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If a neuron has both excitatory and inhibitory chemical synapses providing it with input, is it true in all organisms that the chemicals that cause the inhibition are distinct from those that cause the excitation?

Related question:
What is "synaptic polarity" in a chemical synapse?

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    $\begingroup$ I don't know enough to answer the "all" aspect, but usually L-glutamate is excitatory and GABA inhibitory. $\endgroup$ Nov 22, 2017 at 16:52
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    $\begingroup$ Also, do you count pre-birth and abnormal variations in GABA role? nature.com/articles/nrn3956 $\endgroup$ Nov 22, 2017 at 16:55
  • $\begingroup$ @Fizz: I am concerned that perhaps the question is too broad, now that I think of it. I'm mainly interested in adults who have developed normally $\endgroup$ Nov 22, 2017 at 17:03
  • $\begingroup$ @MichaelStachowsky You can make this question more focused (as you state) by editing it. $\endgroup$
    – Steven Jeuris
    Nov 23, 2017 at 13:42

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Short answer
I cannot give a conclusive answer, but if you are talking about a schoolbook neuron with different neurons synapsing onto it, then excitation is indeed generally mediated by another neurotransmitter than inhibition.

Background

  • First and foremost, in the central nervous system the principal excitatory neurotransmitter is glutamate, and the principal inhibitory neurotransmitter is GABA. However, neurotransmitters in themselves are not excitatory or inhibitory, it is the receptors that govern their workings. In this regard it is important to know that none of the Glu receptors (ionotropic or metabotropic) is inhibitory, at least as faras I am aware, and none of the GABA receptors is excitatory.
  • Secondly, from a postsynaptic point of view, synapses with different input cells are separate. In other words, a glutamatergic neuron never secretes GABA in the same synapse or vice versa. In effect, neurotransmitter systems are separated. Often, a principal neurotransmitter may be accompanied by neuromodulators, but never will one neuron release opposite effector molecules. Instead, opposing inputs are imposed by separate cells, each with their own neurotransmitter system.
  • The neurotransmitter acetylcholine can be excitatory at the neuromuscular junction in skeletal muscle, causing the muscle to contract. In contrast, it is inhibitory in the heart, where it slows heart rate. So Ach can have dualistic and even opposing actions, but the respective receptors are different and geographically segregated in different neurons;
  • But, this is schoolbook, anthropocentric knowledge, and since you are asking about *any creature (anywhere in the multiverse?), then yeah... I must admit I simply don't know. Little is known about deep sea creatures for instance, and it might as well be that one freaky critter deploys its neurochemicals in totally different and unexpected ways.
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    $\begingroup$ Thanks for the answer and the detail. I have a feeling that most questions about the nervous system of...anything really...start with "it depends on..." and then takes 50 years of PhD level research to answer... $\endgroup$ Nov 24, 2017 at 3:28
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    $\begingroup$ There certainly are inhibitory presynaptic metabotropic glutamate receptors; they help regulate overexcitability by self-inhibition. $\endgroup$
    – Bryan Krause
    Aug 2, 2022 at 13:53
  • $\begingroup$ @BryanKrause that's interesting! Didn't know :) $\endgroup$
    – AliceD
    Aug 3, 2022 at 11:40

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