Opioids in the hypothalamic paraventricular nucleus stimulate ethanol intake

Abstract

Background: Specialized hypothalamic systems that increase food intake might also increase ethanol intake. To test this possibility, morphine and receptor-specific opioid agonists were microinjected in the paraventricular nucleus (PVN) of rats that had learned to drink ethanol. To cross-validate the results, naloxone methiodide (m-naloxone), an opioid antagonist, was microinjected with the expectation that it would have the opposite effect of morphine and the specific opioid agonists.

Methods: Sprague-Dawley rats were trained, without sugar, to drink 4 or 7% ethanol and were then implanted with chronic brain cannulas aimed at the PVN. After recovery, those drinking 7% ethanol, with food and water available, were injected with 2 doses each of morphine or m-naloxone. To test for receptor specificity, 2 doses each of the mu-receptor agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-Enkephalin (DAMGO), delta-receptor agonist D-Ala-Gly-Phe-Met-NH2 (DALA), or kappa-receptor agonist U-50,488H were injected. DAMGO was also tested in rats drinking 4% ethanol without food or water available. As an anatomical control for drug reflux, injections were made 2 mm dorsal to the PVN.

Results: A main result was a significant increase in ethanol intake induced by PVN injection of morphine. The opposite effect was produced by m-naloxone. The effects of morphine and m-naloxone were exclusively on intake of ethanol, even though food and water were freely available. In the analysis with specific receptor agonists, PVN injection of the delta-agonist DALA significantly increased 7% ethanol intake without affecting food or water intake. This is in contrast to the kappa-agonist U-50,488H, which decreased ethanol intake, and the mu-agonist DAMGO, which had no effect on ethanol intake in the presence or absence of food and water. In the anatomical control location 2 mm dorsal to the PVN, no drug caused any significant changes in ethanol, food, or water intake, providing evidence that the active site was close to the cannula tip.

Conclusions: The delta-opioid receptor agonist in the PVN increased ethanol intake in strong preference over food and water, while the kappa-opioid agonist suppressed ethanol intake. Prior studies show that learning to drink ethanol stimulates PVN expression and production of the peptides enkephalin and dynorphin, which are endogenous agonists for the delta- and kappa-receptors, respectively. These results suggest that enkephalin via the delta-opioid system can function locally within a positive feedback circuit to cause ethanol intake to escalate and ultimately contribute to the abuse of ethanol. This is in contrast to dynorphin via the kappa-opioid system, which may act to counter this escalation. Naltrexone therapy for alcoholism may act, in part, by blocking the enkephalin-triggered positive feedback cycle.

Fig. 1

Effects of morphine injection (12.7 nmol, 25.4 nmol, n = 7) in the PVN on ethanol, food, and water consumption in the dark. A. Morphine at the lower dose stimulates 7% ethanol intake. B. Morphine does not significantly affect food intake. C. Morphine does not significantly affect water intake. *p < 0.025. The vehicle shown is averaged from both injections.

Fig. 2

Effects of m-naloxone injection (4.3 nmol, 6.4 nmol, n = 6) in the PVN on ethanol, food, and water consumption. A. M-naloxone at the lower dose suppresses 7% ethanol intake. B. M-naloxone does not significantly affect food intake. C. M-naloxone does not significantly affect water intake. *p < 0.025. The vehicle shown is averaged from both injections.

Fig. 3

DAMGO (2 nmol, n = 6), with no food or water available, does not significantly stimulate 4% ethanol intake.

Fig. 4

Effects of DAMGO (2.9 nmol, 5.8 nmol, n = 5) in the PVN on ethanol, food, and water consumption. A. DAMGO does not significantly affect 7% ethanol intake. B. DAMGO does not significantly affect food intake. C. DAMGO does not significantly affect water intake. The vehicle shown is averaged from both injections.

Fig 5

Effects of DALA (7.1 nmol, 14.2 nmol, n = 5) in the PVN on ethanol, food, and water consumption. A. DALA at the higher dose significantly increases 7% ethanol intake. B. DALA does not significantly affect food intake. C. DALA does not significantly affect water intake. *p < 0.025. The vehicle shown is averaged from both injections.

Fig. 6

Effects of U-50,488H (2.2 nmol, 10.7 nmol, n = 5) in the PVN on ethanol, food, and water consumption. A. U-50,488H suppresses 7% ethanol intake at the high dose. B. U-50,488H does not significantly affect food intake. C. U-50,488H does not significantly affect water intake. *p < 0.025. The vehicle shown is averaged from both injections.

Fig 7

Black dots indicate injector placement for animals included in data analysis. Injections were unilateral and counterbalanced, left and right. Coronal sections are -1.8 mm caudal to Bregma. Adapted from The Rat Brain, compact 3^rd edition, G. Paxinos and C. Watson, Copyright 1997, with permission from Elsevier. A. Injection sites for PVN drug injections. B. Injection sites for anatomical control injections.