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Crossed Lines: What can atypical anti-psychotics tell us about schizophrenia? |
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This essay enquires into the contribution the new atypical anti-psychotics have made to our current understanding of the neuropharmacological aspects of schizophrenia - if they have. The atypical anti-psychotics have received much publicity for their reduced side effects compared to typical anti-psychotics. But is it true? And do they actually work? The essay concludes 'probably not' |
Schizophrenia is a disorder affecting (by some estimates) more
than 1.0% of the American population, with over 300,000 acute
episodes reported each year in the United States alone (Stahl
1997). Rates in Europe and Asia are calculated using different
criteria than the USA, and perhaps for this reason are lower, being
of the order of 0.2 to 1.0% lifetime prevalence (Feldman, Meyer
& Quenzer 1997); this still represents a very large number of
people. Schizophrenia is intensely distressing for sufferers,
families, and carers alike, and is usually a chronic disorder (Kolb
& Whishaw 1996). For all these reasons, ongoing efforts are
being made to find effective treatments in the 'magic
bullet' tradition (Feldman, Meyer & Quenzer (1997)), which
would alleviate the symptoms of schizophrenia without causing
unpleasant side effects.
Any such treatment would have benefits for all those afflicted
and affected by the disorder, and also of course for the
shareholders of any company fortunate enough to develop a
'cure'. Unfortunately, '...the biological basis of
schizophrenia remains unknown...' (Stahl 1997); it is hard to
develop a treatment for a disease which has unknown causes.
Moreover it is a diagnosis covering a very heterogeneous set of
symptoms; the simplified model put forward by Crow (1980)) divides
these into two categories, positive symptoms(those 'added
on' to 'normal' behaviour), and negative symptoms (as
if 'subtracted' from 'normal' behaviour). What
contribution can neuropharmacology, particularly by way of study of
the newer atypical anti-psychotics, make to our current
understanding of the neurological basis of schizophrenia?
The aetiology of schizophrenia is very problematic; despite
around a hundred years of experimentation and theorising since
Bleuler's (1908) original introduction of the term, there is no
generally-agreed account for the origins of schizophrenia (Davison
& Neale 1998). Proposals include epidemiological factors,
genetic factors, developmental factors, biopsychosocial factors,
vulnerability-stress factors, structural abnormalities and
functional abnormalities, with various conflicting patterns of
evidence supporting and contradicting each of these proposals
(Feldman, Meyer & Quenzer (1997); Sarason & Sarason (1996);
Davison & Neale (1998)).
Neuropharmacology concerns itself with '...drug-induced
modification of nervous system function...' (Feldman, Meyer
& Quenzer (1997)). Where schizophrenia and other psychoses are
concerned, the neuropharmacological aim is to re-establish
'normal' function by changing brain chemistry by
administration of drugs, specifically the group of drugs known as
neuroleptics. The neuroleptics are drugs of dramatic effect:
...antipsychotic drugs have been termed 'neuroleptics' in that these drugs' actions imitate a neurological disease... Textbook of Psychiatry: American Psychiatric Press (1988)
They can very significantly change the behaviour of those patients to whom they are administered. Notably, the traditional or 'typical' anti-psychotic drugs have specific (and undesirable) side effects (Davison & Neale 1998); more recently, 'atypical' anti-psychotics have been described as having lesser or different, less troublesome, side effects (Stahl 1997). Is it possible, then, to infer something about the changes that occur from 'normal' functioning in the 'schizophrenic' brain by examining the effects of neuroleptic drugs on brain function? Atypical anti-psychotics are defined in terms of their differences from typicals, so we will first examine the effects on brain function of the typical anti-psychotics.
