Bruce E. Hunter

Biocytin: a versatile anterograde neuroanatomical tract-tracing alternative

Biocytin, a naturally occurring low molecular weight analog of biotin, was evaluated as a neuroanatomical tract-tracing marker in the adult rat brain. Since it retains high-affinity binding to avidin, biocytin can be labelled with avidinylated visualization reagents. Iontophoretic or pressure injections resulted in filling of cell bodies and dendrites around the injection site and their efferent axonal processes and boutons. Retrogradely labelled neurons were occasionally observed at a distance but only with large injections. Anterograde tracing with biocytin is successful even in animals that are quite old, in contrast to lectins and HRP conjugates, and offers advantages in delivery, tissue processing, selection of light and/or electron microscopic labels, time to obtain results, and cost over many conventional tracers.

A novel nicotinic agonist facilitates induction of long-term potentiation in the rat hippocampus

Long-term potentiation (LTP) can be modulated by a number of neurotransmitter receptors including muscarinic and GABAergic receptor types. We have found that a novel nicotinic agonist, 2,4-dimethoxybenzylidene anabaseine (DMXB), facilitated the induction of LTP in the hippocampus in a dose-dependent and mecamylamine-sensitive manner. DMXB displaced high affinity nicotinic [125I]alpha-bungarotoxin and [3H]acetylcholine binding in rat brain. Xenopus oocyte studies demonstrated that DMXB has agonist activity at alpha 7 but not alpha 4/beta 2 nicotinic receptor subtypes. These results indicated that DMXB is a novel nicotinic agonist with apparent specificity for the alpha 7/alpha-bungarotoxin nicotinic receptor subtype and indicate that nicotinic receptor activation is capable of modulating the induction of long-term potentiation.

Modulation of Learning and Memory: Effects of Drugs Influencing Neurotransmitters

Drugs have been utilized for many years as experimental tools in the analysis of the biological mechanisms underlying behavior. An assumption implicit in this approach is that an assessment of the actions of various drugs on behavior, coupled with knowledge of mechanisms of drug action, will provide important information concerning the neurobiological bases of behavior.

Ethanol dependence in the rat: a parametric analysis.

Rats were maintained on liquid diets as their sole source of calories and fluid for 10, 15, 20, and 30 days. The diets consisted 35-40% of total calories in the form of ethanol. This procedure resulted in substantial ethanol intake leading to behavioral intoxication. Blood ethanol concentrations were found to be elevated throughout the day with a peak during the dark phase of the light cycle. The removal of ethanol resulted in evidence of physiological dependence, including behavioral manifestation of autonomic and somatic dysfunction and an increased susceptibility to audiogenic convulsions. Ten days of ethanol exposure was found to be sufficient for the reliable induction of ethanol dependence. Further increases in ethanol exposure resulted in increased hyperexcitability as measured by susceptibility to audiogenic convulsions. The severity of withdrawal behavior was found to be correlated with the blood ethanol concentration measured upon ethanol removal. A behavioral rating scale for the evaluation of alcohol withdrawal intensity in rats is described.

Alterations and recovery of dendritic spine density in rat hippocampus following long-term ethanol ingestion

Neuronal loss and dendritic pathology are often observed in humans and animals after long-term ethanol ingestion. It is not known, however, if surviving but damaged neurons can recover normal structure during ethanol abstinence. We quantified dendritic spine density in two neuronal populations in rat hippocampus to investigate whether reversibility from the cellular neurotoxic sequelae of chronic ethanol exposure was possible. Male Long-Evans rats were maintained for 20 weeks on an ethanol-containing liquid diet. Controls were pair-fed a liquid diet with sucrose substituted isocalorically for ethanol. One-half of each group was sacrificed at the end of the 20-week treatment and one-half was given a 20-week ethanol-free recovery period period to sacrifice. Analysis of rapid Golgi material revealed a decreased spine density in CA1 pyramidal cells that increased to control level during abstinence, and an increased spine density in dentate gyrus granule cells that was reduced toward control level during abstinence. Thus, despite the fact that chronic ethanol exposure produced differential initial effects, the return toward normal spine density in each region is consistent with the concept of neuronal recovery and reorganization during abstinence from ethanol.

