chemical senses and memory

Question 1

olfaction is a M_____ sense?

Answer 1

multidimensional sense - unlike sound there aren’t set parameters to measure, like intensity (amplitude) and frequency

Question 2

what is a labelled line vs a combinatorial code?

Answer 2

Labelled line (taste) = specific receptor dedicated/only responding to one specific input

Combinatorial code (olfaction) =
Multiple neurons may respond to varying degrees of intensity, allowing for integration of information; the pattern/combination of receptors activated and to what degree, is what encodes/defines the sensory input
Allows for perception of multidimensional inputs

Question 3

how does olfactory sensory transduction work?

Answer 3

Odorant molecule activates olfactory receptor - GPCR - Golf alpha subunit - adenylate cyclase, cAMP, cation channel opens, Ca2+ activates a Cl- channel

Take home message = use of second messengers amplifies the signal
NOTE - insect olfactory receptors (ORs) are ion channels, not GPCRs

Question 4

do olfactory receptors respond to one odorant only?

Answer 4

Basically receptors respond to a specific kind of odorant AND will also respond to similar molecules, creating their odorant response profile. The better the fit/bind the stronger the response/ the lower the concentration of odorant a receptor is able to respond to

Question 5

as olfactory sensory neurons mature…?

Answer 5

As olfactory sensory neurons mature, they narrow down to each express only one kind of olfactory receptor

Otherwise how would the brain know what odorant caused the response from the receptor

Question 6

How to glomeruli contribute to organisation? Similar structure in insects?

Answer 6

Olfactory sensory neurons (OSNs - mammals) expressing the same receptor are scattered throughout the epithelium (inc. chances of detecting an odourant)

These OSNs expressing same receptor type, converge onto one glomerulus in the olfactory bulb

Insects have an analogous structure - olfactory receptors (ORs) on antenna - converge onto glomeruli in antennal lobe

Question 7

what comes after the glomerulus?

Answer 7

A 2nd order neuron - projection neuron (PN) in drosophila - synapses with the sensory neurons at the glomerulus, one PN to one glomerulus, to keep odorant info separate/distinct
(Its mitral and tufted cells in mammals)

Question 8

give three reasons why a second order neuron is needed

Answer 8

If an odorant persists, the sensory neuron/receptor will continue responding at a regular rate
However senses are important for detecting change
1. The second order neuron fires at a high rate at first, but this drops off over time even as the smell persists, as the presynaptic terminal uses up its readily releasable vesicle population

2. Reduces noise - by allowing for summation of information from the multiple OSNs activated by an odorant, into one signal
3. Increases sensitivity - experiments showed that while the ORNs have low activity at low concentrations of an odorant, the activity of the projection neurons was high due to the spatial summation

Question 9

interneurons are connections
between glomeruli.

explain one of their purposes (G_ C_)

Answer 9

Gain control -
we need to be able to detect differences at low concentrations - hence the spatial summation causing high PN /second order neuron activation in low odour concentrations

But we also need to detect differences when odorants are in high concentrations, so we need to ‘dampen’ size of response so its not
immediately maxed out and differences can still be detected

This is achieved by lateral inhibition via the interneurons

Question 10

interneurons are connections
between glomeruli.

explain one of their purposes (De-c______)

Answer 10

de-correlation -
communication between glomeruli via interneurons is essential for making the response pattern to different odours as different/distinguishable as possible

Experiment showed knocking out the interneurons made the odour correlation between responses from mitral cells more similar/harder to tell apart

Question 11

what structures are involved in innate response to smell (mammals and drosophila)?

give two experiments demonstrating this

Answer 11

Glomeruli send info to the amygdala in mammals, the lateral horn in drosophila

experiments -
1. Mice in quadrant box thing avoided smell of foxes (TMT in fox urine)
Silencing of cortical amygdala (using optogenetics) meant they no longer avoided the fox smell

2. Flies - avoid toxic food with harmful microbes when choosing where to lay eggs
   Silencing of lateral horn neurons removed this avoidance

Question 12

what structures are involved in learned response to smell?

