The nucleus paragigantocellularis latralis (LPGi) is located in the rostral ventrolateral medulla (RVLM), a brainstem region that regulates homeostatic functions such as blood pressure and cardiovascular reflexes, respiration, pain and opiate withdrawal syndrome. LPGi has many anatomical relationships with important nuclei such as periaqueductal gray (PAG), locus coeruleus (LC), dentate and raphe. In the present study we have examined the role of LPGi in the conditioned place preference (CPP) induced by morphine in the presence and absence of clonidine in the rat.
We used 45 male N-MRI rats. Animals were divided into seven groups: (1) control; (2) control + saline; (3) sham; (4) LPGi lesion; (5) lesion + 0.02 mg kg-1 clonidine; (6) lesion + 0.2 mg kg-1 clonidine; and (7) lesion + 2 mg kg-1 clonidine. LPGi nucleus was destroyed bilaterally by electrical lesion under ketamine (100 mg kg-1) and Rampone (3 mg kg-1), I.P., anaesthesia. Data were analysed using Student’s paired t test. After the recovery period, CPP was induced by Hand’s method (Hand et al. 1989). One hour before testing, clonidine was administered (I.P.).
We did not find any significant differences between the results of control, control + saline and sham groups in the CPP test. There was a significant increase in the CPP between sham and lesion + saline groups (P < 0.019). There was a significant decrease between lesion + saline and lesion + clonidine (0.02, 0.2 and 2 mg kg-1) groups (P < 0.002) in the CPP test.
LC is an important noradrenergic nucleus in the brain and is involved in memory, pain and anxiety. LPGi sends its major excitatory projections to the LC. Lesion of LPGi decreases the activity of LC. It seems that increase of CPP in the LPGi lesioned group is related to the decrease of LC activity. LPGi plays a modulatory role in the CPP. In the intact animals, clonidine did not alter CPP at all, but it decreased CPP significantly (P < 0.002) in all the lesioned groups. Our results indicate that LPGi lesion induced α2 adrenergic sensitivity in the LC and follower centres.
Mohammad pour Kargar was a MSc Student at the Ahwaz College of Science.