4-aminobutyrate: 2-oxoglutarate transaminase-containing neurons in the perinuclear zone of the rat supraoptic nucleus

Glutamate microstimulation of local inhibitory circuits in the supraoptic nucleus from rat hypothalamus slices

The hypothesis of a local inhibitory input to the hypothalamic supraoptic nucleus was tested with combined glutamate microstimulation and whole cell patch-clamp recordings in slices from rat hypothalamus. Synaptic activity in supraoptic magnocellular neuroendocrine cells (MNCs) was monitored and glutamate microdrops were applied in the perinuclear region of the supraoptic nucleus to evoke firing of action potentials in putative presynaptic inhibitory cells. The effect of glutamate microdrops applied in the perinuclear region was tested on 57 supraoptic MNCs. In control conditions, spontaneous excitatory (EPSCs) and inhibitory (IPSCs) postsynaptic currents were observed at resting membrane potential in all MNCs tested. Glutamate microstimulation evoked an abrupt increase in the frequency and size of spontaneous IPSCs in eight MNCs. Forty-nine MNCs did not show any change in the inhibitory synaptic input. Microapplication of glutamate in the periphery of the supraoptic nucleus did not modify the amplitude or the frequency of spontaneous EPSCs in any of the 57 MNCs tested. In the group of eight MNCs that responded to glutamate microstimulation by an increase in inhibitory input, two types of responses were observed. Four MNCs showed an increase in both size and frequency of spontaneous IPSCs through the entire range of amplitude. In the other four MNCs, local glutamate stimulation produced a dramatic increase in the size of IPSCs and a lesser increase in the frequency of the smaller IPSCs. The potential effect of the glutamate-evoked increase in inhibitory input on the firing activity of MNCs was tested in current-clamp conditions. Intracellular current injection was applied to evoke firing of action potentials in six MNCs that had responded to local glutamate microstimulation by an increase in inhibitory input. Glutamate microdrop applications inhibited the evoked action potential firing in all six cells. These results suggest 1) that local inhibitory interneurons are present in the periphery of the supraoptic nucleus, 2) that they contain functional glutamate receptors, 3) that they form inhibitory synapses with supraoptic MNCs, and 4) that activation of these interneurons inhibits firing in MNCs. These results support the hypothesis that local inhibitory interneurons play a important role in the firing activity of supraoptic MNCs.

Emerging concepts of structure-function dynamics in adult brain: the hypothalamo-neurohypophysial system

As the first known of the mammalian brain's neuropeptide systems, the magnocellular hypothalamo-neurohypophysial system has become a model. A great deal is known about the stimulus conditions that activate or inactivate the elements of this system, as well as about many of the actions of its peptidergic outputs upon peripheral tissues. The well-characterized actions of two of its products, oxytocin and vasopressin, on mammary, uterine, kidney and vascular tissues have facilitated the integration of newly discovered, often initially puzzling, information into the existing body of knowledge of this important regulatory system. At the same time, new conceptions of the ways in which neuropeptidergic neurons, or groups of neurons, participate in information flow have emerged from studies of the hypothalamo-neurohypophysial system. Early views of the SON and PVN nuclei, the neurons of which make up approximately one-half of this system, did not even associate these interesting, darkly staining anterior hypothalamic cells with hormone secretion from the posterior pituitary. Secretion from this part of the pituitary, it was thought, was neurally evoked from the pituicytes that made the oxytocic and antidiuretic "principles" and then released them upon command. When these views were dispelled by the demonstration that the hormones released from the posterior pituitary were synthesized in the interesting cells of the hypothalamus, the era of mammalian central neural peptidergic systems was born. Progress in developing an ever more complete structural and functional picture of this system has been closely tied to advancements in technology, specifically in the areas of radioimmunoassay, immunocytochemistry, anatomical tracing methods at the light and electron microscopic levels, and sophisticated preparations for electrophysiological investigation. Through the judicious use of these techniques, much has been learned that has led to revision of the earlier held views of this system. In a larger context, much has been learned that is likely to be of general application in understanding the fundamental processes and principles by which the mammalian nervous system works.(ABSTRACT TRUNCATED AT 400 WORDS)