The original and archetypal typical anti-psychotic is
chlorpromazine, which is one of the phenothiazines, a group of
chemicals synthesised as part of the search for new organic dyes in
the late 1800's (Feldman, Meyer & Quenzer (1997)). Many
phenothiazines are antihistamines; chlorpromazine was found by
chance to have anti-psychotic effects when its antihistaminic
properties were being studied by way of administration to a ward of
schizophrenics (Stahl 1997). This group of organic chemicals has a
characteristic nucleus composed of two benzene rings joined with
nitrogen and sulphur atoms, making up a three-ring structure;
various other compounds attach to this, giving rise to differing
activities and pharmacological potencies (Feldman, Meyer &
Quenzer (1997)).
Chlorpromazine was the first drug found to have some definite
clear effect on reducing the symptoms of schizophrenia,
particularly those known (in Crow's (1980) system) as positive
symptoms. As a result of its introduction, there was a revolution
in the treatment of schizophrenia, with many traditional hospitals
being closed and psychiatric patients being returned (for better or
worse) to 'care in the community' whilst being maintained
on a dose of chlorpromazine. These results led to a major research
drive to find out why drugs of this class caused their effects.
Chlorpromazine was demonstrated by X-ray crystallography to be
morphologically similar to dopamine, a principal inhibitory
neurotransmitter, and thus it was proposed that it preferentially
occupied dopamine receptor sites (see Figure 1 
).
This finding was part of the evidence that led to the proposal
of the dopamine hypothesis, a claim that the symptoms of
schizophrenia were caused by (in the original formulation) an
excess of dopamine in the brain. However, the absence of high
levels of dopamine metabolites (particularly homovanillic acid,
HVA) in schizophrenic patients led to a modification of the theory
to suggest that the symptoms were associated with excessive numbers
of - or sensitivity in - dopamine receptors in the schizophrenic
brain (Davison & Neale 1998).
Unfortunately chlorpromazine and related agents are not unimodal in their effects; as well as having the desired result of calming schizophrenic patients and diminishing the expression of their positive symptoms, with chronic application they also have side effects. The principal effects caused are:
Extra-pyramidal effects/responses/symptoms, EPE, EPR or EPS: these involve disorders of movement that mimic Parkinson's disease, and are connected with the basal ganglia. EPE/EPR/EPS are medication dependent, and thus stop if the medication is ceased.
Tardive Dyskinesias: these are various stereotypical movements such as sucking and lip smacking, tremors and rigidities. The special difficulty with these dyskinesias is that, once formed, they usually are permanent (Gardos et al 1994). Thus, the 'cure' for schizophrenia can in some cases effectively cause a different - and permanent - neurological disease.
Galactorrhea: this reversible disorder mostly (but not exclusively) of female recipients of the phenothiazines results in stimulation of the breasts causing tenderness, swelling and in some cases inappropriate lactation.
Other responses can include amenorrhea, impotence and sterility. (after Stahl (1997))
Taken together with the desired effects of the phenothiazines, this pattern of responses maps more-or-less neatly onto three principal dopamine neuronal pathways in the brain. These are, respectively:
The mesolimbic dopamine pathway, projecting from the mid brainstem to the limbic area of the brain: this is considered to be a likely therapeutic target of treatment, with over-activity of dopamine receptors perhaps involved in delusional states and hallucinations.
The nigrostriatal dopamine pathway, which projects from the substantia nigra in the hindbrain to the basal ganglia: this is thought to be associated with the pseudo-Parkinsonian EPE.
The tuberinfundibular pathway, from the hypothalamus to the anterior pituitary: this is part of the prolactin circuit, normally controlling aspects of pregnancy and especially lactation. (after Feldman, Meyer & Quenzer (1997))
There is a fourth dopamine pathway, the mesocortical dopamine
pathway, from the midbrain tegmentum to the limbic cortex. There is
however no consensus as to whether the effects of phenothiazines on
this pathway are beneficial; it is regarded as a cognitive pathway,
and thus reducing its activity may not be of advantage,
particularly in negative symptom patients (Feldman, Meyer &
Quenzer (1997)). The general run of the various dopamine pathways
is shown in schematic form in Figure 2 , below:
Overall there is a strong argument for the involvement of dopamine pathways in the action of phenothiazines, which can be expressed as follows:
The phenothiazines are known to act at dopamine receptor sites
The administration of phenothiazines causes a pattern of (eventual) changes in expression of positive schizophrenic symptoms, as well as a pattern of side effects (as described above)
The dopamine pathways, where dopamine receptors are especially prevalent and so where we can presume the phenothiazines are particularly active, relate to the pattern of therapeutic and side effects seen in chronic administration of the drugs.
