Oxytocin release evoked by electrical stimulation of the medial forebrain in the rat: analysis of stimulus parameters and supraoptic neuronal activity

Near-infrared nanosensors enable optical imaging of oxytocin with selectivity over vasopressin in acute mouse brain slices

Oxytocin plays a critical role in regulating social behaviors, yet our understanding of its function in both neurological health and disease remains incomplete. Real-time oxytocin imaging probes with spatiotemporal resolution relevant to its endogenous signaling are required to fully elucidate oxytocin's role in the brain. Herein, we describe a near-infrared oxytocin nanosensor (nIROXT), a synthetic probe capable of imaging oxytocin in the brain without interference from its structural analogue, vasopressin. nIROXT leverages the inherent tissue-transparent fluorescence of single-walled carbon nanotubes (SWCNT) and the molecular recognition capacity of an oxytocin receptor peptide fragment to selectively and reversibly image oxytocin. We employ these nanosensors to monitor electrically stimulated oxytocin release in brain tissue, revealing oxytocin release sites with a median size of 3 µm in the paraventricular nucleus of C57BL/6 mice, which putatively represents the spatial diffusion of oxytocin from its point of release. These data demonstrate that covalent SWCNT constructs, such as nIROXT, are powerful optical tools that can be leveraged to measure neuropeptide release in brain tissue.

Permissive effect of centrally administered oxytocin on the excitatory response of oxytocin neurones to ventral tegmental stimulation in the suckled lactating rat

The mesencephalic ventral tegmentum has been implicated in the milk-ejection reflex and modulation of inputs from this region could provide a mechanism whereby central oxytocin facilitates synchronous bursting of oxytocin neurones during suckling. Experiments were therefore undertaken to investigate the effect of intracerebroventricular (i.c.v.) oxytocin on the response of oxytocin neurones to ventral tegmental stimulation. Oxytocin neurones were recorded in the supraoptic nucleus of urethane-anaesthetized lactating rats during suckling, and their response to single shock stimulation of the ventral tegmentum was monitored using peri-stimulus time-interval histograms. Before i.c.v. oxytocin, oxytocin neurones were either unresponsive to ventral tegmental stimulation, or displayed a small inhibition. However, after administration of oxytocin (2.2 ng i.c.v.), seven out of eight neurones tested displayed a pronounced excitatory response (onset latency 78.4 +/- 4.8 ms, duration 73.4 +/- 8.3 ms). This permissive effect on the excitatory response was only observed in the presence of suckling, and followed the same time course as facilitation of the milk-ejection reflex, being maximal immediately before each facilitated bursting response in the oxytocin neurones. The response to ventral tegmental stimulation remained unaltered after intraperitoneal administration of hypertonic saline to cause a generalized increase in the excitability of the oxytocin neurones. Moreover, i.c.v. oxytocin had no effect on the response of oxytocin neurones to stimulation of a descending input from the medial septum. In conclusion, administration of i.c.v. oxytocin has a selective permissive effect on the excitation of oxytocin neurones from the ventral tegmentum, and this supports previous in vitro studies suggesting that centrally released oxytocin may act as a modulator of afferent transmission to the magnocellular nuclei. This effect on the afferent excitation of oxytocin neurones may provide a mechanism whereby i.c.v. oxytocin facilitates suckling-evoked bursting activity.

Electrophysiological analysis of the morphofunctional organization of the limbic control of magnocellular neurosecretory nuclei in the rat hypothalamus

The effects of electrical stimulation of the ventral hippocampus, ventral subiculum, and corticomedial amygdala were used to obtain a general and comparative assessment of the organization of efferent outputs of limbic system structures to the magnocellular neurosecretory nuclei of the hypothalamus, i.e., antidromally identified neurosecretory cells and other groups of identified neurons in these nuclei and in the perinuclear zones. These studies showed that different efferent outputs of the hippocampal formation provide differential control of spike activity of neurosecretory cells in the supraoptic nucleus, with excitatory pathways from the ventral hippocampus and inhibitory pathways from the subiculum. The effects of the amygdala on neurosecretory cells of the paraventricular nucleus were shown to be excitatory, though they were less significant than the excitatory and inhibitory effects of the hippocampus. It was demonstrated that in general, the effect of limbic structures are addressed predominantly to cells which do not project to the posterior lobe of the hypophysis. Projections were mostly to interneurons, which, as convergence sites for excitatory influences both from limbic structures and neurosecretory cells, may thus be responsible for the involvement of the latter in integrative brain functions.