Answer 12

Glomeruli send info to the piriform complex in mammals, or the mushroom body in drosophila

Question 13

for innate and learned responses to smell, compare:

1. purpose
2. kind of activity seen
3. what odours affect what neurons?

Answer 13

1. innate = categorise, good or bad
   Learned = discriminate, differ between people, assign good or bad to individual odours
2. innate = dense signalling, requires robust and defined response
   Learned = sparse, each odour activates a few neurons
3. innate = certain neurons pre-determined to react to e.g. food
   Learned = Arbitrary - each neuron could potentially respond to anything (don’t know what the smell will be associated with) so during development connections are pretty much random

Question 14

what is the biased random walk?
Explain how this is seen in E.coli

Answer 14

walking, and if things are improving, keep going straight, if not, turn

E. coli do this, have a chemical pathway to detect food, if conc. Increases the pathway signals flagellum to spin one way causing ‘runs’/keep going, if signal is getting weaker, spins the other way and ‘tumbles’ - changes direction

Question 15

give an example of olfactory search behaviour in drosophila

Answer 15

Cross wind cast in drosophila - when you’re heading towards a smell, the scent could be lost simply due to turbulence, so important not to turn immediately upon loss of scent, flies zigzag instead

Question 16

what is ‘active sensing’ in mammals?

Answer 16

another olfactory search response
Moving head around lets you sample a larger space AND generates fast changes of detected odour concentration

Coordinate the sniff cycle with how you move your head to help localise smell

NOTE - humans are actually quite good at this, and seen in fly larva also

Question 17

explain the labelled line/ areas of brain involved in taste

Answer 17

tongue to
solitary nucleus of brainstem to
hypothalamus and amygdala

from solitary nucleus, also goes to VPM (ventral posterior medial nucleus) of the thalamus, to the insula and parietal cortex

Question 18

how does taste use interneurons?

Answer 18

If something has stuff that should taste bitter AND sweet, the bitter sensory neuron activates an interneuron that inhibits the activated ‘sweet’ sensory neuron, preventing it from passing signal onto second-order neuron, so we still avoid the food

Question 19

different tastes…

Answer 19

activate different parts of the brain

Possibly to link to avoidance vs attract - Optogenetically activating ‘sweet’ or ‘bitter’ areas of insula makes mice approach or avoid the stimulus

Question 20

each organism that can move must decide what to do next, so what must happen when they take in sensory information?

Answer 20

detect the sensory information, then there must be some processing, but also some kind of evaluator that helps decide what to do based on that sensory information - then motor output obvi

Question 21

briefly give an overview of Pavlov’s dogs

Answer 21

Dog sees food (unconditioned stimulus, US) and salivates

Ring bell (conditioned stimulus, CS) before you feed the dog, and after a while the bell is enough to make the dog salivate - associated the sound with getting food

basically memory (f the bell) allows ‘prediction of future’ (food must be coming)

Question 22

what experiment similar to Pavlov’s dogs has been done in drosophila?

Answer 22

Associating an odour with a reward like sugar (or just absence of punishment?), and another odour with a punishment like a shock, see if fly remembers which chamber was safer/didn’t receive a shock based on the different odours

Question 23

give an overview of the cells in the olfactory system of drosophila

Answer 23

olfactory receptor neurons (detect individual odours, organised into glomeruli at antennal lobe) projection neurons (PNs) are the second order neurons - these input into kenyon cells (KCs)

Question 24

explain how kenyon cells work

Answer 24

Multiple projection neurons input to kenyon cells

Kenyon cells require activity from it’s multiple inputs at once to fire (because each PN only causes a small response and the AP threshold in kenyon cells is high)

This results in kenyon cells requiring very specific smells (reacting ‘SPARSELY’), or certain combinations of smells

Question 25

why is the sparse nature of kenyon cells necessary?

