Organum vasculosum laminae terminalis contributes to increased sympathetic nerve activity induced by central hyperosmolality

Abstract

The contribution of the organum vasculosum laminae terminalis (OVLT) in mediating central hyperosmolality-induced increases of sympathetic nerve activity (SNA) and arterial blood pressure (ABP) was assessed in anesthetized rats. Solutions of graded NaCl concentration (150, 375, and 750 mM) were injected (150 mul) into the forebrain vascular supply via an internal carotid artery (ICA). Time-control experiments (n = 6) established that ICA NaCl injections produced short-latency, transient increases of renal SNA (RSNA) and mean ABP (MAP) (P < 0.05-0.001). Responses were graded, highly reproducible, and unaltered by systemic blockade of vasopressin V1 receptors (n = 4). In subsequent studies, stimulus-triggered averaging of RSNA was used to accurately locate the OVLT. Involvement of OVLT in responses to ICA NaCl was assessed by recording RSNA and MAP responses before and 15 min after electrolytic lesion of the OVLT (n = 6). Before lesion, NaCl injections increased RSNA and MAP (P < 0.05-0.001), similar to time control experiments. After lesion, RSNA responses were significantly reduced (P < 0.05-0.001), but MAP responses were unaltered. To exclude a role for fibers of passage, the inhibitory GABA-A receptor agonist muscimol was microinjected into the OVLT (50 pmol in 50 nl) (n = 6). Before muscimol, hypertonic NaCl increased RSNA, lumbar SNA (LSNA), and MAP (P < 0.05-0.001). After muscimol, both RSNA and LSNA were significantly reduced in response to 375 and 750 mM NaCl (P < 0.05). MAP responses were again unaffected. Injections of vehicle (saline) into OVLT (n = 6) and muscimol lateral to OVLT (n = 5) each failed to alter responses to ICA NaCl. We conclude that OVLT neurons contribute to sympathoexcitation by central hyperosmolality.

Fig. 1

Time-control experiments in untreated rats showing effects on arterial blood pressure (ABP), renal sympathetic nerve activity (RSNA), and integrated RSNA of repeated internal carotid artery (ICA) injections of graded concentrations of NaCl. A: ICA NaCl injections elicited graded increases of RSNA and ABP (left). Note that responses to the first series (series 1) and second series (series 2) of ICA injections were highly reproducible. B: group data (n = 6) showing that series 1 (open bars) and series 2 (solid bars) ICA injections each produced similar graded increases of RSNA (left) and mean arterial pressure (MAP) (right). *P < 0.001 vs. 150 mM NaCl, †P < 0.01 vs. 375 mM NaCl, ‡P < 0.001 vs. 375 mM NaCl.

Fig. 2

Effects on ABP, RSNA, and integrated RSNA of repeated ICA injections of graded concentrations of NaCl before and during systemic blockade of vasopressin V1 receptors. A: prior to blockade of V1 receptors, the first series of ICA NaCl injections elicited graded increases of RSNA and ABP (left). Responses to the second series of ICA injections during V1 receptor blockade were unaltered (right). B: group data (n = 4) showing that RSNA (left) and MAP (right) responses were similar before (series 1, open bars) and during (series 2, solid bars) systemic V1 receptor blockade *P < 0.001 vs. 150 mM NaCl, †P < 0.05 vs. 375 mM NaCl, ‡P < 0.01 vs. 150 mM NaCl, §P < 0.001 vs. 375 mM NaCl.

Fig. 3

Stimulus-triggered averages of RSNA used to determine the location of the OVLT. Stimulating sites >0.2 mm rostral to the organum vasculosum laminae terminalis (OVLT) caused no obvious peak in the RSNA average (middle, top trace). Stimulating within ~0.1 mm rostral of OVLT (middle, second trace) produced a constant latency increase of RSNA with a maximal response (middle, third trace) being evoked from a site corresponding to the OVLT (see Fig. 7 for histology). Stimuli directed ≥0.1 mm lateral (right and left) and ~0.1 mm ventral (middle, bottom trace) to the site of maximum response evoked little or no RSNA response. The latency to the peak RSNA average response following OVLT stimulation averaged 157 ± 9 ms (n = 16).

Fig. 4

Effects on ABP, RSNA, and integrated RSNA of ICA injections of graded concentrations of NaCl before and after electrolytic lesion of the OVLT. A: before lesion (left), injection of isotonic NaCl had no effect on recorded variables, whereas hypertonic NaCl (375 and 750 mM) produced graded increases in ABP and RSNA. After lesioning the OVLT (right), RSNA responses to hypertonic NaCl were attenuated. Pressor responses were unaffected. B: group data (n = 6) comparing effects of OVLT lesions on MAP and RSNA responses to ICA NaCl. Compared with control responses before OVLT lesion (open bars), increases in RSNA evoked by 375 mM and 750 mM NaCl were significantly reduced afterward (solid bars). Increases in MAP were not affected. C: histological section (50 μm) through the OVLT region showing a typical lesion. LV, lateral cerebral ventricle; AC, anterior commissure. *P < 0.05 vs. control, **P < 0.01 vs. control.

Fig. 5

Effects on ABP, integrated RSNA, and integrated LSNA of ICA injections of graded concentrations of NaCl before and after chemical inhibition of the OVLT. A: microinjection of vehicle into the OVLT had no effect on responses to ICA hypertonic NaCl (top, left vs. right). In contrast, responses were blunted following microinjection of the GABA-A receptor agonist muscimol into the OVLT (bottom, left vs. right). B: summary data show that increases in LSNA were significantly greater than increases in RSNA. For the group, vehicle injections did not alter responses to ICA NaCl (n = 6), whereas muscimol significantly reduced both LSNA and RSNA responses without altering pressor responses (n = 6). *P < 0.05 vs. control, **P < 0.01 vs. control. †P < 0.001 vs. corresponding RSNA response.

Fig. 6

Effects on ABP, integrated RSNA, and integrated LSNA of ICA injections of graded concentrations of NaCl before and after microinjecting muscimol ~200 μm lateral to the OVLT. A: prior to muscimol microinjection (top), ICA hypertonic NaCl (375 and 750 mM) injections caused graded increases in ABP, RSNA, and LSNA. Isotonic NaCl had no affect. Responses 15 min after microinjection of muscimol lateral to the OVLT were not different from control. B: group data (n = 5) reveal that muscimol microinjected outside the OVLT had no effect on MAP, RSNA, or LSNA responses to ICA hypertonic NaCl. †P < 0.01 vs. corresponding RSNA response.

Fig. 7

Histological verification of OVLT microinjection sites. A: section through the forebrain showing a typical microinjection site. Note that the track of tissue damage caused by the stimulating electrode/glass microinjector ends just dorsal to the OVLT and toward its rostral-most extent. B: schematic drawings of microinjection sites targeting the ventral lamina terminalis/OVLT region. Black regions represent the overlapping distributions of Chicago sky blue dye for all rats receiving muscimol microinjections targeting the OVLT. Vehicle injections had a similar distribution (see Fig. 5 for response data). Gray regions indicate the distribution of muscimol injected lateral to the OVLT (see Fig. 6 for response data). The 0.0 mm plane (middle) represents the site at which OVLT stimulation evoked the peak RSNA response.