The discrete anatomical distribution of arginine vasopressin and corticotropin releasing factor (CRF) immunoreactivity in the paraventricular nucleus (PVN) of the rat hypothalamus is altered after adrenalectomy. Not only is the immunostaining of both peptides enhanced, but vasopressin immunoreactivity, normally confined to the magnocellular subdivision, becomes clear in a large percentage of CRF neurones in the parvocellular subdivision. These changes in immunoreactivity may reflect changes in post-translational events, peptide metabolism or genomic activity that lead indirectly or directly to the enhanced expression of vasopressin. Here we report that levels of transcripts homologous to vasopressin messenger RNA increase in the PVN after adrenalectomy, in parallel with increases in vasopressin immunoreactivity. In fact, after adrenalectomy, vasopressin mRNA can be detected in CRF-immunoreactive neurones. These results indicate that a considerable degree of plasticity is retained by the adult neuronal genome of the rat and that this plasticity may be modulated by the endocrine environment.

Immunocytochemical studies have shown that adrenalectomy produces changes in the content and distribution of [arginine-8]vasopressin (AVP) immunoreactivity in the paraventricular nucleus of the hypothalamus. The purpose of this study was to determine whether manipulation of adrenal hormones affects the levels of AVP mRNA. In situ hybridization assays with highly specific synthetic oligodeoxyribonucleotide probes and immunocytochemistry were used to detect the distribution of AVP mRNA and AVP-immunoreactive perikarya. AVP mRNA is codistributed with AVP immunoreactivity in the posterior magnocellular subdivision of the paraventricular nucleus and its accessory nuclei, the supraoptic nucleus and the suprachiasmatic nucleus. In adrenalectomized rats, the density and distribution of the hybridization signal were increased in the paraventricular nucleus; a 2-fold increase in the area comprising the signal was observed. At the cellular level, silver grains were detected in corticotropin-releasing-factor-immunoreactive neurons throughout the medial parvocellular subdivision of the paraventricular nucleus. No changes were seen in the distribution of AVP mRNA in the supraoptic or suprachiasmatic nuclei. Treatment with dexamethasone prevented the increase in AVP mRNA produced by adrenalectomy. In contrast, adrenalectomy did not alter the hybridization signal obtained with a probe for a-tubulin mRNA. These results suggest, at the cellular level, that adrenalectomy induces a glucocorticoid-sensitive stimulation of AVP mRNA synthesis in the central nervous system. Thus, considerable plasticity in gene expression is retained in the hypothalamus of the adult rat.The discrete anatomical organization of hypothalamic neurosecretory neurons within the paraventricular nucleus (PVN) has established this region as a unique site to study the neurohypophyseal secretory system. [arginine-8]Vasopressin (AVP) and oxytocin are contained within neurons of the PVN and the supraoptic nucleus (SON) (1, 2) and are involved in both autonomic and neurosecretory functions (3, 4). The axons emanating from these perikarya form the hypothalamoneurohypophyseal tracts, which project to the pars nervosa where AVP is released from their terminal endings (5,6). However, AVP has been detected in axon terminals within the external zone of the median eminence (7), suggesting that it may be involved, to some extent, in regulating anterior pituitary functions. Unlike the neurohypophyseal projection, the majority of AVP-immunoreactive axon terminals in the median eminence originate from neurons within the medial parvocellular subdivision of the PVN (8). Biochemical studies have confirmed quantitatively that adrenalectomy increases AVP immunoreactivity (7, 9). After adrenalectomy, corticotropin-releasing factor (CRF) and AVP were reported to be colocalized within neurons in the medial parvocellular region of the PVN (10, 12). This finding is of particular interest because immunocytochemical studies have shown that adrenalectomy produces a dexamethasonesen...

Previous studies have shown that adrenalectomy augments arginine vasopressin (AVP) messenger RNA levels in the adult paraventricular nucleus. It is now demonstrated that unilateral lesions in the lateral septal nucleus enhance the adrenalectomy-induced expression of AVP mRNA. This effect was entirely ipsilateral to the lesion and most prominent in the rostral paraventricular nucleus and related nuclei. Moreover, AVP and AVP mRNA were found to be colocalized with oxytocin in a few neurons. These results indicate that mRNA expression is modulated by synaptic influences and raise the possibility that synaptically mediated selection of neuronal phenotypes is a dynamic feature of the mature central nervous system.

