Low intensity tetanic stimulation induces long-term potentiation in hippocampal neurons

A minimal intensity of the stimulation necessary for the induction of long-term potentiation of synaptic transmission (LTP) was investigated by intracellular recording in guinea pig in vitro hippocampal slices. High frequency stimulation of afferent fibres at intensities evoking in CA 1 neurons control excitatory postsynaptic potentials (EPSPs) of amplitudes 1-5 mV, resulted usually in a long-lasting increase in response amplitude. LTP was not observed at lower stimulus strength. The coactivation of a certain, though small number of synaptic contacts is thus necessary for the production of LTP.     

Early and late post-tetanic potentiation,and post-tetanic block in a monosynaptic reflex pathway

Observations have been made upon the nature of early and late post-tetanic potentiation and upon post-tetanic block of presynaptic collaterals with particular reference to behavior in circumstances of varied duration and frequency of conditioning stimulation. Early potentiation is most conspicuous following brief tetani at high frequency, late potentiation following long tetani, much lower frequencies being all that are needed. The two phenomena thus are distinguishable and separate. Dorsal root electrotonus produced by stimulations of varying duration and frequency is described, and the similarity between behavior of the D.R.IV R. electronic potential and early potentiation demonstrated. It is shown how early potentiation and post-tetanic block are due to the same process (hyperpolarization) at different intensities. The view that the agency for potentiation is associated with augmented presynaptic action due to hyperpolarization is confirmed. A diagram is constructed to indicate the probable temporal courses of early and late potentiation.     

Role of desensitized acetylcholine receptors in the development of post-tetanic potentiation

A hypothesis that desensitized acetylcholine (ACh) receptors are responsible for postsynaptic potentiation (PSP) was verified in experiments performed on the sartorial muscle of the frog using a voltage-clamp recording technique. The PSP was estimated, at active and inhibited acetylcholinesterase, with the use of paired stimulation of the motor nerve at various interpulse intervals, according to an increase in the time constant () of the second end-plate current (EPC) relative to that of the first EPC. The appearance of desensitized receptors on the postsynaptic membrane, caused by the application of a promoter of desensitization proadiphen or by application of exogenous ACh, was followed by acceleration of the decay of the first EPC and by a decrease in its amplitude. However, this did not intensify PSP, which even decreased in the presence of proadiphen under the influence of ACh and membrane hyperpolarization. A sharp increase in calcium concentration after certain period of ACh influence, which increased quantum ACh release, did not enhance PSP, which allows us to suggest the role of the presynaptic factor to be insignificant. The results provide arguments against the direct involvement of desensitized acetylcholine receptors in the PSP development.Neirofiziologiya/Neurophysiology, Vol. 27, No. 5/6, pp. 361–367, September–December, 1995.     

Influence of gender on post-tetanic potentiation in human dorsiflexors

Twitch contractions of the ankle dorsiflexors were evoked before and after 7 s of tetanic stimulation at 100 Hz in young women and men. Torque decreased more in men (18%) than in women (12%) during the tetanus. There was no gender difference in twitch peak torque potentiation over the 5-min post-tetanus. Potentiation was 42% (women) and 45% (men) at 5 s post-tetanus, and still present at 5 min (women 24%, men 25%). The immediate (5 s) shortening of twitch rise time was similar in women (14%) and men (13%), but during the 5-min men's rise time came to exceed whereas women's only approached pretetanus values (e.g., +9% vs. -1% at 5 min). The immediate decrease in half-relaxation time was also similar in women (24%) and men (22%); however, women's but not men's values remained less than pretetanus values for most of the 5-min period. Twitch rate of torque development increased similarly (75%) in women and men at 5 s, with no gender difference over 5 min. In contrast, rate of torque relaxation increased significantly only in men. Rate of torque development normalized to peak torque was similar in women and men pretetanus and increased similarly 5 s post-tetanus, but women had greater values through most of the 5-min post-tetanus. Normalized rate of torque relaxation was similar in women and men and not affected by tetanus. In the dorsiflexor muscles, young women and men show a similar amount and pattern of twitch force potentiation, but there are gender differences in time-related twitch contractile properties in the first 5 min after tetanus.     

Development of long-term post-tetanic potentiation and changes in S-100 protein contents in hippocampal slices of rats in different functional states]

Male rats of the strains with low (LE) high excitability (HE) of the nervous system have been used in this study. Half of the animals of each strain were neurotized in accordance with the Hecht's scheme. In the hippocampal slices of the non-neurotized LE rats there was a significant increase of the populational spike amplitude during development of LTP as compared with the opposite group of the animals. The LTP formation in the LE strain of rats caused a decrease in the S-100 protein content in the water-soluble, and an increase in the membrane-bound fraction of the protein. Similar results we have observed with the non-inbred Wistar rats but not with the HE strain of the animals. The levels of the water-soluble S-100 protein fraction were also higher in the hippocampuses and entorenal cortices, but not in the cerebellae of the LE strain, as compared with the HE strain of the rats. No differences have been found in the membrane-bound fraction of S-100 protein.     

