Short-term changes in incubation temperature: behavioral and physiological effects in the chick embryo from 6 to 20 days

In 3 experiments we have attempted to determine the extent to which the chick embryo is behaviorally and physiologically responsive to short term changes (.5-2 hr) in the normal ambient incubation temperature. Embryos ranging in age from 6-20 days of incubation have been examined after exposure to temperatures ranging from 30.5 degrees to 44.4 degrees C (normal incubator and nesting temperatures are 37-38 degrees C). At 6,9 and 12 days of age the heart rate and the duration of amnion contractions were significantly altered by exposure to temperature either higher or lower than normal although overt neuromuscular activity (motality) remains unchanged. At 15 and 20 days, however, (hatching occurs on Day 21) the rate of neuromusclar activity is altered at both low and high temperatures. At 20 days, beak-clapping, vocalization, and respiration rates also change reliably upon short-term exposure to both high and low temperatures. The possibility is discussed that embryonic responsiveness to temperature changes in the environment during natural incubation may play some role in later behavioral capabilities.

The role of supraspinal input in embryonic motility: a re-examination in the chick

The present experiments represent an attempt to further clarify the role of the brain in embryonic motility and behavior. By making high chronic cervical transections ("gaps") at early prefunctional stages of incubation (i.e., 40-50 hours) and studying the subsequent emergence of motility in the chick it has been possible to determine that supraspinal input is not functional until about the tenth day of incubation. Acute cervical transection results in a modification of the temporal pattern (rhythm) of motility without affecting the frequency of activity. Qualitatively the movements of spinal embryos are indistinguishable from controls up to 16-17 days. At that time there are detectable differences in the character of spontaneous movements, in reflex responsivity and in hatching behavior; spinal embryos are not able to initiate the coordinated Type III movements necessary for escape from the shell. Injection of strychnine into chronic cervical and control embryos at 10 days and at 16-17 days indicates that certain aspects of the typical strychnine response are lost following removal of brain input to the spinal cord. Finally, chronic thoracic gaps result in a clear modificaect at this age. These data suggest that propriospinal integration is present at least several days prior to the onset of supraspinal input in the chick spinal cord.

Ontogeny of behavioral sensitivity to strychnine in the chick embryo: evidence for the early onset of CNS inhibition

The development of behavioral sensitivity to strychnine has been studied in the chick embryo between day 7 of incubation and 1 day posthatching. The earliest response to systemically applied strychnine was a marked depression of spontaneous motility at high concentrations of the drug. Lower concentrations had no effect at this time (6-7 days). About 2 days later, on day 81/2 or 9, strychnine induced a statistically reliable increase in spontaneous motility (hyperactivity). This consisted of a brief (i.e. 1-2 min) excitatory response which was followed by a return to baseline activity levels in the case of low drug concentrations, or a depression of activity in the case of low drug concentrations, or a depression of activity in the case of higher concentrations. By 11 days of incubation the brief excitatory response following strychnine had increased in duration to about 4 min. This was also the case for 13-day embryos. At no time between 7 and 13 days were convulsions produced by strychnine, even at concentrations several times greater than that required to induce hyperactivity. For the first time at 16 days myoclonic convulsions were observed following strychnine. These usually began soon after the initial hyperactivity and frequently lasted for as long as 30 min. At the same time the sensitivity of the embryo to strychnine increased, compared to earlier stages. By 18 days strychnine most often induced an immediate convulsive response without the preceding brief hyperactivity. This was also typical of newly hatched chicks. The systemic application of glycine at 9 and 13 days of incubation produced a slight, but statistically reliable, depression of ongoing spontaneous motility, consistent with what one might expect if glycine were acting as an inhibitory neurotransmitter. It typically took between 3 and 4 min following injection for this glycine response to occur. Biochemical, electrophysiological and neuroanatomical evidence was reviewed in an attempt to support the suggestion that the strychnine and glycine data summarized above may reflect the presence of strychnine-sensitive postsynaptic inhibitory processes in the chick spinal cord. Additionally, comparative data on the relative onset of excitatory and inhibitory processes in the developing spinal cord were discussed. It was concluded that, although the data are still imcomplete concerning the question of whether, developmentally, inhibition is a primary or secondary acquisition, inhibitory mechanisms nevertheless appear rather early during vertebrate neurogenesis.

Amniotic contraction and embryonic motility in the chick embryo

During part of the incubation period of chick embryos the amnion shows spontaneous contractions. Removal of the amnion on days 9, 10, and 11 has no effect on the amount of, or the cyclic aspects of, motility exhibited by the embryo. These observations question the importance of the amnion and yolk-sac as stimulative factors in the initiation and maintenance of cyclic embryonic motility at the ages studied.