Deprivation of afferent inputs in neural circuits leads to diverse plastic changes in both pre- and postsynaptic elements that restore neural activity. The axon initial segment (AIS) is the site at which neural signals arise, and should be the most efficient site to regulate neural activity. However, none of the plasticity currently known involves the AIS. We report here that deprivation of auditory input in an avian brainstem auditory neuron leads to an increase in AIS length, thus augmenting the excitability of the neuron. The length of the AIS, defined by the distribution of voltage-gated Na(+) channels and the AIS anchoring protein, increased by 1.7 times in seven days after auditory input deprivation. This was accompanied by an increase in the whole-cell Na(+) current, membrane excitability and spontaneous firing. Our work demonstrates homeostatic regulation of the AIS, which may contribute to the maintenance of the auditory pathway after hearing loss. Furthermore, plasticity at the spike initiation site suggests a powerful pathway for refining neuronal computation in the face of strong sensory deprivation.

SUMMARY1. Properties of a mechano-electrical transduction channel were studied in enzymatically dissociated chick vestibular hair cells by using a whole-cell recording variation ofthe patch voltage-clamp technique. The apical hair bundle was stimulated by a glass rod which moved along a one-dimensional axis when stimulated by either a triangular or a trapezoidal command voltage. The motion of the glass rod was monitored optically using a photodiode.2. In response to triangular stimuli, the hair cell generated a current of triangular wave form with occasional step-like spiky or zigzag-appearing events.3. Control experiments confirmed that the current was generated only when the hair bundle was displaced towards the tallest stereocilium.4. The mechano-sensitive current was blocked by streptomycin and by neomycin. The blockage by streptomycin was clearly voltage dependent: the reduction of the current became larger with hyperpolarization of the membrane. This suggests that the positively charged antibiotic molecules plug the mechanically gated channels.5. From the evidence presented in 3 and 4 above, the mechano-sensitive current recorded here was identified as the mechano-electrical transduction (m-e.t.) current.6. The permeability of the m-e.t. channel to various monovalent cations was determined from reversal potential measurements. Since a CsCl-EGTA intracellular medium was used, all the permeabilities were calculated relative to PC. The sequence of permeabilities was Li > Na > K > Rb > Cs > choline > TMA > TEA.7. External Ca ions were indispensable for the recording of transduction current and Sr ions could replace Ca ions without loss of the transduction activity. The minimum [Ca]. for stable generation of the m-e.t. current was 20 /M in Cs saline. The addition of 50-200 #tM-Ca to the isotonic Ba saline could maintain the m-e.t. current. 8. The m-e.t. current was observed in isotonic Ca and in Sr salines. Isotonic Ba, Mg and Mn salines were enriched with 1-2 mM-Ca in order to generate the m-e.t. current. The permeabilities of the divalent cations relative to Cs were calculated from the reversal potentials, and the sequence of permeabilities among divalent cations was Ca > Sr > Ba > Mn > Mg.9.Step-like m-e.t. currents were observed in Cs saline. The smallest step amplitude with clear resolution had a conductance of49'7 + 4-5 pS (mean + S.D., n = 7 cells). This H. OHMORI is likely to be an elementary m-e.t. channel conductance. The permeability coefficient of Cs ions calculated from the above conductance was 7 04 x 10-14 cm3/s. 10. The m-e.t. potential was observed in current-clamp experiments and the amplitude was 24 mV at -43 mV when using CsCl-EGTA intracellular medium. The potential change was smooth and indicated strong low-pass filtering by the membrane. The cell's input capacitance ranged from 4-5 to 7-4 pF and the membrane time constant, calculated for voltages between -50 and -70 mV, was in the order of 10 Ms.11. An approximately linear transduction of a mechanical force into an electrical current is d...

