Effects of Unilateral Electrolytic Lesion of the Dorsomedial Nucleus of the Hypothalamus on Milk-ejection Reflex in the Rat

Abstract

Abstract. The dorsomedial nucleus of the hypothalamus (DMH) has been suggested to be a final relay site for the afferent pathway of milk-ejection reflex (MER). The present experiments were undertaken to examine whether unilateral lesion of the DMH abolished MER induced by unilateral suckling by pups. The unilateral DMHs of urethaneanesthetized lactating rats were electrolytically lesioned. Then 5 pups were applied to nipples ipsilateral or contralateral to the lesioned DMH and the intramammary pressure was monitored to observe the occurrence of MER. The effects of bilateral suckling by 10 pups were also examined. Eighteen of the 29 rats displayed MER during ipsi-or bilateral suckling for about 1 h. Seventeen of these 18 rats did not show MER during contralateral suckling for about 1 h. The remaining rat that showed milk ejection during contralateral suckling had a lesion outside the DMH. In 2 additional rats, extracellular action potentials of single oxytocin (OT) cells located in the supraoptic nucleus ipsilateral to the lesioned DMH were recorded during suckling ipsi-and conralateral to the lesioned DMH. OT cells showed milkejection burst during ipsilateral suckling but not during contralateral suckling. The results suggest that the unilateral DMH receives information of suckling stimuli applied to contralateral nipples. Key words: Dorsomedial hypothalamic nucleus, Milk-ejection burst, Oxytocin neurone, Supraoptic nucleus (J. Reprod. Dev. 56: [98][99][100][101][102] 2010) uring suckling, oxytocin (OT) is released intermittently from the neurohypophysial terminals [1]. Each OT release, shown by an increase of intramammary pressure, is preceded by a synchronized short high-frequency burst of action potentials in most OT cells of the paraventricular (PVN) and supraoptic nucleus (SON) [2]. In spite of many studies on the neural mechanisms controlling such periodic and synchronized activation of OT cells during milk-ejection reflex, the exact mechanisms are still not fully understood.The afferent pathway for the milk-ejection reflex has been extensively studied. In the rat spinal cord, Fukuoka et al. [3] demonstrated that suckling stimulus is conveyed rostrally in the ipsilateral lateral funiculus. 5] suggested that information from the suckling stimulus is relayed at the lateral cervical nucleus, crosses the midline and passes through the lateral tegmentum of the midbrain. At the level of the midbrain, Juss and Wakerley [6] showed that the lateral tegmentum is essential for milk-ejection reflex. Furthermore, they showed that unilateral lesion of this area prevents the response to contralateral suckling, but not ipsilateral suckling. It has also been suggested that the Field of Forel and zona incerta might predominantly mediate transmission of the suckling stimulus from the midbrain to hypothalamic oxytocin neurons [7]. However, a retrograde tracing study by Tribollet et al. [8] showed that neurons in these areas are not labeled following injection of tracer into the SON, which suggests the existence of furthe...

Oxytocin Cells in the Supraoptic Nucleus Receive Excitatory Synaptic Inputs from the Contralateral Supraoptic and Paraventricular Nuclei in the Lactating Rat

“…In addition to our previous results indicating that some DMH cells projected to the SON in both sides of the hypothalamus [11,12,13], the results of the present experiment suggesting the existence of excitatory connections between OT cells in the bilateral hypothalamus would be a good base supporting a synchronization of burst discharges of OT cells in separate magnocellular nuclei.…”

“…Our previous studies on the DMH showed that the cells in the DMH on one side projected to the SON in both sides of the hypothalamus. However, there were not many cells in the DMH projecting to the SON in both sides of the hypothalamus [11,12,13]. So it seems unlikely that such a small number of DMH cells fully support synchronization of 9000 OT cells in the hypothalamus.…”

“…The afferent pathway for the milk ejection reflex has been extensively studied in the spinal cord [6], and in the midbrain [7,8,9]. In the hypothalamus, it was suggested that the dorsomedial hypothalamic nucleus (DMH) was a final relay site for the afferent pathways of the milk ejection reflex [10,11,12,13]. …”

“…Our previous studies on the DMH showed that the cells in one side of the DMH projected to the SON in both sides of the hypothalamus. However, there were not many cells in the DMH projecting to the SON in both sides of the hypothalamus [11,12,13]. Although such projections from the DMH to the SON may contribute to the synchronized activation of OT cells during the milk ejection reflex, it seems likely that other mechanisms also contributed to such synchronization.…”

Central Nervous System Control of Oxytocin Secretion during Lactation

“…Electrical stimulation of the DMH facilitates OT neuronal activity in the SON. 154 However, the importance of the DMH per se is still clouded by the possibility that fibers of passage are responsible for both orthodromic and antidromic responses to SON stimulation, as well as lesion effects. 152 Unilateral electrolytic lesions of the DMH completely block suckling to contralateral (but not ipsilateral) suckling.…”