Modifications to synaptic transmission at group Ia synapses on cat spinal motoneurones by 4-aminopyridine

Treating monocularly deprived lambs with 4-aminopyridine produces rapid changes in ocular dominance only after short periods of deprivation

Lambs of various ages (4-30 days) were monocularly deprived for periods ranging from 4 to 51 days after which they were prepared for conventional electrophysiological recording from the striate cortex. Sufficient units were sampled to obtain a representative ocular dominance (OD) histogram. A second sample of units was then obtained after the lambs had been given an i.v. dose of 4-aminopyridine (4-AP; 0.5-5.3 mg/kg) which increases synaptic transmission. For four out of five lambs in which the deprivation had been of 4-5 days duration there was a significant increase in responsiveness of the deprived eye to stimulation after the 4-AP had been given. By contrast, only one out of four lambs which had been deprived for 19-51 days showed a significant recovery after 4-AP treatment. The results suggest that during the initial stages of monocular deprivation the deprived eye remains connected to, but is less effective in driving, cortical cells. One explanation of the failure to reactivate the deprived eye after long periods of deprivation is that the deprived eye becomes anatomically disconnected from cortical cells.

Post-tetanic potentiation and facilitation of synaptic potentials evoked in cat spinal motoneurones

1. Excitatory post-synaptic potentials (e.p.s.p.s) were evoked in spinal alpha-motoneurones of the cat by impulses in single group Ia nerve fibres. 2. The average peak amplitude of some of these e.p.s.p.s was increased by a conditioning tetanus. The maximum increase observed was 54% of the control amplitude. 3. The average peak amplitude of some e.p.s.p.s was increased by a single conditioning stimulus which preceded the test stimulus by 1 or 2 msec. The maximum increase observed was 28% of the control amplitude. 4. The ability of e.p.s.p.s to potentiate following a tetanus was correlated with their ability to be facilitated by a single conditioning stimulus. 5. If an e.p.s.p. could be facilitated prior to a tetanus, the amount of facilitation was reduced after the tetanus, with all facilitation being abolished when post-tetanic potentiation was maximal. 6. The fluctuations of an e.p.s.p. were analysed before and after a tetanus. The peak amplitudes that an e.p.s.p. fluctuated between while potentiated did not gradually diminish as the effect of the tetanus disappeared. Post-tetanic potentiation, when it occurred, was accompanied by a decrease in the probability of occurrence of components with smaller peak amplitudes and an increase in the probability of occurrence of components with larger peak amplitudes. 7. These results are consistent with the suggestion that the magnitude of the synaptic potential generated at a single bouton does not vary from trial to trial (Jack, Redman & Wong, 1981a). Nor does the amplitude of this potential vary following a single conditioning stimulus or a tetanus. Post-tetanic potentiation and facilitation result from a decrease in the probability of failure to release transmitter following the conditioning stimuli.

The components of synaptic potentials evoked in cat spinal motoneurones by impulses in single group Ia afferents

1. Excitatory post-synaptic potentials (e.p.s.p.s) were evoked in cat spinal motoneurones by impulses in single group Ia afferent fibres. The probability density of the fluctuations in peak amplitude of each e.p.s.p. was calculated from the recorded peak amplitude and the probability density of the recording noise. 2. Most e.p.s.p.s fluctuated between different components (i.e. individual e.p.s.p.s of a particular discrete amplitude) with peak amplitudes which were integer multiples of the increment between successive components. The average peak amplitude of this incremental e.p.s.p. was about 90 microV for e.p.s.p.s generated at or near the soma. 3. In general, the probability density of the peak amplitude could not be described using Poisson or binomial distributions. 4. For many e.p.s.p.s the complete time course of each component could be calculated. There was no variability in the amplitude of these components nor in their latency of onset. For some e.p.s.p.s there were differences in the latency and time course of the components. 5. The increments between successive components of e.p.s.p. generated proximally were no larger (at the soma) than the corresponding increments for e.p.s.p.s generated at more distal dendritic sites. 6. These results and those from subsequent papers (Jack, Redman & Wong, 1981; Hirst, Redman & Wong, 1981) reinforce earlier suggestions that each bouton behaves in an all-or-nothing manner with respect to post-synaptic effect, and the probability of failure varies at different boutons arising from the same afferent.