Gailyn A. Howell

Activity of substantia nigra units across the sleep-waking cycle in freely moving cats

The activity of dopamine-containing substantia nigra units was recorded by means of movable 32 or 62 micrometers diameter insulated nichrome wires in freely moving cats. Nigral units displayed a slow, somewhat irregular activity during quiet waking (mean = 4.6 spikes per sec), and showed no significant change in activity during sleep. While nigral unit activity was somewhat higher during active waking, there was no relationship between unit discharge and phasic movement.

Raphe unit activity in freely moving cats: Effects of benzodiazepines

Benzodiazepines (chlordiazepoxide and diazepam) produced a dose-dependent decrease in the discharge rate of serotonin-containing neurons in the dorsal raphe nucleus of freely moving cats. This ranged from no significant change at doses of 0.5 and 1.0 mg/kg (i.p.), to greater than 90% reductions in unit activity at 10 mg/kg. The effects of benzodiazepines on raphe units occurred within 15-30 min of injection and the duration of action was dose-dependent and lasted from 1 to more than 6 hr. Doses of benzodiazepines that significantly decreased raphe unit activity (i.e. 2.5-10 mg/kg) also produced ataxia and decreased EMG activity. These data suggest that benzodiazepine-induced suppression of raphe unit activity is closely related to general motor behavior. Raphe unit activity remained suppressed during phasic increases in EMG activity during eating, grooming, or predatory behavior, suggesting that benzodiazepines also have a direct inhibitory action on raphe cells. The present results are discussed in the context of the serotonergic hypothesis of anxiety.

Cholinergic denervation-induced increase of chelatable zinc in mossy-fiber region of the hippocampal formation

Chelatable zinc has repeatedly been associated with hippocampal mossy fibers, but the neurobiological functions of the zinc have remained unclear. Zinc is a constituent of the 7S Nerve Growth Factor (NGF) molecule. The hippocampal area is rich in neurotrophic factors and cholinergic deafferentation of the hippocampus appears to activate these neurotrophic factors. In this paper we report that cholinergic deafferentation of the hippocampus causes a transient rise in mossy-fiber zinc, as measured by zinc-dithizonate histochemistry. The changes are not significant until 10 days after the septal lesion, and the zinc returns to control levels by 21-28 days. Dithizonate has an association constant of 10(-15) with zinc, while NGF has an association constant of 10(-11). Thus densitometric measurement of zinc dithizone within the hippocampus may be a quantitative histochemical marker of a zinc-associated nerve growth-like substance. The data support recent findings that nerve growth-like factors increase in the hippocampal area following deafferentation, and in addition suggest that zinc is a critical factor in the regulation of trophic phenomena in the hippocampal formation.

Zinc dithizonate staining in the cat hippocampus: Relationship to the mossy-fiber neuropil and postnatal development

Abstract The formation of a colored dithizonate complex in the vicinity of the mossy-fiber axons in the hippocampus was investigated in the cat. Two histochemical tests indicate that the dithizonate complex which is formed is zinc dithizonate. Comparison of Golgi-impregnated material with dithizonate-labeled material indicates that the zinc dithizonate staining is coextensive with the mossy-fiber neuropil throughout the hippocampus. Also, studies using the dithizone reaction with kittens show that the mossy-fiber region contains small but detectable amounts of zinc at 1 week of age and that the adult concentration of zinc in the region is not attained until 13 weeks of age or later. Taken together, the results support the notion that there is a pool of chelatable zinc uniquely associated with the mossy-fiber system of the hippocampus.

A retrograde transport method for mapping zinc-containing fiber systems in the brain

A method for selectively labeling the CNS neurons that give rise to zinc-containing axonal boutons is described. Focal intracerebral infusions of selenium anions are used to produce a precipitate (presumably ZnSe) in zinc-containing axonal boutons at the injection site, and within 24-48 h, the precipitate is transported to the neurons of origin of those boutons. Silver enhancement is used to render the ZnSe visible in sections prepared for the light or electron microscope. No evidence of retrograde labeling is apparent in neurons that do not contain zinc in their boutons. Thus, the method provides a chemically specific technique for retrograde tracing of zinc-containing pathways in the CNS.

Stable-isotope dilution measurement of zinc and lead in rat hippocampus and spinal cord

Abstract Zinc and lead concentrations in hippocampus and spinal cord of rats were measured using the highly-accurate method of stable isotope dilution mass-spectrometry. In hippocampus, average zinc concentration was 72.7 ppm (dry weight), average lead, 0.053 ppm; in spinal cord, zinc averaged 26.1 ppm, lead, 0.018 ppm. Possible explanations for apparent overestimations of rat CNS metal content in previously publisjed work were discussed.

Zinc-containing neuronal innervation of the septal nuclei

A zinc-specific retrograde transport method has been employed to map the zinc-containing neuronal projections to the septal nuclei. Sodium selenite was infused iontophoretically into the lateral or medial septal nuclei to precipitate vesicular zinc as ZnSe in situ, and the neurons that were subsequently labeled by the retrograde transport of ZnSe to their perikarya were mapped. Zinc-containing cells of origin were found only in the hippocampal formation and predominantly in two regions thereof: (i) in s. oriens and deep s. pyramidale of fields CA3a and CA2 and (ii) in s. pyramidale of distal CA1 and adjacent prosubiculum.

Zinc containing projections to the bed nucleus of the stria terminalis

A retrograde tracing method that selectively labels the perikarya of zinc-containing neurons was used to identify the neurons that supply zinc-containing fibers to the bed nucleus of the stria terminalis in the rat. In agreement with prior lesion studies, retrograde tracing indicates that neurons in amygdalar and periamygdalar regions are the major sources of the zinc-containing innervation of the bed nucleus complex. Zinc-containing neurons in the presubiculum and prosubiculum were also retrogradely labeled from the BNST, whereas cells of the subiculum proper did not label. Light and occasional retrograde labeling of some CA1 and CA2 neurons and limbic cortical neurons was also observed, but the possibility of transport from regions bordering BNST injections (septum, caudate-putamen) could not be excluded in the latter cases.

Effects of Subcutaneous Injections of Zinc Chloride on Seizures Induced by Noise and by Kainic Acid

Summary Several lines of evidence implicate zinc in the pathogenesis of epileptic seizures, and administration of zinc salts has been shown to affect seizure susceptibility. In the present work, we studied the effects of subcutaneous (s.c.) injections of ZnCl2 on seizures induced by intraperitoneal (i.p.) kainic acid (10 mg/kg) in rats and by noise (80–120 dB) in the DBA/2J mouse. Previous administration of zinc salt (20–200 mg/kg) substantially reduced the frequency of noise‐induced running fits, clonic and tonic seizures, and deaths in mice, but had no significant effect on the incidence or severity of kainic acid‐induced seizures in rats. Together with findings in the literature, our results suggest that zinc plays multiple, sometimes antagonistic roles in seizure development.

In vitro recording of raphe unit activity: Evidence for endogenous rhythms in presumed serotonergic neurons

The spontaneous activity of single neurons in the nucleus raphe dorsalis was recorded in vitro in mouse brain slices. The neurons displayed the slow and regular discharge pattern characteristic of raphe neurons recorded in vivo. When magnesium ion was added to increase the medium concentration to 20-30 mM for the purpose of inhibiting all synaptic transmission, raphe neurons continued to display the same discharge pattern and rate. The data suggest that the steady rhythmic firing of nucleus raphe dorsalis neurons is generated by an intracellular pacemaker mechanism.