Dinah Weissmann

Regional Distribution of High‐Affinity γ‐[3H] Hydroxybutyrate Binding Sites as Determined by Quantitative Autoradiography

Abstract: The distribution of high‐affinity binding sites for γ‐[3H]hydroxybutyrate in coronal sections of rat brain was studied by quantitative autoradiographic techniques. Binding sites for this naturally occumng substance, which may possibly have a neurotransmitter role, are concentrated in some restricted areas of the brain, particularly in the limbic system. The hippocampus (especially field CAI of Ammons horn, at 292 fmol/mg of tissue), septum (72 fmol/mg of tissue), and cortex (frontal, 113 fmol/mg of tissue; parietal, 103 fmol/mg of tissue; cingulate, 114 fmol/mg of tissue; and entorhinal, 134 fmol/mg of tissue) show pronounced labeling with γ‐[3H]hydroxybutyrate. Binding is much lower in caudatus‐putamen (50 fmol/mg of tissue), thalamus, and hypothalamus. Caudal parts of the brain (cerebellum, pons, and medulla) are practically devoid of binding sites. These results strongly support a functional role of endogenous y‐hydroxybutyrate in particularly restricted areas of the rat brain.

Evidence for hyperdensity of 5HT1B binding sites in the substantia nigra of the rat after 5,7-dihydroxytryptamine intraventricular injection

High affinity serotonin (5HT) binding sites have been found highly concentrated in the substantia nigra (SN) of the rat brain in each classical anatomical subdivisions of this structure, SN reticulata (SNR), SN lateralis (SNL), SN compacta (SNC). In all of the anatomical samples examined along the posteroanterior brain axis (at 200 um intervals), they corresponded to 5HT1B binding sites. The analysis of their distribution performed in rats 15 days after 5,7-DHT intraventricular injection has revealed : (1) the post-synaptic localization of these 5HT1B sites ; (2) the selective increase in their density at the level of SNR. This increase was found heterogeneously distributed inside the SNR and clearly differentiated in external and internal portions of this structure. This hyperdensity in 5HT1B sites in the SNR likely explains the functional hypersensitivity previously demonstrated by local injection of exogenous 5HT into the SN and systemic administration of RU 24969, a preferential 5HT1B agonist.

Biochemical and autoradiographic measurements of brain serotonin synthesis rate in the freely moving rat: a reexamination of the alpha-methyl-L-tryptophan method.

Abstract: Biochemical approaches were used in freely moving rats to determine, under steady‐state conditions, the brain/arterial plasma partition coefficients of L‐tryptophan and α‐[3H]methyl‐L‐tryptophan, from which the lumped constant for the α‐methyl‐L‐tryptophan method of estimating the rate of brain serotonin synthesis is calculated. The lumped constants were significantly different in the various structures examined: 0.149 ± 0.003 in the raphe dorsalis, 0.103 ± 0.002 in the raphe centralis, 0.087 ± 0.003 in the reticular formation, and 0.62 ± 0.08 in the pineal gland. From these data we proposed a two‐compartment model to calculate the rate of serotonin synthesis by quantitative autoradiography using a three‐time point experiment. Rates of synthesis for the raphe dorsalis and the reticular formation (620 ± 57 and 80 ± 35 pmol/g of tissue/min, respectively) were similar to those measured simultaneously by biochemical means, but rates were 50% higher for the raphe centralis (568 ± 90 vs. 381 ± 31 pmol/g of tissue/min). The lack of dynamic equilibrium of the tracer between plasma and tissue pools may explain the discrepancy between the two methods. Our findings did not confirm previous data, indicating that the application of the autoradiographic method to measure the rate of brain serotonin synthesis using α‐methyl‐L‐tryptophan as tracer has limitations.

Ontogeny of tyrosine hydroxylase levels in the neuropil close to locus caeruleus.

We aimed to characterize tyrosine hydroxylase (TH) expression within the pericaerulean area (PCA) during postnatal development. Levels of TH along the caudorostral axis of the locus caeruleus (LC) showed a dramatic increase in the PCA beyond day 21. This was due to the extension of the TH-containing area, particularly organized in the ventrolateral and longitudinal directions. As dendrites of LC neurones were observed at long distances within this PCA, such an increase in TH distribution could affect functions related to the LC.

RU24722 induces spatially organized phenotypic plasticity in the locus coeruleus

The plasticity of tyrosine hydroxylase (TH) expression in rat locus coeruleus (LC) was evaluated after RU24722 TH induction using, as a new parameter of characterization, the quantitative topology of LC defined by TH-positive cells. This new phenotype was spatially organized into cell subpopulations in the medial LC, dorsal and ventro-lateral to the initial perikaryal space. Reserpine and parachlorophenylalanine, which elicited a similar increase in the TH content, failed to induce a significant change in the number of TH-expressing cells. Activation of TH expression is not sufficient to reveal the existence of such a plasticity and some original but still unknown mechanism(s) of control of TH expression is affected by RU24722.

Benzodiazepine binding sites in the cingulate cortex after lesion of the noradrenaline and dopamine containing afferents

The distribution of benzodiazepine binding sites was analysed in the cingulate cortex of the rat brain by quantitative radioautography of brain sections incubated with a full agonist benzodiazepine ligand, 3H-flunitrazepam (3H-FLU), or with a partial agonist with non benzodiazepine structure, (7-3H)-4hydroxy-N(4,5-dihydroxy-2-thiazolyl)-6 methoxy-3-quinoline (3H-RU 43028), after lesion of noradrenaline (NA) and dopamine (DA) containing afferents to this structure. NA denervation was obtained by systemic administration of N-(2-chlorethyl)-N ethyl-2-bromobenzylamine (DSP4) and destruction of both NA and DA containing afferents was induced by unilateral injection of 6-hydroxydopamine (6-OHDA) in the middle forebrain bundle (MFB). A similar caudo-rostral pattern of distribution was found in the cingulate cortex after incubation with these two ligands which bound a greater number of sites in the anterior portion of the structure. In spite of a very precise anatomical sampling (200 micron intervals along the postero-anterior axis) no significant difference was observed when intact and lesioned brains were compared. It is concluded that benzodiazepine binding sites eventually localized on catecholaminergic afferents to the cingulate cortex do not represent a significant proportion of the total population of these sites in this structure.