Mariana Morris

Plasma oxytocin levels in autistic children.

BACKGROUND Social impairments are central to the syndrome of autism. The neuropeptide oxytocin (OT) has been implicated in the regulation of social behavior in animals but has not yet been examined in autistic subjects. METHODS To determine whether autistic children have abnormalities in OT, midday plasma samples from 29 autistic and 30 age-matched normal children, all prepubertal, were analyzed by radioimmunoassay for levels of OT. RESULTS Despite individual variability and overlapping group distributions, the autistic group had significantly lower plasma OT levels than the normal group. OT increased with age in the normal but not the autistic children. Elevated OT was associated with higher scores on social and developmental measures for the normal children, but was associated with lower scores for the autistic children. These relationships were strongest in a subset of autistic children identified as aloof. CONCLUSIONS Although making inferences to central OT functioning from peripheral measurement is difficult, the data suggest that OT abnormalities may exist in autism, and that more direct investigation of central nervous system OT function is warranted.

Stress increases oxytocin release within the hypothalamic paraventricular nucleus

Evidence indicates that the hypothalamic paraventricular nucleus (PVN) and oxytocin (OT) neurons in particular play a role in the physiological response to stress. Microdialysis (MD) experiments were performed to determine whether OT is released into the PVN during shaker stress. Male rats were prepared with venous catheters and PVN guide cannulae. OT and vasopressin (VP) release into PVN and peripheral blood were measured under basal conditions and during and after shaker stress (10 min at 110 cycles/min). Stress produced a specific increase in PVN and plasma OT. Dialysate OT levels were 0.3+/-0.1, 2.8+/-1.2 and 1.3+/-0.6 pg/sample (control, stress and recovery, respectively). Plasma OT was significantly increased during stress (3.7+/-1.2 vs. 11.7+/-2.3 pg/ml, basal vs. stress, respectively). When MD probes were located outside the PVN, there was no increase in OT release, demonstrating site specificity. Stress produced no change in VP levels, either in dialysate or plasma. These results show that OT, but not VP, is released into the PVN and peripheral blood in response to shaker stress. The data raise the possibility that local release of OT into the PVN plays a role in the neuroendocrine stress cascade.

Systemic osmotic stimulation increases vasopressin and oxytocin release within the supraoptic nucleus

Vasopressin (VP) and oxytocin (OT) are released within the hypothalamic nuclear region in response to direct microdialysis with hypertonic solutions. Experiments were performed to determine whether systemic osmotic stimulation causes changes in intranuclear peptide release within the supraoptic nucleus (SON). A hypertonic sodium chloride solution was injected intraperitoneally (ip) or intravenously (iv) and microdialysis techniques were used to simultaneously monitor central and peripheral peptide release in urethane anesthetized rats. Systemic osmotic stimuli elicited increases in intranuclear peptide release which were delayed and long‐lasting, occurring over a 2.5 h period. In contrast, plasma peptide levels peaked at 30‐min after the stimulus. The results demonstrate that increased plasma sodium elicits an increase in VP and OT release into the extracellular space of the hypothalamic SON. The different patterns of peptide release in plasma and brain point toward the possibility of independently regulated release into the different compartments.

Neuropeptide gene expression in hypothalamic magnocellular neurons of normal and hypophysectomized rats: A combined immunohistochemical and in situ hybridization study

