Effect of testosterone on medial preoptic/anterior hypothalamic neurone responses to stimulation of the lateral septum

Hypothalamic regulation of pancreatic secretion is mediated by central cholinergic pathways in the rat

The vago-vagal reflex plays an important role in mediating pancreatic secretion evoked by cholecystokinin and non-cholecystokinin-dependent luminal factors. We hypothesize that the vago-vagal reflex mediating pancreatic secretion in the rat is under central control and regulated by cholinergic pathways in the hypothalamus. To test this hypothesis, we demonstrated that chronic decerebration decreased basal pancreatic enzyme secretion from 318 +/- 12 to 233 +/- 9 mg h-1 and reduced the net increase in pancreatic secretion stimulated by intraduodenal infusion of 5 % peptone and hypertonic NaCl by 54 % and 45 %, respectively. Intracerebroventricular administration of methscopolamine (MSCP, 50 nmol (5 mul)-1), a blood-brain barrier-impermeant cholinergic muscarinic receptor antagonist, evoked results similar to those achieved by chronic decerebration. To localize the sites of action, we demonstrated that microinjection of MSCP (20 nmol) into the lateral hypothalamic nucleus or the paraventricular nucleus resulted in inhibition of both basal pancreatic protein secretion and luminally stimulated pancreatic secretion by 48 % and 52 %, respectively. Intracerebroventricular injection of hemicholinium-3 at doses known to deplete the endogenous ACh store produced similar inhibitory results. In addition, microinjection of ACh (5 pmol) or the muscarinic M1 receptor agonist McN-A-343 (30 ng) into the lateral hypothalamic nucleus increased pancreatic secretion over basal levels by 46 % and 40 %, respectively. Selective lesions of lateral septal cholinergic neurons decreased basal pancreatic secretion and inhibited peptone-induced pancreatic secretion by 30 %. Destruction of the lateral parabrachial nucleus produced a 44 % inhibition of peptone-induced pancreatic section. Finally, microinjection of glutamate into the lateral septum or the lateral parabrachial nucleus stimulated vagal pancreatic efferent nerve firings from a basal level of 0 +/- 0.5 impulses (30 s)-1 to 4.5 +/- 0.5 and 14 +/- 2 impulses (30 s)-1, respectively, and pancreatic protein output increased 50 % and 84 % over basal levels. Administration of MSCP to the paraventricular nucleus eliminated these effects. These observations suggest that cholinergic neurons of the lateral septum and lateral parabrachial nucleus regulate pancreatic secretion. Further, cholinergic input from the lateral parabrachial nucleus to the hypothalamus plays a major role in the modulation of vagal pancreatic efferent nerve activity and pancreatic secretion evoked by the vago-vagal reflex.

Heterosexual housing increases the retention of courtship behavior following castration and elevates metabolic capacity in limbic brain nuclei in male whiptail lizards, Cnemidophorus inornatus

In male vertebrates the display of courtship behavior depends on the presence of testicular androgens. However, social experiences in adulthood can alter the hormonal dependence of courtship behavior in a variety of species, and we have previously proposed that these behavioral changes are linked to changes in neural metabolic capacity (cytochrome oxidase activity). Here we investigated the effects of prior social experience (housing with females vs housing in isolation) on the retention of courtship behavior following gonadectomy and on cytochrome oxidase (CO) activity in male little striped whiptail lizards, Cnemidophorus inornatus. In Experiment 1, we found that males that were previously housed with females (HWF males) continued to display courtship behavior longer after castration than males previously housed in isolation (ISOLATE males). This is similar to the behavioral plasticity found in rodents and cats. On the other hand, courtship behavior while gonadally intact was indistinguishable between HWF and ISOLATE males. Because all males were housed individually following castration, the difference is due to different social experiences prior to castration. In Experiment 2, we found that gonadally intact HWF males had significantly elevated CO activity in the preoptic area, amygdala, and anterior and ventromedial hypothalamic areas relative to intact ISOLATE males. No significant differences in metabolism were found in the lateral septum, lateral hypothalamus, and habenula or in hindlimb muscle, suggesting that the increase in metabolism is specific to brain nuclei involved in courtship behavior. Altogether, this demonstrates that elevations in metabolic capacity correlate with experience-dependent increases in robustness to castration.

