PS 3010 Behavioural Pharmacology Semester 2: 20042005 Lecture 7 a

1
PS 3010 Behavioural Pharmacology Semester 2
2004-2005Lecture 7 a bAnxiety, anxiety
disorders and anxiolytics

• Prof Michael H. Joseph,
• School of Psychology

2
What is anxiety ?

• Certain stimuli are innately aversive
• pain (shock), restraint ..
• or stimuli are innately fear provoking
• snakes (for many species), loud noise.
• These produce certain physiological (generalised
  stress) responses and behavioural responses.
• Opposing behavioural responses of fight/ flight,
  and behavioural inhibition (freezing)

3
The human stress/anxiety response

• In addition, humans can learn associations
  between initially neutral stimuli and these
  situations, so that these stimuli come to evoke
  these responses.
• IN HUMANS then, stressful stimuli gt responses at
  different levels OHD
• 1. Peripheral stress response (may be centrally
  mediated)
• 2. Changes in behaviour
• 3. Aversive emotional state (subjective anxiety)

4
Human emotion James-Lange theory

• 1. Peripheral stress response (may be centrally
  mediated)
• 2. Changes in behaviour
• 3. Aversive emotional state (subjective anxiety)
•  These normally occur together, and can be
  collectively conditioned to external stimuli.
• Some theories (e.g. J-L) suggest that 3 results
  from 1 2

5
Classification of anxiety

• In humans, there may be no readily identifiable
  stimulus classification (from Peter Tyrer)
• Episodic Persistent 
• A. Predictable PHOBIA STRESS
• stimulus ADJUSTMENT
• REACTION
•  
• B. No specific PANIC GENERALISED
  stimulus (fear) ANXIETY

6
Classification of anxiety (cont)

• We can understand A as a heightened reaction (in
  size or duration) to stimuli which normally
  induce some anxiety response
• - reaction is resistant to learning, or at least
  to habituation.
• While B is either autonomous activation, or
  perhaps a learned response to stimuli which do
  not normally evoke any anxiety.
• - either enhanced learning, or a generally
  sensitised response.

7
Human and animal anxiety

• panic (fear) as primarily the stress response,
  anxiety as primarily the central state, but each
  will evoke the other
• WHEN we now come on to consider animal studies,
  we can observe 1 2 above, but 3 can only be
  inferred, and may have no useful meaning.
• Rather anthropomorphic we can say animals
  behave as if they were anxious, but we dont
  know how they feel

8
Animal models

• We will see that all the models used involve
  specific stimuli presented acutely, hence their
  face validity is to A rather than B.
• Although learned responses can be studied, as
  indeed they are, the responses are all
  "reasonable", e.g. fear of stimuli associated
  with mild shock.
• Can only use normal variation, genetic factors or
  environmental manipulations to study abnormal
  sensitivity of an endogenous response, or
  enhanced sensitivity of learning, or enhanced
  persistence of learning.

9
Animal models II

• Use of learned responses can help to distinguish
  brain systems involved in anxiety from those
  involved in the primary stress response
  mobilisation etc.
•  
• Animal tests can be used to find other
  anxiolytics at a behavioural level (gives
  predictive validity), but only human trials can
  tell us if they reduce subjective anxiety. 

10
Animal models of anxiety (Behavioural) OHD

• 1) STRESS IN GENERAL - too inclusive -
  (no.1, peripheral stress response, may
  predominate)
• 2) RESPONSE TO NOVELTY (? mild stress)
• a) new form of food - inhibition of eating
• b) novel environment (light, noise)
• i) inhibition of exploration dark /light box,
  elevated plus maze
• ii) increased defaecation
• iii) inhibition of social interaction

11
Animal models of anxiety (Behavioural) (contd.)

• 3) RESPONSE TO SIGNALS PREDICTIVE OF PUNISHMENT
• a) predictive of shock - inhibiting (CER)
• b) predictive of non-reward - activating (PREE)
• 4) RESPONSE TO CONFLICT (food/shock or a)
  direct lick/shock) 
• b) to signal predictive of conflict period 
• As we descend the list, there is higher face
  validity, although more laborious to set up, but
  still all are appropriate anxiety (stimulus
  related)

12
these tests have predictive validity

• because they give positive results across
  different classes of drugs effective in treating
  human anxiety
• benzodiazepines, barbiturates, alcohol
• used at low (not merely sedative) doses
• The anti-conflict effectiveness of anxiolytics is
  proportional to clinical dose required OHD
• (indeed this is why this conflict test has been
  so popular in the past).

