Neural control circuits in C. Elegans 3

Conservation of foraging behaviour

• A behavioural state consisting of a sustained period of intensive searching after food encounter is highly conserved

• Understanding the molecular and circuit mechanisms underlying this searching state may provide insight into the basis of ancient conserved behaviours

mgl-1 is required for local search behaviour

• 42 mutants lacking ionotropic and metabotropic glutamate receptors in the neurons of the foraging circuit screened

• Those which did not impact feeding or locomotion taken and calculated the average fold change in reorientations relative to the wildtype during local search

• Mutant strain CX17083 showed the strongest defect in local search behaviour (with only mild defects during global search) → biggest difference in reorientations relative to the wildtype

• unmapped background mutation responsible → deletion that disrupted several exons of mgl-1 which encodes a G- protein coupled inhibitory metabotropic glutamate receptor

• Full length mgl-1 transgene rescued the local search behaviour, implicating mgl-1 as the causative mutation

mgl-1 mutants

• Persistent pharyngeal pumping after food removal

• altered physiological response to prolonged starvation

• therefore, mgl-1 is necessary for local search behaviour after food removal

identifying sites of mgl-1 action

• activation of the mgl-1 fragment is able to rescue the local search defect

• activation of mgl-1 in a subset of cells and observed the phenotype —> ASI, IL1, ADE, AIA, RMD and NSM were able to rescue the phenotype

• Of these, only activating mgl-1 in AIA, ADE or in both, was sufficient to rescue the phenotype

hyperactivity of AIA and ADE in the mgl-1 mutants

• MGL-1 is mst similar to the mammalian group 2 metabotropic glutamate receptors, which couple to inhibitory G proteins

• therefore, hypothesised that the loss of inhibition of the AIA and ADE neurons may result in hyperactive neurons which interfere with local search

• Used tetanus toxin light chain to prevent synaptic vesicle release and silence these neurons —> expressing tetanus toxin in AIA and ADE in mgl-1 mutants was able to rescue the local search deficits with minimal impact on locomotion

• This indicates that when animals are removed from food, MGL-1 suppresses AIA and ADE to release local search behaviour

• AIA and ADE both release multiple neurotransmitters and peptides which could mediate this behavioural effect —> dopamine and ins-1 both tested but neither explain local search behaviour

Sources of glutamate which supress AIA and ADE through MGL-1 to generate local search

• AIA and ADE post-synaptic to multiple glutamatergic neurons

  • AIA receives glutamatergic input from chemosensory neurons

  • ADE receives glutamatergic input from mechanosensory neurons regulated by food texture

• Knockouts of the eat-4 vesicular transporter (responsible for loading glutamate into the vesicles) in both the chemosensory and mechanosensory pathways resulted in an identical phenotype to the mgl-1 mutants

• glutamatergic chemosensory and mechanosensory neurons each independently drive full local search behaviour by suppression of either AIA or ADE

negatively correlated activity of ASK and AIA

• ASK identified to have a behavioural defect when eat-4 was knocked out and is directly activated by food removal

• In the absence of food ASK and AIA both exhibit spontaneous activity:

  • ASK decreases calcium levels from a high baseline

  • AIA increases calcium activity from a low baseline

• Activity levels of these neruons also recorded in the early and late times after food removal and concluded that in the shift between local to global search behaviour AIA becomes more active and ASK becomes less active

Glutamate controlling AIA activity at multiple timescales

• the activity of these neurons were negatively correlated, suggesting that they were glutamate release from ASK may inhibit AIA

• However, mgl-1 not required for the temporal coupling of ASK and AIA activity suggesting that glutamate signalling through MGL-1, alongside another glutamate receptor to modulate AIA activity

summary - circuitry driving local search behaviour

• when food was last encountered encoded using chemosensory and mechanosensory pathways, either of which can drive the local search behvaiour

• Removal from food leads to the activation of multiple sensory neurons which release glutamate onto AIA and ADE

• glutamate acts through MGL-1 (slow GPCR) and also on fast glutamate gated channels in AIA and other neurons

• MGL-1 activation (due to its similarity with inhibitory group 2 metabotropic receptors) supresses AIA and ADE neurotransmitter release during the period of local search

• During this period the reduction of either AIA or ADE release is sufficient to incraese reversals and reorientations → nt release from these neurons may inhibit the reversal promoting AIB and AVA neurons

summary - circuitry driving the switch between local and global search

• after 10-20 minutes away from food, glutamatergic signalling from ASK slowly decreases and AIA activity slowly increases, representing a food memory

• At the same time, the reversal circuit becomes resistant to sensory input through additional mechanisms, resulting in global search

• However, even when sensory glutamate onto AIA is absent (removed by tetanus toxin) you are still able to exhibit local search behaviour showing that glutamatergic memory is not essential and additional clocks can represent food memory

parallel processing in circuitry conserved behaviour

• either AIA or ADE modules are sufficient to generate the local search behaviour, not additive

• parallel processing and nutritional cues characteristic of mammalian feeding and satiety circuits → suggestion that this redundant organisation may be common in circuits involved in survival behaviours