COINCIDENT STIMULATION WITH PHEROMONE COMPONENTS IMPROVES TEMPORAL PATTERN RESOLUTION IN CENTRAL OLFACTORY NEURONS

Male moths must detect and resolve temporal discontinuities in the sex pheromonal odor signal emitted by a conspecific female moth to orient to and locate the odor source. We asked how sensory information about two key components of the pheromone influences the ability of certain sexually dimorphic projection (output) neurons in the primary olfacto ry center of che male moth's brain to encode the frequency and duratio n of discrete pulses of pheromone blends. Most of the male-specific pr ojection neurons examined grave mixed postsynaptic responses, consisti ng of an early suppressive phase followed by activation of firing, to stimulation of the ipsilateral antenna with a blend of the two behavio rally essential pheromone components. Of 39 neurons tested, 33 were ex cited by the principal (most abundant) pheromone component but inhibit ed by another, less abundant but nevertheless essential component of t he blend. We tested the ability of each neuron to encode intermittent pheromonal stimuli by delivering trains of 50-ms pulses of the two-com ponent blend at progressively higher rates from 1 to 10 per second. Th ere was a strong con-elation between 1) the amplitude of the early inh ibitory postsynaptic potential evoked by the second pheromone componen t and 2) the maximal rate of odor pulses that neuron could resolve (r = 0.92). Projection neurons receiving stronger inhibitory input encode d the temporal pattern of the stimulus with higher fidelity. With the principal, excitatory component of the pheromone alone as the stimulus , the dynamic range for encoding stimulus intermittency was reduced in nearly 60% of the neurons tested. The greatest reductions were observ ed in those neurons that could be shown to receive the strongest inhib itory input from the second behaviorally essential component of the bl end. We also tested the ability of these neurons to encode stimulus du ration. Again there was a strong correlation between the strength of t he inhibitory input to a neuron mediated by the second pheromone compo nent and that neuron's ability to encode stimulus duration. Neurons th at were strongly inhibited by the second component could accurately en code pulses of the blend from 50 to 500 ms in duration (r = 0.94), but that ability was reduced in neurons receiving little or no inhibitory input (r = 0.23). This study confirms that certain olfactory projecti on neurons respond optimally to a particular odor blend rather than to the individual components of the blend. The key components activate o pposing synaptic inputs that enable this subset of central neurons to copy the duration and frequency of intermittent odor pulses that are a fundamental feature of airborne olfactory stimuli.