Strain differences in Fos expression following airpuff startle in Spontaneously Hypertensive and Wistar Kyoto rats

The airpuff startle stimulus elicits both a behavioral and a concurrent sympathetic and parasympathetic activation, which have been shown to differ between inbred normotensive Wistar Kyoto and Spontaneously Hypertensive rat strains. Neither the brain sites responsible for the cardiovascular and motor responses, nor the origins of the strain differential responses, have yet been elucidated. The goals of the present study were (i) to define the neuronal pattern of immunoreactive Fos expression to the airpuff stimulus, and (ii) to determine whether this pattern of expression differed between the two contrasting inbred rat strains, thereby relating to observed differences in response. The airpuff stimulus induced Fos protein expression in discrete nuclei within the hypothalamus, thalamus, midbrain, pons and medulla of both strains, with strain-dependent differences evident in the hypothalamus (lateral, ventromedial and dorsomedial), pons (locus coeruleus) and medulla (rostroventrolateral medulla and solitary tract nuclei). To remove Fos expression arising from test chamber novelty, which was observed in both strains, a subset of animals was habituated to the test chamber for four days prior to testing. Habituation reduced Fos expression in several brain regions in the Wistar Kyoto, but failed to do so in the Spontaneously Hypertensive rat. The present results are the first to identify a set of brain regions likely to be responsible for the mediation of the cardiovascular and motor responses associated with the airpuff startle stimulus. Several of the identified areas contain neurotransmitters implicated by prior pharmacological studies. Further, these data identify differences in the degree of activation of specific neuronal structures that probably underlie strain differences in the cardiovascular response to the airpuff. Additionally, the results provide a cellular correlate to reported deficits in behavioral habituation by the Spontaneously Hypertensive rat and suggest a potentially profound difference between the ability of these two strains to adapt to repeated mild stress stimuli.