Sex differences in hypothalamic-mediated tonic norepinephrine release for thermal hyperalgesia in rats

Prazosin as an Adjuvant to Increase Effectiveness of Duloxetine in a Rat Model of Oxaliplatin-Induced Peripheral Neuropathy

OBJECTIVES

Duloxetine, the only American Society of Clinical Oncology (ASCO) treatment recommended for chemotherapy-induced peripheral neuropathy (CIPN) in cancer survivors, is not effective for 40% of survivors. This study examined the ability of a duloxetine-prazosin combination to prevent the development of allodynia and hyperalgesia in a rat model of oxaliplatin-induced peripheral neuropathy (OPIN).

METHODS

Female (n = 24) and male (n = 41) rats were started on duloxetine (15 mg), prazosin (2 mg), or a duloxetine-prazosin combination one week prior to administration of the chemotherapy drug, oxaliplatin, and continued the duloxetine-prazosin combination for 32 days. Behavioral testing for mechanical allodynia and mechanical hyperalgesia was done with selected von Frey filaments over the course of the study.

RESULTS

Overall percent paw withdrawal for rats that received the duloxetine-prazosin combination was significantly lower in female (p < .001 for both conditions) and male (p = .029 for allodynia; p < .001 for hyperalgesia) than those that received water. No significant posttreatment differences were found for allodynia or hyperalgesia between rats treated with duloxetine and rats that received the duloxetine-prazosin combination in either sex.

CONCLUSIONS

These finding provide preliminary evidence that a duloxetine-prazosin combination can prevent the posttreatment development of allodynia and hyperalgesia in both male and female rats; however, the results suggest that the duloxetine-prazosin combination is no more efficacious than duloxetine alone in preventing chronic OIPN.

IMPLICATIONS FOR NURSING PRACTICE

The profession of nursing is built on clinical practice supported by scientific research. The current study addressed the clinical practice problem of prevention and management of painful OIPN, which is a priority area in oncology nursing.

Effectiveness of Duloxetine on Oxaliplatin-induced Allodynia and Hyperalgesia in Rats

Development of painful oxaliplatin-induced peripheral neuropathy (OIPN) is a major problem in people who receive oxaliplatin as part of cancer treatment. The pain experienced by those with OIPN can be seriously debilitating and lead to discontinuation of an otherwise successful treatment. Duloxetine is currently the only recommended treatment for established painful OIPN recommended by the American Society of Clinical Oncology, but its preventative ability is still not clear. This study examined the ability of duloxetine to prevent signs of chronic OIPN in female (n = 12) and male (n = 21) rats treated with the chemotherapeutic agent oxaliplatin. Using an established model of OIPN, rats were started on duloxetine (15 mg) one week prior to oxaliplatin administration and continued duloxetine for 32 days. Behavioral testing for mechanical allodynia and mechanical hyperalgesia was done with selected von Frey filaments. Significant posttreatment differences were found for allodynia in female (p = .004), but not male rats. Duloxetine was associated with significant differences for hyperalgesia in both female (p < .001) and male (p < .001) rats. These findings provide preliminary evidence of the preventative effects of duloxetine on both oxaliplatin-induced allodynia and hyperalgesia in male and female rats, with a difference noted in response between the sexes.

The role of medial prefrontal cortex projections to locus ceruleus in mediating the sex differences in behavior in mice with inflammatory pain

