Release of endogenous norepinephrine from rat hypothalamus in vitro

Combined electrochemical and electrophysiological studies of monoamine overflow in rat hippocampal slices

In vivo electrochemical measurements of chronoamperometric recordings from Nafion-coated electrodes were used to investigate monoamine overflow from selected regions of the rat hippocampal slice. Concurrent electrophysiological measurements of evoked CA1 pyramidal cell population spike responses were used to characterize changes in the electrical activity in the slices that occur during potassium-induced neurotransmitter overflow. Superfusion with elevated K+ (10-50 mM, 5 min) elicited consistent concentration-dependent increases in the electrochemical responses recorded from the dentate gyrus. At the onset of K+ perfusion, there was an initial increase in the population spike response, followed by electrical silence, which usually lasted 5-10 min following the return to normal medium, and required 20-30 min for complete recovery of the response. The potassium-induced electrochemical signal always increased following the decline in the electrophysiological response. Although the electrochemical signal usually returned to baseline much before the electrophysiological response (usually within 5 min), both signals remained refractory for some time. Cocaine pretreatment (10-50 microM) caused a dose-dependent augmentation of the electrochemical responses. Local pressure ejection of K+ via a micropipette elicited dose-dependent increases in the electrochemical signals that were of relativity brief duration as compared to superfusion with K+. Such potassium-evoked responses were highly localized, and were attenuated in amplitude in animals that had been previously treated with the selective noradrenergic neurotoxin, DSP-4. In addition to K+, local applications of methyl-amphetamine, tyramine and veratridine also elicited electrochemical signals, and the time courses of these responses were specific to the releasing agent that was used. Preliminary data obtained using high-speed electrochemical recordings of both oxidation and reduction current suggested that tyramine ejections evoked primarily norepinephrine overflow, while K+ evoked the overflow of both norepinephrine and serotonin. The present experiments demonstrate that simultaneous electrophysiological and electrochemical experiments can be used in an isolated preparation of brain such as the hippocampal slice to characterize the electrophysiological events that occur during stimulated transmitter release.

In vivo electrochemical demonstration of potassium-evoked monoamine release from rat cerebellum

In vivo electrochemical methods were employed to study the potassium (K+)-evoked release of monoamines from the cerebellum of the chloral hydrate anesthetized rat. K+-evoked releases were elicited using micropipette-Nafion-coated graphite epoxy electrode arrays in the granule/Purkinje cell layer, molecular layer, and white matter. These recorded releases were generally found to be reversible, moderately dose-dependent, and reproducible. However, the temporal dynamics of the releases were different for the cell layer versus molecular layer records. Releases were infrequently observed in cerebellar white matter, an area which is relatively devoid of monoamine containing terminals. The signals recorded from the cell and molecular layers were significantly attenuated by pretreatment with the selective noradrenergic neurotoxin, N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4). Local pretreatment with nomifensine, a potent catecholamine reuptake blocker, significantly prolonged the K+-evoked signals observed in both the granule/Purkinje cell and molecular layers. These data, taken together with earlier reports on the electrophysiological responses to activation of cerebellar noradrenergic inputs, support the conjecture that in vivo electrochemical recording methods have the sensitivity and spatial resolution for studies of functional monoamine release from brain regions that have a diffuse or laminated monoamine innervation.

Stimulation by gastrin-releasing peptide, neurotensin and DN1417, a novel TRH analog, of dopamine and norepinephrine release from perifused rat hypothalamic fragments in vitro

Both dopamine (DA) and norepinephrine (NE) release from perifused rat hypothalamic fragments was increased by depolarizing concentrations of potassium (K+, 20 mM and 56 mM) in a dose-related and Ca2+-dependent manner. DA and NE release induced by high K+ (20 mM) was further enhanced by gastrin-releasing peptide (GRP, 10(-5)-10(-6) M) and DN 1417 (10(-5) M), a thyrotropin-releasing hormone (TRH) analog. GRP (10(-5) M), neurotensin (NT, 10(-5) M) and DN 1417 (10(-5) M) also stimulated spontaneous release of DA and NE from the hypothalamus. These results suggest that GRP, NT and DN 1417 act at the hypothalamic catecholamine nerve terminals and stimulate the release of DA and NE in the rat.