Developmental effect of testosterone on estrogen sensitivity of the rat preoptic neurons with axons to the ventral tegmental area

The neonatal effect of testosterone on neuronal sensitivity to estrogen was examined in the medial preoptic area of the rat when adult. Electrical stimulation of the ventral tegmental area induced antidromic action potentials in 683 neurons in 106 urethane-anesthetized animals which consisted of 38 males castrated on the day of birth (day 1), 30 females androgenized by testosterone injection on day 5, and 38 normal females. All females were used after ovariectomy when adult. Histological localization of the antidromically activated neurons was similar in all the animal groups. The latency jump in 40% of the potentials indicated that activated preoptic axons terminate in the stimulation site. The latency for activation was in the range 1.6-43.7 ms and the threshold was as low as 60 microA. The absolute refractory period did not exceed 1700 microseconds. Estrogen increased the threshold among 278 cells in the normal females and 181 cells in the neonatally castrated males. Preoptic neurons in the androgenized females (n = 224) differ from those in others in that their threshold, regardless of estrogen treatment, was at the ceiling values seen in the other groups in the presence of estrogen. Estrogen also prolonged the absolute refractory period in the females but not in the androgenized females. The reduced excitability and the lack of estrogen sensitivity may be responsible for the insensitivity of the androgenized females to feminine actions of estrogen, that may presumably underlie a sex difference in certain behavioral functions.

A re-evaluation of the effects of gonadal steroids on neuronal activity in the male rat

Single unit activity (SUA) was recorded from 77 cells located in the arcuate nucleus (ARC) and medial preoptic area (MPA) of anesthetized, intact male rats. Animals were administered vehicle, testosterone (T; 5 or 50 micrograms) or 17 beta-estradiol (E; 0.5 microgram) intravenously and SUA was monitored for 8-12 min. T (50 micrograms) reduced SUA in 50% of ARC units and 44% of MPA units within 2.1 +/- 0.46 and 3.3 +/- 0.92 min, respectively. Inhibition of ARC SUA was more pronounced than MPA SUA. A small percentage (9%) of ARC units were excited by T. E reduced SUA in 29% of ARC units and 27% of MPA units. Single doses of 5 micrograms T did not affect ARC activity. However, when followed within 10 min by an additional dose of 5 or 50 micrograms T, 30% and 43% of ARC units were inhibited, respectively. Doses (10 micrograms) of T produced plasma T concentrations within physiological limits, although 50 micrograms doses produced supraphysiological T levels. Neither dose affected circulating LH concentrations. We conclude that physiological and supraphysiological concentrations of T can rapidly affect SUA within the ARC.

Influences from different areas of the cerebral cortex on preoptic neurons: morphological and electrophysiological data

Spatial organization of neurons in the prefrontal and cingulate cortex, cortex of the piriform lobe and the hippocampus which project axons to the preoptic region, has been studied in cats using horseradish peroxidase tracing. Cortical areas were selected taking into consideration their phylogenetical distinctions. The prefrontal cortex was found to send a major portion of fibres to the preoptic region, while the density of units forming such connections was maximal in the cingulate cortex. Field potentials and neuronal reactions of the medial and lateral divisions of the preoptic region and the adjacent hypothalamic zones were studied in ketamine-anaesthetized cats. The most pronounced field potentials were recorded in the preoptic region upon stimulation of the cortex of the piriform lobe and cingulate cortex. There was a close correlation between the responses of single neurons and components of the field potentials. The majority of neurons responding to cortical stimuli were located mainly in the lateral preoptic region, where the larger amount of primary excitatory reactions were recorded. The medial preoptic region contained a smaller number of responsive neurons prevailingly generating the primary inhibitory reactions. For the lateral preoptic region the inhibition/excitation ratio was 0.6:1 at all cortical stimulations, but for the medial region it was 5.8:1. In the preoptic division adjoining the bed nucleus of the stria terminalis, the primary inhibitory reactions considerably prevailed over the primary excitatory; on the other hand, in the supraoptic nucleus the primary excitatory reactions prevailed weakly (ratios of 4.9:1 and 0.7:1, respectively). The preoptic region was found to be a zone of wide convergence of cortical inputs to single cells, where three-quarters of neurons responded to the stimulation of two, three or even four cortical areas.

Responses in the diagonal band of Broca, adjacent septal nuclei and the islands of Calleja of cats to stimulation of the subcallosal fornix, medial basal hypothalamus and medial forebrain bundle

The projection of neurons in the septal nuclei and the insula magna of the islands of Callaja (IC) was explored together with their response to stimulation of the fornix. The septal nuclei all contained neurons projecting in the medial forebrain bundle (MFB). Only the diagonal band of Broca (DBB) and the lateral septal nucleus (LS) contained many neurons projecting toward the medial basal hypothalamus (MBH). The spatial distribution of neurons excited by stimulation of the fornix in the DBB was almost identical with the distribution of neurons projecting toward the MBH and there was considerable overlap (10/28 cells). In the medial septal nucleus the spatial distribution of neurons excited by stimulation of the fornix and neurons projecting in the MFB was similar and there was considerable overlap (6/21 cells). The connectivity of the IC resembled that of the MS but there was little overlap between the neurons excited by fornix stimulation and those projecting in the MFB (1/27 cells). In the LS there were almost equal numbers of neurons projecting in the MFB and toward the MBH but there was very little input from the fornix. Neurons were significantly more often excited by stimulation of the lateral fornix, carrying axons from the subiculum, than they were by medial stimulation exciting axons from Ammon's horn. Axons projecting toward the MBH or in the MFB had conduction velocities less than 1 m/s.

