Pierre J. Magistretti

Immunohistochemical distribution of pro-somatostatin-related peptides in cerebral cortex

Mammalian brain contains 3 peptides related to the pro-somatostatin molecule: somatostatin-14 (SS14), the form originally identified from hypothalamic extracts, somatostatin-28 (SS28) and somatostatin 28 (1-12) (SS28 (1-12)). By using antibodies which selectively recognize one or more of these 3 somatostatin-related peptides, we have characterized their immunohistochemical distribution in neocortex. These somatostatin-related peptides have a specific laminar distribution in cortex and are differentially distributed such that SS28 is largely restricted to cell bodies, whereas SS28 (1-12) is preferentially localized in neuronal processes and terminals in a density which far exceeds that revealed by SS-14 immunoreactivity. These data suggest that there may be an intraneuronal transformation from a SS28-like peptide to a SS28 (1-12)-like peptide. In addition, the enriched distribution of nerve fibers containing the antigenic determinant, SS28 (1-12), strongly implies that somatostatin-related peptides constitute a major neurotransmitter system in neocortex. The morphological characteristics of this system are homologous with long projection pathways such as the cortico-cortical specific intrinsic systems as well as projections.

The distribution and morphological characteristics of the intracortical VIP-positive cell: an immunohistochemical analysis

Using a sensitive immunohistochemical procedure, we have undertaken a detailed morphological characterization of the vasoactive intestinal peptide-positive (VIP-positive) neuron in the cerebral cortex of the rat. VIP-positive neurons are present in all regions of cortex, and are usually strongly bipolar, possessing long, radially directed processes with very limited branching in the tangential plane. The most extensive dendritic branching occurs in layers I and deep IV-superficial V, and the density of axonal varicosities is highest in layers II-IV. In the visual cortex, approximately 50% of the labeled cell bodies are in layer II and III and 80% of the labeled cell bodies are contained in layers I-IV (superficial 600 microns of cortex). In order to determine the density and 3-dimensional distribution pattern of these cells, we prepared serial tangential sections through the rat visual cortex, mapped the distribution of all labeled cells in each section on transparent acetate sheets, and compressed these superimposed maps. This analysis demonstrated that: (1) approximately 1% of the cortical neurons are VIP-positive, (2) their distribution is fairly uniform and statistically random, (3) there are no large areas (i.e. with a diameter greater than 100 microns) that lack a VIP-positive cell, (4) on the average, there is one VIP-positive cell per column of 30 microns diameter, and (5) the average nearest neighbor distance on the compressed display is 15 microns. Given the morphological characteristics of VIP-positive cells, these data indicate that each VIP-containing cell is identified with a unique radial volume, which is generally between 15 and 60 microns in diameter, and overlaps with the contiguous domains of neighboring VIP-positive cells. These morphological data support the notion that VIP-containing neurons play an important functional role within radially oriented columns of cerebral cortex.

Immunohistochemical distribution of pro-somatostatin-related peptides in hippocampus ☆

By using immune sera which recognize one or more of the 3 peptides, somatostatin-14 (SS14), somatostatin-28 (SS28) and somatostatin-28(1-12) (SS28(1-12)) we have characterized their immunohistochemical distribution in the hippocampal formation. There exist at least two independent neuronal systems containing pro-somatostatin-related peptides: an intrinsic system of cells in the polymorphic layers which branch locally, and a dense terminal field in the molecular layer of the dentate gyrus that may constitute a portion of the entorhinal-dentate projection. In addition, SS28 is the dominant form present in cell bodies, whereas SS28(1-12) is preferentially localized in neuronal processes and terminals. We could not detect SS14 immunohistochemically.

Functional receptors for vasoactive intestinal polypeptide in cultured astroglia from neonatal rat brain

The effects of vasoactive intestinal polypeptide (VIP) were assessed on astroglia cultured from rat CNS. In these cultures VIP (500 nM) promoted the hydrolysis of [3H]glycogen newly synthesized from [3H]glucose. This effect on [3H]glycogen levels was also observed with the structurally related peptide PHI-27 and with other substances which had been demonstrated to promote glycogenolysis in rodent CNS in vitro such as: norepinephrine (NE), serotonin, histamine, adenosine, K+ and dibutyryl cyclic-AMP (dbcAMP). Furthermore, VIP (500 nM) and PHI 27 (500 nM), when applied to astroglial cultures in serum-free medium, displayed marked effects on the morphological appearance of the cell population: they converted the flat cells present in the cultures into cells with typical astrocytic morphology. As previously reported, this effect on the cellular morphology of the cultures was also observed, under identical experimental conditions, after NE and dbcAMP application. These studies demonstrate that cultured rat neonatal astroglia possess receptors for VIP, and suggest that a cyclic AMP accumulation may mediate both the metabolic and morphologic components of this response.

Effects of chronic lithium treatment on dopamine receptors in the rat corpus striatum. II. No effect on denervation or neuroleptic-induced supersensitivity.

