Differential effects of 4-aminopyridine on acetylcholine release triggered by K+ depolarization, veratridine, or A23187 in rat cerebral cortical synaptosomes

Acetylcholine release and choline uptake by cuttlefish (Sepia officinalis) optic lobe synaptosomes

Acetylcholine (ACh), which is synthesized from choline (Ch), is believed to hold a central place in signaling mechanisms within the central nervous system (CNS) of cuttlefish (Sepia officinalis) and other coleoid cephalopods. Although the main elements required for cholinergic function have been identified in cephalopods, the transmembrane translocation events promoting the release of ACh and the uptake of Ch remain largely unsolved. The ACh release and Ch uptake were quantitatively studied through the use of in vitro chemiluminescence and isotopic methods on a subcellular fraction enriched in synaptic nerve endings (synaptosomes) isolated from cuttlefish optic lobe. The ACh release evoked by K+ depolarization was found to be very high (0.04 pmol ACh.s(-1).mg(-1) protein). In response to stimulation by veratridine, a secretagogue (a substance that induces secretion) that targets voltage-gated Na+ channels, the release rate and the total amount of ACh released were significantly lower, by 10-fold, than the response induced by KCl. The high-affinity uptake of choline was also very high (31 pmol Ch.min(-1).mg(-1) protein). The observed ACh release and Ch uptake patterns are in good agreement with published data on preparations characterized by high levels of ACh metabolism, adding further evidence that ACh acts as a neurotransmitter in cuttlefish optic lobe.

Effects of 4-aminopyridine on motor evoked potentials in patients with spinal cord injury: a double-blinded, placebo-controlled crossover trial

4-Aminopyridine (4-AP) is a potassium (K+) channel blocking agent that has been shown to reduce the latency and increase the amplitude of motor evoked potentials (MEPs) elicited with transcranial magnetic stimulation (TMS) in patients with chronic spinal cord injury (SCI). These effects on MEPs are thought to reflect enhanced conduction in long tract axons brought about by overcoming conduction deficits due to focal demyelination and/or by enhancing neuroneuronal transmission at one or more sites of the neuraxis. The present study was designed to obtain further evidence of reduced central motor conduction time (CMCT) and to determine whether MEPs could be recorded from paretic muscles in which they were not normally elicited. MEPs were elicited with TMS being delivered to subjects (n = 25) pre- and post-administration of 4-AP (10 mg capsule) or placebo. The principal finding was that 4-AP lowered the stimulation threshold, increased the amplitude and reduced the latency of MEPs in all muscles tested, including those that were unimpaired, but did not alter measures of the peripheral nervous system (i.e., M-wave, H-reflex, F-wave). These 4-AP-induced changes in MEPs were significantly greater than those seen with placebo (p < 0.05). The primary implication of these results is that a low dose of 4-AP (immediate-release formulation) appears to improve the impaired central motor conduction of some patients with incomplete SCI. This is most likely attributable to overcoming conduction deficits at the site of injury but may also involve an increase in cortical excitability.

Effects of peroxidation and aging on rat neocortical ACh-release and protein kinase C

Cerebral cortical synaptosomes were prepared from 2- or 24-month-old Fischer 344 rats and exposed to a peroxidizing condition (50 microM ferrous ions and 2 microM ascorbate ions) before measuring either the release of newly synthesized [3H]acetylcholine (ACh) or protein kinase C activity (PKC). Several secretagogues with different mechanisms of action and different responses to aging were used to trigger release: K+ depolarization (5 mM-60 mM), calcium ionophore A23187 (1-10 micrograms/ml), and 4-aminopyridine (0.1-10 mM). Aging reduced K+ depolarization-induced release at every K+ concentration studied, reduced A23187-induced release at low but not high concentrations and did not affect 4-aminopyridine-induced release. Membrane peroxidation of synaptosomes from 2-month-old rats altered the response to secretagogues to match that seen in 24-month-old rats. Membrane peroxidation also attenuated the A23187-stimulated translocation of free to bound synaptosomal PKC activity in 2-month-old but not 24-month-old animals. These results suggest that membrane peroxidation may mimic some age-related deficits in secretagogue-induced [3H]ACh release.