Thus, a picture has been constructed of the logic behind the dopamine hypothesis and the presumed action of the typical anti-psychotics. It must be emphasised that there are other classic anti-psychotic drugs apart from the phenothiazines, but pressure of space means that this group has been selected here as embodying typical properties; all the classical anti-psychotics tend to block dopaminergic synapses (Stahl 1997).
Dopaminergic systems present contradictory aspects; dopamine
constitutes up to 80% of the total brain catecholamine content, yet
dopaminergic cells are rather rare, there being no more than about
a million in the whole human brain (Feldman, Meyer & Quenzer
(1997)). Moreover, although the dopamine hypothesis preceded any
detailed understanding of the dopamine (DA) receptor, work
commencing in the 1970's (Kebabian & Calne 1979) showed
that there was more than one type of DA receptor; at least six
types are now identified, though these are still categorised as
generally similar to the first two recognised, that is D1-like and
D2-like receptors.
The D1-like include D1 and D5 receptors, the
D2-like include D2S and D2L, D3, and D4. These can be broadly
characterised in that activated D1-like receptors stimulate
adenylate cyclase enzymatic response; activated D2-like receptors
either inhibit it or do not affect it (Kalat 1996). The obvious
question arises, if the phenothiazines affect dopaminergic synapses
by blocking them, then which DA receptor is involved? The balance
of evidence is that it is the D2-type receptors that are involved
in the anti-psychotic effects of the typical drugs (Feldman, Meyer
& Quenzer (1997), Davison & Neale (1998), Sarason &
Sarason (1996), Stahl (1997)). An obvious corollary is that the
therapeutic effect may be connected with inhibition of the second
messenger cAMP system (Vallar & Meldolesi 1989).
Stahl (1997) makes clear the point that the blocking of two of the four dopamine pathways in the brain (the nigrostriatal and tuberinfundibular pathways) is definitely not beneficial, whilst blocking the third (the mesocortical pathway) is of doubtful utility. The problem with the typical anti-psychotics is that they affect them all. Given that there are various different types of DA receptors, the possibility arises that they are unevenly distributed in the brain and - if that should be the case - it might be possible to selectively block them. In this way, it could be possible to alter the therapeutically relevant pathways without causing undesirable side effects - just the claim that is put forward for the atypical anti-psychotics. As with the typicals, here the pressure of space means that we will concentrate on one archetypal atypical anti-psychotic, clozapine.
What is atypical about an atypical anti-psychotic? Kinon & Lieberman (1996) in a comprehensive review, point out that the term 'atypical' is problematic. Some of the criteria used to ascribe atypicality include:
| Criteria for atypicality in anti-psychotics | |
|---|---|
| 1 | Decrease or absence of acute EPSE and TD |
| 2 | Increase of therapeutic efficacy toward positive, negative or cognitive symptoms |
| 3 | Decrease or absence of capacity to elevate prolactin |
Clozapine was first recognised as atypical for its high
therapeutic efficiency without concomitant extra-pyramidal
side-effects, arising from its low propensity to block D2 receptors
(Kerwin 1994). Kerwin also points out that the simplest definition
of 'atypical' is '...an effective neuroleptic that does
not cause catalepsy in rats...'. But how does clozapine achieve
its effects? The available evidence gives a contradictory picture.