13
these tests are behaviourally selective

• Reversal of behavioural effects
• - specific to conflict period
• - allows check on non-specific sedation
• - need to check for analgesia
• Hence how these drugs, especially benzodiazepines
  (BDZs), work, and where they work, will provide
  important clues to the biological substrate of
  anxiety.

14
anxiolytic (anxiety reducing) drugs

• Alcohol is the oldest anxiolytic (anxiety
  reducing) drug used in man.
• Obvious problems of dependence.
• Anaesthetic at high dose, but therapeutic index
  is narrow, and duration of action short.
• Barbiturates represented a considerable advance
  on this, and had
• sedative, and anti-anxiety effects,
• later ones had hypnotic and anticonvulsant
  effects also.

15
anxiolytic (anxiety reducing) drugs - barbiturates

• Barbiturates were introduced from early this
  century the first, barbital, had a slow onset,
  and extremely prolonged action.
• Advances since have led to structural analogues
  which penetrate to the brain more rapidly, giving
  successively more rapid and shorter duration of
  action.
• These include phenobarbital, pentobarbital,
  thiopental - useful for different purposes in
  anaesthesia or anaesthetic induction.

16
anxiolytic (anxiety reducing) drugs
barbiturates and benzodiazepines

• However barbiturates also have considerable
  problems with tolerance - define - (which is both
  metabolic and cellular) and dependence, plus poor
  therapeutic index, and respiratory depression
  (dangerous) in overdose.
• From the mid 1950s, the first BDZ anxiolytics
  were introduced, following work at
  Hoffman-LaRoche. Chlodiazepoxide (Librium) was
  reported to have profound sedative, muscle
  relaxant, taming and anticonvulsant activity.

17
anxiolytic (anxiety reducing) drugs
benzodiazepines

• More potent, and shorter acting congeners
  (chemical analogues) were developed - diazepam
  (Valium) and oxazepam.
• Other variants, nitrazepam (Mogadon) and
  flurazepam, were found to have a different
  balance of actions, being somewhat more
  hypnotic. OHD
• In fact there is no hard and fast distinction
  between anxiolytic and hypnotic BDZs, - mainly a
  matter of dose. NB also metabolic
  inter-relationships can prolong action.

18
anxiolytic (anxiety reducing) drugs
benzodiazepines II

• Thus similar to earlier drugs, but BDZs had
  enormous advantages in that anxiolytic dose was
  not sedative, or if so, tolerance developed to
  the latter but not the former.
• In addition the therapeutic index was extremely
  high (very difficult to overdose), and there
  seemed to be no problems with abuse potential
• (contrast alcohol and barbiturates).

19
anxiolytic (anxiety reducing) drugs
benzodiazepines III

• In addition to anxiolytic and hypnotic actions
  they were useful muscle relaxants, pre-
  medication for surgery and dentistry, and
  anticonvulsant (like barbiturates) but w/o
  sedative action they are still the treatment of
  choice for status epilepticus (i.v.).
• Hence became extremely popular more than 50 of
  psychotropic drug prescriptions in US at one
  time. However usage peaked in the 70s, and
  recently usage as anxiolytics has declined (but
  not as hypnotics)

20
anxiolytic (anxiety reducing) drugs
benzodiazepines IV

• This is due to concerns over dependence -
  (but much milder than alcohol or barbiturates).
• After treatment for some time, accumulation,
  especially in the elderly, can lead to confusion,
  dementia-like effect.
• Withdrawal, especially if abrupt, can lead to
  rebound anxiety and insomnia.
• Depression of REM sleep can lead to rebound vivid
  dreams. In extreme cases can get paranoia and
  even seizures.

