D. D. Potter

Seizure-like activity and cellular damage in rat hippocampal neurons in cell culture

Neurons dissociated from the hippocampal formations of neonatal rats were grown in medium containing kynurenic acid (a glutamate receptor antagonist) and elevated Mg2+. Such chronically blocked neurons, when first exposed to medium without blockers (after 0.5-5.0 months), generated intense seizure-like activity. This consisted of bursts of synchronous electrical responses that resembled paroxysmal depolarization shifts and sustained depolarizations that, in some neurons, nearly abolished the resting potential. Sustained depolarizations were usually reversed by timely application of kynurenate or 2-amino-5-phosphonovalerate, indicating that continuous activation of glutamate receptors was required for their maintenance. Prolonged periods of intense seizure-like activity usually killed most neurons in the culture. This system allows seizure-related cellular mechanisms to be studied in long-term cell culture.

Studies on rat sympathetic neurons developing in cell culture. I. Growth characteristics and electrophysiological properties.

In this series of three papers, we describe electrophysiological and pharmacological studies on sympathetic principal neurons developing in cell culture. This paper is concerned with the methods for growing and recording from the neurons and with observations on some of their electrical properties. The succeeding papers are concerned with functional synapses which the neurons form with one another. Superior cervical ganglia of newborn rats were dissociated into single cells and small cell clusters, and the resulting cell suspension of principal neurons and a much smaller number of non-neuronal cells was cultured at low density in medium containing nerve growth factor (D. Bray, 1970, Proc. Nat. Acad. Sci. USA. 65, 905–910; R. E. Mains and P. H. Patterson, 1973a, J. Cell Biol. 59, 329–345). As in the previous studies the multiplication of the non-neuronal cells could be controlled so that the neurons grew in the presence of an increasing number of non-neuronal cells or in the virtual absence of other cell types. Another method for obtaining mixed cultures was to plate the initial cell suspension onto a preexisting layer of cells dissociated from some other tissue (e.g., heart). Neurons grown for 3 weeks or longer in the presence of non-neuronal cells had resting potentials, passive electrical properties, and action potentials generally similar to those reported for principal neurons of the superior cervical ganglia of adult rats. Through the use of tetrodotoxin, tetraethylammonium, and cobalt, evidence was obtained for the presence of potential-sensitive sodium, potassium, and calcium channels. Frequently the action potential was followed by a prolonged after-hyperpolarization whose properties suggested the presence of potassium channels controlled by calcium ions. When the neurons were grown in the absence of non-neuronal cells, the action potentials were similar, but the prolonged after-hyperpolarization was rarely seen, and the neurons usually discharged repetitively in response to a steady depolarization.

Desipramine Reduces Stress-Activated Dynorphin Expression and CREB Phosphorylation in NAc Tissue

The nucleus accumbens (NAc) is a critical brain area for reward and motivated behavior. Accumulating evidence suggests that altered function of the transcription factor cAMP response element binding protein (CREB) within the NAc is involved in depressive behavior. In rats, stress activates CREB within the NAc, and elevation of CREB expression in this region produces depressive-like behaviors that are accompanied by activation of CREB-regulated target genes. The depressive-like behaviors seem to be due, at least in part, to CREB-mediated increases in dynorphin function, because they are mimicked by κ-opioid receptor (KOR) agonists and attenuated by KOR antagonists. We hypothesized that if CREB-mediated dynorphin expression in the NAc contributes to depressive behavior, then antidepressants might reduce dynorphin function in this region. Here, we demonstrate that desipramine (DMI), a norepinephrine reuptake inhibitor that has been used for decades to treat clinical depression, blocks swim stress-induced activation of prodynorphin (encodes dynorphin) in the NAc. In primary cultures of NAc and striatum, DMI decreases basal and stimulated CREB phosphorylation by causing reductions in intracellular calcium (Ca2+) availability that are independent of norepinephrine or other monoaminergic inputs, identifying a potential mechanism for alterations in CREB-mediated gene expression. Fluoxetine (FLX), a selective serotonin reuptake inhibitor, has similar effects in culture, suggesting a common intracellular effect of these antidepressants. These findings raise the possibility that a therapeutically relevant mechanism of action of DMI occurs through attenuation of CREB-mediated gene transcription, which is mediated via previously uncharacterized mechanisms that occur directly within the NAc.