Connections from the subfornical organ to the oxytocin and vasopressin systems in the lactating rat. A study using electrical stimulations, lesions and electrophysiological recordings

The medial septal area has been implicated in the control of the magnocellular neurosecretory cells of the hypothalamus, and in particular, in the regulation of neurons secreting oxytocin. The present study investigated the hypothesis that this medial septal pathway originates in the subfornical organ. Brief electrical stimulation of the subfornical organ or of the medial septum both evoked a transient rise in intramammary pressure equivalent to that caused by an i.v. injection of 1 mU oxytocin. The optimal frequency was 5-20 Hz for 5-10 s. Prolonged stimulation also elicited at its onset a single transient response, similar to that evoked by brief stimulation. Extracellular recordings were made from neurosecretory cells of the supraoptic nucleus identified by antidromic stimulation of the neural stalk and further classified as vasopressinergic and oxytocinergic by their reaction at the time of reflex milk ejection induced by suckling. Single-pulse stimulation of the subfornical organ rarely produced excitation, but short trains of stimuli evoked a large excitation in most oxytocinergic and vasopressinergic neurons. To delineate further the pathway from the subfornical organ to the magnocellular neurons, stimulations were combined with various lesions of the medial forebrain. The effects of stimulation of the subfornical organ were abolished after a section immediately rostral to the organ, and in most cases after lesion of the medial septum. Stimulation of the medial septum no longer had an effect after the subfornical organ had been lesioned a week prior to experiments, a period sufficient to allow degeneration of subfornical efferents. This study shows that the excitatory afferent input to the oxytocin and vasopressin-secreting neurons of the hypothalamus from the medial septal area originates in the subfornical organ. This input is not involved in the main afferent control of the milk ejection reflex since lesions of the subfornical organ and of the medial septum had no effect on the reflex. It is suggested, therefore, that the subfornical input to both oxytocin and vasopressin cells intervenes to facilitate synergistic action of both hormones in non-reproductive functions.

GABA receptor mediation of median preoptic nucleus-evoked inhibition of supraoptic neurosecretory neurones in rat

1. This study evaluated the influence of focal electrical and chemical microstimulation in the median preoptic nucleus (MnPO) on the excitability of putative vasopressin and oxytocin neurones recorded in the supraoptic nucleus of urethane- or pentobarbitone-anaesthetized rats. 2. In vasopressin neurones, single 1 Hz stimulation reduced the excitability of 120/139 cells. Trains of repetitive 5-30 Hz stimulation, or microinfusion of glutamate into the MnPO, similarly induced a cessation in spontaneous phasic or continuous firing in 17/18 and 17/20 vasopressin neurones, respectively. In 20/21 cells, locally applied bicuculline (100 microM) attenuated MnPO-evoked depressant responses whereas strychnine (100 microM) and timolol (20 microM) were without effect on 5/5 vasopressin neurones. In three cells, bicuculline applications were associated with marked increases in MnPO-evoked excitations. 3. In oxytocin neurones, single-pulse (1 Hz) electrical stimulation in MnPO evoked an increase in the excitability in 51/59 cells. However, in 6/7 oxytocin cells tested, glutamate microinfusions into MnPO induced prolonged suppression in firing. During trains of stimuli (5-30 Hz), 26/44 cells displayed an initial increase in firing associated with the first few impulses but this was then replaced by suppression of activity; another ten cells displayed excitation alone, and eight cells demonstrated only suppression. The depressant responses evoked during trains of MnPO stimulation were blocked by 100 microM bicuculline (6/6 cells tested) whereas strychnine was ineffective (2/2 cells tested). 4. These results suggest that the MnPO provides a mainly depressant influence on supraoptic vasopressin and oxytocin neurones, perhaps through the activation of postsynaptic GABAA receptors.

Further Evidence that the Septum is not part of the Main Pathway of the Milk Ejection Reflex in the Rat

Abstract The possible role of the medial septum in the control of oxytocin release and of the milk ejection reflex induced by suckling was investigated in lactating rats by using electrical stimulations and lesions. In anaesthetized animals, brief electrical stimulation of the medial septum at 5 to 50 Hz elicited a single brief milk ejection similar to natural reflex milk ejections, whereas prolonged low frequency stimulations (5 to 10 Hz) induced a prolonged inhibition of the reflex. In acute experiments under anaesthesia, lesions of the medial septum did not impair the amplitude and pattern of reflex milk ejections. In chronic experiments, lesions of the medial septum resulted first in a loss of body weight of the mothers and a parallel reduction in growth of the litters. After a few days, the litters gained weight normally, and the pattern of milk ejections was normal. Thus, the pathways which pass through or originate from the medial septum and which are excitatory for oxytocin release appear not to be involved in the regulation of the milk ejection reflex. In view of these results and those from our previous study on the lateral septum, we conclude that the whole septum is not essential to the milk ejection reflex. However, the effects of septal stimulation suggest that the medial and lateral septum may be involved in a secondary neural circuitry that can inhibit the reflex.

Activation of supraoptic neurosecretory cells by osmotic stimulation of the median preoptic nucleus

In urethane-anesthetized male rats extracellular recordings were obtained from neurosecretory cells in the supraoptic nucleus (SON) while each of electrical stimulation and local osmotic stimulation produced by pressure injection of hypertonic saline was applied to the median preoptic nucleus (MnPO) or to the medial septal nucleus (MS). Electrical stimulation of the MnPO produced orthodromic excitation or initial inhibition followed by strong excitation in most SON cells (59/61), and local osmotic stimulation of the MnPO excited majority of cells (39/51). On the contrary, local osmotic stimulation of the MS did not excite SON cells, although electrical stimulation excited all the cells (5/5). The results suggest that the MnPO is one of the osmosensitive sites controlling electrical activity of SON neurosecretory cells.