In part, clozapine has low occupancy of the nigrostriatal sites, in
the range 30-60% (Farde et al 1992), thus accounting for its low
EPE. However, Pilowsky et al (1992) showed that here was no direct
connection between D2 occupancy and therapeutic effect, thus
introducing other possible sites of action for clozapine. The
effects of clozapine are amongst the most complex known (Stahl
1997). As well as DA receptors, clozapine also affects the
serotinergic 5HT2, muscarinic acetylcholine, adrenergic, NMDA
glutamate, GABA, and cholecystokin systems.
Notably, while affecting only some 20-60% of D2 receptors it
also affects some 85-90% of 5HT2 receptors (Stahl 1997), and this
finding has led to proposals that this balance of blockade of
different systems (or perhaps some other pairing out of the nine
currently-known affected NT receptors) is responsible for its
enhanced therapeutic profile (Feldman, Meyer & Quenzer (1997)).
Thus, serotonin-dopamine antagonists (SDA) have been developed to
mimic this aspect of clozapine's function without some of its
side-effects.
Clozapine also has atypical side effects (Feldman, Meyer & Quenzer (1997)). These include those resulting from its broad effects in adrenergic and cholinergic systems, such as orthostatic hypotension, tachycardia, hypersalivation, weight gain, and most seriously aganulocytosis, the destruction of bone marrow (the aetiology of this being unknown). This means that clozapine is often only administered as a drug of last resort, and efforts are being made to develop drugs with its advantages but without the dangerous aspects.
Neurochemical hypotheses of the aetiology of schizophrenia now involve the dopamine hypothesis, the neurodevelopmental model, a noradrenalin (norepinephrine) model, a serotonin hypothesis, a glutamate hypothesis and more but the evidence available does not presently enable clear acceptance or rejection of any of them (Feldman, Meyer & Quenzer (1997), Kerwin 1994). It is difficult to make psychological sense out of the maze of potential interactions; the argumentation (and potential supporting evidence) here is complex, because NT systems in their inhibitory, excitatory, and facilitatory interactions are knitted into a complex skein. The dynamic, interactive, and self-regulating feedback networks inherent to the CNS further complicate what is already difficult to understand.
Recently evidence has been accumulating that perhaps points to an important involvement of the cortex in the neuropathology of schizophrenia. Van Tol et al (1991) and Seeman (1992) described the D4 receptor (which is common in the cortex) as a potential site of action of clozapine. However, Roth, Tandra, Burgess & Sibley (1994) found that there was no way reliably to distinguish between typical and atypical anti-psychotics based on their affinities for the D4 receptor. Lidow, Williams and Golman-Rakic (1998) suggest an alternate mechanism involving upregulation of cortical D2 receptors (which are highly occupied by clozapine in the cortex; Pilowsky et al 1997) without downregulating D1 receptors at the same time. They argue that the clozapine-D4 hypothesis is not supported on evidence including the lack of psychosis in individuals genetically lacking D4 receptors; clozapine unlike chlorpromazine occupies cortical sites but not striatal sites. The psychological implication here is straightforward. Schizophrenia is a thought disorder (Feldman, Meyer & Quenzer (1997)), the cortex is generally regarded as implementing higher-order cognitive and social functions (Kolb & Whishaw 1997), and thus there is intuitive appeal in a theory that links a thought disorder with molecular biological dysfunction in the cortex.
There is not universal agreement that the claims made for clozapine can be substantiated; various practitioners have found little or no beneficial effect. Baldachino & McKnight (1994), for example, report on 14 patients who fitted the profile of not responding well to typical anti-psychotics and were prescribed clozapine over a three-year period. They point out that '... the treatment outcome reported in scientific studies would have a significant impact on the number of long-term rehabilitation beds required if it were reflected in a clinical setting...'. However, out of the 14, only three remained on the drug at the end of the period; three had moved to living in the community, of whom two had already stopped taking clozapine. One patient did move into the community as a result of clozapine administration. Their conclusion was that the clinical outcome in practice does not reflect the controlled study outcome, and that the perceived benefit in various patients was '...due to a decrease in their arousal...' (Baldachino & McKnight 1994); in other words, they troubled staff less. Kerwin (1994) pointed out that '...the acute drowsiness noted for the drug was considered as an advantage in acute cases...'.
