Comparison of the Upper Marginal Neurons of Cortical Layer 2 with Layer 2/3 Pyramidal Neurons in Mouse Temporal Cortex.

Layer 2/3 (L2/3) excitatory neurons in the neocortex make major contributions to corticocortical connections and therefore function to integrate information across cortical areas and hemispheres. Recent evidence suggests that excitatory neurons in L2/3 can have different properties. Sparse evidence from previous studies suggests that L2 neurons located at the border between L1 and L2 (referred to as L2 marginal neurons, L2MNs), have a morphology distinct from a typical pyramidal neuron. However, whether the membrane properties and input/output properties of L2MNs are different from those of typical pyramidal neurons in L2/3 is unknown. Here we addressed these questions in a slice preparation of mouse temporal cortex. We found that L2MNs were homogeneous in intrinsic membrane properties but appeared diverse in morphology. In agreement with previous studies, L2MNs either had oblique apical dendrites or had no obvious apical dendrites. The tufts of both apical and basal dendrites of these neurons invaded L1 extensively. All L2MNs showed a regular firing pattern with moderate adaptation. Compared with typical L2/3 pyramidal neurons that showed regular spiking (RS) activity (neurons), L2MNs showed a higher firing rate, larger sag ratio, and higher input resistance. No difference in the amplitude of excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs, respectively), evoked by stimulation of L1, was found between the two types of neurons, but the IPSPs in L2MNs had a slower time course than those in L2/3 RS cells. In paired recordings, unitary EPSPs showed no significant differences between synapses formed by L2MNs and those formed by L2/3 RS neurons. However, short-term synaptic depression (STSD) examined with a L2MN as the presynaptic neuron was greater when another L2MN was the postsynaptic neuron than when a L2/3 RS neuron was the postsynaptic neuron. The distinct morphological features of L2MNs found here have developmental implications, and the differences in electrophysiological properties between L2MNs and other L2/3 pyramidal neurons suggest that they play different functional roles in cortical networks.

The insular auditory field receives input from the lemniscal subdivision of the auditory thalamus in mice.

Here we studied the auditory thalamic input to the insular cortex using mice as a model system. An insular auditory field (IAF) has recently been identified in mice. By using retrograde neuronal tracing, we identified auditory thalamic neurons projecting to the IAF, primary auditory cortex (AI), and anterior auditory field (AAF). After mapping the IAF, AAF, and AI by using optical imaging, we injected a distinct fluorescent tracer into each of the three fields at frequency-matched locations. Tracer injection into the IAF resulted in retrogradely labeled cells localized ventromedially in the lemniscal division, i.e., the ventral subdivision of the medial geniculate body (MGv). Cells retrogradely labeled by injections into the AAF were primarily found in the medial half of the MGv, whereas those from AI injections were located in the lateral half, although some of these two subsets were intermingled within the MGv. Interestingly, retrogradely labeled cells projecting to the IAF showed virtually no overlap with those projecting to the AAF or the AI. Dual tracer injections into two sites responding to low- and high-frequency tones within each of the three auditory fields demonstrated topographic organizations in all three thalamocortical projections. These results indicate that the IAF receives thalamic input from the MGv in a topographic manner, and that the MGv­IAF projection is parallel to the MGv­AAF and MGv­AI projections.

Identification and characterization of an insular auditory field in mice.

We used voltage-sensitive-dye-based imaging techniques to identify and characterize the insular auditory field (IAF) in mice. Previous research has identified five auditory fields in the mouse auditory cortex, including the primary field and the anterior auditory field. This study confirmed the existence of the primary field and anterior auditory field by examining the tonotopy in each field. Further, we identified a previously unreported IAF located rostral to known auditory fields. Pure tone evoked responses in the IAF exhibited the shortest latency among all auditory fields at lower frequencies. A rostroventral to dorsocaudal frequency gradient was consistently observed in the IAF in all animals examined. Neither the response amplitude nor the response duration changed with frequency in the IAF, but the area of activation exhibited a significant increase with decreasing tone frequency. Taken together, the current results indicate the existence of an IAF in mice, with characteristics suggesting a role in the rapid detection of lower frequency components of incoming sound.

Handedness: dependent asymmetrical location of the human primary gustatory area, area G.

In magnetoencephalogram studies, the primary gustatory area, area G, is not always seen in the same coronal plane in both hemispheres. We investigated possible asymmetry in right-handed and left-handed individuals by functional MRI. Group analyses revealed a significant difference in the antero-posterior coordinates of the area G between the right and left hemispheres in the right-handed group, but not in the left-handed group, indicating significant morphometric asymmetry in the former group and ambiguous morphometric asymmetry in the latter. However, in left-handed individuals with motor speech areas detected in the right hemisphere, area G was more posteriorly located in the right than in the left hemisphere. These findings suggest that the motor speech area contributes to the asymmetric location of area G.

Effects of acetylcholine on coding of taste information in the primary gustatory cortex in rats.

Acetylcholine (ACh) receptors are widely distributed throughout the cerebral cortex in rats. Recently, cholinergic innervation of the gustatory cortex (GC) was reported to be involved in certain taste learning in rats. Here, the effects of iontophoretic application of ACh on the response properties of GC neurons were studied in urethane-anesthetized rats. ACh affected spontaneous discharges in a small fraction of taste neurons (11 of 86 neurons tested), but influenced taste responses in 27 of 43 neurons tested. No correlations with ACh susceptibility were noted for spontaneous discharges and taste responses. Among the 27 neurons, ACh facilitated taste responses in 13, inhibited taste responses in 13 and either facilitated or inhibited taste responses depending on the stimuli in 1. Furthermore, ACh affected the responses to best stimuli that produced the largest responses among four basic tastants (best responses) in 7 of 27 taste neurons, to non-best responses in 9, and to both best and non-best responses in 11. ACh mostly inhibited the best responses (13 of 18 neurons). Thus, ACh often decreased the response selectivity to the four basic tastants and changed the response profile. Atropine, a general antagonist of muscarinic receptors, antagonized ACh actions on taste responses or displayed the opposite effects on taste responses to ACh actions in two-thirds of the neurons tested. These findings indicate that ACh mostly modulates taste responses through muscarinic receptors, and suggest that ACh shifts the state of the neuron network in the GC, in terms of the response selectivities and response profiles.