Olfactory coding: tagging and tuning odor-activated synapses for memory

Abstract

A recent study in the locust olfactory system shows how neuromodulators can alter the rules of synaptic plasticity to form associative memories through the use of 'tagged' synapses.

Figure 1

Neuromodulation of STDP in the insect olfactory system. (A) The locust olfactory system receives sensory input from olfactory receptor neurons (ORNs) mainly on the antennae. These neurons synapse with both local neurons (LNs) and projection neurons (PNs) in the antennal lobe (AL), the first olfactory center within the brain. The PNs alone carry olfactory information to other areas of the brain, including the Kenyon cells (KCs) of the mushroom body calyx (MB calyx), which in turn synapse on the β-lobe neurons (bLNs). (B) Spike-timing-dependent plasticity (STDP) was elicited at the KC–bLN synapse. Two well-separated groups of KCs in the mushroom body (MB; S1 and S2, one for pairing, one for control) could be activated extracellularly, and individual bLNs could be activated intracellularly. To elicit STDP, stimuli to KCs and bLNs were paired, forward (dt > 0) or backward (dt < 0), within narrow temporal windows (±30 ms, upper sets of traces). Induction of STDP was sometimes followed by injection of octopamine (OCT) into the β lobe (middle trace). The results of these manipulations were tested afterward (bottom traces): STDP facilitated the KC-elicited response in the bLN (rising phase of the EPSP is shown) when dt > 0, and diminished the response when dt < 0; delayed delivery of OCT decreased EPSPs elicited only at STDP-tagged synapses (S1). (C) In a more naturalistic test, firing in the bLNs elicited by an odor (odor 1) was reduced after that odor specifically (odor 1, but not odor 2) had been paired with OCT injection into the β-lobe.