Comparative effects of convulsant drugs on the sensory responses of neurons in the amygdala and brainstem reticular formation

Amygdala hyperactivity and tonotopic shift after salicylate exposure

The amygdala, important in forming and storing memories of aversive events, is believed to play a major role in debilitating tinnitus and hyperacusis. To explore this hypothesis, we recorded from the lateral amygdala (LA) and auditory cortex (AC) before and after treating rats with a dose of salicylate that induces tinnitus and hyperacusis-like behavior. Salicylate unexpectedly increased the amplitude of the local field potential (LFP) in the LA making it hyperactive to sounds≥60 dB SPL. Frequency receptive fields (FRFs) of multiunit (MU) clusters in the LA were also dramatically altered by salicylate. Neuronal activity at frequencies below 10 kHz and above 20 kHz was depressed at low intensities, but was greatly enhanced for stimuli between 10 and 20 kHz (frequencies near the pitch of the salicylate-induced tinnitus in the rat). These frequency-dependent changes caused the FRF of many LA neurons to migrate towards 10-20 kHz thereby amplifying activity from this region. To determine if salicylate-induced changes restricted to the LA would remotely affect neural activity in the AC, we used a micropipette to infuse salicylate (20 μl, 2.8 mM) into the amygdala. Local delivery of salicylate to the amygdala significantly increased the amplitude of the LFP recorded in the AC and selectively enhanced the neuronal activity of AC neurons at the mid-frequencies (10-20 kHz), frequencies associated with the tinnitus pitch. Taken together, these results indicate that systemic salicylate treatment can induce hyperactivity and tonotopic shift in the amygdala and infusion of salicylate into the amygdala can profoundly enhance sound-evoked activity in AC, changes likely to increase the perception and emotional salience of tinnitus and loud sounds. This article is part of a Special Issue entitled: Tinnitus Neuroscience.

Electrical stimulation therapies for CNS disorders and pain are mediated by competition between different neuronal networks in the brain

CNS neuronal networks are known to control normal physiological functions, including locomotion and respiration. Neuronal networks also mediate the pathophysiology of many CNS disorders. Stimulation therapies, including localized brain and vagus nerve stimulation, electroshock, and acupuncture, are proposed to activate "therapeutic" neuronal networks. These therapeutic networks are dormant prior to stimulatory treatments, but when the dormant networks are activated they compete with pathophysiological neuronal networks, disrupting their function. This competition diminishes the disease symptoms, providing effective therapy for otherwise intractable CNS disorders, including epilepsy, Parkinson's disease, chronic pain, and depression. Competition between stimulation-activated therapeutic networks and pathophysiological networks is a major mechanism mediating the therapeutic effects of stimulation. This network interaction is hypothesized to involve competition for "control" of brain regions that contain high proportions of conditional multireceptive (CMR) neurons. CMR regions, including brainstem reticular formation, amygdala, and cerebral cortex, have extensive connections to numerous brain areas, allowing these regions to participate potentially in many networks. The participation of CMR regions in any network is often variable, depending on the conditions affecting the organism, including vigilance states, drug treatment, and learning. This response variability of CMR neurons is due to the high incidence of excitatory postsynaptic potentials that are below threshold for triggering action potentials. These subthreshold responses can be brought to threshold by blocking inhibition or enhancing excitation via the paradigms used in stimulation therapies. Participation of CMR regions in a network is also strongly affected by pharmacological treatments (convulsant or anesthetic drugs) and stimulus parameters (strength and repetition rate). Many studies indicate that treatment of unanesthetized animals with antagonists (bicuculline or strychnine) of inhibitory neurotransmitter (GABA or glycine) receptors can cause CMR neurons to become consistently responsive to external inputs (e.g., peripheral nerve, sensory, or electrical stimuli in the brain) to which these neurons did not previously respond. Conversely, agents that enhance GABA-mediated inhibition (e.g., barbiturates and benzodiazepines) or antagonize glutamate-mediated excitation (e.g., ketamine) can cause CMR neurons to become unresponsive to inputs to which they responded previously. The responses of CMR neurons exhibit extensive short-term and long-term plasticity, which permits them to participate to a variable degree in many networks. Short-term plasticity subserves termination of disease symptoms, while long-term plasticity in CMR regions subserves symptom prevention. This network interaction hypothesis has value for future research in CNS disease mechanisms and also for identifying therapeutic targets in specific brain networks for more selective stimulation and pharmacological therapies.

Steroid hormone effects on picrotoxin-induced seizures in female and male rats

Picrotoxin (1 mg/kg, i.p.), evoked a single generalized seizure in 75% of ovariectomized rats. Pretreatment of matched pairs with silastic implants containing 100% estradiol had an anticonvulsant effect; it protected all rats against such seizures. Implants containing 10% estradiol in cholesterol were less effective in protecting against picrotoxin-induced seizures. With 2 mg/kg picrotoxin, 85% of the seizure-affected ovariectomized controls had multiple seizures. The incidence of seizures and the ratio of single to multiple seizures induced by the higher dose of picrotoxin were unaffected by estradiol silastic implants, intraperitoneal injections of progesterone (0.5 mg, 4-5 h before convulsant) or the combination of both hormones. At the 2 mg/kg dose, 8/8 intact males had no seizures while all paired ovariectomized females had seizures. By contrast, the incidence of seizures in pairs of gonadectomized males and females did not differ. Testosterone treatment improved the ratio of single to multiple seizures in males but not in females. Males had statistically fewer multiple seizures than did females after testosterone treatment. The distribution of latencies to a single seizure is statistically different from the distribution of latencies to the first of multiple seizures irrespective of dose, sex and hormone treatment. This suggests that the population of rats responding with a single seizure at the higher dose of picrotoxin have a higher threshold for acquiring multiple seizures and that testosterone predisposes males but not females to this population.