Insights from an ancient gymnosperm lineage: ambient temperature and light and the timing of thermogenesis in cycad cones.

PREMISE: Although maintaining the appropriate mid-day timing of the diel thermogenic events of cones of the dioecious cycads Macrozamia lucida and M. macleayi is central to the survival of both plant and pollinator in this obligate pollination mutualism, the nature of the underlying mechanism remains obscure. We investigated whether it is under circadian control. Circadian mechanisms control the timing of many ecologically important processes in angiosperms, yet only a few gymnosperms have been studied in this regard. METHODS: We subjected cones to different ambient temperature and lighting regimens (constant temperature and darkness; stepwise cool/warm ambient temperatures in constant darkness; stepwise dark/light exposures at constant temperature) to determine whether the resulting timing of their thermogenic events was consistent with circadian control. RESULTS: Cones exposed to constant ambient temperature and darkness generated multiple temperature peaks endogenously, with an average interpeak-temperature period of 20.7 (±0.20) h that is temperature-compensated (Q10 = 1.02). Exposure to 24-h ambient temperature cycles (12 h cool/12 h warm, constant darkness) yielded an interpeak-temperature period of 24.0 (±0.05) h, accurately and precisely replicating the ambient temperature period. Exposure to 24-h photo-cycles (12 h light/12 h dark, constant ambient temperature) yielded a shorter, more variable interpeak-temperature period of 23 (±0.23) h. CONCLUSIONS: Our results indicate that cycad cone thermogenesis is under circadian clock control and differentially affected by ambient temperature and light cycles. Our data from cycads (an ancient gymnosperm lineage) adds to what little is known about circadian timing in gymnosperms, which have rarely been studied from the circadian perspective.

T-type calcium channels cause bursts of spikes in motor but not sensory thalamic neurons during mimicry of natural patterns of synaptic input.

Although neurons within intact nervous systems can be classified as 'sensory' or 'motor,' it is not known whether there is any general distinction between sensory and motor neurons at the cellular or molecular levels. Here, we extend and test a theory according to which activation of certain subtypes of voltage-gated ion channel (VGC) generate patterns of spikes in neurons of motor systems, whereas VGC are proposed to counteract patterns in sensory neurons. We previously reported experimental evidence for the theory from visual thalamus, where we found that T-type calcium channels (TtCCs) did not cause bursts of spikes but instead served the function of 'predictive homeostasis' to maximize the causal and informational link between retinogeniculate excitation and spike output. Here, we have recorded neurons in brain slices from eight sensory and motor regions of rat thalamus while mimicking key features of natural excitatory and inhibitory post-synaptic potentials. As predicted by theory, TtCC did cause bursts of spikes in motor thalamus. TtCC-mediated responses in motor thalamus were activated at more hyperpolarized potentials and caused larger depolarizations with more spikes than in visual and auditory thalamus. Somatosensory thalamus is known to be more closely connected to motor regions relative to auditory and visual thalamus, and likewise the strength of its TtCC responses was intermediate between these regions and motor thalamus. We also observed lower input resistance, as well as limited evidence of stronger hyperpolarization-induced ('H-type') depolarization, in nuclei closer to motor output. These findings support our theory of a specific difference between sensory and motor neurons at the cellular level.