Saundra L. Patrick

Initiation, Propagation, and Termination of Epileptiform Activity in Rodent Neocortex In Vitro Involve Distinct Mechanisms

Waves of epileptiform activity in neocortex have three phenomenological stages: initiation, propagation, and termination. We use a well studied model of epileptiform activity in vitro to investigate directly the hypothesis that each stage is governed by an independent mechanism within the underlying cortical circuit. Using the partially disinhibited neocortical slice preparation, activity is induced and modulated using neurotransmitter receptor antagonists and is measured using both intracellular recordings and a linear array of extracellular electrodes. We find that initiation depends on both synaptic excitation and inhibition and entails a slow process of recruitment at discrete spatial locations within cortical layer 5 but not layer 2/3. Propagation depends on synaptic excitation but not inhibition and is a fast process that involves neurons across the spatial extent of the slice and in all cortical layers. Termination is modulated by synaptic excitation and inhibition. In space, termination occurs reliably at discrete locations. In time, termination is characterized by a strong depolarizing shift (block) and recovery of neurons in all cortical layers. These results suggest that the phenomenological stages of epileptiform events correspond to distinct mechanistic stages.

Physical withdrawal in rats tolerant to Δ9-tetrahydrocannabinol precipitated by a cannabinoid receptor antagonist

Tolerance to delta 9-tetrahydrocannabinol (delta 9-THC) was produced in rats by twice daily injections (15 mg/kg i.p.) for 6.5 days. Administration of the cannabinoid antagonist SR141716A (i.p. or i.c.v.) induced a profound precipitated withdrawal syndrome in delta 9-THC-tolerant animals. The syndrome was characterized by a disorganized pattern of constantly changing brief sequences of motor behavior. Autonomic signs were not evident. THC-tolerant animals that were treated with vehicle remained quiet throughout the observation period.

An examination of the central sites of action of cannabinoid-induced antinociception in the rat

Microinjections of low doses of the potent and selective cannabinoids WIN 55,212-2 and CP 55,940 into the lateral ventricle produce long-lasting reduction in sensitivity to noxious thermal stimuli (1). To determine the central distribution of ventricularly administered WIN 55,212-2, we microinjected an analgesic dose of the drug with [3H]WIN 55,212-2. At the peak time of antinociception, the radiolabeled drug was confined to periventricular sites throughout the brain. The contribution of particular periventricular structures to the antinociceptive effect was evaluated using intracerebral microinjection techniques and the tail-flick test. Guide cannulae were implanted above the following periventricular structures: the medial septal area, lateral habenlua, perihypothalamic area, arcuate nucleus of the hypothalamus, dorsal raphe nucleus and the dorsolateral and ventrolateral aspects of the periaqueductal gray. Microinjections of WIN 55,212-2 (5 micrograms/0.5 microliter) into the medial septal area, lateral habenula, perihypothalamic area, arcuate nucleus, and ventrolateral periaqueductal gray did not significantly affect tail-flick latencies. By contrast, microinjections of WIN 55,212-2 into the dorsolateral periaqueductal gray and the dorsal raphe significantly elevated tail-flick latencies. The results of this study indicate that at least two periventricular structures within the brain are involved in cannabinoid antinociception.

Effects of intranigral cannabinoids on rotational behavior in rats: interactions with the dopaminergic system

The effect of unilateral intranigral cannabinoid receptor stimulation on rotational behavior was explored. The potent cannabinoid agonist CP 55,940 (5 and 10 micrograms/0.5 microliter) induced contralateral turning when microinjected unilaterally into the substantia nigra pars reticulata. In addition, the cannabinoid agonist markedly attenuated the contralateral rotation induced by the dopamine D1 agonist SKF 82958 and completely reversed the ipsilateral rotation induced by the dopamine D2 agonist quinpirole. In both cases, the coadministration of the cannabinoid agonist together with the D1 or D2 agonist induced contralateral rotation. It appears that cannabinoids may exert different effects depending on the state of the ongoing chemical activation of this brain circuitry.

Concomitant sensitization of amphetamine-induced behavioral stimulation and in vivo dopamine release from rat caudate nucleus

Rats were treated twice daily either with saline or d-amphetamine (5 mg/kg) for 5 days. When challenged approximately 15 days later with an injection of 0.5 mg/kg amphetamine, the chronic amphetamine animals showed (1) an augmented release of dopamine in the caudate nucleus in vivo and (2) an increase in stereotyped behavior compared to the chronic saline animals. These results suggest that an increase in dopamine release from the caudate may contribute to amphetamine-induced behavioral sensitization.

