The role of the nigro-striatal dopamine system and the effect of age on prolactin-induced yawning behavior

Abstract

The material presented in this dissertation examines prolactin induced yawning behavior with emphasis on the nigro-striatal dopamine system and aging.

A subcutaneous injection of a low dose of prolactin (PRL) induced yawning in young male rats. Prolactin induced yawning was blocked by scopolamine pretreatment indicating the involvement of muscarinic receptors. When PRL was injected into male rats greater than 12 months old, no yawning was detected.

Bilateral 6-OH-dopamine lesions of the substantia nigra did not affect physostigmine-induced yawning whereas both apomorphine- and prolactin-induced yawning were significantly reduced. Simultaneous recording of yawning behavior and DOPAC release from the corpus striatum (CS) was accomplished with push-pull perfusion of the CS. Yawns and penile erections induced by PRL were associated with rapid momentary decreases in DOPAC efflux in these living animals.

Prolactin increased in vitro basal DA release from CS fragments and decreased amphetamine (AMPH) stimulated DA release. The effects of PRL on DA release were independent of extra-cellular calcium. Spiperone, a DA D2-receptor antagonist, stimulated basal DA release similar to PRL but did not attenuate (AMPH)-stimulated DA release. However, spiperone blocked the ability of PRL to attenuate AMPH-stimulated DA release.

In vitro potassium (K$\sp+$)-evoked DA release from the CS was significantly lower in old compared to young animals under control conditions. Leu-enkephalin (ENK) significantly reduced K$\sp+$-evoked DA release in young animals, but did not affect DA release in old animals. Naloxone, which had no effect in young animals, raised the K$\sp+$-evoked DA response in old animals to levels seen in young control preparations. In young castrates leu-ENK inhibited K$\sp+$-evoked DA release but K$\sp+$-evoked DA release and was unchanged by the addition of naloxone. However, in the aged castrates naloxone inhibited K$\sp+$-stimulated and leu-ENK increased both basal and K$\sp+$-stimulated DA release.

In summary, PRL can induce yawning by acting on the nigro-striatal DA system through DA terminals that inhibit a central muscarinic system. However, prolactin's ability to induce yawning is dependant on the age and gonadal condition of the animal which can alter opiate modulation of the nigro-striatal dopamine system.