| Abstract No.: | A-D1136 |
| Country: | Canada |
| | |
| Title: | THERMOSENSITIVITY OF A VERTEBRATE CPG MODULATED BY NITRIC OXIDE |
| | |
| Authors/Affiliations: | 1 R. Meldrum Robertson, Keith T. Sillar;
1 Queen's University, Kingston, ON; 2 University of St. Andrews, Fife, United Kingdom |
| | |
| Content: | Objectives: We are interested in the mechanisms that determine the constraints on neural circuit function under extreme environmental conditions and in particular the role of neuromodulators. In insects, inhibition of the NO/cGMP/PKG pathway delays shutdown of the ventilatory CPG during hyperthermia. We tested whether similar manipulations would affect operation of the swimming CPG at high temperatures in Xenopus tadpoles.
Materials and Methods: Using glass suction electrodes to record extracellularly from three ventral roots of immobilized stage 42 Xenopus larvae, we monitored swimming motor patterns generated either in response to a 1ms electrical stimulation of the tail or following superfusion of NMDA (50µM). The saline was locally heated just before it flowed into the preparation chamber. Motor patterns were recorded while preparations were exposed to repeatable temperature ramps from room temperature (~20°C) to 38°C over 6 minutes. Turning the heater off allowed a gradual return to room temperature in around 10 minutes. We bath-applied standard pharmacological agents to manipulate the NO/cGMP/PKG pathway: SNAP (NO donor, 100µM); carboxy-PTIO (NO scavenger, 100µM); L-NAME (NO synthase inhibitor, 1mM); ODQ (guanylyl cyclase inhibitor, 20µM); 8-bromo cGMP (PKG activator, 40µM).
Results: At room temperature the swimming motor pattern had a frequency around 14 cycles/s. NMDA swimming was continuous and showed slow wave modulation at 0.2 Hz whereas tail stimulation evoked discrete episodes of swimming of variable duration which depended on the time since the end of the previous episode and prevailing pharmacological manipulation. Increasing bath temperature increased the motor pattern frequency to around 33 cycles/s just prior to motor pattern failure which occurred at 32±0.4 °C (mean±SE; n = 20). Heat increased the frequency of NMDA slow wave modulation (up to 0.5 Hz) and dramatically shortened the duration of evoked swimming episodes (down to ~1s). Hyperthermia also evoked short bursts of escape swimming with the motor characteristics of C-starts (i.e. initiated with near simultaneous activation of myotomes along one side). Motor pattern recovery occurred 4±0.4 minutes after the heater was switched off, when the bath temperature had dropped to 27±0.6°C. Drug treatments that activated the NO/cGMP/PKG pathway (SNAP and 8-bromo cGMP) reduced the failure temperature and markedly slowed recovery whereas inhibitory treatments (carboxy-PTIO, L-NAME and ODQ) increased the failure temperature and speeded recovery.
Conclusion: Hyperthermia increases swimming frequency, reduces swim episode duration and triggers C-start escape behaviour before the swim CPG shuts down. Manipulations of the NO/cGMP/PKG pathway show that this system is involved in setting the threshold temperatures for these effects. In Xenopus tadpoles, pharmacologically applied NO is known to potentiate glycinergic and GABAergic connections and thus to act as a brake on swimming by decreasing swimming frequency and shortening swim episodes. We suggest that under natural conditions this is part of a stress response that would conserve energy and prevent potentially damaging overexcitation of the circuit.
|
| | |
| Back |
|