Diagonal band projection towards the hypothalamic supraoptic nucleus: light and electron microscopic observations in the rat

Supraoptic nucleus (SON) neurons receive a prominent gamma-amino-butyric acid (GABA) input. This study evaluated the hypothesis, partly on the basis of recent electrophysiological data, that this innervation might arise from GABAergic neurons located in the ventral diagonal band of Broca area. For retrograde transport studies, pentobarbital-anesthetized male Long-Evans rats received 0.03-0.20-microliter injections of a suspension of rhodamine tagged latex microspheres into the SON. In two cases where such injections were confined to the SON, less than 60 retrogradely labeled neurons were detected in the ipsilateral diagonal band. In three animals where injections extended into the perinuclear zone around the SON, more than 2,000 retrogradely labeled cells were counted in the ipsilateral diagonal band. For anterograde transport studies, another group of animals received either 30% horseradish peroxidase (HRP) in 0.5% poly-L-ornithine (0.05-0.10 microliter injections) or Phaseolus vulgarus (iontophoresed from a 2% solution) into the diagonal band. After survivals of 18-24 hours (HRP) or 5 days (PHAL-L) labeled axon terminals invested the perinuclear zone above the SON. The presence of just a single fiber within the nucleus indicated a minor projection to the SON itself. The HRP-injected material was processed for ultrastructural examination and revealed dense HRP-labeled axon terminals in this perinuclear zone, most often (98%) forming axodendritic appositions. A postembedding colloidal gold technique to visualize GABA-synthesizing terminals revealed that fewer than 5% of these perinuclear HRP-labeled terminals also exhibited GABA-like immunoreactivity. Within the SON, where GABAergic axon terminals are abundant, few (less than 5%) GABAergic terminals contained HRP.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuronal plasticity in the hedgehog supraoptic nucleus during hibernation

The purpose of the present study was to identify processes of plasticity in the receptive field of neurosecretory neurons of the supraoptic nucleus during hibernation in the hedgehog, in order to correlate them with the increased neurosecretory activity observed in this nucleus during this annual period. Using the Rapid Golgi method, a quantitative study was conducted in the receptive field of bipolar and multipolar neurons (the main components of the nucleus). Results indicate a generalized increase in the following characteristics: (1) number of dendritic spines per millimeter along the dendritic shafts; (2) degree of branching in the dendritic field; and (3) dendritic density around the neuronal soma. These data demonstrate modification of the dendritic field in the supraoptic nucleus during hibernation, a change undoubtedly related to functional conditions. Since the observed changes affect structures such as dendritic spines which are directly related to the arrival of neural afferences, the discussion is centered on the types of stimuli which may be responsible for the observed processes.

Cardiovascular input to hypothalamic neurosecretory neurons

In vivo extracellular recordings from rat supraoptic and paraventricular magnocellular neurosecretory cells (MNCs) indicate that putative vasopressin-secreting MNCs may be identified by an abrupt and brief cessation in firing consequent to a transient drug-induced rise in arterial pressure sufficient to activate arterial baroreceptors. In the diagonal band of Broca (DBB), a population of neurons projecting towards the supraoptic nucleus are activated during this drug-induced hypertension. Electrical stimulation in DBB selectively depresses supraoptic vasopressin-secreting MNCs. Intracellular recordings in perfused hypothalamic explants confirm a DBB-evoked bicuculline-sensitive and chloride-dependent postsynaptic inhibition, similar to that associated with the application of gamma-aminobutyric acid (GABA) in approximately half of supraoptic MNCs. Since bicuculline also selectively blocks baroreceptor-induced inhibition in supraoptic MNCs, it is proposed that the depressant baroreflex input to vasopressin-secreting MNCs involves a population of DBB neurons and GABAergic interneurons located close to MNCs. An excitatory and selective input to vasopressin-secreting MNCs follows chemoreceptor activation, possibly mediated by the A1 noradrenergic cell group in the ventrolateral medulla. Another excitatory input to both vasopressin- and oxytocin-secreting MNCs is triggered by circulating angiotensin II and appears to be relayed centrally through an angiotensinergic projection from the subfornical organ.