The posterior hypothalamus exerts opposing effects on nociception via the A7 catecholamine cell group in rats

Abstract

Stimulation of the posterior hypothalamic area (PH) produces antinociception in rats and humans, but the precise mechanisms are unknown. The PH forms anatomical connections with the parabrachial area, which contains the pontine A7 catecholamine cell group, a group of spinally projecting noradrenergic neurons known to produce antinociception in the dorsal horn. The aim of the present study was to determine whether PH-induced antinociception is mediated in part through connections with the A7 cell group in female Sprague-Dawley rats, as measured by the tail flick and foot withdrawal latency. Stimulation of the PH with the cholinergic agonist carbachol (125 nmol) produced antinociception that was blocked by pretreatment with atropine sulfate. Intrathecal injection of the α(2)-adrenoceptor antagonist yohimbine reversed PH-induced antinociception, but the α(1)-adrenoceptor antagonist WB4101 facilitated antinociception. Intrathecal injection of normal saline had no effect. In a separate experiment, cobalt chloride, which reversibly arrests synaptic activity, was microinjected into the A7 cell group and blocked PH-induced antinociception. These findings provide evidence that the PH modulates nociception in part through connections with the A7 catecholamine cell group through opposing effects. Antinociception occurs from actions at α(2)-adrenoceptors in the dorsal horn, while concurrent hyperalgesia occurs from actions of norepinephrine at α(1)-adrenoceptors. This hyperalgesic response likely attenuates antinociception from PH stimulation.

Fig. 1

The location of carbachol microinjection sites in the PH for tail response latencies in Experiment 1. Most of the injection sites were located within the border of the PH between AP −3.60 and −3.80 mm from bregma. The symbols represent the values of the differences between baseline tail response latencies and those at 15 min after microinjection of carbachol (125 nmol) as follows: (△) 2–3.9 sec; (△) 4–5.9 sec; (□) 6–8 sec. Injection sites located in the lateral hypothalamus (●) and the hippocampus (●) elicited antinociceptive responses and were excluded from data analysis. AMG, amygdala; ic, internal capsule; LH, lateral hypothalamus; LV, lateral ventricle; PH, posterior hypothalamus; VM, ventromedial thalamic nucleus; VPL, ventral posterolateral thalamic nucleus; VPM; ventral posteromedial thalamic nucleus; ZI, zona incerta.

Fig. 2

Microinjection of carbachol in the PH produced time-dependent increases in nociceptive tail response latencies. Following a baseline response latency measurement, normal saline or carbachol (125 nmol) was microinjected into the PH at time 0. Time-course changes in (A) tail response latencies or (B) foot withdrawal latencies produced by microinjection of saline (□; n = 6) or carbachol (○; n = 6). In a separate group of rats (◇; n = 9), pretreatment with atropine in the PH reduced tail flick latencies and blocked foot withdrawal latencies following carbachol microinjection. Mean response latencies ± SEM. *p< 0.05, carbachol vs saline; #p < 0.05, carbachol vs. atropine.

Fig. 3

Antinociception from microinjection of carbachol (125 nmol) in the PH was blocked by intrathecal injection of the α₂-adrenoceptor antagonist yohimbine, but facilitated by injection of the α₁-adrenoceptor antagonist WB4101. (A) Carbachol (Carb) was injected at -17 min and three post drug tail response measurements were made at -15, -10 and -5 min. Either Yohimbine (YOH, □, n = 10), WB4101, (○, n = 9) or saline (◆, n = 7) was injected intrathecally at time -1. (B) Time course of the effects of intrathecal injection of either YOH, WB4101, or saline following carbachol microinjection in the PH on foot withdrawal response latencies. Mean response latencies ± SEM. * p < 0.05.

Fig. 4

The location of microinjection sites in the PH and A7 cell group for tail response latencies in Experiment 3. Most of the injection sites for carbachol/cobalt chloride (A) and carbachol/saline (B) were located within the PH between AP −3.60 and −4.16 from bregma at 10 min post carbachol injection. The symbols represent the values of the differences between baseline tail response latencies and those at 15 min after microinjection of carbachol (125 nmol) as follows: (△) 2–3.9 sec; (○) 4–5.9 sec; (□) 6–8 sec. Injection sites located in the cortex, dorsal thalamus, zona incerta (■) and ventral hypothalamus (●) elicited antinociceptive responses and data were excluded from analysis. Microinjections of cobalt chloride (C; ○) and saline (D; ●) were located in the A7 area (✶) approximately AP −8.72 – 10.04 from bregma. A7, A7 catecholamine cell group; Aq, Aqueduct of Sylvius; AMG, amygdala; Ic, inferior colliculus ic, internal capsule; LH, lateral hypothalamus; LV, lateral ventricle; MCP, middle cerebellar peduncle; PH, posterior hypothalamus; pyr, pyramids; SCP, superior cerebellar peduncle;VM, ventromedial thalamic nucleus; VPL, ventral posterolateral thalamic nucleus; VPM; ventral posteromedial thalamic nucleus; ZI, zona incerta.

Fig. 5

Antinociception produced by microinjection of carbachol in the PH was blocked by microinjection of cobalt chloride in the A7 area. (A) Following a baseline measurement at -25 min, carbachol (Carb) was microinjected in the PH and four tail response latencies were measured. Either cobalt chloride (○; CoCl; n = 11) or saline (●; n = 13) was microinjected in the A7 area. Cobalt chloride significantly reduced tail flick latencies across all time points compared to saline controls (p < 0.05). Mean latency values ± SEM are plotted on ordinate as a function of time (min). (B) Cobalt chloride microinjected in the A7 area also blocked PH-mediated antinociception on the foot withdrawal test (○; p < 0.05). Normal saline microinjected near the A7 area (●) did not alter nociceptive responses.