The influence of chronic dietary lithium administration was evaluated on dopamine receptor supersensitivity in the rat corpus striatum. Supersensitivity was induced with either unilateral destruction of dopamine-containing fibers in the nigrostriatal pathway or with 3 weeks of treatment with haloperidol (HAL). Both treatments elevated [3H]spiroperidol binding sites, but in neither case was this increase in ligand binding affected by chronic dietary Li (brain levels 0.8 to 1.2 mEq/1 tissue). Our rats receiving 21 daily injections of HAL did show a behavioral supersensitivity to the dopamine agonist, apomorphine, and this effect was attenuated by concurrent treatment with dietary Li (accompanying paper). However, in contrast to previous data, this behavioral attenuation could not be linked to the prevention of increased [3H]spiroperidol binding in the corpus striatum. Furthermore, co-administration of dietary Li to subjects injected with HAL for 3 weeks did not reverse the increased density of [3H]spiroperidol binding sites which developed in the corpus striatum. Neither HAL nor Li treatment altered the affinity of the radioligand for its binding site. In the same animals, neostriatal dopamine-sensitive adenylate cyclase was not affected by either long-term dietary Li or chronic neuroleptic treatment, supporting the view that membrane antagonist and agonist sites differentially adapt to chronic alterations of synaptic input. Taken together, the results are incompatible with the hypothesis that the anti-manic action of Li is related to its ability to prevent dopamine receptor supersensitivity.

Effects of chronic lithium treatment on dopamine receptors in the rat corpus striatum. I. Locomotor activity and behavioral supersensitivity

Spontaneous locomotor activity and dopaminergic responsivity were assessed after long-term dietary treatment with the anti-manic drug lithium. Chronic dietary Li administration was not accompanied by the toxicities often reported with other modes of administration. In addition, the diet reliably yields serum and brain Li levels in the prophylactic range for manic-depressive illness. After 4 weeks exposure to Li, spontaneous locomotor activity was reduced as compared to subjects on the control diet whether or not food intake was restricted. The depression of locomotor activity following an injection of the dopamine agonist, apomorphine, was less severe in animals that ingested Li compared to those with free access to the control diet. Finally, in confirmation of the findings of Pert et al., chronic Li administration led to a partial attenuation of apomorphine-evoked stereotyped behaviors in subjects rendered supersensitive to the drug by daily injections of haloperidol (HAL) for 3 weeks. The findings suggest that the commonly reported suppressant action of Li on spontaneous behavior is not attributable to overt toxicity or to diminished growth rate. Similarly, these health factors do not account for the ability of chronic Li treatment to suppress the behavioral manifestation of dopaminergic supersensitivity associated with long-term HAL exposure.

Chemical and physiological aspects of the actions of lithium and antidepressant drugs

The possible mechanisms underlying the anti-manic actions of lithium have been examined in a variety of interdisciplinary experiments. The possibility that lithium can regulate the sensitivity changes in dopaminergic transmission produced by chronic treatment with haloperidol has been tested. Although a modest modification of behavioral responses to the dopamine agonist apomorphine was found, there was no evidence that this action of lithium reflected alterations of the binding parameters of dopamine-related ligands. In other studies, consistent, dose-dependent increases in brain enkephalin content were found after rats consumed a specially manufactured lithium diet for 2-3 weeks. Not only were brain enkephalin levels increased after this treatment, but some signs of basal analgesic responsiveness also suggested that the elevated levels of enkephalins were functionally significant. To test the possibility that the effects of lithium may not be seen in normal rats, the effects of lithium were compared on spontaneously hypertensive and unaffected, normotensive rats of a related strain. Treatment with lithium altered blood pressure in the hypertensive strain but did not affect blood pressure in the controls. These studies suggest that multiple brain systems may be regulated by treatment with lithium but that the critical pathophysiological process may not be demonstrable in the normal rat.

Morphological and functional correlates of VIP neurons in cerebral cortex

Vasoactive Intestinal Polypeptide (VIP) promotes the hydrolysis of 3H-glycogen newly synthesized from 3H-glucose by mouse cortical slices. This effect occurs rapidly, approximately 50% of the maximal effect being reached within one minute. The maximal effect is achieved after 5 minutes and maintained for at least 25 minutes. Furthermore the glycogenolytic effect of VIP is reversible, and pharmacologically specific. Thus several neuropeptides present in cerebral cortex such as cholecystokinin-8, somatostatin-28, somatostatin-14, met-enkephalin, leu-enkephalin, do not affect 3H-glycogen levels. VIP fragments 6-28, 16-28 and 21-28 are similarly inactive. Furthermore, among the peptides which share structural homologies with VIP, such as glucagon, secretin, PHI-27 and Gastric Inhibitory Peptide, only secretin and PHI-27 promote 3H-glycogen hydrolysis, with EC50 of 500 and 300 nM respectively, compared to an EC50 of 25 nM for VIP. Immunohistochemical observations indicate that each VIP-containing bipolar cell is identified with a unique radical cortical volume, which is generally between 15-60 micrograms in diameter and overlaps with the contiguous domains of neighbouring VIP-containing bipolar cells. Thus this set of biochemical and morphological observations support the notion that VIP neurons have the capacity to regulate the availability of energy substrates in cerebral cortex locally, within circumscribed, contiguous, radial domains.

Effect of 6-hydroxydopamine lesions on norepinephrine-induced [3H]glycogen hydrolysis in mouse cortical slices

The effects of norepinephrine (NE) on in vitro [3H]glycogenolysis were assessed in slices of cerebral cortex from mice whose cortical noradrenergic innervation had been severely reduced by intracisternal 6-hydroxydopamine (6-OHDA) injections. A supersensitive response to NE was observed, as demonstrated by a decrease in the EC50 of the catecholamine in the lesioned mice from 533 +/- 88 nM to 39.3 +/- 7.9 nM. This supersensitive response, observed two weeks after the lesion, was post-synaptic since isoproterenol, a beta-adrenergic agonist not accumulated by pre-synaptic uptake mechanisms, also gave an equally supersensitive response.