Responses of cat preoptic neurons to stimulation of the medial frontal cortex and the medial basal hypothalamus

Responses of single preoptic neurons to electrical stimulation of the medial frontal cortex, the mediobasal hypothalamus (MBH) and the medial forebrain bundle (MFB) were recorded in anaesthetised cats. Single pulse stimulation of the medial frontal cortex orthodromically drove 96 otherwise quiescent preoptic neurons, which were found more frequently in the dorsal preoptic region, inhibited 53% of the spontaneously active preoptic neurons and excited 16%. Testing of cortically influenced preoptic neurons with MBH or MFB stimulation resulted in antidromic invasion of 6% (MBH) and 9% (MFB). Convergence of orthodromic inputs from medial frontal cortex and MBH was detected in 78% of spontaneously active preoptic neurons, and three-way convergence including input from MFB was noted in 17% of neurons tested with all stimulators. Some cortex-response neurons were found to also respond to vaginal or anal probing, paw squeezing and haemorrhage. The role of this input to the preoptic region from medial frontal cortex remains to be elucidated, but may include neuroendocrine, behavioural and homeostatic functions.

Projections of oestrogen-sensitive neurones from the ventromedial hypothalamic nucleus of the female rat

Stimulation of the mesencephalic central grey matter caused antidromic activation of 199 neurones in the hypothalamic ventromedial nucleus of forty-six urethane-anaesthetized female rats, of which twenty-two (ten ovariectomized and twelve ovariectomized and oestrogen treated) had the lateral projection of the nucleus disrupted by a small parasagittal knife-cut. The remaining twenty-four rats (ten ovariectomized and fourteen ovariectomized and oestrogen treated) had a cut in the frontal plane to interrupt caudal projection of the nucleus. In eight animals with the lateral knife-cut, eighteen cells were antidromically activated from the central grey matter as well as from the dorsal longitudinal fascicle. Collision between the potentials activated from these two sites revealed that seventeen of them descended to the central grey matter by way of the periventricular system, one cell having a bifurcated axon to these sites. Likewise, eleven cells in another group of eight animals with the posterior cut were found to project to the central grey matter via the perifornical region. Pre-treatment with oestrogen significantly lowered the activation thresholds and absolute refractory periods of neurones antidromically stimulated from the central grey in ovariectomized animals bearing posterior but not lateral knife-cuts. These results suggest that the ventromedial hypothalamic neurones responsible for oestrogen-dependent autonomic and behavioural functions project to the central grey via a lateral rather than a posterior pathway.

Properties of antidromically identified neurons in the enkephalinergic magnocellular dorsal nucleus of the guinea pig hypothalamus

In the guinea pig, immunocytochemical and neuroanatomical studies have demonstrated that enkephalin-containing neurons in the hypothalamic magnocellular dorsal nucleus (MDN) terminate in the lateral septum (LS). In the present investigation, 114 MDN neurons, studied with extracellular recording techniques, were identified by antidromic activation from the LS. Latencies of responses from ipsilateral and contralateral LS were 13.5 and 18.78 ms, respectively, corresponding to an axonal conduction velocity of 0.1 m/s. By using the reciprocal collision test, evidence is presented for bilateral projection of individual MDN neurons to the LS. Fifty-one (44.73%) MDN-LS neurons discharged in a slow irregular pattern. Interspike time histograms were very similar and had a mode of about 280 ms. Peristimulus time histograms were compiled from 15 active MDN-LS neurons. Stimulation which elicited antidromic spikes resulted in a brief silent period in the spontaneous activity which was related to the normal interspike interval pattern of the firing. Prolonged silent periods as well as silent period occurring after subthreshold stimulus and increasing with the stimulus intensity were attributed to inhibitory synaptic effects. On the other hand, some MDN-LS neurons displayed orthodromic excitatory responses following LS stimulation. These observations provide electrophysiological evidence of a direct MDN-LS pathway, in all likelihood of enkephalinergic nature, and indicate that some MDN-LS neurons receive inhibitory and excitatory afferents from the LS.

Different electrophysiological effects of testosterone on medial preoptic/anterior hypothalamic neurons have similar time courses

The time courses of different electrophysiological effects of testosterone on a specific subpopulation of medial preoptic/anterior hypothalamic neurons and sexual behavior were measured in castrated rats. Testosterone reduced the absolute refractory period of those neurons projecting into the medial forebrain bundle with the same time course as it restored maximal responsiveness to orthodromic stimulation of either the lateral septum or the fimbria and sexual responses (5 days). The similar time-courses for these different electrophysiological effects of testosterone argue for a common mechanism of action.