Imaging odor coding and synaptic plasticity in the mammalian brain with a genetically-encoded probe.

We have used the genetically-encoded fluorescent exocytosis indicator synaptopHluorin (spH), expressed selectively in mouse olfactory receptor neurons, to image odor representations at the input to the olfactory bulb. The olfactory bulb is a powerful system for in vivo fluorescence imaging because its inputs are segregated into receptor-specific functional units (glomeruli) that are optically accessible and receive massively convergent input from sensory neurons. In a line of transgenic mice expressing spH under the control of a receptor neuron-specific promoter (OMP), odorant-evoked patterns of receptor neuron input to approximately 10% of the olfactory bulb can be imaged with excellent spatial resolution and sensitivity during single brief odorant presentations. Odor representations are similar across mice and can be imaged repeatedly in the same animal for months. In olfactory bulb slices from OP-spH mice, shock-evoked spH signals are rapid and linear reporters of transmitter release, although control for changes in extracellular pH is critical for proper interpretation of the spH signals. These features have allowed us to characterize the functional organization and mechanisms of presynaptic modulation of transmitter release at the first olfactory synapse. The capacity for long-term chronic imaging permits the direct visualization of the function regeneration and remapping of input to the olfactory bulb after lesions of the nasal epithelium.

Predominance of late-spiking neurons in layer VI of rat perirhinal cortex.

Recent work demonstrated the importance of perirhinal cortex (PR) in a variety of behavioral tasks and disease processes. Studies from our laboratory revealed that some layers of PR contain neurons with unusual properties. Here we report a detailed examination of the cellular neurobiology of layer VI of PR, using whole-cell recordings and biocytin cell fills in horizontal rat brain slices. The most striking finding is that an overwhelming majority ( approximately 86%) of neurons are late-spiking (LS) cells, which can delay the onset of their spike trains by several seconds or more relative to the onset of a depolarizing current step. LS neurons previously have been shown to exist only in very small numbers in a limited number of other cortical regions. Anatomical reconstructions have revealed that the LS neurons vary greatly in morphology, including both pyramidal and nonpyramidal cells. Another surprising physiological finding is the fact that single-spiking (SS) neurons are the second most common cell type ( approximately 7%). SS neurons issue only a single action potential even in response to extreme depolarization. They have been seen previously in the amygdala, but never in cortex. A third remarkable finding is that there are almost no regular spiking (RS) neurons, unlike all other cortical regions that have been studied. This unique abundance of LS neurons in layer VI, along with the presence of SS neurons and the absence of RS neurons, demonstrates that layer VI of PR is unlike any other cortical region that has been studied to date.

Prolonged synaptic integration in perirhinal cortical neurons.

Layer II/III of rat perirhinal cortex (PR) contains numerous late-spiking (LS) pyramidal neurons. When injected with a depolarizing current step, these LS cells typically delay spiking for one or more seconds from the onset of the current step and then sustain firing for the duration of the step. This pattern of delayed and sustained firing suggested a specific computational role for LS cells in temporal learning. This hypothesis predicts and requires that some layer II/III neurons should also exhibit delayed and sustained spiking in response to a train of excitatory synaptic inputs. Here we tested this prediction using visually guided, whole cell recordings from rat PR brain slices. Most LS cells (19 of 26) exhibited delayed spiking to synaptic stimulation (>1 s latency from the train onset), and the majority of these cells (13 of 19) also showed sustained firing that persisted for the duration of the synaptic train (5-10 s duration). Delayed and sustained firing in response to long synaptic trains has not been previously reported in vertebrate neurons. The data are consistent with our model that a circuit containing late spiking neurons can be used for encoding long time intervals during associative learning.