Exposure to the Selective κ-Opioid Receptor Agonist Salvinorin A Modulates the Behavioral and Molecular Effects of Cocaine in Rats

Stress and chronic exposure to drugs of abuse can trigger addictive and depressive disorders. Both stimuli increase activity of dynorphin, a neuropeptide that acts at κ-opioid receptors (KORs). In humans, KOR agonists cause dysphoria, raising the possibility that dynorphin modulates the depressive-like effects of stress and chronic drug use. We examined if KOR activation alters sensitivity to stimulant drugs by assessing the effects of the selective KOR agonist, salvinorin A (SalvA), on cocaine-induced locomotor activity and c-Fos expression. Acute administration of SalvA blocked the locomotor-stimulant effects of cocaine, whereas repeated SalvA together with concomitant exposure to activity testing chambers potentiated the locomotor response to a cocaine challenge. In contrast, repeated SalvA administered in home cages rather than the activity chambers failed to potentiate the locomotor response to a cocaine challenge. One potential explanation for these findings is that activation of KORs disrupts context conditioning: acute locomotor responses to SalvA alone did not fully habituate with repeated testing in the activity chambers. The effects of SalvA on locomotor activity paralleled its effects on cocaine-induced c-Fos expression in the dorsal striatum: acute SalvA attenuated cocaine-induced c-Fos, whereas repeated SalvA potentiated it when administered in the activity chambers but not the home cage. Acute SalvA also blocked the locomotor stimulant effects of the D1 receptor agonist SKF 82958, whereas repeated SalvA potentiated these effects when administered in the activity chambers. These findings suggest that SalvA regulates the stimulant effects of cocaine through interactions with D1 receptor-mediated signaling in the dorsal striatum.

Studies on rat sympathetic neurons developing in cell culture: III. Cholinergic transmission

Abstract Principal neurons were dissociated from the superior cervical ganglia of newborn rats and grown in culture with several types of non-neuronal cells. As described in the second paper of this series, the neurons in such mixed cultures formed two types of excitatory synapses with each other, electrical and chemical. Evidence is presented here that transmission at the chemical synapses was cholinergic. Four nicotinic ganglionic blocking agents (curare, hexamethonium, tetraethylammonium, and mecamylamine) strongly attenuated or eliminated the excitatory postsynaptic potentials (e.p.s.p.s) at moderate concentrations; atropine at relatively high concentrations also blocked transmission. Iontophoretic application of acetylcholine (ACh) to the surface of the neurons gave rise to depolarizations that could be made to resemble the e.p.s.p.s in size and time course; the ACh potentials and the e.p.s.p.s were then similarly affected by nicotinic blocking agents. The sensitivity to ACh was often distributed nonuniformly on the neuronal surface; it was common to find small, sharply localized regions of high sensitivity. Catecholamines (norepinephrine, epinephrine, and dopamine) had only inhibitory actions; in a few experiments adrenergic blocking agents (phenoxybenzamine, propranolol) were found to have no effect on the e.p.s.p.s. These observations leave no doubt that the neurons released ACh and had ganglionic, nicotinic ACh receptors on their surfaces. The significance of the fact that a high proportion of the sympathetic neurons in mixed cultures formed cholinergic synapses is discussed.

Studies on rat sympathetic neurons developing in cell culture: II. Synaptic mechanisms

Abstract Sympathetic neurons dissociated from superior cervical ganglia of newborn rats were grown in culture either alone or with non-neuronal cells, as described in the preceding paper. In the presence of the non-neuronal cells, but rarely in their absence, neurons formed functional synapses with each other de novo. The synapses were of two types, both excitatory. One type operated by nonrectifying electrical transmission and comprised only a few percent of the interactions; it was characterized by negligible synaptic delay and the transfer of steady depolarizations or hyperpolarizations from one cell to the other. At the second type of synapse which was chemical, there was a synaptic delay (minimum, 1 msec) and the amplitudes of the chemically mediated postsynaptic potentials (e.p.s.p.s) were dependent on the concentrations of Ca2+ and Mg2+ in the extracellular medium. As described in the following paper, the e.p.s.p.s were sensitive to nicotinic-cholinergic blocking agents. The incidence of chemical transmission increased markedly with age in culture. This increase was associated with the formation of networks in which the neurons were extensively connected to each other. In such cultures an action potential evoked in one neuron often gave rise in other neurons and in the stimulated neuron to a volley of synaptic activity (“complex wave”) which occurred nearly synchronously, though not identically, in each neuron. The complex waves depended on chemical transmission since they, like the simple e.p.s.p.s, were abolished by nicotinic blocking agents.

Observations on the Neurosecretory System of Portunid Crabs

A study has been made of the inclusions found in living and fixed nerve cells in the central nervous systems of two genera of portunid crabs, Callinectes and Carcinus. In order to discover if any of the types of inclusions found in neuro- secretory cells is unique to these cells, and therefore, by inference, directly associated with the production, Iransport or storage of hormonally active substances, the inclusions of nerve cells whose endocrine function is not in doubt have been compared with inclusions of other nerve cells in the eyestalk, brain and thoracic ganglion.

