can be blocked by Ro15-1788, a partial inverse agonist (Hunkeler et al., 1981). High concentrations however decreased the amount of chloride uptake. This effect was also already seen with flurazepam in electrophysiological studies of the frog spinal cord (Nistri et al., 1984; Choi et al., 1981). The GABA-stimulated chloride uptake, inhibited by 30 I~M alprazolam, was not antagonized by Ro15-1788 indicating that the inhibitory effect of alprazolam was due to a direct blockade of the GABA-gated chloride channel and not to a blockade of benzodiazepine receptors (Obata et al., 1988). The effect of alprazolam on the benzodiazepine receptors can contribute to the anti-panic effect because other, non-triazolobenzodiazepines, can inhibit a panic attack, especially in high dose. Clonazepam (2.5 rag/day) (Tesar, 1987,1991) and lorazepam (6 mg/day) (Charney el al., 1989) have been shown to be as effective as alprazolam. The following effects are specific for benzodiazepines with a triazologroup. At the moment there is no evidence for other benzodiazepines to have these properties. It is clear that especially these mechanisms are responsible for the antidepressant action of the triazolobenzodiazepines. The effectiveness of alprazolam as an antidepressant is confirmed in double-blind comparative studies with imipramine (Overall et al., 1987a,b), desimipramine (Remick et al., 1985), doxepin (Ansseau et al., 1984), amitriptyline (Singh et al., 1988: Eriksson et al., 1987: lmlah, 1985), imipramine

CH3 I

Triazolam

Alprazolam

Adinazolarn

Vig. 1. Structures of lria/olobcnzodia×epincs.

and placebo (Mendels et al., 1986" Feiglmer et al., 1983), amitriptylinc, doxepin and placebo (Rickels et al., 1985). diazepam, imipramine and placebo (Rickels et al., 1987). As an anti-panic drug alprazolan-t has an earlier response (Albus. 1990: Mellergard el al., 1991) but after a longer treatment period (4 8 weeks) it is not much more effectivc than imipramine (Charney et al., 1986: Rizley et al., 1986; Leon et al., 1990; Andersch et al., 1991). Somewhat peculiarly the anti-panic eft'oct of imipraminc and alprazolam seems not to depend on the existence of depressive symptoms (Lesser et al., 1988: Klerman, 1990). The triazolo group, which differentiates the triazolobenzodiazepines (see Fig. 1) is a heterocyclic ring with three nitrogens. Platelet-activating factor (PAF, l-O-alkyl-lacetyl-sn-glyceryl-3-phosphorylcholine) plays an important role in different biological processes. It is an extracellular messenger in the communication of a variety of cells (Lee et al., 1986). PAl-' is released during anaphylaxis induced by IgE (Vargaftig el al.. 1981" PinckaM el al., 1979). It can also be released in in vitro experiments from different types of cells: from basophils during immunological challengc (Varhaftig et al., 1981), and in response to specific stimuli from platelets (Chignard et al., 1979), from neulrophils (Paulson el al., 1990) and macrophages (Lynch ct al., 1979). Some studies have shown that phytohemagglulinin-induced lymphocyte proliferation (Rola-Pleszesynski el al., 1987: I)ulioust et al., 1988) and interleukm-2 production by lymphocytes (RolaPleszesynski et al., 1987) were inhibited by PAl-'. There is an influence on the synthesis of tumor necrosing factor-alpha in mice treated with endotoxin as well (Ferguson-Chanowitz ct al., 1990). PAF-receptors were visualised in human rnononuclear lymphocytes (Ng et al., 1988). Thus it was suggested that PAl-" may have irnmunorcgulatory functions in the immune syslem. PAl,, is one of the most powerful platelet activators known and it

435

1CH2--O--C H2-- [CH2]n-- CH3

H3C--C--O-- CH

CH~-O--P--O--CH~--C.~--N Cc.~ O_

CN3

Fig. 2. Structure of platelet-activating factor (PAF).

