The supramammillary nucleus mediates primary reinforcement via GABA(A) receptors

Abstract

The supramammillary nucleus (SUM), a dorsal layer of the mammillary body, has recently been implicated in positive reinforcement. The present study examined whether GABA(A) receptors in the SUM or adjacent regions are involved in primary reinforcement using intracranial self-administration procedures. Rats learned quickly to lever-press for infusions of the GABA(A) antagonist picrotoxin into the SUM. Although picrotoxin was also self-administered into the posterior hypothalamic nuclei and anterior ventral tegmental area, these regions were less responsive to lower doses of picrotoxin than the SUM. The finding that rats learned to respond selectively on the lever triggering drug infusions is consistent with picrotoxin's reinforcing effect. Coadministration of the GABA(A) agonist muscimol disrupted picrotoxin self-administration, and another GABA(A) antagonist, bicuculline, was also self-administered into the SUM; thus, the reinforcing effect of picrotoxin is mediated by GABA(A) receptors. Since rats did not self-administer the GABA(B) antagonist 2-hydroxysaclofen into the SUM, the role of GABA(B) receptors may be distinct from that of GABA(A) receptors. Pretreatment with the dopamine receptor antagonist SCH 23390 (0.05 mg/kg, i.p.) extinguished picrotoxin self-administration into the SUM, suggesting that the reinforcing effects of GABA(A) receptor blockade depend on normal dopamine transmission. In conclusion, the blockade of GABA(A) receptors in the SUM is reinforcing, and the brain 'reward' circuitry appears to be tonically inhibited via supramammillary GABA(A) receptors and more extensive than the meso-limbic dopamine system.

Figure 1

Self-administration of the GABA_A receptor antagonist picrotoxin. (a) Injection sites of picrotoxin. Numbers indicate distances (mm) posteriorly from bregma. 3V, third ventricle; at-VTA, anterior tip of ventral tegmental transition area; aVTA, anterior VTA; LHA, lateral hypothalamic area; MM, medial mammillary nucleus; PHN, posterior hypothalamic nucleus; SUM, supramammillary nucleus. Drawings were adapted from the atlas by Paxinos and Watson (1997). (b) Picrotoxin self-administration rates in various brain regions. Each infusion was delivered in a 75 nl volume; therefore, the doses of picrotoxin examined were 0.75, 2.25, 7.5, and 22.5 pmoles per infusion for 0.01, 0.03, 0.1, and 0.3 mM, respectively. *Different from respective vehicle treatment, P < 0.05. (c) Event records of a representative rat self-administering picrotoxin or vehicle (VEH) into the SUM. Each horizontal line indicates the length of session. Each vertical line indicates when an infusion was delivered. The numbers on the right of horizontal lines are total numbers of infusions obtained in sessions.

Figure 2

Photomicrograph showing a representative injection site in the SUM. The arrowhead indicates the tip of injection cannula track. FR, fasciculus retroflexus; for other abbreviations, see the legend of Figure 1a.

Figure 3

Two lever discrimination for supramammillary picrotoxin reward. Four rats received vehicle in session 1 and picrotoxin (0.1 mM) in sessions 2–9. They were trained on operant conditioning schedules of a fixed-ratio 1 with a 20-s timeout in sessions 1–5 and a partial progressive ratio (up to 6) in sessions 6–9. (a) Mean response rates (±SEM) of the two levers are summarized over nine sessions. Active lever-presses in each of sessions 7, 8, and 9 were greater than inactive lever-presses in respective sessions, and they were also greater than active lever-presses in each of sessions 2, 3, 4, and 5 (*P < 0.05). (b) Mean infusion rates (±SEM) are shown over nine sessions. The infusions in session 6 were lower than those in sessions 2, 4, and 5 (*P < 0.05). (c) Cumulative response records and infusion event records of a representative rat are shown. Each line moves up a unit vertically every time the rat pressed the active lever. Each perpendicular slit indicates the point of an infusion delivery. Each arrow accompanied by a number indicates the point at which the response requirement was incremented, and the number indicates required lever-presses for each infusion. The horizontal lines in the bottom indicate session length with vertical lines again indicating the points of infusions delivered.

Figure 4

GABA_A receptors mediate picrotoxin self-administration. Mean infusions per min (±SEM) are shown in (a), (b), and (c). (a) When the GABA_A receptor agonist muscimol (MUS; 0.3 mM) was coadministered with picrotoxin (PIC; 0.1 mM), self-administration rates were decreased markedly and were not distinguishable from self-administration rates of vehicle (VEH) (N = 6). *Different from picrotoxin treatment, P < 0.05. Self-administration patterns of a representative rat are shown in (d). (b) Rats (N = 6) learned to self-administer the GABA_A receptor antagonist bicuculline (BICUC; 0.1 mM) into the SUM. *Different from vehicle treatments, P < 0.05. (c) The rats that self-administered bicuculline did not self-administer the GABA_B receptor antagonist 2-hydroxysacrofen into the SUM. (d) and (e) Each set of event records show self-administration patterns of a representative rat.

Figure 5

Effects of the dopamine D₁ antagonist SCH 23390 on picrotoxin self-administration. (a) When rats (N = 8) were treated with SCH 23390 (SCH; 0.05 mg/kg, i.p.), self-administration rates of picrotoxin (PIC) were markedly higher than when they were treated with saline (SAL) or when they received vehicle (VEH) substituted for picrotoxin. Data are mean infusions per min (SEM). *Different from PIC + SAL, P < 0.001. (b) Data are mean square-root infusion intervals (s) plus SEM with the first two intervals or the last two intervals averaged. *Different from the first intervals of PIC + SAL, PIC + SCH, or VEH + SAL or the last intervals of PIC + SAL, P < 0.01. (c) Event records show typical self-administration patterns of a representative rat.