Anatomy and physiology of multipolar cells in the rat inferior collicular cortex using the in vitro brain slice technique

Abstract

Coronal brain slices from 21–50-d-old hooded rats were used to characterize intracellular responses of cells in both the external and dorsal cortices of the inferior colliculus (IC). These cells could generate both sodium and calcium spikes. Depending on current amplitude, depolarizing current pulses could elicit either phasic or tonic firing patterns, with spike frequency adaptation. Spiking also occurred at the offset of a hyperpolarizing pulse. These patterns were due primarily to the activation of calcium conductances. Stimulation of the commissural pathway connecting the left and right IC produced a short-latency monosynaptic IPSP followed by an EPSP(s) and a late polysynaptic IPSP(s). Non-NMDA glutamate antagonists eliminated or reduced the amplitude of the EPSP and the late portion of the inhibition, while both IPSPs were blocked by GABAA antagonists. As described previously in guinea pig (Smith, 1986) and rat (Pierson et al., 1989), a large NMDA-mediated depolarizing event (paroxysmal depolarizing shift, or PDS) could be elicited by shocking the commissure of the IC in the presence of picrotoxin or bicuculline, NMDA, 4-aminopyridine, or in 0 Mg2+ Ringer's. The picrotoxin-induced PDS was significantly reduced or abolished in Ringer's containing aminophosphonovalerate. Cells displaying the responses described were labeled with neurobiotin. Those labeled are medium-sized multipolar cells. Their dendrites are usually spiny and can extend superficially up to the cortical surface. Their thin axons give rise to collaterals that branch profusely within the cortex. The main axons project laterally along the circumference of the IC or medially into the commissure separating the collicular hemispheres.

Neuronal Responses to Lemniscal Stimulation in Laminar Brain Slices of the Inferior Colliculus

“…As in ICC, the balance of excitation by NMDA receptors and inhibition due to GABA A receptors is the key mechanism to regulate hippocampal excitability (Dingledine et al 1986). When regulation by GABA synapses is lost, epileptiform bursting occurs (Dingledine et al 1986;Smith 1992;Wagner 1996), and this may be related to pathological seizure activity in both ICC (Smith 1992;Li et al 1994) and the hippocampus (Coulter 2001;Avoli et al 2002;Nadler 2003).…”

“…Most brain slice experiments in the rat ICC have been performed on transverse slices (Smith 1992;Li et al 1994Li et al , 1998Li et al , 1999Bal et al 2002;Kraushaar and Backus 2002;Ma et al 2002a, b;Wu et al 2002;N'Gouemo and Morad 2003;Wu et al 2004), cut roughly perpendicular to the ICC layers. Because that plane cuts the laminae, it is likely to sever the dendrites of the flat cells and will cut many inputs to the ICC, particularly those entering from the lateral lemniscus.…”

“…Because that plane cuts the laminae, it is likely to sever the dendrites of the flat cells and will cut many inputs to the ICC, particularly those entering from the lateral lemniscus. This has not been a problem for previous studies focused on the dorsal cortex of IC, because commissural inputs were not cut (Smith 1992;Li et al 1994Li et al , 1998Li et al , 1999. When using the transverse plane, the stimulating electrodes must be placed inside the ventrolateral part of ICC (Wu et al 2002(Wu et al , 2004 or just outside it (Wagner, 1996;Moore et al 1998).…”

Identification of cell types in brain slices of the inferior colliculus

“…The rebound was similar to a Ca 11 -based rebound potential described previously in the dorsal cortex of the IC. 55 These neurons had sustained ®ring to depolarizing currents. The slow rebound was not seen in buildup±pauser neurons (Fig.…”

“…55 Male or female rats were used under NIH and institutional animal care guidelines, and all efforts were made to minimize animal suffering, to reduce the number of animals used, and to utilize alternatives to in vivo techniques if available. Rats were a mean age of 25.8 days (range 14±44 days) and weight of 78.2^36.2 g (mean^S.D.).…”