Neurotensin Broadly Recruits Inhibition via White Matter Neurons in the Mouse Cerebral Cortex: Synaptic Mechanisms for Decorrelation.

The neuropeptide, neurotensin (NT), inhibits UP state generation in the cerebral cortex and temporally restricts the response to thalamic input, likely by a generalized increase in inhibition. To investigate the cellular and circuit substrate(s) for how a neuropeptide can shift the balance between cortical excitation and inhibition, we performed whole-cell recordings on slice preparations from mice expressing enhanced green fluorescent protein under control of the promoter for the homeobox gene, lhx6 (lhx6-EGFP mice). These mice identify the 2 largest classes of cortical interneurons; FS and low-threshold-spiking inhibitory neurons. In the presence of NT, both types of lhx6-EGFP neurons were excited through a direct, Na+-dependent depolarization, and through an increase in synaptic excitation. Paired recordings identified cortical white matter (WM) neurons as a source of this excitatory input, which was strengthened in the presence of NT. NT-driven increased synaptic input caused a functional decorrelation of gap junction transmission between lhx6-EGFP neuron pairs. Finally, the synaptic transmission between pyramidal cells and lhx6-EGFP neurons was modulated by addition of NT in favor of stronger inhibition and weaker excitation. These findings demonstrate the existence and functional consequences of an intracortical WM neuron projection, and suggest mechanisms underlying NT-induced promotion of wakefulness.

Desynchronization of the Rat Cortical Network and Excitation of White Matter Neurons by Neurotensin.

Cortical network activity correlates with vigilance state: Deep sleep is characterized by slow, synchronized oscillations, whereas desynchronized, stochastic discharge is typical of the waking state. Neuropeptides, such as orexin and substance P but also neurotensin (NT), promote arousal. Relatively little is known about if NT can directly affect the cortical network, and if so, through which mechanisms and cellular targets. Here, we addressed these issues using rat in vitro cortex preparations. Following NT application specifically to deeper layers, slow oscillation activity was attenuated with a significant reduction in UP state frequency. The cortical response to thalamic stimulation exhibited enhanced temporal precision in the presence of NT, consistent with the transition in vivo from sleep to wakefulness. These changes were associated with a relative shift toward inhibition in the excitation/inhibition balance. Whole-cell recordings from layer 6 revealed presynaptically driven NT-induced inhibition of pyramidal neurons and excitation of fast-spiking interneurons. Deeper in the cortex, neurons within the white matter (WM) were strongly depolarized by NT application. The colocalization of NT and tyrosine hydroxylase immunoreactivities in deep layer fibers throughout the cortical mantle indicates mediation via dopaminergic systems. These data suggest a cortical mechanism for NT-induced wakefulness and support a role for WM neurons in state control.

A method for visually guided whole-cell recordings in brain slices exhibiting spontaneous rhythmic activity.

In vitro brain slice electrophysiology is a powerful method to investigate the network and cellular bases of brain function. Ideally, slices should be able to spontaneously express the ensemble rhythms that characterize the intact brain, but this is only rarely the case in the submerged configuration required for visualization of cells. In contrast, the interface configuration often preserves in vivo-like activity but does not allow optically guided whole-cell recording. Here we present a chamber design that, when used with a heated air objective, offers the experimenter the benefits of both visualization and the interface environment. The chamber is based on the design of the traditional Oslo-style interface chamber but modified to fit an upright microscope. Spontaneous slow (0.1-1 Hz) oscillations could be recorded extracellularly from slices of the rat somatosensory cortex with similar success, duration and frequency as the traditional interface chamber. Slow oscillations could also be readily recorded in the whole-cell configuration from visually selected pyramidal neurons. In hippocampal slices spontaneous gamma oscillations (20-80 Hz) were observed both extracellularly and in whole-cell recordings. The design presented here may be useful to the in vitro study of a range of brain circuits where the combination of visualization and spontaneous patterned network activity is desired.