We tested the hypothesis that the cognitive impairment associated with inflammatory pain may result from dysregulation of the top-down control of locus ceruleus's (LC) activity by the medial prefrontal cortex (mPFC). Injection of complete Freund's adjuvant (CFA) served as a model for inflammatory pain. The CFA injection decreased the thermal thresholds in both sexes but only the male mice showed increased anxiety-like behavior and diminished cognition, while the females were not affected. Increased calcium fluorescence, a marker for neuronal activity, was detected by photometry in the mPFC of males but not in females with CFA. Next, while chemogenetic inhibition of the projections from the mPFC to the LC improved the object recognition memory of males with pain, the inhibition of the mPFC to LC pathway in female mice produced anxiolysis and spatial memory deficits. The behavior results prompted us to compare the reciprocal innervation of mPFC and LC between the sexes. We used an anterograde transsynaptic tagging technique, which relies on postsynaptic cre transfer, to assess the innervation of LC by mPFC efferents. The males showed a higher rate of postsynaptic cre transfer into LC neurons from mPFC efferents than the females. And vice versa, a retrograde tracing experiment demonstrated that LC to mPFC projection neurons were more numerous in females when compared to males. In conclusion, we provide evidence that subtle differences in the reciprocal neuronal circuit between the LC and mPFC may contribute to sex differences associated with the adverse cognitive effects of inflammatory pain.

Pain condition and sex differences in the descending noradrenergic system following lateral hypothalamic stimulation

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LH stimulation produced pronociceptive and antinociceptive effects from alpha-adrenoceptors in naïve male and female rats.

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LH stimulation produced pronociceptive and antinociceptive effects from alpha-adrenoceptors in male CCI rats.

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LH stimulation produced alpha-adrenoceptor-mediated pronociception, but not antinociception in female CCI rats.

The lateral hypothalamus (LH) is known to modulate nociception via the descending noradrenergic system in acute nociception, but less is known about its role in neuropathic pain states. In naïve females, LH stimulation produces opposing effects of α-adrenoceptors, with α₂-adrenoceptors mediating antinociception, while pronociceptive α₁-adrenoceptors attenuate the effect. Whether this opposing response is seen in neuropathic conditions or in naïve males is unknown. We used a mixed factorial design to compare male and female rats with chronic constriction injury (CCI) to naïve rats, measured by Total Paw Withdrawal (TPW) responses to a thermal stimulus. Rats received one of three doses of carbachol to stimulate the LH followed by intrathecal injection of either an α₁- or an α₂-adrenoceptor antagonist (WB4101 or yohimbine, resp.) or saline for control. Overall, naïve rats showed a more pronounced opposing alpha-adrenergic response than CCI rats (p < 0.04). Naïve male and female rats demonstrated antinociception following α₁-adrenoceptor blockade and hyperalgesia following α₂-adrenoceptor blockade. Male CCI rats also showed dose dependent effects from either WB4101 or yohimbine (p < 0.05), while female CCI rats had significant antinociception from WB4101 (p < 0.05), but no effect from yohimbine. These results support the idea that peripheral nerve damage differentially alters the descending noradrenergic modulatory system in male and female rats, and notably, that female CCI rats do not show antinociception from descending noradrenergic input. These findings are suggestive that clinical therapies that recruit the descending noradrenergic system may require a different approach based on patient gender.

Anatomical evidence for lateral hypothalamic innervation of the pontine A7 catecholamine cell group in rat

Substantial behavioral evidence exists to support the idea that the lateral hypothalamus (LH) makes axonal connection with spinally-projecting noradrenergic neurons of the A7 catecholamine cell group in the pons. Through this putative projection, the LH modulates nociception via α_1- and α₂-adrenoceptors in the dorsal horn. We used double-label immunocytochemistry to demonstrate that axons from the LH labeled with the anterograde tracer biotinylated dextran amine (BDA) appose tyrosine hydroxylase-immunoreactive (TH-ir) neuron profiles in the A7 area. Other pontine areas labeled with BDA included the dorsomedial tegmental area, the pontine reticular nucleus, oral part, the caudal aspect of the dorsal raphe, the periaqueductal grey and the A6 area. To confirm the findings of the brightfield experiment, we used confocal microscopy to identify axons from the LH labeled with the anterograde tracer Fluoro-Ruby co-localized with TH-ir dendrites and cell bodies in the A7 cell group. These findings provide an anatomical substrate for behavioral studies in which stimulation of the LH modifies nociception in the spinal cord via norepinephrine.