Effect of intracerebroventricular injection of GABA receptors antagonists on morphine-induced changes in GABA and GLU transmission within the mPFC: an in vivo microdialysis study

Abstract

Objectives: Many studies have focused on ventral tegmental area than of other mesocorticolimbic areas, and implicated a key role for the medial prefrontal cortex (mPFC) in the development of addictive behaviors. So far, the role of gamma-aminobutyric acid (GABA) receptors in the discriminative properties of morphine has received little attention and few studies evaluated the role of these receptors in drug dependence. Hence, we investigated the role of this receptor on morphine- induced GABA/ glutamate (GLU) changes in the mPFC following morphine administration using in vivo microdialysis.

Materials and methods: In this study, 60 rats weighing 270-300 g were divided into six groups. First, microdialysis probe was inserted into the mPFC and was perfused with artificial cerebrospinal fluid and collected the baseline samples in all groups. In saline and morphine groups, the saline, in phaclophen and (phaclofen+morphine) groups, phaclofen (100 nmol), and in bicuculline and (bicuculline+morphine) groups, bicuculline (20 nmol) was injected intracerebroventricular. In saline, phaclofen and bicuculline groups 20 min later, animals received saline (0.2 ml, IP) and others groups received morphine (20 mg/kg, IP).

Results: Our results showed that morphine increased the average concentration of GABA and decreased the concentration of GLU within mPFC. Pretreatment with phaclofen and bicuculline 20 min before morphine administration had no effect on GABA and GLU release for 100 min.

Conclusion: The present study indicated that morphine influence the GABA and GLU transmission in mPFC. Therefore evaluation of neurochemistry changes of this neural circuitry may provide further insight into the mechanisms underlying drug dependence.

Keywords: Addiction; GABA-A receptor-antagonists; GABA-B receptor-antagonists; Morphine; Prefrontal cortex.

Figure 1

Schematic coronal section of rat brain adapted from an atlas (Paxinos and Watson, 2005). (A) Photomicrograph scan of a coronal section (50 μm) showing the probe site in the medial prefrontal cortex (mPFC). (B) The animals included in all groups equally distributed regarding to the place where the probe was implanted. The placements of probes implanted in the mPFC of rats included in the statistical analysis

Figure 2

A) Effect of intracerebroventricular infusion of gamma-aminobutyric acid (GABA) receptors antagonists on the average concentration of GABA in medial prefrontal cortex (mPFC) following intraperitoneal morphine administration. Administration of morphine (20 mg/kg, IP) significantly increased the average concentration of GABA in comparison with saline group (*P<0.05). Pretreatment with Phaclofen (100 nmol, ICV) and bicuculline (20 nmol, ICV) 20 min before morphine administration had no effect on the concentration of GABA increased by morphine (*P<0.05). Also, administration of the same doses of phaclofen and bicuculline alone had no effect on the average concentration of GABA. Data are expressed as mean±SEM (n=10 rats in each group). B) Effect of intracerebroventricular infusion of GABA receptors antagonists on GABA release in mPFC following intraperitoneal morphine administration. Administration of morphine (20 mg/kg, IP) (second arrow) significantly increased GABA release in comparison with saline group (*P<0.05). Pretreatment with Phaclofen (100 nmol, ICV) and bicuculline (20 nmol, ICV) 20 min before morphine administration (first arrow) had no effect on GABA release during 100 min. Also, administration of the same doses of phaclofen and bicuculline alone had no effect on GABA release. Data are expressed as mean±SEM (n=10 rats in each group)

Figure 3

A) Effect of intracerebroventricular infusion of GABA receptors antagonists on the average concentration of GLU in mPFC following intraperitoneal morphine administration. Administration of morphine (20 mg/kg, IP) significantly decreased the average concentration of GLU in mPFC in comparative with saline group. Pretreatment with Phaclofen (100 nmol, ICV) and bicuculline (20 nmol, ICV) 20 min before morphine administration had no effect on concentration of GABA increased by morphine. Also, administration of the same doses of phaclophen and bicuculline alone had no effect on the average concentration of GLU (*P<0.05). Data are expressed as mean±SEM (n=10 rats in each group). B) Effect of intracerebroventricular infusion of GABA receptors antagonists on GLU release in mPFC following intraperitoneal morphine administration. Administration of morphine (20 mg/kg, IP) (second arrow) significantly decreased GLU release in comparative with saline group (*P<0.05). Pretreatment with phaclofen (100 nmol, ICV) and bicuculline (20 nmol, ICV) 20 min before morphine administration (first arrow) had no effect on GABA release during 100 min. Also, administration of the same doses of phaclofen and bicuculline alone had no effect on GLU release. Data are expressed as mean±SEM (n=10 rats in each group)