Neurons initiate spikes in the axon initial segment or at the first node in the axon. However, it is not yet understood how the site of spike initiation affects neuronal activity and function. In nucleus laminaris of birds, neurons behave as coincidence detectors for sound source localization and encode interaural time differences (ITDs) separately at each characteristic frequency (CF). Here we show, in nucleus laminaris of the chick, that the site of spike initiation in the axon is arranged at a distance from the soma, so as to achieve the highest ITD sensitivity at each CF. Na+ channels were not found in the soma of high-CF (2.5-3.3 kHz) and middle-CF (1.0-2.5 kHz) neurons but were clustered within a short segment of the axon separated by 20-50 microm from the soma; in low-CF (0.4-1.0 kHz) neurons they were clustered in a longer stretch of the axon closer to the soma. Thus, neurons initiate spikes at a more remote site as the CF of neurons increases. Consequently, the somatic amplitudes of both orthodromic and antidromic spikes were small in high-CF and middle-CF neurons and were large in low-CF neurons. Computer simulation showed that the geometry of the initiation site was optimized to reduce the threshold of spike generation and to increase the ITD sensitivity at each CF. Especially in high-CF neurons, a distant localization of the spike initiation site improved the ITD sensitivity because of electrical isolation of the initiation site from the soma and dendrites, and because of reduction of Na+-channel inactivation by attenuating the temporal summation of synaptic potentials through the low-pass filtering along the axon.

We cloned a cDNA (HAC4) that encodes the hyperpolarization-activated cation channel (I f or I h ) by screening a rabbit sinoatrial (SA) node cDNA library using a fragment of rat brain I f cDNA. HAC4 is composed of 1150 amino acid residues, and its cytoplasmic N-and C-terminal regions are longer than those of HAC1-3. The transmembrane region of HAC4 was most homologous to partially cloned mouse I f BCNG-3 (96%), whereas the C-terminal region of HAC4 showed low homology to all HAC family members so far cloned. Northern blotting revealed that HAC4 mRNA was the most highly expressed in the SA node among the rabbit cardiac tissues examined. The electrophysiological properties of HAC4 were examined using the whole cell patch-clamp technique. In COS-7 cells transfected with HAC4 cDNA, hyperpolarizing voltage steps activated slowly developing inward currents. The half-maximal activation was obtained at ؊87.2 ؎ 2.8 mV under control conditions and at ؊64.4 ؎ 2.6 mV in the presence of intracellular 0.3 mM cAMP. The reversal potential was ؊34.2 ؎ 0.9 mV in 140 mM Na These results indicate that HAC4 forms I f in rabbit heart SA node.

SUMMARY1. The properties of the Ca channel in tissue cultured clonal cells (GH3) isolated from a rat anterior pituitary tumour were studied with the patch electrode voltageclamp technique.2. To isolate the current through the Ca channel, the currents through the Na channel, the delayed K channel and the Ca2+ induced K channel were minimized by replacing the external Na+ with TEA+ and adding EGTA to the K-free solution inside the patch electrode.3. The selectivity ratios through the Ca channel with different cations were 2-7 (Ba2+): 1-6 (Sr2+): 1.0 (Ca2+) and the m2 form of the activation kinetics and the relationships between the time constant and the membrane potential were common to the three divalent cations.4. The amplitude of the Ba2+ current increased linearly with [Ba2+]0 up to 25 mM and thereafter tended to show saturation.5. The current-voltage relation showed a positive shift along the voltage axis as [Ba2+]0 increased, probably due to the screening effect of Ba2+ on the negative surface charges.6. The time constant of activation as a function of the membrane potential showed a parallel shift as [Ba2+]0 was increased, suggesting that the activation kinetics were independent of the permeant ion concentration.7. The time constant of the tail current was consistent with m2 kinetics for opening and closing of the Ca channel.8. The extrapolated 'instantaneous' tail current rapidly increased as the activating membrane potential became more positive and reached an apparent saturation at membrane potentials substantially more positive than the potential that gave the maximum peak inward current, and suggested that the single channel has a sigmoidal current-voltage relationship.9. The power density spectrum obtained during the steady-state inward Ba2+ current had a cut-off frequency which was nearly voltage independent; this is expected if the fluctuation of the current originates from m2 activation kinetics.10. The results of noise analysis suggest that the amplitude of the single Ca channel current was about 0-2 pA at 25 mM-Ba2+ and 0-7 pA at 100 mM-Ba2+ for membrane potentials in the vicinity of the maximum inward current.