Contrasting Ca²⁺ channel subtypes at cell bodies and synaptic terminals of rat anterioventral cochlear bushy neurones

Abstract

Whole‐cell patch clamp recordings were made from bushy cells of the anterioventral cochlear nucleus (aVCN) and their synaptic terminals (calyx of Held) in the medial nucleus of the trapezoid body (MNTB). Both high voltage‐activated (HVA) and low voltage‐activated (LVA) calcium currents were present in acutely dissociated aVCN neurones and in identified bushy neurones from a cochlear nucleus slice. The transient LVA calcium current activated rapidly on depolarization (half‐activation, −59 mV) and inactivated during maintained depolarization (half‐inactivation, −89 mV). This T‐type current was observed in somatic recordings but was absent from presynaptic terminals. On the basis of their pharmacological sensitivity, P/Q‐type Ca2+ channels accounted for only 6 % of the somatic HVA, while L‐, N‐ and R‐type Ca2+ channels each accounted for around one‐third of the somatic calcium current. The divalent permeabilities of these native calcium channels were compared. The Ba2+/Ca2+ conductance ratios of the somatic HVA and LVA channels were 1.4 and 0.7, respectively. The conductance ratio of the presynaptic HVA current was 0.9, significantly lower that that of the somatic HVA current. We conclude that LVA currents are expressed in the bushy cell body, but are not localized to the excitatory synaptic terminal. All of the HVA current subtypes are expressed in bushy cells, but there is a strong polarity to their localization; P‐type contribute little to somatic currents but predominate at the synaptic terminal; L‐, N‐ and R‐types dominate at the soma, but contribute negligibly to calcium currents in the terminal.

The Role of Physiological Afferent Nerve Activity duringIn VivoMaturation of the Calyx of Held Synapse

“…A possible caveat of the use of Ca V 1.3 Ϫ/Ϫ mice could be that, in addition to controlling release in IHCs, Ca V 1.3 L-type channels expressed in cell bodies and dendrites of auditory neurons could exert important regulatory functions linked to synapse development and refinement. Indeed, L-type Ca 2ϩ channels account for one-third of the somatic high-voltage-activated Ca 2ϩ current in bushy cells of the rat anteroventral cochlear nucleus (Doughty et al, 1998), but the molecular identity of those channels is unknown.…”

The calyx of Held

“…Note that the transient Ca 2+ current is only apparent in the bushy cell body and no current is evoked at −50 mV in the calyx (HP holding potential). Modified, with permission, from Doughty et al (1998). f The metabotropic glutamate receptor agonist L-AP4 reduces the amplitude of the presynaptic Ca 2+ current.…”

Voltage-gated and background K+ channel subunits expressed by the bushy cells of the rat cochlear nucleus

“…Bushy neurones were isolated by using a technique described earlier in details (Doughty et al, 1998;Rusznák et al, 2001). In short, 4-11-day-old rats were decapitated, their brains removed in low Na + artificial cerebrospinal fluid (aCSF), and the ventral cochlear nuclei incubated in aCSF containing 0.03 mg/ml collagenase (type IA) and 0.12 mg/ml pronase (type XIV) for 40 min at 31°C.…”

“…Moreover, bushy cells possess a hyperpolarisation-activated nonspecific cationic conductance (I h ), whose activity may also contribute to the relatively low input resistance of these cells (Cuttle et al, 2001). Bushy neurones express various types of depolarisation-activated Ca 2+ currents too, and it was noted that the channels situated on the cell body and on the presynaptic terminals are not identical (Doughty et al, 1998). The presence of a transient outward current was also noted on bushy neurones, as it has been described that isolated bushy cells expressed high-threshold potassium conductances (Manis and Marx, 1991), showing some degree of inactivation.…”