Absence of a circadian rhythm in persisting corticosterone fluctuations following surgical isolation of the medial basal hypothalamus

Regulation of basal ACTH secretion by corticosterone is mediated by both type I (MR) and type II (GR) receptors in rat brain

The mechanisms involved in the physiology of the secretion of ACTH are reviewed. The secretion is regulated by the biological consequences of the occupancy of high affinity mineralocorticoid (MR) and lower affinity glucocorticoid receptors (GR) for corticosterone at specific sites of the rat brain. The regulation by this mechanism of basal secretion during the circadian rhythm, the effect of adrenalectomy and of corticosterone replacement is discussed. Experiments with RU486, a specific glucocorticoid antagonist, suggest that occupancy of both MR and GR is required for normal control of ACTH at the time of peak activity. The occupancy of the GR for a few hours per day apparently suffices to maintain steady levels of the products of GR-responsive genes throughout the body.

Differential fornix ablations and the circadian rhythmicity of adrenal corticosteroid secretion

Suction ablations of the medial or lateral fornix were performed in order to transect selectively the medial corticohypothalamic tract (mcht) which originates in the anteroventral subiculum and travels in the lateral fornix terminating in the basal hypothalamus. The circadian rhythmicity of plasma adrenal corticosteroid levels was assessed in individual animals 1--2 weeks postoperatively. Ablation of the lateral fornix disrupted the periodicity of corticosteroid secretion which is normally synchronized with the light--dark cycle, whereas medial fornix ablation or neocortical ablation caused no such disruption. Group mean levels of plasma adrenal corticoids were higher in the lateral fornix-ablated animals than in the medial fornix-ablated, neocortically ablated, or intact control animals. These findings suggest that the anteroventral subiculum is important in the regulation of adrenal corticosteroid rhythmicity, and that it exerts an inhibitory influence upon corticosteroid release.

Participation of brain catecholaminergic neurons in a self-sustained circadian oscillation of plasma corticosterone in the rat

Circadian rhythms of locomotor activity and plasma corticosterone were determined simultaneously in individual rats which were injected intraventricularly with 6-hydroxydopamine (6-OHDA), a specific depletor of catecholamines. In light-dark cycles (LD), 6-OHDA-treated rats showed essentially normal circadian rhythms of both functions. In 200 lux continuous light (LL) the locomotor activity in the drug-treated rats ran freely with a period slightly longer than 24 h: free-running parameters (mesor, amplitude and period) of the locomotor activity were the same as those control rats. The circadian rhythm of plasma corticosterone in control rats also ran freely in LL, resulting in a reversal of their phase after 12 days exposure to LL. However, the rhythm disappeared in 6-OHDA-treated rats under LL: instead in these rats 3--4 episodes of secretion were obsered over a 24 h period. These results suggest a difference in the biochemical background between the regulatory mechanism of circadian rhythm of locomotor actitivity and that of plasma corticosterone. It is also apparent that brain catecholaminergic components are indispensable for the circadian rhythm of plasma corticosterone to freerun normally in LL, but are dispensable in LD. A plausible model consisting of two oscillators is proposed to explain these findings.

Participation of limbic-hypothalamic structures in circadian rhythm of slow wave sleep and paradoxical sleep in the rat

The effect of brain lesion or surgical isolation of the neural circuit on SWS and PS circadian rhythm have been studied in female rats under a 14/10 light-dark schedule. Cortical EEG'S AND DORSAL NECK EMG were used to monitor SWS, PS and alertness in female rats. Intact and operated controls showed regular 4-5-day vaginal cycles and nocturnal sleep rhythm, but the night PS value on proestrus was lower than in other cycles. Following septal lesion, MPO roof cut, vaginal cycles and SWS rhythm were regularly maintained; however, the PS appearance at night, except during proestrus, increased (night PS peak). These results were similar to those for pinealectomized or ovariectomized female rats. A frontal cut of the MBH produced persistent estrus and disturbed both SWS and PS circadian rhythm. The suprachiasmatic-lesioned rats showed persistent estrus and disrupted SWS rhythm, but regularly maintained the circadian PS rhythm. The vaginal cycles and SWS rhythm in the fornical-transected rats were regularly maintained, but the PS rhythm was disturbed during diestrus and showed ultradian rhythm. From these results, it is suggested that the pineal hormone and the gonadal feedback mechanisms may be involved in the night PS peak and this mechanism may involve the septal- and amygdaloid-hypothalamic systems. A different neural mechanism exist for SWS and PS circadian rhythm; SWS rhythm involves the suprachiasmatic-basal hypothalamic system and PS circadian rhythm is related, in part, to the hippocampal-hypothalamic system.

