Role of GABAA receptors in the retrorubral field and ventral pallidum in rat jaw movements elicited by dopaminergic stimulation of the nucleus accumbens shell
Introduction
It is well known that dopaminergic neurons in the retrorubral field primarily project to the striatum (Gerfen et al., 1987). This region also contains neural population that projects to the parvicellular reticular formation, a region that contains orofacial premotor nuclei and gives rise to neurones that converge onto neurones in the trigeminal motor nucleus (Fort et al., 1990, Travers and Norgren, 1983). The retrorubral field is known to receive γ-aminobutyric acid (GABA)ergic input from the nucleus accumbens (Zahm and Heimer, 1993). Activation of dopamine receptors in the nucleus accumbens has been found to elicit repetitive jaw movements in rats (Koshikawa et al., 1990a). Subsequently, it has been found that these dopamine receptor-mediated jaw movements are elicited predominantly by co-activation of dopamine D1 and D2 receptors in the shell, but not the core, of the nucleus accumbens (Cools et al., 1995, Koshikawa et al., 1996a).
Until now, it is not clear whether or not the retrorubral field plays any role in the transmission of these accumbens shell-specific, dopamine D1/D2 receptor-mediated jaw movements. Such a role can be expected in view of the fact that (1) it is known that the retrorubral field is involved in the control of oral behaviours in cats (Arts et al., 1998) and (2) the retrorubral field contains dopaminergic neurons projecting to the region of the striatum that encompasses its ventrolateral part (Gerfen et al., 1987), where repetitive jaw movements can be elicited by dopamine D1/D2 receptor stimulation (Koshikawa et al., 1989, Delfs and Kelley, 1990, Adachi et al., 2002, Adachi et al., 2003).
In the first part of this study, like in our previous studies (Adachi et al., 2002, Adachi et al., 2003), electromyography and a phototransduction system were used to characterise the pattern of jaw movements induced by injections of GABAergic drugs, muscimol and bicuculline to activate and inhibit GABAA receptors respectively, into the retrorubral field in order to compare these with those elicited by injections of dopamine receptor agonists into the nucleus accumbens shell (Cools et al., 1995). Once the nature of the behavioural response to injections into the retrorubral field of GABAergic drugs was established, we studied the specificity of the response using joint injections of the GABAA receptor agonist muscimol and the GABAA receptor antagonist bicuculline.
To analyse the role of the retrorubral field in the jaw movements that are elicited by stimulation of dopamine receptors in the nucleus accumbens shell, muscimol and bicuculline were injected bilaterally into the retrorubral field of rats treated with bilateral injections into the nucleus accumbens shell of a mixture of the dopamine D1 receptor agonist, (±)-6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol (SKF 82958; 5 μg), and the dopamine D2 receptor agonist, quinpirole (10 μg); the latter dopaminergic treatments have been found to be highly effective in eliciting the above-mentioned dopamine receptor-mediated jaw movements (Cools et al., 1995).
Like the retrorubral field, the ventral pallidum is also known to receive a GABAergic input from the nucleus accumbens (Groenewegen and Russschen, 1984, Zahm and Brog, 1992). Although GABAergic manipulation of the ventral pallidum does not elicit repetitive jaw movements (Adachi et al., 2002), it has been found to modulate the jaw movements induced by bilateral injections of a mixture of SKF 82958 (5 μg) and quinpirole (10 μg) into the ventrolateral striatum (Adachi et al., 2002).
For that reason, it became of interest to compare the role of the retrorubral field with that of the ventral pallidum. The final part of this study therefore examined a role of GABAA receptors in the ventral pallidum by injecting muscimol and bicuculline bilaterally into this brain structure of rats treated with a mixture of SKF 82958 (5 μg) and quinpirole (10 μg) into the nucleus accumbens shell.
Section snippets
Surgical procedures
Male Sprague–Dawley rats weighing 260–330 g were housed in cages (27×45×20 cm) that were kept at constant room temperature (23±2 °C) and relative humidity (55±5%) under a 12 h light/dark cycle (lights on at 0700 h), with free access to food and water.
Rats were anaesthetised with halothane (0.5–4.0% when appropriate) and supplemented with ketamine HCl (10.0 mg/kg, i.p.). The surgical and recording procedures were as described previously (Koshikawa et al., 1989, Koshikawa et al., 1990a, Koshikawa
Pattern of jaw movements elicited by bilateral injections of GABAergic drugs into the retrorubral field
Muscimol (50 ng) alone had no effect. In contrast, bicuculline (150 ng) was effective in eliciting jaw movements. Fig. 2 shows the results of the electromyographic activity recorded from digastric and masseter muscle together with the vertical component of jaw movements. Bilateral injections of bicuculline into the retrorubral field (Fig. 2, left parts) increased electromyographic activity in the digastric muscle that was associated with the movement of jaw opening, but did not change the
Discussion
The present study demonstrates that bilateral injections of the GABAA receptor antagonist bicuculline produced characteristic repetitive jaw movements when injected into the retrorubral field. The behavioural response to bicuculline was specific for GABAA receptors, because the GABAA receptor agonist muscimol well inhibited this response (Fig. 3). Using an identical experimental set up (Adachi et al., 2002), we have previously shown that none of the GABAergic drugs, injected into the ventral
Acknowledgements
These studies were supported by grants from Nihon University (NK), the Dental Research Centre (NG), the Sato Fund (NK) and the Promotion and Mutual Aid Corporation for Private Schools of Japan (NK); and a grant for promotion of multidisciplinary research projects (NK, ARC) and a grant-in-aid for scientific research (#14370609 to NK) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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