Subcortical influences upon prefrontal cortex of the cat

Excitatory responses evoked in prefrontal cortex by mediodorsal thalamic nucleus stimulation: influence of anaesthesia

The prefrontal cortex and the mediodorsal thalamic nucleus are reciprocally connected through excitatory amino acid pathways. Cortical excitatory responses resulting from activation of either the mediodorsal thalamic nucleus-prefrontal cortex pathway (short latency) or the recurrent collaterals of prefrontal cortex-mediodorsal thalamic nucleus neurons (long latency) can be discriminated mainly by their latency. The present study was undertaken to compare the effects of halothane and ketamine anaesthesia on these cortical excitatory responses and to establish their pharmacological characteristics using microiontophoretic application of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D,L-2-amino-5-phosphonovaleric acid (APV), the specific antagonists of D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxalone propionic acid (AMPA) and N-methyl-D-aspartic acid (NMDA) receptors respectively. The number of prefrontal cortex cells which presented short or long latency excitatory responses was smaller in halothane- than in ketamine-anaesthetized rats. Whatever the anaesthetic used, short latency responses were blocked by CNQX and not affected by APV. Long latency responses were mainly blocked by APV and occasionally by CNQX in halothane-anaesthetized rats, while they were only blocked by CNQX in ketamine-anaesthetized animals. Therefore, halothane seems to preferentially reduce evoked responses mediated by AMPA receptors while ketamine completely abolishes evoked responses involving NMDA receptors. Moreover, the present data confirm that excitatory responses resulting from the activation of the mediodorsal thalamic nucleus-prefrontal cortex pathway are mainly mediated by AMPA receptors. In addition, they demonstrate that cortical responses linked to the activation of recurrent collaterals from prefrontal cortex-mediodorsal thalamic nucleus neurons involve both AMPA and NMDA receptors.

Anatomical and electrophysiological evidence for an excitatory amino acid pathway from the thalamic mediodorsal nucleus to the prefrontal cortex in the rat

This study was undertaken to identify the neurotransmitter of the projection from the thalamic mediodorsal nucleus (MD) to the prefrontal cortex (PFC) using both retrograde transport of D-[3H]aspartate and electrophysiological approaches in the rat. Unilateral microinjections of D-[3H]aspartate performed into the prelimbic area of the PFC resulted in dense labelling of numerous cells in the ipsilateral MD. Excitatory responses were observed in PFC neurons after electrical stimulation of the MD. However, since cortical neurons project to the MD, these excitatory responses could have resulted either from the activation of the MD-PFC pathway and/or from the activation of recurrent collaterals of antidromically driven cortico-thalamic fibres. The conduction time of each of these two reciprocal pathways was determined by antidromic activation. Short latency excitatory responses resulted from activation of the MD-PFC pathway. They were predominantly observed in PFC neurons located in layer III and evoked at low frequency stimulation (0.3-1 Hz). These excitatory responses disappeared or were replaced by longer latency responses when higher frequency stimulations (3-10 Hz) were used. MD-evoked responses were blocked by the iontophoretic application of the AMPA receptor antagonist CNQX into the PFC. These results indicate that the MD-PFC pathway utilizes glutamate and/or aspartate as the neurotransmitter and that its activation induces excitation in PFC neurons through AMPA receptors. Even though the local application of the NMDA receptor antagonist APV was ineffective, a contribution of these receptors in MD-PFC transmission cannot be excluded.

Glutamatergic hippocampal formation projections to prefrontal cortex in the rat are regulated by GABAergic inhibition and show convergence with glutamatergic projections from the limbic thalamus

Anatomic and physiologic studies in the rat have shown projections from the hippocampal formation (HF) and mediodorsal (MD) thalamic nucleus to the medial prefrontal cortex (mPFC). The authors used multi-barrel iontophoresis to: confirm the neurotransmitter used in the projection from HF to mPFC; investigate the role of GABAergic inhibition in the regulation of this projection; and examine the functional convergence of projections from HF and MD onto single mPFC neurons. During HF stimulation, nine cells (6%) showed excitation followed by prolonged inhibition, 39 cells (26%) showed prolonged inhibition alone and 100 cells (68%) showed no clear response. In a further 12 cells that showed no predrug excitation to HF stimulation (representing 16% of the cells in this category), iontophoresis of the GABAA antagonist bicuculline methiodide (BMI) revealed excitatory responses. A total of six mPFC cells (38% of the cells showing excitatory responses to HF stimulation) showed convergent excitation to HF and MD thalamic (or adjacent paratenial nucleus) stimulation. Five out of eight (63%) of the predrug or BMI-revealed excitatory responses of mPFC neurons to HF stimulation were selectively decreased after AMPA antagonist iontophoresis (either CNQX or DNQX). These data confirm that the HF projection to prefrontal cortex is, at least in part, glutamatergic; suggest that the responses of mPFC neurons to activity in this HF pathway are regulated by GABAergic inhibition; and indicate that projections from HF and MD converge onto single mPFC neurons.