Abolition of plasma growth hormone response to stress and of the circadian rhythm in pituitary-adrenal function in female rats with preoptic-anterior hypothalamic lesions

The chronic effects of lesions placed in the medial or lateral preoptic-anterior hypothalamic area (PO-AHA) on plasma growth hormone (GH) and corticosterone (cpd B) responses to stress, non-stress levels of these hormones and somatic growth were studied in adult female rats. Responses to 3-min immobilization--blood withdrawal stress uere tested at 2 and 11 weeks and to 3-min ether--blood withdrawal at 5 weeks after surgery. Non-stress plasma GH levels were further assessed in serial blood samples collected at 4-h intervals during a 24-h period at 15 weeks, and cpd B levels were studied similarly for a 44-h period at 22 weeks. Rats with lateral PO-AHA lesions showed normal GH suppression and cpd B elevation in response to both stress procedures. In contrast, neither stressor suppressed plasma GH levels in rats with medial PO-AHA lesions, but both produced normal increments in cpd B. The 24-h mean non-stress level of plasma GH in the group with medial lesions was higher and that of the group with lateral lesions was lower than those of controls. Cpd B levels were reduced during the 44-h study, and circadian rhythmicity was abolished in rats with medial lesions. The group with lateral lesions showed normal rhythmicity in plasma cpd B levels, Linear growth was not affected in either lesioned group. The GH findings suggest that the medial but not lateral PO-AHA is essential for stress-induced inhibition of GH secretion and that this region may exert a tonic inhibitory influence on non-stress GH secretion. The data also suggest that the medial PO-AHA is importantly involved in producing the circadian rhythm in pituitary-adrenal function.

Changes in diurnal fluctuations of plasma thyroid stimulating hormone and corticosterone following anterior hypothalamic deafferentation in the rat

Systemic thyroid stimulating hormone (TSH) and corticosterone (B) levels were measured in serial plasma samples withdrawn from 8 adult female rats before and for several weeks after anterior hypothalamic cuts (FC), made with a modified Halász knife (1.5 mm radius). With lights on from 5:00 to 19:00 h the rats were bled by rapid venipuncture at 9:00 and 20:00 h 14 days before and 5 days after FC and by chronic intra-atrial cannulas at 9:00, 13:00, 17:00 and 20:00 h on days 25, 30 and 40 or 50 post-FC. The frontal cuts, localized later by histology to sites just rostral to the suprachiasmatic nucleus in 3 rats and just caudal to the nucleus in the other 5 brains, altered the charactersitic diurnal drop in plasma TSH and rise in B but did not eliminate marked fluctuations in hormone levels. The new hormonal platterns could not be related specifically to FC loci and appeared to be a dynamic process for each rat. At 5 days after FC the diurnal patterns were either obscured or shifted in phase. The most consistent finding at subsequent sampling intervals was that TSH and B plasma levels in each animal showed parallel fluctuations (during 9:00--20:00 h) as though synchronized in phase. The data suggest that interruption of anterior hypothalamic connections in the female rat is compatible with fluctuations in plasma TSH and B which are unique to a particular animal and may be obscured by group analysis.