Hypothalamic magnocellular neurons of the paraventricular and supraoptic nuclei contain several peptides and non-peptide putative neurotransmitters co-existing with vasopressin and oxytocin. However, the functional role of these substances is still unknown. In the present paper the temporal course of changes in the expression of vasopressin, oxytocin, galanin, cholecystokinin, dynorphin and tyrosine hydroxylase in magnocellular hypothalamic neurons of rats subjected to hypophysectomy was examined. Following different survival times the animals were processed either for immunohistochemistry with antibodies against the above mentioned peptides or for in situ hybridization with synthetic oligonucleotide probes complementary to the mRNAs encoding for the peptides. The results obtained showed a marked rise in vasopressin mRNA levels at two days followed by a decrease up to 36 days of survival. Oxytocin mRNA responded to the lesion with a transient decrease, with its lowest values between five and seven days. This was followed by a recovery which almost reached normal values at 36 days of survival. The results also showed a marked, transient activation of the synthetic pathway for galanin and cholecystokinin. The numbers of cells expressing these peptides were maximal between five and seven days, and the respective mRNA levels were significantly increased at these survival times. This was followed by a decrease in the amount of galanin- and cholecystokinin-like immunoreactivity as well as in the levels of their respective mRNAs. Dynorphin-like immunoreactivity showed a course similar to that of galanin and cholecystokinin in operated animals. However, the amounts of dynorphin mRNA were significantly increased at two days, but were followed by a reduction at five days and remained low throughout the different survival times tested. The experiments performed with the tyrosine hydroxylase antibodies and probe showed undetectable levels of the enzyme and its mRNA in normal and hypophysectomized animals. These results demonstrate that, in magnocellular hypothalamic neurons, expression of several peptides occur in differential ways after hypophysectomy. The possibility is discussed that these changes represent part of the mechanisms underlying the process of degeneration and regeneration known to occur in magnocellular hypothalamic neurons after hypophysectomy.

Single and Repeated Environmental Stress: Effect on Plasma Oxytocin, Corticosterone, Catecholamines, and Behavior

Rat studies were done to further characterize an environmental model of stress designated shaker stress (SS). Plasma oxytocin (OT), corticosterone (CS), norepinephrine (NE), and epinephrine (E) were measured before and after 5 or 30 min of SS applied one time or applied 10 times over a 2-week period. The major findings were partial adaptation of plasma E within 30 min of acute SS, adaptation of plasma CS baselines but not responses to chronic SS, and complete adaptation of plasma OT responses to chronic SS. Poststress behavior during chronic SS was affected in the following ways: freezing time habituated, defecation and rearings increased, and grooming and teeth chattering remained relatively constant. The results show that SS produces consistent patterns of hormonal and behavioral responses; some aspects of the patterns are similar to those elicited by other environmental stresses, whereas some aspects are unique to SS. We conclude that rats do not adapt to repeated SS but rather that most hormonal and behavioral defense mechanisms are renewable on a daily basis.

Extracellular oxytocin in the paraventricular nucleus: hyperosmotic stimulation by in vivo microdialysis.

The effect of central osmotic stimulation on oxytocin (OT) secretion from the paraventricular nucleus (PVN) was examined using a newly developed in vivo microdialysis technique. A dialysis probe was inserted into the PVN region and microdialysis was performed in conscious animals. Hyperosmotic solutions were delivered via the dialysis probe, and perfusate and blood samples were collected. OT was consistently detected in the PVN dialysate. Hyperosmotic sodium chloride (1 M) produced a significant increase in dialysate and plasma OT, whereas D-mannitol (2 M) had no effect. These results suggest that (1) in vivo microdialysis may provide a useful technique for the evaluation of neuropeptide secretion from specific brain regions and (2) there are sodium-sensitive cells in the PVN region which respond to increases in extracellular sodium, resulting in an increase in central and peripheral oxytocin secretion.

Central Oxytocin Mediates Stress-Induced Tachycardia

To address the role of oxytocin in the control of cardiovascular reactivity, we examined the effect of central injection of oxytocin, vasopressin and mixed base antisense oligodeoxynucleotides on stress‐induced cardiovascular and endocrine changes. Antisense oligomers were injected into the paraventricular nucleus (PVN), 4 h prior to the stress test. The oxytocin antisense abolished the tachycardia produced by 5 min of shaker stress. The blood pressure and plasma oxytocin responses were not different between the groups. PVN levels of OT were reduced in the oxytocin antisense‐treated group while brain stem levels were increased. These results demonstrate the importance of a specific peptide system, the PVN/oxytocin axis, in stress‐induced tachycardia. Further, the data illustrate the effectiveness of short‐term treatment with antisense oligomers on physiological responses.

Baroreceptor influences on oxytocin and vasopressin secretion.