Organization of cerebral cortical afferent systems in the rat. II. Hypothalamocortical projections

The organization of hypothalamic projections to the cerebral cortex in the rat has been studied using retrograde and anterograde tracer methods. Four separate populations of hypothalamic neurons, which constitute a major source of diffuse cortical innervation, were identified: Tuberal lateral hypothalamic (LHAt) neurons which innervate the cerebral cortex tend to cluster in the perifornical region, in the zona incerta, and along the medial edge of the cerebral peduncle, at levels roughly coextensive with the ventromedial hypothalamic nucleus. Most of these neurons project to the ipsilateral cortex; a small percentage innervate the contralateral cortex, but this varies among cortical terminal fields. The perifornical neurons are organized in a roughly topographic medial-to-lateral relationship with respect to their cortical terminal fields. Field of Forel (FF) neurons, which project primarily to the frontal cortex of the ipsilateral hemisphere, are located just ventral to the medial edge of the medial lemniscus, at the level of the ventromedial basal thalamic nucleus. The more laterally placed neurons innervate the lateral frontal, insular and perirhinal cortex; the more medial neurons, around the mammillothalamic tract, innervate the medial frontopolar, prelimbic, infralimbic, and anterior cingulate cortex. Posterior lateral hypothalamic (LHAp) neurons form a dense cluster spanning the lateral hypothalamus, from the cerebral peduncle to the posterior hypothalamic area at premammillary levels, and extending into the supramammillary nucleus and the adjacent ventral tegmental area. LHAp neurons innervate the entire cerebral cortex, predominantly on the ipsilateral side. Populations of LHAp neurons projecting to different cortical target areas show considerable spatial overlap, but computer plots of the centers of these populations demonstrate a strict topographic relationship with respect to the cerebral cortex. Tuberomammillary (TMN) neurons form a sheet along the ventrolateral surface of the premammillary hypothalamus. About twice as many TMN neurons innervate the ipsilateral, as compared to the contralateral hemisphere; it is not known whether single neurons project to both hemispheres. No topographic organization of the TMN cortical projection is apparent. Injections of different-colored fluorescent dyes into various cortical areas demonstrate that hypothalamic neurons in general have rather restricted cortical terminal fields. Only occasional neurons are found, primarily in LHAt, which are double labeled by injections into different cytoarchitectonic areas.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibitory influence of the mesocortical dopaminergic system on spontaneous activity or excitatory response induced from the thalamic mediodorsal nucleus in the rat medial prefrontal cortex

Since the medial prefrontal cortex receives converging projections from the mediodorsal nucleus of the thalamus (MD) and the dopaminergic neurons located in the ventromedial mesenscephalic tegmentum (VMT) the responses of cortical neurons to ipsilateral VMT and MD stimulation (50-150 microA; 0.2-0.5 ms duration) were analyzed in ketamine anaesthetized rats. MD stimulation at 1 Hz blocked the firing of 99% of the spontaneously active cortical units tested (mean latency, 15 ms; mean duration, 182 ms). MD stimulation at 5-10 Hz evoked single spike responses (mean latency, 16 ms) in 80% of the units tested. Ten to 15 days after kainic acid injection into the MD the number of cortical neurons inhibited (1 Hz) or excitated (5-10 Hz was reduced to 57 and 18%, respectively. Following stimulation of the VMT (at a frequency of 1-5 Hz), 85% of cortical neurons showed an arrest of spontaneous firing occurring after a mean latency of 17 ms and lasting 109 ms on the average. Most of the cells displaying the VMT inhibitory effect were excitated by MD stimulation. Moreover VMT stimulation, applied 3-45 ms before that of MD, blocked the excitation induced by MD in 75% of the units tested. After injection of 6-hydroxydopamine into the medial forebrain bundle or intraperitoneal administration of alpha-methyl-paratyrosine (alpha-MpT), the number of units tested responding to VMT stimulation was of 19 and 35%, respectively. Moreover in these treated rats, the proportion of excitatory responses to MD blocked by VMT stimulation was reduced to 5 and 6%.(ABSTRACT TRUNCATED AT 250 WORDS)