Functional considerations of enkephalinergic projections from the hypothalamic ventromedial nucleus of the rat

231 FUNCTIONAL CONSIDERATIONS OF ENKEPHALINERGIC PROJECTIONS FROM THE HYPOTHALAMIC VENTROMEDIAL NUCLEUS OF THE RAT. Priest CA, Pfaff DW, Laboratory of Neurobiology and Behavior, Rockefeller University, New York, New York, USA. The ventromedial nucleus of the hypothalamus (VMH) has been implicated in the modulation of physiological regulatory mechanisms and behaviors such as food and water intake, temperature regulation, autonomic nervous system functioning and aggressive and sexual behaviors. The nucleus is centrally located as a key component in limbic-hypothalamic neural circuitry and receives extensive afferent connections that allow it to serve as an integrative site for a wide range of physiological, hormonal and environmental stimuli (1,2). In addition, efferent projections have been traced from the VMH to neural sites in the limbic forebrain, hypothalamus and midbrain (3,4), many of which concentrate tritiated estrogen (5-8), a hallmark of estrogen-sensitive cells. Among the neurotransmitters and neuropeptides which are produced by neurons in the VMH, the opioid peptide methionine-enkephalin (mENK), which is encoded by preproenkephalin (PPE), may play a particularly important modulatory role in the neural sites to which the VMH sends efferent connections, as suggested by the dramatic sensitivity of PPE at the VMH to changing levels of circulating gonadal steroids. For example, estrogen treatment of ovariectomized rats markedly increases levels of PPE mRNA and MENK protein within one hour (9-11). Furthermore, when in situ hybridization histochemistry is used to examine levels of PPE mRNA expression in the VMH after acute exposure to estrogen, a biphasic pattern of expression is seen. Although the levels of PPE mRNA expression per cell do not change, the number of VMH cells expressing PPE mRNA is significantly higher than that counted in ovariectomized females at 1 hour and 48 hours after estrogen treatment, with a nadir in the number reached 4 hours after estrogen treatment (11). Distributions of mENK-like immunoreactive cells (12,13), PPE mRNA-expressing cells (14) and estrogen-concentrating, mENKlike immunoreactive cells (15) overlap extensively in the VMH, especially in the ventrolateral subdivision of the nucleus (VMHvl), thus suggesting an anatomical basis for the direct regulation of PPE mRNA by estrogen. Because levels of mENK in the VMH vary across the estrous cycle of female rats (16), the regulation of mENK production at the VMH may have behavioral relevance. The following report will review some of the opioidergic VMH connections and their possible functions. Through combining tract-tracing and in situ hybridization histochemical techniques, a strong PPE projection has been identified from the VMH to the medial preoptic area (MPOA); approximately 30-40% of the cells in the VMHvl which express PPE mRNA project to the lateral subdivision of the medial preoptic nucleus (17), which is involved in the regulation of lordosis behavior in female rats. Electrical stimulation of the MPOA decreases the display of lordosis and lesioning the nucleus facilitates the behavior (18,19). Therefore, the enkephalinergic projection from the VMH may be important for the inhibition of lordosis outside the period of sexual receptivity. Indeed, the distribution of/.t-opioid receptors is estrogen-sensitive in the MPOA (20), the activation of which has been reported to inhibit lordosis (21,22). Conversely, inhibition of MPOA neurons may act to disinhibit lordosis behavior, thus increasing its display. Additionally, opioid inhibition of neural activity at the MPOA may be involved in the control of maternal behavior by the nucleus (23), although a second major enkephalinergic projection to the MPOA originates from the central amygdala (24) which may affect maternal behavior at the MPOA in response to information from the vomeronasal system. Another well-defined enkephalinergic projection system of the VMH is to the midbrain periaqueductal gray (PAG). Notably, lesions of the VMH reduce mENK-like immunoreactivity in the PAG (25), suggesting that PPE cells in the VMH have long projections to the PAG. The PAG also has been implicated in the control of lordosis behavior by enkephalin in female rats. Infusion of enkephalin and an inhibitor to its catabolism into the PAG inhibits the display of lordosis (26), although this may reflect a desensitization of post-synaptic opioid receptors. However, since substance P (27), prolactin (28) and LHRH (29) have been shown to increase lordosis behavior when infused

232 into the PAG, mENK may act to inhibit the release of one, or a combination, of these facilitatory neuropeptides. Thus, the up-regulation of PPE mRNA expression by estrogen at the VMH may represent a possible mechanism through which lordosis behavior could be inhibited when its display would be inappropriate. It is important to note, however, that enkephalin-like immunoreactive fibers that project into the PAG have been identified in the associated area surrounding the PAG (25) which also may regulate PAG activity. Alternatively, the mENK produced in the VMH may act locally within the VMH. Opioidergic neurons are often interneurons and anatomical studies have described a number of VMH neurons with short projections which remain intranuclear (30,31). In support of this hypothesis, infusions of DADLE ([D-Ala2, D-Leu~]-enkephaiin) into the VMH have been reported to increase food intake, suggesting a local effect of the opioid agonist within the VMH (32). It is possible that mENK could be released at the VMH to inhibit GABA or mENK interneurons (33), which, in turn, could disinhibit neurons that may facilitate lordosis behavior. Indeed, up-regulation of PPE mRNA expression in the VMH by estrogen has been correlated positively with increased lordosis behavior (34). Thus, evidence suggests that estrogen-sensitive, cells in the VMH which express PPE mRNA may participate both in the regulation of VMH intranuclear circuitry and of other limbic-hypothalamic structures. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34.

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