No abstract

Intracellular recordings revealed that in neocortical explant cultures prepared on the day of birth and examined 3-6 weeks later, neurons mature and establish complex synaptic relationships that lead to spontaneous and triggered synchronous discharge. The spontaneous synchronous activity took several forms, including periodic generation of epileptiform depolarizing waves, prolonged periods of seizure-like discharge, and periodic, intense barrages of IPSPs. Synchronous depolarizations were associated with a marked increase in membrane conductance. Intracellular injection of currents of varying polarity and intensity affected their amplitudes and polarities without influencing the probability of their occurrence, indicating that the discharge reflected the synchronous activities of a neuronal population. This conclusion was confirmed with simultaneous recordings from pairs of neurons. Effects of the GABAa receptor antagonist, bicuculline, and the NMDA receptor antagonist, 2-aminophosphonvalerate (2APV), were used to assess the contributions of impairment of inhibition and enhancement of excitation to the initiation of synchronous discharge. The frequency with which spontaneous depolarizations were generated in normal medium was markedly reduced by 2APV. Moreover, seizure-like activity was induced by removing Mg++ from the medium, a condition that enhances conductance through NMDA receptor-coupled channels. This behavior was also attenuated by 2APV. Perfusion of bicuculline was potently epileptogenic. 2APV cut short the late, voltage-dependent phase of bicuculline-induced paroxysmal depolarizations, indicating a role of NMDA receptors in generating this component of the wave. Epileptiform activities induced by withdrawal of Mg++ were greatly augmented by bicuculline, indicating that blockade of inhibition was not a prerequisite for seizure-like activity. This conclusion is supported by the finding that in many neurons in untreated cultures, paroxysmal generation of trains of IPSPs was the primary manifestation of spontaneous, synchronous population discharge.

The distribution of mRNA with high sequence homology to somatostatin mRNA within the periventricular hypothalamus of rat was assessed using in situ hybridization techniques with synthetic oligodeoxyribonucleotide probes, complementary to the 3' coding region of rat somatostatin mRNA. The probes (22- and 24-mers) were 5'-end labeled using T4 polynucleotide kinase and gamma-32P-ATP. They were used either individually or after ligation with T4 DNA ligase to form a 46-mer. Serial tissue sections (less than 10 microns) were taken from the level of the preoptic/anterior hypothalamus through the paraventricular hypothalamus. In situ hybridizations were conducted at room temperature in hybridization buffer. Neurons immunoreactive with antiserum raised against somatostatin were identified in alternate sections using standard immunocytochemical procedures. The anatomical location of the hybridization signal was determined by autoradiography. Our results show that the peri- and paraventricular hypothalamus is rich in transcripts putatively coding for somatostatin and that these transcripts are co-distributed with neurons immunoreactive with antisomatostatin immunoglobulin.

We examined the electrophysiological and morphological properties of neocortical neurons maintained in explant cultures prepared from the parietal cortex of newborn Sprague-Dawley rats. After 3-6 weeks in vitro, cultures showed regional differences in cellular density reminiscent of cortical layering, and an abundance of axonal processes. Pyramidal-shaped neurons with spinous dendrites were the dominant elements revealed by Lucifer yellow injections. Intracellular recordings revealed that many electrophysiological properties of neurons in the explants resembled those of neocortical neurons in vivo and in slice preparations. In response to depolarizing current injection, neurons in the explants showed the same three patterns of repetitive firing described in neocortical slices, as well as a similar array of responses. Spontaneous synaptic potentials were recorded from all neurons and complex PSPs were evoked in response to focal extracellular stimulation. GABAa receptors mediated a significant component of the evoked responses. Fifteen of sixty neurons generated action potentials that arose spontaneously from resting potentials. Neurons in many slices generated large, prolonged depolarizing potentials that reflected coordinated synaptic activity within the explants. These results underscore the usefulness of the neocortical explant as a valuable model for studying aspects of the behavior of circuits of cortical neurons.

No abstract