Serotonin, anxiety and depression Flashcards
How can anxiety be measured in animal models
Using unconditioned, ethological tests of anxiety such as the elevated plus maze or food hyponeophagia
What is the elevated plus maze
A cross shaped maze raised above the ground featuring two open arms and two closed arms
Anxiety is captured by approach/avoid conflict between the competing goals/drives of the dark enclosed safe spaces and exploratory drive.
Measured by open arm entries/ total entries.
What are the effects of benzodiazepines on EPM (Pellow et al., 1985)
validation of the EPM paradigm as a measure of anxiety (two open two closed arms in a + maze design)
confinement to the open arms was associated with higher levels of plasma corticosterone and more anxiety-related behaviour than rats confined to the closed arms.
administration of anxiolytic drug diazepam was associated with more time spent in the open arms
administration of the anxiogenic drug amphetamine was associated with less time spent in the open arms.
what is the difference between axiolytic and anxiogenic drugs?
anxiolytic drugs such as benzodiazepine reduce anxiety
anxiogenic drugs such as amphetamine and haloperidol increase anxiety
what is hyponeophagia
Novelty-suppressed feeding
Mice and rats cannot vomit so to overcome the problem of potential food toxicity they have evolved a strategy of first ingesting only very small amounts of novel substances.
Bait shyness is now used in the behavioural laboratory as a way of measuring anxiety.
A highly palatable but novel substance is offered in a novel situation, such as a new cage.
The latency to consume a defined amount of the new food is then measured.
Fossat et al., 2014
Demonstrated that serotonin increases anxiety in crayfish using an aquatic plus maze.
When allowed to freely explore the plus maze, lobsters show a preference for the dark arms.
Administering a shock increases the time spent in the dark arms, demonstrating this is an anxiety-related behaviour.
HPLC chromatography which quantitively measured the composition of plasma (drugs and their metabolites) found that the shock and subsequent anxiety-related behaviour were associated with higher levels of 5-HT.
Administration of 5-HT antagonists (e.g minaserin and methysergide) block 5-HT receptors, and blocked the increase in anxiety-related behaviour following shock.
Injection of 5-HT increased anxiety-related behaviours (increased time spent in the dark arm) in the absence of a shock.
Thus, stress increases 5-HT, which increases anxiety-related behaviours.
Brilley et al., 1990
demonstrated that selective serotonin lesions reduce anxiety-related behaviour on the EPM in rats.
Administered 5,7, hydroxytryptamine as a selective toxin of 5-HT neurons to produce selective 5-HT depletion.
2 weeks later rats tested on the EPM, those with selective 5-HT lesions spent more time on the open arms of the maze compared to controls. Post-mortem HPLC confirmed that 5-HT was depleted in the HPC and cortex.
Ohmura et al., 2015
Optogenetic technique used to selectively activate MRN and DRN 5-HT neurons to release serotonin and measured effect on EPM.
ChR2 expression is controlled by Tryptophan hydroxylase promoter, only expressed in 5-HT neurons. ChR2 is light sensitive, light induces a conformation change resulting in opening of an ion channel, influx of Na+ and so depolarisation. Thus, blue light activation should cause the release of serotonin.
Validated this histologically: stained THP and FYP (which correspond to 5-HT neurons and ChR2 expressing neurons respectively) demonstrated overlap.
Electrophysiologically: blue light activation demonstrated to cause an increase in firing rate (so depolarisation and subsequent action potential is occurring)
in vivo micro-dialysis in ventral HPC of CSF fluid demonstrated corresponding increase in 5-HT levels following blue light stimulation.
Behaviourally, activation of MRN neurons resulted in more time spent in the enclosed arms (so more anxiety-related behaviour) compared to controls. Not seen for DRN stimulation.
What determines the emotional response to a given aversive situation
the proximity and intensity of danger (defensive distance) and whether an animal is approaching or leaving the danger (defensive direction). An important caveat is then whether it is possible to avoid the threat given the environmental constraints, i.e. is there an escape route.
Each emotional response is associated with a distinct behavioural profile which has evolved for the demands of the situation
Explain defensive distance
Defensive distance can be considered as a graded scale moving from potential, to imminent, and current threat.
When threat is potential, such as if a mouse were to detect the odour of a cat, it induces anxiety (passive avoidance).
if the mouse now sees the cat, the threat becomes imminent inducing fear (active avoidance).
