Synchronized clusters of action potentials can increase or decrease the excitability of the axons of magnocellular hypothalamic neurosecretory cells.

Extracellular recordings were made from supraoptic nucleus (SON) cells in urethane anaesthetized male rats in vivo. Two stimulating electrodes were positioned to activate the cells antidromically, one in the mid axon region of the cells and the other at the axon terminals. Trains of 5-20 just-subthreshold stimuli at 5 s intervals decreased the threshold for antidromic activation from both sites. Whereas neither single stimuli, nor the stimuli at the beginning of a train of 20 stimuli evoked antidromic action potentials, later action potentials did so. Paradoxically, trains of 20 just-suprathreshold stimuli increased the threshold for activation of both axons and terminals. In recordings from the same cells, stimuli were applied singly at 5 s intervals at an intensity which almost invariably evoked an antidromic action potential. Identical stimuli were then applied in trains of 20 stimuli at 50 Hz. After the first train, the initial stimulus pulses of the trains frequently fell below threshold. Following a conditioning train of five stimuli applied to one electrode, the period of decreased threshold (increased excitability) at the other electrode lasted less than 100 ms and the period of increased threshold (decreased excitability) after 12 trains of 20 stimuli lasted between 5 and 10 s. Both decreased and increased excitability were seen at axons and terminals of both putative oxytocin and vasopressin cells. Since the excitability changes were shown in vivo at frequencies encountered during recordings, it is likely that they influence the probability of spike propagation and hormone secretion under physiological conditions.

GHRP-6-induced changes in electrical activity of single cells in the arcuate, ventromedial and periventricular nucleus neurones [correction of nuclei] of a hypothalamic slice preparation in vitro.

Previously, we demonstrated that systemic injection of the growth hormone secretagogue, growth hormone-releasing peptide (GHRP)-6, selectively activated cells in the hypothalamic arcuate nucleus, as reflected by increased electrical activity and induction of the immediate early gene c-fos. The growth hormone secretagogue receptor distribution is not confined to the arcuate nucleus, suggesting that additional sites of action may exist. In the present study we characterized the electrophysiological responses of cells in the arcuate nucleus, ventromedial nucleus and periventricular nucleus in an in-vitro hypothalamic slice preparation, following bath application of GHRP-6. Additionally, since central somatostatin administration has been shown to attenuate the induction of the c-fos gene by GHRP-6, we sought to determine whether the arcuate cells activated by GHRP-6 are also somatostatin-sensitive. Male Wistar rats (100-150 g body weight (BW)) were anaesthetized (urethane; 1.2 g/kg BW) and the brains removed. Coronal sections (400 microm thickness) were cut through a block of hypothalamus and were transferred to a slice chamber perfused with artificial cerebrospinal fluid. Forty-one arcuate nucleus cells were tested with bath application of 15 microm GHRP-6 for 10 min, 16 of which were tested subsequently (>30 min later) with application of 10 microM somatostatin. Following GHRP-6 administration, 19 cells (46. 3%) showed a significant increase in firing rate during the 15-min period after GHRP-6 application (P<0.001), 17 cells (41.5%) did not respond and the remaining five cells (12.2%) were significantly inhibited. Six of the eight arcuate nucleus cells that were excited by GHRP-6 were significantly inhibited by somatostatin. By contrast, five of the six arcuate nucleus cells that were unresponsive to GHRP-6 were also unresponsive to somatostatin. In the ventromedial nucleus, of 19 cells tested, eight cells (42.1%) were excited by GHRP-6, eight cells (42.1%) were unresponsive and the remaining three cells (15.8%) were significantly inhibited. Of 19 cells recorded in the periventricular nucleus, 13 (68.4%) were unresponsive to GHRP-6 and six (31.6%) were significantly inhibited. Thus, electrophysiological studies in vitro suggest that: (1) neurones in the hypothalamic arcuate nucleus, ventromedial nucleus and periventricular nucleus show changes in electrical activity in response to GHRP-6; and (2) the arcuate nucleus cells excited by GHRP-6 are also subject to inhibitory control by somatostatin.

Electrophysiological evidence for mutual excitation of oxytocin cells in the supraoptic nucleus of the rat hypothalamus.

1. Using the ventral surgical approach in vivo, extracellular recordings were made from seventy-nine cells in the supraoptic nucleus of urethane-anaesthetized male, virgin female or lactating female rats while stimulating the pituitary stalk. Cells were classed according to their spontaneous firing activity as: continuous (putative oxytocin), phasic (putative vasopressin) and silent. 2. Stimulation of the neural stalk produced an excitation (up to 25 ms poststimulus) in eleven of the seventy-nine antidromically identified magnocellular neurones, consistent with the existence of excitatory collaterals or dendritic contacts between such cells. In these recordings a second spike could frequently be seen, following the antidromic spike, with a variable latency. Such spikes consistently collided with subsequent antidromically evoked spikes. Poststimulus excitation was only seen in silent and continuously firing (putative oxytocin) cells, suggesting that oxytocin and vasopressin cells have different connections. 3. Excitatory connections were seen more frequently in lactating females (8 out of 22 cells) than in males (1 out of 15 cells) or virgin females (2 out of 10 cells), and thus may make an important contribution to the bursts of firing which precede reflex milk ejection.

Osmoresponsiveness of the rat supraoptic nucleus in vivo depends on glutamatergic inputs.

Intracellular recording from supraoptic nucleus (SON) neurones in hypothalamic slices revealed that a 40 mOsm change in osmolality had little effect on membrane conductance. However, intracellular recordings in vivo revealed a significant increase in EPSP frequency after the plasma osmolality had been raised by approximately 10 mOsM. We recorded extracellularly in vivo from 18 antidromically identified SON neurones in urethane-anaesthetised male Wistar rats, while hypertonic saline was infused intravenously (1.05M NaCl, 50 microliters/min for 35 minutes). In 9 experiments an intracerebroventricular (ICV) injection of 5 microliters 10mM kynurenic acid was given 5 minutes prior to start of the infusion. Kynurenate significantly reduced the osmotic response (P < 0.001, Student's t-test); the rat of increase in spike frequency was reduced from 0.089 +/- 0.004 Hz/min (n = 9) to 0.035 +/- 0.003 Hz/min (n = 9). Kynurenic acid did not reduce the basal firing rate of SON neurones, but in osmotically stimulated neurones, it reduced the firing rate to basal levels. Hence the osmoresponsiveness of SON neurones depends on a glutamatergic input that is independent of mechanisms that maintain basal electrical activity.