The effects of iontophoretic application of putative neurotransmitters on the electrical activity of rat mediobasal hypothalamic neurons in relation to their steroid sensitivity

Differential involvement of dopamine D1 and D2 receptors and inhibition by dopamine of hypothalamic VMN neurons in early postnatally overfed juvenile rats

Dopamine is among the neurotransmitters involved in central regulation of food intake, and body weight control. To study possible changes in neuronal responses to dopamine, single unit activity of the ventromedial hypothalamic nucleus (VMN) was recorded in brain slices of normal and obese rats. The latter had developed overweight throughout juvenile life (p < 0.05) by early postnatal over-nourishment due to a reduction of litter size from 3rd to 21st day of life (small litters, SL). With effective concentrations of about 100-500 nM/I dopamine inhibited significantly more VMN neurons in obese than normal rats (Chi-square p < 0.05). While D2 receptors in the VMN are reported to mediate inhibition of food intake, the responses to dopamine were blocked by D2 receptor antagonists in significantly fewer neurons of SL than normal rats (p < 0.05). Furthemore, including results of action of D1 receptor agonists we found that significantly more neurons in SL than NL rats seem to express D1 receptors. Thus, increased suppression by dopamine of firing of VMN neurons that signal satiety with a rise in the discharge rate, and changed expression or activity of dopamine receptors might contribute to increased feeding behavior in juvenile rats hyperphagic and overweight due to early postnatal overfeeding.

Non-genomic effects of glucocorticoids in the neural system. Evidence, mechanisms and implications

Complementing the classical concept of genomic steroid actions, here we (i) review evidence showing that important neural effects of glucocorticoids are exerted by non-genomic mechanisms; (ii) describe known mechanisms that may underlie such effects; (iii) summarize the functions and implications of non-genomic mechanisms and (iv) outline future directions of research. The role of non-genomic mechanisms is to shape the response of the organism to challenges that require a substantial reorganization of neural and somatic functions and involve massive behavioral shifts. Non-genomic effects may (i) prepare the cell for subsequent glucocorticoid-induced genomic changes, (ii) bridge the gap between the early need of change and the delay in the expression of genomic effects and (iii) may induce specific changes that in some instances are opposite to those induced by genomic mechanisms. The latter can be explained by the fact that challenging situations require different responses in early (acute) and later (chronic) phases. Data show that non-genomic mechanisms of glucocorticoid action play a role in both pathological phenomena and the expression of ameliorative pharmacological effects. Non-genomic mechanisms that underlie many glucocorticoid-induced neural changes constitute a for long overlooked controlling factor. Despite the multitude and the variety of accumulated data, important questions remain to be answered.

Cortisol exerts site-, context- and dose-dependent effects on agonistic responding in hamsters

Abstract Site-, context- and dose-dependent actions of intrahypothalamic cortisol administration on the agonistic behaviors of adult male golden hamsters (n = 128 dyads) were examined. When cortisol-treated animals were tested in paired encounters with aggressive cholesterol-treated opponents, chronic (>/= 24 h) cortisol treatment (1 mm implants) induced significant (P < 0.05) submission in three medial hypothalamic areas (anterior hypothalamic area > medial preoptic area > ventromedial hypothalamus), but aggression in the paraventricular nucleus or third ventricle. In contrast, chronic cortisol treatment in the anterior hypothalamic area resulted in high levels of aggression during paired encounters with submissive opponents, and during territorial aggression tests with juvenile male intruders. Acute (>/= 20 min) cortisol treatment in the anterior hypothalamic area (100 nl injections) induced significant submission after 10(-2) M, but significant aggression after 10(-6) M microinjections in paired encounters with aggressive vehicle-injected opponents. These findings suggest glucocorticoid-sensitive mechanisms within the anterior hypothalamus modulate aggressive responding during intrasexual social encounters.

Iontophoretic application of glucocorticoids inhibits identified neurones in the rat paraventricular nucleus

In an electrophysiological study designed to examine the negative feedback effects of glucocorticoid hormones, we have recorded the electrical activity of 147 neurones in the paraventricular nucleus of the rat hypothalamus. 37 (25%) of the neurones were antidromically identified as projecting to the median eminence and were located at a mean depth of 2.35 +/- 0.08 mm from the base of the brain, corresponding with the corticotropin-releasing factor-rich region of the nucleus. The mean firing rate of the identified cells was 4.7 +/- 0.6 Hz which was not significantly different from that of adjacent, unidentified cells (5.6 +/- 0.6 Hz). Most (17/18, 94%) of these cells tested responded to painful somatosensory stimuli and 26 (74%) of the identified cells were inhibited by iontophoretic application of corticosterone and/or hydrocortisone, whereas only one cell was excited and 8 unaffected. Of the identified cells, only 18 (20%) were inhibited, 36 (41%) were excited and 34 (39%) were non-responsive. The proportion of inhibitory responses was thus greater for the identified cells (P less than 0.005; chi 2-test). For the identified cells, whose spontaneous activity was unaffected by glucocorticoid application, glutamate-evoked responses could usually be depressed by the application. The time course of all responses usually showed an immediate onset, increasing in magnitude and continuing for extended periods following cessation of iontophoresis. Electrophysiologically identified magnocellular neurones were also tested and the majority (7/12, 58%) of vasopressin-secreting neurons were also found to be inhibited, whilst all (8/8, 100%) of the oxytocin-secreting neurones were excited by the glucocorticoid application. These results may represent an electrophysiological correlate of the negative feedback control of adrenocortical secretion and are discussed within this context.

