Crowder NA, Dickson CT, Wylie DR
J. Neurophysiol. 2004 Jan;91(1):274-85
The pretectal nucleus lentiformis mesencephali (LM) and the nucleus of the basal optic root (nBOR) of the avian accessory optic system (AOS) are retinal-recipient visual nuclei involved in the analysis of optic flow that results from self-motion, and in the generation of the optokinetic response. Neurons in these nuclei show direction selectivity in response to large-field motion and are tuned in the spatiotemporal domain. In addition to retinal afferentation, both the nBOR and LM receive afferents from the Wulst, which is thought to be the avian homolog of the primary visual cortex. We examined the effects of Wulst electrical stimulation on the activity of LM neurons and recorded the directional and spatiotemporal tuning of LM neurons in pigeons before, during, and after the Wulst was temporarily inactivated by lidocaine injection. In response to Wulst electrical stimulation, LM neurons showed either short-latency excitation followed by longer-latency inhibition (W+ cells), or only a longer-latency inhibition (W- cells). The average response latencies for W+ and W- cells were 13.5 and 28.3 ms, respectively. The effects of Wulst stimulation did not correlate with either the directional or spatiotemporal tuning of the LM neurons. Injection of lidocaine into the nBOR reduced the longer-latency oscillations of W+ and W- cells. When the Wulst was temporarily inactivated by lidocaine neither the directional nor spatiotemporal response properties of LM neurons were affected. The possible functions of the projection from the Wulst to the LM are discussed.
Fransén E, Alonso AA, Dickson CT, Magistretti J, Hasselmo ME
A multicompartmental biophysical model of entorhinal cortex layer II stellate cells was developed to analyze the ionic basis of physiological properties, such as subthreshold membrane potential oscillations, action potential clustering, and the medium afterhyperpolarization. In particular, the simulation illustrates the interaction of the persistent sodium current (I(Nap)) and the hyperpolarization activated inward current (Ih) in the generation of subthreshold membrane potential oscillations. The potential role of Ih in contributing to the medium hyperpolarization (mAHP) and rebound spiking was studied. The role of Ih and the slow calcium-activated potassium current Ikappa(AHP) in action potential clustering was also studied. Representations of Ih and I(Nap) were developed with parameters based on voltage-clamp data from whole-cell patch and single channel recordings of stellate cells (Dickson et al., J Neurophysiol 83:2562-2579, 2000; Magistretti and Alonso, J Gen Physiol 114:491-509, 1999; Magistretti et al., J Physiol 521:629-636, 1999a; J Neurosci 19:7334-7341, 1999b). These currents interacted to generate robust subthreshold membrane potentials with amplitude and frequency corresponding to data observed in the whole cell patch recordings. The model was also able to account for effects of pharmacological manipulations, including blockade of Ih with ZD7288, partial blockade with cesium, and the influence of barium on oscillations. In a model with a wider range of currents, the transition from oscillations to single spiking, to spike clustering, and finally tonic firing could be replicated. In agreement with experiment, blockade of calcium channels in the model strongly reduced clustering. In the voltage interval during which no data are available, the model predicts that the slow component of Ih does not follow the fast component down to very short time constants. The model also predicts that the fast component of Ih is responsible for the involvement in the generation of subthreshold oscillations, and the slow component dominates in the generation of spike clusters.