Enhancement of recurrent inhibition of the spinal monosynaptic reflex by preceding stimulation of the medullary raphé in rats

Functional alteration of inhibitory influences on spinal motor output in painful diabetic neuropathy in rats

Diabetes is frequently accompanied by painful polyneuropathies that are mediated by enhanced neuronal excitability in the spinal cord, partly because of decrease in spinal intrinsic inhibitory influences. Changes in spinal excitatory-inhibitory balance may alter spinal segmental motor output. In the study presented here, the mono- and disynaptic (the fastest polysynaptic) reflexes (MSR and DSR, respectively) were recorded from L5 ventral roots in response to stimulation of the ipsilateral L5 dorsal root in spinalized streptozotocin (STZ)-induced diabetic rats with a reduced withdrawal threshold to mechanical stimuli. The diabetic rats generally exhibited larger spinal reflex amplitudes, the DSR being influenced in particular. We addressed whether recurrent and presynaptic inhibition of the spinal reflexes were altered in STZ-treated animals. The recurrent inhibition of the MSR and DSR elicited by preceding antidromic conditioning stimulation delivered to the recorded L5 ventral root was markedly suppressed in diabetic rats. By contrast, the presynaptic inhibition of the MSR and DSR elicited by preceding conditioning stimulation to the ipsilateral L4 dorsal root was not impaired. Thus, in diabetic painful neuropathy, reduced spinal intrinsic inhibition in the ventral horn contributes to an enhanced spinal segmental motor output.

Decorrelating actions of Renshaw interneurons on the firing of spinal motoneurons within a motor nucleus: a simulation study

A simulation of spinal motoneurons and Renshaw cells was constructed to examine possible functions of recurrent inhibition. Recurrent inhibitory feedback via Renshaw cells is known to be weak. In our model, consistent with this, motoneuron firing was only reduced by a few pulses per second. Our initial hypothesis was that Renshaw cells would suppress synchronous firings of motoneurons caused by shared, dynamic inputs. Each motoneuron received an identical pattern of noise in its input. Synchrony coefficients were defined as the average motoneuron population firing relative to the activity of selected reference motoneurons; positive coefficients resulted if the motoneuron population was particularly active at the same time the reference motoneuron was active. With or without recurrent inhibition, the motoneuron pools tended to show little if any synchronization. Recurrent inhibition was expected to reduce the synchrony even further. Instead, it reduced the variance of the synchrony coefficients, without a comparable effect on the average. This suggests-surprisingly-that both positive and negative correlations between motoneurons are suppressed by recurrent inhibition. In short, recurrent inhibition may operate as a negative feedback mechanism to decorrelate motoneurons linked by common inputs. A consequence of this decorrelation is the suppression of spectral activity that apparently arises from correlated motoneuron firings due to common excitatory drive. Without recurrent inhibition, the power spectrum of the total motoneuron pool firings showed a peak at a frequency corresponding to the largest measured firing rates of motoneurons in the pool. Recurrent inhibition either reduced or abolished this peak, presumably by minimizing the likelihood of correlated firing among pool elements. Renshaw cells may act to diminish physiological tremor, by removing oscillatory components from aggregate motoneuron activity. Recurrent inhibition also improved coherence between the aggregate motoneuron output and the common drive, at frequencies above the frequency of the "synchronous" peak. Sensitivity analyses demonstrated that the spectral effect became stronger as the duration of inhibitory synaptic conductance was shortened with either the magnitude or the spatial extent of the inhibitory conductances increased to maintain constant net inhibition. Overall, Renshaw inhibition appears to be a powerful way to adjust the dynamic behavior of a neuron population with minimal impact on its static gain.

Central neurophysiologic mechanisms of the regulation of inhibition

The neuronal populations of the cuneate nucleus of the midbrain, the medial parabrachial nucleus, the median and magnus nuclei of the raphé, the electrical and local chemical stimulation of which elicits the inhibition of the motor activity of animals, were determined in chronic experiments on freely-moving white male mongrel rats. It was established that when each of the enumerated regions of the brain are stimulated electrically in other zones which elicit motor inhibition, multineuronal responses with a latent period of less than 2.5 msec were recorded. At the same time, multidirectional bilateral changes in muscle tone of the flexors and extensors of the hind limbs are observed in sodium ethaminal anesthetized and unanesthetized animals. The electrolytic destruction of the inhibitory zones of the median raphé nucleus and raphé nucleus magnus blocks the motor inhibition elicited by electrical stimulation of the cuneate nucleus of the midbrain and the medial parabrachial nucleus.

Effect of nucleus raphe magnus stimulation on recurrent inhibition of the monosynaptic reflex in the cat

Experiments were performed on 8 cats anesthetized with urethan-chloralose. The effects of nucleus raphe magnus (NRM) conditioning stimulation on recurrent inhibition of posterior biceps semitendinosus nerve (PBSt) monosynaptic reflex elicited by electric stimulation of a part of L7 ventral root were investigated in the cat. It was found that (1) the PBSt monosynaptic reflex was facilitated by NRM conditioning stimulation at 30, 50 and 80 ms conditioning-test stimulus intervals, but that (2) the inhibited monosynaptic reflex by recurrent inhibition was further inhibited by NRM conditioning stimulation at the same conditioning-test stimulus intervals, and that (3) the activity of Renshaw cells as recorded by glass microelectrode was enhanced by NRM conditioning stimulation. These facts indicated that recurrent inhibition pathways are enhanced by NRM conditioning stimulation.

A biphasic excitability change in hindlimb motoneurons evoked by stimulation of the nucleus raphes magnus in the cat

1. The influence of electrical stimulation of the nucleus raphes magnus (RM) on spinal segmental systems were examined. 2. RM stimulation produced an initial increase and a subsequent suppression of the amplitude of both fiextor and extensor lumbar monosynaptic reflex potentials (MSRs). 3. Intracellular recordings were made from alpha-motoneurons of the common peroneal nerve (flexor) and the tibial nerve (extensor). RM stimulation evoked postsynaptic potentials with a time course similar to that of MSR facilitation. 4. RM stimulation inhibited the aggregate excitatory synaptic potential (EPSP) evoked by stimulation of group I afferent fibers without apparent changes in the motoneuronal membrane potential. 5. These data suggest that the RM-evoked biphasic effect on MSR consists of early facilitation due to EPSP, and late inhibition possibly due to presynaptic inhibition of group I afferent fibers.