The objective of these studies was to investigate the role of arterial baroreceptors in the control of neurohypophyseal secretion. The effect of sinoaortic denervation on basal and osmoticinduced release of oxytocin and vasopressin and on blood pressure was determined. Hypertonic or isotonic saline was infused intravenously into sham-operated or denervated rats 3 days after surgery. Plasma oxytocin and vasopressin were measured at 5 and 15 minutes after the infusion. The control levels of oxytocin were increased in the denervated rats, but vasopressin levels were not significantly altered. The vasopressin and oxytocin responses to hypertonic saline were greater after baroreceptor denervation. Plasma oxytocin was increased from 4.7±0.9 to 72.2±8.7 pg/ml in the denervated rats and from 1.8±0.3 to 39.9±6.7 pg/ml in the shamoperated control group at 5 minutes after the infusion (p < 0.01). The plasma vasopressin response to hypertonic saline was 7.1 ±0.6 pg/ml in the sham-operated versus 11.1±1.6 pg/ml in the denervated rats (p < 0.05). There was no difference between sham-operated and denervated rats hi the effect of hypertonic saline on plasma sodium and hematocrit. Mean arterial blood pressure was increased after sinoaortic denervation (116.3±4.2 mm Hg in the sham-operated vs. 138.2±8.3 mm Hg in the denervated rats, p < 0.05); however, there was no difference in the pressor response to hypertonic saline. These results show that the baroreceptor system influences the secretion of both oxytocin and vasopressin, with effects on basal secretion as well as the response to an osmotic stimulus. These changes may be important in the regulation of cardiovascular and fluid balance under conditions of baroreceptor deficiency.

Central and systemic oxytocin release : a study of the paraventricular nucleus by in vivo microdialysis

The mechanisms controlling central and systemic oxytocin (OT) release were examined using in vivo microdialysis of the paraventricular (PVN) region. Dialysate and plasma samples were collected from conscious male rats and stimuli were administered via the dialysate fluid. Characterization studies showed that microdialysis was a viable technique for the study of peptide secretion in the conscious animal. OT was consistently detected in the PVN dialysate and a partially purified extract crossreacted in parallel fashion with the synthetic peptide. In vitro studies showed that peptide recovery was positively correlated with the pore size of the dialysis membrane and that there was an inverse relationship between flow rate and recovery. Hypertonic saline administered centrally caused an increase in dialysate and plasma oxytocin while the intravenous injection affected only plasma oxytocin. The excitatory amino acid, glutamate (0.05-0.5 M), caused an increase in plasma, but not dialysate oxytocin, while depolarization with potassium chloride (0.05-0.15 M) had no significant effects. Histological examination showed that the dialysis probe was located in the rostral, lateral PVN. Our results show that in vivo microdialysis provides a method for the delivery of drugs into specific brain regions as well as a useful technique for the evaluation of in vivo neuropeptide release.

Excitotoxin paraventricular nucleus lesions: stress and endocrine reactivity and oxytocin mRNA levels

Electrolytic lesion of the paraventricular nucleus (PVN) of the hypothalamus blocks the tachycardia response to stress. The current study examined the effects of chemical lesion of PVN parvocellular neurons on the cardiovascular and endocrine responses to stress and on the content of hypothalamic oxytocin (OT) mRNA levels. Acute footshock stress increased heart rate in both ibotenic acid lesion and control groups of animals; however, the tachycardia was significantly lower in animals with a PVN lesion than the controls. Lesion of the PVN also attenuated the increase in plasma OT induced by stress, 4-fold in the lesion group versus 20-fold for the controls. There was not a generalized decrease in hormonal responsiveness since the OT response to an osmotic challenge was exaggerated in the lesion group. There was no difference between the groups in the arterial pressure and vasopressin responses to acute stress. Neurotoxin lesions of the PVN also resulted in significant depletions of VP and OT in all levels of the spinal cord and decreased OT levels in the dorsal brainstem. Ibotenic acid lesions of the PVN resulted in no significant changes in OT mRNA in the PVN, SON and PP. In addition, the 48-h dehydration resulted in a significant increase in plasma OT and OT mRNA in the PVN. These data indicate that the parvocellular neurons of the PVN play a role in integration of cardiovascular and endocrine responses to both stressful and osmotic stimuli and provide further evidence that parvocellular OT and VP neurons project to the brainstem and spinal cord.