induces aggregation, secretion and also platelet shape change (Chesney et al., 1982). It is also a mediator of inflammation (Varhaftig et al., 1981), it induces bronchoconstriction and contraction of the smooth muscle (Varhaftig et al., 1981; Findlay et al., 1981). There is also increased vascular permeability (Hwang et al., 1985). In animal studies, PAF can have different cardiovascular effects like hypotension, decreasing cardiac output, decreasing renal bloodflow, myocardial ischemia, etc. (Tanaka et al., 1983; Lichey et al., 1984: Jackson et al., 1986; H6bert et al., 1987; Baer et al., 1987: Scherf et al., 1986). It also seems to play a role in experimental animal models of certain renal diseases like nephrotic syndrome (Egido et al., 1990) or acute renal failure (Lopez-Farre et al., 1990). The biochemical pathway of PAF is extensively studied. Lyso-PAF is formed through the action of phospholipase A2 which removes arachidonic acid from position 2 (see Fig. 3) on the precursor, an acyl-PAF analog. Through the action of acetyltransferase PAF is formed. A specific acetylhydrolase causes the degradation of PAF back to lysoPAF (Snyder et al., 1983). The enzymes for the metabolism of PAF are the highest in the kidney but are also present in the brain (Blank et al., 1981). PAF

A•cetat

Acetyltransfecase Acetylhydrolase Acetyl-C oAJ~ /~Acetate U

.coo,7

-,c

Phospholipase A

, cy, co,

Acetylt ransferase

Acyl OR PAF ANALOG

Fig. 3. The biochemical pathway of PAF.

Triazolobenzodiazepines are potent enough to inhibit specifically the PAF-induced activation of rabbit (Cox et al., 1987), canine (Tahraoui et al., 1988) and human platelets (Kornecki et al., 1985). The concentration of alprazolam, producing a 50% decrease in the initial velocity of the PAF-induced aggregation in platelet-rich plasma, is 5 /~M (Kornecki et al., 1985) or even 1 #M (Chesney et al., 1987). Alprazolam causes a competitive displacement of labeled [3H]PAF from the specific binding places on human and canine platelets (Chesney et al., 1987). In vivo, triazolobenzodiazepines are also strong antagonists (Darius et al., 1986; Kornecki et al., 1987; Casals-Stenzel, 1987). Benzodiazepines which have no triazologroup do not inhibit the PAF-induced platelet activation (Chesney et al., 1987; Kornecki et al., 1984, 1987). The inhibitory effect of the triazolobenzodiazepines on the platelet activation, secretion, and changes in shape is caused by the PAF system. The triazolobenzodiazepines had no relevant inhibitory activity on the platelet activation by adenosine diphosphate, thrombin, epinephrine, collagenic or arachidonic acid (Kornecki et al., 1984). Alprazolam has also a competitive inhibitory effect on the PAF binding with human mononuclear leucocytes (Ng et al., 1988). Platelets do not only have PAF receptors but also receptors for different types of prostaglandins and noradrenergic receptors. PGE1 stimulates the G-protein adenylate cyclase activity that causes a cAMP increase followed by diminishing the intracellular calcium concentrations and so inactivation of platelet aggregation. To the contrary the ~2-noradrenergic receptors inhibit the G-protein adenylate cyclase activity and thus cause platelet aggregation. The c~2-noradrenergic activity also inhibits the augmented G-protein adenylate cyclase activity after PGEI stimulation (Anton, 1990). It has been shown that in patients with depression, the stimulation by PGEj and the suppression by norepinephrine of PGEi-stimulated platelet G-protein-adenylate cyclase are reduced (Siever et al., 1984). More recently, it was found that in patients with panic disorders, there was a reduced basal, PGEl-stimulated, and fluoride-stimulated platelet G-protein adenylate cyclase activity (Charney et al., 1989). Treatment of patients with alprazolam produces an enhanced coupling between the platelet receptors and adenylate cyclase through the G-proteins (Mooney et al., 1985; Schildkraut et al., 1990). This results in an enhanced transduction of transmembrane

