Electrophysiology of the afferent input to oxytocin- and vasopressin-secreting neurones. Facts and problems

Gene regulation of vasopressin and vasopressin receptors in cancer

It is proposed that neuropeptide production by tumours is an important part of a special process of oncogenic transformation rather than a pre-existing condition of progenitor cells; this concept is called Selective Tumour gene Expression of Peptides essential for Survival (STEPS). All small-cell lung cancers and breast cancers evidently express the vasopressin gene, and this gene seems to be structurally normal in all but exceptional cases. Vasopressin gene expression in cancer cells leads to the production of both normal and abnormal forms of tumour vasopressin mRNA and proteins. Although the necessary post-translational processing enzymes are expressed in these cells, most processing seems to be extragranular, and most of the protein products become components of the plasma membrane. Small-cell lung cancer and breast cancer cells also express normal genes for all vasopressin receptors and produce normal vasopressin receptor mRNAs and V1a and V1b receptor proteins, and the vasopressin-activated calcium mobilising (VACM) protein; plus both normal and abnormal forms of the V2 receptor. Through these receptors, vasopressin exercises multifaceted effects on tumour growth and metabolism. A normal protein vasopressin gene promoter seems to be present in small-cell lung cancer cells, and this promoter contains all of the transcriptional elements known to be involved in gene regulation within hypothalamic neurones. Since these elements largely account for regulation of tumour gene expression observed in vitro, it is likely that as yet unknown factors are selectively produced by tumours in vivo to account for the observed seemingly autonomous or unregulated production of hormone in tumour patients. Promoter elements thought to be responsible for selective vasopressin gene expression in small-cell lung cancer probably include an E-box and a neurone restrictive silencer element close to the transcription start site. It is possible that transcription factors acting at these same elements can explain selective vasopressin expression, not only in small-cell tumours, but also in all other tumours such as breast cancer. By extrapolation, similar mechanisms might also be responsible for the expression of additional features that characterize the 'neuroendocrine' profile of these cancers.

Characterization of carbachol-induced rhythmic bursting discharges in neurons from guinea pig lateral septum slices

A brain slice preparation from guinea pigs and intracellular recording techniques were used to examine the effects of carbachol application on the three classes (A, B, and C) of neurons (n = 68, 40 of class A, 12 of class B, 16 of class C) within the mediolateral part of the lateral septum (LSml). Bath application of carbachol elicited a sustained depolarization associated with an increase in membrane input resistance, action-potential firing and triggered rhythmic bursting discharges in 59% of recorded neurons. According to the configuration of these bursts, LSml neurons were classified into type I, II, and III neurons with reference to their response to carbachol. The frequency of spontaneous bursts was increased by depolarization caused by applied DC current in the three types of neurons. Bursts in type II and III neurons were voltage and dose dependent. These dependences were responsible for a continuum of variation in carbachol responses in these two types of neurons. As the neuron depolarized in the presence of carbachol, spontaneous action potentials increased in frequency and slow afterdepolarizing potentials (sADPs) appeared and preceded the occurrence of the first burst. These sADPs from adjacent action potentials appeared to progressively increase to initiate a burst. In the presence of carbachol, sADPs and bursts were also observable after action potentials evoked by depolarizing current pulses at the resting membrane potential (RMP) in LSml neurons. Evoked sADPs and bursts were associated with an apparent increase in input conductance. Application of low Na+ medium blocked both the sADP and bursts. Application of zero Ca2+ medium either 1) blocked completely the generation of sADPs and bursts (n = 16), or 2) did not block bursts (n = 14). Evoked sADPs and bursts were blocked by tetraethylammonium but were resistant to external Cs+. The results indicate that the activation of cholinergic receptors does not differentially affect the three classes of LSml neurons. The responses to carbachol in type II and III neurons form a continuum of variation, whereas these of type I neurons constitute a discrete entity. The selective cholinergic induction of a sADP, and the progressive activation of these sADPs in LSml neurons are thought to be responsible for the onset of the three types of rhythmic bursting discharges. We propose that sADPs and bursts induced by carbachol are generated by a cationic conductance largely permeable to Na+. In a subpopulation of LSml neurons (n = 16), the bursts are dependent on the presence of Ca2+ in the medium.

Release of oxytocin into blood and into cerebrospinal fluid induced by naloxone in anaesthetized morphine-dependent rats: the role of the paraventricular nucleus

