Effects of neonatal capsaicin desensitization on sensory functions.

<p>Rats were treated with saline (Control) or with capsaicin (Desensitized) at 14 days of life. Sensory functions were measured 10 weeks after capsaicin treatment. (A) Eye wiping (a measure of capsaicin evoked sensory irritation) was reduced in desensitized rats (n = 10 in both groups, p<0.001). (B) Plasma extravasation was similarly reduced (Evans blue accumulation, a measure of capsaicin mediated neurogenic inflammation) (n = 9 for Control and n = 5 for Desensitized, p = 0.03).</p

Effects of neonatal capsaicin desensitization on sensory TRPV1 expression.

<p>TRPV1 expression was examined in the dorsal root ganglia of control and capsaicin desensitized (neonatal capsaicin treatment) adult rats. Tissue sections were probed with antibodies specific to TRPV1 (red) and neurofilament (green). TRPV1 expression was present in a subset of sensory neurons in control rats, which was missing in desensitized rats within the dorsal root ganglia. However, TRPV1 immunoreactivity was also found in non-neuronal cell types, located at the outer regions of ganglia (regions occupied by sensory neurons are indicated by the freehand drawing). Images are representative of at least 10 sections.</p

Functional effects of neonatal capsaicin desensitization on arteriolar TRPV1.

<p>Effects of neonatal capsaicin desensitization were characterized on the functional properties of isolated, cannulated skeletal muscle arterioles. Endothelial responses were tested by the application of acetylcholine (Panel A, values normalized to the maximal acetylcholine mediated dilations, symbols represent the mean ± SEM, n = 4 for the control and n = 4 for the desensitized). Smooth muscle functions were assessed by norepinephrine (Panel B, symbols represent the mean ± SEM of n = 4 determinations in both cases). The spontaneous myogenic tone (Panel C, symbols represent the mean ± SEM of n = 4 determinations for all groups) was tested by the contractile response developing in response to increasing intraluminal pressure (from 20 mmHg to 120 mmHg in 20 mmHg increments). Finally, capsaicin mediated responses were also tested by the cumulative application of the drug. Capsaicin evoked a dose-dependent constriction, supporting a physiological role for this receptor in vascular smooth muscle cells, which was not affected by the desensitization protocol (Panel D, symbols represent the mean ± SEM of n = 4 determinations for both groups).</p

TRPV1 immunoreactivity in vascular smooth muscle cells.

<p>Rat dorsal root ganglia sections were co-stained by anti TRPV1 (red) and anti vascular smooth muscle actin (SMA, green) antibodies. Some of the TRPV1 positive cells were co-stained by the antibody specific to vascular smooth muscle actin, suggesting TRPV1 expression in this cell type. Moreover, vascular TRPV1 expression was not affected by neonatal capsaicin treatment (note similar TRPV1 staining in SMA positive regions of control and desensitized rats). Images are representative of at least 6 sections.</p

Effects of neonatal capsaicin desensitization on vascular TRPV1 expression.

<p>Skeletal muscle (gracilis muscle of the rat) tissue sections were stained with anti-TRPV1 (red) and anti-vascular smooth muscle actin (SMA, green) antibodies. There appeared a complete overlap of the staining patterns (merged images). Arteriolar TRPV1 expression was not affected by neonatal capsaicin desensitization, similarly to the vascular TRPV1 in the dorsal root ganglia. Images are representative of at least 10 sections.</p

Functional effects of acute capsaicin desensitization on vascular TRPV1.

<p>The short-term effects of capsaicin were tested here. Arterioles were isolated from control rats and were treated with capsaicin (1 µM, First treatment). Capsaicin was washed away after the 20 min long treatment and the arterioles were incubated in the physiological buffer alone for 40 min (regeneration). Then capsaicin effects we re-tested (Second treatment) to estimate the level of tachyphylaxis (n = 6 arteries were tested, symbols represent the mean ± SEM of time-matched arteriolar diameter). Acute desensitization to capsaicin was apparent upon the first treatment (decrease and then increase in arteriolar diameter in the continuous presence of capsaicin, maximal decrease was 70±3% at 95 s after capsaicin application, Panel A). The magnitude of the response was decreased upon the second capsaicin treatment on the same arterioles (tachyphylaxis, decreased response to repeated capsaicin stimuli, maximal decrease was 27±9% at 125 s after capsaicin application, Panel A). This acute capsaicin desensitization (20 min treatment) did not affect norepinephrine mediated constrictions (symbols represent the mean ± SEM of n = 6 determinations, Panel B) or acetylcholine mediated dilations (symbols represent the mean ± SEM, n = 6, Panel C).</p