Focal transcranial magnetic stimulation in awake rats: Enhanced glucose uptake in deep cortical layers

BACKGROUND

Transcranial magnetic stimulation (TMS) is an emerging neuromodulation tool. However, preclinical models of TMS are limited.

OBJECTIVE

To develop a method for performing TMS in awake rats and to characterize neuronal response to TMS by mapping glucose uptake following TMS administration.

METHODS

A headpost was implanted into rat skull serving as a refence to guide TMS target. Motor threshold measurement was used as the metric to assess the consistency in TMS delivery across animals and across sessions. Using a fluorescent glucose analogue (2-NBDG) as a marker of neuronal activity, we mapped glucose uptake in response to TMS of the rat motor cortex.

RESULTS

The average motor threshold (n = 41) was 34.6 ± 6.3 % of maximum stimulator output (MSO). The variability of motor threshold across animals was similar to what has been reported in human studies. Furthermore, there was no significant difference in motor threshold measured across 3 separate days. Enhancement in fluorescent signals were TMS dose (power)-dependent, which centered around the motor cortex, covering an area medial-laterally 2 mm, rostral-caudally 4 mm at 55 % MSO, and 3 mm at 35 % MSO. The count of total cells with significant fluorescent signal was: 107 ± 23 (55 % MSO), 73 ± 11 (35 % MSO) and 42 ± 11 (sham, 5% MSO).

CONCLUSIONS

Our method allows for consistent motor threshold assessment for longitudinal studies. Notably, cells with fluorescent signal enhancement were consistently aggregated in deep cortical layers, with minimal enhancement in superficial layers COMPARISONS WITH EXISTING METHOD(S): To our knowledge, this is the first study of focal TMS in awake rodents.

Ventral tegmental area neurons are either excited or inhibited by cocaine's actions in the peripheral nervous system

Cocaine's multiple pharmacological substrates are ubiquitously present in the peripheral and central nervous system. Thus, upon its administration, cocaine acts in the periphery before directly acting in the brain. We determined whether cocaine alters ventral tegmental area (VTA) neuronal activity via its peripheral actions. In urethane-anesthetized rats, we recorded VTA neuron's responses to intravenous injections of two cocaine analogs: cocaine-hydrochloride (HCl, 0.25 mg/kg), which readily cross the blood-brain barrier (BBB), and cocaine-methiodide (MI, 0.33 mg/kg), which does not cross the BBB. Both cocaine analogs produced sustained changes in discharge rates that began 5 s after the initiation of a 10-s drug infusion. Within the first 90 s post-injection, the magnitudes of neuronal responsiveness of both cocaine analogs were comparable, but later the effects of cocaine-HCl were stronger and persisted longer than those of cocaine-MI. The proportion of neurons responsive to cocaine-HCl was twice that of cocaine-MI (74% and 35%, respectively). Both analogs also differed in their response onsets. Cocaine-MI rarely evoked responses after 1 min, whereas cocaine-HCl continued to evoke responses within 3 min post-injection. VTA neurons were either excited or inhibited by both cocaine analogs. Most units responsive to cocaine-MI, regardless of whether they were excited or inhibited, had electrophysiological characteristics of putative dopamine (DA) neurons. Units inhibited by cocaine-HCl also had characteristics of DA neurons, whereas excited neurons had widely varying action potential durations and discharge rates. Cocaine-MI and cocaine-HCl each produced changes in VTA neuron activity under full DA receptor blockade. However, the duration of inhibition was shortened and the number of excitations increased, and they occurred with an earlier onset during DA receptor blockade. These findings indicate that cocaine acts peripherally with a short latency and alters the activity of VTA neurons before its well-known direct actions in the brain.

Sex differences in models of temporal lobe epilepsy: role of testosterone

Kainic acid and pilocarpine were used to assess sex differences in temporal lobe seizures. Adult Sprague-Dawley rats were injected with kainic acid (10-12 mg/kg) or with pilocarpine (380 mg/kg) and behavior was recorded for the next 3 h. Trunk blood was collected for hormonal measurements. Our data indicate that the male is more susceptible to the convulsant effects of agents that produce temporal lobe-like seizures. Males presented a higher amount of full limbic convulsions than females. To assess the role of plasma testosterone levels in kainate-induced seizures, a group of males was gonadectomized and half received testosterone replacement. The presence of testosterone, in intact and in gonadectomized males with testosterone replacement, increased the susceptibility to seizure. Seizures were either stronger (full limbic) or more frequent in animals with testosterone compared to animals devoid of testosterone. These results suggest that differences in plasma levels of testosterone may be partially responsible for the observed gender differences in seizure susceptibility. Our data reveal a reciprocal relationship between kainic acid-induced temporal lobe seizures and plasma testosterone. Testosterone enhances the occurrence and the severity of seizures. Conversely, kainic-acid-induced seizures decrease plasma testosterone. The higher plasma corticosterone levels found in these males suggest that kainic acid-induced seizures activate the hypothalamic-pituitary-adrenal axis which may induce alterations in plasma levels of male reproductive hormones.

Effects of cocaine administration on VTA cell activity in response to prefrontal cortex stimulation

The repeated use of psychostimulants in humans has been associated with progressive enhancement of anxiety, panic attacks, and eventually paranoid psychosis. The appearance of such behaviors has been termed behavioral sensitization, which forms part of the basic pathological mechanisms involved in drug addiction. Psychostimulants act via a circuit involving the ventral tegmental area (VTA), prefrontal cortex (PFC), and nucleus accumbens. The PFC sends glutamatergic projections that activate dopaminergic neurons in the VTA. These projections provide an extremely important excitatory drive necessary for the development of sensitization. The effects of cocaine administration on the response of dopaminergic VTA cells to activation of the PFC have not been reported. Here the effects of acute cocaine administration on VTA cell response to PFC stimulation are examined. Statistical analysis of the changes in spontaneous activity and evoked response revealed a significant decrease in spontaneous activity at 1.0 mg/kg i.v. after cocaine treatment compared to baseline levels. The net effect was an increase in signal-to-noise ratio. Treatment with MK-801 at a dose of 2 mg/kg showed that the excitatory response was, at least partially, NMDA-mediated. Prazosin pretreatment (0.5 mg/kg i.p.) did not prevent a significant decrease in spontaneous activity brought about by cocaine (15 mg/kg, i.p.). Nonetheless, prazosin alone induced a significant decrease in the response to PFC stimulation when compared to baseline. In addition, iontophoretic application of norepinephrine (NE) onto VTA cells revealed that NE potentiated (19.2%), enhanced (26.9%), or suppressed (46.2%) the glutamate-evoked response in VTA cells. The results suggest that a possible role of cocaine in the process of sensitization might be to amplify the PFC-induced excitation at the VTA. Since the iontophoretic release of NE in almost half of the sampled cells produced similar effects to those of cocaine it may suggest a possible NE-mediated mechanism for cocaine actions.