What sorts of things can psychopharmacology tell us? Given a particular disorder of the brain, psychoneuropharmacology can look at a disordered brain, examine its workings at a molecular level, and if irregularities of function can be identified, suggest potential agents to modify or rectify those irregularities. Clearly, the whole process depends on accurate clinical diagnosis, description and discrimination - but this is problematic. For a start, the schizophrenic brain is inside the patient's head, and only available post-mortem, so most work is done with rats. There are other problems.
A diagnosis of schizophrenia cannot stand alone; the 'disease' does not exist outside the individual showing symptoms of schizophrenia, unlike a pathogen. Abstracting a disease - 'schizophrenia' - from a set of symptoms is reification, the conversion of an idea to a thing. But is schizophrenia one disease? There are at least two distinct patterns, positive and negative symptoms, which in any modern analysis would hardly be clustered together as one disease. We might remember the criticisms of Freud's theories, that they cannot be disproven or falsified (in the Popperian mode) because they make causal claims that embody both positive and negative outcomes of both positive and negative causes - rather like schizophrenia.
If a claim is to be made that (say) D2 receptors are associated with positive symptoms of schizophrenia, then final testing of this needs not a set of animals but access to suitable schizophrenic patients, diagnosed as schizophrenic by DSM IV or ICD 10 criteria. But these criteria embed certain assumptions about what constitutes 'schizophrenia', which in part include responsiveness to drugs that are D2 antagonists. This embeds a clear circularity and a potential self-fulfilling prophecy.
Examining the neuropathology of the 'schizophrenic
brain' in the hope of developing a 'cure' is only
useful if we assume a particular casual chain - that disordered behaviour is the result of disordered brains, Figure 3 .
But of course this is not the only feasible causal chain. As
long as we admit the possibility that mental states (whatever they
are) affect the physiology of the brain (something well established
in psychoneuroimmunology, for example, and see Feldman, Meyer &
Quenzer (1997)) -- then this other chain, from disordered mentality to disrupted biology is possible: Figure 4 . Or indeed, why should there not be a feedback mechanism between biology, brain, behaviour, mental acts and attitudes, thus: Figure 5
Should this last suggestion have any truth in it, it's conceivable that anti-psychotics may sometimes perpetuate any
disorder, rather than necessarily helping to reorder the brain. In other cases they may break the chain, allowing the system to reorder itself. Of course, all these causal chains are much easier to speculate on than prove, or disprove.
Schizophrenia, as the author has emphasised repeatedly, is a thought disorder (Feldman, Meyer & Quenzer (1997)). Yet the basic screening method for detecting potential anti-psychotic drugs uses rats (Kerwin 1994). Can we reasonably expect rats to have thought disorders? The answer, obviously, is 'no'. Various stratagems are adopted; schizophrenia-analogues are induced in animals by amphetamine or PCP psychosis (Feldman, Meyer & Quenzer (1997)). However, amphetamine psychosis in humans is distinguishable from schizophrenia by an absence of low Responsive Search Score in a visual tracking task; plainly, the two are not identical. PCP psychosis is not principally dopaminergic in its effects (Stahl 1997). Thus, a major problem of the ecological validity of extrapolating from animal studies is introduced.
We still know very little about how the brain works; only a few years ago, large parts of the brain now known for specific functions were labelled as 'association areas'. Recent study has been addressing the cerebellum, which used to be thought to be involved only in fine motor functions. Schmahmann (1998), in a review of the implication of the cerebellum in cognitive disorders, points to the possibility that cerebellar disorders can lead to disorder of thought - the cerebellar cognitive affective syndrome. Middleton & Strick (1998) in another review concur that this has implications for schizophrenia. If this should prove to be substantiated, then yet another set of neuropathological considerations enter.
