Takao Akaishi

Estrogen excites oxytocinergic, but not vasopressinergic cells in the paraventricular nucleus of female rat hypothalamus

Extracellular antidromic potentials were recorded from the paraventricular nucleus of the ovariectomized female rat hypothalamus following electrical stimulation of the neurohypophysis. Effects of estrogen-treatment were investigated after classifying the antidromically identified cells into tonically-firing, phasically-firing, or silent groups according to their patterns of spontaneous discharge. Estrogen significantly decreased the antidromic activation threshold and shortened the refractory period as well as the antidromic spike latency in the tonic-firing cells. We suggest that estrogen selectively and directly excited the tonically-firing, presumably oxytocinergic cells.

Responses of paraventricular and supraoptic units to angiotensin II, SAR1-ILE8-angiotensin II and hypertonic NaCl administered into the cerebral ventricle

Extracellular recordings of action potentials were made from neurones antidromically identified as neurosecretory cells in the paraventricular and supraoptic nuclei of urethane-anesthetized female rats. Eighty-six neurones were examined for their responsiveness to 10 ng of angiotensin II (AII) injected into the third cerebral ventricle and 78 (91%) of them increased their firing rate following the AII injection. None of the neurosecretory cells tested showed a response to the intraventricular (IVT) injection of isotonic NaCl. Thalamic neurones and non-neurosecretory hypothalamic neurones did not respond to the AII given IVT. Firing activity of 13 neurosecretory neurones was recorded during reflex milk ejection induced by suckling pups in the lactating rats. Seven of them were classified as oxytocinergic cells because they showed a burst of activity before reflex milk ejections and the remaining 6 neurones which gave no burst of firing before milk ejections were classified as nonoxytocinergic neurones. The IVT application of AII resulted in activation of all the oxytocinergic neurones and 5 of the 6 non-oxytocinergic neurones. The effect of AII on the firing of the neurosecretory cell was inhibited by the simultaneous application of Sar1-Ile8-AII (1 microgram), a competitive AII antagonist. The IVT injection of the antagonist alone inhibited the spontaneous firing of the neurosecretory cells, but it did not affect the firing of thalamic or non-neurosecretory hypothalamic neurones. Hypertonic NaCl (0.85 M NaCl, 1 mu1 IVT) also activated 13 of 20 neurosecretory cells tested. Combined application of AII and hypertonic NaCl elicited a marked potentiation of the response of neurosecretory cells to each of the stimuli. These findings indicate that AII activates neurosecretory cells stimulating specific AII receptors in the brain and that AII has a synergistic action with hypertonic NaCl. Inhibition of spontaneous activity of neurosecretory cells by a competitive AII antagonist suggests that endogenous AII may participate in the maintenance of basal activity of neurosecretory cells.

Electrophysiological evidence for multiple sites of actions of angiotensin II for stimulating paraventricular neurosecretory cells in the rat.

Microelectrophoretically (MEPh) applied angiotensin II (AII) excited about half of the PV neurosecretory cells recorded. The excitation was blocked by MEPh applied Sar1-Ala8-AII. Intraventricular (IVT) injection of AII excited both the sensitive cells and insensitive cells to the MEPh applied AII. MEPh-applied Sar1-Ala8-AII, however, blocked the IVT AII induced excitation only in the former type of the cells.

Projections of oestrogen‐sensitive neurones from the ventromedial hypothalamic nucleus of the female rat.

Stimulation of the mesencephalic central grey matter caused antidromic activation of 199 neurones in the hypothalamic ventromedial nucleus of forty‐six urethane‐anaesthetized female rats, of which twenty‐two (ten ovariectomized and twelve ovariectomized and oestrogen treated) had the lateral projection of the nucleus disrupted by a small parasagittal knife‐cut. The remaining twenty‐four rats (ten ovariectomized and fourteen ovariectomized and oestrogen treated) had a cut in the frontal plane to interrupt caudal projection of the nucleus. In eight animals with the lateral knife‐cut, eighteen cells were antidromically activated from the central grey matter as well as from the dorsal longitudinal fascicle. Collision between the potentials activated from these two sites revealed that seventeen of them descended to the central grey matter by way of the periventricular system, one cell having a bifurcated axon to these sites. Likewise, eleven cells in another group of eight animals with the posterior cut were found to project to the central grey matter via the perifornical region. Pre‐treatment with oestrogen significantly lowered the activation thresholds and absolute refractory periods of neurones antidromically stimulated from the central grey in ovariectomized animals bearing posterior but not lateral knife‐cuts. These results suggest that the ventromedial hypothalamic neurones responsible for oestrogen‐dependent autonomic and behavioural functions project to the central grey via a lateral rather than a posterior pathway.

Responses of neurons in the nucleus basalis of Meynert to various afferent stimuli in rats

The effects of innocuous and noxious mechanical stimulation of skin, and of baroreceptor and chemoreceptor stimulation, on the activity of single neurons in the nucleus basalis of Meynert (NBM), whose axons project to the cortex, were examined in urethane-anesthetized adult rats. Most of the neurons were not significantly influenced by innocuous mechanical cutaneous stimulation or baroreceptor stimulation, while they were excited by noxious mechanical cutaneous stimulation and chemoreceptor stimulation. The NBM neurons were excited more intensely and frequently by nociceptive mechanical stimulation to a fore- or hindpaw than by that to the back or face. The function of these NBM neurons is discussed.