436 signals. Elicitation of the relaxation response by various relaxation and meditation techniques causes the same increases in platelet G-protein adenylate cyclase activity after 28 days of therapy (Schildkraut et al., 1990). These findings suggest that triazolobenzodiazepines and nonpharmacologic, cognitively induced, elicitation of relaxation response may share a common mechanism of action involving enhanced signal transduction by the receptor-G-protein-adenylate cyclase enzyme complex. In psychiatry the effect of triazolobenzodiazepines on the PAF receptors was not so important for a long time. The recent observation however, that PAF is a potent stimulator of the hypothalamo pituitary adrenal axis (HPA) stimulated the research on PAF's possible role in depression and anxiety disorders. The HPA axis is thought to play an important role in these diseases (Bernardini et al., 1989). Alprazolam, as was found, inhibited the effect that PAF had on the HPA axis (Bernardini et al., 1989). The instrument used in these studies is the dexamethasone suppression test (DST). In patients with depressiom the test is not always positive but can be used to separate different types of depression and to follow up the effect of therapy (Arana et al., 1985). For anxiety disorders, there is still no clear answer to what the role of the HPA axis is. Recently however, a number of studies suggest that there is a connection between panic disorder and HPA axis hyperactivity. An increased cortisol activity in panic patients was measured (Goldstein et al., 1987). When withdrawal symptoms occurred alter longtime alprazolam administration an increased plasma cortisol was measured as well (Mellman et al., 1986). Another study found that adrenocorticotropic hormone (ACTH) responses to corticotropin-release hormone (CRH) were blunted in panic patients (Roy-Byrne et al., 1986), a pattern of response also reported in depressed patients (Gold et al., 1984). Another specific mode of action of alprazolam is the acting as an z2-adrenoreceptor agonist (Eriksson et al., 1986). This could not be seen with diazepam and could be antagonised by the known ~.2-adrenoreceptor antagonists like yohimbine and idazoxane. Long term treatment with alprazolam induces no changes in density of the ~lor :~2-receptors (Rick-Brand et al., 1988). A stimulation of the :~e-adrenoreceptors inhibits the activity of the locus coeruleus (Aghajanian, 1987).

Relation to panic attack It is possible to place the different modes of action ot" triazolobenzodiazepines in a hypothetical model of the etiopathogenesis of panic attacks. Alprazolaln, seen as a benzodiazepine, has a stimulating effect on the GABA turnover and GABA is a global inhibitor of the central nervous system. It has been postulated that, in patients with panic attacks there could be an abnormal sensitivity of the GABA-receptor complex. It has been shown that benzodiazepines can induce a shift in the sensitivity of the receptor towards increased sensitivity to inverse agonists (inverse agonist shift). Such a shift could occur in patients with panic attacks due to a differential genetic expression of the receptor assemblies or changes in the conformation of the receptor itself (Paul, 1991). A1wazolam is also a competitive inhibitor of the PAF. PAF is known to be a strong stimulator of the HPA-axis. There is growing evidence that the activity in the HPA-axis is elevated in patients with panic disorder: reducing the activity of PAF can reduce the activity in the HPA-axis. Indeed, treatment with alprazolam increased hypothalamic CRH concentrations by inhibiting the turnover and this effect was associated with low adrenocorticotropin hormone plasma concentrations. In contrast, the CRH concentration in the locus coeruleus, amygdala, and several other cortical regions was markedly reduced (Owens et al., 1989, 1991). There is also an important link between the CRH activity and lhe activity in the locus coeruleus. Norepinephrine stimulates CRH release in the hypothalamus (Gold et al., 1990) and, on the other side, CRH enhances the locus coeruleus firing rate in experimental models (Valentino et al., 1983). The locus coeruleus seems to play an important role in the pathogenesis of the panic attacks. The activity in the locus coeruleus is controlled by calcium and calcium-dependenl currents (Llinas, 1988). The cells of the locus coeruleus show spontaneous phasic activity mediated by calcium action potentials. CRH changes the phasic discharge pattern of the neurons into one of higher and more irregular neuronal activity (Valentino et al.. 1983). The calcium-dependent potassium current is reduced by CRH. This causes secondarily increased calcium currents. It is important to note that stimulation of the ~ adrenoreceptors in the locus coeruleus causes a stimulation of the calcium-dependent potassium

437 current. That mechanism can partly explain the therapeutic efficacy of ~2-agonists like clonidine (Uhde et al., 1989) and alprazolam. Imipramine produces a down regulation of the c~2-adrenoreceptots (Ventulani et al., 1985). Agents which provoke panic, act in a similar way. Caffeine decreases potassium currents and increases calcium currents (Greene et al., 1985). Yohimbine may decrease potassium currents via :~2-receptor antagonism (Aghajanian et al., 1983). PH changes, in both directions, increase calcium currents (Konnerth et al., 1987). To the contrary corticosterone enhances the inhibitory capacity of pyramidal neurons in the hippocampus, such as after-hyperpolarisations and inhibitory postsynaptic potentials (Aldenhoff et al., 1988). This noradrenergic mechanism is not without important problems and controversy. Although imipramine, which blocks spontaneous panic, may indeed reduce the locus coeruleus firing rate, it has been found to have no effect on yohimbine-induced anxiety (Charney et al., 1985), or to make it worse (Garfield et al., 1967). Natural occurring panic attacks (Woods et al., 1987), CO2-induced panic (Woods et al., 1988), as well as treatment with alprazolam (Zemishlany et al., 1990) are not associated with M H P G (3-methoxy-4-hydroxyphenyl glycol) concentration changes. M H P G is a central metabolite of noradrenaline. Buspirone is a stimulator of the discharge pattern of the locus coeruleus but it is used as an anxiolytic (Sanghera et al., 1983). High doses of buspirone, however, can provoke panic and anxiety (Ludwig et al., 1986). Finally, direct electrical stimulation of the locus coeruleus did not produce a panic reaction (Kaitin et al., 1986). The HPA axis related to the locus coeruleus is however, most probably a very important underlying neuronal network in inducing and generating the panic attack. Diverse panic provocation tests give clues about that neuronal network. The lactate panic provocation test can influence the network in two ways. Lactate induces an augmentation of bicarbonate, which can not pass the blood brain barrier but is further metabolised to carbondioxide which permeates quickly to the central nervous system. In the central nervous system, pH changes cause locus coeruleus activation as mentioned above. On the other hand, lactate causes a stimulation of the serotonin reuptake which reduces the central serotonergic transmission (Lingjaerde, 1985). The serotonergic system is supposed to have an inhibitory activity on