For this author, one of the critical elements illustrating the
inadequacy of current hypothesising on the aetiology of
schizophrenia is the delay that occurs between the administration
of an anti-psychotic drug (which changes NT levels in minutes) and
the development of changes in extrinsic behaviour (which takes
months). The author knows of no satisfactory proposal to explain
this.
The cognitive revolution in psychology was based on an
information-processing paradigm based on analogy with computer
systems. The brain is not a computer system, but nonetheless
psychopharmacology seems to ignore the potential distinction
between two levels of analysis - hardware and software. A thought
disorder (as in schizophrenia) can be analogised as a
software-level error. Giving an anti-psychotic drug, however, is
the equivalent of rearranging the wiring. This would not achieve
much in a typical computer system, when faced with a program error.
The whole validity of the information-processing approach is based
on the recognition of the programmability and flexibility of the
neural substrate in (at least) higher-order organisms - the
capacity to learn and modify behaviour, to abstract from the
intrinsic stimuli. Increasingly it is recognised that those systems
once thought to be autonomous are, in fact, in a complex
feedback/feedforward relationship with levels of cognition that
were once seen as quite abstracted and detached from the pure
biological level - an example would be the interactions between
appraisal, attitude and mood with the activity and effectivity of
the immune system (Feldman, Meyer & Quenzer (1997)). Thus, the
attempt to permanently change a thought disorder by messing with
the wiring may be inherently doomed to failure.
This of necessity has been a very selective, simplified and
compressed review of but a few of the issues surrounding
psychopharmacology and schizophrenia. Let us return to the opening
statement: what is 'the contribution of atypical
anti-psychotics to our current understanding of the
neuropharmacological basis of schizophrenia'? The truthful
answer is 'not a lot' - though this is not due to any fault
in psychopharmacology, but because of the tremendous complexities
inherent in the issues.
What we do know is that the cluster of symptoms labelled as
'positive schizophrenia' is (perhaps) associated with a set
of changes in the mesolimbic dopaminergic pathway - or perhaps the
mesocortical, or perhaps it is the cortex or cerebellum that is
central. It may be that the dopaminergic system is critical, or the
5HT2 system, or glutamate, or something else. It is definite that
something changes somewhere in the brain, though we do not know if
that change is causal or a response. We also know that neuroleptics in
general affect the brain, and that atypical anti-psychotics
reportedly sometimes in some cases work in patients that have not
responded to typical anti-psychotics, and perhaps (sometimes) help
sufferers from negative systems. The available treatments are not
however any sort of a cure, merely a palliative for some of the
worst symptoms and an aid to hospital staff in patient management,
in that giving neuroleptic drugs to 'schizophrenics'
quietens them down, shuts them up, and (sometimes) helps them to
feel better.
What we do not know - and psychopharmacology alone is unlikely
to tell us - is
the origin of the causal chain that leads to 'schizophrenia'
where in the brain the critical functions are located, or
what we can do to reliably help sufferers without risking damaging them at the same time.
Overall, it may be apparent that the author is deeply unconvinced by claims that there is any one simple cause-effect relation in schizophrenia. There is substantial evidence that this may not be one 'disease', but a clinically-useful but aetiologically-nonsensical clustering together of similar symptoms with dissimilar causes. Until these are clearly discriminated there seems little realistic prospect of progress beyond the palliative level -- though this is not to discount the tremendous value of any palliative option that is available for this distressing disorder. It is of course easy to be cleverly critical in an essay such as this, at a safe remove from any ward or lab, but we need to actually understand why schizophenia happens in order to help victims and sufferers, even if this means a Kuhnian-style paradigm-shift with its associated career costs and fall-out.
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