Determinants of the slow acquisition of medical and sulcal prefrontal cortex self-stimulation: an individual differences approach

Severe seizures as a side effect of deep brain stimulation in the dorsal peduncular cortex in a rat model of depression

Deep brain stimulation (DBS) has shown to have antidepressant effects in both human trials and animal studies. However, the optimal target and the underlying therapeutic mechanisms remain to be determined. In this study, we investigated if high frequency (HF) DBS in the dorsal peduncular cortex (DPC) alleviates depressive-like behavior in an experimental model of depression. Surprisingly, HF DBS in the DPC caused acute induction of seizures in ~40% of animals stimulated with clinically relevant stimulation parameters. Reducing the stimulation's amplitude by 50% did not alter seizure occurrence. Electroencephalographic (EEG) recordings showed seizures up to Racine stage IV lasting up to 4 min after cessation of stimulation. We conclude that HF DBS in the DPC is not suitable for mood-related experiments in rats but could be a potential model for seizure induction.

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

The prefrontal cortex (PFC) has long been known to be involved in the mediation of complex behavioral responses. Considerable research efforts are directed towards refining the knowledge about the function of this brain area and the role it plays in cognitive performance and behavioral output. In the first part, this review provides, from a pharmacological perspective, an overview of anatomical, electrophysiological and neurochemical aspects of the function of the PFC, with an emphasis on the mesocortical dopamine system. Anatomy of the mesocortical system, basic physiological and pharmacological properties of neurotransmission within the PFC, and interactions between dopamine and glutamate as well as other transmitters within the mesocorticolimbic circuit are included. The coverage of these data is largely restricted to what is relevant for the second part of the review which focuses on behavioral studies that have examined the role of the PFC in a variety of phenomena, behaviors and paradigms. These include reward and addiction, locomotor activity and sensitization, learning, cognition, and schizophrenia. Although the focus of this review is on the mesocortical dopamine system, given the intricate interactions of dopamine with other transmitter systems within the PFC and the importance of the PFC as a source of glutamate in subcortical areas, these aspects are also covered in some detail where appropriate. Naturally, a topic as complex as this cannot be covered comprehensively in its entirety. Therefore this review is largely limited to data derived from studies using rats, and it is also specifically restricted to data concerning the medial PFC (mPFC). Since in several fields of research the findings concerning the function or role of the mPFC are relatively inconsistent, the question is addressed whether these inconsistencies might, at least in part, be related to the anatomical and functional heterogeneity of this brain area.

Fos expression following self-stimulation of the medial prefrontal cortex

Self-stimulation of the medial prefrontal cortex and medial forebrain bundle appears to be mediated by different directly activated fibers. However, reward signals from the medial prefrontal cortex do summate with signals from the medial forebrain bundle, suggesting some overlap in the underlying neural circuitry. We have previously used Fos immunohistochemistry to visualize neurons activated by rewarding stimulation of the medial forebrain bundle. In this study, we assessed Fos immunolabeling after self-stimulation of the medial prefrontal cortex. Among the structures showing a greater density of labeled neurons in the stimulated hemisphere were the prelimbic and cingulate cortex, nucleus accumbens, lateral preoptic area, substantia innominata, lateral hypothalamus, anterior ventral tegmental area, and pontine nuclei. Surprisingly, little or no labeling was seen in the mediodorsal thalamic nucleus or the locus coeruleus. Double immunohistochemistry for tyrosine hydroxylase and Fos showed that within the ventral tegmental area, a substantial proportion of dopaminergic neurons did not express Fos. Despite previous suggestions to the contrary, comparison of the present findings with those of our previous Fos studies reveals a number of structures activated by rewarding stimulation of both the medial prefrontal cortex and the medial forebrain bundle. Some subset of activated cells in the common regions showing Fos-like immunoreactivity may contribute to the rewarding effect produced by stimulating either site.

Aversive effects of the synthetic cannabinoid CP 55,940 in rats

A series of experiments investigated the behavioral and hedonic effects of the synthetic cannabinoid CP 55,940 in male Wistar rats. CP 55,940 had a biphasic effect on locomotor activity, with a 10 micrograms/kg dose causing locomotor stimulation and a 100 micrograms/kg dose causing profound hypoactivity. CP 55,940 (100 micrograms/kg) also caused a marked hypothermia for at least 3 h following administration, while lower doses (2.5 and 10 micrograms/kg) had no effect. CP 55,940 (100 micrograms/kg) had anorexic and hyperdipsic effects for up to 24 h following administration and caused significant reductions in body weight. CP 55,940 (100 micrograms/kg) also caused significant avoidance to a flavoured fluid (saccharin) with which it was paired. In the conditioned place preference paradigm both the 10 micrograms/kg and 100 micrograms/kg doses of CP 55,940 produced significant place avoidance. It is concluded that CP 55,940 is aversive to rats. The possible mechanisms underlying this aversion are discussed.

Characteristics of stimulation-induced feeding sites in the sulcal prefrontal cortex

Double pulse tests were used to infer the refractory periods of the substrate underlying stimulation-induced feeding in the sulcal prefrontal cortex of the rat. Eleven sites were examined, of which five supported the behaviour at currents of 250 to 400 microA; pulse duration was 100 microseconds. The average profile indicates a recovery function that begins at 0.5 ms and ends at 3.0 ms, with no apparent contribution from local potential summations. The mean effectiveness value corresponding to the asymptotic portion of the curves was 91%. These results suggest that there is substantial overlap in the excitability of neurons underlying stimulation-induced feeding in the sulcal prefrontal cortex and that reported for the medial forebrain bundle, but unlike the latter structure, there is no evidence of self-stimulation from the same sulcal prefrontal cortex placements. Response rates were collected for a maximum of 21 days from each of the cortical sites at which feeding was recorded and at the same currents used to evaluate the refractory periods underlying stimulation-induced feeding. The rates averaged between 0 and 3 responses per minute whether the current was available or not. These data represent the first demonstration of a site that supports stimulation-induced feeding in the absence of brain stimulation reward, at least at these specific placements and stimulation parameters.