Identification of Stress‐Activated Inputs to the Dorsomedial Hypothalamus

Abstract

Acute psychological stress elicits a typical response characterized in the short term by elevated sympathetic nerve activity and increases in heart rate and blood pressure. While the primary neural circuitry that underpins these responses is still uncertain, the dorsomedial hypothalamus (DMH) is known to play a major role.1 However, the antecedent inputs responsible for DMH activation in response to stress are unknown. In the present study, retrograde tracing was combined with Fos immunohistochemistry following air jet stress (AJS) to identify neurons that both project to the DMH and are activated by stress.Sprague Dawley rats received a unilateral injection of 15–30 nl of retrograde tracer choleratoxin B subunit‐AlexaFluor 555 into the DMH. One week later rats were exposed to a 15 minute AJS protocol (n=4), while controls (n=2) were left undisturbed. Following perfusion, brain sections were processed for Fos immunohistochemistry and photographed. The distribution of retrogradely and/or Fos‐labeled neurons was mapped using a rat brain atlas.2 The number of labeled cells in key regions was counted. Data are expressed as mean ± SE cells per section.Retrogradely labeled neurons were found throughout the brainstem and forebrain (Table 1). The majority of labeled cells were located in the forebrain, in particular in the medial prefrontal cortex, lateral septal nucleus, claustrum and amygdala. Following AJS, Fos‐positive neurons were located in many, but not all, of the regions in which retrogradely labeled neurons were found. There was a significant increase in the number of Fos‐positive neurons in the Kolliker Fuse nucleus (KF) in the pons, the dorsomedial, dorsolateral and ventrolateral midbrain periaqueductal gray (PAG), amygdala, the cingulate cortex, lateral septal nucleus and the claustrum. Double‐labeled neurons were found in the brainstem in the KF and ventrolateral PAG, and in the forebrain in the amygdala, cingulate cortex, lateral septal nucleus and claustrum. However, overall there were very few double‐labeled neurons seen throughout the brain.Neurons that both project to the DMH and are activated by AJS were identified in several brain regions, particularly in the forebrain. The number of such neurons was very small, suggesting that these neurons may play a minor role in stress‐induced activation of the DMH. An alternate hypothesis is that recruitment of DMH in response to stress may be dependent upon disinhibition. Number of labeled cells in key nuclei Region CTb Fos CTb + Fos RVLM 0.7±0.4 9.2±5.1 0.0±0 RVMM 0.3±0.2 6.1±4 0.0±0 PBC 27.6±14.2 14.4±5.9 1.5±0.8 KF 20.2±14.8 11.0±4.0* 0.6±0.4* dmPAG 4.4±2.1 7.3±1* 0.2±0.2 dIPAG 4.2±1.5 10.7±0.6* 0.5±0.4 IPAG 8.6±2.7 19.9±2.4 0.3±0.1 vIPAG 12.0±1.6 18.4±3.2* 1.0±0.5* Amy 124.4±22.7 55.9±18.3* 2.4±0.7* Cingular Cor 88.8±32.1 17.0±0.3* 3.2±0.8* Lat SN 177.9±62.1 38.5±5.5* 4.9±1.8* Claustrum 148.3±55.2 35.8±11* 2.1±0.4* mPFC 192.0±79 42.4±21.1 3.3±0.8 Values are mean ± SE