Dorsomedial hypothalamic sites where disinhibition evokes tachycardia correlate with location of raphe-projecting neurons

Abstract

Disinhibition of neurons in the region of the dorsomedial hypothalamus (DMH) elicits sympathetically mediated tachycardia in rats through activation of the brain stem raphe pallidus (RP), and this same mechanism appears to be largely responsible for the increases in heart rate (HR) seen in air jet stress in this species. Neurons projecting to the RP from the DMH are said to be concentrated in a specific subregion, the dorsal hypothalamic area (DA). Here, we examined the hypothesis that the location of RP-projecting neurons in the DA correspond to the sites at which microinjection of bicuculline methiodide (BMI) evokes the greatest increases in HR. To determine the distribution of RP-projecting neurons in the DA, cholera toxin B was injected in the RP in four rats. A consistent pattern of retrograde labeling was seen in every rat. In the hypothalamus, RP-projecting neurons were most heavily concentrated midway between the mammillothalamic tract and the dorsal tip of the third ventricle dorsal to the dorsomedial hypothalamic nucleus approximately 3.30 mm caudal to bregma. In a second series of experiments, the HR response to microinjections of BMI (2 pmol/5 nl; n = 76) was mapped at sites in the DA and surrounding areas in 22 urethane-anesthetized rats. All injection sites were located from 2.56 to 4.16 mm posterior to bregma, and the microinjections that evoked the largest increase in HR (i.e., >100 beats/min in some instances) were located in a region where RP-projecting neurons were most densely concentrated. Thus RP-projecting neurons in the DA may mediate DMH-induced tachycardia and thus play a role in stress-induced cardiac stimulation.

Panic-Prone State Induced in Rats with GABA Dysfunction in the Dorsomedial Hypothalamus Is Mediated by NMDA Receptors

“…This area then was divided into dorsal and ventral halves. The DMH, as described, is particularly sensitive to BMI-induced cardioexcitatory response (Samuels et al, 2004), receives projections from circumventricular organs (CVOs) such as the organum vasculosum lamina terminalis (OVLT) (Thompson and Swanson, 1998), and has selective neurochemical changes that follow stress (Lowry et al, 2003). The single c-Fos-immunoreactive (c-Fos-IR) and c-Fos/NR1 or c-Fos/GluR2/3-IR cell counts were done in the dorsal DMH region [i.e., mainly made up of the dorsal hypothalamic area (DA) but also including part of the subincertal region (SubI)] and ventral DMH region [i.e., mainly made up of the dorsal part of the dorsomedial hypothalamic nucleus (DMD) but also including part of the perifornical region (PeF)] at Ϫ3.14 mm bregma.…”

“…1 A) (Shekhar and Keim, 1997;Samuels et al, 2004) and were verified physiologically by injecting BMI under anesthesia and obtaining increases in HR (i.e., in which BMI elicited Ն50 beats/min in HR). Although there was some tissue damage at the site of implantation, this damage was minimal and was observed equally in the control and experimental groups, with the majority of cells in the DMH/PH still intact surrounding the infusion site and stained with methyl green (data not shown).…”

“…Each rat then was anesthetized as previously indicated before the implantation of a 28 gauge cannula attached to a T-shaped Alzet osmotic minipump (catalog #3260PG, 26 gauge; Durect, Cupertino, CA) as described previously (Shekhar et al, 1996). Briefly, cannulas were directed at cardioexcitatory regions of the DMH (Samuels et al, 2004) based on the following coordinates from bregma: 1.2 mm posterior, ϩ0.5 mm lateral, ϩ8.5 mm ventral, and adjusted for approaching at a ϩ10°a ngle toward the midline, with the stereotaxic incisor bar elevated 5 mm above the interaural line. A T-shaped pump cannula with a side arm attached to a small Tygon tube was used for pump implantations.…”

Activation of the Orexin 1 Receptor is a Critical Component of CO2-Mediated Anxiety and Hypertension but not Bradycardia

“…The area of the DMH/PeF where single ORX-A-ir neurons and double c-Fos/ORX-ir neurons was counted was roughly square in dimension with the corners being the mammillothalamic tract, the fornix, the top of the third ventricle and a point located halfway down the third ventricle (immediately medial from the fornix). The DMH/ PeF, as described, is particularly sensitive to BMI-induced cardioexcitatory response (Samuels et al, 2004). All single ORX-A-ir neurons and double c-Fos/ORX-ir neurons counted that were lateral to the DMH/PeF area were considered to be in the LH region.…”

Neural Pathways Underlying Lactate-Induced Panic

“…no. 3260PG, Plastics One, Ranoake, VA) were directed at cardioexcitatory regions of the DMH (Samuels et al, 2004) based on the following stereotaxic coordinates relative to bregma (using a 101 angle from the vertical plane with the incisor bar set at + 5 mm were: anterior À1.2 mm, lateral + 2.5 mm and ventral + 7.0 mm. A total of 50 pmol per 100 nl of bicuculline methiodide (BMI), a GABA A receptor antagonist, was injected through the vertical arm of the cannula to ascertain that the tip of the cannula was placed in a cardioexcitatory region (ie where BMI elicits X50 beats per min in HR).…”