Answer 25

If too many Kenyon cells were active for loads of odours - e.g. loads of overlap for different smells, - different odours wouldn’t have such distinct behavioural responses - e..g if the smell associated with a shock activated same kenyon cells as the smell with no shock, no difference in behaviour/preference would be seen

Question 26

what are the three steps of the GAL4/UAS system?

Answer 26

1. Make a transgenic line of flies with GAL4 + promoter -
   Gal4 = a yeast transcription factor
   Promotor = attaching GAL4 to a promoter is done to direct GAL4’s expression to one tissue/cell type - control location of expression
2. Make UAS driver line -
   UAS is the DNA sequence the TF ‘GAL4’ binds to
   Put UAS upstream of your gene of interest (more on what this might be later)
3. Cross the lines -
   Cross the two transgenic lines, resulting in your gene of interest only being expressed in your location of choice (based on the promoter you choose)

Question 27

what are some possible uses of the GAL4/UAS system?

Answer 27

Possible ‘genes of interest’ - fluorescent proteins to visualise morphology of specific neurons

Express proteins to induce cell death or modify neural activity

***Use optogenetics to silence specific neurons in certain situations and evaluate function in various behaviours, physiological processes, or disease states

Question 28

what is a slight drawback, and solution, of the GAL4/UAS system?

Answer 28

Drawback - While use of a promoter helps localise expression of your gene of interest to a certain area/cell type, you often want to be even more specific to a single cell or a very small number of cells

Solution - narrow down which cells will express the gene of interest by using two promoters -
by splitting the GAL4 transcription factor into two parts, and adding zipper domains so that they can zip together and form a functional protein, you can put each half under the control of different promoters
Now a cell has to express both of those promoters to express both halves of GAL4 and cause production of your protein of choice

Question 29

what is the mushroom body? what three neurons are important for what we are looking at?

Answer 29

The site of associative learning in flies

Kenyon cells are to do with detecting very specific smells

DANs = dopaminergic neurons, that carry the reward/punishment information

Mbons = mushroom body output neurons = the ones that send out avoid or approach signals

Question 31

how are Kenyon cells organised by Mbons and DANs?

Answer 31

KCs send their axons along two lobes

Mbons receive input from specific areas of KCs, sectioning the lobes

DANS also section the lobes of KC axons identically, by where the DANs provide input to the KCs

Question 31

when a fly smells a certain odour, and is then given a shock, what happens to these neurons?

Answer 31

Kenyon cell is activated by the specific odour

the DAN (dopaminergic neuron) signals that there was a punishment/pain/ this smell is bad

and what the DAN does is weaken the KC connection to the approach output/Mbon, making the avoid output synapse relatively stronger

forward learning - DAN causes depression of KC synapse with the ‘wrong’ output

Question 32

in FORWARD learning, dopamine signalling…

Answer 32

Dopamine signalling locally depresses (LTD) KC-MBON synapses for active KCs, for the ‘wrong’ output

i.e. if smell is associated with a reward, the DAN causes depression of the KC – avoid Mbon synapse

Question 33

give a brief overview of the experiment that implanted ‘fake’ memories

Answer 33

Some DANs, when activated optogenetically, can entrain aversive or appetitive memory

You can implant fake memories in a fly, in that use of optogenetics in a punishment DAN (i.e. certain light activates the neuron) paired with a certain odour can result in avoidance of that odour, despite there not being an actual punishment

Question 34

scientists activated KCs THEN DANs, and then the other way around, measuring the strength of mbon stimulation using GCaMP

what happens to the neurons in the backward pairing? (DAN activated first then KC, like shock THEN odour)

Answer 34

potentiation of KC-MBON synapses

scientists stimulated DAN then KC, mimicking shock then odour. Saw LTP of the approach output (the odour is signalling relief/absence of pain/shock

So learning isn’t all LTD

Question 35

scientists activated KCs THEN DANs, and then the other way around
what happens to the neurons in the forward pairing? (KC activated first then DAN, like odour THEN shock)

Answer 35

as already discussed, you get depression of the KC-MBON synapse involved in the approach output

i.e. weakening the ‘wrong’ response to make the correct one relatively stronger

Question 36

molecular mechanisms - how do KCs have two different reactions to dopamine (LTD vs LTP)?

