A Postsynaptic Mechanism Underlying Long-Lasting Changes in the Excitability of Pyramidal Tract Neurones in the Anaesthetized Cat
1. Changes in the excitability of pyramidal tract (PT) neurones that last for hours can be induced by altering their firing rates for brief periods, in cats that are anaesthetized and paralyzed.
2. Long-lasting increases in excitability can be induced by trains of antidromic conditioning shocks (e.g. 100Hz, 0.2 ms width, train duration 6 sec to 10 min). In this experimental situation the site of the underlying change could be pre- or postsynaptic. However, we have evidence that a postsynaptic mechanism exists, shown in the following way. Synaptic transmission was blocked by the application of MgCl2 solution to the pial surface. Antidromic conditioning trains were then given to the PT at the medulla; an increase in the excitability of PT cells resulted, and persisted undiminished, for up to 3 hours (ref. 1). This observation needs to be taken into account when considering the cellular basis of conditioning and learning.
3. Long-lasting decreases in excitability of PT neurones were produced by stimulation of the contralateral PT when synaptic transmission was not impaired by the application of Mg (1). The decreases in excitability persisted without decrement for more than 1 hour. Conditioning stimuli given to the contralateral tract affect cells whose axons run in the ipsilateral tract via synapses alone, either via axon collaterals or more circuitous pathways. The site of the underlying change in this case may be pre or postsynaptic; it could be due to a prolonged decrease in excitability of PT cells, or to a prolonged increase in excitability of inhibitory interneurones.
KeywordsConditioning Stimulus Firing Rate Synaptic Transmission Pyramidal Tract Test Shock
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- 2.Bindman, L.J., Lippold, O.C.J. & Milne, A.R. (1976). Long-lasting changes of post-synaptic origin in the excitability of pyramidal tract neurones. J. Physiol. 258, 71–72P.Google Scholar
- 3.Bindman, L.J., Lippold, O.C.J. & Milne, A.R. (1976). Prolonged decreases in excitability of pyramidal tract neurones. J. Physiol. 263, 141–142P.Google Scholar
- 4.Bindman, L.J. & Milne, A.R. (1977). The reversible blocking action of topically applied magnesium solutions on neuronal activity in the cerebral cortex of the anaesthetized rat. J. Physiol. 269, 34P.Google Scholar
- 5.Bliss, T.V.P. & L$mo, T. (1973). Long-lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit following stimulation of the perforant path. J. Physiol. 232, 331–356.Google Scholar
- 7.SBmjen, G.G. & Kato, G. (1968) Effects of magnesium and calcium on neurons in the central nervous system. Brain Res. 9, 161–164Google Scholar
- 8.Tzebelikos, E. & Woody, C.D. (1979) Intracellularly studied excitability changes in coronal-pericruciate neurons following low frequency stimulation of the corticobulbar tract. Brain Res. Bull. 4, 635–641.Google Scholar
- 9.Woody, C.D. & Black-Cleworth, P. (1973) Differences in excitability of cortical neurones as a function of motor projection in conditioned cats. J. Neurophysiol. 36, 1104–1116.Google Scholar
- 10.Woody, C.D., Swartz, B.E. & Gruen, E. (1978) Effects of acetylcholine and cyclic GMP on input resistance of cortical neurones in awake cats. Brain Res. 158, 373–395.Google Scholar