If the cat were to move to attack the mouse, the threat is current and the mouse would enter a state of panic resulting in an undirected escape attempt.
what is active and passive avoidance
If the mouse withholds from entering, anxiety has facilitated passive avoidance of danger where the defensive direction is ‘not towards’ threat. In contrast, if the mouse now sees the cat, the threat becomes imminent inducing fear. The emotional response of fear drives the animal to actively avoid the danger, moving away from threat.
what are the neural correlates of aversive emotional responses (Graeff, 1994)
Analysis of the underlying neural correlates to these emotional responses has led to the conception of a hierarchical defence system where progressively more central structures deal with progressively more complex aspects of defence (Graeff, 1994).
Here, the lowest level of the hierarchy is the periaqueductal grey (PAG) which coordinates undirected escape, then the medial hypothalamus mediates directed escape, the amygdala mediates freezing and active avoidance, while at the highest level the hippocampus mediates anxiety and risk assessment behaviour.
How do studies of pain support the theoretical distinction between fear and anxiety.
Fanselow 1979: shuttle box experiment where rats show a preference for side of cage associated with predictable shocks. This preference is removed if nalaxone is administered. Indicates that anelgesic endorphins are released in anticipation of the shock to reduce subjective pain.
Bolles & Fanselow 1979
This effect is dose-dependent and learning dependent and is not seen if naloxone is administered after the shock, or if the animal has only received a single shock
Bolles & Fanselow 1980
This behaviour helps the animal to avoid danger because analgesics suppress recuperative behaviours such as wound licking which could interfere with fight, flight or freeze responses in the face of danger.
Ploghaus et al., 2001
anxiety increases sensitivity to pain - signalling reduced fitness and so increasing avoidance behaviour. E.g human fMRI where HA stimulus is associated with either painful hot stimulus to wrist as well as low temp stimulus, LA stimulus only associated with low temp. Greater subjective pain reported for same low temp stimulus when cued by HA stimulus compared to LA stimulus. Associated with activity in HPC formation (before we were able to measure deep brain structures like ventral HPC). Implications for pain management. Highlights importance of preparatory cues.
What are the neural correlates of learning about ambiguous and unambiguous cues
Need to learn about cues that indicate increasing proximity of danger.
Some are innate → unconditioned anxiety.
Others need to be learnt/ acquired.
Cues that signal potential vs imminent threat are differentially disrupted by lesions.
Phillips & LeDoux, 1992
Measured conditioned freezing in response to tone and chamber following administration of shock. Control rats freeze in the chamber before and after the tone. HPC lesion rats only freeze to tone. Amygdala lesion rats freeze to neither shock and tone.
McNish et al., 1997
Rats with dorsal HPC lesions unimpaired on contextual fear-potentiated startle. Explained by unconditioned effect on activity that impaired freezing such that relatively weaker conditioning is associated with contextual learning and is not sufficient to induce freezing.
Richmon et al., 1999
Cytotoxic lesions to dorsal vs ventral HPC on freeze response to context measured. Here, it was demonstrated that lesions of the ventral hippocampus but not dorsal impair contextual fear conditioning. Dorsal hippocampal lesions led to impairment on the watermaze task, a test of spatial reference learning. Thus, the ventral hippocampus seems to support anxiety, and its disruption leads to deficits in learning about more distal, ambiguous cues that suggest potential danger.
Bannerman et al., 2004
Effects of ventral HPC lesions on unconditioned anxiety
Glover et al. 2017
In a genetically modified mouse which expresses thymidine kinase selectively in newborn cells, valganciclovir can be used to selectively inhibit neurogenesis.
Enables comparison of the relative contribution of newborn cells in the HPC to learning about ambiguous cues.
Tone-shock conditioning to CS+ and 50%CS.
Transgenic mice without neurogenesis showed reduced defensive behaviour than control mice for 50%CS.
Fos gene analysis of network activity in HPC and amygdala showed fewer activated mature granule cells in CA3 and CA1 in ambiguous relative to unambiguous fear condition. Not seen in amygdala or controls.
Control mice use the predictability of prior threat to adapt levels of cautious anxiodepressive-like behaviour in subsequent feeding environments. Mice lacking neurogenesis showed no difference in novelty-suppressed feeding latency in either condition, taking longer following predictable cues, and less time following ambiguous cues relative to controls.
General increase in anxiodepressive-like behaviours irrespective of the shock predictability. Furthermore, mice who were adrenalectomized (blocking stress-induced glucocorticoids release) failed to show the increase in novelty-suppressed feeding latency in control mice following learning about an ambiguous predictor of danger. Therefore, learning about ambiguous predictors of threat result in an increase in defensive behaviour and this generalises to subsequent novel contexts and this process depends on hippocampal neurogenesis and glucocorticoids.
