Effects of strychnine on fictive swimming in the lamprey: evidence for glycinergic inhibition, discrepancies with model predictions, and novel modulatory rhythms

Abstract

1. Inhibitory postsynaptic potentials (ipsps) produced by two classes of interneurons, CC (Contralateral and caudal projecting) and lateral interneurons, were tested for strychnine sensitivity using paired intracellular recordings in the lamprey spinal cord. The ipsps were partially blocked by 0.2-0.5 microM strychnine and were completely blocked by 5 microM strychnine. Thus, the ipsps may be glycinergic. 2. These interneurons are key participants in a proposed circuit model for fictive swimming. A connectionist-type computer simulation of the model demonstrated that the cycle period of the network increased with decreasing ipsp strength. 3. Application of strychnine (0.1-0.5 microM) to the spinal cord during fictive swimming induced by an excitatory amino acid increased cycle period, consistent with previous reports, but at odds with stimulation predictions. 4. Strychnine also produced slow rhythmic modulation of fictive swimming (period = 12 s) which maintained left-right alternation and rostral-caudal coordination. Auto- and cross-correlation analyses revealed that the slow modulation was present in a weaker form in most control preparations during fictive swimming. 5. Since the proposed model for the swimming pattern generator in the lamprey spinal cord does not predict the observed speeding with strychnine, nor the slow modulatory rhythm, it appears to be deficient in its present formulation.

Glycinergic Inhibition Contributes to the Generation of Rostral Scratch Motor Patterns in the Turtle Spinal Cord

“…We are not aware of comparable studies in other scratch reflex preparations; however, a great deal of work has investigated the role of inhibitory transmitters in the spinal control of fictive locomotion. Our results are consistent with fictive locomotion studies in which strychnine consistently increased ventral root burst frequencies (lamprey: Grillner and Wallén, 1980;McPherson et al, 1994;frog embryos: Dale, 1995). In other locomotor work, strychnine produced biphasic effects, either increasing burst frequency without affecting right-left alternation, or decreasing burst frequency while synchronizing the right and left sides (lamprey: Cohen and Harris-Warrick, 1984;Hagevik and McClellan, 1994;neonatal rat: Cowley and Schmidt, 1995).…”

Neuronal control of swimming behavior: Comparison of vertebrate and invertebrate model systems

“…MNs then activate ipsilateral myotomal muscle (Teräväinen and Rovainen, 1971). Some CCINs produce monosynaptic glycinergic inhibition in contralateral MNs, and therefore contribute to the rhythmic hyperpolarizations of MN oscillations (Buchanan, 1982; McPherson et al, 1994). The LINs only rarely inhibit ipsilateral MNs (Rovainen, 1974a), although small local inhibitory interneurons that inhibit ipsilateral motoneurons have been described (Buchanan and Grillner, 1988).…”

Neural mechanisms potentially contributing to the intersegmental phase lag in lamprey

“…strychnine) of the inhibitory synaptic transmission are administered or the commissural interneurons are photo-ablated or cut. This leads to a progressive increase of the burst frequency followed by a breakdown of organized bursting (Grillner and Walle n 1980;McPherson et al 1994;Buchanan and McPherson 1995). This may apply to already ongoing ®ctive locomotor activity initiated by bath application of excitatory amino acids.…”