Pulsatile growth hormone, prolactin, and thyrotropin secretion in rats with hypothalamic deafferentation

Fifteen minute sequential blood sampling of male rats via right atrial cannulae was used to examine the patterns of anterior pituitary hormone secretion following hypothalamic deafferentations. Growth hormone (rGH) profiles from rats with complete hypothalamic deafferentation (CC) showed persistence of episodic rGH secretion with greatly increased frequency and elevated trough values, but without apparent entrainment to the light-dark cycle. Anterior (AC) and posterior (PC) deafferentation had no effect on the normal 3 h periodicity of rGH secretory episodes, but light-dark entrainment was lost in AC rats. Prolactin (rPRL) secretion in CC rats remained at normal low baseline levels without secretory episodes, while in AC and PC rats, as in normal rats, bursts of rPRL secretion occurred, often temporally related to rGH bursts. Thyrotropin (rTSH) levels in CC rats were less variable, and lower than in normal rats, while rTSH secretory profiles appeared normal in AC and PC rats. These results suggest that the isolated medial basal hypothalamus (MBH) contains neural substrates for episodic rGH secretion, and to a lesser extent, for rTSH. It is hypothesized that excessive rGH secretion in CC rats results from surgery damaging the rhythmicity of the rGH releasing factor (GRF) michanism, so that both sustained secretion and more frequent bursts of GRF occur. An effective rPRL inhibitory mechanism remains intact within complete hypothalamic deafferentations, while episodic rPRL and rTSH secretion require intact lateral afferents to the MBH. Visual pathway connections to the MBH, which mediate light-dark cycle entrainment of episodic rGH secretion, appear to pass through the region of the anterior hypothalamic and suprachiasmatic nuclei.

Influence of medial preoptic-anterior hypothalamic area stimulation of the excitability of mediobasal hypothalamic neurones in the rat

1. Extracellular action potentials recorded from 798 neurones in the mediobasal hypothalamus (MBH) of pentobarbitone anaesthetized male rats were analysed for a change in excitability following stimulation in the medial preoptic and anterior hypothalamic areas. 2. An increase in excitability characteristic of orthodromic excitation was observed from 11-5% (n=92) of MBH neurones. Latencies for excitation were shorter for cells tested with anterior hypothalamic area stimulation (n=42; mean 5-4 +/- 2-6 msec S.D.) than for cells tested with medial preoptic stimulation (n=50; mean 15-2 +/- 7-2 msec S.D.). With spontaneously active neurones, excitation was followed by a decrease in excitability lasting 150-250 msec. An initial decrease in excitability, suggestive of post-synaptic inhibition, over a wide latency range (4-30 msec) and with duration of 100-400 msec was observed from 3-6% of MBH neurones. 3. Features of antidromic invasion were observed from 149 MBH neurones. From the medial preoptic area, the latency range was 0-5-38 msec (mean 7-8 +/- 5-5); from the anterior hypothalamic area the latency range was 0-4-9-5 msec (mean 3-1 +/- 2-3). Occasionally an abrupt decrease in latency followed an increase in stimulus intensity. Most cells followed paired stimuli at frequencies up to 500 Hz. Axon conduction velocities were estimated to be under 2-0 m/sec. Antidromic invasion was usually followed by a decrease in excitability lasting approximately 100-150 msec. 4. Twenty MBH neurons displayed antidromic invasion from both the medial preoptic or anterio hypothalamic areas and one other stimulation site: the median eminence (five cells); the amygdala (six cells); the region of thalamic nucleus medialis dorsalis (three cells) and the midbrain periaqueductal gray (six cells). Interaction studies indicated that the axons of these cells branched close to the origin of the axon itself. 5. Antidromic invasion from the surface of the median eminence identified thirty-nine tuberoinfundibular neurones. Stimulation in the medial preoptic and anterior hypothalamic area produced orthodromic excitatory (n = 5) and inhibitory (n = 4) actions on HVM neurones, but was without an action on most other neurones (n = 30). Tuberoinfundibular neurones in the ventromedial nucleus also responded to stimulation in the amygdala, but usually at latencies greater than that for medial preoptic area evoked responses. 6. These observations indicate a close relationship between MBH neurones and cells located in both the amygdala and the medial preoptic-anterior hypothalamic area. The data for tuberoinfundibular neurones indicates that several extrahypothalamic areas may send fibres to these cells. These pathways may be important for the adaptive neuroendocrine responses reported in the literature.