Modulatory effects of alpha1-,alpha2-, and beta -receptor agonists on feline spinal interneurons with monosynaptic input from group I muscle afferents

Abstract

Previous studies have shown that monoamines may modulate operation of spinal neuronal networks by depressing or facilitating responses of the involved neurons. Recently, activation of interneurons mediating reciprocal inhibition from muscle spindle (Ia) afferents and nonreciprocal inhibition from muscle spindle and tendon organ (Ia/Ib) afferents in the cat was found to be facilitated by noradrenaline (NA). However, which subclass membrane receptors are involved in mediating this facilitation was not established; the aim of the present experiments was to investigate this. Individual Ia- and Ia/Ib-inhibitory interneurons were identified in the cat lumbar spinal cord, and NA agonists were applied close to these neurons by ionophoresis. The agonists included the alpha1-receptor agonist phenylephrine, the alpha2-receptor agonists clonidine and tizanidine, and the beta-receptor agonist isoproterenol. Effects were measured by comparing changes in the number of extracellularly recorded spike potentials evoked by electrical stimulation of muscle nerves and changes in the latency of these potentials before, during, and after application of the tested compounds. Results show that the facilitatory effect of phenylephrine is as strong as that of NA, whereas the facilitatory effect of isoproterenol is weaker. Clonidine depressed activity of both Ia- and Ia/Ib-inhibitory interneurons, whereas tizanidine had no effect. These findings lead to the conclusion that beneficial antispastic effects of clonidine and tizanidine in humans are unlikely to be associated with an enhancement of the actions of Ia- and Ia/Ib-inhibitory interneurons, and the findings also support previous proposals that these compounds exert their antispastic actions via effects on other neuronal populations.

Fig. 1.

Experimental setup used for identification of interneurons and ionophoresis. Left, Diagram showing an interneuron mediating Ia-reciprocal inhibition (Ia, gray) and examples of records used for the identification of Ia interneurons: monosynaptic excitation from Q, their ability to follow activation at 400 Hz, and inhibition via Renshaw cells after stimulation of the L6 ventral root at intensities supramaximal for α-motoneuronal axons. Middle, Diagram showing interneurons mediating Ia/Ib excitation (white) and nonreciprocal inhibition (gray) and records used to identify the inhibitory interneurons by antidromic activation from the L4 segment and lack of antidromic activation from the thoracic segments after stimulation up to 1 mA of either the ipsilateral (iTh) or contralateral (coTh) lateral funiculus. Right, The arrangement of the two micropipettes used for the ionophoresis (Ionophor.) and recording. They were attached to a double manipulator with separate microdrives. For details, see Jankowska et al. (1997, 2000). In this and the following figures, the top traces in each pair show microelectrode records (with the negativity down) and bottom traces show records from the cord dorsum and the time of arrival of the afferent volleys (with the negativityup). The time calibration in the middle panel is for records in both panels.MN, Motoneuron; R, Renshaw cell;DSCT, dorsal spinocerebellar tract neuron;VR, ventral root.

Fig. 2.

Examples of the depression of responses of an Ia interneuron (A–C) and an Ia/Ib interneuron (D–F) by clonidine. A, D, Control responses. B, E, Peristimulus time histograms aligned with the records in A and D, respectively, showing responses before, during, and after ionophoresis.C, F, Cumulative sums of the same responses. Note the decrease in the number of responses during ionophoresis, as indicated both by the histograms and by the height of the cumulative sums. Note also that the latencies are longer during ionophoresis than during the control and recovery periods. Dotted vertical linesindicate minimal latencies in the control records. The time calibration in F applies to all records. Vertical scale bars are for C and F.

Fig. 3.

Examples of negligible effects of tizanidine on responses of an Ia interneuron (left) and an Ia/Ib interneuron (right). Top, Neuronal responses to stimulation of the peripheral nerves and simultaneously obtained records of afferent volleys from cord dorsum.Bottom (and aligned with the neuronal responses), Cumulative sums showing responses before, during, and after the end of ionophoresis. Note that the number of responses remained practically unaffected during ionophoresis. The latencies also were practically unaffected, although, as illustrated on the left, shorter latencies were noted occasionally. Other indications are as in Figure 2.

Fig. 4.

Comparison of effects of clonidine and tizanidine on samples of Ia and Ia/Ib interneurons. The effects of the two agonists are indicated by changes in the number of responses evoked by 20 consecutive stimuli (single or the first of two stimuli). The ordinate mean number of responses with SEM is shown. Control 1, Responses evoked before placement of the drug-containing microelectrode. Control 2, Responses evoked after placement of the drug-containing microelectrode. Other data are for the time periods indicated below the bars during ionophoresis and during periods of 1–5 and 6–15 min of recovery after the end of ionophoresis. Statistically significant changes with respect to the first control data are indicated by an asterisk(p < 0.05). Dotted linesindicate mean number of responses in control 1.

Fig. 5.

Examples of facilitation of responses of an Ia interneuron and an Ia/Ib interneuron by phenylephrine (Phe). Top records show responses of the neurons aligned with records of afferent volleys and with cumulative sums of these responses before, during, and after ionophoresis, as in Figure 3. Note the marked increase in the number of responses during ionophoresis and shorter latencies. Dotted lines indicate mimimal latencies in control records.

Fig. 6.

Examples of facilitation of responses of an Ia interneuron and an Ia/Ib interneuron by isoproterenol (Iso). Top records show responses of the neurons aligned with simultaneously recorded afferent volleys and cumulative sums of these responses, as in Figure 3. Dotted lines indicate mimimal latencies in control records.

Fig. 7.

Comparison of effects of phenylephrine and isoproterenol on samples of Ia and Ia/Ib interneurons. The effects of the agonists are indicated by changes in the number of responses evoked by 20 consecutive stimuli, as in Figure 4. Ordinate mean number of responses with SEM. Statistically significant changes with respect to the first control data are indicated by an asterisk(p < 0.05). Dotted linesindicate mean number of responses in control 1.