Changes in response properties of rostral ventromedial medulla neurons during prolonged inflammation: modulation by neurokinin-1 receptors

Abstract

Activation of neurokinin-1 (NK-1) receptors in the rostral ventromedial medulla (RVM) can facilitate pain transmission in conditions such as inflammation, and thereby contribute to hyperalgesia. Since blockade of NK-1 receptors in the RVM can attenuate hyperalgesia produced by prolonged inflammation, we examined the role of NK-1 receptors in changes of response properties of RVM neurons following four days of hind paw inflammation with complete Freund's adjuvant. Recordings were made from functionally identified ON, OFF and NEUTRAL cells in the RVM. Spontaneous activity and responses evoked by a series of mechanical (10, 15, 26, 60, 100, and 180 g) and heat (34-50 °C) stimuli applied to the inflamed and non-inflamed hind paws were determined before and at 15 and 60 min after injection of the NK-1-antagonist L-733,060 or vehicle into the RVM. Prolonged inflammation did not alter the proportions of functionally-identified ON, OFF and NEUTRAL cells. ON cells exhibited enhanced responses to mechanical (60-100g) and heat (48-50 °C) stimuli applied to the inflamed paw, which were attenuated by L-733,060 but not by vehicle. Inhibitory responses of OFF cells evoked by mechanical stimuli applied to the inflamed paw were also inhibited by L-733,060, but responses evoked by stimulation of the contralateral paw were increased. Heat-evoked responses of OFF cells were not altered by L-733,060. Also, neither L-733,060 nor vehicle altered spontaneous ongoing discharge rate of RVM neurons. These data indicate that NK-1 receptors modulate excitability of ON cells which contribute to both mechanical and heat hyperalgesia, whereas NK-1 modulation of OFF cells contributes to mechanical hyperalgesia during prolonged inflammation.

Figure 1

Prolonged inflammation increases responses of ON cells evoked by mechanical stimulation of the inflamed paw. A) Examples of responses of a single ON cell and location of its recording site in the RVM. Responses to mechanical stimuli applied to the inflamed paw were greater than those produced by stimulation of the contralateral paw. Structures of the brain stem are indicated by dashed lines according to the atlas of the rat brain (Paxinos and Watson, 1998), GiA – Gigantocellular Reticular nucleus, pars Alpha, RMg – Raphe Magnus nucleus, py – pyramidal tract. The site of recording, marked by prussian blue, is indicated by arrow. Responses are presented as frequency histograms. Mechanical stimuli of 5 sec and bending force of applied von Frey monofilaments are indicated above histograms. B) Mean (± sem) number of impulses evoked by different intensities of mechanical stimulation with von Frey monofilaments (each of 5 s duration) applied to the plantar surface of the inflamed and contralateral paws for all ON cells (n=32). * indicates a significant difference between the inflamed and contralateral (* p < 0.05, ** p < 0.01, **** p < 0.001).

Figure 2

Responses of OFF cells to mechanical stimuli during prolonged inflammation. A) Examples of mechanically-evoked responses (frequency histograms) and the recording site for a single OFF cell. Responses to mechanical stimuli did not differ between inflamed and contralateral paws. Structures of the brain stem are indicated as in Figure 1A. The recording site is indicated by the arrow. Periods of mechanical stimuli and bending force of applied by the von Frey monofilaments are indicated above the histograms. B) Mean (± sem) number of impulses evoked by different intensities of mechanical stimulation with von Frey monofilaments (each of 5 s duration) applied to the plantar surface of the inflamed and contralateral paws (n=17). There were no differences in responses evoked by stimulation of the inflamed and contralateral paws.

Figure 3

Responses of ON cells evoked by heat during prolonged inflammation. A) Examples of responses (frequency histograms) of a single ON cell to heat stimuli and the location of its recording site in the RVM. Responses were greater for stimuli applied to the inflamed paw. Structures of the brain stem are indicated as in Figure 1A. The recording site is indicated by arrow. Traces of stimulus temperatures (5-s duration) are indicated above the histograms. B) Mean (± sem) number of impulses evoked by heat stimuli applied to the inflamed and contralateral paws, Responses evoked by 48° and 50°C were greater for stimuli applied to the inflamed paw (n=27). **** indicates a significant difference between the inflamed and contralateral paws (p < 0.001).

Figure 4

Responses of OFF cells evoked by heat stimuli during prolonged inflammation. A) Examples of responses (frequency histograms) to heat and the recording site for a single OFF cell. Responses to heat stimuli did not differ between paws. Brain stem structures are indicated as in Figure 1A. The recording site is indicated by arrow. Traces of stimulus temperature (5-s duration) are provided above each histogram. B) Mean (± sem) number of impulses evoked by different intensities of heat stimulation (each of 5 s duration) applied to the plantar surface of the inflamed and contralateral paws (n=16). No differences in responses evoked by heat applied to the inflamed and contralateral paws.

Figure 5

L-733,060 decreased responses of ON cells to mechanical stimuli during prolonged inflammation The mean (± sem) number of impulses evoked by mechanical stimulation of the inflamed (A) but not the contralateral (B) paw were reduced following injection of L-733,060 in the RVM (n=20). Responses evoked by stimulation of the inflamed (C) or contralateral (D) paw were not altered following injection of vehicle into the RVM (n=6). * indicates a significant difference from baseline value at 15 min and # indicates a significant difference from baseline at 60 min (*p < 0.05, ****p < 0.001, ####p < 0.001).

Figure 6

Effects of L-733,060 injection into the RVM on responses of OFF cells to mechanical stimuli. The inhibitory response evoked by mechanical stimulation decreased when stimuli were applied to the inflamed paw (A) but increased when applied to the contralateral paw (B) following injection of L-733,060 into the RVM (n=10). Responses evoked by stimulation of the inflamed (C) or contralateral (D) paw were not altered following injection of vehicle into the RVM (n=6). Data are expressed as the mean (± sem) number of impulses. # indicate significant differences from baseline values (# p < 0.05, ### p < 0.005, #### p < 0.001).

Figure 7

Attenuation of responses of ON cells to heat stimuli following injection of L-733,060 into the RVM. The mean (± sem) number of impulses evoked by heat stimuli of 44°–50°C applied to the inflamed paw (A) but t not the contralateral paw (B) was decreased at 15 min following injection of L-733,060 (n=20). Responses to heat stimuli following injection of vehicle (n=6) into the RVM were not altered (C and D). * and # indicate a significant difference from baseline at 15 and 60 min after injection, respectively (*p < 0.05, #p < 0.05, ***p < 0.005, ****p < 0.001).

Figure 8

Effects of L-733,060 injection into the RVM on responses of OFF cells to heat stimuli. Responses of OFF cells evoked by heat stimuli applied to either paw were not altered at any time following injection of either 733,060 (A and B, n=10) or vehicle (C and D, n=6) into the RVM.