L-type calcium channel-mediated plateau potentials in barrelette cells during structural plasticity

Abstract

Development and maintenance of whisker-specific patterns along the rodent trigeminal pathway depends on an intact sensory periphery during the sensitive/critical period in development. Barrelette cells of the brain stem trigeminal nuclei are the first set of neurons to develop whisker-specific patterns. Those in the principal sensory nucleus (PrV) relay these patterns to the ventrobasal thalamus, and consequently, to the somatosensory cortex. Thus PrV barrelette cells are among the first group of central neurons susceptible to the effects of peripheral damage. Previously we showed that membrane properties of barrelette cells are distinct as early as postnatal day 1 (PND 1) and remain unchanged following peripheral denervation in newborn rat pups (Lo and Erzurumlu 2001). In the present study, we investigated the changes in synaptic transmission. In barrelette cells of normal PND 1 rats, weak stimulation of the trigeminal tract (TrV) that was subthreshold for inducing Na(+) spikes evoked an excitatory postsynaptic potential-inhibitory postsynaptic potential (EPSP-IPSP) sequence that was similar to the responses seen in older rats (Lo et al. 1999). Infraorbital nerve transection at birth did not alter excitatory and inhibitory synaptic connections of the barrelette cells. These observations suggested that local neuronal circuits are already established in PrV at birth and remain intact after deafferentation. Strong stimulation of the TrV induced a sustained depolarization (plateau potential) in denervated but not in normal barrelette neurons. The plateau potential was distinct from the EPSP-IPSP sequence by 1) a sustained (>80 ms) depolarization above -40 mV; 2) a slow decline slope (<0.1 mV/ms); 3) partially or totally inactivated Na(+) spikes on the plateau; and 4) a termination by a steep decay (>1 mV/ms) to a hyperpolarizing membrane level. The plateau potential was mediated by L-type Ca(2+) channels and triggered by a N-methyl-D-aspartate (NMDA) receptor-mediated EPSP. gamma-aminobutyric acid-A (GABA(A)) receptor-mediated IPSP dynamically regulated the latency and duration of the plateau potential. These results indicate that after neonatal peripheral damage, central trigeminal inputs cause a large and long-lasting Ca(2+) influx through L-type Ca(2+) channels in barrelette neurons. Increased Ca(2+) entry may play a key role in injury-induced structural remodeling, and/or transsynaptic cell death.

FIG. 1

Morphological changes in denervated principal sensory nucleus (PrV). A: whisker-specific trigeminal afferent arbor patches in normal PrV revealed by ganglionic injection of biocytin. B: disappearance of afferent terminal patches in denervated PrV. C: examples of biocytin-labeled barrelette cells in normal PrV showing polarized dendritic trees. D: examples of biocytin-labeled barrelette cells in denervated PrV showing symmetrical dendritic trees. Both normal (E) and denervated (F) barrelette cells show a prominent A-type K⁺ conductance (I_A) (denoted by A).

FIG. 2

Postsynaptic responses to weak stimulation of trigeminal tract (TrV) in barrelette neurons during postnatal development and after infraorbital (IO) nerve transection. A: stimulation of TrV at a subthreshold intensity for postsynaptic Na⁺ spikes (<50 µA) induced an excitatory postsynaptic potential–inhibitory postsynaptic potential (EPSP-IPSP) sequence in a barrelette cell at PND 1. Note that the IPSP is in hyperpolarizing direction at resting potential (−60 mV). It is reversed into depolarizing direction at −105 mV. B: at PND 10, weak stimulation induced an EPSP-IPSP sequence. The IPSP is reversed by membrane hyperpolar-ization (lower trace). C: weak stimulation of TrV induced an EPSP-IPSP sequence in denervated barrelette cells at PND 5.

FIG. 3

Strong stimulation of TrV results in a plateau potential in denervated barrelette cells. A: single electrical shock at the maximal intensity (300–500 µA) induces an EPSP-IPSP sequence with a Na⁺ spike riding on it in a normal barrelette cell. B and C: denervated barrelette cells respond to strong stimulation with a plateau potential that appears either immediately after the Na⁺ spike (B) or after the IPSP (C). D: in the same normal barrelette cell, 3 shocks at 50 Hz evoked 3 Na⁺ spikes riding on 3 EPSP-IPSP sequences. E and F: in the same denervated barrelette cells, 3 shocks elicited long-lasting plateau potentials. Note that Na⁺ spikes are totally or partially inactivated during plateau potentials. G: in normal barrelette cell, single shock induces a prolonged EPSP with 2 spikes riding on it in the presence of bicuculline (10 µM) (E vs. A). H and I: in the same denervated barrelette cells, application of bicuculline either prolonged the duration of the plateau potential (H vs. B) or shortened its latency (I vs. C).

FIG. 4

Mechanisms underlying the generation of the plateau potential. A: stimulation of TrV with increasing intensity increases the EPSP amplitude (trace 1 vs. trace 2) in a denervated barrelette neuron. When the late EPSP reaches above −40 mV, plateau potential is evoked (trace 3). Further increase of intensity causes an increase in amplitude and duration of the plateau potential (traces 4 and 5). B: when the membrane potential is held at −60 mV, strong stimulus results in a plateau potential (upper trace). However, at —85 mV, the same stimulus fails to induce a plateau potential (lower trace). C: single shock at moderate intensity (<250 µA) does not evoke the plateau potential (trace 1). In the presence of bicuculline, the same stimulus induces a plateau potential (trace 2). D: application of bicuculline shortens the latency of plateau potentials (trace 1: before; trace 2: after drug application). E: application of bicuculline prolongs the duration of the plateau potential (trace 1 vs. trace 2). Note that the IPSP is blocked in trace 2. F: nitrendipine blocks the plateau potential at −60 mV (trace 1 vs. trace 2). G: nitrendipine does not alter the size of the EPSPs in the same cell (trace 1 vs. trace 2). H: the plateau potential is blocked by d-APV (100 µM trace 1 vs. trace 2).