Membrane excitability in the neurohypophysis

Electrophysiological events recorded at presynaptic terminals of the crayfish neuromuscular junction with a voltage indicator

The water-soluble voltage indicator JPW1114 was used to stain thin axons and terminal varicosities of the crayfish neuromuscular junction. A slow, overnight injection protocol was developed to brightly stain fine structures without cytotoxicity. Fluorescence transients filtered at 2 kHz showed that the duration of terminal action potentials was shorter than that of those recorded in the main trunk of the axons. In addition, the repolarization phases of the terminal and axonal action potentials overlapped in time, suggesting that the entire axonal arborization repolarizes simultaneously. Manipulating resting membrane potential, +/-15-20 mV, did not alter the peak level or duration of action potentials if they fired in isolation. A prolongation of action potential, by 23%, could be induced if a 10-spike burst at 100 Hz was fired from depolarized membrane potential. No such change was observed when the high frequency train was fired from resting or hyperpolarized levels. Microelectrodes in the main trunk of axons typically recorded a depolarizing after-potential (DAP) following an action potential initiated from resting membrane potential. The DAP could be inverted and enlarged by depolarization and hyperpolarization, respectively. Fluorescence transients recorded from terminals exhibited similar DAP characteristics. The ratio of DAP to action potential amplitude recorded from terminals was similar to that recorded from the main axon. Thus, the entire axonal arborization returned to resting level in a spatially uniform manner during the DAP. The functional significance of DAP is discussed in the light of these observations.

Nitric oxide modulation of the spontaneous firing of rat medial vestibular nuclear neurons

Modulation of the spontaneous activity of rat medial vestibular nuclear neurons by nitric oxide was investigated using the whole-cell patch-clamp technique. The spike frequency was increased by sodium nitroprusside (SNP), a nitric oxide liberating agent, and it was also increased by another nitric oxide liberating agent, sodium-nitroso-N-acetylpenicillamine. L-Arginine, the substrate of nitric oxide synthase, increased the firing of the neurons. The increased SNP-induced firing was inhibited by 1H-[1,2,4]oxadiazolo[4,3-a]quinozalin-1-one (ODQ), a specific inhibitor of guanylate cyclase. These results suggest that nitric oxide increases the neuronal excitability of the neurons by a cGMP-dependent mechanism.

Distinct rat neurohypophysial nerve terminal populations identified by size, electrophysiological properties and neuropeptide content

Voltage-gated ion channels are critical to excitation-secretion coupling in nerve terminals. We have identified two distinct populations of rat neurohypophysial (NHP) terminals distinguished by size, neuropeptide content and electrophysiological properties, including resting membrane potential, action potential (AP) properties, and K+ current and Na+ current characteristics. In large terminals (10-16 microm diameter), resting membrane potential was more negative than in small terminals (5-9.9 microm; -61+/-4 mV vs. -55+/-3 mV; p<0.01), action potential amplitude was larger (69+/-4 mV vs. 53+/-3 mV; p<0.01), peak IK was larger (1460+/-90 pA vs. 1140+/-70 pA; p<0.05) with a more negative V1/2 for activation (-3.1 mV vs. -0.6 mV; p<0.05), and Na+ current density was greater (approximately 470 pA/pF vs. approximately 300 pA/pF; p<0.01) with more negative V1/2 values for activation from -70 or -90 mV holding potentials (-44 mV vs. -24 mV; p<0.01). A positive linear correlation between INa amplitude and terminal size showed an inflection at a diameter of 9.2 microm. Neuropeptide content was generally segregated into a population of small terminals (<10 microm diameter) containing predominantly vasopressin and a population of large terminals (> or =10 microm diameter) containing predominantly oxytocin (OT); a small fraction of terminals in each group contained both peptides. These findings suggest that electrophysiological differences between small vasopressin-containing and large oxytocin-containing neurohypophysial terminals may contribute to their observed differential firing and peptide release patterns.

Cortical injury affects short-term plasticity of evoked excitatory synaptic currents

The hypothesis that plastic changes in the efficacy of excitatory neurotransmission occur in areas of chronic cortical injury was tested by assessing short-term plasticity of evoked excitatory synaptic currents (EPSCs) in neurons of partially isolated neocortical islands (undercut cortex). Whole cell recordings were obtained from layer V pyramidal neurons of sensorimotor cortical slices prepared from P36-P43 control and undercut rats. AMPA/kainate receptor-mediated EPSCs elicited by stimuli delivered at 40 to 66.7 Hz exhibited more paired-pulse depression (PPD) in undercut cortex than control, the time constant of depression evoked by trains of 20- to 66.7-Hz stimuli was faster, and the steady-state amplitude of EPSCs reached after five to seven EPSCs was lower. An antagonist of the glutamate autoreceptor, group II mGluR, increased the steady-state amplitude of EPSCs from undercut but not control cortex, suggesting that activation of presynaptic receptors by released glutamate is more prominent in undercut cortex. In contrast, the GABA(B) receptor antagonist (2S)-3-[[(1S)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl](phenylmethyl)phosphinic acid had no effect. Increasing [Ca(2+)](o) from 2 to 4 mM increased PPD, with a smaller effect in neurons of the undercut. The I-V relationship of AMPA/kainate receptor-mediated EPSCs was close to linear in both control and undercut neurons, and spermine had no significant effect on the EPSCs, suggesting that decreases in postsynaptic glutamate receptors containing the GluR2 subunit were not involved in the alterations in short-term plasticity. Results are compatible with an increase in the probability of transmitter release at excitatory synapses in undercut cortex due to functional changes in presynaptic terminals.

Loose-patch clamp currents from the hypothalamo-neurohypophysial system of the rat

The loose-patch clamp technique was used to study voltage-activated currents from the surface of rat neurohypophysial and hypothalamic regions in situ. In the neurohypophysis, depolarizing pulses of 4-8 ms duration yielded tetrodotoxin (TTX)-sensitive sodium currents, a 4-AP-sensitive "A"-type potassium current, and a long-lasting outward TEA- and tetrandrine-sensitive Ca(2+)-activated potassium current. All of these currents were elicited during the application of the pulse. With high external calcium there were long-lasting inward currents blocked by Ni(2+) and Cd(2+), identifying them as voltage-gated calcium currents. Depolarizing pulses of 0.3-0.7 ms duration yielded fast biphasic responses, of 1-3 ms duration, composed of mostly sodium and "A"-type potassium currents. With high external calcium there were fast inward currents blocked by Ni(2+) and Cd(2+), indicating that these were voltage-gated calcium currents. These responses have the characteristics of action potential currents: they were elicited after the cessation of the applied pulse and the "A" component is eliminated together with the sodium component upon application of TTX. Similar responses to long and short pulses were obtained from the surface of the associated magnocellular somata in the supraoptic nucleus, and their projections. The explant currents are similar to those previously characterized using conventional methods from somata and terminals.