Effect of cortisol on a model of stereotyped behavior of rabbits in the form of thumping

Experiments conducted on 10 adult male Chinchilla rabbits showed that single 5-sec electrostimulation of the region of the medial hypothalamus induces a series of stereotyped behavioral responses in the form of synergic thumping on the floor of the experimental chamber. When the stimuli are repeated every 1.5-2 min adaptation to stimulation is observed, which is expressed in a gradual decrease of the number of thumps. To study the pharmacological and hormonal effects on the described behavior, an experimental model is proposed which consists of two series of stimulation, each of which consists of 10 successive stimuli. The investigated pharmacological agent (cortisol [hydrocortisone] hemisuccinate) was injected into the animals in the interval between the two series of stimulation. A statistical analysis of the experimental results showed that intraperitoneal injection of cortisol in doses of 10 and 20 mg/kg enhances the simulation-induced behavioral response and shortens the latent period of its manifestation compared with the control. Key words: rabbit, electrostimulation, medial hypothalamus, stereotyped behavior, cortisol.

Estrogen effects on neuronal responsiveness to electrical and neurotransmitter stimulation: an in vitro study on the ventromedial nucleus of the hypothalamus

To investigate the effects of estrogen on the activity of the neurons in the hypothalamic ventromedial nucleus (VMN), the single-unit activity was recorded from in vitro brain tissue slices prepared from ovariectomized rats that were either treated or untreated with estrogen. In tissue slices continuously perifused with estrogen-free solution, half of the 348 VMN neurons studied were silent or nearly silent, and the rest fired spontaneously at low rates. Electrical stimulation of the periventricular region medial to the VMN evoked not only 3 types of orthodromic responses, but also antidromic responses from many VMN neurons, indicating that the VMN and the periventricular region are intimately and reciprocally connected. Neuronal responses to a battery of 6 neurotransmitters were studied by microinjecting 50 microliters of a transmitter solution directly into the chamber where the tissue slice was being continuously perifused. When effective, glutamate (Glut) was almost exclusively excitatory; acetylcholine (ACh) and norepinephrine (NE) could be excitatory and/or inhibitory; 5-hydroxytryptamine (5-HT) and dopamine (DA) were predominantly inhibitory; and gamma-amino-butyric acid (GABA) was exclusively inhibitory on neuronal activity. The overall resting neuronal activity and responses to the transmitter were very similar to those observed in in vivo studies, indicating that, for VMN at least, in vitro tissue slices can serve as a simplified model for studying neuronal activity and responses. Comparisons of results between estrogen-treated and untreated preparations showed that chronic, in vivo estrogen treatments, while having no effect on resting firing rate, selectively facilitated the responsiveness of silent (but not spontaneously active) neurons to excitatory orthodromic inputs, and the responsiveness to the excitatory action of ACh, the overall action of NE and the inhibitory action of 5-HT. These selective estrogen effects would no doubt alter the input-output relationships of VMN neurons evoked by the orthodromic inputs and ACh, NE and 5-HT. Since the VMN is crucial to many biological functions, and the cholinergic, adrenergic and serotonergic inputs to the ventromedial hypothalamus are involved in the estrogenic regulation of sexual behavior, reproductive neuroendocrine, and other biological functions, the alteration of the input-output relationships evoked by the orthodromic and transmitter inputs would affect the regulation of many functions.

Inhibitory action of adrenocorticotrophin on rat arcuate neurons in vitro

Extracellular recordings were made from 52 units in perifused 300 microns thick slices of rat hypothalamus taken parallel to the ventricular surface of the arcuate nucleus and kept at 37 degrees C in an organ bath. Addition of 8 nM adrenocorticotrophic hormone (ACTH) to the perfusate reversibly and rapidly inhibited 15 of the 38 units found within 300 microns of the ventricular surface (in the arcuate nucleus), and there were no comparable excitations. A prompt and reversible excitation of 6/14 units by ACTH was found when records were made from units found 300-600 microns lateral to the ventricle in the lateral arcuate region. It is suggested that ACTH may be an inhibitory transmitter in the arcuate nucleus and that this action may be an inhibition of ACTH-containing neurons.