Fig. 4. Effects of intrathecal administration of clonidine on heroin self-administration after spinal nerve ligation or sham treatment. Mean (SEM) number of infusions administered for each dose is shown after intrathecal administration of saline or 10 μg clonidine or for sessions immediately preceding each intrathecal manipulation (baseline) in either spinal nerve–ligated ( A ) or sham ( B ) rats. Duplicate determinations were made for each intrathecal manipulation. The order of heroin dose presentation was descending and clonidine was administered 10 min before the start of the self-administration session. Administration of 30 μg produced significant sedation in both groups of animals (data not shown). Average paw withdrawal threshold was 2.8 (1.2) g for ligated rats and 14.4 (2.4) g for sham-treated rats on the days during which intrathecal manipulations were performed before intrathecal infusions or self-administration sessions. Paw withdrawal thresholds were 15 (0) g for both groups of animals immediately after all self-administration sessions. * Significantly different from baseline , P ≤ 0.05. n = 6/group.