Location of muscarinic type 2 receptors within the synaptic circuitry of the cat visual thalamus

A cholinergic projection from the parabrachial region (PBR) of the brainste m to the visual thalamus has been studied in great detail during the past 2 0 years. A number of physiological studies have demonstrated that this proj ection causes a dramatic change in thalamic activity during the transition from sleep to wakefulness. Additionally, the PER may mediate more subtle ch anges in thalamic activity as attentional levels fluctuate during the wakin g state. The synaptic circuitry underlying these events has been identified in the cat thalamus. However, there is currently no anatomical information regarding the distribution of cholinergic receptors in relation to this ci rcuitry To begin to understand how the PER projection modulates thalamic ac tivity, we used immunocytochemical techniques to examine the distribution o f muscarinic type 2 (M2) receptors in the visual thalamus of the cat. The d istribution of M2 receptors correlates well with previous reports of the di stribution of cholinergic terminals in the visual thalamus. At the light mi croscopic level, dense M2 staining was seen in the neuropil of the dorsal l ateral geniculate nucleus (dLGN) and pulvinar nucleus and in somata and pro ximal dendrites of cells in the thalamic reticular nucleus (TRN). In the dL GN and pulvinar nucleus, we quantitatively analyzed the distribution of M2 receptors using electron microscopy. Postembedding immunocytochemistry for gamma aminobutyric acid (GABA) was used to determine whether M2 receptors a re present on interneurons or thalamocortical cells. In particular, we exam ined the distribution of M2 receptors with respect to the known sites of PE R terminations. The dendrites of both thalamocortical cells and interneuron s were stained for the M2 receptors in both the glomerular and extraglomeru lar neuropil. However, the densest staining was found in glomerular GABAerg ic profiles that displayed the morphology associated with interneuron dendr itic terminals (F2 profiles). Our data suggest that M2 receptors play an im portant role both in blocking thalamic spindle oscillations and in increasi ng the efficacy of signal transmission during increased attentional states. (C) 1999 Wiley-Liss, Inc.