Antinociceptive actions of cannabinoids following intraventricular administration in rats

Intraventricular administration of 5 or 20 micrograms of the cannabinoids WIN55,212-2 and CP-55,940 markedly reduced rats responses to noxious thermal stimuli in the tail-flick test; no significant effect was found at 1 micrograms. The dose-response curves were steep and monotonic, the onset was rapid, and the effect lasted about an hour at the highest dose. In contrast to their antinociceptive actions, WIN55,212-2 and CP-55,940 failed to alter the latency of righting reflexes at the highest dose, suggesting that motor impairment did not cause the decreased responsiveness to the thermal stimulus. Finally, an assessment of the biodistribution of intraventricularly administered [3H]WIN55,212-2 in brain and spinal cord at the time of maximal antinociception revealed that the drug was confined to the brain. The levels of [3H]WIN55,212-2 found in S3-S4, the location of the spinal mechanisms for tail-flick, were below the limit of detectability. Together, these findings provide direct evidence that the antinociceptive effects of cannabinoids are mediated, at least in part, by their actions in the brain.

Evidence for a role of endogenous cannabinoids in the modulation of acute and tonic pain sensitivity

The competitive CB1 receptor antagonist SR141716A was used to test the hypothesis that endogenous cannabinoids modulate tonic pain sensitivity. Pretreatment with the antagonist significantly enhanced the response to a chemical nociceptive stimulus in the formalin test. Postreatment with the antagonist 5 min following the induction of tonic pain produced hyperalgesia during the tonic phase only. These findings suggest that endogenous cannabinoids serve naturally to modulate the maintenance of pain following repeated noxious stimulation.

Thalamic Control of Layer 1 Circuits in Prefrontal Cortex

Knowledge of thalamocortical (TC) processing comes mainly from studying core thalamic systems that project to middle layers of primary sensory cortices. However, most thalamic relay neurons comprise a matrix of cells that are densest in the “nonspecific” thalamic nuclei and usually target layer 1 (L1) of multiple cortical areas. A longstanding hypothesis is that matrix TC systems are crucial for regulating neocortical excitability during changing behavioral states, yet we know almost nothing about the mechanisms of such regulation. It is also unclear whether synaptic and circuit mechanisms that are well established for core sensory TC systems apply to matrix TC systems. Here we describe studies of thalamic matrix influences on mouse prefrontal cortex using optogenetic and in vitro electrophysiology techniques. Channelrhodopsin-2 was expressed in midline and paralaminar (matrix) thalamic neurons, and their L1-projecting TC axons were activated optically. Contrary to conventional views, we found that matrix TC projections to L1 could transmit relatively strong, fast, high-fidelity synaptic signals. L1 TC projections preferentially drove inhibitory interneurons of L1, especially those of the late-spiking subtype, and often triggered feedforward inhibition in both L1 interneurons and pyramidal cells of L2/L3. Responses during repetitive stimulation were far more sustained for matrix than for core sensory TC pathways. Thus, matrix TC circuits appear to be specialized for robust transmission over relatively extended periods, consistent with the sort of persistent activation observed during working memory and potentially applicable to state-dependent regulation of excitability.

A comparison of (−)-deoxybenzomorphans devoid of opiate activity with their dextrorotatory phenolic counterparts suggests role of σ2 receptors in motor function

Three novel benzomorphans, (+)-N-benzylnormetazocine, (-)-deoxy-N-benzylnormetazocine, and (-)-deoxypentazocine were tested for their ability to produce circling behavior in rats following intranigral microinjections. Dose studies revealed the following rank order of potency: (-)-deoxypentazocine > (-)-deoxy-N-benzylnormetazocine > (+)-N-benzylnormetazocine. This rank order approximates that for affinities for sigma 2 receptors but not sigma 1 receptors. It is very unlikely that the effects of the (-)-deoxybenzomorphans were mediated by opiate receptors for the following reasons: (1) consistent with the known requirement for the phenolic hydroxyl group for opiate activity, both (-)-deoxy compounds showed very low affinity for opiate receptors; (2) naloxone (4 micrograms) co-administered with (-)-deoxy-N-benzylnormetazocine failed to reduce its efficacy; (3) both (-)-deoxy compounds failed to produce marked analgesic effects in the tail flick test following systemic injections of 20 mg/kg s.c. These finding suggest that sigma 2 receptors mediate the motor effects of sigma ligands in rats.

Cannabinoid effects in basal ganglia in a rat model of Parkinson's disease

Cannabinoid receptors in the brain are highly concentrated in the basal ganglia, which is in accordance with their well known effects on motor behavior. In this study, rats with 6-hydroxydopamine lesions of the nigrostriatal pathway were implanted with cannulae in the striatum, globus pallidus and substantia nigra. The effect of unilateral infusion of the potent cannabinoid agonist CP55,940 on turning behavior was studied for each structure. Lesioned animals responded to intrapallidal and intrastriatal administration of the cannabinoid in a manner that was similar to that of unlesioned animals. However, lesioned animals showed greater contralateral turning in response to the cannabinoid infusions in the substantia nigra than unlesioned animals.