Preliminary investigation of potential links between pigmentation variants and opioid analgesic effectiveness in horses during cerebrospinal fluid centesis

Abstract

Background: The pleiotropic effects of the melanocortin system show promise in overcoming limitations associated with large variations in opioid analgesic effectiveness observed in equine practice. Of particular interest is variation in the melanocortin-1-receptor (MC1R) gene, which dictates pigment type expression through its epistatic interaction with the agouti signalling protein (ASIP) gene. MC1R has previously been implicated in opioid efficacy in other species; however, this relationship is yet to be explored in horses. In this study, analgesic effectiveness was scored (1-3) based on noted response to dura penetration during the performance of cerebrospinal fluid centisis after sedation and tested for association with known genetic regions responsible for pigmentation variation in horses.

Results: The chestnut phenotype was statistically significant (P < 0.05) in lowering analgesic effectiveness when compared to the bay base coat colour. The 11bp indel in ASIP known to cause the black base coat colour was not significant (P>0.05); however, six single nucleotide polymorphisms (SNPs) within the genomic region encoding the ASIP gene and one within MC1R were identified as being nominally significant (P<0.05) in association with opioid analgesic effectiveness. This included the location of the known e MC1R variant resulting in the chestnut coat colour.

Conclusions: The current study provides promising evidence for important links between pigmentation genes and opioid effectiveness in horses. The application of an easily identifiable phenotype indicating variable sensitivity presents a promising opportunity for accessible precision medicine in the use of analgesics and warrants further investigation.

Keywords: ASIP; Horse; MC1R; Opioid; Pigmentation; Sensitivity.

Fig. 1

Distribution of sex, stratified by breed^†. Purple = Quarter horse (n = 23), Grey = Warmblood (n = 9), Orange = Thoroughbred (n = 13), Stripes = female, Solid = male^†. Iberian, Standardbred, & Arabian horses were excluded (n = 4)

Fig. 2

Boxplots showing a) Hydromorphone dosage rate^† stratified by breed^‡, b) Hydromorphone dosage rate^† stratified by sex, c) age (in years) stratified by sex, and d) age (in years) stratified by analgesic effectiveness score. Purple = Quarter horse, Grey = Warmblood, Orange = Thoroughbred, Stripes = female, Solid = male., Black = analgesic effectiveness score 1, Pink = analgesic effectiveness score 2, Brown = analgesic effectiveness score 3.^† Target range 0.01–0.02 mg/kg BW intravenously. ‡ Iberian, Standardbred, & Arabian horses were excluded (n = 4)

Fig. 3

Distribution of analgesic effectiveness (AE) stratified by breed^† (a) and sex (b). ^† Iberian, Standardbred, & Arabian horses were excluded (n = 4). Grey = Warmblood, Orange = Thoroughbred, Purple = Quarter horse, Stripes (red) = female, Solid (blue) = male

Fig. 4

SNP association analysis on chromosome 3 before (a) and after (b) the addition of covariates (age, breed^†, sex, and hydromorphone dosage rate) using a generalized linear model. The significant MC1R location on chromosome 3 (g.36979560) (outlined in red) alone accounted for 14.8% of explained variance before the addition of covariates. Covariates alone accounted for 8.7% of explained variance. The MC1R SNP with the addition of covariates accounted for 18%. ^† Iberian, Standardbred, & Arabian horses were excluded (n = 4)

Fig. 5

SNP association analysis on chromosome 22 before (a) and after (b) the addition of covariates (age, breed^†, sex, and hydromorphone dosage rate) using generalizes linear model. ASIP significant SNPs alone accounted for 15% of variation before the addition of covariates. ASIP significant SNPs in addition to covariates accounted for 45% explained variance. ^† Iberian, Standardbred, & Arabian horses were excluded (n = 4)