Ablation of Kappa-Opioid Receptors from Brain Dopamine Neurons has Anxiolytic-Like Effects and Enhances Cocaine-Induced Plasticity

Brain kappa-opioid receptors (KORs) are implicated in states of motivation and emotion. Activation of KORs negatively regulates mesolimbic dopamine (DA) neurons, and KOR agonists produce depressive-like behavioral effects. To further evaluate how KOR function affects behavior, we developed mutant mice in which exon 3 of the KOR gene (Oprk1) was flanked with Cre-lox recombination (loxP) sites. By breeding these mice with lines that express Cre-recombinase (Cre) in early embryogenesis (EIIa-Cre) or only in DA neurons (dopamine transporter (DAT)-Cre), we developed constitutive KOR knockouts (KOR−/−) and conditional knockouts that lack KORs in DA-containing neurons (DAT-KORlox/lox). Autoradiography demonstrated complete ablation of KOR binding in the KOR−/− mutants, and reduced binding in the DAT-KORlox/lox mutants. Quantitative reverse transcription PCR (qPCR) studies confirmed that KOR mRNA is undetectable in the constitutive mutants and reduced in the midbrain DA systems of the conditional mutants. Behavioral characterization demonstrated that these mutant lines do not differ from controls in metrics, including hearing, vision, weight, and locomotor activity. Whereas KOR−/− mice appeared normal in the open field and light/dark box tests, DAT-KORlox/lox mice showed reduced anxiety-like behavior, an effect that is broadly consistent with previously reported effects of KOR antagonists. Sensitization to the locomotor-stimulating effects of cocaine appeared normal in KOR−/− mutants, but was exaggerated in DAT-KORlox/lox mutants. Increased sensitivity to cocaine in the DAT-KORlox/lox mutants is consistent with a role for KORs in negative regulation of DA function, whereas the lack of differences in the KOR−/− mutants suggests compensatory adaptations after constitutive receptor ablation. These mouse lines may be useful in future studies of KOR function.

Transmitter status in cultured sympathetic principal neurons: plasticity, graded expression and diversity.

Publisher Summary This chapter summarizes the recent investigations of the transmitter status of sympathetic principal neurons derived from neonatal or adult rats and grown singly in microcultures with cardiac cells. The work began out of an interest in the status of individual neonatal sympathetic neurons during a transition from an initial (at least) adrenergic state to a predominantly cholinergic state, under the influence of non-neuronal cells. Under the influence of nerve growth factor, neurites grow progressively over the microculture but not beyond its borders. Many microcultures survive for 1–3 months; after such periods, the density of neurites over the myocytes is often greater than that of the normal innervation of sympathetic target tissues in vivo . The classical view of transmitter status in adult mammalian sympathetic principal neurons is that two transmitters—norepinephrine and acetylcholine—are expressed. Each neuron secretes only one transmitter (monofunction), and that transmitter is expressed approximately full-on; once the appropriate transmitter is adopted, the neuron does not change status.

Relative Timing Between Kappa Opioid Receptor Activation and Cocaine Determines the Impact on Reward and Dopamine Release.

Negative affective states can increase the rewarding value of drugs of abuse and promote drug taking. Chronic cocaine exposure increases levels of the neuropeptide dynorphin, an endogenous ligand at kappa opioid receptors (KOR) that suppresses dopamine release in the nucleus accumbens (NAc) and elicits negative affective states upon drug withdrawal. However, there is evidence that the effects of KOR activation on affective state are biphasic: immediate aversive effects are followed by delayed increases in reward. The impact of KOR-induced affective states on reward-related effects of cocaine over time is not known. We hypothesize that the initial aversive effects of KOR activation increase, whereas the delayed rewarding effects decrease, the net effects of cocaine on reward and dopamine release. We treated rats with cocaine at various times (15 min to 48 h) after administration of the selective KOR agonist salvinorin A (salvA). Using intracranial self-stimulation and fast scan cyclic voltammetry, we found that cocaine-induced increases in brain stimulation reward and evoked dopamine release in the NAc core were potentiated when cocaine was administered within 1 h of salvA, but attenuated when administered 24 h after salvA. Quantitative real-time PCR was used to show that KOR and prodynorphin mRNA levels were decreased in the NAc, whereas tyrosine hydroxylase and dopamine transporter mRNA levels and tissue dopamine content were increased in the ventral tegmental area 24 h post-salvA. These findings raise the possibility that KOR activation—as occurs upon withdrawal from chronic cocaine—modulates vulnerability to cocaine in a time-dependent manner.