Ionic and pharmacological properties of reciprocal inhibition in Xenopus embryo motoneurones.

Abstract

SUMMARY1. Properties of rhythmic, compound mid-cycle inhibitory post-synaptic potentials (i.p.s.p.s), which constitute one of the three main synaptic drives to motoneurones during fictive swiming in Xenopus embryos, have been examined using ionic and pharmacological manipulation.2. Mid-cycle i.p.s.p.s are Cl-dependent. They are reversed by intracellular Clinjection and attenuated by lowered extracellular Cl-concentration.3. In response to bath application of 100 /iM-glycine or 100 /uM-y-aminobutyric acid (GABA), motoneurones show a decrease in cell input resistance of 24+ 2-9 MQ (mean + S.E. of mean) or 16 + 3-7 % and 26 + 6-0 MQ or 14 + 2-0% respectively. This is associated with a weak hyperpolarization or depolarization of 0 + 1-5 mV and -3+1-4 mV respectively. Both responses can be made strongly depolarizing by intracellular Cl-injection.4. The response to glycine is blocked by 1 Lm-strychnine but is largely unaffected by bicuculline below 50 /M. The response to GABA is largely blocked by 10 fMbicuculline but is unaffected by 1 /m-strychnine. Both strychnine and bicuculline are therefore specific antagonists in the amphibian embryo preparation. Glycine and GABA are both partially antagonized by 10 /LM-picrotoxin. 5. Mid-cycle i.p.s.p.s recorded in motoneurones during fictive swimming are reduced in amplitude by 05-t1 /M-strychnine but are largely unaffected by 40 #tM-bicuculline. In embryos immobilized by ventral root transaction, 100 /Mtubocurarine, a likely GABA antagonist in the embryo, has no effect on mid-cycle inhibition. Glycine is suggested to be the probable transmitter released by commissural interneurones and mediating mid-cycle inhibition during fictive swimming, acting to increase conductance of Cl-.

Metamodulation of a Spinal Locomotor Network by Nitric Oxide

“…Therefore, we expected during recordings from presumed motor neurons that there would be a selective potentiation of GABAergic over glycinergic inhibition by SNAP in the presence of phentolamine. It has been shown previously that GABA inhibitory IPSPs coincide with the termination of swimming episodes (Reith and Sillar, 1999), whereas glycinergic IPSPs coincide with the midcycle component of swimming (Soffe, 1987). Bath application of phentolamine alone did not appear to have any significant effect on the frequency of IPSPs after the end of each episode (Fig.…”

“…The neural circuitry driving this behavior is relatively simple (Roberts, 1990). The frequency of swimming is primarily determined by the balance between synaptic excitation (Dale and Roberts, 1985;Sillar and Roberts, 1993;Perrins and Roberts, 1995) and inhibition (Soffe, 1987;Dale, 1995;Reith and Sillar, 1999). Once initiated, swimming will continue until either the animal's head contacts an obstacle and cement gland afferents trigger inhibitory GABA release from reticulospinal neurons onto spinal neurons (Boothby and Roberts, 1992a,b;Perrins et al, 2002;Li et al, 2003), or locomotion gradually runs down as a result of adenosine accumulation (Dale and Gilday, 1996;Dale, 2002).…”

The synaptic drive from the spinal locomotor network to motoneurons in the newborn rat

“…This was confirmed indirectly by bicuculline's lack of effect on motoneuronal inhibition, which in our study was monitored directly by performing intracellular recordings (data not shown). A similar chloride-sensitive rhythmic inhibition of the motoneurons has been observed in the lamprey (Russell and Wallen, 1983) the tadpole (Roberts et al, 1983(Roberts et al, , 1986Soffe, 1987Soffe, , 1989, and the cat (Orsal et al, 1986). In the adult cat, Pratt and Jordan (1987) have noted that intravenous injection of strychnine abolishes the inhibition exerted on motoneurons without modifying the locomotor rhythmicity.…”

GABAergic inactivation of the central pattern generators for locomotion in isolated neonatal rat spinal cord.

“…These results are in agreement with what has been observed in various vertebrate species. In the lamprey (Cohen & Harris-Warrick, 1984) and *the tadpole (Dale, 1985;Soffe, 1987), it has been reported that glycine mediates the reciprocal inhibition between the left and right sides. Kudo, Ozaki & Yamada (1991) have established that in the rat, the inhibitory connections between the right and left sides are glycinergic and described how they develop in the embryo.…”