Chronic morphine exposure induces degradation of receptive field properties of LGN cells in cats1

Abstract

Aim: To investigate the effect of chronic morphine exposure on the receptive field properties of lateral geniculate nucleus (LGN) neurons in cats. Methods: Cats were injected with morphine (10 mg/kg) or saline twice daily, for 10 d. Subsequently, extracellular single‐unit recording techniques were used to examine the sensitivity of LGN neurons to visual stimuli in chronic morphine‐treated and saline‐treated cats. Results: Compared with saline‐treated cats (as controls), LGN neurons in morphine‐treated cats had decreased signal‐to‐noise ratios (1.9 vs 3.1), and orientation and direction sensitivity (0.103 vs 0.135 and 0.074 vs 0.10, respectively), accompanied by significant increases in spontaneous (27.4 vs 17.5) and evoked activity (preferred: 42.2 vs 38.1; average: 28.1 vs 22.3). Conclusion: Chronic morphine exposure can lead to the functional degradation of LGN neurons in cats, which might result from the effects of chronic morphine exposure on inhibitory neurotransmission.

Chronic Morphine Exposure Affects Visual Response Latency of the Lateral Geniculate Nucleus in Cats

“…Visual latency is affected by many of these inhibitory and excitatory mechanisms 24 . It has been reported that spontaneous activity is higher and that the signal‐to‐noise ratio is lower in LGN cells from morphine‐treated cats, which may be due to a decline in GABAergic inhibition 11 . We also observed these changes in response properties.…”

“…Previous research has shown that morphine‐like drugs can alter visual discrimination performance in rats, 7 evoke cortical potentials in cats 8 and reduce visual sensitivity in humans 9 . Recent studies have also revealed that chronic morphine exposure induces degradation of the receptive field properties of the LGN and primary visual cortex (V1) cells in cats 10,11 . However, it is not known whether chronic morphine exposure affects the temporal properties of LGN cells and whether morphine has similar effects on different types of neurons.…”

“…9 Recent studies have also revealed that chronic morphine exposure induces degradation of the receptive field properties of the LGN and primary visual cortex (V1) cells in cats. 10,11 However, it is not known whether chronic morphine exposure affects the temporal properties of LGN cells and whether morphine has similar effects on different types of neurons.…”

GABA increases stimulus selectivity of neurons in primary visual cortices of cats chronically treated with morphine

“…Additionally, decreased response modulation, extended time course of response and visual response latencies were also found in the visual pathway of cats given chronic morphine treatment (He et al, 2005c;Long et al, 2008). Most notably, orientation and direction selectivity, which play important roles in the perception of form (Hubel, 1988) and motion (Albright and Stoner, 1995),were also found to be impaired in the LGN (He et al, 2005a) and V1 (He et al, 2005b). All these findings suggest that chronic morphine exposure has a substantial influence on neurons in the visual pathway.…”

“…Previous studies in our laboratory have revealed that, when compared with normal or saline-treated cats, cats treated chronically with morphine exhibit significantly different neuronal response properties, such as higher spontaneous activities, higher visually evoked responses and lower signal-to-noise ratios in both the lateral geniculate nucleus (LGN) (He et al, 2005a) and V1 (He et al, 2005b). A degraded signal-to-noise ratio leads to decreased ability of the neural system to discriminate signal from background, which therefore may have some relationship with the disrupted visual functions, such as the reduced visual sensitivity, described above.…”