Neurochemistry of neurons in the ventrolateral medulla activated by hypotension: Are the same neurons activated by glucoprivation?

Abstract

Previous studies have demonstrated that a range of stimuli activate neurons, including catecholaminergic neurons, in the ventrolateral medulla. Not all catecholaminergic neurons are activated and other neurochemical content is largely unknown hence whether stimulus specific populations exist is unclear. Here we determine the neurochemistry (using in situ hybridization) of catecholaminergic and noncatecholaminergic neurons which express c-Fos immunoreactivity throughout the rostrocaudal extent of the ventrolateral medulla, in Sprague Dawley rats treated with hydralazine or saline. Distinct neuronal populations containing PPCART, PPPACAP, and PPNPY mRNAs, which were largely catecholaminergic, were activated by hydralazine but not saline. Both catecholaminergic and noncatecholaminergic neurons containing preprotachykinin and prepro-enkephalin (PPE) mRNAs were also activated, with the noncatecholaminergic population located in the rostral C1 region. Few GlyT2 neurons were activated. A subset of these data was then used to compare the neuronal populations activated by 2-deoxyglucose evoked glucoprivation (Brain Structure and Function (2015) 220:117). Hydralazine activated more neurons than 2-deoxyglucose but similar numbers of catecholaminergic neurons. Commonly activated populations expressing PPNPY and PPE mRNAs were defined. These likely include PPNPY expressing catecholaminergic neurons projecting to vasopressinergic and corticotrophin releasing factor neurons in the paraventricular nucleus, which when activated result in elevated plasma vasopressin and corticosterone. Stimulus specific neurons included noncatecholaminergic neurons and a few PPE positive catecholaminergic neuron but neurochemical codes were largely unidentified. Reasons for the lack of identification of stimulus specific neurons, readily detectable using electrophysiology in anaesthetized preparations and for which neural circuits can be defined, are discussed.

Insulin‐responsive autonomic neurons in rat medulla oblongata

“…No information was provided about cross‐reactivity with the protein product of FRA1. In further confirmation of the specificity of sc‐253, we have found that this antibody shows a distribution of 2DG‐activated C1 and C3 neurons in rats (Korim, Llewellyn‐Smith, & Verberne, ; Menuet et al, ) that is very similar to the distribution of 2DG‐responsive neurons localized with an Arnel anti‐Fos antiserum that we have used previously (Ritter et al, ) and to the distribution detected by others with a different SCBT anti‐Fos that was raised against amino acids 3–16 of the protein encoded by human c‐fos (Parker et al, ; Parker et al, ). Staining with SCBT anti‐Fos sc‐253 also reveals virtually the same distribution of hypotension‐responsive C1 neurons in normotensive WKY rats as our sheep anti‐Fos (Minson, Arnolda, Llewellyn‐Smith, Pilowsky, & Chalmers, ) and produces a similar staining pattern to other anti‐Fos antisera that have been used for functional neuroanatomical studies on barosensitive C1 neurons in rats (e.g., Chan & Sawchenko, ; Stornetta, Sevigny, Schreihofer, Rosin, & Guyenet, ; Sved, Mancini, Graham, Schreihofer, & Hoffman, ).…”

“…These results are consistent with the conclusion that RVLM non‐C1 neurons do not respond to insulin treatment and therefore may not play a significant role in glucoregulatory responses. In contrast, Parker et al have reported that treatment with 2DG induces Fos synthesis in a significant number of non‐C1 neurons (Parker et al, ; Parker et al, ). There are several possible methodological causes for the divergence between our Fos results with insulin and those of Parker et al with 2DG.…”

“…Observations made here suggest a trend for insulin‐sensitive C1 RVLM neurons to be located in the medial and middle portions of the C1 column whereas our experience is that hypotension‐sensitive C1 RVLM neurons localized with the same anti‐Fos antibody often occur in lateral parts of the column (for example, Burman et al, ; Chan & Sawchenko, ; Minson et al, ). Arguing against the existence of distinct populations of RVLM C1 neurons that respond to metabolic versus cardiovascular stimuli is a recent comparison of the neurochemical phenotypes of neurons in the ventrolateral medulla that were activated by hydralazine versus 2DG (Parker et al, , ). This analysis suggested that at least some of the RVLM neurons were activated by both stimuli.…”

“…Indeed, plasma adrenaline has been shown to increase following hydralazine administration (Vollmer, Balcita‐Pedicino, Debnam, & Edwards, ). Another possible reason for Parker et al’s (, ) finding that C1 neurons respond to more than one stimulus is that C1 neurons are interconnected (Agassandian, Shan, Raizada, Sved, & Card, ). Because Fos is a poor marker for timing of activation, a connection between a “cardiovascular” C1 neuron and a “glucoregulatory” C1 neuron could lead to Fos expression in a 2DG‐responsive C1 neuron when a hydralazine‐responsive C1 neuron is activated.…”

“…Falls in blood pressure activate a significant number of non‐C1 neurons in the RVLM (Burman et al, ; Parker et al, ; Schreihofer & Guyenet, ), implicating them in cardiovascular control. We have found previously that, in the RVLM, non‐C1 neurons identified by the fast conduction velocities of their spinal axons do not respond to 2DG (Verberne & Sartor, ).…”