Effect of temperature on post-tetanic potentiation in human dorsiflexor muscles

The effect of temperature on post-tetanic potentiation (PTP) has been examined in the muscles of small mammals but not in human skeletal muscle. We examined PTP in the ankle dorsiflexor muscles of 10 young men by evoking twitches before and after a 7-second tetanus at 100 Hz in a control (room air approximately 21 degrees C) condition and after immersion of the lower leg in warm (45 degrees C) and cold (10 degrees C) water baths for 30 min. Exposure to cold decreased tetanus and pre-tetanus twitch peak torque, but increased rise time, half-relaxation time, and muscle action potential (M-wave) amplitude; exposure to warm water had little effect. PTP was smallest in cold exposure 5 s post-tetanus, but persisted throughout the 12 min test period, whereas PTP had subsided by 6 min post-tetanus in control and warm exposures. M-wave amplitude initially decreased after exposure to warm water, recovered, then decreased again by 11 min post-tetanus. In contrast, exposure to cold had no initial effect but did increase the M-wave amplitude during the last half of the 12 min test period, similar to that seen in the control. The greatest immediate decrease in rise time and half-relaxation time was observed in the control; however, by 12 min post-tetanus warm exposure showed the greatest increase in rise time and half-relaxation time above pre-tetanus values. The decrease in the unpotentiated twitch torque with cooling in human dorsiflexors is typical for muscles with a predominance of type I (slow) fibres. The effect of cold on PTP is similar to that seen previously in mammalian muscles with a predominance of type II (fast) fibres, although the underlying mechanism of the cooling effect appears to differ.     

Effects of post-tetanic potentiation induced by whole-body electrostimulation and post-activation potentiation on maximum isometric strength

It is currently unknown the most effective potentiation protocol to increase maximum strength. Hence, we investigated the separated and combined effects of post-tetanic potentiation (PTP) induced by whole-body electrostimulation (WB-EMS) and post-activation potentiation (PAP) induced by voluntary maximum isometric contractions on maximum isometric strength. Ten trained males were randomly evaluated on four occasions. In session A, maximum isometric strength (split squat) was measured in minutes 1, 4, and 8. In session B, the measurements were taken in minutes 2, 6, and 10. In session C, a WB-EMS protocol was applied to elicit PTP and the measurements were performed in minutes 1, 4, and 8. In session D, the same WB-EMS protocol was applied and the measurements were taken in minutes 2, 6, and 10. No significant differences in maximum isometric strength were observed between: (i) the control and WB-EMS in minutes 1 vs. 1 and 2 vs. 2; (ii) the control and PAP in minutes 1 vs. 4, 1 vs. 8, 2 vs. 6, and 2 vs. 10; and (iii) the PAP and WB-EMS plus PAP in minutes 4 vs. 4, 8 vs. 8, 6 vs. 6, and 10 vs. 10. In contrast, the WB-EMS plus PAP revealed a significant increase of 54% (~450 N) compared to the WB-EMS in minutes 4 and 8 compared to the minute 1 (p < 0.001), but not between minutes 2 vs. 6 and 2 vs. 10. The present results showed that PTP induced by WB-EMS in isolation or combined with PAP induced by voluntary maximum isometric contractions did not produce a significant increase in maximum isometric strength compared to the control and PAP alone, respectively.     

Simulation of post-tetanic potentiation and fatigue in muscle using a visco-elastic model

Post-tetanic potentiation (PTP) in single motor units was simulated using a simple visco-elastic model. Single isometric twitches and unfused tetani were obtained using a wide range of physiological input rates. Values of model parameters were chosen to simulate contraction times close to those of fast and slow muscle fibers. PTP has been attributed either to i) an augmented plateau level of active state or ii) an increase in time constant of active state decay. Our results show that a prolonged decay time of active state can account for most of the experimental data obtained in amphibian and mammalian preparations. In particular, potentiation is more marked in unfused tetani than in single twitches. Moreover the model accounts for PTP even in the case of a reduction of active state plateau due to fatigue.     

Coexistence of twitch potentiation and tetanic force decline in rat hindlimb muscle

An experimental protocol designed to assess fatigability in motor units has been applied to two hindlimb muscles of anesthetized adult rats to study the effects of whole-muscle fatigue on the isometric twitch. Both soleus and extensor digitorum longus exhibited a linear relationship between fatigability (i.e., force decline after a 360-s fatigue test) and the magnitude of the twitch force following the fatigue test. Twitch force after the fatigue test was potentiated (i.e., greater than the value before the fatigue test) in many muscles, despite the development of considerable fatigue. This coexistence of fatigue and twitch potentiation was observed in 7% (5/70) of soleus and 48% (31/64) of extensor digitorum longus muscles. The coexistence was exhibited only by the least fatigable muscles of the fast-contracting extensor digitorum longus. The extensor digitorum longus muscles that did not exhibit twitch potentiation probably experienced a higher proportion of muscle-fiber inactivation, such as due to failure of neuromuscular propagation, that was induced by the fatigue regimen.     

Dopaminergic regulation of theta activity of septohippocampal neuron in the awake rabbit

The effects of dopamine reuptake blocker nomifensine and nonselective antagonist of dopamine receptors haloperidol on the theta rhythmicity of the medial septal neurons and hippocampal EEG were investigated in the rabbit. Bilateral intracerebroventricular infusion of nomifensine (9 micrograms in each ventriculus) produced an increase in both the rate of firing and the theta modulation of medial septal neurons; the theta power of the hippocampal EEG also augmented. The degree of neuronal theta stability (time constant of damping, tao theta) significantly increased. The frequency of rhythmic bursts in the neuronal firing also substantially elevated. The amplitude, regularity and frequency of theta waves in the hippocampal EEG also increased. The antagonist haloperidol (12.5 mg) caused the opposite effect. The theta activity of medial septal neurons and the theta power of the hippocampal EEG decreased after haloperidol injection. Theta rhythmicity of septal neurons significantly diminished, the rate of rhythmic bursts in the neuronal firing also decreased, although not substantially. The theta amplitude and regularity in the hippocampal EEG also decreased. Effects of both drugs built up rapidly and then gradually attenuated. Nomifensine infusion against the background of exposure to haloperidol provoked neither increasing neuronal firing rate, nor elevating theta activity. These finding suggest that dopaminergic system produces activation of the septohippocampal system in situations that require selective attention to functionally important information.