Answer 36

they have two different dopamine receptors, DopR1 and DopR2, each with different downstream consequences

Question 37

what does DopR1 do/what processes is it involved in?

Answer 37

DopR1 is associated with forward learning, meaning odour predicts shock/reward

this means activation of DopR1 likely causes LTD

it is Gs coupled, so produces cAMP

Question 38

what does DopR2 do/what processes is it involved in?

Answer 38

this mediates forgetting, which is an active process

likely causes LTP

Gq coupled, so causes IP3 production and Ca2+ release from the ER

Question 39

explain the experimental evidence behind the idea that DopR1 causes LTD and DopR2 causes LTP, depending on order of activation of KCs vs DANs

Answer 39

1. measured levels of cAMP (2nd messenger of DopR1) and Ca2+ release (2nd messenger of DopR2)

note - Used an optical sensor for cAMP, and expressed G-CAMP in the ER so reduced signalling from GCAP = more Ca2+ release

Result = production of second messengers relies on the coincidence of DA and KC activation i.e. the order/timing

cAMP - highest when the DA and KC signals happened at same time, then symmetrically drops off in either direction the further apart the signals occur
Ca2+ release - asymmetrical - only get Ca2+ release from the ER when dopamine comes before KC activation… continued

Question 40

youve explained that production of second messengers from DopR1 s DopR2 relies on the coincidence of DA and KC activation i.e. the order/timing

now explain how this links to LTD and LTP?

Answer 40

Subtracting the cAMP profile from the Ca2+ release one - matched the graph measuring LTD and LTP (or forward vs backward conditioning)

Combo of Ca2+ release and cAMP production determines LTP and LTD, deciding what the neuron will do, approach or avoid

Question 41

the two dopamine receptors cause LTD vs LTP

how come both effects aren’t seen equally in every scenario if they’re activated by the same NT?

Answer 41

theory - the IP3 receptor is a two factor switch…

Ca2+ then IP3 -
Odour detection then teaching signal (forward/LTD)
KC activity 1st causes Ca2+ influx, Ca2+ binds to the IP3 receptor, locking it in a conformation where IP3 cannot bind, preventing activation and Ca2+ release despite DopR2 activation

IP3 then Ca2+ -
Teaching signal (shock/reward), followed by odour (backward/LTP)
Dopamine first, then Kenyon cell activity, meaning IP3 then Ca2+, IP3 binds to receptor, changing conformation and exposing a binding site for Ca2+ from the KC activity, Ca2+ binds and the receptor is activated, ER Ca2+ release (two factor switch)

Question 42

explain the similarity between this circuit in the mushroom body and the cerebellum in mammals

Answer 42

Circuit has same organisation, and same purpose in that ‘training’ reduces the ‘wrong’ behaviour (excpet cerebellum is for motor fucntion)

mushroom body PNs = cerebellum mossy fibres

mushroom body KCs = cerebellum Granule cells

mushroom body DANs = cerebellum climbing fibres

mushroom body mbons = cerebellum purkinje cells

Question 43

explain the similar circuitry in fish with an electrosensory lobe

Answer 43

PNs = mossy fibres

KCs = granule cells

DANs = efferent copy of signal that the electric organ in the fish tail sends out, to prevent the electric sensory lobe of CNS from mistaking the fish’s own signals for that of its prey. Also uses LTD

Mbons = purkinje-like cells

This circuit allows the fish to learn to ignore electric signals generated by its own electric organ (i.e., “wrong” signals)