Abstract Opioid actions on oxytocin secretion into blood and cerebrospinal fluid (CSF) were investigated in urethane-anaesthetized female rats after intracerebroventricular (icv) infusion of morphine sulphate or vehicle for 5 days. Serial femoral arterial blood samples and cisterna magna CSF samples were collected for radioimmunoassay. Naloxone was given to assess endogenous opioid tone in icv vehicle-infused rats and to precipitate withdrawal in morphine-dependent animals. Initial plasma oxytocin concentration was not affected by icv morphine infusion. In control rats receiving icv vehicle, naloxone increased plasma oxytocin 11-fold within 5 min, and in icv morphine-infused rats, naloxone increased plasma oxytocin 80-fold within 5 min. In both groups, 90 min after naloxone plasma oxytocin was still 5 and 10 times, respectively, the initial concentration. Without naloxone, neither plasma nor CSF oxytocin concentration changed significantly with time (up to 90 min) in either icv treatment group. In the icv vehicle group, there was a 2-fold increase in CSF oxytocin 90 min after naloxone. In the icv morphine-infused group, CSF oxytocin was increased 5-fold 40 min after naloxone. In another group of icv morphine-infused rats, intravenous infusion of oxytocin to achieve plasma levels similar to those seen after naloxone, did not significantly increase CSF oxytocin. In a further group of icv morphine-infused rats, [(3)H]oxytocin was infused intravenously immediately after naloxone was given; in these rats oxytocin transfer from blood to CSF could account at most for only 20% of the increase in CSF oxytocin after naloxone. A further group of rats underwent bilateral microknife ablation of the paraventricular nuclei (PVN) 9 days before icv vehicle or morphine infusions were started; blood and CSF samples were collected under urethane anaesthesia. Initial concentrations of oxytocin in CSF and in plasma were similar in both groups with PVN ablation. In all PVN-lesioned rats initial plasma concentrations of oxytocin were undetectable (<5 pg/ml) and thus less than in intact rats. In contrast, initial levels of oxytocin in CSF were 8-fold greater in PVN-lesioned rats than in intact animals. Naloxone increased plasma oxytocin concentration in the icv vehicle group at least 10-fold within 30 min and in the icv morphine group at least 100-fold within 5 min. CSF oxytocin in the icv vehicle group was not altered by naloxone, but in the icv morphine group CSF oxytocin was increased 5-fold 40 min after naloxone. There were no consistent differences between the icv vehicle- and icv morphine-treated groups in the initial plasma levels of vasopressin, growth hormone and adrenocorticotrophin; PVN ablation did not affect adrenocorticotrophin levels. After naloxone growth hormone levels did not change, vasopressin concentration rose moderately only after 90 min and only in the icv vehicle-treated group, and adrenocorticotrophin concentrations decreased with time whether or not naloxone was given. The results imply an endogenous opioid tone on neurons releasing oxytocin into CSF, and morphine-dependence of these neurons. Furthermore, in PVN-lesioned rats, magnocellular supraoptic neurons could be a source of oxytocin release into CSF.

Fastigial inputs to paraventricular neurosecretory neurones studied by extra- and intracellular recordings in rats

1. The effects of stimulation of the cerebellar fastigial nucleus (FN) on the activity of neurosecretory neurones in the hypothalamic paraventricular nucleus (PVN) of rats, anaesthetized with urethane and alpha-chloralose, were investigated by extracellular and intracellular recordings. 2. With extracellular recording, 139 PVN neurosecretory neurones were identified by antidromic activation following stimulation of the pituitary stalk, of which 120 were spontaneously firing and 19 were silent. Three types of responses to FN stimulation (1 or 2 pulses at 333 Hz) were observed in 43% of spontaneously firing PVN neurosecretory neurones: inhibition (type I) with a latency of 4.3 +/- 4.1 ms (mean +/- S.D., 32 of 120 neurones, 27%); excitation (type E) with 22.3 +/- 8.1 ms latency (14 of 120, 11%); and inhibition-excitation type (I-E) with 5.5 +/- 3.4 ms latency (6 of 120, 5%). Silent neurosecretory neurones did not respond to FN stimulation. Twelve per cent of non-neurosecretory cells (three out of twenty-six tested) responded to FN stimulation (one was inhibited, and two were excited). 3. Repetitive stimulation (60 Hz, 10 s) of the FN, which evoked a stimulus-locked pressor response, suppressed on-going activity of PVN neurosecretory neurones in 43% (twenty-nine of sixty-eight) of neurones tested. In 40% of these neurones, the activity was also inhibited by intravenous injection of phenylephrine (3 micrograms in 0.3 ml Ringer solution), while in other neurones the injection had no effect. Rebound excitation of neurone activity lasting for 1-5 min after termination of repetitive stimulation was observed in 28% of the neurones. Ten neurones (14%) were excited by repetitive stimulation. 4. Successful intracellular recordings were made from seventy-two PVN neurones, of which thirty-seven were neurosecretory cells. The mean resting membrane potential was -51 mV (n = 72; range from -40 to -75 mV). The input resistance of neurosecretory cells was 117 +/- 21 M omega (range 93-153 M omega; n = 8). This value was higher than that for non-neurosecretory cells which was found to be 53 +/- 10 M omega (range 35-72 M omega; n = 9). The difference was statistically significant (P less than 0.01, Student's t test). 5. In response to FN stimulation, sixteen (43%) of the thirty-seven neurosecretory neurones showed IPSPs with latencies of 7.4 +/- 2.8 ms and three (8%) exhibited EPSPs with latencies of 13.3 +/- 4.2 ms.(ABSTRACT TRUNCATED AT 400 WORDS)