Short-term memory impairment following chronic alcohol consumption in rats

Abstract Prolonged alcohol consumption (20 weeks), concomitant with adequate nutrition, was found to result in a residual short-term memory deficit after a 2-month alcohol-free period. Alcohol was administered in a liquid diet containing 35–37% ethanol-derived calories. One control group was pair-fed the same diet, except that sucrose was isocalorically substituted for ethanol. A second control group received pelleted laboratory food. After a 60-day alcohol-free period, short-term memory was assessed by training the rats on a discrete-trial temporal-alternation task in which bar presses were reinforced on alternate trials. Performance of the alternation problem was evaluated under conditions of short (20 sec) and long (50 sec) between-trial retention intervals. Although alcohol-treated-rats were relatively unimpaired when the retention interval was short, they were severely impaired with the long retention interval. In addition, the performance of alcohol-treated rats was severely disrupted when a distractor task was introduced during the short retention interval. The results were discussed relative to the similar short-term memory impairments of chronic alcoholic and alcoholic Korsakoff patients.

Alcohol withdrawal syndrome in rats: Neural and behavioral correlates

Abstract Rats were chronically implanted with electrodes in the ventral hippocampus, amygdala and anterior cortex and maintained on liquid diets as their only source of calories and fluid for 15 days. The diet consisted of 35–40% of the calories in the form of ethanol while a control group was pair-fed identical diets with sucrose isocalorically substituted for ethanol. On the sixteenth day the diets were removed and electrographic activity and behavior were simulataneously observed for 8–10 hr. Withdrawal symptoms were observed beginning 2–4 hr following alcohol abstinence and included tail-stiffening, tremors, severe ataxia and auditory-induced convulsions. EEG epileptiform activity was observed and initially consisted of transient spike events, which usually became progressively organized into brief spike burst sor sustained paroxysmal activity. The results suggested that cortical bioelecric activity may not play a primary role in the genesis of behavioral hyperexcitability during alcohol withdrawal. The utility of the method of combined observations of neural bioelectric activity and behavior for the delineation of the neural substrates of alcohol withdrawal symptoms was discussed.

Dissociation between physical dependence and volitional ethanol consumption: Role of multiple withdrawal episodes

Abstract The experiment examined the effects of single and multiple episodes of forced administration of a liquid diet containing ethanol on subsequent volitional ethanol consumption. Rats were subjected to a series of 3 sequences of forced liquid diet consumption lasting 20, 50 and 50 days. One group (AD) received a liquid diet with 35–42% of the calories in the form of ethanol. Another group (SD) received identical diets except sucrose was isocalorically substituted for ethanol. Following each sequence a free-choice test was given in which the rats were allowed to choose between an alcohol diet, a sucrose diet and water. After 20 days of alcohol consumption, rats in the AD group rejected the alcohol diet, despite the occurrence of severe withdrawal symptoms including tail-stiffening, ataxia, tremors and hyperreactivity. During subsequent preference tests, a substantial, but transient, increase in alcohol self-selection was observed. It was concluded that rats may learn the association between alcohol and relief of withdrawal symptoms, but a number of withdrawal episodes are required.

Augmentation of short-term plasticity in CA1 of rat hippocampus after chronic ethanol treatment

The neurotoxic effects of chronic ethanol exposure were investigated in rat hippocampus by electrophysiological analysis of the Schaffer collateral-commissural input to stratum radiatum of CA1. Experimental animals were fed an ethanol-containing liquid diet for 20 weeks but were withdrawn from the special diet at least eight weeks prior to acute electrophysiological recordings. Ethanol treatment had no effect on input-output relationships for either the population EPSP or the population spike (PS). During paired-pulse stimulation, the ethanol group exhibited a greater facilitation of the test pulse PS relative to the control group, although potentiation of the EPSP was unchanged. In addition, the ethanol group showed a trend toward greater facilitation of the PS during 5 and 10 Hz tetani. No differences between groups were observed in the magnitude or duration of the long-term potentiation produced by 5, 10 or 100 Hz stimulus trains. Ethanol treatment did significantly reduce the transient spike depression after low frequency stimulation. This pattern of results is similar to that found for treatments which reduce hippocampal recurrent inhibition. Thus, chronic ethanol treatment may produce a lasting disruption of intrinsic inhibitory neurotransmission in the rat hippocampus.

Chronic ethanol consumption reduces the neurotrophic activity in rat hippocampus

The effect of chronic ethanol treatment (CET) for 21-26 weeks on the neurotrophic activity contained in the rat hippocampus (HPC) was determined with a bioassay in cultures of dissociated dorsal root ganglion cells (DRG) obtained from E7-8 chick embryos. Extracts of the HPC from CET or pair-fed control rats were used as experimental media, and neuronal survival and neurite-outgrowth of DRG cultures were determined. Both neuronal survival (-25%) and neurite-outgrowth (-50%) were reduced in the presence of HPC extracts from CET rats relative to controls. These data suggest that CET reduces the neurotrophic content of the HPC which may result in damage to septohippocampal neurons.