Abstract
A neuron’s sensitivity profile is fundamental to functional classification of cell types, and underlies theories of sensory coding. Here we show that gustatory neurons in the nucleus of the solitary tract (NTS) and parabrachial nucleus of the pons (PbN) of awake rats spontaneously change their tuning properties across days. Rats were surgically implanted with a chronic microwire assembly into the NTS or PbN. Following recovery, water-deprived rats had free access to a lick spout that delivered taste stimuli while cellular activity was recorded. In 12 rats for the NTS and 8 rats for the PbN, single units could be isolated at the same electrode on consecutive days (NTS, 14 units for 2–5 consecutive days, median = 2 days; PbN, 23 units for 2–7 days, median = 2.5 days). Waveforms were highly similar (waveform template correlation > 0.99) across days in 13 units in NTS and 13 units in PbN. This degree of similarity was rare (0.3% of pairs in NTS, 1.5% of pairs in PbN) when the waveforms were from presumed-different neurons (units recorded on nonconsecutive days with at least one intervening day in which there were no spikes, or from different wires or rats). Analyses of multi-day recordings that met this criterion for “same unit” showed that responses to taste stimuli appeared, disappeared, or shifted in magnitude across days, resulting in changes in tuning. These data imply, generally, that frameworks for cell classification and, specifically, that theories of taste coding, need to consider plasticity of response profiles.
Highlights
The neural representation of sensory stimuli or events is mediated by neurons that are specialized to respond to a subset of features within the sensory domain
While taste-responsive cells in the CNS are more broadly tuned across taste qualities than are cells in the periphery, functionally distinct cell types can be defined by the tastant that evokes the most vigorous response, called the “best” stimulus [2]
To determine whether Consecutive recordings were likely to represent recordings of the same neuron, we examined the similarity of the recorded waveform templates
Summary
The neural representation of sensory stimuli or events is mediated by neurons that are specialized to respond to a subset of features within the sensory domain. While taste-responsive cells in the CNS are more broadly tuned across taste qualities than are cells in the periphery, functionally distinct cell types can be defined by the tastant that evokes the most vigorous response, called the “best” stimulus [2] These taste-quality-specific cell types are evident as a chemotopic map in the gustatory cortex [3,4,5]. Long-term recordings (up to 21 days) from single fibers in the chorda tympani nerve (mediating taste on the rostral tongue) show that their response profiles change over time [7] These recordings were made from rejuvenated chorda tympani (CT) fibers, the authors hypothesized that these effects were likely due to reorganized input resulting from the turnover in taste receptor cells. Central systems for coding and processing taste stimuli must function in the face of a dynamically changing set of tuning properties in their brainstem afferents
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