Neural inputs from the uterus to the paraventricular magnocellular neurons in the rat

Extracellular action potentials were recorded from antidromically identified, tonically firing cells in the hypothalamic paraventricular nucleus (PVN) of ovariectomized, estrogen-treated female rats under urethane anesthesia. Genital or somatic sensory stimuli, or electrical stimulation of the nerves innervating the pelvis were applied. Uterine horn or vaginal distension each excited 33% of the neurons tested. Probing of the cervix had no effect. Hindpaw pinch produced excitation in 39% and inhibition in 11% of the neurons tested. Non-noxious somatic stimuli had no effect. Stimulation of the uterine afferent nerves, the hypogastric and pelvic nerves, excited 55% and 30% of the neurons tested, respectively. Stimulation of a somatic nerve of the hindleg, the sciatic nerve, activated 80% of the neurons tested. These results indicate that specific sensory afferents arrive at the PVN from the uterus; in addition, somatic afferents converge in this hypothalamic nucleus.

Nerve growth factor-mediated sexual differentiation of the rat hypothalamus

Injection of antibody to nerve growth factor into the cerebral lateral ventricle blocked testosterone-induced behavioral defeminization of neonatal female rats. When tested as adults following ovariectomy and combined estrogen-progesterone treatment, the injected animals showed a significantly higher lordosis quotient than the testosterone-treated, normal rabbit serum-infused controls. Failure of vaginal opening and clitoral enlargement manifested the well-documented masculinizing effect of testosterone on the genitalia in the experimental as well as the control animals. Estrogen sensitivity of hypothalamic neurons which are responsible for the induction of lordosis was retained in the experimental animals. Recordings of the antidromic action potentials from neurons in the ventromedial nucleus of the hypothalamus following stimulation of the midbrain central gray revealed that estrogen decreased the antidromic activation threshold and shortened the absolute refractory period of the hypothalamic efferents along with the estrogen-induced behavioral activation in the experimental animals. In the control group, the estrogen-induced neuronal activation was lost altogether with the behavioral activation.

Effects of microelectrophoretically applied acetylcholine- and angiotensin-antagonists on the paraventricular neurosecretory cells excited by osmotic stimuli

Extracellular action potentials were recorded from neurons identified antidromically as neurosecretory cells in the paraventricular nucleus of urethane-anesthetized rats. An intracarotid injection of 0.6 M NaCl increased the firing rate of 26 out of the 33 neurosecretory cells tested. The excitation induced by hypertonic NaCl was blocked by hexamethonium in 6 of 10 neurosecretory cells with nicotinic-cholinergic receptors, by atropine in 4 of 5 cells with muscarinic-cholinergic receptors and by [Sar1, Ala8]-angiotensin II in 5 of 12 cells with angiotensin II receptors. These results suggest that nicotinic- or muscarinic-cholinergic receptors and/or angiotensin II receptors appear to be involved in the final transmission of osmotic stimuli in the paraventricular neurosecretory cells.

Interaction of hypertonic NaCl, hemorrhage and angiotensin II in stimulating paraventricular neurosecretory cells in the rat.

SummaryAntidromically identified paraventricular neurosecretory cells were recorded extracellularly in urethane-anesthetized female rats. Their activity was examined for response to 0.3 M NaCl (0.1 ml, intracarotid injection) and hemorrhage (10 ml/kg b.w.) applied separately or in combination, and was also investigated for the interaction of these stimuli with angiotensin II (AII). About half (51%) of the 106 neurosecretory cells recorded were excited both by osmotic stimuli and by hemorrhage. The remaining cells exhibited various combinations of responses such as no change after 0.3 M NaCl and excitation after hemorrhage (24.5%), no change after either stimulus (19.8%), no change after 0.3 M NaCl and inhibition after hemorrhage (3.8%), and excitation after 0.3 M NaCl and no change after hemorrhage (0.9%). When the neurosecretory cells which did not respond to the 0.3 M NaCl were tested for response to the combined application of 0.3 M NaCl and subthreshold bleeding stimulus (2.5–5.0 ml/kg b.w.), 65.4% of them (n = 26) showed an excitatory response. A subthreshold dose (5 ng) of AII injected into the third ventricle (IVT) potentiated the response to 0.3 M NaCl in 61.8% of the neurosecretory cells (n = 34), whereas the same dose of AII had no effect on the response to hemorrhage (10 ml/kg b.w.). An AII antagonist, saralasin (1 μg, IVT), inhibited a response to 0.3 M NaCl in 80.8% of the neurosecretory cells (n = 26), while it affected a response to hemorrhage (10 ml/kg b.w.) in none of them. However, when the dose of saralasin was increased to 4 or 5 μg, it inhibited not only a response to 0.3 M NaCl but also a spontaneous firing activity and a response to hemorrhage of the neurosecretory cells (n = 5). These results suggest that an osmotic stimulus interacts with hemorrhage or AII in stimulating the neurosecretory cell and that endogenous AII is involved in the mechanism for osmotic activation of the neurosecretory cell. On the other hand, hemorrhage seemed hardly to interact with AII, although a dose of saralasin large enough to inhibit the spontaneous activity of neurosecretory cells suppressed their excitatory response to hemorrhage.

Leumorphin, a novel opioid peptide, promotes lordosis in female rats

The actions of leumorphin, a recently characterized endogenous opioid peptide, oppose of most opioid peptides in facilitating lordosis reflex, a major component of female sexual behavior in the rat. Maximal lordosis appeared promptly after infusion of 1 nmol leumorphin into the ventromedial hypothalamus of ovariectomized estrogen-primed rats. This facilitation lasted for as long as 5 h, unless interrupted by midbrain infusion of an antiserum to prolactin. The result is a discovery of a novel substance of remarkable strength in facilitating lordosis, an effect presumably mediated by midbrain release of prolactin.