21
anxiolytic (anxiety reducing) drugs
benzodiazepines V

• However most of these problems can be avoided by
  slow withdrawal, and the problems have perhaps
  been overstated, given the enormous benefits that
  many patients have derived
• (recall they are still used as hypnotics).

22
Mechanism of action of BDZs

• Binding sites for BDZs have been found in the
  brain and are widely distributed.
• Also found in all vertebrate species so far
  examined (inc. lower mammals, fish, reptiles).
• This is encouraging if we want a
  phylogenetic-ally old system and want to use
  rodent models of anxiety.
• GABA and its receptors are also widely
  distributed in the brain.

23
Receptors for BDZs

• Binding for BDZs was found to be saturable and
  stereospecific, suggesting a specific protein was
  acting as the binding site. i.e. it was a
  specific receptor.
• This was found to be one of the subunits of the
  GABA receptor complex. OHD
• BDZs do not compete with GABA agonists or
  antagonists at the GABA site, but rather bind to
  another site on a different subunit.
• Binding of BDZ increases the affinity for GABA at
  its own site.

24
Receptors for BDZs II

• Thus BDZs potentiate the action of active GABA,
  but do not themselves act as direct agonists.
• This, coupled with lack of receptors
  peripherally, or in the autonomic system, perhaps
  accounts for their extreme safety.
• Binding of BDZ increases the frequency of channel
  opening (analogous to increased GABA levels),
• Conversely, binding of barbiturates, (which bind
  to another sub-unit) increases the length of
  chloride channel opening.

25
Receptors for BDZs III

• This may account for their general similarities
  of action, but also for their differences.
• Alcohol also influences the activity of the GABA
  mediated chloride channel, but there is no
  specific receptor.
• Why should brains have receptors for artificial
  drugs such as barbiturates or BDZs? OHD
• Discovery of opiate receptors (q.v.) led to
  expectations that endogenous ligand could be
  found for BDZ also.

26
Ligands at BDZ Receptors

• Beta-carboline (BCCE) was purified from human
  urine, and found to bind to BDZ receptors, but
  was found to be an artefact of purification.
• However this class of compounds ( BCCM, DMCM)
  was of great experimental interest, since rather
  than blocking (antagonists) or simulating
  (agonists) the effects of BDZs, they had exactly
  the opposite effects, i.e. they were
  proconvulsant, and anxiogenic.

27
Ligands at BDZ Receptors II

• When given to chair adapted primates they
  increased agitation and arousal. Similar results
  obtained in cautious human experiments (since
  discontinued).
• Other compounds, e.g. flumazenil, were found
  which were true antagonists, and blocked the
  binding of BDZs and of the so-called "inverse
  agonists".
• This was the first example of a receptor which
  can be stimulated in opposite directions by
  different agonists acting at the same site.

28
Ligands at BDZ Receptors III

• It raises the question of whether any putative
  endogenous ligand would be anxiolytic or
  anxiogenic.
• Endogenous ligand has still not been identified,
  although one account is that small amounts of
  BDZs are present in the diet from plants, so that
  BDZs themselves could be endogenous also.
•  
• This deals with how BDZs etc act in the brain.
  Now we will turn to where they act.

29
where do BDZs act in the brain?

• BDZ receptors are especially dense in the cortex,
  olfactory bulbs, hippocampus (HC) and limbic
  system.
• HC and related limbic system is of particular
  interest, as independent psychological evidence
  implicates this structure.
• What is this evidence ?
• Lesions to the Septo-HC system (SHS) produce in
  animal studies, a spectrum of effects very
  similar to those of anxiolytic drugs, especially
  BDZ, in low doses.