the locus coeruleus (Gray, 1982). The role of serotonin in the pathophysiology of panic attacks remains unclear. Facilitation of serotonergic neurotransmission is essential when antidepressant drugs are used in the treatment of panic disorders. This is confirmed by the findings that a selective serotonin reuptake inhibitor, fluvoxamine, is superior to the noradrenaline reuptake inhibitor, maprotiline, in the treatment of panic disorder (Den Boer et al., 1988). The effect of fluvoxamine or fluoxetine is, as is not yet explained, biphasic with an initial worsening (Den Boer et al., 1988; Gorman et al., 1987). Also in favor of this theory is the observation that there is a beneficial effect of monoamine oxidase inhibitors and serotonin precursors in the treatment of panic attacks (Eriksson et al., 1990). The triazolobenzodiazepines also have a stimulatory effect on serotonin activity in the brain and this can be another mode of action of these drugs (Turmel et al., 1989). Chronic treatment with alprazolam seems to decrease significantly the reactivity to lactate (Cowley et al., 1991). The carbon dioxide provocation test is also used many times to induce a panic attack. The fact that only persons with a history of panic attacks can get an attack during the test (Gorman et al., 1984) raised the presumption that these persons had supersensitive carbon dioxide receptors. The question if there are effective supersensitive receptors remains unclear (Woods et al., 1986) but as mentioned above, it is not necessary to explain the mode of action of carbon dioxide in inducing panic attacks. Carbon dioxide provocation tests or lactate provocation tests enhance the cerebral bloodflow in the brain, bilaterally in the temporal poles, the insular cortex, claustrum, lateral putamen, in the vicinity of the superior colliculus and in the left anterior cerebellar vermis. Positron emission tomographic studies have shown that these changes are less pronounced in persons who suffer from panic attacks than in normal controls (Reiman et al., 1986, 1989: Steward et al., 1988). These studies also found, during the nonpanic state, an asymmetry in bloodflow in the right parahippocampal area (an asymmetry that appears to reflect increased measurements in the right rather than decreased measurements in the left) and also a higher oxygen metabolism in this area and even a higher oxygen metabolism in the whole brain (Reiman et al,, 1984). Perhaps it is possible that the increased bloodflow in the right para-

438 hippocampal area is associated with the increased production of CRH and that the mild hyperventilation, seen in patients with panic attacks during the nonpanic state (Gorman et al., 1986), is a compensation mechanism. In a panic provocation test, where the cerebral carbon dioxide is increasing, the bloodflow must also increase. These patients try to avoid this by hyperventilation and in this way they try to protect the right parahippocampal area from further vasodilatation which causes CRH induction and locus coeruleus stimulation. In many cases, that method is unsuccessful and a panic attack is generated. It should be interesting to measure if there is indeed a link between the vascular defect in the right parahippocampal area and the HPA-axis hyperactivity.

Concluding remarks Triazolobenzodiazepines intervene in 4 different sites in the neuronal mechanism of panic attacks. They work as competitive inhibitors of the PAF stimulated CRH action, they work as inhibitors at the locus coeruleus, they potentiate the action of the serotonin action on the locus coeruleus and finally, they potentiate G A B A transmission. All these actions are interrelated and contribute to the potent anti-panic effect of triazolobenzodiazepines.

Acknowledgement

We sincerely thank Raf Sel (M.D.) for his very helpful suggestions regarding the major revision of this paper.

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