Elsevier

Neuroscience

Volume 142, Issue 3, 27 October 2006, Pages 885-892
Neuroscience

Systems neuroscience
Serotonin-induced in vitro long-term facilitation exhibits differential pattern sensitivity in cervical and thoracic inspiratory motor output

https://doi.org/10.1016/j.neuroscience.2006.06.036Get rights and content

Abstract

Intermittent hypoxia induces 5-HT-dependent, pattern-sensitive long-term facilitation (LTF) of spinal respiratory motor output. We used a split-bath in vitro neonatal rat brainstem–spinal cord preparation to test whether: 1) intermittent spinal 5-HT exposure (without hypoxia) is sufficient to induce LTF in phrenic and intercostal inspiratory motor outputs; 2) LTF magnitude is greater in intercostal versus phrenic activity; and 3) phrenic and intercostal motor output exhibits differential pattern sensitivity to 5-HT application. With a barrier at spinal segment C1, 5-HT (5 μM) was applied episodically (3 min 5-HT, 5 min wash, ×3) to the spinal cord (C2–L1) while recording inspiratory bursts in cervical (C4 or C5) and thoracic (T5 or T6) ventral roots. Episodic 5-HT application increased cervical and thoracic burst amplitudes to 136±22% and 150±22% of baseline, respectively, at 120 min post-drug (P<0.01). Continuous 5-HT application (5 μM, 9 min) had no effect on cervical burst amplitude at 120 min post-drug, but increased thoracic burst amplitude to 142±11% of baseline at 120 min post-drug (P<0.001). Methysergide pretreatment abolished both cervical and thoracic 5-HT-induced LTF. Quantitative reverse transcriptase–polymerase chain reaction and immunocytochemistry revealed that 5-HT2A and 5-HT7 receptor subtypes (receptors known to influence LTF expression in adult rats) are expressed in ventral cervical and thoracic spinal cord with no differences in expression levels due to spinal segment or age. Thus, 5-HT is sufficient to induce spinal LTF in neonatal rats and differences in pattern sensitivity suggest heterogeneity in underlying mechanisms.

Section snippets

Brainstem–spinal cord preparations

The University of Wisconsin Institutional Animal Care and Use Committee approved all experimental procedures. All experiments conformed to International Guiding Principles for Biomedical Research Involving Animals as established by the Council for International Organizations of Medical Sciences. Statistical calculations were performed during the study to minimize the number of animals used to obtain valid scientific results. Sprague–Dawley (Colony 217, Harlan Sprague Dawley, Inc., Indianapolis,

Results

To avoid studies on unstable preparations, criteria were established to quantify the stability of the inspiratory burst frequency. If the inter-burst period increased to greater than three standard deviations above the average inter-burst period during baseline, it was taken as an indication that the preparation was not stable, and data from these experiments were excluded from analysis (23 of 49 preparations were excluded). The average inter-burst period at the beginning and end of experiments

Discussion

To our knowledge, this is the first demonstration of spinal 5-HT-induced LTF in vitro. Spinal 5-HT receptor activation, without hypoxia, is sufficient to induce LTF in both cervical and thoracic respiratory motor output in vitro. Although the magnitude of LTF was similar in cervical and thoracic motoneurons following episodic spinal 5-HT application, there were profound differences in pattern sensitivity between these motor outputs. Whereas cervical LTF exhibited pattern sensitivity similar to

Conclusions

5-HT is sufficient to elicit LTF in spinal respiratory motor output in vitro. The LTF magnitude is relatively similar between these motor pools. However, differential pattern sensitivity between cervical and thoracic LTF may provide a novel and highly useful model to study the mechanistic basis of pattern sensitivity in respiratory plasticity, or other important forms of pattern sensitivity in both invertebrate and vertebrate nervous systems (Pittenger and Kandel, 2003).

Acknowledgments

We would like to thank Dr. Mark Brownfield for the generous gift of the 5-HT2A and 5-HT7 antibodies. This work was supported by NIH 69064 and HL 080209. M. R. L.-B. was supported by an NIH training grant (NRSA) GM007507.

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