30
Septo-HC system and BDZs

• We can summarise these effects as
• 1) Reduction of response to signals of
  punishment (but not to punishment itself)
• e.g. release of suppression of responding by a
  tone previously associated with punishment or
  conflict, but unimpaired escape behaviour
• 2) Reduction of response to signals of
  non-reward - abolition of the PREE
• 3) Reduction of response to novel stimuli
  (open-field, new food)
• 4) Reduction of response to innate fear stimuli

31
Gray's BEHAVIOURAL INHIBITION SYSTEM

• On this basis, Jeffrey Gray postulated that one
  function of the SHS and associated limbic
  structures is a
• BEHAVIOURAL INHIBITION SYSTEM
• Gray's BIS
• INPUTS any of
• signals of punishment
• signals of non-reward
• novel stimuli
• innate fear stimuli

32
Gray's BEHAVIOURAL INHIBITION SYSTEM II

• OUTPUTS all of
• inhibition of ongoing behaviour
• increment in arousal
• increased attention and checking
• SHS associated limbic system act as a
  comparator, receiving information about incoming
  stimuli comparing this with predicted events. If
  mismatch, or predicted aversive event, the BIS is
  activated.
• This is anxiety. Result is inhibition of ongoing
  
• behaviour, increased arousal, increased checking.

33
Gray's BEHAVIOURAL INHIBITION SYSTEM III

• ANTI-ANXIETY DRUGS gt REDUCED ACTIVITY IN THE
  B.I.S.
• How does potentiation of gaba bring about this
  inhibition of the BIS (anxiolysis)?
• Firstly, as indicated, there are many GABA
  interneurones in the HC, i.e. many GABA
  receptors, accounting for the high density of BDZ
  binding sites, i.e. sites for BDZs to act.
  Secondly the SHS receives substantial projections
  from the LC (NA) and median RN (5HT) in brain
  stem.

34
Amines and the BIS

• Activity of these systems is increased in stress,
  and inhibited by gaba in both cell body and
  terminal areas.
• Lesions to these pathways reproduce subsets of
  the effects of SHS lesions
• lesions to NA (DB) impair response to non-reward,
  novelty
• lesions to 5HT (RN) impair response to conflict,
  stimuli associated with punishment

35
Amines and the BIS II

• Broadly, the NA projection from LC is alerting
  tells SHS that this information is important,
  check carefully. Tagging of information entering
  SHS
• (NA projection to hypothalamus primes motor
  response, while SHS simultaneously inhibits it
  makes rapid switch possible)
• 5HT projections from MR tags stimuli associated
  with punishment, and is particularly effective in
  activating Behavioural Inhibition.

36
Catecholamines and the BIS

• THIS FORMULATION subsumes earlier evidence for
  involvement of NA system.
• This is an attractive concept, in that the
  neurochemical basis of preparation for
  fight/flight response involves catecholamines
• (Ad, NA) acting as hormones (from adrenal
  medulla), and NA as transmitter in the
  sympathetic branch of the autonomic nervous
  system, i.e. outside the CNS in the endocrine and
  PNS.

37
Catecholamines and anxiety

• What evidence suggests that the central component
  of the response (fear, anxiety) is subserved by
  CA pathways in the brain ?
• Clearly defined NA projection via dorsal bundle
• Evidence for involvement of central NA system -
  OHD
• Only (2) is nec specific to central NA in anxiety
  (NB spinal projections)

38
Catecholamines and anxiety II

• Appears that there is increased NA function
  especially in panic attacks
• Clonidine is effective treatment - stimulates
  autoreceptors. (Concept of autoreceptors)
• Tricyclics are also effective, which is
  paradoxical, unless their actions are via actions
  on 5HT systems
• Might be a longer term adaptive effect, possibly
  by down regulation of receptors.

39
anxiety and serotonin (5HT)

• Evidence suggesting a positive link between the
  5HT system and anxiety is also of long standing.
  
