Electrical Activities of Neurones in the Dorsomedial Hypothalamic Nucleus Projecting to the Supraoptic Nucleus During Milk-ejection Reflex in the Rat

Abstract

Abstract. Using urethane-anesthetized lactating rats, extracellular action potentials were recorded from single cells in the dorsomedial hypothalamus (DMH), which were identified as projecting to the supraoptic nucleus (SON). Sixty-two DMH cells were identified as projecting to the SON. Of these 53, 4 and 5 cells had ipsilateral, contralateral and bilateral projections, respectively. Two cells (1 ipsilaterally and 1 contralaterally projecting cell) showed bursting activities preceding milk ejection that were similar to those of oxytocin (OT) cells in the SON or paraventricular nucleus. Two ipsilaterally and 2 bilaterally projecting cells reduced their firing rates preceding milk ejection. The results suggest that some of the projections from the DMH to the SON are contralateral or bilateral and that these projections may contribute to synchronized activation of OT cells bilaterally distributed in the hypothalamus during milk-ejection reflex. Key words: Dorsomedial hypothalamic nucleus, Milk-ejection burst, Supraoptic nucleus (J. Reprod. Dev. 56: [336][337][338][339][340] 2010) xytocin (OT) is synthesized in the cell body of the OT cell in the hypothalamic paraventricular (PVN) and supraoptic nuclei (SON) and released into blood circulation from axon terminals in the neurohypophysis. Various kinds of physiological stimuli such as parturition, lactation, osmotic stimulus and stress are known to stimulate OT secretion [1][2][3][4]. In lactating rats, OT is released intermittently as a result of periodic and synchronized activation of OT cells distributed in both sides of the hypothalamus, whereas pups suck nipples continuously [5]. The afferent pathway for the milkejection reflex has been extensively studied. Using urethane-anesthetized rats, Fukuoka et al. [6] demonstrated that lesions of the lateral funiculi of the spinal cord blocked the milk-ejection reflex and that lesion of the dorsal and ventral funiculi was ineffective. Furthermore, these studies showed that suckling stimulus ascends the lateral funiculus ipsilaterally. Using neuroanatomical and lesioning experiments, Dubois-Dauphin et al. [7,8] suggested that information of suckling stimulus was relayed at the lateral cervical nucleus, crossed the midline and passed through the lateral tegmentum of the midbrain. At the level of the midbrain, Juss and Wakerley [9] showed that the lateral tegmentum was essential for milk-ejection reflex. Furthermore, they showed that unilateral lesion of this area prevented the response to contralateral suckling, but not ipsilateral suckling. It has also been suggested that the Field of Forel and zona incerta might mainly mediate transmission of suckling stimulus from the midbrain to hypothalamic OT cells [10]. However, a retrograde tracing study by Tribollet et al. [11] showed that cells in these areas were not labeled following injection of tracer into the SON, which suggested the existence of a further relay site before suckling stimulus reaches OT cells. We have previously shown that cells in the dorsomedial hypoth...

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

“…Takano et al 151 recorded hypothalamic neurons near the PVN during the reflex, all of which could be antidromically identified from ipsilateral SON stimulation but did not project to the neurohypophysis. 152,153 Even though the antidromic DMH projections to the region of the SON are more strongly ipsilateral, 142 DMH stimulation can activate the SON bilaterally. Electrical stimulation of the DMH facilitates OT neuronal activity in the SON.…”