The stopping response of Xenopus laevis embryos: pharmacology and intracellular physiology of rhythmic spinal neurones and hindbrain neurones.

1. Xenopus laevis embryos stop swimming in response to pressure on the cement gland. This behaviour and 'fictive' stopping are blocked by bicuculline (10 mumol 1(-1)), tubocurarine (110 mumol 1(-1)) and kynurenic acid (0.5 mmol 1(-1)). 2. Intracellular recordings from spinal neurones active during swimming have shown that pressure on the cement gland evokes compound, chloride-dependent inhibitory postsynaptic potentials (IPSPs). These are blocked by bicuculline, tubocurarine and kynurenic acid, but are unaffected by strychnine (2 mumol 1(-1)). 3. When the cement gland is pressed, trigeminal ganglion activity precedes both the IPSPs and the termination of 'fictive' swimming activity recorded in rhythmic spinal neurones. The trigeminal discharge is unaffected by the antagonists bicuculline, tubocurarine, kynurenic acid and strychnine. 4. Intracellular recordings from the hindbrain have revealed neurones that are normally silent, but rhythmically inhibited during 'fictive' swimming. In these neurones pressure on the cement gland evokes depolarising potentials, often with one or more spikes. 5. We propose that the stopping response depends on the excitation of pressure-sensitive trigeminal receptors which innervate the cement gland. These release an excitatory amino acid to excite brainstem GABAergic reticulospinal neurones, which inhibit spinal neurones to turn off the central pattern generator for swimming. There may also be a less direct pathway.

The stopping response of Xenopus laevis embryos: behaviour, development and physiology.

1. When Xenopus laevis embryos swim into an obstruction they usually stop. This stopping response to stimulation on the head is present from stage 28 to 45. At stage 37/38 it is more reliable in restrained than in free-swimming animals, and to stimuli to the cement gland than to the head skin. 'Fictive' swimming also stops reliably after the same stimuli but struggling and 'fictive' struggling do not. 2. Discharge of deformation-sensitive trigeminal sensory neurons in response to pressure on the cement gland or head skin precedes the 'fictive' stopping response. When the embryo hangs from cement gland mucus, trigeminal neurons are active and the embryo is less responsive to stimulation. 3. Lesions of the central nervous system have allowed us to draw the following conclusions about this inhibitory pathway: (a) either the cement gland or the head skin must be intact; (b) one trigeminal ganglion is both sufficient and necessary; (c) the pathway is independent of the forebrain and midbrain; (d) it can take an ipsilateral or contralateral route through the hindbrain; (e) at least two hindbrain interneuron components are involved. 4. A similar stopping response is present in embryos and larvae of the urodele Ambystoma mexicanum.

CL-proteins and the regulation of lipoprotein lipase activity in locust flight muscle.

Lipoprotein lipases in the flight muscles of Locusta migratoria show a marked substrate specificity: diacylglycerols associated with the adipokinetic hormone (AKH)-induced lipoprotein, A+, are hydrolysed at 4 to 5 times the rate of those associated with the lipoprotein in resting (non-hormone-stimulated) locusts, Ayellow. To determine the basis for this discrimination, the effect on the activity of flight muscle lipoprotein lipase of CL-proteins, a major constituent of lipoprotein A+, but not of Ayellow, has been investigated; they inhibit the flight muscle enzyme in a competitive manner whether activity is measured with a natural lipoprotein substrate, a lipid emulsion or a water soluble substrate. Experiments in vivo suggest that the flight muscle enzyme is normally inhibited in resting (non-AKH-stimulated) locusts but, interestingly, injection of synthetic AKH-I relieves the inhibition and increases the activity by 30 to 40%. This is not a direct effect of the hormone on the enzyme, but appears to be related to the hormone-induced formation of lipoprotein A+, so that the majority of CL-proteins in the haemolymph become bound to this lipoprotein and the concentration of free CL-proteins is markedly reduced. We suggest that CL-proteins play a major role in the regulation of lipoprotein lipase in locust flight muscle.