• Even before BDZ receptors were discovered, 5HT
  was implicated by finding that anticonflict
  effects of BDZ could be replicated by
• - lesions to 5HT system (5,7-DHT of
  ascending pathways)
• - inhibition of 5HT synthesis

40
anxiety and serotonin (5HT) II

• Led to the theory that one relevant action of BDZ
  was to reduce activity in raphe projections.
• Recent studies using in vivo techniques have
  indeed confirmed that extracellular 5HT in HC is
  reduced by systemic BDZs, after acute or chronic
  treatment
• Extracellular 5HT in HC is also increased in
  elevated plus maze (work of Marsden's and
  Zeterstrom's groups)

41
anxiety and serotonin (5HT) III

• HOWEVER anticonflict actions of BDZ persist after
  lesioning of projection from MR to HC, and thus
  other projections are involved too.
• 5HT projections from the DR innervate many of the
  DA innervated areas, in particular other limbic
  areas such as amygdala, NAC and fCx
• Hence possibility of indirect effects via 5HT
  action on DA.
• BDZ is known to reduce increased turnover/release
  in mesocortical/mesolimbic DA pathways in stress.
  

42
anxiety and serotonin (5HT) IV

• Some actions of BDZs on 5HT may be in the
  amygdala of known importance in fear
  conditioning.
• Issue of whether 5HT lesion simply releases
  behaviour in a non-specific way - disinhibition
  when offered a choice, animal goes for a small
  immediate reward in preference to a large delayed
  one.
• Actions of 5HT may be behavioural suppression
  whatever the cause - lesions release feeding in
  satiated rats, or for food made bitter by
  adulteration.

43
anxiety and serotonin receptors 2

• SSRIs are indirect 5HT agonists, and
  paradoxically are also effective against panic,
  like the NA specific ones.
• Interestingly, SSRIs also effective against OCDs
  (which have anxiety basis).
• Now have agents specific for different classes of
  5HT receptors.
• Receptors in the DR projection areas are of the
  5HT-2 type, and ritanserin, a 5HT-2A/C antagonist
  is reported to have anxiolytic properties in man

44
anxiety and serotonin receptors 2 1

• .and 5HT-2C agents are reported to have
  anxiolytic effects in animal models.
• Activity of MR neurones is controlled by 5HT-1A
  receptors on cell bodies
• A number of 5HT-1A partial agonists (buspirone,
  gepirone, ipsapirone) are also anxiolytic, and
  reported not to have dependence problems, but
  slower onset of action.

45
anxiety and serotonin receptors 1 3

• In animal tests the -1A agonists are most
  effective in neophobia and social interaction
  tests, less so in conflict, but recall that
  conflict test was optimised for BDZs
• (NB -1A also in HC - post-synaptic)
• The anxiolytic potential of 5HT-3 receptor
  blockers such as ondansetron is also discussed.
• Animal studies on 5HT-3 antagonists show effect
  in elevated plus maze.

46
anxiety and serotonin receptors 3

• Also, effects in light/dark box, which do not
  show tolerance or rebound on withdrawal for
  either -1A or -3 agents.
• Ondansetron, but not buspirone, blocks rebound
  effect of discontinuing BDZs.
• 5HT-3 effect could be mediated by post-synaptic
  receptors found especially in ERCx and dentate
  gyrus of HC
• Clinical trials have not yielded clear results.

47
Conclusions

• Convergence of the actions of a number of
  anxiolytic (anxiety reducing) agents on the GABA
  receptor (major inhibitory) has pointed to its
  central role in anxiety.
• Work on the BDZ site on the GABA receptor has
  resulted in useful pharmacological probes for
  both clinical and experimental work.
• A critical brain structure in anxiety appears to
  be the septo-hippocampal formation

48
Conclusions II

• Anxiolytic drugs may act directly on GABA
  receptors within this structure (local
  inhibition).
• May act also on the noradrenergic and
  serotonergic projections to it (inhibition of
  inputs), and to other limbic areas (interacting
  with its outputs).
• The activity of these systems in anxiety can now
  be investigated directly in animal models.
• More selective anti-anxiety drugs are available,
  and are being developed, working on subtypes of
  5HT receptors (5HT-1, -2, -3), although
  paradoxically, dirty drugs may be better.